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Journal of Experimental and Theoretical Physics

Efficient inversionless amplification of infrared radiation using quantum dots with quasi-discrete energy levels

Vladimir A. Kukushkin

Published Aug 1, 2009• 1 citation
Publisher

Abstract

A model of inversionless gain on a transition between quasi-discrete excited levels of a quantum system is proposed that minimizes the absorption of a coherent optical pump beam due to the finite lifetimes of the corresponding states. A quantum-dot system with transition frequency between quasi-discrete levels lying in the mid-infrared (mid-IR) range is used as an example to determine an optimal pump frequency at which the pump absorption rate is minimized and to calculate the gain for a mid-IR field. The proposed model system can be considered as a prototype for creating highly efficient mid- and far-IR quantum-dot amplifiers and lasers operating on transitions between quasi-discrete levels at room temperature.

DOI
10.1134/s1063776109080020

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A model of inversionless gain on a transition between quasi-discrete excited levels of a quantum system is proposed that minimizes the absorption of a coherent optical pump beam due to the finite lifetimes of the corresponding states. A quantum-dot system with transition frequency between quasi-discrete levels lying in the mid-infrared (mid-IR) range is used as an example to determine an optimal pump frequency at which the pump absorption rate is minimized and to calculate the gain for a mid-IR field. The proposed model system can be considered as a prototype for creating highly efficient mid- and far-IR quantum-dot amplifiers and lasers operating on transitions between quasi-discrete levels at room temperature.
A scheme for infrared generation without population inversion between subbands in quantum-well and quantum-dot lasers is presented. The scheme is based on the resonant nonlinear mixing of the optical laser fields on the two interband transitions that are generated in the same active region and that serve as the coherent drive for the infrared field. This mechanism for frequency down-conversion does not rely upon any ad hoc assumptions of long-lived coherences in the semiconductor active medium, and it should work efficiently at room temperature with injection current pumping. For optimized waveguide and cavity parameters, the intrinsic efficiency of the down-conversion process can reach the limiting quantum value corresponding to one infrared photon per one optical photon. Due to the parametric nature of infrared generation, the proposed inversionless scheme is especially promising for long-wavelength (far-infrared) operation.
A method is suggested of efficient mid- to far-infrared (ir) pulse amplification via driving optical pulse energy conversion in waveguiding quantum well (QW) heterostructures at room temperature. It is based on the optical pulse creating transient population inversion at a long-wavelength transition in the three-level scheme formed by QW levels of dimensional quantization. As a result, efficient amplification of a weak mid- to far-ir pulse, propagating simultaneously with the driving one, becomes possible. A waveguide and QW heterostructure design that is optimal for optical-long-wavelength pulse conversion is proposed and shown to be much simpler than that used in quantum cascade lasers. For the typical input peak power of a picosecond driving pulse and mid- to far-ir pulse of $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{W}$ and 100−−0.1mW100--0.1\phantom{\rule{0.3em}{0ex}}\mathrm{mW}100−−0.1mW, respectively, the present scheme is able to produce output ir pulses with peak powers of at least several tens of watts or several tens of milliwatts in the mid- or far-ir range, respectively, i.e., to convert an appreciable part of the optical pulse energy into the mid-ir signal.
We propose and study resonant ladder schemes of inversionless lasing on a fast-decaying transition when the population inversion is not possible or very difficult to achieve. The driving field on a neighboring, slowly decaying transition is not imposed externally, but is self generated in the same active medium. Inversionless lasing in such schemes avoids the problem of driving-field absorption and leads to considerable reduction in the required pumping rate. Applications of such schemes include inversionless ultraviolet and soft x-ray lasing in gas lasers and inversionless generation of coherent mid/far-infrared emission on intersubband (interlevel) transitions in multiple quantum-well or quantum-dot laser diodes.
The emergence of laser action from an atomic system without population inversion is beginning to attract theoretical and experimental attention as a possible way of generating useful coherent radiation at unusual wavelengths. The goal of this communication is to clarify the physical origin of this phenomenon using a four-level system whose ground state consists of two quasidegenerate levels that are coupled to a higher-lying state by a Raman-like transition. In the presence of the ground-state coherence induced by the Raman field, we calculate the absorption spectrum of a weak probe whose tunable frequency scans across the transition frequencies between the two ground-state levels and another higher-lying state that may or may not be weakly excited by an incoherent pump mechanism. Our calculation, based on a dressed atomic state approach, clarifies the conditions that must be satisfied to produce gain, identifies the frequency (or frequencies) at which the gain is maximum, and elucidates the functions of the incoherent pump, the Raman field, and the coherence between the ground-state levels in this process. An appropriate set of Maxwell- Bloch equations for a ring-cavity model of this system confirms the appearance of laser action when the small-signal gain conditions are satisfied.
A quantum dot infrared (IR) photodetector based on intraband optical transitions among the various states within the valence band by considering the principle of electromagnetically induced transparency (EIT) is proposed and theoretically investigated. Absorption spectra of the probe beam and its dependence on the control beam and IR signal under the conditions of EIT have been studied. The incident IR signal itself does not generate any photocurrent. However, profound modification of the absorption characteristics of the probe beam by incident IR signal intensity leads to photocurrent generation in the proposed photodetector. Thermal characterization of the photodetector has been carried out through the evaluation of the temperature dependence of the quantum efficiency of the device.