Dissertations / Theses on the topic 'TeraHertz phonons'
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Li, Xian Ph D. Massachusetts Institute of Technology. "Terahertz-field-induced nonlinearity in phonons, electrons and spins." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/122713.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 189-210).
In this thesis, I describe work aimed at understanding nonlinear material responses initiated by strong terahertz (THz) field excitation. I discuss two aspects of nonlinear THz spectroscopy in condensed-matter materials: developments of experimental THz capabilities and spectroscopy methods and their applications in investigating ultrafast nonlinear dynamics in different classes of materials. I first describe the THz generation, detection and spectroscopy methods, which are the basis of all of our studies. We have generated strong single- and multi-cycle THz pulses covering several spectral ranges using inorganic and organic crystals and developed linear and nonlinear THz spectroscopy techniques to interrogate light-matter interactions based on different observables and/or symmetry criteria.
We have demonstrated a new method for studying time-domain electron paramagnetic resonance that allows us to measure THz-frequency fine structures of spin energy levels on a tabletop and have developed nonlinear two-dimensional (2D) magnetic resonance spectroscopy to distinguish nonlinear THz-spin interaction pathways. We also show that THz-pump, optical-probe spectroscopy, including THz field-induced second-harmonic generation spectroscopy and THz Kerr effect spectroscopy, can be extended to study phase transitions in quantum paraelectric and topological materials. We have employed the THz methods to drive and detect nonlinear responses from several degrees of freedom in the materials. We have demonstrated collective coherent control over material structure by inducing a quantum paraelectric to ferroelectric phase transition using intense THz electric fields in strontium titanate.
We show that a single-cycle THz field is able to drive ions along the microscopic pathway leading directly to their locations in a new crystalline phase on an ultrafast timescale. We have driven highly nonlinear lattice and electronic responses in a topological crystalline insulator by dynamically perturbing the protecting crystalline symmetry through THz phonon excitation. We have observed oscillations in optical reflectivity that may be associated with electronic gap opening and modulation in the topological surface states. Finally, we have demonstrated nonlinear manipulation of collective spin waves in a canted antiferromagnet using strong THz magnetic fields and we have observed full sets of the second- and third-order nonlinear responses in 2D THz magnetic resonance spectra, which are accurately reproduced in our numerical simulations.
by Xian Li.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Chemistry
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Warren, Matthew Timothy. "Time-Domain Terahertz Studies of Strongly Correlated GeV4S8 and Osmate Double-Perovskites." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1512066420271281.
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Kasai, S., T. Katagiri, J. Takayanagi, K. Kawase, and T. Ouchi. "Reduction of phonon resonant terahertz wave absorption in photoconductive switches using epitaxial layer transfer." American Institite of Physics, 2009. http://hdl.handle.net/2237/12632.
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Hibberd, Morgan. "Studying low frequency vibrational modes using ultrafast techniques." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/studying-low-frequency-vibrational-modes-using-ultrafast-techniques(4f2da8db-befe-4c37-a6ae-42d069c54dd5).html.
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In this thesis, I report on the investigation of the low frequency vibrational modes in a number of different systems using ultrafast spectroscopic techniques. These consist of biological systems, including the enzyme, morphinone reductase (MR) and the related biomolecules, riboflavin (Rb) and flavin mononucleotide (FMN), as well as non-biological systems, including the semiconductor gallium nitride (GaN) and gold nanoparticles (Au NPs). The term low frequency refers to terahertz (THz) frequencies, where vibrational modes exist at the molecular level, with molecular rotations, lattice vibrations and inter- and intra-molecular vibrations occurring in the THz spectral range. These vibrational modes occur on sub-picosecond timescales and therefore ultrafast techniques utilising femtosecond laser pulses provide a means of studying these modes, and are employed throughout this thesis. The two ultrafast techniques of transient absorption (TA) spectroscopy and terahertz time-domain spectroscopy (THz-TDS) were used. Firstly, a high-repetition rate transient absorption (HRRTA) spectrometer was commissioned to perform pump-probe measurements with an ultraviolet pump and broadband visible probe. The performance of the HRRTA spectrometer was benchmarked using Au NPs and used to investigate the existence of a promoting vibration in MR contributing to the catalysis process, predicted to occur at THz frequencies. Weak oscillations were detected in the charge-transfer absorption band of MR bound to the non-reactive cofactor 1,4,5,6-tetra-hydro-nicotinamide adenine dinucleotide (NADH4), with a frequency of approximately 1.5 THz and provide evidence of the first direct observation of a promoting vibration in an enzyme. To complement the TA measurements, THz-TDS was also used to obtain direct measurements of the absorption at THz frequencies. Due to the challenge of studying water-based biological samples, an initial investigation was performed on a wurtzite GaN wafer, which exhibited optical phonon modes in the THz frequency range that were found to determine the dielectric response of the semi-insulating semiconductor wafer. Use of a non-polar m-plane wafer allowed the anisotropic nature to be observed and values of 9.22 ± 0.02 and 10.32 ± 0.03 for the static dielectric constants were obtained for the THz electric field polarised both perpendicular and parallel to the c-axis of the wurtzite GaN wafer, respectively. Finally, biological studies using THz-TDS were performed with measurements on Rb pellets and films revealing vibrational modes in the THz region. The sharp absorption features were not observed in FMN, despite a small difference in molecular content from Rb, and dehydration was required to reveal small amplitude absorption features. Final measurements on MR and MR-NADH4 films were carried out and evidence of absorption features in the THz frequency range were observed, however further work is required to determine the precise origin of these features.
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Freeman, Will. "Terahertz quantum cascade structures using step wells and longitudinal optical-phonon scattering." Monterey, Calif. : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/dissert/2009/Jun/09Jun%5FFreeman%5FPhD.pdf.
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Dissertation (Ph.D. in Physics)--Naval Postgraduate School, June 2009.Dissertation supervisor: Karunasiri, Gamani. "June 2009." Description based on title screen as viewed on July 14, 2009. Author(s) subject terms: Terahertz, THz, Quantum cascade structure, QC structure, Quantum cascade laser, QCL, Step well, Longitudinal optical-phonon, LO-phonon, Electron-phonon scattering, Electronelectron scattering, Impurity scattering, Interface roughness scattering, Optical transition, Electron transport, Monte Carlo method, Metal-metal waveguide, Surface plasmon waveguide Includes bibliographical references (p. 103-108). Also available in print.
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Mährlein, Sebastian F. [Verfasser]. "Nonlinear Terahertz Phononics: A Novel Route to Controlling Matter / Sebastian F. Mährlein." Berlin : Freie Universität Berlin, 2017. http://d-nb.info/1135608059/34.
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Knighton, Brittany E. "Nonlinear Ultrafast Excitation and Two-Dimensional Terahertz Spectroscopy of Solids." BYU ScholarsArchive, 2021. https://scholarsarchive.byu.edu/etd/9190.
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Ultrafast spectroscopy allows us to probe and understand material properties. With it, we can measure phonon-polaritons (optical phonons coupled with light) and the resulting dispersion curve in lithium niobate. Customizing the excitation source in ultrafast measurements can excite phonon modes to large amplitudes, allowing the experimental exploration of the Potential Energy Surface in solids. However, stronger pump fluences and bigger signal isn't always the answer in ultrafast spectroscopy. When sample signals and their nonlinear and mechanisms cannot be distinguished with 1D measurements, simple 2D THz measurements are a great place to start searching for distinct factors as was the case in cadmium tungstate. 2D measurements when paired with modeling and first principles calculations can reveal cutting edge information about exciting materials.
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Nguema, Agnandji Edwin. "Génération et détection Terahertz : application à la caractérisation de matériaux en couches minces." Thesis, Bordeaux 1, 2009. http://www.theses.fr/2009BOR13796/document.
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Ce travail porte sur la caractérisation de matériaux en couches minces par spectroscopie terahertz dépendant du temps. Dans ce but, nous avons élaboré un banc d’analyse spectroscopique dont l’émission et la détection terahertz reposent sur l’utilisation de laser femtoseconde, de semi-conducteurs, de photocommutateurs ultrarapides ou de cristaux électro-optiques. La réponse diélectrique quantitative de matériaux ferroélectriques (titanate de baryum/ - Ba1-xSrxTiO3) déposés sous forme de couches minces, a permis de mettre en évidence l’importance des modes mous de phonon par une étude en température. Enfin, le comportement électromagnétique de polymères conducteurs à base de polyaniline a été effectué notamment leur efficacité de blindage en bande millimétrique et submillimétriqueThis work concerns the characterization of thin film materials by terahertz time domain spectroscopy. For this purpose, we elaborated a terahertz setup in which the terahertz emission and terahertz detection are based on the use of femtosecond laser, semiconductors, ultrafast photoswitches or electro-optic crystals. The study of dielectric function of ferroelectrics thin film (barium titanate/-Ba1-xSrxTiO3) with temperature, give the importance of soft phonon mode. Finally, the electromagnetic behavior of conducting polymers based on polyaniline was made, in particular their shelding effectiveness in millimeter and sub-millimeter length
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Souza, Fabricio Macedo de. "Dinâmica de plasma e fônon e emissão de radiação terahertz em superfícies de GaAs e telúrio excitadas por pulsos ultracurtos." Universidade de São Paulo, 2000. http://www.teses.usp.br/teses/disponiveis/76/76131/tde-18112013-111000/.
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Após a excitação de uma amostra semicondutora por um pulso ultracurto, os fotoporadores interagem com a rede excitando modos longitudinais ópticos. Essa interação provoca variações no índice de refração do material, produzindo modulações na resposta óptica do meio (efeito eletro-óptico). Por outro lado, esta dinâmica origina polarizações dependentes do tempo o que gera emissão de radiação terahertz. Experimentos recentes (pump-probe) observaram modulações do campo através de medidas da refletividade resolvidas no tempo. A refletividade e o campo estão relacionados segundo o efeito eletro-óptico. Também se resolve temporalmente o campo irradiado pela amostra, através de antenas que operam na faixa de terahertz. Tanto as medidas eletro-ópticas quanto de emissão terahertz fornecem informações sobre a interação dinâmica do plasma com a rede após a excitação óptica. Nesse trabalho simulamos a interação dinâmica de plasma e fônons em n-GaAs e Telúrio (\"bulk\") após estes serem excitados por um pulso ultracurto. Utilizamos equações hidrodinâmicas para descrever transporte de cargas e uma equação fenomenológica de oscilador harmônico forçado, para descrever oscilações longitudinais ópticas da rede. Complementando nossa descrição temos a equação de Poisson, com a qual calculamos o campo gerado pelo plasma e pela polarização da rede semicondutora. Essas equações constituem um sistema de seis equações diferencias (quatro parciais) acopladas. Para resolvê-las utilizamos o método das diferenças finitas. Do cálculo numérico obtemos a evolução temporal do campo elétrico no interior do material. Com esse campo determinamos as freqüências de oscilação do sistema e calculamos o campo irradiado. Nossos resultados apresentam acordo qualitativo com os experimentosAbove-band-gap optical excitation of semiconductors generates highly non-equilibrium photocarriers which interact with phonons thus exciting vibrational modes in the system. This interaction induces refractive-index changes via the electro-optic effect. Moreover it gives rise to electromagnetic radiation at characteristic frequencies (terahertz). Both effects have been measured by time-resolved ultra fast spectroscopy. Recent pump-probe experiments have found strong modulations of the internal electric field through electro-optic measurements. The emitted electromagnetic radiation has also been detected by a terahertz dipole antenna. Both electro-optic and terahertz emission measurements provide information about the coupled dynamics of photocarriers and phonons. In this work we simulate the dynamics of plasmon-phonon coupled modes in n-GaAs and Tellurium (bulk) following ultrafast laser excitation. The time evolution of the photocarrier densities and currents is described semi classically in terms of the moments of the Boltzmann equation. Phonon effects are accounted for by considering a phenomenological driven-harmonic-oscillator equation, which is coupled to the electron-hole plasma via Poisson\'s equation. These equations constitute a coupled set of differential equations. We use finite differencing to solve these equations. From the numerical results for the evolution of internal fields we can calculate both the characteristic frequencies of system and its terahertz radiation spectrum. Our results are consistent with recent experimental data
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Vassant, Simon. "Contrôle optique et électrique de réflectivité THz assistée par phonon-polaritons de surface." Phd thesis, Ecole Centrale Paris, 2011. http://tel.archives-ouvertes.fr/tel-00601767.
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Le travail de thèse porte sur la conception et la réalisation de deux modulateurs optiques assistés par phonon-polaritons de surface fonctionnant en réflectivité, autour de 8.5 THz, à température ambiante. Nous avons dans un premier temps validé expérimentalement la description théorique du couplage du champ propagatif aux phonons polaritons de surface pour un réseau de GaAs grâce à des mesures de réflectivité THz résolues angulairement. Nous montrons l'importance de la géométrie de la structure pour une description quantitative du couplage. Nous avons réalisé un modulateur de réflectivité THz contrôlé optiquement. La structure est un réseau lamellaire de GaAs dopé , de période inférieure à la longueur d'onde supportant un mode composé de plasmon-phonon-polaritons de surface se propageant le long des murs du réseau. L'éclairement de la structure dans le visible modifie la fréquence de résonance THz de ce mode en créant des photo-porteurs dans les murs du réseau et permet ainsi un contrôle actif de la réflectivité.Enfin nous étudions et réalisons un modulateur de réflectivité THz contrôlé électriquement. Nous proposons une structure permettant d'exciter un mode de phonon-polaritons d'interface dans un puits quantique. Ce mode est très confiné dans le puits et présente une forte sur-intensité de champ. Cet effet original est lié à la permittivité du puits proche de zéro à la fréquence du mode d'interface. La perturbation engendrée par des transitions intersous-bandes dans le puits quantique unique permet, en appliquant une tension de l'ordre du volt, de contrôler l'intensité du couplage du champ propagatif au mode du puits, ce qui donne un contrôle actif de la réflectivité de la structure.
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Archambault, Alexandre. "Optique des ondes de surface : super-résolution et interaction matière-rayonnement." Phd thesis, Université Paris Sud - Paris XI, 2011. http://tel.archives-ouvertes.fr/tel-00678073.
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Il existe au niveau d'interfaces séparant des milieux de constantes diélectriques de signes opposés des ondes électromagnétiques confinées à proximité de ces interfaces. On parle d'ondes de surface. C'est notamment le cas des métaux et des cristaux polaires : on parle alors de plasmons-polaritons de surface et de phonons-polaritons de surface respectivement. L'objectif de cette thèse est de revisiter certains aspects théoriques associés à ces ondes de surface.Dans un premier temps, en nous basant sur le formalisme de Green, nous donnons un moyen d'obtenir une expression du champ des ondes de surface sous forme de somme de modes. En présence de pertes, ces ondes ont nécessairement un vecteur d'onde ou une pulsation complexe. Nous donnons ainsi deux expressions de leur champ, correspondant à chacun de ces deux cas, et discutons de l'opportunité d'utiliser l'une ou l'autre de ces expressions.Nous posons par la suite les bases d'une optique de Fourier et d'une optique géométrique des ondes de surface. Nous montrons comment obtenir une équation de Helmholtz à deux dimensions pour les ondes de surface, un principe d'Huygens-Fresnel pour les ondes de surface, ainsi qu'une équation eikonale pour les ondes de surface, qui s'applique sous certaines hypothèses. Nous nous intéressons également à la superlentille proposée par Pendry, qui s'appuie sur les ondes de surface. Nous étudions notamment le fonctionnement de cette superlentille en régime impulsionnel, et montrons qu'en présence de pertes, il est possible d'obtenir une meilleure résolution avec certaines formes d'impulsion par rapport au régime harmonique, au prix d'une importante baisse de signal toutefois.Nous développons ensuite un traitement quantique des ondes de surface. Nous calculons au préalable une expression de leur énergie, et nous donnons une expression de leur hamiltonien et de leurs opérateurs champ. Sans pertes, nous montrons que le facteur de Purcell prédit par notre théorie quantique est rigoureusement égal au facteur de Purcell calculé avec des outils classiques. Nous comparons ensuite ce facteur de Purcell à celui calculé classiquement avec pertes, et montrons sur un exemple que les pertes peuvent être négligées dans de nombreux cas. Nous donnons enfin une expression des coefficients d'Einstein associés aux ondes de surface permettant d'étudier la dynamique de l'inversion de population d'un milieu fournissant un gain aux ondes de surface. Nous appliquons par la suite ce formalisme quantique à l'interaction électrons-phonons-polaritons de surface dans les puits quantiques, notamment leur interaction avec un mode de phonon du puits particulièrement confiné grâce à un effet de constante diélectrique proche de zéro (epsilon near zero, ENZ).
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Böttge, Christoph N. [Verfasser], and Mackillo [Akademischer Betreuer] Kira. "On the Phonon Interactions and Terahertz Excitations among Coulomb-correlated Charge Carriers of Semiconductors / Christoph N. Böttge. Betreuer: Mackillo Kira." Marburg : Philipps-Universität Marburg, 2013. http://d-nb.info/1043316590/34.
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Dolguikh, Maxim. "MONTE CARLO SIMULATION OF HOLE TRANSPORT AND TERAHERTZ AMPLIFICATION IN MULTILAYER DELTA DOPED SEMICONDUCTOR STRUCTURES." Doctoral diss., University of Central Florida, 2005. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3882.
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Monte Carlo method for the simulation of hole dynamics in degenerate valence subbands of cubic semiconductors is developed. All possible intra- and inter-subband scattering rates are theoretically calculated for Ge, Si, and GaAs. A far-infrared laser concept based on intersubband transitions of holes in p-type periodically delta-doped semiconductor films is studied using numerical Monte-Carlo simulation of hot hole dynamics. The considered device consists of monocrystalline pure Ge layers periodically interleaved with delta-doped layers and operates with vertical or in-plane hole transport in the presence of a perpendicular in-plane magnetic field. Inversion population on intersubband transitions arises due to light hole accumulation in E B fields, as in the bulk p-Ge laser. However, the considered structure achieves spatial separation of hole accumulation regions from the doped layers, which reduces ionized-impurity and carrier-carrier scattering for the majority of light holes. This allows remarkable increase of the gain in comparison with bulk p-Ge lasers. Population inversion and gain sufficient for laser operation are expected up to 77 K. Test structures grown by chemical vapor deposition demonstrate feasibility of producing the device with sufficient active thickness to allow quasioptical electrodynamic cavity solutions. The same device structure is considered in GaAs. The case of Si is much more complicated due to strong anisotropy of the valence band. The primary new result for Si is the first consideration of the anisotropy of optical phonon scattering for hot holes.Ph.D.
Department of Physics
Arts and Sciences
Physics
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Weerasekara, Aruna Bandara. "Electrical and Optical Characterization of Group III-V Heterostructures with Emphasis on Terahertz Devices." Digital Archive @ GSU, 2007. http://digitalarchive.gsu.edu/phy_astr_diss/16.
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Electrical and optical characterizations of heterostructures and thin films based on group III-V compound semiconductors are presented. Optical properties of GaMnN thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on GaN/Sapphire templates were investigated using IR reflection spectroscopy. Experimental reflection spectra were fitted using a non - linear fitting algorithm, and the high frequency dielectric constant (ε∞), optical phonon frequencies of E1(TO) and E1(LO), and their oscillator strengths (S) and broadening constants (Γ) were obtained for GaMnN thin films with different Mn fraction. The high frequency dielectric constant (ε∞) of InN thin films grown by the high pressure chemical vapor deposition (HPCVD) method was also investigated by IR reflection spectroscopy and the average was found to vary between 7.0 - 8.6. The mobility of free carriers in InN thin films was calculated using the damping constant of the plasma oscillator. The terahertz detection capability of n-type GaAs/AlGaAs Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) structures was demonstrated. A threshold frequency of 3.2 THz (93 µm) with a peak responsivity of 6.5 A/W at 7.1 THz was obtained using a 0.7 µm thick 1E18 cm−3 n - type doped GaAs emitter layer and a 1 µm thick undoped Al(0.04)Ga(0.96)As barrier layer. Using n - type doped GaAs emitter layers, the possibility of obtaining small workfunctions (∆) required for terahertz detectors has been successfully demonstrated. In addition, the possibility of using GaN (GaMnN) and InN materials for terahertz detection was investigated and a possible GaN base terahertz detector design is presented. The non - linear behavior of the Inter Pulse Time Intervals (IPTI) of neuron - like electric pulses triggered externally in a GaAs/InGaAs Multi Quantum Well (MQW) structure at low temperature (~10 K) was investigated. It was found that a grouping behavior of IPTIs exists at slow triggering pulse rates. Furthermore, the calculated correlation dimension reveals that the dimensionality of the system is higher than the average dimension found in most of the natural systems. Finally, an investigation of terahertz radiation efect on biological system is reported.
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Mai, Thuc T. "Optical spectroscopy of cooperative phenomena and their symmetries in solids." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555629359625425.
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Legrand, Romain. "Acoustique - étude et utilisation de nouvelles sources et transducteurs aux longueurs d'onde nanométriques." Phd thesis, Université Pierre et Marie Curie - Paris VI, 2013. http://tel.archives-ouvertes.fr/tel-00945558.
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Ce manuscrit porte sur l'étude de transducteur acoustique couvrant la plage de fréquence allant de 100 GHz à 1 THz. Un laser impulsionnel pico-seconde est à l'origine de ces hautes fréquences. Les structures utilisées pour la génération sont un empilement bicouches GaAs/AlAs. La sensibilité de génération et de détection d'onde acoustique en fonction de la longueur d'onde est étudiée expérimentalement. Afin d'améliorer l'efficacité de transduction, une micro-cavité optique est utilisée pour augmenter l'interaction entre le rayonnement laser et le super-réseau. Ces transducteurs opto-acoustiques sont utilisés pour mesurer les temps de vie des phonons subterahertz et terahertz. Une anomalie dans le comportement en fréquence des temps de vie a été mise en évidence dans l'arséniure de gallium. L'utilisation de courbes de dispersion issues de calculs ab initio associée à l'approche de Landau-Rumer pour les temps de vie ultra-sonores permet d'expliquer ce comportement inattendu.
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Chen, I.-Ju, and 陳怡如. "Ultrafast Carrier Relaxation and Sub-Terahertz Longitudinal Coherent Acoustic Phonon Generation Studies in Graphene." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/09872783133772347645.
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碩士國立臺灣大學
光電工程學研究所
101
Graphene is a two-dimensional honeycomb lattice of carbon atoms. Due to its superior qualities, for example, it’s transparent, its thermal conductivity outperforms any other materials, its charge-carrier mobility is extremely high, and it is mechanically extremely strong, it has tremendous potential for future applications. For example, it’s a promising candidate for the conducting channel of future ultrafast field effect transistors and the replacement for the transparent electrode in touching screens, liquid crystal display (LCD), and solar cells. In this thesis, one of graphene’s undiscovered potential will be studied: graphene’s atomic thickness is one order thinner than any realistic metal films, which are usually used as the acoustic transducers in ultrafast acoustic devices and acoustic microscopy. Hence, graphene is a promising material to generate extremely broad band-width coherent acoustic phonons, because metal film transducers typically generate longitudinal coherent acoustic phonons with band-width inversely proportional to the film thickness. Moreover, unlike other broad band-width acoustic transducers, graphene is easily fabricated can be transferred onto all kinds of substrates. Thus, the usage of graphene can help applying acoustic microscopy to all kinds of materials. In the above applications, clear understanding about the carrier dynamics of supported monolayer graphene is critical. In this thesis, femtosecond infrared pump-probe spectroscopy together with theoretical analyses were used to study the ultrafast carrier dynamics of graphene. The characteristic times, sequences, and dominance of several hot carriers’ efficient relaxation channels, including carrier-carrier, carrier-phonon, and phonon-phonon interaction were studied. Moreover, by comparing the ultrafast carrier dynamics of our own graphene samples and pristine graphene, which was theoretically predicted or experimentally observed in previous literatures, we can have a clue of the differences between our own sample and pristine graphene due to fabrication methods and the usage of substrates. Then, pump-probe spectroscopy was also used to study whether coherent acoustic phonons that propagate into the substrate can be generated by photo-excitation of the supported monolayer graphene. First, through the observation of backward Brillouin oscillations with graphene deposited on glass, sapphire and GaN, it was verified that via photo-excitation of the monolayer graphene, coherent acoustic phonons with momentum in the out-of-plane direction were generated and had propagated into the substrate. This result suggested that by depositing monolayer graphene on top of a substrate, it can serve as an acoustic transducer. Afterwards, in order to reveal the full spectrum of the generated coherent acoustic phonons, a GaN crystal with a piezoelectric InGaN quantum well buried inside was used as the substrate. With our 3nm InGaN quantum well, detection band-width up to 2THz was achieved. The experimental results showed that the generated coherent acoustic phonon pulse had a bipolar shape and frequency components extending up to 1 THz (with the peak at 200GHz). In summary, by photo-exciting supported monolayer graphene, in spite of the two-dimensional nature of graphene, THz band-width coherent acoustic phonons that propagate perpendicularly to the surface and into the substrate can be generated. Hence, our study has confirmed the feasibility of using supported monolayer graphene as a THz acoustic transducer. A hypothesis based on the ultrafast carrier dynamics study of our supported graphene sample was brought up to elucidate the generation mechanism. A good correspondence was reached between the characteristics of the experimentally observed coherent acoustic phonon pulse and the prediction of the hypothesis.
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Chen, Ting-Yang, and 陳廷揚. "Annealing Temperature Dependence of Phonon Modes and Crystallinity on Perovskites Studied by Terahertz Spectroscopy." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/6c9pyv.
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碩士國立交通大學
光電工程研究所
107
Hybrid halide perovskites have become one of the most promising solar cell materials, due to their great advantages of simple fabrication, lower cost, strong absorption, and good mobility. There have been tremendous efforts to improve the conversion efficiency of perovskite-based photovoltaic devices and now it reaches over 22.5 %. Recently, two-dimensional (2D) perovskites have attracted great attention in light emitting device community. Some studies have pointed out that 2D perovskites have higher stability under atmospheric environment. The thermal stability is another very important property for the out-door applications. In this work, we employed the broadband time-domain terahertz spectroscopy to study the low frequency phonon vibration mode of the alkylammonium lead iodides (XAPbI3); ethylammonium (EAPbI3), propylammonium ((PA)2PbI4), and butylammonium lead iodide ((BA)2PbI4). We found that the phonon mode corresponding to the bending vibration of I-Pb-I disappears for three layered perovskites and those at 1.5 and 2 THz due to the bonding length variety of Pb-I blueshift as the length of alkylammonium chain increases. In order to understand the thermal stability of layered perovskites, we measured the phonon modes under different thermal annealing temperatures. For three-dimensional perovskite, CH3NH3PbI3 (MAPbI3), the onset of blueshift of phonon mode is observed when the temperature reaches at about 65 ℃, where tetragonal-to-cubic phase transition occurs. When covered with monolayer graphene or transition metal dichalcogenides, the blueshift of mode frequency was not observed, implying the improvement thermal stability. For layered perovskites, the phonon modes at 1.5 and 2 THz is nearly unchanged as the temperature increases upto 100 ℃, indicating high thermal stability of 2D perovskites. The x-ray diffraction measurements show that the thermal annealing of 2D perovskites improves the crystallinity to ordered phase.
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Gerecht, Eyal. "Development of niobium nitride hot electron bolometric mixer for terahertz frequencies: The phonon-cooled version." 1998. https://scholarworks.umass.edu/dissertations/AAI9841871.
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NbN HEB mixers represent a promising approach for achieving receiver noise temperatures of a few times the quantum noise limit at frequencies above 1 THz. NbN HEB devices have been shown to have sufficient bandwidth for applications in astronomy, remote sensing, and plasma diagnostics in the FIR range. The NbN HEB is a phonon cooled bolometer in which the energy is transfered from the hot electrons to the substrate via inelastic collisions with phonons. The development of an NbN HEB mixer contained two steps: (1) implementing mixing in a comparatively large "direct-coupled" prototype device which required LO power of a few milliwatts, and (2) optimization of the first step by the development of an "antenna-coupled" (quasi-optically coupled) device with an LO power level of less than one $\mu W$. The LO power was coupled to the antenna via an extended hemispherical lens (1.3 mm in diameter). The design, fabrication, and measurement stages were performed by a collaborative effort between a Russian team from the Department of Physics at Moscow State Pedagogical University in Moscow, the Submillimeter Technology Laboratory at UMass/Lowell and the Department of Electrical and Computer Engineering at UMass/Amherst. Mixing at 2.5 THz was demonstrated for the first time using the direct-coupled device achieving an intrinsic conversion loss of 23 dB. Sufficient level of LO power coupling at four different frequencies was demonstrated with the antenna-coupled device. The antenna/lens configuration has performed as well as expected insuring coupling to LO power of less than one $\mu$W. A 3 dB conversion gain was demonstrated with the antenna-coupled device using a laser LO at 1.56 THz with an IF frequency of 500 KHz. A second laser was utilized as the rf source. Noise temperature for the NbN HEB mixer receiver of 5800 K has been demonstrated over the 1.25-1.75 GHz IF band. The mixer temperature was 2500 K and the total conversion loss was 27 dB. Further optimization of the receiver configuration and device fabrication as well as additional development in mixer modeling will gradually lead to lower noise temperatures.
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Kübler, Carl [Verfasser]. "Femtosecond terahertz studies of many body correlations : from ultrafast phonon plasmon dynamics to an insulator metal transition / vorgelegt von Carl Kübler." 2007. http://d-nb.info/986197246/34.
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