Low-threshold room-temperature operation of injectorless quantum-cascade lasers: influence of doping density

The influence of the doping density in the active sections of InP-based injectorless quantum cascade lasers, emitting at 6.8 mum, is investigated. The doping sheet density is varied in the range 2.5-8.6times10¹⁰ cm^-2. Lasing is observed in the whole range, with a threshold current density as low as 1.2 kA/cm² at 300 K for the smallest doping sheet density of 2.5times10 ¹⁰ cm^-2. Further improvement has been made by additionally increasing the number of periods in the active region from 40 to 60. With the same doping level of 2.5times10¹⁰ cm^-2 record low threshold current densities of 0.73 kA/cm² at 300 K were achieved     

High-power terahertz quantum-cascade lasers

Demonstration of quantum-cascade lasers at /spl sim/4.4 THz (/spl lambda//spl sim/68 /spl mu/m), which are measured to emit 248 mW peak power in pulsed mode, and 138 mW continuous-wave power at heatsink temperatures of 10 K, is reported. These lasers are based on a resonant-phonon depopulation scheme, and use a semi-insulating surface-plasmon waveguide.     

Effects of extraction barrier width on performance of terahertz quantum-cascade lasers

Experimental results are presented on the effects of extraction barrier on the performance of resonant phonon terahertz quantum-cascade lasers. The extraction barrier is varied from 48 to 39 A in an otherwise identical 3.4 THz laser structure. It is found that, when the extraction barrier width decreases from 48 to 39 A, the threshold current density at 10 K increases monotonously from 900 A/cm to 1.4 kA/ cm , and the maximum lasing temperature increases from 95 to 110 K. No optimum extraction barrier width was found for maximum lasing temperature.     

Continuous-wave operation of terahertz quantum-cascade lasers

We report continuous-wave (CW) operation of a 4.4-THz quantum-cascade laser grown in the GaAs-AlGaAs materials system by molecular beam epitaxy. The device operates at 4 K with a threshold current of 160 mA, and an output power of /spl sim/25 /spl mu/W. In pulsed mode, the maximum operating temperature is 52 K, with a threshold current of 108 mA at 4 K. CW lasing was achieved by using a small cavity ridge area (60/spl times/600 /spl mu/m), and by coating one of the laser facets. These two features allowed for a substantial decrease of the threshold current and therefore reduced detrimental heating effects. The role played by the lateral resistance of the 800-nm GaAs layer underneath the active region was also investigated. Experimental data is presented showing that the temperature of the active region, which eventually hinders CW lasing, can be substantially influenced by the value of this lateral resistance.     

Analysis of terahertz surface emitting quantum-cascade lasers

An analysis of surface-emitting terahertz quantum-cascade lasers operating at wavelengths near 100 /spl mu/m is presented. The devices use distributed feedback through second-order Bragg metal gratings to generate strong emission of radiation normal to the laser surface. The analysis is based on coupling between the exact Floquet-Bloch eigenmodes of infinite periodic structures in finite length devices. The results show performance of surface-emitting terahertz lasers comparable to edge-emitting devices, with high radiative efficiencies and low threshold gains. Using phase-shifts in the grating, high-quality single-lobe beams in the farfield are obtained.     

Beam steering in high-power CW quantum-cascade lasers

We report the light-current (L-I), spectral, and far-field characteristics of quantum cascade lasers (QCLs) with seven different wavelengths in the /spl lambda/=4.3 to 6.3 /spl mu/m range. In continuous-wave (CW) mode, the narrow-stripe (/spl ap/13 /spl mu/m) epitaxial- side-up devices operated at temperatures up to 340 K, while at 295 K the CW output power was as high as 640 mW with a wallplug efficiency of 4.5%. All devices with /spl lambda//spl ges/4.7 /spl mu/m achieved room-temperature CW operation, and at T=200 K several produced powers exceeding 1 W with /spl ap/10% wallplug efficiency. The data indicated both spectral and spatial instabilities of the optical modes. For example, minor variations of the current often produced nonmonotonic hopping between spectra with envelopes as narrow as 5-10 nm or as broad as 200-250 nm. Bistable beam steering, by far-field angles of up to /spl plusmn/12/spl deg/ from the facet normal, also occurred, although even in extreme cases the beam quality never became worse than twice the diffraction limit. The observed steering is consistent with a theory for interference and beating between the two lowest order lateral modes. We also describe simulations of a wide-stripe photonic-crystal distributed-feedback QCL, which based on the current material quality is projected to emit multiple watts of CW power into a single-mode beam at T=200 K.     

Investigation of thermal effects in quantum-cascade lasers

The development of a thermal model for quantum cascade lasers (QCLs) is presented. The model is used in conjunction with a self-consistent scattering rate calculation of the electron dynamics of an InGaAs-AlAsSb QCL to calculate the temperature distribution throughout the device which can be a limiting factor for high temperature operation. The model is used to investigate the effects of various driving conditions and device geometries, such as epilayer down bonding and buried heterostructures, on the active region temperature. It is found that buried heterostructures have a factor of eight decrease in thermal time constants compared to standard ridge waveguide structures in pulsed mode and allow a /spl sim/78% increase in heat sink temperature compared to epilayer down mounted devices in continuous-wave mode. The model presented provides a valuable tool for understanding the thermal dynamics inside a quantum cascade laser and will help to improve their operating temperatures.     

Characterizing the beam properties of terahertz quantum-cascade lasers

Terahertz quantum-cascade lasers (QCLs) are very promising radiation sources for many scientific and commercial applications. Shaping and characterizing the beam profile of a QCL is crucial for any of these applications. Usually the beam profile should be as close as possible to a fundamental Gaussian TEM₀₀ mode. In order to completely characterize the laser beam the power and the wavefront have to be measured. We describe methods for characterizing the beam properties of QCLs. Several QCLs with single-plasmon waveguide and emission frequencies between 2 and 5 THz are investigated. The beam profiles of these lasers are shaped into almost fundamental Gaussian modes using dedicated lenses. The beam propagation factor M² is as low as 1.2. The wavefront is measured along the axis of propagation with a THz Hartmann sensor. Its curvature behaves as expected for a Gaussian beam. The applied methods can be transferred to any other THz beam.     

High-resolution emission spectra of pulsed terahertz quantum-cascade lasers

The spectra of pulsed terahertz quantum-cascade lasers were measured with high spectral resolution. The characteristic line width at half maximum was 0.01 cm⁻¹; it is controlled by laser temperature variations during the supply voltage pulse. It was shown that an increase in the laser temperature leads to a decrease in the emission frequency, which is caused by an increase in the effective refractive index of the active region. It was also found that a decrease in the supply voltage results in a decrease in the emission frequency, which is caused by a change in the energy of diagonal transitions between lasing levels.     

Low-voltage terahertz quantum-cascade lasers based on LO-phonon-assisted interminiband transitions

A report is presented on GaAs/(Al,Ga)As terahertz (THz) quantum-cascade lasers (QCLs) based on alternating photon and longitudinal optical (LO) phonon-assisted transitions between quasi-minibands. The design is optimised for low threshold current densities and low operating voltages, resulting in reduced ohmic heating during continuous-wave (CW) operation. The QCLs exhibit lasing over a broad range of frequencies from 2.91 to 3.21 THz. Using single-plasmon waveguides, pulsed operation up to 114 K and CW operation up to 65 K has been achieved.     

Study of postgrowth processing in the fabrication of quantum-cascade lasers

The special postgrowth processing of structures for quantum-cascade lasers is studied. The processing includes regrowth with a high-resistivity material (indium phosphide) with a carrier concentration of n ≈ 5 × 10¹⁰ cm^?3, photolithography with various wet chemical etchants, and the fabrication of special contacts providing enhanced heat removal. The use of modified postgrowth processing provides necessary parameters satisfying requirements for high-quality devices.     

Multilayer heterostructures for quantum-cascade lasers operating in the terahertz frequency range

The results obtained in a study of the structural and optical properties of GaAs/AlGaAs heterostructures with 228 quantum cascades, grown by molecular-beam epitaxy, and in a simulation of interband optical transitions and transitions between the energy levels of a cascade are presented.     

Self-consistent scattering model of carrier dynamics in GaAs-AlGaAs terahertz quantum-cascade lasers

Intersubband electron scattering transport in terahertz GaAs-AlGaAs quantum cascade lasers is analyzed, using a full 13-level self-consistent rate equation model. The approach includes all relevant scattering mechanisms between injector-collector and active region states in the cascade structures. Employing an energy balance equation which includes the influence of both electron longitudinal optical phonon and electron-electron scattering, the method also enables evaluation of the average electron temperature of the nonequilibrium carrier distributions in the device. The electron temperature is found to give a strong influence on the output characteristics, particularly at very low temperatures. The threshold currents and electric field-current density characteristics are in very good agreement with experiment, implying that the model has a strong predictive capability.     

高性能激光单脉冲选择器

本文论述了采用三种类型雪崩晶体管驱动器的高性能激光单脉冲选择器的工作原理及设计思想,并分析了它们各自的特点。这类激光单脉冲选择器的抖动<1ns,寿命>10~7次,延迟时间约为20ns,单脉冲选出率为100％,信噪比>10~3。     

Long-wavelength (9.5-11.5 μm) microdisk quantum-cascade lasers

Quantum-cascade whispering-gallery-mode disk lasers emitting at 9.5-μm and 11.5-μm wavelength are reported. Taking advantage of the high-quality resonator (Q≈200), the threshold current density of disk lasers emitting at 9.5 μm is reduced below the value of the corresponding ridge waveguide geometry (J_th,disk=2.39 kA.cm ^-2 versus J_th,ridge=3.0 kA.cm^-2). Additionally, the increase in wavelength compared to previously reported disk lasers at 5.0 μm is a significant step toward the microcavity regime (by an effective scaling factor of 2.5, comparing identical disk sizes), disk diameters from 125 μm down to 20 μm are used to study the approach to the microcavity regime by size reduction. Far-field pattern measurements identify scattering from the pedestal as an important outcoupling mechanism for microdisk lasers. An excellent agreement between the measured and calculated free spectral range of the whispering gallery modes allows us to estimate the beta factor of the microdisks, resulting in β≈0.05 for a 20-μm diameter disk. A two-level rate equation model is evaluated for the quantum-cascade disk laser as a tool for a direct measurement of β. Nevertheless, the actual measurement is at present blurred by luminescence (light-emitting diode) from the disk center accompanied by an unbalanced carrier distribution between the whispering gallery laser and the center light-emitting diode     

Second-harmonic generation in GaAs-based quantum-cascade lasers grown on <100> substrates

Emission of coherent light at 5.1 /spl mu/m wavelength from GaAs-based quantum cascade lasers is reported. This was achieved by integrating nonlinear cascades with large second-order susceptibility in the active regions of the laser.     

Realization of GaAs/AlGaAs quantum-cascade lasers with high average optical power

Quasi-continuous-wave operation of GaAs/AlGaAs quantum-cascade lasers with high average optical power is demonstrated. Double X-ray diffraction has been used to investigate the quality of the epitaxial material. The compositional gradients and the interface quality are controlled effectively. The corrected average power of per facet about 17 mW and temperature tuning coefficient of the gain peak about 0.91 nm/K from 83 K to 140 K is achieved in pulse operation. Best value of threshold current density is less than 3.0 kA/cm² at 83 K.