Добірка наукової літератури з теми "Photonic crystal cavity (PhC cavity)"

We propose a new type of refractive index sensing based on the transverse magnetic (TM) modes in the photonic crystal (PhC) nanobeam (NB) cavity with a horizontal air gap. The electric field of the resonant TM mode is strongly confined within the horizontal air gap present at the PhC NB cavity. In order to increase the quality (Q) factor and the sensitivity (S) of the refractive index change in the air simultaneously, the cavity structure is fully optimized. Because of the trade-off between the Q-factor and S of the TM mode in the PhC NB cavity with an air gap, there is an optimal thickness of the air gap in the dielectric slot. From the numerical simulation results, S can exceed 1000 nm/RIU with Q > 40,000. When the dielectric slot becomes thin, S could be higher than 1200 nm/RIU. For practical applications, we suggest an Si-based PhC NB cavity with a horizontal SiO2 slot structure which can also provide high S with a high Q-factor after a very fine selective wet etching process. This new type of TM resonant mode in the PhC NB cavity can be an ideal platform for compact sensors in photonic integrated circuits for TM waveguide systems.

Abstract In this work, we performed a study of an interferometric high sensitivity photonic crystal (PhC) sensor applied to measure glucose concentration in human urine samples. The architecture of the photonic sensor numerically simulated is based on a Mach–Zehnder interferometer formed by a coupled waveguide-cavity system. The sensor operates using photonic mode transitions detected from the photonic band diagram analysis, driven by changes in the glucose concentration in the urine samples that affect the refractive index of the optical sampling cavity. The photonic mode transition causes a phase shift between the wave traveling in the reference arm of the interferometer and the propagating electromagnetic wave in the sensing cavity containing the probing sample. As a consequence, the transmittance at the output of the interferometer can be modulated, making it extremely sensitive to changes in the refractive index of the sensing cavity. Since the PhC sensor studied works under variations in transmittance, we proposed a sensitivity coefficient as a function of the change in transmittance per unit change in the refractive index. The sensitivity achieved by the photonic sensor presents a value of 7000%/RIU, which indicates a variation in transmittance of 70% for changes in the refractive index of 0.01. These results demonstrate the feasibility of using photonic transitions between modes as a sensing tool in integrated photonic devices.

Abstract In this work, we study a composite zinc oxide photonic crystal (PhC) that includes a meso-cavity coupled to a PhC L3 microcavity to obtain a double resonance effect and second-harmonic generation (SHG) conversion efficiency as high as 468 W−1. This exceptional conversion efficiency was attributed to the high quality-factors Q found in the fundamental and second-harmonic (SH) modes whose values were of the order of 105 and 106, respectively. Since the L3 microcavity plays a relevant role in the SHG of the composite PhC, we performed a calculation of its photonic band structure to observe the induced modes in its bandgap. Furthermore, we also found that the resonant mode adjusted to the frequency of the SH exhibits high Purcell factors of the order of 105. Hence, in a semiconductor material, it can be easily enhanced the light emission at the SH frequency using an adequate driving fundamental frequency light beam. These results can stimulate the engineering of photonic nanostructures in semiconductor materials to achieve highly efficient non-linear effects with applications in cavity quantum electrodynamics.

In this paper, a new ultra-compact optical biosensor based on photonic crystal (phc) resonant cavity is proposed. This sensor has ability to work in chemical optical processes for the determination and analysis of liquid material. Here, we used an optical filter based on two-dimensional phc resonant cavity on a silicon layer and an active area is created in center of cavity. According to results, with increasing the refractive index of cavity, resonant wavelengths shift so that this phenomenon provides the ability to measure the properties of materials. This novel designed biosensor has more advantage to operate in the biochemical process for example sensing protein and DNA molecule refractive index. This nanoscale biosensor has quality factor higher than 1.5 × 104 and it is suitable to be used in the home healthcare diagnostic applications.

We present a design methodology for hybrid lasers to realise mode-hop free operation by controlling the cavity mode spacing. In this study, a compact hybrid photonic crystal laser (H-PhCL) was employed which allowed a reduction of the Fabry–Perot length of the laser cavity and eliminated the need for an active mode stabilisation mechanism in order to realise mode-hop free operation. The H-PhCL was formed by butt-coupling a reflective semiconductor optical amplifier (RSOA) with a two-dimensional silicon (Si) photonic crystal (PhC) cavity. Continuous stable single frequency operation with >40 dB side-mode suppression ratio (SMSR) of the laser was achieved for gain currents of up to 100 mA, i.e., up to four times the threshold current. The shorter length of the laser cavity enabled single frequency operation due to the selection of a single longitudinal mode by the PhC narrowband reflector. Various longitudinal mode spacing regimes were studied to explain the mode-hop free characteristics of the H-PhCL. The proposed hybrid laser design methodologies can be adapted to eliminate mode-hopping in laser wavelength.

Abstract A design for a high quality factor photonic crystal ring resonator (PCRR) is presented. The PCRR is based on pillar type photonic crystals, which consist of a hexagonal array of silicon rods. The cavity is created by removing elements from the regular photonic crystal (PhC) grid. Achieving strong confinement of light intensity in the low index region is the advantage of this PCRR. In that manner, the interaction of light and analyte, which can be a liquid or a gas, will be enhanced. The high quality factor of the cavity (Q=1.0229\times {10}^{5}), along with strong overlap between the field of the resonant mode and the analyte as well as the low group velocity of PCRR modes yield enhanced light-matter interaction. An enhancement factor of \gamma =2.127\times {10}^{4} compared to the bulk light absorption in a homogenous material provides the potential for highly sensitive gas detection with a photonic crystal ring resonator.

Increasing the quality factor (Q-factor) of an optical resonator device has been a research focus utilized in various applications. Higher Q-factor means light is confined in a longer time which will produce a sharper peak and higher transmission. In this paper, we introduce a novel technique to further increase the Q-factor of a one-dimensional photonic crystal (1D PhC) cavity device by using an end loop-mirror (ELM). The technique utilizes and recycles the transmitted light from the conventional 1D PhC cavity design. The design has been proven to work by using the 2.5D FDTD simulation with Lumerical FDTD and MODE software. By using the ELM technique, the Q-factor of a 1D PhC design has been shown to increase up to 79.53% from the initial Q value without the ELM. The experimental result shows that the device is measurable by adding a Y-branch component to the one-port structure and able to get a high Q result. This novel design technique can be combined with any high Q-factor and very high Q-factor designs to increase more Q-factor values of photonic crystal cavity devices or any other suitable optical resonator devices.

A simple scheme to generate an integrated, nanoscale optical frequency comb (OFC) is numerically studied. In this study, all optical intensity modulators based on photonic crystal (PhC) cavities are cascaded both in series and parallel. By adjusting the modulation parameters, such as the repetition rate, phase, and coupling efficiency of the modulating wave, it is possible to produce combs with a variety of different characteristics. Unique to PhC intensity modulators, in comparison with standard lithium niobate modulators, is the ability to control the amplitude of the light via a cavity rather than controlling the phase through one arm of a Mach–Zehnder interferometer. This opens up modulation-based OFC generation to new possibilities in both nanoscale operation and cavity-based schemes.

A new class of hybrid systems that couple optical and mechanical nanoscale devices is under development. According to their interaction concepts, two groups of opto-mechanical systems are summarized as mechanically tunable and radiation pressure-driven optical resonators. On account of their high-quality factors and small mode volumes as well as good on-chip integrability with waveguides/circuits, photonic crystal (PhC) cavities have attracted great attention in sensing applications. Benefitting from the opto-mechanical interaction, a PhC cavity integrated opto-mechanical system provides an attractive platform for ultrasensitive sensors to detect displacement, mass, force, and acceleration. In this review, we introduce basic physical concepts of opto-mechanical PhC system and describe typical experimental systems for sensing applications. Opto-mechanical interaction-based PhC cavities offer unprecedented opportunities to develop lab-on-a-chip devices and witness a promising prospect to further manipulate light propagation in the nanophotonics.

In this report, using two-dimensional photonic crystals (PhC) and a one-dimensional PhC nano-beam cavity, we realized the development of all-polymeric dye-lasers on a dye-doped, suspended poly-methylmethacrylate film with a wavelength-scale thickness. In addition to the characterization of basic lasing properties, we also evaluated its capacity to serve as an attachable strain sensor. Through experimentation, we confirmed the stable lasing performances of the dye-laser attaching on a rough surface. Moreover, we also theoretically studied the wavelength responses of the utilized PhC resonators to stretching strain and further improved them via the concept of strain shaping. The attachability and high strain sensing response of the presented thin film PhC dye-lasers demonstrate their potential as attachable strain sensors.

In recent years, Silicon Photonics has emerged as a promising technology for cost-effective fabrication of photonic components and integrated circuits, the application of which is recently expanding in technological fields beyond tele- and data-communications, such as sensing and biophotonics. Compact, energy-efficient laser sources with precise wavelength control are crucial for the aforementioned applications. However, practical, efficient, electrically-pumped lasers on Silicon or other group IV elements are still absent, owing to the indirect bandgap of those materials. Consequently, the integration of III-V compounds on Silicon currently appears to be the most viable route to the realization of such lasers. In this thesis, I present and explore the potential of an External Cavity (EC) hybrid III-V/Silicon laser design, comprising a III-V-based Reflective Semiconductor Optical Amplifier (RSOA) and a Silicon reflector chip, based on a two-dimensional Photonic Crystal (PhC) cavity vertically coupled to a low-refractive-index dielectric waveguide. The vertically coupled system functions as a wavelength-selective reflector, determining the lasing wavelength. Based on this architecture mW-level continuous-wave (CW) lasing at room temperature was shown both in a fiber-based long cavity scheme and die-based short cavity scheme, with SMSR of > 25 dB and > 40 dB, respectively. Furthermore, by electrically modulating the refractive index of the PhC cavity in the reflector chip, tuning of the emitted wavelength was achieved in the die-based short cavity EC laser configuration. In this way, I demonstrated the suitability of the examined EC configuration for direct frequency modulation. The proposed scheme eliminates the need for wavelength matching between the laser source and a resonant modulator, and reveals the potential of employing low-power-consumption resonant modulation in practical Silicon Photonics applications.

Today's information and communication industry is confronted with a serious bottleneck due to the prohibitive energy consumption and limited transmission bandwidth of electrical interconnects. Silicon photonics offers an alternative by transferring data optically and thereby eliminating the restriction of electrical interconnects over distance and bandwidth. Due to the inherent advantage of using the same material as that used for the electronic circuitry, silicon photonics also promises high volume and low cost production plus the possibility of integration with electronics. In this thesis, I introduce an all-silicon optical interconnect architecture that promises very high integration density along with very low energy consumption. The basic building block of this architecture is a vertically coupled photonic crystal cavity-waveguide system. This vertically coupled system acts as a highly wavelength selective filter. By suitably designing the waveguide and the cavity, at resonance wavelength of the cavity, large drop in transmission can be achieved. By locally modulating the material index of the cavity electrically, the resonance wavelength of the cavity can be tuned to achieve modulation in the transmission of the waveguide. The detection scheme also utilizes the same vertically coupled system. By creating crystal defects in silicon in the cavity region, wavelength selective photodetection can be achieved. This unique vertical coupling scheme also allows us to cascade multiple modulators and detectors coupled to a single waveguide, thus offering huge channel scalability and design and fabrication simplicity. During this project, I have implemented this vertical coupling scheme to demonstrate modulation with extremely low operating energy (0.6 fJ/bit). Furthermore, I have demonstrated cascadeability and multichannel operation by using a comb laser as the source that simultaneously drives five channels. For photodetection, I have realized one of the smallest wavelength selective detector with responsivity of 0.108 A/W at 10 V reverse bias with a dark current of 9.4 nA. By cascading such detectors I have also demonstrated a two-channel demultiplexer.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2019
Cataloged from PDF of thesis. "The Table of Contents does not accurately represent the page numbering"--Disclaimer page.
Includes bibliographical references (pages 45-47).
The growing demand for efficient infrared sensors for light ranging, thermal-cameras, and soon, free-space optical communications has yet to be answered. In this study, we use polycrystalline silicon in conjunction with a photonic crystal cavity (PhCC) to enhance light absorption for efficient sensing. We present a cost-effective alternative to the current III-V detectors. By adding a 2D-PhC resonator layer, surface-illuminated light can be confined within a 10 micron region with great intensity, leading to a higher effective path-length and improved detector responsivity. More than 1000 variants of this detector are designed and implemented in a 65nm CMOS process. Using a nearest neighbor method, we find the optimized designs. We validate experimental findings by simulating mode behavior of the PhCC structures using FDTD models. In addition, a numerical study on cavity parameter optimization for achieving high Q-factors and extinction ratios specifically for surface-illumination is presented. We report polysilicon PhCC-enhanced sensors with Q-factors of 6500 resulting in responsivities at 1300nm up to 0.13mA/W -a 25x improvement over non-resonant surface-illuminated Silicon detectors.
by Ebrahim Dakhil Al Johani.
S.B.
S.B. Massachusetts Institute of Technology, Department of Physics

High quality factor, small mode volume photonic crystal cavities and single emitter quantum dots are the topic of this dissertation. They are studied as both a combined system with InAs quantum dots grown in the center of a 2D GaAs photonic crystal slab nanocavity as well as individually. The individual studies are concerned with passive 1D silicon photonic crystal nanobeam cavities and deterministic, site-selectively grown arrays of InAs quantum dots.For the combined system, strong light matter coupling in a quantum dot photonic crystal slab nanocavity is discussed. Vacuum Rabi splitting is seen when the interaction strength exceeds the dissipative processes of the coupled system. In order to increase the probability of a spectral matching between cavity modes and quantum dot transitions, a technique for condensing an inert gas onto a sample is used. This can lead to a spectral tuning of up to 4 nm of the cavity mode with minimal change in the cavity quality factor while maintaining cryogenic temperatures down to 4 K. The effect of a large density of quantum dots within a quantum dot photonic crystal slab nanocavity is also addressed. Gain and absorption effects are found to occur, changing the cavity emission linewidth from that of its intrinsic value, as well as lasing with a low number of quantum dots and with high spontaneous emission coupling factors. Additionally, methods for improving the quality factor of GaAs photonic crystal cavities and better understanding different loss mechanisms are discussed.In the individual studies, the site-selective growth of InAs quantum dots on pre-structured GaAs wafers is shown as a promising method for the eventual deterministic fabrication of photonic crystal cavities to single quantum dots. An in-situ annealing step is used to reduce quantum dot density, helping ensure that dots are not grown in unwanted locations.Given silicon's potential for achieving higher quality factors than its GaAs counterpart, a study of 1D passive silicon photonic crystal nanobeam cavities is carried out. Transmission through a coupled microfiber is used to measure quality factors of the cavities and compared with that of a crossed polarized resonant scattering measurement.

The topic of this dissertation is photonic crystal nanocavities for semiconductor cavity quantum electrodynamics. For the purposes of this study, these nanocavities may be one dimensional (1D) or two dimensional (2D) in design. The 2D devices are active and contain embedded InAs quantum dots (QDs), whereas the 1D devices are passive and contain no active emitters. The 2D photonic crystal nanocavities are fabricated in a slab of GaAs with a single layer of InAs QDs embedded in the slab. When a cavity mode substantially overlaps the QD ensemble, the dots affect the linewidths of the observed modes, leading to broadening of the linewidth at low excitation powers due to absorption and narrowing of the linewidths at high excitation powers due to gain when the QD ensemble absorption is saturated. We observe lasing from a few QDs in such a nanocavity. A technique is discussed with allows us to tune the resonance wavelength of a nanocavity by condensation of an inert gas onto the sample, which is held at cryogenic temperatures. The structural quality at the interfaces of epitaxially grown semiconductor heterostructures is investigated, and a growth instability is discovered which leads to roughness on the bottom of the GaAs slabs. Adjustment of MBE growth parameters leads to the elimination of this roughness, and the result is higher nanocavity quality factors. A number of methods for optimizing the fabrication of nanocavities is presented, which lead to higher quality factors. It is shown that some fundamental limiting factor, not yet fully understood, is preventing high quality factors at wavelengths shorter than 950 nm. Silicon 1D devices without active emitters are investigated by means of a tapered microfiber loop, and high quality factors are observed. This measurement technique is compared to a cross-polarized resonant scattering method. The quality factors observed in the silicon nanocavities are higher than those observed in GaAs, consistent with our observation that quality factors are in general higher at longer wavelengths.

A photonic crystal (PC) structure for trapping a 50nm radius dielectric particle at a precise location on a silicon surface in an organic solvent environment has been designed and all of its key components have been fabricated. The high gradient of electric field intensity in a PC cavity mode, with wavelength ~ 1.5 microns, exerts a radiation force toward the center of the cavity. The Finite Difference Time Domain (FDTD) modeling method was used to design a symmetric (input/output) structure that consists of two grating couplers, two parabolic tapered waveguides, two single mode ridge waveguides, two photonic crystal waveguides and a single three-missing-hole (L3) PC cavity. The radiation force on the dielectric sphere was exactly calculated using FDTD simulations to evaluate the Maxwell Stress Tensor (MST) in the presence of the particle to be trapped. This result was compared to that obtained using the simpler dipole approximation, and good agreement between them was found. The fabrication of the structure was done by electron beam lithography and chlorine plasma etching. The Q factors for some of the fabricated samples were measured from the cavity enhanced photoluminescence emission of PbSe quantum dots deposited on the sample surface. A vertical Q factor of 3600 (in vacuum environment) was measured for an isolated cavity, which corresponds to a Qv of 3800 ( in solvent environment) in the FDTD simulations. Also, the Q, of the overall structure (cavity and the waveguides) was measure to be 1050 in vacuum, which from simulations is equivalent to a Q of 1800 in a solvent. These Q values and the resonant frequencies of the modes are in close, but not perfect agreement with the simulation results.

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 31-34).
We propose a design of photonic crystal cavity with self-similar electromagnetic boundary conditions, that achieve ultrasmall mode volume (Vff). The electric energy density of a cavity mode can be maximized in the air or dielectric region, depending on the choice of boundary conditions. We illustrate the design concept with a silicon-air ID photon crystal cavity that reaches an ultrasmall mode volume of Vff ~ 7.01 x 10- 5 [lambda]3 at [lambda] ~ 1550 nm. We show that the extreme light concentration in our design can enable ultra-strong Kerr nonlinearities, even at the single photon level. These features open new directions in cavity quantum electrodynamics, spectroscopy, and quantum nonlinear optics.
by Hyeongrak Choi.
S.M.

Dans le premier chapitre de la thèse, nous étudions la possibilité d’améliorer le couplage opto- mechanique photon-phonon entre le mode de résonance d’une cavité Fabry-Pérot de haute finesse et les vibrations mécaniques des éléments diélectriques (membranes) à l’intérieur de la cavité. En introduisant un défaut quadratique dans la disposition des membranes, nous montrons que le deux couplages (linéaire et quadratique) augmentent. Enfin, nous proposons un modèle très simple avec lequel on cherche à simuler un cristal photonique quasipériodique. Dans le deuxième chapitre de cette thèse, nous présentons nos résultats de recherche sur le transport d’excitons à travers une cavité visant à augmenter l’efficacité du transport. Le modèle que l’on étudie est une chaîne unidimensionnelle d’atomes froids comprenant chacun deux niveaux énergétiques. Grâce au couplage entre exciton et photon, ces deux quanta s’hybrident et forment deux branches de polariton à l’intérieur de la cavité. Nous avons observé qu’à résonance avec un des deux modes de polariton, on peut transmettre l’exciton via le mode polaritonique dans un temps très court. En outre, le désordre n’affecte la propagation excitonique que de façon algébrique. Dans le troisième chapitre de cette thèse, nous présentons nos résultats de recherche sur la réalisa- tion d’interactions entre photons grâce à la médiation d’atomes ultrafroids piégés dans un réseaux optique unidimensionnelle et placés à l’intérieur d’une fibre à cristaux photoniques. Nous avons détecté un régime dans lequel on peut réaliser le “bunching” photon-photon.Dans le quatrième et dernière chapitre de cette thèse, nous étendons les résultats du chapitre précédent aux atomes de Rydberg
In the first chapter of this thesis, we study a quasiperiodic array of dielectric membranes inside a high-finesse Fabry-Pérot cavity. We work within the framework of the transfer matrix formal- ism. We show that, in a transmissive regime, the introduction of a quadratic spatial defect in the membrane positions enhances both the linear and quadratic optomechanical couplings between optical and mechanical degrees of freedom. Finally, we propose a theoretical model to simulate a one-dimensional quasiperiodic photonic crystal. In the second chapter of this thesis, we consider the problem of the transport of an exciton through a one-dimensional chain of two-level systems. We embed the chain of emitters in a transverse optical cavity and we show that, in the strong coupling regime, a ultrafast ballistic transport of the exciton is possible via the polaritonic modes rather than ordinary hopping. Due to the hybrid nature of polaritons, the transport efficiency is particularly robust against disorder and imperfections in the system. In the third chapter of this thesis, we consider an ordered array of cold atoms trapped in an optical lattice inside a hollow-core photonic crystal fiber. We study photon-photon interactions mediated by hard-core repulsion between excitons. We show that, in spite of underlying repulsive interac- tion, photons in the scattering states demonstrate bunching, which can be controlled by tuning the interatomic separation. We interpret this bunching as the result of scattering due to the mismatch of the quantization volumes for excitons and photons, and discuss the dependence of the effect on experimentally relevant parameters. In the fourth chapter of the thesis, we extend the results of the previous chapter to Rydberg atoms

Optical microcavities can find broad application areas, like tunable and compact sources, dynamic filters for optical communications, biological and chemical sensors, etc. Optical microcavities are key component allowing one to obtain compact laser devices exhibiting small cavity volume and low threshold. Among the different resonators architectures for microcavities laser, photonic crystal (PC) structures are one of the most promising. These structures feature the periodicity in one or more dimensions, and are resonant for light waves of a specific wavelength. Two-dimensional PCs are planar dielectric waveguide where photons are vertically confined by the vertical profile of the optical index, while the crystal periodicity acts only in the slab plane. In photonic crystals the refractive index contrast of the periodic structure is high enough to open a full band gap, and thus to fully confine light at very small scale. Strong coupling is theoretically feasible, and quality factor of more than 106 have been experimentally achieved for small modal volumes. Substantial additional gains are possible with confinement improvement in microfabrication techniques and with implementation of low loss design. Photonic crystal whose index contrast is lower can be used as DFB gratings. In a DFB device, the laser modes receive feedback at one specific wavelength, determined by the grating period of the structure. The feedback strength is related to the coupling constant, ?, which in turn depends on the grating index contrast, and to the grating length, L. The ?L product must be high enough to ensure the feedback required for lasing. In an optically pumped laser, an external source supplies the excitation energy necessary to get the population inversion. To do that, it must be on resonance with one of the absorption transitions of the active medium. When the external source provides enough energy, the active material exhibits gain: more photons are generated than lost. For intense incoming beams, i.e. intense laser sources, multiple-photon absorption processes become appreciable. It is thus possible to have absorption also by pumping with sources having photon energies lower than the resonance energy of the active medium. The two-photon absorption (TPA), described from the 3rd order susceptibility, involves the simultaneous absorption of two photons with energy: E_exc-E_ground=2?? The absorption of the first photon causes the promotion of the electron to the virtual level. Here the simultaneous absorption of the second photon promotes the electron to the real excited state. The system can then decay to the ground state emitting an up-converted photon, i.e., a photon having energy higher than that of the exciting ones. A large part of this work is devoted to the realization and characterization of active microresonators behaving as laser sources. Two main research subjects will be pursued: An integrated InP semiconductors photonic crystal microcavity laser, operating in the telecommunication wavelength. A distributed feedback laser for two-photon induced IR-to-visible up-conversion lasing. Within the second subject, particular attention will be devoted to the characterization of the two-photon induced emission properties of organic push-pull dyes and II-VI semiconductors quantum dots (QDs) to evaluate their potentiality as candidates for all-solid-state up-converted laser devices. The third reported research subject is the exploitation of hybrid silica-titania sol-gel films for UV lithography application, finalized to the production of surface relief gratings. MICROLASER BASED ON EFFECTIVE INDEX CONFINED, SLOW-LIGHT MODES IN PC WAVEGUIDES This research regards the study of photonic crystal microcavity having small mode volume V and high quality factor Q, for the production of low threshold integrated laser devices. The light propagation inside the PC is modified because of its periodicity. In this study we exploit the low-light guided modes at the high symmetry point of the dispersion curve of a PC-W1 waveguide. The PC-W1 waveguide is a PC having triangular symmetry with a missing row of hole along the ?K direction. The linear defect entails the appearance of defect modes with frequencies localized inside the unperturbed PC band gap, and thus modes that exponentially decay inside the PC. The band associated with the defect mode becomes flat at the K point of the band diagram, leading to slow-modes whose group velocity goes to zero. The lateral confinement of low-group velocity modes is controlled by locally changing the refractive index of the two dimensional photonic crystal waveguides. The index modulation is induced by post-processing a dielectric strip on top of the two-dimensional PC waveguide. This results in a photonic heterostructure whose confinement properties are the result of the effective index shift and the local curvature of the band associated with the waveguide mode. In this thesis the results of the device simulation, experimental realization and characterization will be reported. Computational tools, such as MPB and 3D-FDTD software have been used for the device design and for the study the electromagnetic field behavior inside the cavity. The realization of the PC structure has been accomplished through lithography techniques like e-beam lithography and reactive ion etching. Intense clean-room activities have been necessary to reach optimized structure quality. The characterization of the microcavity laser has been pursued with a proper optical set-up, in such a way to determine its performance. UP-CONVERTED LASING The up-converted lasing is an alternative method to convert the emission of a cheap, easily available IR laser into that of a more technological valuable visible laser. It involves the two photon pumping (TPP) mechanism, i.e., the NLO system is excited through the simultaneously absorption of two photons in the near-IR range. In this work we report our effort towards the realization of a solid-state visible laser device based on a TPP induced emission. The starting point of this technology is to find a system able to efficiently convert the IR incoming radiation to a visible one. We have studied the up-conversion process both in push-pull organic dyes embedded in sol-gel hybrid films and in semiconductor core-shell CdSe-CdS-ZnS quantum dots embedded in zirconia films. The excitation source is an amplified Ti:Sapphire laser at 800 nm. Concerning the organic compounds, it has been possible to characterize the emission properties only in solution, because of their poor photostability when they are embedded in sol-gel matrices. On the opposite, quantum dots embedded in zirconia films show promising amplified emission properties, with interesting gain value and extremely long time stability. We have investigated the possibility to implement this material with a distributed feedback optical resonator for obtaining compact and integrated laser devices. The grating parameters have been determined with MPB software, and first attempts of e-beam lithography of the pattern have been done. We have also prepared a devoted optical set-up for the optical characterization of the laser devices. SILICA-TITANIA SOL-GEL FILM FOR DIRECT PHOTOPATTERNIG APLICATIONS The possibility to exploit the photocatalytic action of hybrid silica-titania sol-gel film towards the decomposition of their organic component, for the direct patterning of surface structure has been investigated. These films have been characterized to study their microstructural properties, and to confirm the presence of crystalline titanium oxo-clusters. Their photocatalytic efficiency has been measured using stearic acid as reference material. To test the potentiality of this system for UV-lithography, it has been exposed to a UV-lamp. The organic component decomposition leads to a film shrinkage of about 60%, accompanied by a refractive index increases of about 0.1 By irradiating the spin-coated films through an UV-mask, structures of different shapes and micrometer dimension have been achieved.
Lo studio delle microcavità ottiche riveste un grande interesse per applicazioni in svariati campi, quali la ricerca di sorgenti laser tunabili e compatte, filtri per le telecomunicazioni, sensori chimici e biologici, etc. Le microcavità ottiche sono fondamentali per l’ottenimento di dispositivi laser compatti, aventi bassa soglia di emissione laser, ove il campo elettromagnetico è confinato in volumi estremamente ridotti, con conseguente aumento dell’interazione radiazione-materia,. Tra le possibili architetture della cavità risonante, per dispositivi pompati otticamente, i cristalli fotonici rappresentano una delle soluzioni più promettenti. Questi ultimi sfruttano la periodicità in una o più direzioni e sono risonanti con determinate lunghezze d’oda della radiazione elettromagnetica. In un cristallo fotonico bidimensionale il confinamento verticale è garantito dal profilo verticale dell’indice di rifrazione, mentre il confinamento nel piano del cristallo è opera della strutture periodica. Nei cristalli fotonici il contrasto di indice di rifrazione della struttura periodica è tale da aprire un intervallo completo di energie proibite per la propagazione della radiazione nel mezzo. Essa può quindi essere confinata in volumi molto piccoli, dell’ordine del cubo della lunghezza d’onda, con fattori di qualità sperimentali superiori a 106. Inoltre i valori ottenuti sperimentalmente sono inferiori a quelli previsti teoricamente, e ulteriori passi in avanti saranno possibili con lo sviluppo delle tecniche litografiche e di produzione del materiale attivo. I cristalli fotonici nei quali il contrasto di indice di rifrazione è insufficiente per aprire un band-gap completo si comportano come reticoli distributed feedback, DFB. In un dispositivo DFB, i modi risonanti ricevono il feedback a lunghezze d’onda specifiche, determinate dal periodo del reticolo. La forza dell’accoppiamento è legata alla costante di accoppiamento ?, la quale, a sua volta, dipende dal contrasto di indice nel reticolo e all’estensione totale del reticolo. Il prodotto ?L deve essere sufficiente per garantire il feedback richiesto per l’emissione laser. In un laser a pompaggio ottico, una sorgente esterna fornisce al mezzo attivo l’energia di eccitazione richiesta per raggiungere l’inversione di popolazione, requisito necessario per ottenere il guadagno all’interno del mezzo e quindi l’amplificazione. Affinché si verifichi assorbimento, l’energia del fascio di pompa deve essere in risonanza con una delle transizioni del mezzo attivo. Per campi incidenti molto intensi, come possono essere quelli legati a fasci laser focalizzati, diventano tuttavia apprezzabili anche fenomeni di assorbimento multi fotonici. Si può quindi avere assorbimento anche utilizzando sorgenti di pompa aventi energie inferiori all’energia di risonanza del mezzo attivo. L’assorbimento a due fotoni (TPA), legato alla suscettibilità non lineare al terzo ordine, comporta l’assorbimento simultaneo di due fotoni, con energia: E_exc-E_ground=2?? L’assorbimento del primo fotone promuove l’elettrone dallo stato fondamentale a uno stato virtuale, dal quale esso passa immediatamente allo stato eccitato attraverso l’assorbimento simultaneo di un secondo fotone incidente. Infine il sistema può tornare allo stato fondamentale, attraverso l’emissione di un fotone a energia superiore rispetto ala pompa. Gran parte del lavoro di dottorato è incentrato sulla realizzazione e caratterizzazione di microcavità attive per l’ottenimento di sorgenti laser. All’interno di tale attività sono stati studiati due sistemi differenti: Una microcavità laser a semiconduttore, realizzata sfruttando le proprietà dei cristalli fotonici bi-dimensionali, che emette alla lunghezza d’onda delle telecomunicazioni. Un dispositivo laser DFB, pompato oticamente a due fotoni, per la conversione di emissione laser dall’infrarosso al visibile. All’interno della seconda tematica, particolare attenzione è stata rivolta alla caratterizzazione delle proprietà di emissione indotta a due fotoni di un cromoforo organico e di quantum dots di un semiconduttore II-VI, il CdSe, entrambi inglobati in matrice sol-gel. Un terzo soggetto è costituito dallo studio delle proprietà foto catalitiche di film sol-gel ibridi a base di silica e titania, in vista di possibili applicazioni per il patterning diretto tramite radiazione UV. CONFINAMENTO DI MODI LENTI IN GUIDA D’ONDA A CRISTALLO FOTONICO PER L’OTTENIMENTO DI MICROCAVITA’ LASER Questa ricerca riguarda lo studio di cavità, ottenute sfruttando cristalli fotonici bidimensionali, a basso volume modale e alto fattore di qualità Q, finalizzate all’ottenimento di dispositivi laser integrati a bassa soglia. Questo lavoro si basa sull’utilizzo dei modi guidati lenti corrispondenti al punto ad elevata simmetria K della curva di dispersione di una guida d’onda W1-PC. Una guida d’onda W1-PC si ottiene da un cristallo fotonico a simmetria triangolare, attraverso la rimozione di una fila di buche lungo la direzione ?K. In questo modo si introduce un difetto lineare, il quale si riflette nella comparse di modi del difetto, aventi frequenze localizzate all’interno del band-gap del cristallo fotonico, che pertanto decadono esponenzialmente all’interno del cristallo. Le bande associate ai modi del difetto hanno curvatura nulla in corrispondenza dei punti a elevata simmetria, e ciò implica una velocità di gruppo del modo nulla in corrispondenza di tali punti. L’estensione laterale dei modi lenti viene controllata agendo sull’indice di rifrazione del cristallo fotonico, in modo da creare una etero struttura in grado di confinarli efficacemente. L’indice effettivo della guida viene modificato localmente depositando un film di polimero all’interfaccia superiore della guida. La forza del confinamento dipende dall’entità della variazione dell’indice e dalla curvatura della banda associata al modo lento. L’attività svolta all’interno di questo progetto consiste nel design della struttura, nella sua realizzazione sperimentale e infine nella caratterizzazione ottica del dispositivo. Per ottimizzare i parametri del dispositivo e comprendere il comportamento della radiazione elettromagnetica all’interno della cavità, sono stati impiegati strumenti di calcolo computazionale, quali i software MPB e TESSA 3D-FDTD. I parametri delle simulazioni sono stati poi utilizzati per la realizzazione del cristallo fotonico, effettuata tramite tecniche litografiche, quali la litografia con fascio elettronico e l’etching ionico. La caratterizzazione ottica del dispositivo è stata effettuata con un apposito set-up, al fine di determinarne le prestazioni. EMISSIONE LASER CON CONVERSIONE DI FREQUENZA La conversione di frequenza laser fornisce l’interessante possibilità di convertire una sorgente laser economica e di facile reperibilità nell’infrarosso, in una sorgente laser nel visibile di enorme interesse tecnologico. Essa si basa sull’emissione indotta a seguito di processi di assorbimento a due fotoni nel vicino IR. In questo lavoro verranno presentati gli sforzi profusi e i risultati preliminari ottenuti nella ricerca di un dispositivo laser allo stato solido per la conversione di frequenza. A tal fine sono state investigate le proprietà di conversione di un cromoforo push-pull organico disperso in matrici sol-gel ibride, e di quantum dots di semiconduttore II-VI, CdSe-CdS-ZnS, dispersi in una matrice inorganica a base di zirconia. Il composto organico presenta interessanti proprietà di emissione indotta a due fotoni in soluzione. Tuttavia la sua scarsa resistenza al pompaggio ottico in matrice solida preclude un suo possibile impiego e rende estremamente problematica la stessa caratterizzazione ottica. Al contrario i film di QDs-ZrO2 mostrano una buona efficienza di conversione di frequenza, con valori di guadagno per l’emissione spontanea amplificata interessanti, e elevata stabilità del segnale emesso nel tempo. E’ stata pertanto studiata la possibilità di implementare i film di QDs-ZrO2 all’interno di una cavità risonante di tipo distributed feedback per ottenere un dispositivo laser compatto e integrabile. I parametri del reticolo sono stati determinati con il software MPB e sono stati fissati in modo da avere amplificazione in corrispondenza del massimo di emissione dei QDs. Sono tutt’ora in corso delle prove di realizzazione del reticolo DFB tramite litografia elettronica su film sol-gel appositamente sviluppati per il patterning diretto. Infine è stato messo appunto un set-up dedicato per la caratterizzazione ottica dei dispositivi prodotti. FILM SOL-GEL IBRIDI A BASE DI SILICA-TITANIA PER IL PATTERNING DIRETTO CON LUCE UV E’ stata studiata l’attività fotocatalitica di film sol-gel ibridi a base di silica-titania, promossa dalla radiazione UV. I film sono stati caratterizzati a livello micro strutturale tramite spettroscopia infrarossa, e sono stati osservati al microscopio elettronico per confermare la presenza di cluster di titanio cristallino al loro interno. L’efficienza del processo di fotocatalisi è stata determinata mediante test standard che si avvalgono dell’acido stearico come materiale di riferimento. Quest’ultimo infatti è in grado di simulare efficacemente i comuni inquinanti organici, è può essere depositato facilmente per spin-coating. Successivamente è stata valutata la possibilità di sfruttare l’attività foto catalitica per il patterning diretto dei film. Tale studio parte dall’osservazione che la fotocatalisi si manifesta anche nei confronti della componente organica dei film sol gel ibridi.. Questo fenomeno è accompagnato da una diminuzione dello spessore del film, fino al 60% sullo spessore iniziale, e può pertanto essere sfruttato per la realizzazione di strutture a rilievo. Test di patterning diretto sono stati effettuati irradiando il film con una lampada UV attraverso una maschera in quarzo, ottenendo strutture di dimensione micrometrica ben definite.

Quantum dots in optical nanocavities are interesting as a test-bed for fundamental studies of light–matter interaction (cavity quantum electrodynamics, QED), as well as an integrated platform for information processing. As a result of the strong field localization inside sub-cubic-wavelength volumes, these dots enable very large emitter–field interaction strengths. In addition to their use in the study of new regimes of cavity QED, they can also be employed to build devices for quantum information processing, such as ultrafast quantum gates, non-classical light sources, and spin–photon interfaces. Beside quantum information systems, many classical information processing devices, such as lasers and modulators, benefit greatly from the enhanced light–matter interaction in such structures. This chapter gives an introduction to quantum dots, photonic crystal resonators, cavity QED, and quantum optics on this platform, as well as possible device applications.

This article describes a holographic laser-processing method for independently controlling the lattice symmetry and lattice constant in three-dimensional photonic lattices. With this approach, optical periodicity is created in lower dimensions and three-dimensional periodicity is obtained by a combination of several lower-dimensional periodic structures. The proposed holographic laser-processing method is compared with the standard four-beam technique. Examples of experimental demonstration achieved in photosensitive polymers are given. The article also introduces a multiphoton direct-writing technique for creating defect structures in lattices towards production of defect cavity-functionalized photonic crystal devices. It shows that all Bravais lattices can be produced by choosing proper incident vectors of laser beams. The lattice constant of the structure can be changed without distorting its lattice symmetry and lattice elements.