Influence of a single quantum dot state on the characteristics of a microdisk laser

We report a quantum dot microcavity laser with a cw sub-microW lasing threshold, where a significant reduction of the lasing threshold is observed when a single quantum dot (QD) state is aligned with a cavity mode. The quality factor exceeds 15,000 before the system lases. When no QD states are resonant, below threshold the cavity mode initially degrades with increasing pump power, after which saturation occurs and then the cavity mode recovers. We associate the initial cavity mode spoiling with QD state broadening that occurs with increasing pump power.     

Single quantum dot nanolaser

Recent theoretical and experimental progress on nanolasers is reviewed with a focus on the emission properties of devices operating with a few or even an individual semiconductor quantum dot as a gain medium. Concepts underlying the design and operation of these devices, microscopic models describing light‐matter interaction and semiconductor effects, as well as recent experimental results and lasing signatures are discussed. In particular, a critical review of the “self‐tuned gain” mechanism which gives rise to quantum‐dot mode coupling in the off‐resonant case is provided. Furthermore recent advances in the modeling of single quantum dot lasers beyond the artificial atom model are presented with a focus on the exploration of similarities and differences between the atomic and semiconductor systems.     

Temperature dependent characteristics of InAs/GaAs quantum dot laser grown by gas source molecular beam epitaxy

We report on the temperature dependent lasing characteristics of InAs/GaAs quantum dot lasers under continuous wave mode. The five-stacked InAs quantum dots were grown by gas-source molecular beam epitaxy with slightly different thickness. Ridge waveguide laser with stripe width of 6 μm was processed on the growth structure. The characteristic temperature was measured as high as infinity in the temperature range of 80–180 k. With the increase of injection current, the lasing spectra of laser diode broaden gradually at low temperature of 80 k. However, when the operation temperature increases from 80 to 300 K, the width of lasing spectrum reduces gradually from 40 to 2.0 nm. The lasing process is obviously different from that of a reference quantum well laser which widens its width of lasing spectra by increasing operation temperature. These experiments demonstrate that a carrier transfer from the smaller size of dots into larger dots caused by thermal effect play an important role in the lasing characteristic of quantum dot lasers. In addition, the laser can operate at maximum temperature of 80 °C under continuous wave mode with a maximum output power of 52 mW from one facet at 20 °C. A wavelength thermal coefficient of 0.196 nm/K is obtained, which is 2.8 times lower than that of QW laser. The low wavelength thermal coefficient of quantum dot laser is mainly attributed to its broad gain profile and state filling effects.     

Random lasing of CsPbBr3 perovskite thin films pumped by modulated electron beam

Aiming at the application requirements of information optics,this Letter proposed a perovskite quantum dot random lasing pumping method suitable for high-speed modulation.At the same time,the luminescence characteristics of perovskite quantum dot films under electron beam pumping conditions are analyzed,and the random lasing mechanism of electron beam pumping CsPbBr3 quantum dot films is revealed.Finally,it is confirmed that perovskite quantum dots are easy to realize random lasing under electron beam pumping conditions.     

Lasing spectra of 1.55 μm InAs/InP quantum dot lasers: theoretical analysis and comparison with the experiments

In this paper, a theoretical model is used to investigate the lasing spectrum properties of InAs/InP (113)B quantum dot (QD) lasers emitting at 1.55 μm. The numerical model used is based on a multi-population rate equation (MPRE) analysis. It takes into account the effect of the competition between the inhomogeneous broadening (due to the QD size dispersion) and the homogenous broadening as well as a nonlinear gain variation associated to a multimode laser emission. The double laser emission and the temperature dependence of lasing spectra of self-assembled InAs/InP quantum dot lasers is studied both experimentally and theoretically.     

InAs–GaAs self-assembled quantum dot lasers: physical processes and device characteristics

The gain characteristics of InAs–GaAs self-assembled quantum dot lasers are studied using two complementary techniques. The modal gain is derived from a measurement of the normal incidence, inter-band photoconductivity. For a device containing a single layer of dots the maximum modal gain of the ground state transition is found to be insufficient for lasing action. As a consequence lasing occurs for excited state transitions, which have a larger oscillator strength, with the precise transition being dependent upon the device cavity length. The second technique uses the Hakki–Paoli method to determine the spectral and current dependence of the gain. A quasi-periodic modulation of the below threshold gain is observed. This modulation is shown to be responsible for the form of the lasing spectra, which consist of groups of lasing modes separated by non-lasing spectral regions. Possible mechanisms for this behaviour are discussed.     

太赫兹量子级联激光器光束特性分析

用有限元法计算太赫兹量子级联激光器激光模式的阈值增益.结果表明:接触层厚度和掺杂浓度对阈值增益的影响远远大于波导宽度和激射波长;接触层厚度较小(大)和掺杂浓度较低(高)时,TM₁(TM₀)模的阈值增益较小.在此基础上,用矢量衍射理论分析岀射光束的远场特性,得到光束的远场光斑基本是椭圆;x方向的远场散射角随波导宽度或激射波长的增加分别线性减小或增加,尽管对应的接触层厚度和掺杂浓度不同,但TM₀和TM₁模x方向的远场散射角相同;另外,还得到y方向远场散射角不受波导宽度或少受激射波长的影响.在阈值增益和光束质量方面,TM₁模都优于TM₀模.     

Thresholds of quasi-supercontinuum interband quantum dot lasers

We present the analysis of threshold conditions that produces wideband- stimulated emission in semiconductor quantum-dot laser. Our theoretical model reveals critical occurrence of broadband lasing when the energy spacing between quantized energy states (ΔE) is comparable to the inhomogeneous broadening of quantum dot nanostructures.     

Continuous-wave high-power (320 mW) single mode operation of electronic vertically coupled InAs/GaAs quantum dot narrow-ridge-waveguide lasers

High-power 3 μm-wide narrow-ridge-waveguide lasers with ten stacks of electronic vertically coupled InAs/GaAs quantum dots in the active region were demonstrated. Unlike that from conventional uncoupled InAs quantum dot lasers, a narrow lasing spectrum was observed because the carriers tunneled in the vertical direction. Continuous-wave operation in single lateral mode yielded a kink-free output power of 320 mW with an efficiency of 0.46 W/A , and a sensitivity of lasing wavelength to temperature of 0.28 nm/K. PACS 42.55.Px; 42.60.Jf; 78.55.Cr     

Impact of gain compression on modulation response and dynamic properties of three state lasing InGaAs/GaAs quantum dot lasers

In this paper, we have theoretically studied dynamics of a semiconductor quantum dot (QD) laser for enhancing its small signal and large signal modulation as a function of compression gain. We have considered InGaAs/GaAs QD laser rate equations and solved this equation system numerically. We have revealed that a diminution in compression gain leads to an improvement in frequency bandwidth for this three state lasing system. We also have calculated turn on delay and output power that obviously indicates the effect of compression gain on relaxation oscillations.     

Low threshold laser cavities of two-dimensional photonic crystal line defect mode

Two-dimensional (2D) rod-type photonic crystal (PC) line defect waveguide (LDW) laser cavities based on three types of line defect modes with zero group velocity are studied by using finite-difference time-domain (FDTD) method. These laser cavities have high quality (Q) factor, better localization of light, non-uniform gain distribution and small overlap between gain medium and light field. Therefore, they have the advantages over conventional and air-bridge PC cavities with uniform gain, such as low threshold, single mode lasing and effectively avoiding thermal effect. From their comparison, one can find the mode at middle Brillouin zones (BZ) is the best one to be used as lasing mode. Its dynamic lasing process and lasing features are demonstrated by the numerical experiment where the FDTD method coupling Maxwell's equations with the rate equations of electronic population is used.     

Oscillator Laser Model

A laser model is formulated in terms of quantum harmonic oscillators. Emitters in the low lasing states are usual harmonic oscillators, and emitters in the upper states are inverted harmonic oscillators. Diffusion coefficients, consistent with the model and necessary for solving quantum nonlinear laser equations analytically, are found. Photon number fluctuations of the lasing mode and fluctuations of the population of the lasing states are calculated. Collective Rabi splitting peaks are predicted in the intensity fluctuation spectra of the superradiant lasers. Population fluctuation mechanisms in superradiant lasers and lasers without superradiance are discussed and compared with each other.     

Numerical analysis of the effects of carrier dynamics on the switch-on and gain-switching of quantum dot lasers

A systematic analysis of the influence of the capture, inter-level relaxation and exciton dephasing time constants on the dynamic behavior of quantum dot Fabry-Perot semiconductor lasers is done taking into account the lasing from the ground and excited states. The simulation results show that the carrier time constants studied influence significantly the static characteristic of the laser, its switch-on response and the pulses generated by gain-switching.     

Microscopic study of relaxation oscillations in quantum-dot VCSELs

We propose a theoretical model to investigate the switch-on dynamics of electrically pumped quantum dot vertical-cavity surface-emitting lasers. The model is based on the self-consistently combined quantum dot-wetting layer Maxwell–Bloch equations incorporating microscopically calculated Coulomb and phonon-assisted scattering processes between the quantum dot and the quantum dot-embedding wetting layer states. Our approach allows the calculation of the time delay before lasing as well as of the frequency and damping rate of the appearing relaxation oscillation. We study their dependence on the strength of the injection current density and on the number of QD-layers and DBR-layers by using the finite-difference time-domain method. The results show that switch-on delay time is in inversely proportion to the injection current density while the frequency and the damping rate of relaxation oscillation are proportional to the current density. If we increase the numbers of QD-layers and DBR-layers the delay time becomes shorter, and the frequency and the damping rate become larger.     

Second harmonic FEL oscillation

We have produced and measured for the first time second harmonic oscillation in the infrared region by a free electron laser. Although such lasing is ideally forbidden, since the gain of a plane wave is zero on axis for an electron beam perfectly aligned with a wiggler, a transverse mode antisymmetry allows sufficient gain in this experiment for lasing to occur. We lased at pulse rates up to 74.85 MHz and could produce over 4.5 W average and 40 kW peak of IR power in a 40 nm FWHM bandwidth at 2925 nm. In agreement with predictions, the source preferentially lased in a TEM01 mode.     

Regulated and entangled photons from a single quantum Dot

We propose a new method of generating nonclassical optical field states. The method uses a semiconductor device, which consists of a single quantum dot as active medium embedded in a p- i- n junction and surrounded by a microcavity. Resonant tunneling of electrons and holes into the quantum dot ground states, together with the Pauli exclusion principle, produce regulated single photons or regulated pairs of photons. We propose that this device also has the unique potential to generate pairs of entangled photons at a well-defined repetition rate.     

Laser cooling of a nanomechanical resonator mode to its quantum ground state

We show that it is possible to cool a nanomechanical resonator mode to its ground state. The proposed technique is based on resonant laser excitation of a phonon sideband of an embedded quantum dot. The strength of the sideband coupling is determined directly by the difference between the electron-phonon couplings of the initial and final states of the quantum dot optical transition. Possible applications of this scheme include generation of nonclassical states of mechanical motion.     

Strong optical field study of a single self-assembled quantum dot

We review the investigation of a single quantum dot driven by a strong optical field. By coherent pump-probe spectroscopy, we demonstrate the Autler–Townes splitting and Mollow absorption spectrum in a single neutral quantum dot. Furthermore, we also show the typical Mollow absorption spectrum by driving a singly charged quantum dot in a strong optical coupling regime. Our results show all the typical features of an isolated atomic system driven by a strong optical field, such as the AC stark effect, Rabi side bands and optical gain effect, which indicate that both neutral and charged quantum dots maintain the discrete energy level states even at high optical field strengths.     

Analysis of various optical pumping schemes for liquid oxygen lasers

Singlet oxygen generated by optical pumping in liquid oxygen/air medium has recently been reported as a potential gain medium for high power lasers at 1580 nm, by USA’s defense agency DARPA. However, the details with reference to the underlying physics of this laser and the potential pumping techniques for achieving lasing are still unclear. The present paper investigates numerically the population kinetics of both upper and lower lasing levels viz. O₂ and O₂ states in liquid oxygen and discusses the criticality involved in obtaining lasing with this medium. Isotopic liquid oxygen and liquid air medium where improved conditions for lasing are anticipated, as compared to that in natural liquid oxygen, have also been considered in the present study. The studies have been carried out for optical pumping by both continuous and pulsed mode lasers at 1064 nm and 634 nm wavelengths. The temporal variation of small signal gain in each case has been investigated along with limitations of the same from point of view of lasing. The available and extractable power from all three medium for an optimum pumping case has also been discussed. The studies reveal that liquid air has high potential for very high power lasers as compared to both natural and isotopic liquid oxygen. PACS 31.15.ag     

Major features of frequency sweeping of emission from heterojunction quantum lasers

We have used numerical modeling to establish the major features of the variation (sweeping) of the “ instantaneous” laser frequency of heterojunction quantum lasers as a function of the pump current modulation frequency and tuning of the lasing frequency within the gain band. The active medium is described within a two-band model with identical distribution of levels for the electron and hole subbands, assuming transitions with no selection rule between the ground-state subbands. Sweeping of the laser frequency occurs due to variation of the refractive index of the active medium, as a result of the variation in the concentration of nonequilibrium charge carriers. Laser frequency sweeping does not occur for low current modulation frequencies, corresponding to realization of the quasisteady-state lasing regime. In the other limiting case of relatively high current modulation frequencies, the modulation depth of the laser output also tends toward zero. The magnitude of the sweep is greatest in the intermediate current modulation frequency region. For a specified current modulation frequency, the dynamic shift of the laser mode depends on the position of the lasing frequency within the gain band. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 223–229, March–April, 2007.