Ultra-bright omni-directional collective emission of correlated photon pairs from atomic vapors

Spontaneous four-wave mixing can generate highly correlated photon pairs from atomic vapors. We show that multi-photon pumping of dipole-forbidden transitions in a recoil-free geometry can result in ultra-bright pair-emission in the full 4\pi solid angle, while strongly suppresses background Rayleigh scattering and associated atomic heating, Such a system can produce photon pairs at rates of ~ 10 ^12 per second, given only moderate optical depths of 10 ~ 100, or alternatively, the system can generate paired photons with sub-natural bandwidths at lower production rates. We derive a rate-equation based theory of the collective atomic population and coherence dynamics, and present numerical simulations for a toy model, as well as realistic model systems based on 133 Cs and 171 Yb level structures. Lastly, we demonstrate that dark-state adiabatic following (EIT) and/or timescale hierarchy protects the paired photons from reabsorption as they propagate through an optically thick sample

Phonon-mediated and weakly size-dependent electron and hole cooling in CsPbBr3 nanocrystals revealed by atomistic simulations and ultrafast spectroscopy

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over similar to 10 ps) of "warm" electrons in the vicinity (within similar to 0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps(-1) carrier(-1), increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers

Development of terahertz quantum-cascade lasers as sources for heterodyne receivers

Die vorliegende Arbeit beschäftigt sich mit der Entwicklung und Optimierung von Terahertz-Quantenkaskadenlasern (THz-QCLs) für die Anwendung als Lokaloszillator in THz-Heterodyndetektoren, insbesondere für die Detektion der astronomisch wichtigen Sauerstoff (OI) Linie bei 4.75 THz. Hierfür wurden zunächst unterschiedliche QCL-Heterostrukturen untersucht. Basierend auf einer Heterostruktur, welche schnelle Intersubbandübergänge über Streuung an Phononen ausnutzt, konnten QCLs mit hoher Ausgangsleistung und niedriger Betriebsspannung bei 3 THz erzielt werden. Während diese Laser auf dem Materialsystem GaAs/Al_xGa_(1-x)As mit $x=0.15$ basieren, führt die Erhöhung des Al-Anteils auf x=0.25 für ähnliche Strukturen zu sehr niedrigen Schwellstromdichten. Durch schrittweise Optimierungen gelang es, QCLs zu realisieren, die bei 4.75 THz emittieren. Mit Hilfe von lateralen Gittern erster Ordnung für die verteilte Rückkopplung (DFB) konnten Einzelmoden-Dauerstrichbetrieb mit hoher Ausgangsleistung, sowie Einzelmoden-Betrieb innerhalb des spezifizierten Frequenzbereichs bei 4.75 THz erzielt werden. Eine allgemeine Methode zur Bestimmung der DFB-Kopplungskonstanten erlaubt eine gute Beschreibung der Laser innerhalb der etablierten Theorie der gekoppelten Moden für DFB-Laser mit reflektiven Endfacetten. Oft steht das Auftreten negativer differentieller Leitfähigkeit bei höheren Feldstärken und die damit verbundenen Bildung von elektrischer Felddomänen (EFDs) im Konflikt mit einem stabilen Betrieb der THz-QCLs. Es wird gezeigt, dass stationäre EFDs mit Diskontinuitäten in der statischen Licht-Strom-Spannungskennlinie verbunden sind, während Selbstoszillationen, verursacht durch nicht-stationäre EFDs, eine zeitliche Modulation der Ausgangsleistung bewirken. Mit Hilfe einer effektiven Driftgeschwindigkeit für QCLs lassen sich viele der beobachteten Phänomene durch die nichtlinearen Transportgleichungen für schwach gekoppelte Übergitter beschreiben.This thesis presents the development and optimization of terahertz quantum-cascade lasers (THz QCLs) as sources for heterodyne receivers. A particular focus is on single-mode emitters for the heterodyne detection of the important astronomic oxygen (OI) line at 4.75 THz. Various active-region designs are investigated. High-output-power THz QCLs with low operating voltages and emission around 3 THz are obtained for an active region, which involves phonon-assisted intersubband transitions. While these QCLs are based on a GaAs/Al_xGa_(1-x)As heterostructure with x=0.15, similar heterostructures with x=0.25 allowed for very low threshold current densities. By successive modifications of the active-region design, THz QCLs have been optimized toward the desired frequency at 4.75 THz. To obtain single-mode operation, first-order lateral distributed-feedback (DFB) gratings are investigated. It shows that such gratings allow for single-mode operation in combination with high continuous-wave (cw) output powers. A general method is presented to calculate the coupling coefficients of lateral gratings. In conjunction with this method, the lasers are well described by the coupled-mode theory of DFB lasers with two reflective end facets. Single-mode operation within the specified frequency bands at 4.75 THz is demonstrated. Stable operation of THz QCLs is often in conflict with the occurrence of a negative differential resistance (NDR) regime at elevated field strengths and the formation of electric-field domains (EFDs). Stationary EFDs are shown to be related to discontinuities in the cw light-current-voltage characteristics, while non-stationary EFDs are related to current self-oscillations and cause a temporal modulation of the output power. To model such effects, the nonlinear transport equations of weakly coupled superlattices are adopted for QCLs by introducing an effective drift velocity-field relation

Analysis of double laser emission occuring in 1.55 μm InAs-InP (113)B quantum dot laser

In this paper, a theoretical model based on rate equations is used to investigate static and dynamic behaviors of InAs–InP (113)B quantum-dot (QD) lasers emitting at 1.55 m. More particularly, it is shown that two modelling approaches are required to explain the origin of the double laser emission occurring in QD lasers grown on both, GaAs and InP substrates. Numerical results are compared to experimental ones by using either a cascade or a direct relaxation channel model. The comparison demonstrates that when a direct relaxation channel is taken into account, the numerical results match very well the experimental ones and lead to a qualitative understanding of InAs–InP (113)B QD lasers. Numerical calculations for the turn-on delay are also presented. A relaxation oscillation frequency as high as 10 GHz is predicted which is very promising for the realization of directly modulated QD lasers for high-speed transmissions

Time Domain Traveling Wave analysis of the multimode dynamics of Quantum Dot Fabry-Perot Lasers

In this paper we investigate with numerical simulations the rich multi-mode dynamics of Quantum Dot Fabry-Perot Lasers. We have used a Time Domain Traveling Wave approach including the electron and hole carrier dynamics in the various Quantum Dot confined states, the inhomogeneous broadening of the complex gain spectrum, the polarization dynamics and the effect of the carrier-photon interaction in the cavity. The role of the various non-linear interaction mechanism on the broadening of optical spectrum of the Quantum Dot laser has been investigated and the main parameters responsible for the phase locking between the longitudinal modes have been identified. We show that in some cases it is possible obtaining pulses after simulating the propagation of the laser output field in a dispersive medium. Many of the obtained simulation results are in good agreement with the experiments reported in the iterature

Direct Nanoscale Imaging of Evolving Electric Field Domains in Quantum Structures

The external performance of quantum optoelectronic devices is governed by the spatial profiles of electrons and potentials within the active regions of these devices. For example, in quantum cascade lasers (QCLs), the electric field domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially nonuniform and temporally unstable. Unfortunately, there exists no prior means of probing the inner potential profile directly. Here we report the nanoscale measured electric potential distribution inside operating QCLs by using scanning voltage microscopy at a cryogenic temperature. We prove that, per the EFD hypothesis, the multi-quantum-well active region is indeed divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules does not continuously increase in proportion to gradual increases in the applied device bias, but rather hops between discrete values that are related to tunneling resonances. We also report the evolution of EFDs, finding that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region.Natural Sciences and Engineering Research Council of CanadaCanadian Foundation for InnovationCMC Microsystems (Firm)Ontario Research Foundatio

Frequency combs induced by phase turbulence

Wave instability—the process that gives rise to turbulence in hydrodynamics1—represents the mechanism by which a small disturbance in a wave grows in amplitude owing to nonlinear interactions. In photonics, wave instabilities result in modulated light waveforms that can become periodic in the presence of coherent locking mechanisms. These periodic optical waveforms are known as optical frequency combs2–4. In ring microresonator combs5,6, an injected monochromatic wave becomes destabilized by the interplay between the resonator dispersion and the Kerr nonlinearity of the constituent crystal. By contrast, in ring lasers instabilities are considered to occur only under extreme pumping conditions7,8. Here we show that, despite this notion, semiconductor ring lasers with ultrafast gain recovery9,10 can enter frequency comb regimes at low pumping levels owing to phase turbulence11—an instability known to occur in hydrodynamics, superconductors and Bose–Einstein condensates. This instability arises from the phase–amplitude coupling of the laser field provided by linewidth enhancement12, which produces the needed interplay of dispersive and nonlinear effects. We formulate the instability condition in the framework of the Ginzburg–Landau formalism11. The localized structures that we observe share several properties with dissipative Kerr solitons, providing a first step towards connecting semiconductor ring lasers and microresonator frequency combs13

Optical and transport properties of GaN and its lattice matched alloys

The study of carrier dynamics in wide band gap semiconductors is of great importance for UV detectors and emitters which are expected to be the building blocks for optoelectronic applications and high voltage electronics. On the experimental side, the progress made in the past two decades in generating subpicosecond laser pulses, resulted in numerous experiments that gave insight into the carrier dynamics in semiconductors. From the theoretical standpoint, the study of carrier interactions together with robust simulation methods, such as Monte-Carlo, provided great progress toward explaining the experimental results. These studies immensely improve our understanding of time scales of carrier recombination, relaxation and transport in semiconductor materials and devices which lead to optimizing the operation of optoelectronic devices, more specifically, emitters and detectors. Wide band gap materials having high breakdown field, wide band gap energy and high saturation velocity are among the most important semiconductors employed in the active layer of LEDs and lasers. GaN , its alloys, and ZnO are among the most important materials in semiconductor devices. Moreover, the use of lattice matched layers based on InAlN or InAlGaN is an alternative design approach which could mitigate the effect of polarization and enable growing thicker layers due to the higher structural quality. We first perform the study of carrier dynamics generated by ultrafast laser pulses in bulk GaN and ZnO materials to investigate the temperature dependent luminescence rise time. The obtained results are compared to the experimental results which show an excellent agreement. In this work, we use Monte Carlo method to evaluate the distribution of carriers considering the interaction of carriers with other carriers and also with polar optical phonons in the system. Considering the ongoing research about the advantages of lattice matched nitride based material systems, we also studied the properties of GaN layers lattice matched to InAlN and InAlGaN. As an application, we utilized the GaN/InAlGaN material system to study the carrier dynamics in Quantum Cascade Lasers. Furthermore, due to the superior properties of GaN which makes it an excellent candidate in power electronic applications, we also design and simulate an advanced vertical trench power MOSFET using drift diffusion and Monte Carlo models and characterize the performance of the device