We investigate the requirements for the pulse width and intensity of the pump beam, for the Fe:ZnSe lasing medium to reach threshold. The rate equation is solved for the time dependence of the population density, N2 of the 5T2 manifold which includes the upper level for the lasing transition.
Focused ion beam milling was used to fabricate on-chip unstable resonator cavity quantum well laser devices. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Pérot cavity device, the unstable resonator cavity device exhibits a 2x diffraction limited beam. The preliminary results demonstrate that a much higher brightness can be reached in this class of broad area devices.
Significant attention has recently been given to photoluminescence (PL) spectra and lifetime measurements on InAs/InAsSb superlattices, as high quality optical material with long carrier lifetimes are required for infrared detectors. The standard sample structure for PL measurements includes energy barriers to block photo-generated carriers from being lost through non-radiative recombination at interfaces between the superlattice and the surface or between the superlattice and the buffer/substrate. However, defect, surface, and/or interface states in AlSb, a commonly used barrier material, are known to contribute carriers to InAs quantum wells. Due to the similarity of the AlSb interface with the InAs/InAsSb superlattice, the effects of the barriers on the electrical and optical properties of the superlattice were investigated. Structures with AlSb barriers at the top and bottom of the superlattice, with no AlSb barriers, and with an AlSb barrier only at the top of the superlattice structure were studied. Hall Effect measurements revealed little change in the sheet carrier concentration at 10 K due to the barriers, but significant increases in low temperature mobility and a two-dimensional-like mobility temperature dependence were observed when barriers were present. Further high magnetic field measurements are necessary, however, to understand the transport properties of these samples due to the likelihood that multiple carriers are present. The photoluminescence (PL) spectra were almost identical regardless of the barriers, except for a 15% increase in intensity with the AlSb barrier between the buffer layer and the superlattice. The surface AlSb barrier had little effect on the intensity. The barriers are therefore recommended for PL measurements to increase the signal intensity; however, they complicate the analysis of single-field Hall Effect measurements.
The analysis of I-V and I-L curves in mid-IR quantum cascade lasers operating at room temperature is performed. When the ohmic component of the device resistance in the I-V curve is subtracted, the current I can be approximated by the exponential function, I = Is exp(eVj/ε) where Is is the saturation current, Vj is the n-n junction voltage, and ε is an energy parameter related to the tunneling mechanism which enables filling of upper states and emptying of lower states of the laser transition. Values of εare found to be in 0.68-1.45 eV range, and when divided by the number of stages in the cascade, the tunneling parameter of each stage is determined. The threshold related “kink” of differential I-V curves is shown. The effect of voltage saturation above the laser threshold is observed. Thus, the possibility of determination of the threshold using electrical measurements in quantum cascade lasers has been demonstrated.
We present experimental comparison of Type-I diode lasers emitting <3 μm wavelength in room temperature with increased strain in quantum wells (QWs). Due to diminishing hole confinement in the barrier, the performance of mid- IR Type-I diode laser is generally poor. Here we improve the hole confinement using quinary alloy in the barrier in conjunction with highly strained QWs. By using molecular beam epitaxial growth method, we achieve up to 2.3% strain in the QWs. At near room temperature, highly strained laser structure shows approximately 4 times improved laser performance than regular strained laser under the same testing condition. The study demonstrates significant improvement in laser efficiency using highly strained QWs in the GaSb-based type-I mid-infrared laser diodes.
While conventional semiconductor lasers employ electrical injection for carrier excitation, optically pumped
semiconductor lasers (OPSLs) have demonstrated high output powers and high brightness in the mid-infrared. An
important consideration for optically pumped lasers is efficient absorption of the pump beam, which can be achieved
through increasing the number of periods in the active region, by placing the active region in a cavity with an optical
thickness of twice the pump wavelength between distributed Bragg reflectors (Optical Pumping Injection Cavity), or by
periodically inserting the active quantum wells into an InGaAsSb waveguide designed to absorb the pump radiation
(Integrated Absorber). A tunable optical pumping technique is utilized by which threshold intensities are minimized and
efficiencies are maximized. The near-IR idler output of a Nd:YAG-pumped optical parametric oscillator (10 Hz, ~4 ns)
is the tunable optical pumping source in this work. Results are presented for an OPSL with a type-II W active region
embedded in an integrated absorber to enhance the absorption of the optical pump beam. Emission wavelengths range
from 4.64 μm at 78 K to 4.82 μm at 190 K for optical pump wavelengths ranging from 1930-1950 nm. The effect of
wavelength tuning is demonstrated and compared to single wavelength pumping (1940 nm) at a higher duty cycle (20-
30%). Comparisons are also made to other OPSLs, including a discussion of the characteristic temperature and high
temperature performance of these devices.
A new approach to tunable mid-infrared lasers, an optically pumped, type-II, InGaSb/InAs gain medium with a chirped
distributed feedback grating, has been developed. The chirped grating is patterned using an interferometric lithography
(IL) technique with spherical wave fronts and etched into the top cladding of the laser slab waveguide structure. Because
the period of grating increases gradually laterally, wavelength tuning is implemented by shifting pump stripe to different
positions on the device with different grating periods. Fabry-Perot modes from the cleaved facets are successfully suppressed
by fabricating the grating 6° tilted with respect to facets and adjusting the pump stripe normal to the grating.
Continuous tuning of 30 nm around 3.1 μm with 320 mW single facet output power at 80K and a 1.6 nm FWHM is reported.
The present device is designed in the 3- to 4-μm range which matches a low loss atmospheric transmission window,
and covers an important region of molecular vibration spectra, in particular, the hydrocarbon C-H stretch at ~ 3.3
μm, making it suitable for atmospheric pressure remote gas sensing of industrially important small molecules such as
methane, hydrogen chloride and ammonia.
In this paper, we briefly review optically pumped type-II "W" quantum-well semiconductor lasers that emit in the midinfrared
wavelengths. In addition, we demonstrate on-chip unstable resonator cavity devices that exhibit excellent lateral
Mid-infrared light-emitting diodes with InGaSb/AlGaAsSb triple-quantum-well active region have been integrated into
arrays of either 200×200 μm2 or 40×40 μm2 square pixels. Two generations of arrays have been designed, fabricated
and tested. The first, "sparse" 6×6 array provided valuable information on optimal electrode design and fabrication
parameters that was used in the design and processing of the second generation "dense" 11×11 array.
We report on optically pumped mid-IR semiconductor lasers that are based on type-II wells. A systematic study of the effect of increasing the In-content in the InxGa1-xSb hole-well suggests that improved hole confinement results in improved power conversion efficiency at elevated temperatures that is also accompanied by a reduction in threshold power and a reduction in T0, the characteristics for threshold.
An active double heterostructure diode structure is employed as a nonlinear medium to demonstrate soliton waveguiding effects. It was observed that the nonlinearities due to reverse bandfilling in active semiconductor amplifiers give rise to a spectral region where self-focusing takes place for photon energies corresponding to the peak of the gain. By monitoring the mode-profile at the output of the slab waveguide as a function of wavelength, a district narrowing of the output beam lateral dimension was observed and the beam profile appeared to stay stable for a range of input intensities. The slab waveguide was 650 micrometer long and each of the contact pads for electrical carrier injection was 60 micrometer wide. The experiments showed that the lateral dimension of the near field profile output beam changed from a FWHM width of 32 micrometer to 5.5 micrometer as the wavelength of the laser was tuned into the optimum range for self-focusing nonlinearities. This corresponds to a peak nonlinear coefficient of n2 equals 2.8 X 10-10 cm2/watt.
In this paper, recent results on the use of low-energy ions in molecular beam epitaxy are described. Mechanisms for ion damage formation are discussed and conditions where ion irradiation can be used without introducing damage are reported. Three main applications are discussed. First, the use of ions to suppress 3D island nucleation during the early stages of strained-layer growth is presented, with particular attention paid to the ion-induced prevention of extended defect formation and strain relaxation. The current understanding of the mechanisms by which ion irradiation affects nucleation is also summarized. Second, ion- induced suppression of phase separation in InGaAsSb alloys during growth on lattice-matched to InP substrates is described. Third, the application of very-low-energy (approximately equals 50 eV) and glancing-angle 1 keV Ar ions to damage-free sputter cleaning and etching of GaAs is discussed.