Circumventing the Manley-Rowe quantum efficiency limit in an optically pumped terahertz quantum-cascade amplifier

Using a microscopic theory based on the Maxwell-semiconductor Bloch equations, we investigate the feasibility of an optically pumped electrically driven terahertz (THz) quantum-cascade laser as a pathway to room-temperature THz generation. In optical conversion schemes the power conversion efficiency is limited by the Manley-Rowe relation. We circumvent this constraint by incorporating an electrical bias in a four level intersubband scheme, thereby allowing coherent recovery of the optical pump energy. The observed THz radiation is generated through both stimulated emission and automatically phase-matched quantum coherence contributions--making the proposed approach both a promising source for THz radiation and a model system for quantum coherence effects such as lasing without inversion and electromagnetically induced transparency.     

Second-harmonic generation in GaAs-based quantum-cascade lasers

We report second-harmonic (SH) and sum-frequency generation in GaAs-based quantum-cascade lasers. A doping dependence study of the second-order susceptibility in one of the investigated structure is shown. We also demonstrate that grating-coupled surface emission is a highly efficient way to couple out the SH radiation.     

Second-harmonic generation in GaAs-based quantum-cascade lasers

We report second-harmonic (SH) and sum-frequency generation in GaAs-based quantum-cascade lasers. A doping dependence study of the second-order susceptibility in one of the investigated structure is shown. We also demonstrate that grating-coupled surface emission is a highly efficient way to couple out the SH radiation.     

Novel design concepts of widely tunable germanium terahertz lasers

We explore the full parameter space for laser operation and compile guidelines for building improved Ge lasers, especially for continuous wave applications. We present laser emission from p-type Ge lasers with small volumes at high repetition rates and with inter-contact distances as low as 750 μm. The emission is analyzed as a function of the current–voltage characteristic. The Poisson equation is solved to determine the electric field distribution and two new laser designs are presented.     

Distributed-feedback terahertz quantum-cascade lasers with laterally corrugated metal waveguides

We report the demonstration of distributed-feedback terahertz quantum-cascade lasers based on a first-order grating fabricated via a lateral corrugation in a double-sided metal ridge waveguide. The phase of the facet reflection was precisely set by lithographically defined facets by dry etching. Single-mode emission was observed at low to moderate injection currents, although multimode emission was observed far beyond threshold owing to spatial hole burning. Finite-element simulations were used to calculate the modal and threshold characteristics for these devices, with results in good agreement with experiments.     

Synchronization and time shifts of dynamical patterns for mutually delay-coupled fiber ring lasers

A pair of coupled erbium doped fiber ring lasers is used to explore the dynamics of coupled spatiotemporal systems. The lasers are mutually coupled with a coupling delay less than the cavity round-trip time. We study synchronization between the two lasers in the experiment and in a delay differential equation model of the system. Because the lasers are internally perturbed by spontaneous emission, we include a noise source in the model to obtain stochastic realizations of the deterministic equations. Both amplitude synchronization and phase synchronization are considered. We use the Hilbert transform to define the phase variable and compute phase synchronization. We find that synchronization increases with coupling strength in the experiment and the model. When the time series from two lasers are time shifted in either direction by the delay time, approximately equal synchronization is frequently observed, so that a clear leader and follower cannot be identified. We define an algorithm to determine which laser leads the other when the synchronization is sufficiently different with one direction of time shift, and statistics of switches in leader and follower are studied. The frequency of switching between leader and follower increases with coupling strength, as might be expected since the lasers mutually influence each other more effectively with stronger coupling.     

共振声子太赫兹量子级联激光器研究

本文综述了共振声子太赫兹(THz)量子级联激光器(QCL)载流子输运及其光电特性的研究结果。我们采用Monte Carlo方法模拟了不同有源区结构的共振声子THz QCL,如四阱有源区和三阱有源区结构等,优化了THz QCL的器件参数。计算表明,器件的增益依赖于注入势垒的宽度、掺杂浓度和声子抽运能级间隔等参数。单阱注入的THz QCL可以获得低的激射频率,而三阱有源区结构的THz QCL具有更高的工作温度。我们数值模拟了这两种器件结构在不同寄生电压下的电子输运、增益以及温度特性等。同时,我们实验测量了THz QCL的发射谱偏压和温度效应,计算结果与实验相吻合。研究结果表明,随着偏压增加发射谱出现了明显的频率蓝移,多模激射仅出现在大电流注入情形。与偏压的密切依赖关系不同,激射频率对温度的变化并不敏感。     

Spectral Characteristics of Half-Ring Quantum-Cascade Lasers

Optics and Spectroscopy - A cavity scheme based on a half-ring with different radii is proposed and fabricated for 7–8 μm quantum-cascade lasers. A quantum-cascade laser with a half-ring...     

光子晶体激光器阈值特性的理论研究

针对光子晶体激光器的结构特点,并考虑到非辐射复合及增益饱和效应的影响,运用速率方程研究了光子晶体激光器的阈值特性。由爱因斯坦A、B系数的关系,得出了自发辐射耦合因子的变化并不独立于腔体积的结论,并给出其具体数学表达式。分析了自发辐射耦合因子、表面复合速率、有源区体积及品质因子对光子晶体激光器阈值电流的影响。结果表明,经过优化处理,可以较好地降低光子晶体激光器的阈值电流,为无阈值激光器的实现提供了理论依据。     

Functional organic single crystals for solid‐state laser applications

Because of long‐range order and high chemical purity, organic crystals have exhibit unique properties and attracted a lot of interest for application in solid‐state lasers. As optical gain materials, they exhibit high stimulated emission cross section and broad tunable wavelength emission as similar to their amorphous counterpart; moreover, high purity and high order give them superior properties such as low scattering trap densities, high thermal stability, as well as highly polarized emission. As electronic materials, they are potentially able to support high current densities, thus making it possible to realize current driven lasers. This paper mainly describes recent research progress in organic semiconductor laser crystals. The building molecules, crystal growth methods, as well as their stimulated emission characteristics related with crystal structures are introduced; in addition, the current state‐of‐the‐art in the field of crystal laser devices is reviewed. Furthermore, recent advances of crystal lasers at the nanoscale and single crystal light‐emitting transistors (LETs) are presented. Finally, an outlook and personal view is provided on the further developments of laser crystals and their applications.     

Scaling Q-switched microchip lasers for shortest pulses

We evaluate the design of Q-switched microchip lasers to examine the limit of their scaling to shorter picosecond pulses. A numerical model based on the laser rate equations is developed and used to predict the output characteristics of prospective laser designs. Based on the modeled results, we employ a SESAM as a passive Q-switch and thin crystals of Nd:YVO₄ as a gain medium to experimentally demonstrate the production of pulses as short as 22?ps.     

给-受体材料系统中光致发光的峰值波长随浓度变化的理论

高效的给-受体(DA)能量传输材料体系被公认为有机激光器中的重要组成部分。基于Förster传输和受激发射的理论,我们提出了一种在DA系统下发射光谱的数值模型。该模型较好地描述了在DA材料系统中发射光谱随着受主浓度变化的最主要特征,即光致发光谱峰值波长随着受体浓度的增加而非线性增大,并在浓度较高时趋于受体的单体发射波长。     

Analytical and visual modeling of InGaN/GaN single quantum well laser based on rate equations

An analytical, visual and open source model based on solving the rate equations for InGaN/GaN single quantum well (QW) lasers has been carried out. In the numerical computations, the fourth-order Runge–Kutta method has been used for solving the differential rate equations. The rate equations which have been considered in this simulation include the two level rate equations for the well and separate confinement heterostructure (SCH) layers. We present a new and inexpensive modeling method with analytical, visual and open source capabilities to investigate and comprehend the QW laser characteristics such as time behavior of carriers in SCHs and QW, photon density, output power and gain, and also the output power versus current which presents the threshold current of the laser. The characteristics of the QW lasers, which include laser time response (P–t), turn-on delay time of lasing and output power–current (P–I) characteristic and related features such as threshold current and slope efficiency have been investigated. Our model accurately computes the P–t and P–I characteristics such as turn-on delay time, threshold current and slope efficiency, and also illustrates the effects of parameters such as the injection current and geometry.     

Design of nonlinearity-enhanced quantum-cascade lasers

We present simulations of mid-infrared quantum-cascade lasers (QCL) with optimized second-harmonic generation (SHG). The optimized design was obtained utilizing techniques from supersymmetric quantum mechanics with both material-dependent effective mass and band nonparabolicity. Two-photon processes are analyzed for resonant cascading triple levels designed for enhancing SHG. Nonunity pumping efficiency from one period of the QCL to the next is taken into account by including all relevant carrier scattering mechanisms between the injector/collector and active regions. Carrier transport and power output of the structure are analyzed by self-consistently solving rate equations for the carriers and photons. Current-dependent linear optical output power is derived based on the steady-state photon population in the active region. The SH power is derived from the Maxwell equations with the phase mismatch and modes overlapping included. Due to stronger coupling between lasing levels, the optimized structure has both higher linear and SH output powers. The optimized structure can be fabricated through digitally grading the submonolayer alloys by molecular beam epitaxy (MBE) technique.     

Effects on spontaneous emission on dynamic characteristics of semiconductor lasers

Dynamic characteristics of semiconductor lasers are calculated with single-mode rate equations, taking into account spontaneous emission factor β and bias current     

Study of dielectric corona pre-ionization in hybrid TEA lasers

In this paper a theoretical analysis of single mode hybrid CO₂ lasers is studied that to describe the process of the dynamic emission in this lasers types and based on the Landau – Teller six-temperature model for the CO₂-N₂-He-CO system. The main discharge region is considered as a time dependent nonlinear RLC circuit. The electric circuit equations (including the ionization rate equations), the equations of laser (including stored energy density in CO₂ modes) and equations of laser intensities are coupled and solved numerically. Then the effects of the ionizer dielectric parameters on the output laser intensity are obtained. Application of this model gives more output energy than that have obtained by the previous works.     

Frequency sweeping of emission from heterojunction quantum lasers with pump current modulation

We have used numerical modeling to study the dynamic shift of the “instantaneous” laser frequency (sweeping) as a function of the pump current modulation frequency for heterojunction quantum lasers when the lasing frequency is tuned within the gain band. As the model for the active medium, we used a two-band model with identical distribution of levels for the electron and hole subbands, in which we take into account direct transitions between the ground-state subbands. We have established that the variation in the sweep qualitatively corresponds to the behavior of the amplitude detuning characteristics of the laser. Depending on the parameters of the system during emission of pulses, the variation in the laser frequency can be monotonic or nonmonotonic. By selecting the pump current modulation frequency and setting the laser frequency within the gain band, we can realize the cases of maximum or minimum dynamic shift of the lasing frequency. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 370–374, May–June, 2006.