Trions or so-called charged excitons are formed by optical excitation of electron-hole pairs in quantum wells with background carrier densities. They consist of three carriers - for n-doping two electrons and one hole - and are hence fermion-like, whereas the optical transition is excitonic. For the first time, optical gain and lasing at the trion resonance have been observed on delta-doped ZnSe quantum wells.

- Proc. 10th Intern. Conf. on II-VI Compounds, Bremen 2001, phys. stat. sol. (b) 229, 637 (2002)
- Phys. Rev. Letters 28, 287402 (2002)

The optical transition takes place between states of equal momentum in the electron and trion dispersion with the consequence that no degeneracy is required for achieving optical gain. Equally important is the fact that the trion band mass is by definition larger than the electron mass. Therefore, the trion distribution f_t spreads out deeper in momentum space than that of the electron f_e. As a result, optical gain can be formed even if the total number of trions does not exceed the number of electrons, i.e. without inversion in the total particle numbers. The absorption-gain crossover (f_t(k) > f_e(k), vertical yellow lines) is related to the chemical potentials of electrons and trions by

E_cr = E _t(0) - µ_e + µ_t.

The samples are binary ZnSe QWs with background electron densities of 3x10¹⁰ .. 10¹¹cm^-2. Additional cladding layers ensure efficient wave guiding in the QW plane. The low-density photoluminescence (PL) and reflectivity spectra exhibit narrow features due to both heavy-hole exciton X and trion T. With increased excitation intensity (2 kW/cm²), the PL from the exciton becomes stronger, whereas a low-energy wing appears for T due to recombination of trions at larger k-vectors. In parallel, a pronounced emission from the sample edges located just on this low-energy wing is found. The optical gain is elaborated by means of stripe-length variation technique. The gain of the QW has been recalculated from the measured modal gain via a numerical analysis of the wave guide structure. Gain spectra for different excitation levels (a: 5 kW/cm², b: 20 kW/cm²) are summarized in the insert. In accord with the estimated chemical potentials, the absorption-gain crossover appears about 2 meV below the T resonance.
Laser action of the trion transition is directly demonstrated on a cleaved 240 µm resonator structure (upper part). Distinct laser modes appear in the region of positive modal gain. The observation of lasing under resonant T excitation directly evidences that stimulated emission of the trion transition occurs without inversion at the zone center k=0.