Tunnel-coupled Wells Research Articles

Temperature Gradient Driven Lasing and Stimulated Cooling

A laser can be understood as a thermodynamic engine converting heat to a coherent single mode field close to Carnot efficiency. To achieve lasing, spectral shaping of the excitation light is used to generate a higher effective temperature on the pump than on the gain transition. Here, using a toy model of a quantum well structure with two suitably designed tunnel-coupled wells kept at different temperatures, we predict that lasing can also occur on an actual spatial temperature gradient between the pump and gain regions. Gain and narrow band laser emission require a sufficiently large temperature gradient and resonator quality. Lasing appears concurrent with amplified heat flow between the reservoirs and points to a new form of stimulated solid state cooling. In addition, such a mechanism could reduce intrinsic heating and thus extend the operating regime of quantum cascade lasers by substituting phonon emission driven injection by a phonon absorption step.

Diagnostics of quantum cascade structures by optical methods in the near infrared region

The spectra and kinetics of the low-temperature interband photoluminescence of epitaxial structures of terahertz quantum cascade lasers is studied under conditions of strong pulsed excitation. Photoluminescence corresponding to transitions between both the ground and excited states of two tunnel-coupled wells is observed at high excitation levels (600 mW; spot diameter ∼200 μm). Kinetic measurements show that the rise and decay times significantly decrease up to the time resolution of the measuring system at wavelengths <770 nm with decreasing photoluminescence observation wavelength.

Terahertz quantum-cascade lasers based on a three-well active module

The authors report on a design of terahertz quantum-cascade lasers based on three-well active modules. Each module consists of two tunnel-coupled wells for the two lasing states and another well for both resonant-phonon depopulation and carrier injection. This design is the simplest so far among the various published working devices. The test device has a lasing frequency of 3.4THz and maximum operating temperature of 142K.

Coherent oscillations of electrons in tunnel-coupled wells under ultrafast intersubbandexcitation

Ultrafast intersubband excitation of electrons in tunnel-coupled wells is studied inrespect of its dependence on the structure parameters, the duration of the infraredpump and the detuning frequency. The temporal dependences of the photoinducedcarrier concentration and dipole moment are obtained for two cases of transitions:from the single ground state to the tunnel-coupled excited states and from thetunnel-coupled states to the single excited state. The peculiarities of dephasingand population relaxation processes are also taken into account. The nonlinearregime of the response is also considered when the splitting energy between thetunnel-coupled levels is renormalized by the photoexcited electron concentration. Thedependences of the period and the amplitude of oscillations on the excitation pulseare presented with a description of the damping of the nonlinear oscillations.

Negative intersubband absorption in biased tunnel-coupled wells

We investigate the use of intersubband transitions between two weakly coupled shallow quantum wells to generate terahertz radiation. The wells are designed for independent contacting and biasing. The maximum gain (absorption) scales with the wavefunction overlap and difference between the well carrier densities. The absorption line shape is studied taking into account the interplay between inhomogeneous and homogeneous broadening mechanisms, including both large-scale variations in the coupled levels and also short-range scattering. The intersubband transitions are strongly renormalized due to Coulomb interactions (depolarization and exchange effects). Population inversion can be achieved through independent variation of the upper and lower well Fermi energy with applied bias. Numerical estimates of the negative absorption coefficient are obtained using realistic parameters, indicating the possible application of such a THz resonator to achieve stimulated emission.

Terahertz injection electroluminescence in multiperiod quantum-cascade AlGaAs/GaAs structures

Terahertz electroluminescence in the range ≈1.5 THz was observed in a quantum-cascade GaAs/AlGaAs structure containing 40 periods of tunnel-coupled wells. The luminescence is caused by the spatially indirect optical electron transitions between the ground states of neighboring quantum wells.