In spite of rather impressive achievements in intersubband quantum cascade lasers their current parameters are still far
from the needs of practical implementation. We compare theoretical prospects of THz gain for two cases: intersubband
GaAs-based quantum cascade lasers and interband laser based on coupled quantum wells InAs-GaSb. Our
methodology of such a comparison is reduced to following: The most typical design of GaAs-based QCL is compared
with an InAs-GaSb coupled quantum well laser operating in the same frequency range. The detailed density matrix
based calculation shows that the maximal possible gain for CQWL can be three orders of magnitude higher. We
present details on LO-phonon emission rates in typical QCL structures. This calculation supports the statement that
low depopulation selectivity might be an essential feature of QCL in few THz spectral range.
We present a theoretical overview of key physical limitations for application-oriented nanostructure design. We focus
on such promising applications as: nanodot-assisted optical imaging, and photo-thermal therapy with the help of
nanostructures. For these applications we consider the following nanostructures: metal-coated nanoshells and metal
nanoparticles. The actual design of relevant nanoobjects for particular applications must include consideration of such
phenomena as: plasmon resonance, light scattering, light absorption. These phenomena are considered for model
systems of various designs for different parameters of radiation. Our model estimations are compared with
experimental results when such results are available. The conclusions are formulated as a paradigm "desired vs.
We present here a bird eye view on the basic physical factors limiting the efficiency of nanostructure-based THz lasing. The origin of these limitations is the contradiction between the requirements of THz gaps, high radiative matrix element value, and selective depopulation. Various ways were suggested to go out of these limitations. They include: sophisticated nanostructure layout, the usage of QCLs, and switch to bipolar THz lasing. We present the results of detailed density matrix based calculations comparing these approaches. In theory InAs-GaSb bipolar THz lasers are the most promising.
We argue that nanostructure based THz lasers of standard design have a principal limitations of gain value. These limitations rise from the obvious necessity to engineer both THz gap and population inversion simultaneously. Typical approach to the gap engineering inherited from midIR lasers utilizes intersubband transitions. However, contrary to midIR range, for THz lasing selective depopulation is problematic. The problem is that the selectivity of
both depopulation mechanisms, LO phonon emission and electron - electron scattering, in THz region is substantially weaker than in midIR region. We suggest to use InAs/GaSb coupled quantum wells as a way to overcome this fundamental limitation. This is the only heterostructure where THz lasing can be based not on intersubband but on interband transitions. A proper design of this structure leads to a hybridization gap coming from anti-crossing of the GaSb valence band and InAs conduction band naturally appearing in the THz range. Two more advantages of this design are (i) a large value of the interband dipole matrix element and (ii) W-shaped spectrum leading to a singular density of states. These advantages lead to a gain much higher than for intersubband THz lasing.
High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-μm and In(Al)GaAsSb/GaSb arrays operating at 2.3-μm. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-μm. We have obtained a CW output power of 10 W from the 2.3-μm laser array. The 1.5-μm arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time.
The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-μm laser arrays.
We suggest a new structure for THz generation based on coupled quantum wells of InAs-GaSb. This structure uniquely combines the advantages of both the p-n junction laser and the cascade laser. Actual generation results from optical transitions between the e3 and e2 levels in InAlAs quantum well, and resonant with them effective band gap between the conduction band of InAs and the valence band of GaSb quantum well, e1-hh1. The separation between e1 and e2 equals to LO phonon energy that provides population inversion between e3 and e2. We consider two ways of structure design that differ by the carrier dispersion: W-shaped dispersion of the carriers in ground states and regular V-shaped dispersion. All these structures bring in the advantages of the system with equidistant levels, i.e., good temperature characteristics and high probability of radiative transition leading to low threshold current compared to alternative designs. We present a comparative analysis of various mechanisms of carrier relaxation (LO phonons and electron-electron scattering) and point out an optimal band structure favoring high efficiency of THz emitter. Corresponding band structure calculations supply one with the range of quantum well parameters providing all the features presented above.