QUANTUM INSPIRED SYMMETRIES IN LASER ENGINEERING

In this thesis, quantum inspired symmetries including Parity-Time (PT) symmetry and Supersymmetry (SUSY) have been studied in the context of non-Hermitian engineered laser systems. This thesis starts with a short review of semiconductor lasers theory in second chapter, followed by an introduction to quantum inspired symmetries: PT symmetry and SUSY in optics and photonics in chapter three. In chapter four, we have studied the robustness and mode selectivity in PT symmetric lasers. We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity. Our results, supported by numerically integrating the nonlinear rate equations and performing linear stability analysis, reveals the following: (1) In principle a second threshold exists after which single mode operation becomes unstable, signaling multimode oscillatory dynamics, (2) For a wide range of design parameters, single mode operation of PT lasers having relatively large free spectral range (FSR) can be robust even at higher gain values, (3) PT symmetric photonic molecule lasers are more robust than their counterpart structures made of single microresonators; and (4) Extending the concept of single longitudinal mode operation based on PT symmetry in millimeter long edge emitting lasers having smaller FSR can be challenging due to instabilities induced by nonlinear modal interactions. Finally, we also present a possible strategy based on loss engineering to achieve more control over the mode selectivity by suppressing the mode that has the highest gain (i.e. lies under the peak of the gain spectrum curve) and switch the lasing action to another mode. In chapter five a new scheme for building two dimensional laser arrays that operate in the single supermode regime is proposed. This is done by introducing an optical coupling between the laser array and a lossy pseudo-isospectral chain of photonic resonators. The spectrum of this discrete reservoir is tailored to suppress all the supermodes of the main array except the fundamental one. This spectral engineering is facilitated by employing the Householder transformation in conjunction with discrete supersymmetry. The proposed scheme is general and can in principle be used in different platforms such as VCSEL arrays and photonic crystal laser arrays. Finally, in chapter six we have investigated the laser self-termination (LST) in trimer photonic molecules. It is shown that under the appropriate conditions, LST can exist in complex discrete structures made of three-cavity photonic molecule lasers. We have shown that the phenomenon of LST is a purely linear effect associated with avoided level crossings. Furthermore, our simulations show that the predicted behavior is persistent when gain saturation nonlinearity is taken into account. Conclusion remarks and future works are discussed in the last chapter

Smart control of light in edge-emitting lasers

Tesi en modalitat de compendi de publicacions, amb diferents seccions retallades per drets dels editorsThe invention of the laser triggered the study of light-matter interactions. In turn, the advent of artificial structured materials on micro- and nanometer scales has become a fruitful playground to tailor the propagation and generation of light, even in exotic or counterintuitive ways, uncovering novel physical phenomena. In this thesis, we precisely propose using recently discovered properties of artificial photonic materials, and new schemes, to control the spatiotemporal dynamics of broad area semiconductor lasers and improve their performance. Semiconductor lasers are replacing other laser sources due to their efficiency, compactness and affordable prices, however suffering from a major drawback. The quality of the emitted beam intrinsically deteriorates when power increases, if the aperture of the laser is very broad as compared to wavelength. The highly multimode and unstable emission limits possible applications of these lasers. Although different mechanisms have been proposed to save this obstacle, obtaining a stable and bright emission remains a longstanding open question. This thesis aims at contributing to this goal without compromising their compact design, and to the new field of non-Hermitian Photonics providing new insights into the control of wave dynamics in artificial complex media. Indeed, the physics of open-dissipative, non-Hermitian systems offers new possibilities to utilize the gain and loss for steering optical processes, and is beyond the recent focus on non-Hermitian Photonics. As initially demonstrated in the frame of Quantum Mechanics, systems with gain and losses may still present real eigenvalues of the Hamiltonian (energy) as the purely conservative ones, yet holding other unexpected physical behaviors, derived from an asymmetric coupling between modes. In particular, this was first observed in systems invariant under parity (P-) and time (T-) symmetry ¿ referred as PT-symmetric ¿. Optical systems with complex permittivity are flexible and achievable classical analogs of such quantum systems to realize and explore these effects. As a first step, we propose to use a chirped modulation of the refractive index (chirped photonic crystal) for intracavity filtering the multimode emission of EELs. To numerically assess the filtering performance, we developed a full (2+1)-dimensional spatio-temporal model, including both transverse and longitudinal dimensions plus time, for the evolution of the electric field and carriers. The good agreement between predictions with actual experimental results demonstrates the proposal while validating the model which is used throughout the thesis, with corresponding modifications. We then analyze the effect of intrinsically imposing in phase refractive index and gain modulations within the semiconductor laser, and use the interplay between real and imaginary parts of the non-Hermitic potential to achieve spatial and temporal stabilization. Taking one step further, we propose to divide the EEL cavity into two mirror-symmetric half-spaces, both holding PT-symmetry but with opposite mode coupling. With this geometry, we expect to obtain a two-fold benefit: on the one hand, achieving a spatial-temporal stabilization of the laser, and on the other, localizing the generated field along the symmetry axis. We numerically demonstrate regimes of simultaneous localization and stabilization leading to an enhanced output and improved beam quality. Finally, while thinner lasers show a more stable new temporal and synchronization instabilities arise in EELS arrays (bars) from the coupling between neighboring lasers, leading again to irregular spatiotemporal behaviors. We show that the proposed mirror symmetric non-Hermitian configuration may be extended to couple individual EELs in the array, by a lateral shift between the pump and index profiles. In all cases, the obtained localized and stable output beam may facilitate a direct coupling of the emitted beam to optical fibers.La invenció del làser va representar el tret de sortida per nombrosos estudis de la interacció entre la llum i la matèria. A banda, el desenvolupament de nous materials fotònics artificials en escales micro i nanomètriques ha esdevingut un camp fructífer per al control de la propagació i generació de la llum, fins i tot de maneres exòtiques o contra intuïtives, revelant nous fenòmens físics. En aquesta tesi proposem, precisament, utilitzar nous materials fotònics artificials i nous esquemes per controlar la dinàmica espai-temporal dels làsers de semiconductor d'apertura ampla, per millorar-ne les propietats. Els làsers de semiconductor estan reemplaçant altres fonts de llum làser gràcies a la seva eficiència, format compacte i preu assequible. Malgrat tot, pateixen un gran inconvenient: el deteriorament del feix emès en augmentar la potència, especialment si l'amplada del làser és molt gran respecte la longitud d'ona. Quan l'emissió esdevé altament multimode i inestable en limita les possibles aplicacions. Encara que s'han proposat diferents mecanismes per superar aquest problema, aconseguir una emissió estable sense comprometre'n el format compacte, és encara una qüestió oberta. Aquesta tesi té com a objectiu contribuir a la millora dels làsers de semiconductor i a l'estudi del control de la dinàmica dels làsers mitjançant el nou camp de la fotònica no hermítica. De fet, la física dels sistemes oberts no Hermítics ofereix noves possibilitats per utilitzar la permitivitat complexa per dominar processos òptics i és la causa del recent interès en la fotònica no hermitiana. Primer, es va demostrar en el marc de la Mecànica Quàntica que els sistemes oberts o no Hermítics, tot i tenir guanys o pèrdues poden presentar autovalors reals del Hamiltonià (valors constants de l'energia) i altres comportaments físics inesperats, derivats d'acoblaments asimètrics entre modes. Efecte observat inicialment en sistemes invariants sota la paritat (P-) i la simetria de temps (T-), anomenats PT-simètrics. Els sistemes òptics amb permitivitat complexa són anàlegs clàssics, flexibles i assequibles d’aquests sistemes quàntics per realitzar i explorar aquests nous efectes. Primer, proposem fer servir una modulació de l’índex de refracció per al filtrat, intacavitat, de l'emissió multimode d'amplificadors i làsers de semiconductor. Per l'anàlisi numèrica desenvolupem un model espai-temporal complet, que inclou dues dimensions espacials, transversal i longitudinal, més l'evolució temporal del camp elèctric i dels portadors. Aquest model s'utilitza al llarg de tota la tesis amb les modificacions corresponents i és contrastat també experimentalment. A continuació, analitzem l'efecte d'imposar modulacions intrínseques de l’índex de refracció i el guany, en fase i, dins del làser de semiconductor. Gràcies a la interacció entre parts reals i imaginàries del potencial no hermític s’aconsegueix una estabilització espacial i temporal. Fent un pas més, dividim la cavitat làser en dos espais PT-simètrics (simetria de mirall) amb un acoblament en sentit oposat. Amb aquesta geometria, esperem obtenir un doble benefici: d'una banda, aconseguir una estabilització espacial-temporal del làser, i per una altra banda, localitzar el camp generat en l'eix de simetria. Es demostra numèricament règims de localització i d'estabilització simultànies, augmentant la potència emesa tot millorant la qualitat dels feix. Finalment, tot i que els làsers més estrets mostren una emissió més estable, els làsers propers s’acoblen quan formen part d'una matriu. Demostrem que l'acoblament asimètric també pot ser utilitzat en barres de làsers de semiconductor per estabilitzar-los temporalment i concentrar-ne l’emissió. L'acoblament asimètric es produeix mitjançant un desplaçament lateral entre el bombeig i l'índex de refracció. En tots els casos, el feix de sortida localitzat i estable obtingut pot facilitar un acoblament directe del feix emès a les fibres òptiques.Postprint (published version

Non-Hermitian aspects of coherently coupled vertical cavity laser arrays

Two by one (2 × 1) optically coupled electrically isolated vertical cavity surface emitting laser (VCSEL) arrays have been studied both theoretically and experimentally. Because of the tunable gain/loss profile in the array, the coupled laser system is non-Hermitian in analogy with non-Hermitian quantum mechanics. The experimentally observed optical mode tuning and beam steering are inherently connected to the non-Hermiticity of the system. Theoretical investigation of the mode tuning mechanism is conducted first by coupled mode analysis, and then in a more comprehensive coupled rate equation analysis. The theoretical analysis reveals the unique mode tuning mechanism in coupled VCSEL arrays and is shown to be in excellent agreement with experimental characterization. Experimentally, 2 × 1 optically coupled electrically isolated VCSEL arrays have been designed, fabricated, and characterized. We perform two-dimensional characterizations by varying the two independently controlled injection currents into each array and recording the laser output power, spectra, near-field intensity profile, and far-field intensity profile. Two-dimensional maps of the output optical power, interference visibility, and beam steering angles versus the two injection currents are plotted as concise representations of the mode tuning behavior controlled by the current tuning. Arrays with built-in asymmetry between the two lasers demonstrate that the mode tuning behavior can also be engineered by the degree of asymmetry. The coupling coefficient is extracted from the characterizations. The theoretical and experimental investigations presented in this work reveal the unique mode tuning mechanism in weakly coupled diode laser arrays and will guide the future pursuit of improved functionalities in coupled VCSEL arrays

Robustness and mode selectivity in parity-time (PT) symmetric lasers

We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity. Our results, supported by numerically integrating the nonlinear rate equations and performing linear stability analysis, reveals the following: (1) In principle a second threshold exists after which single mode operation becomes unstable, signaling multimode oscillatory dynamics, (2) For a wide range of design parameters, single mode operation of PT lasers having relatively large free spectral range (FSR) can be robust even at higher gain values, (3) PT symmetric photonic molecule lasers are more robust than their counterpart structures made of single microresonators; and (4) Extending the concept of single longitudinal mode operation based on PT symmetry in millimeter long edge emitting lasers having smaller FSR can be challenging due to instabilities induced by nonlinear modal interactions. Finally we also present a possible strategy based on loss engineering to achieve more control over the mode selectivity by suppressing the mode that has the highest gain (i.e. lies under the peak of the gain spectrum curve) and switch the lasing action to another mode

Artificial Magnetism and Topological Phenomena in Optics

Recent years have witnessed intense research activities to effectively control the flow of photons using various classes of optical structures such as photonic crystals and metamaterials. In this regard, optics has benefited from concepts in condensed matter and solid-state physics, where similar problems concerning electronic wavefunctions arise. An important example of such correspondence is associated with the photon dynamics under the effect of an artificial magnetic field. This is especially important since photons, as neutral bosons, do not inherently interact with magnetic fields. One way to mitigate this issue is to exploit magneto-optical materials. However, as is well known, using such materials comes with several issues in terms of optical losses and fabrication challenges. Therefore, clearly of interest would be to devise certain schemes, which employ conventional dielectric materials, and yet provide an artificial magnetic field e.g. through geometric phases imprinted in the photonic wave amplitudes. Here, we utilize such schemes to observe various optical wave phenomena arising from the associated artificial magnetism. First, we show that light propagation dynamics in a twisted multicore optical fiber is governed by the Schrödinger equation in the presence of a magnetic potential. Using this, we experimentally observe Aharonov-Bohm suppression of optical tunneling in these structures. Moreover, we use notions from topological insulators to demonstrate the first dielectric-based topological lasers both in 1- and 2-dimensional lattices of microring resonators. Our measurements show that such laser arrays exhibit significant improvement in terms of robustness against defects and disorder, as well as higher slope efficiencies as compared to conventional laser arrays. Finally, we show both theoretically and experimentally, that the cooperative interplay among vectorial electromagnetic modes in coupled metallic nanolasers can be utilized as a means to emulate the classical XY Hamiltonian. In particular, we discern two phases in these systems, akin to those associated with ferromagnetic (FM) and antiferromagnetic (AF) materials

Bound states in the continuum

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work.National Science Foundation (U.S.) (Grants DMR-1307632)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-13-D- 0001)United States. Department of Energy. Office of Science. Solid-State Solar Thermal Energy Conversion Center (Grant DE-SC0001299)United States-Israel Binational Science Foundation (Award 2013508

Emerging Nanophotonic Applications Explored with Advanced Scientific Parallel Computing

The domain of nanoscale optical science and technology is a combination of the classical world of electromagnetics and the quantum mechanical regime of atoms and molecules. Recent advancements in fabrication technology allows the optical structures to be scaled down to nanoscale size or even to the atomic level, which are far smaller than the wavelength they are designed for. These nanostructures can have unique, controllable, and tunable optical properties and their interactions with quantum materials can have important near-field and far-field optical response. Undoubtedly, these optical properties can have many important applications, ranging from the efficient and tunable light sources, detectors, filters, modulators, high-speed all-optical switches; to the next-generation classical and quantum computation, and biophotonic medical sensors. This emerging research of nanoscience, known as nanophotonics, is a highly interdisciplinary field requiring expertise in materials science, physics, electrical engineering, and scientific computing, modeling and simulation. It has also become an important research field for investigating the science and engineering of light-matter interactions that take place on wavelength and subwavelength scales where the nature of the nanostructured matter controls the interactions. In addition, the fast advancements in the computing capabilities, such as parallel computing, also become as a critical element for investigating advanced nanophotonic devices. This role has taken on even greater urgency with the scale-down of device dimensions, and the design for these devices require extensive memory and extremely long core hours. Thus distributed computing platforms associated with parallel computing are required for faster designs processes. Scientific parallel computing constructs mathematical models and quantitative analysis techniques, and uses the computing machines to analyze and solve otherwise intractable scientific challenges. In particular, parallel computing are forms of computation operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently. In this dissertation, we report a series of new nanophotonic developments using the advanced parallel computing techniques. The applications include the structure optimizations at the nanoscale to control both the electromagnetic response of materials, and to manipulate nanoscale structures for enhanced field concentration, which enable breakthroughs in imaging, sensing systems (chapter 3 and 4) and improve the spatial-temporal resolutions of spectroscopies (chapter 5). We also report the investigations on the confinement study of optical-matter interactions at the quantum mechanical regime, where the size-dependent novel properties enhanced a wide range of technologies from the tunable and efficient light sources, detectors, to other nanophotonic elements with enhanced functionality (chapter 6 and 7)

Exciton-polariton trapping and potential landscape engineering

Exciton-polaritons in semiconductor microcavities have become a model system for the studies of dynamical Bose-Einstein condensation, macroscopic coherence, many-body effects, nonclassical states of light and matter, and possibly quantum phase transitions in a solid state. These low-mass bosonic quasiparticles can condense at comparatively high temperatures up to 300 K, and preserve the fundamental properties of the condensate, such as coherence in space and time domain, even when they are out of equilibrium with the environment. Although the presence of an in-plane confining potential is not strictly necessary in order to observe Bose-Einstein condensation, engineering of the polariton confinement is a key to controlling, shaping, and directing the flow of polaritons. Prototype polaritonbased optoelectronic devices rely on ultrafast photon-like velocities and strong nonlinearities exhibited by polaritons, as well as on their tailored confinement. Nanotechnology provides several pathways to achieving polariton confinement, and the specific features and advantages of different methods are discussed in this review. Being hybrid exciton-photon quasiparticles, polaritons can be trapped via their excitonic as well as photonic component, which leads to a wide choice of highly complementary trapping techniques. Here we highlight the almost free choice of the confinement strengths and trapping geometries that provide powerful means for control and manipulation of the polariton systems both in the semi-classical and quantum regimes. Furthermore, the possibilities to observe effects of the polariton blockade, Mott insulator physics, and population of higher-order energy bands in sophisticated lattice potentials are discussed. Observation of such effects could lead to realization of novel polaritonic non-classical light sources and quantum simulators.PostprintPeer reviewe