Дисертації з теми "TERAHERTZ (THZ) RADIATION"

Thesis (MSc)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Terahertz radiation is currently used in security, information and communication technology (ICT), and biomedical sciences among others. The usability of terahertz (THz) radiation, in many of its applications depends on characteristics of the materials being investigated in the THz range. At the heart of THz usage is a THz spectroscopy system necessary for the generation and detection of the THz radiation. In this thesis, we characterise such a THz spectroscopy system. In our typical THz spectrometric system, we make use of femtosecond (fs) laser technology and pump-probe principles for emission and detection of THz radiation. Background about the principles of generation THz radiation using fs triggered antennas and the principles of the spectroscopy technique and appropriate literature references are presented. Using an assembled commercially available kit, we reproduce known spectra in order to confirm correct functionality (for calibration) of the assembled spectroscopy system and to gain experience in interpreting these spectra. By introducing a suitable x - y scanning device we construct a crude THz imaging device to illustrate the principle.
AFRIKAANSE OPSOMMING: Terahertsstraling word deesdae wyd in die sekuriteits, inligting-en-kommunikasie en biomediese sektore aangewend. Die gepastheid van terahertsstraling (THz) vir ’n spesifieke toepassings hang af van die eienskappe van die materiale wat ondersoek word. Vir die uitvoer van sulke eksperimente word ’n THz-spektroskopie sisteem benodig vir die opwekking en meting van THz-straling. In hierdie tesis word so ’n THz-spektroskopie sisteem beskou en gekarakteriseer. In die sisteem word van ’n femtosekondelaser (fs) gebruik gemaak in ’nn pomp-en-proef opstelling vir die uitstraling en meting van THz-straling. Die beginsels rakende die opwekking van THz-straling, deur gebruik te maak van ’n antenna wat deur ’n fs-laser geskakel word, asook die beginsels van die spektroskopiese tegniek, met toepaslike verwysings, word in die tesis aangebied. Deur gebruik te maak van’n kommersiële THz opstelling is bekende spektra gemeet om die korrekte funksionering (vir kalibrasie doeleindes) na te gaan en om ondervinding op te doen in die interpretasie van hierdie spektra. ’n X-Y-translasie toestel is tot die opstelling bygevoeg om THz-afbeelding moontlik te maak en sodoende hierdie beginsel te illustreer.

Creation of ultrafast semiconductor components is inconceivable without understanding various processes of picoscond duration in semiconductors. These processes, as electron energy relaxation time or nonequiriblium carrier capture are very important for semiconductor photonics and terahertz range devices. Since now, the most popular tool of measuring ultrafast processes in semiconductors was picosecond or femtosecond laser pulses. In spite of excellent time resolution, optical pump – probe methods have a significant imperfection. Interpretation of the results can be very complicate. Also, the measured result can be affected by few variable parameters or interaction of various physical phenomenon. Therefore determinate results can be hardly related with electron time dependent characteristic. The aim of this dissertation was to measure electron energy relaxation times and electron life times by using terahertz pulses in narrow – gap semiconductors used for photoconductive terahertz emitters or detectors. In this dissertation, electron characteristic times witch describe various processes in semiconductor, were studied. These measurements were performed by optical pump – terahertz probe technique and time domain terahertz spectroscopy. The emission of terahertz pulses from the semiconductor surface, illuminated by femtosecond laser pulses, was investigated.
Ultrasparčių puslaidininkinių komponentų kūrimas reikalauja gilesnio supratimo apie tai, kaip puslaidininkiuose vyksta fizikiniai procesai, trunkantys kelias pikosekundes ar net mažiau nei vieną pikosekundę. Tokie reiškiniai, kaip elektronų impulso ir energijos relaksacija bei nepusiausvyrųjų krūvininkų pagavimas yra labai svarbūs puslaidininkinių fotonikos ir terahercinio diapazono prietaisų veikimui. Iki pastarojo meto pagrindinis ultrasparčiųjų procesų puslaidininkiuose tyrimo įrankis buvo optiniai metodai, kuriuose elektronų dinamikai stebėti buvo pasitelkiami pikosekundinių ar femtosekundinių lazerių impulsai. Nepaisant išskirtinai didelės šių metodų laikinės skyros, optinio kaupinimo-zondavimo matavimų rezultatus yra palyginti sudėtinga interpretuoti. Šie rezultatai dažniausiai yra įtakojami kelių sistemos parametrų kitimo ir įvairių fizikinių reiškinių tarpusavio sąveikos, todėl sunkiai susiejamas su kuria nors elektronų laikine charakteristika. Disertacijos darbo tikslas – naudojant terahercinės spinduliuotės impulsus išmatuoti elektronų impulso ir energijos relaksacijos trukmes keliuose siauratarpiuose puslaidininkiuose bei jų gyvavimo trukmes medžiagose, skirtose fotolaidžių terahercinės spinduliuotės emiterių ir detektorių gamybai. Šioje disertacijoje yra pateikiami įvairių charakteringų elektroninius procesus puslaidininkiuose apibūdinančių trukmių matavimų naudojant terahercinės spinduliuotės impulsus rezultatai. Tokie tyrimai atlikti ir optinio žadinimo –... [toliau žr. visą tekstą]

Cette thèse est consacrée à l'étude des détecteurs de radiation THz basés sur des transistors à effet de champ qui ont été fabriqués en utilisant les technologies différentes. La photo-réponse de transistors à effet de champ a été étudiée dans une large gamme d'intensités de radiation: de 0,5 mW/cm2 à 500 kW/cm2, et pour des fréquences allant de 0,13 THz à 3,3 THz. Les détecteurs montrent la photo-réponse linéaire en fonction de l'intensité du rayonnement dans une large gamme d'intensités, jusqu'à plusieurs kW/cm2. Pour toutes les fréquences, nous avons observé que la région linéaire a été suivie par une partie non linéaire et ensuite par une saturation. Cet effet a conduit à un nouveau modèle de détecteurs FET à large bande qui est basé sur la connaissance phénoménologique de caractéristiques statiques de transistor. Le modèle prend en compte le comportement non linéaire du courant dans le canal dans toute une plage de fonctionnement du transistor, ce qui est particulièrement important à des intensités élevées de rayonnement THz. Les données expérimentales ont été interprétées avec succès dans le cadre du modèle développé
This thesis is devoted to study of terahertz detectors based on field-effect transistors fabricated using silicon technology and they comparison to InGaAs/InP ones. The main research effort was devoted to the problem of detectors linearity at high radiation intensities. The photoresponse of field effect transistors to terahertz radiation in a wide range of intensities: from 0.5 mW/cm2 up to 500 kW/cm2 and for frequencies from 0.13 THz to 3.3 THz was studied. This work shows that the photoresponse of all studied detectors increases linearly with increasing radiation intensity up to a few kW/cm2 range and is followed by the nonlinear and saturation parts for higher radiation intensities. This effect has led to the new model of broadband field-effect transistor detectors. The model is based on the phenomenological knowledge of the transistor static transfer characteristic and explains the photoresponse nonlinearity as related to non-linearity and saturation of the transistor channel current. The developed model explains consistently experimental data both in linear and nonlinear regions of terahertz detection

Les travaux de cette thèse consistent à développer des outils de spectroscopie et d'imagerie térahertz pour des applications médicales. L'objectif est de déterminer le potentiel et l'efficacité de la spectroscopie térahertz et de l'imagerie dans la détection des régions cancéreuses et la distinction entre les tissus malades et sains pour le cancer du sein chez les femmes. La spectroscopie térahertz est une technique sans contact, non ionisante pour obtenir des résultats rapides, comparée à l'analyse clinique standard. Les études expérimentales sont divisées en deux sections principales :Section I :Cette partie se concentre sur la spectroscopie en utilisant un rayonnement THz. La maîtrise de cette technique permet de travailler en mode réflexion ou transmission avec des fréquences dans la bande passante térahertz. Plusieurs types de matériaux ont été utilisés comme fantômes pour la calibration de l'expérience : des solides (silice, téflon, saphir et verre), des liquides (méthanol, eau et alcool) et des tissus biologiques (cancer, fibres et gras), ainsi qu'un mélange (eau-méthanol). Les indices de réfraction, les coefficients d'absorption et les fonctions diélectriques complexes ont d'abord été mesurés et extraits puis fittés avec un modèle de Debye. Les tissus biologiques sont apparus hétérogènes en épaisseur et avec des surfaces qui peuvent être irrégulières, ce qui rend difficile l'extraction d'informations précises, en raison d’artefacts induits. Les signaux ont été traités en suivant un protocole rigoureux : Les mesures sont effectuées sur un support parfaitement caractérisé en transmission pour réduire les incertitudes sur la phase lors des mesures en réflexion. Les signaux THz réfléchis aux interfaces entre l'air / échantillon, air / fenêtre, eau / fenêtre et fenêtre / fenêtre sont utilisés comme signal de base pour estimer et améliorer le rapport signal-bruit dans les mesures de spectroscopie. L'avantage de cette méthode est sa précision, sa simplicité et sa facilité d'application pour un système de réflexion avec un angle d'incidence. La mesure des indices de réfraction et des coefficients d'absorption des échantillons avec des tissus tumoraux et sains a révélé que les régions tumorales présentent des différences significatives par rapport au tissu normal lors de l’interaction tissu-rayonnement térahertz.Section II :La deuxième partie de cette étude porte sur l'imagerie THz pour la détection du cancer du sein, à la fois dans les modes de transmission et de réflexion. Plusieurs types d'échantillons ont été étudiés. Les coupes utilisées comprenaient des tissus inclus en paraffine, des tissus frais sortis du bloc opératoire, fixés au formol et des blocs. Pour cela le spectromètre a été déplacé à l'hôpital. Plus de 50 échantillons ont été ainsi inspectés. TroisIVméthodes de traitement d'image ont été utilisées : le découpage, l'automatisation et le tri d'images manuel. De plus, les images obtenues dans le domaine temporel et dans le domaine fréquentiel ont été analysées pour décrire et identifier les différentes régions du tissu mammaire étudiées et déterminer le contraste entre le tissu sain et le tissu malade. La quantité d'eau différentielle présente dans les tissus malades peut être l'une des origines de contraste. En effet, le tissu cancéreux possède une teneur en eau plus élevée que celle des fibres ou des tissus adipeux normaux, ce qui permet de discriminer les régions cancéreuses, fibreuses et graisseuses sur les images THz
The work of this thesis consists in developing terahertz spectroscopy and imaging tools for medical applications. The goal is to determine the potential and effectiveness of terahertz spectroscopy and imaging in the detection of cancer regions and the distinction between diseased and healthy tissue for breast cancer in women. Terahertz spectroimaging is a non-contact, non-ionizing technique for rapid results compared to standard clinical analysis. Experimental studies are divided into two main sections:Section IThis part focuses on THz spectroscopy using THz radiation. The mastery of this technique makes it possible to work in reflection or transmission mode with frequencies in the terahertz bandwidth. Several types of materials have been used as ghosts for the calibration of the experiment: solids (silica, teflon, sapphire and glass), liquids (methanol, water and alcohol) and biological tissues (cancer, fiber and fat), as well as a mixture (water-methanol). The refractive indices, the absorption coefficients and the complex dielectric functions were first measured and extracted and then fitted with a Debye model. Biological tissues have appeared heterogeneous in thickness and with surfaces that may be irregular, making it difficult to extract accurate information because of induced artifacts. The signals have been processed according to a rigorous protocol: The measurements are carried out on a perfectly characterised substratet in transmission to reduce the uncertainties on the phase during the measurements in reflection. The THz signals reflected at the interfaces between the air / sample, air / window, water / window and window / window are used as a basic signal to estimate and improve the signal-to-noise ratio in the spectroscopy measurements. The advantage of this method is its accuracy, simplicity and ease of application for a reflection system with an angle of incidence. Measurement of refractive indices and absorption coefficients of samples with tumor and healthy tissue revealed that the tumor regions showed significant differences from normal tissue during terahertz tissue-radiation interaction.Section II:The second part of this study focuses on THz imaging for breast cancer detection in both transmission and reflection modes. Several types of samples have been studied. Sections used included paraffin-embedded tissue, fresh tissues removed from the OR, formalin-fixed, and blocks. For this the spectrometer has been moved to the hospital. More than 50 samples were inspected. Three image processing methods were used: cutting, automation and manual image sorting. In addition, time domain and frequency domain images were analyzed to describe and identify the different regions of mammary tissue studied and to determine the contrast between healthy tissue and diseased tissue. The amount of differential water present in diseased tissue can be one of the sources of contrast. In fact, the cancerous tissue has a higher water content than that of normal fibers or adipose tissue, which makes it possible to discriminate the cancerous, fibrous and fatty regions on the THz images

As the amount of study into the terahertz (THz) region of the electromagnetic spectrum steadily increases, the parallel plate waveguide has emerged as a simple and effective fixture to perform many experiments. The ability to concentrate THz radiation into a small area or volume enables us to analyze smaller samples and perform more repeatable measurements, which is essential for future research. While the fundamental physics of PPW transmission are understood mathematically, the practical knowledge of building such a fixture for the THz domain and taking measurements on it with a real system needs to be built up through experience. In this thesis, multiple PPW configurations are built and tested. These include waveguides of different lengths and opening heights, using lenses and antennas to focus and collect radiation from the input and output, and different amounts of polish on the waveguide surface. A basic resonator structure is also built and measured as a proof of concept for future research. The two most useful propagation modes through the waveguide, the lowest order transverse magnetic (TEM) and transverse electric (TE) modes, were characterized on all of the setups. Additionally, a flexible fixture was designed and measured which will allow future work in the THz field to be much more reliable and repeatable.

Depuis les années 1975, les atomes de Rydberg sont étudiés et maintenant utilisés en information quantique pour leurs propriétés particulières d’interaction. Cependant, ces objets physiques peuvent se retrouver impliqués dans différentes autres applications, où leurs caractéristiques remarquables en font de parfaits outils. Dans ce mémoire, nous nous intéresserons à deux applications distinctes faisant intervenir des atomes de Rydberg de césium. Tout d’abord, nous verrons comment utiliser de tels atomes pour produire une source d’électrons monocinétiques, grâce au mécanisme d’ionisation singulier de ce type d’atomes à une valeur précise de champ électrique dépendante du niveau d’excitation. Les électrons ainsi produits sont ensuite extraits et leur dispersion en énergie mesurée. On montrera notamment de façon théorique et d’après les premières mesures expérimentales réalisées pendant la thèse, que l’on peut espérer obtenir une dispersion en énergie des électrons produits par cette technique de l’ordre du meV, résolution jamais atteinte à ce jour. Ce type de source devient aujourd’hui un outil indispensable pour accéder à la mise au point et l’étude de nouveaux matériaux par contrôle de réactions chimiques à l’échelle moléculaire, et à la cartographie des phonons. Dans un second temps, nous verrons qu’il est possible de désexciter un nuage d’atomes de Rydberg de niveaux variés grâce à une source externe dans le domaine térahertz. Ce projet s’inscrit dans le cadre des expériences d’étude de l’antimatière menées actuellement au CERN, qui visent à élucider le mystère de l’asymétrie matière/antimatière. Les méthodes actuelles de production de l’antihydrogène, forment des nuages de ces anti-atomes dans différents états de Rydberg. Pour les étudier, il est alors nécessaire de désexciter le plus d’atomes d’antihydrogène possible vers le niveau fondamental. Nous présenterons la méthode envisagée, ainsi que les résultats obtenus expérimentalement sur un dispositif créé pendant la thèse pour montrer la faisabilité de la technique. Ces premiers résultats montrent qu’il est possible d’accélérer la désexcitation d’un atome de Rydberg sur un état très élevé grâce à une lampe se comportant comme un corps noir. Nous détaillerons les améliorations envisagées, en particulier pour adapter le spectre des fréquences THz à utiliser et empêcher la photoionisation des atomes, par des filtres ou par le façonnage spectral via l’utilisation d’un photomixer
Since 1975, Rydberg atoms have been studied and now used in quantum information for their particular interaction properties. However, these physical objects can be involved in various other applications, where their remarkable characteristics make them perfect tools. In this paper, we will focus on two distinct applications involving cesium Rydberg atoms. First, we will see how to use such atoms to produce a source of monocinetic electrons, thanks to the singular ionization mechanism of this type of atoms at a precise value of electric field dependent on the excitation level. The electrons thus produced are then extracted and their energy dispersion measured. Theoretically and according to the first experimental measurements made during the thesis, we will show that we can hope an energy dispersion of the electrons produced by this meV technique, a resolution never reached before. Today, this type of source is becoming an indispensable tool for the development and study of new materials by molecular scale chemical reaction control and for phonon mapping. In a second step, we will see that it is possible to de-energize a cloud of Rydberg atoms of various levels thanks to an external source in the tera-hertz domain. This project is part of the ongoing anti-matter experiments at CERN, which aim to unravel the mystery of the matter/anti-matter asymmetry. The current methods of production of antihydrogen, forms clouds of these anti-atoms in different Rydberg states. To study them, it is then necessary to de-energize as many antihydrogen atoms as possible to the fundamental level. We will present the method envisaged, as well as the results obtained experimentally on a device created during the thesis to show the feasibility of the technique. These first results show that it is possible to accelerate the deenergization of a Rydberg atom on a very high state thanks to a lamp behaving like a black body. We will detail the improvements envisaged, in particular to adapt the spectrum of the THz frequencies to use and prevent the photoionization of atoms, by filters or by spectral shaping via the use of a photomixer

Parmi les approches possibles pour réaliser des sources térahertz dans la gamme0,2 - 2 THz, nous nous sommes intéressés à la voie optoélectronique qui consiste à générerl'onde térahertz par le photomélange de deux ondes lasers à des fréquences optiques. Letravail présenté dans cette thèse concerne l'étude de lasers bi-fréquence capables d'émettreles deux ondes requises simultanément. Nous commençons par développer un modèlethéorique décrivant la compétition de gain entre les modes laser grâce au calcul de différentscoefficients de couplage. Sur le plan expérimental, nous montrons tout d'abord qu'endésalignant légèrement un des miroirs de la cavité laser, il est possible d'obtenir un régimestable d'émission sur deux fréquences pourtant en compétition dans le milieu à gain, ici uncristal dopé néodyme. Nous nous intéressons ensuite au régime impulsionnel et montronsque les impulsions peuvent être synchronisées grâce à l'action d'un laser externe. Enfin, leprocessus de photomélange a été réalisé et des ondes électromagnétiques ont été généréesdans le domaine des radio-fréquences autour de 20 GHz.

The acceleration of charged particles with ultrafast terahertz electromagnetic radiation could enable new, and improve many of aspects of, accelerator applications. These include providing shorter electron bunches for ultrafast time-resolved pump-probe spectroscopy, enabling complex longitudinal profiles to be imparted onto charged particle bunches and significantly improving the ability to synchronise an accelerator to an external laser. In this thesis I present investigations into terahertz radiation sources that enabled the generation of terahertz radiation with attractive properties for accelerator based applications. Specific attention has been paid to temporally tunable sources that generate strong longitudinally polarised electric field components as these enable a free-space co-linear interaction geometry to be implemented. A simulation describing the propagation of radiation from such sources has been developed. Terahertz sources have been designed and the radiation generated characterised via electro optic detection. These include a radially biased photoconductive antenna (PCA) based source of which the longitudinally polarised terahertz electric field component was found to have an amplitude of 2.22 kVcm-1 as well as a near-single cycle temporal profile. This radially biased PCA was used in conjunction with the Accelerators and Lasers in Combined Experiments (ALICE) energy recovery linear accelerator at the Daresbury Laboratory in an electron acceleration experiment. To enable higher longitudinally polarised terahertz electric field strengths to be obtained, as well as the ability to temporally tune the terahertz radiation, generation within non-linear optical crystals was investigated. Magnesium-oxide doped stoichiometric lithium niobate (MgO:SLN) was investigated as a possible candidate due to its high non-linear susceptibility tensor and reported ability to impose temporal tuning directly from the pump laser beam. A scheme consisting of two MgO:SLN crystals each generating a separate linear polarised terahertz pulse which were then combined via a lens was designed and built. Electro optic detection techniques were used to characterise the radiation generated from this source. Peak terahertz electric fields amplitudes of 11.6 kVcm-1 and 47 kVcm-1 were measured for both the longitudinally and transversely polarised field components respectively. Temporal profiles measured from both the longitudinally and transversely polarised electric field components showed electric field periods of approximately 300 fs. This method of generating terahertz radiation employed a pulse-front tilt technique. Allowing for the same scaling as recently reported in the literature for MgO:SLN generation techniques, which will in principle allow this method to scale to longitudinally polarised terahertz electric field profiles in excess of 1 MVcm-1.

This thesis tackles issues of particular interest regarding analysis and design of passive components at the mm-wave and Terahertz (THz) bands. Innovative analysis techniques and modeling of complex structures, design procedures, and practical implementation of advanced passive devices are presented. The first part of the thesis is dedicated to THz passive components. These days, THz technology suffers from the lack of suitable waveguiding structures since both, metals and dielectric, are lossy at THz frequencies. This implies that neither conventional closed metallic structures used at microwave frequencies, nor dielectric waveguides used in the optical regime, are adequate solutions. Among a variety of new proposals, the Single Wire Waveguide (SWW) stands out due to its low attenuation and dispersion. However, this surface waveguide presents difficult excitation and strong radiation on bends. A Dielectric-Coated Single Wire Waveguide (DCSWW) can be used to alleviate these problems, but advantages of the SWW are lost and new problems arise. Until now, literature has not given proper solution to radiation on bends and, on the other hand, rigorous characterization of these waveguides lacks these days. This thesis provides, for the first time, a complete modal analysis of both waveguides, appropriated for THz frequencies. This analysis is later applied to solve the problem of radiation on bends. Several structures and design procedures to alleviate radiation losses are presented and experimentally validated. The second part of the thesis is dedicated to mm-wave passive components. These days, when implementing passive components to operate at such small, millimetric wavelengths, to ensure proper metallic contact and alignment between parts results challenging. In addition, dielectric absorption becomes significant at mm-wave frequencies. Consequently, conventional hollow metallic waveguides and planar transmission lines present high attenuation so that new topologies are being considered. Gap Waveguides (GWs), based on a periodic structure introducing an Electromagnetic Bandgap effect, result very suitable since they do not require metallic contacts and avoid dielectric losses. However, although GWs have great potential, several issues prevent GW technology from becoming consolidated and universally used. On the one hand, the topological complexity of GWs difficulties the design process since full-wave simulations are time-costly and there is a lack of appropriate analysis methods and suitable synthesis procedures. On the other hand, benefits of using GWs instead of conventional structures are required to be more clearly evidenced with high-performance GW components and proper comparatives with conventional structures. This thesis introduces several efficient analysis methods, models, and synthesis techniques that will allow engineers without significant background in GWs to straightforwardly implement GW devices. In addition, several high-performance narrow-band filters operating at Ka-band and V-band, as well as a rigorous comparative with rectangular waveguide topology, are presented.
Esta tesis aborda problemas actuales en el análisis y diseño de componentes pasivos en las bandas de onda milimétrica y Terahercios (THz). Se presentan nuevas técnicas de análisis y modelado de estructuras complejas, procedimientos de diseño, e implementación práctica de dispositivos pasivos avanzados. La primera parte de la tesis se dedica a componentes pasivos de THz. Actualmente no se disponen de guías de onda adecuadas a THz debido a que ambos, metales y dieléctricos, introducen grandes pérdidas. En consecuencia, no es adecuado escalar las estructuras metálicas cerradas usadas en microondas, ni las guías dieléctricas usadas a frecuencias ópticas. Entre un gran número de recientes propuestas, la Single Wire Waveguide (SWW) destaca por su baja atenuación y casi nula dispersión. No obstante, como guía superficial, la SWW presenta difícil excitación y radiación en curvas. El uso de un recubrimiento dieléctrico, creando la Dielecric-Coated Single Wire Waveguide (DCSWW), alivia estos inconvenientes, pero las ventajas anteriores se pierden y nuevos problemas aparecen. Hasta la fecha, no se han encontrado soluciones adecuadas para la radiación en curvas de la SWW. Además, se echa en falta una caracterización rigurosa de ambas guías. Esta tesis presenta, por primera vez, un análisis modal completo de SWW y DCSWW, adecuado a la banda de THz. Este análisis es aplicado posteriormente para evitar el problema de la radiación en curvas. Se presentan y validan experimentalmente diversas estructuras y procedimientos de diseño. La segunda parte de la tesis abarca componentes pasivos de ondas milimétricas. Actualmente, estos componentes sufren una importante degradación de su respuesta debido a que resulta difícil asegurar contacto metálico y alineamiento adecuados para la operación a longitudes de onda tan pequeñas. Además, la absorción dieléctrica incrementa notablemente a estas frecuencias. En consecuencia, tanto guías metálicas huecas como líneas de transmisión planares convencionales presentan gran atenuación, siendo necesario considerar topologías alternativas. Las Gap Waveguides (GWs), basadas en una estructura periódica que introduce un efecto de Electromagnetic Bandgap, resultan muy adecuadas puesto que no requieren contacto entre partes metálicas y evitan las pérdidas en dieléctricos. No obstante, a pesar del potencial de las GWs, varias barreras impiden la consolidación y uso universal de esta tecnología. Por una parte, la compleja topología de las GWs dificulta el proceso de diseño dado que las simulaciones de onda completa consumen mucho tiempo y no existen actualmente métodos de análisis y diseño apropiados. Por otra parte, es necesario evidenciar el beneficio de usar GWs mediante dispositivos GW de altas prestaciones y comparativas adecuadas con estructuras convencionales. Esta tesis presenta diversos métodos de análisis eficientes, modelos, y técnicas de diseño que permitirán la síntesis de dispositivos GW sin necesidad de un conocimiento profundo de esta tecnología. Asimismo, se presentan varios filtros de banda estrecha operando en las bandas Ka y V con altas prestaciones, así como una comparativa rigurosa con la guía rectangular.
Aquesta tesi aborda problemes actuals en relació a l'anàlisi i disseny de components passius en les bandes d'ona mil·limètrica i Terahercis. Es presenten noves tècniques d'anàlisi i modelatge d'estructures complexes, procediments de disseny, i implementació pràctica de dispositius passius avançats. La primera part de la tesi es focalitza en components passius de THz. Actualment no es disposen de guies d'ona adequades a THz causa que tots dos, metalls i dielèctrics, introdueixen grans pèrdues. En conseqüència, no és adequat escalar les estructures metál·liques tancades usades en microones, ni les guies dielèctriques usades a freqüències òptiques. Entre un gran nombre de propostes recents, la Single Wire Waveguide (SWW) destaca per la seua baixa atenuació i quasi nul·la dispersió. No obstant això, com a guia superficial, la SWW presenta difícil excitació i radiació en corbes. L'ús d'un recobriment dielèctric, creant la Dielecric-Coated Single Wire Waveguide (DCSWW), alleuja aquests inconvenients, però els avantatges anteriors es perden i nous problemes apareixen. Fins a la data, no s'han trobat solucions adequades per a la radiació en corbes de la SWW. A més, es troba a faltar una caracterització rigorosa d'ambdues guies. Aquesta tesi presenta, per primera vegada, un anàlisi modal complet de SWW i DCSWW, adequat a la banda de THz. Aquest anàlisi és aplicat posteriorment per evitar el problema de la radiació en corbes. Es presenten i validen experimentalment diverses estructures i procediments de disseny. La segona part de la tesi es centra en components passius d'ones mil·limètriques. Actualment, aquests components pateixen una important degradació de la seua resposta a causa de que resulta difícil assegurar contacte metàl·lic i alineament adequats per a l'operació a longituds d'ona tan menudes. A més, l'absorció dielèctrica incrementa notablement a aquestes freqüències. En conseqüència, tant guies metàl·liques buides com línies de transmissió planars convencionals presenten gran atenuació, sent necessari considerar topologies alternatives. Les Gap Waveguides (GWs), basades en una estructura periòdica que introdueix un efecte de Electromagnetic Bandgap, resulten molt adequades ja que no requereixen contacte entre parts metàl·liques i eviten les pèrdues en dielèctrics. No obstant, tot i el potencial de les GWs, diverses barreres impedixen la consolidació i ús universal d'aquesta tecnologia. D'una banda, la complexa topologia de les GWs dificulta el procés de disseny atés que les simulacions d'ona completa consumeixen molt de temps i no existeixen actualment mètodes d'anàlisi i disseny apropiats. D'altra banda, és necessari evidenciar el benefici d'utilitzar GWs mitjançant dispositius GW d'altes prestacions i comparatives adequades amb estructures convencionals. Aquesta tesi presenta diversos mètodes d'anàlisi eficients, models, i tècniques de disseny que permetran la síntesi de dispositius GW sense necessitat d'un coneixement profund d'aquesta tecnologia. Així mateix, es presenten diversos filtres de banda estreta operant en les bandes Ka i V amb altes prestacions, així com una comparativa rigorosa amb la guia rectangular.
Berenguer Verdú, AJ. (2017). Analysis and design of efficient passive components for the millimeter-wave and THz bands [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/84004
TESIS

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2009.
Includes bibliographical references (p. 147-155).
In this thesis research, I implemented a terahertz generation scheme that enables high-field near-single-cycle terahertz (THz) pulse generation via optical rectification in a LiNbO3 (LN) crystal. I also developed a method for the non reconfigurable generation of high-intensity multiple-cycle THz fields aimed to more efficiently deliver THz energy to resonant samples. A novel free-space THz-pump/THz-probe setup enabled time-resolved measurement of ultrafast nonlinear electronic responses in doped bulk semiconductor samples. The ability to spectrally and temporally resolve the response of the semiconductor sample using a THz probe allowed us to uncover both the dynamics of impact ionization and interesting phonon-plasma interactions in indium antimonide (InSb) for the first time. Nonlinear vibrational responses in LN were ob-served first in a dual THz beam setup on an integrated LN waveguide platform with optical probing to reveal the generation of terahertz second harmonic signals and also in THz transmission measurements in cooled LN where THz intensity-dependent self-phase modulation and harmonic generation were observed. An outlook and preliminary results toward implementation and observation of high-field THz-driven responses in ferroelectric materials are given in the final chapter of this thesis.
by Ka-Lo Yeh.
Ph.D.

Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...

The research in the subject of terahertz (THz) radiation has gained a great awareness since nineties to fill the THz gap in the electromagnetic spectra due to the existing significant properties and resulting applications in diverse areas such as medical, spectroscopy, communications and others. The non-ionizing and high penetration potential of the THz radiation has enabled numerous techniques involving both optics and electronics to be proposed to study the emission and detection of THz radiation. The thesis focuses on designing of analytical models to study the emission of high power THz radiation by employing premodulated electron beams using free electron laser (FEL) as a classical device. The role of premodulation of the beam in generation of the THz radiation is discussed. To study the efficient emission of the radiation, plasma based medium such as surface plasma wave, Langmuir wave are used as wigglers in FEL. Slow wave structure involving a waveguide with dielectric lining is also used as a Cerenkov FEL for THz emission. The power, amplitude and efficiency of the radiation wave are evaluated to have an analysis of the output radiation. We have studied the effect of parameters of the electron beam, wiggler, plasma and other factors affecting the tunability of the THz radiation. The route followed in the analytical treatment of the analysis involves fluid theory which deals with the interaction of premodulated electron beam, electromagnetic wave and a pump signal. The softwares used for obtaining the results are MATLAB and MATHEMATICA.

In this thesis, I present studies in the field of terahertz (THz) spectroscopy. These studies are divided into three areas: Development of a narrowband THz source, the study of carrier transport in metal thin films, and the exploration of coherent dynamics of quasi-particles in semiconductor nanostructures with both broadband and narrowband THz sources. The narrowband THz source makes use of type II difference frequency generation (DFG) in a nonlinear crystal to generate THz waves. By using two linearly chirped, orthogonally polarized optical pulses to drive the DFG, we were able to produce a tunable source of strong, narrowband THz radiation. The broadband source makes use of optical rectification of an ultra-short optical pulse in a nonlinear crystal to generate a single-cycle THz pulse. Linear spectroscopic measurements were taken on NiTi-alloy thin films of various thicknesses and titanium concentrations with broadband THz pulses as well as THz power transmission measurements. By applying a combination of the Drude model and Fresnel thin-film coefficients, we were able to extract the DC resistivity of the NiTi-alloy thin films. Using the narrowband source of THz radiation, we explored the exciton dynamics of semiconductor quantum wells. These dynamics were made sense of by observing time-resolved transmission measurements and comparing them to theoretical calculations. By tuning the THz photon energy near exciton transition energies, we were able to observe extreme nonlinear optical transients including the onset of Rabi oscillations. Furthermore, we applied the broadband THz waves to quantum wells embedded in a microcavity, and time-resolved reflectivity measurements were taken. Many interesting nonlinear optical transients were observed, including interference effects between the modulated polariton states in the sample.
Graduation date: 2012

Quantum materials are a class of systems that host low energy emergent properties due to the strong correlations between the lattice, charge, spin and orbital degrees of freedom. In recent years, this classification has encompassed the world of strongly correlated materials, like high-temperature superconductors and Mott-Hubbard insulators, and the topological states of matter, like Dirac/Weyl semimetals and topological insulators. This doctoral thesis discusses the study of the optical characterization of novel quantum materials, with a special focus on the terahertz (THz) spectral range, where low energy emergent features arise as a consequence of strong electronic correlations, symmetry breaking and/or topological transitions. The low energy photons associated to THz radiation grant an easy access to the quasiparticles occupying the low energy modes of quantum materials. The recent progress in the generation and detection of THz radiation have enabled the production of ultrashort pulses at the picosecond scale and their exploitation in time-domain spectroscopy measurements, like pump-probe and nonlinear spectroscopy, from which it is possible to study the out-of-equilibrium dynamics of electrons and dipole-coupled low energy states, along with the appearance of nonlinear responses at the presence of high external electric fields. This thesis shows that quantum materials can host a plethora of THz-related responses which can be tuned in terms of their thickness, temperature and external drivings, like the perturbation associated to an ultrashort optical pulse. It addresses the optical and THz study of novel magnetic quantum materials like Co2MnGa, a magnetic nodal line semimetal, MnBi2Te4, the first intrinsic magnetic topological insulator discovered, and CrI3, a layered ferromagnetic insulator with research interests ranging from spintronics to topological Majorana modes. In particular, the interplay between the magnetic, electronic and phononic states is addressed through linear and nonlinear spectroscopy, along with the direct sampling of the topological features by optical means. As a final purpose, this thesis also reports the novel findings for the direct optical sampling of the superconductive gap in the Sr-doped nickelate NdNiO2, a strain-induced superconductor. The results show how the electrodynamic properties of this material are quantitatively different from the ones found in cuprate superconductors, sharing a similar infinite layer structural phase.

碩士
國立交通大學
光電工程系所
96
THz wave was generated from dipole antenna-type devices made by using oxygen-ion implanted GaAs (GaAs:O) and low-temperature-grown GaAs (LT-GaAs). We compared the emission properties of LT-GaAs photoconductive (PC) antennas with GaAs:O fabricated condition (2.5 X 10^13 ions/cm^2 (500 keV & 800 keV ), 4 X 10^13 ions/cm^2(1200 keV ) in the　pulse and CW mode. The absolute power of THz wave was also measured by a bolometer for comparison of the relative radiation power. Compared GaAs:O with LT-GaAs at same structure of PC antennas. The material GaAs:O can generate higher THz power than LT-GaAs both in pulsed mode and CW mode. The THz power of GaAs:O (2.27uW) is higher than that of LT-GaAs(1.27uW) in CW mode, and in pulse mode, the THz peak amplitude of GaAs:O (5.2mW) is higher than that of LT-GaAs (3.6mW). The bandwidth of GaAs:O and LT-GaAs are measured about 1THz both under pulse (TDS) and CW (photomixing) pumping. However, the THz power of LT-GaAs is saturated on CW mode, while GaAs:O doesn’t. The result shows that GaAs:O is a proper THz emitter compare with LT-GaAs which is hardly to reproduce.

In this thesis, superconducting nanostructures for quantum detection of electromagnetic radiation are studied. In this regard, electrodynamics of topological excitations in 1D superconducting nanowires and 2D superconducting nanostrips is investigated. Topological excitations in superconducting nanowires and nanostrips lead to crucial deviation from the bulk properties. In 1D superconductors, topological excitations are phase slippages of the order parameter in which the magnitude of the order parameter locally drops to zero and the phase jumps by integer multiple of 2\pi. We investigate the effect of high-frequency field on 1D superconducting nanowires and derive the complex conductivity. Our study reveals that the rate of the quantum phase slips (QPSs) is exponentially enhanced under high-frequency irradiation. Based on this finding, we propose an energy-resolving terahertz radiation detector using superconducting nanowires. In superconducting nanostrips, topological fluctuations are the magnetic vortices. The motion of magnetic vortices result in dissipative processes that limit the efficiency of devices using superconducting nanostrips. It will be shown that in a multi-layer structure, the potential barrier for vortices to penetrate inside the structure is elevated. This results in significant reduction in dissipative process. In superconducting nanowire single photon detectors (SNSPDs), vortex motion results in dark counts and reduction of the critical current which results in low efficiency in these detectors. Based on this finding, we show that a multi-layer SNSPD is capable of approaching characteristics of an ideal single photon detector in terms of the dark count and quantum efficiency. It is shown that in a multi-layer SNSPD the photon coupling efficiency is dramatically enhanced due to the increase in the optical path of the incident photon.

碩士
國立成功大學
物理學系碩博士班
91
Terahertz (THz) radiation (THz waves), electromagnetic radiation in a frequency interval from 0.1 to 30 THz, is the next frontier in imaging science and technology. THz waves, or T-rays, occupy a large portion of the electromagnetic spectrum between the infrared and microwave bands. However, compared to relatively well-developed medical imaging at microwaves and optical frequencies, basic research, new initiatives and advanced technological developments in the THz band are very limited. In this thesis, we present the details of our work which has successfully set up a system to generate and detect the terahertz waves. Photoconductive technique is used for radiation generation while electro-optical sampling is employed in the radiation detection. Electro-optical crystal ZnTe is used as the radiation sensor while GaAs wafer and various semiconductor surface intrinsic-N+ (SIN+) microstructures are used as the radiation emitters. The dependences of the intensities and frequency ranges of the THz waves, radiated from InP SIN+ structures, on the built-in electric field, top layer thickness of the structures and pumping power density are investigated. Ti: Sapphire pulse laser with 80 femtosecond (fs) pulse width is used to pump and probe the THz radiation. Since the radiation power and spectrum are also strong related to the pulse width (80 femtosecond), an autocorrelator is also set up to monitor the pulse width of the laser pulse width.

Waveguides are desirable components for energy transmission throughout the electromagnetic spectrum. This thesis experimentally examines a thick slot waveguide for THz guiding and field enhancement. The waveguide is machined from planar copper sheets using the novel technique of femtosecond laser micromachining. In-plane photoconductive THz coupling to a thick slot waveguide is demonstrated using Discontinuous Galerkin Time Domain (DGTD) simulation. The results reveal positive implications for broadband low-loss/dispersion transmission lines up to 1.5 THz.
Graduate
0544
0607
0756
levismith3@hotmail.com

碩士
國立成功大學
物理學系碩博士班
94
The terahertz (THz) radiation is the electromagnetic wave with frequency between 0.1THz to 10THz. In this thesis, we employ the photoconductive mode and the free-space electro-optical sampling to generate and detect the terahertz radiations. Terahertz radiation from InAlAs SIN+ and InAlAs SIP+ hetrostructures and InAs wafer are measured at different pump power. The critical electric field, stored energy and optical absorption saturation in the emitters have played important roles in the amplitude of terahertz radiation. The decrease of terahertz radiation under the high pump power involves various radiation mechanisms. Finally, the enhancement of terahertz radiation by conjugate polymers (DB-PPV) deposited on n-doped GaAs wafer is observed and discussed.

碩士
國立交通大學
光電工程系所
96
In this thesis, role of chirp properties within mode locked pulses to generate efficiency and broadband THz radiation is investigated. By integration of pulse shaping technique and pump-probe measurement, behavior of increasing carrier relaxation time with chirp and positive sign is observed and explained by frequency dependent excess energy. Meanwhile, chirp- and power- dependence of THz radiation are studied using chirp controlled PC antenna based THz radiation system. We confirm the nature of asymmetry and positive preference of THz radiation for mode locked chirp pulse. We further observe an anomalous and nonlinear increase of THz radiation with increasing power of transform-limited and tight focusing exciting pulse. This could be due to distribution of carries from two photon absorption and is the first experimental report of two photon absorption induced THz radiation enhancement which have been predicted theoretically. In addition, we analyze phenomenon observed by Drude-Lorentz model with obtained chirp- and power- dependent carrier lifetime and influence of chirp within sampling. Finally, we suggest possible approaches for generating high efficiency and broadband THz radiation：(1) positive chirp exciting can enhance radiation power but depress the high frequency part. (2) transform-limited exciting pulse can provide broader spectrum and further enhance power under highly tight focusing condition due to TPA contribution. (3) antenna with short carrier and momentum lifetime exciting by transform-limited pulse to decrease influence of PC sampling.

In this work the generation of tunable continuous-wave (CW) terahertz (THz) radiation via photomixing of two modes emitted from a frequency-tunable dual-mode external cavity semiconductor laser is investigated. In order to realize frequency-tunable dual-mode semiconductor lasers (SLs), two concepts are developed. They are based on two double-external-cavity configurations comprising spectrally filtered feedback mechanisms: Double-Littman-configuration and Double-Littrow-configuration. Stable dual-mode emission is realized by employing narrow stripe SLs as gain medium in these configurations. Each of the modes is a single external cavity mode that can be tuned independently over the entire gain spectrum. The difference frequency of the two modes can be changed from 100 GHz to 10 THz. By using these laser-configurations, frequency-tunable CW THz radiation is generated via photomixing of the two laser modes on micro-structured antennas. Two types of antennas are employed to generate THz radiation: a log-periodic antenna and a H-dipole antenna. The generated THz radiation is temporally and spectrally characterized. Measurements using a Fourier-Transform-Spectrometer confirm a single THz spectrum with a narrow linewidth and a frequency corresponding to the difference frequency of the two incident laser modes. The frequency dependence of the performance and polarization of THz radiation follows the frequency-characteristics of the used antennas. Generated THz radiation is applied for spectroscopic demonstrations, including THz transmission of non-metallic materials and identification of absorption lines of HCl and water vapor. To achieve high power dual-mode emission, broad area semiconductor lasers (BALs) are used as the gain medium in a Double-Littman-configuration. Also with BALs in this configuration frequency-tunable dual-mode operation is achieved. Furthermore, longitudinal and lateral modes of the BAL can be controlled by applying a mechanism of spectrally and spatially filtered feedback in the external cavity. By photomixing of two modes emitted from the dual-mode BAL on antennas, THz radiation is generated and characterized. In addition, the simultaneous oscillation of two modes induces the four-wave mixing (FWM) process due to nonlinear interactions of the intense optical field and the semiconductor medium. FWM phenomena observed for dual-mode operation of narrow stripe SL are studied in detail. Ultrafast carrier dynamics such as carrier heating and spectral hole burning are responsible for FWM process in the detuning range of several hundred GHz. Finally, direct generation of THz radiation in dual-mode-BAL is investigated. The results achieved in this work show that frequency-tunable dual-mode external cavity semiconductor lasers provide generation of temporally and spectrally stable, frequency-tunable CW THz radiation via photomixing.

碩士
國立成功大學
光電科學與工程研究所
94
Abstract Terahertz (THz) radiation can be generated from a variety of semiconductors illuminated by femtosecond optical pulses. It is widely believed that three different mechanisms result in THz radiation from surface of bulk semiconductors, i.e. the photoconductivity, the photo-Dember effect, and the optical rectification. In this thesis, we use free space electro-optic sampling (FS-EOS) to characterize the THz radiation from a series of GaAs surface intrinsic-N+ (SIN+) structures. By THz time-domain spectroscopic technique we directly measure the electric field of the THz radiation. In addition, the radiation mechanism of the sample under the experiment condition is identified as the photoconductivity. In the photoconductive process, the THz radiation from semiconductor surface is a consequence of the motion of photo-excited carriers accelerated by the local or bias field which may be an external applied field or an internal field from charge depletion layers, Schottkey barriers, strained layers, and p-n junctions of semiconductors. The modulation spectroscopy of photoreflectance (PR) is used to measure the surface electric field of a series of GaAs SIN+ structures with various intrinsic layers, followed immediately by the measurement of the electric field amplitude of the THz radiation. Experimental results indicate that as the surface field is lower than the so called “critical electric field”, the amplitude of THz radiation field is proportional to the product of the surface field and the number of photo-excited carriers. As the surface field exceeds the critical field, the amplitude is independent of the surface field but is proportional to the product of the critical field and the number of the photo-excited carriers. The critical field corresponds to the field at which the drift velocity is maxima in semiconductor and depends on the energy difference between the Γ to L valley of the semiconductor.

博士
國立成功大學
物理學系碩博士班
95
Terahertz radiation (THz) represents the electromagnetic waves with frequencies around 1012 Hz. In comparison with other electromagnetic waves, the characteristics of terahertz radiation have not been clearly understood and very few scientific research and practical applications have been reported. Therefore, the generation, detection, application and mechanism of terahertz radiation have become important area of studies. In this thesis we first introduce the generation and detection system and studies of the mechanism of terahertz radiation. The effects of critical electric field, conjugate polymer (DB-PPV) and saturation effect on the intensity of terahertz radiation radiated from semiconductor microstructures will also be reported. It is widely believed that, in photoconductive mode, intensity of terahertz radiation is proportional to local field (bias) by the external applied or built-in electric field. However, when GaAs surface intrinsic-N+ (SIN+) structures are used as the terahertz emitter, the larger the bias electric field, the smaller the terahertz radiation intensity is observed. There exists the so-called “the critical electric field” in semiconductors. As the local field is below the critical electric field Ec, the maximum drift velocity of free charged photo-excited carriers in a semiconductor is proportional to the electric field in the semiconductor. However, as the field rises above the critical electric field Ec, the maximum drift velocity declines slightly as the field increases. The maximum drift velocity of the free charged carriers peaks at the critical electric field, which depends on the energy difference between the Γ to L valley (intervalley threshold, L valley offset) in the semiconductor. While the intrinsic layer thickness is less than 200 nm, the electric field of GaAs is larger than the critical field thus the drift velocity is approximately constant. The amplitude of THz radiation above the critical electric field Ec is not proportional to nphEloc but proportional to nphEc. An effective electric field Eeff can be defined, which equals to the critical field Ec as the Eloc is larger than the critical field and equals to Eloc as the Eloc is smaller than the critical field. Then, the terahertz radiation intensity can be expressed by . The dependence of THz and nphEeff on the thickness of the intrinsic layer obtained experimentally are almost identical to each other implying that there is indeed a critical electric field, Ec , in the semiconductor such that ETHz is dependent on Eloc when Eloc is smaller than Ec and is independent of Eloc when Eloc exceeds Ec. As the field is lower than the critical field, the amplitude is proportional to the product of the surface field and the number of photo-excited carriers. In the high field limit where the surface field exceeds the critical field, the amplitude of THz is independent of the surface field but is proportional to the product of the critical field and the number of the photo-excited carriers. Since the critical electric field depends on the energy difference between Γ and L valley or the L valley offset in semiconductors. The L valley offset can be estimated from critical electric field determined from THz radiation. In the second part of this study, THz radiation from the surfaces of various semiconductor wafers and microstructures is investigated. Various polymer films are spin-cast on the surfaces of semiconductors and semiconductor heterostructures to enhance the terahertz radiation. The conjugate polymer (2, 3- dibutoxy-1, 4-polyphenylenevinylene (DB-PPV)) is found to effectively enhance the terahertz radiation intensity from the surface intrinsic GaAs wafer by as much as 50%. Changes in surface field and the density of interfacial states are detected associated with the enhancement of THz radiation by DB-PPV conjugate polymer surface-coating. For the semiconductors in which drift current dominates the diffusion current, THz radiation is enhanced by coating DB-PPV on their surfaces to increase their surface fields. For the semiconductors in which the diffusion current dominates the drift current, THz radiation is not enhanced by increasing in surface fields. The contactless and nondestructive modulation spectroscopy of photoreflectance is employed to determine the changes in surface field and the density of interfacial states which are closely related to the enhancement of THz radiation. Finally, we study the dependence of intensity of terahertz radiation on pump beam power in InAlAs SIN+ samples with different doping concentrations in the buffer layer. The intensity of terahertz radiation increases linearly with the pump power; reaches its saturation intensity; and declines slightly in the high power region. It is found that the intensity of the terahertz radiation is restricted by the critical electric field and the energy stored in the semiconductors, which depends on the surface field within the surface intrinsic layer. The variation in the THz intensity with pump power can be attributed to the change in the mobility of the photo-excited charged carriers.