Optically pumped room-temperature GaAs lasers

Abstract

Room-temperature pulsed operation of GaAs cleaved platelet lasers bulk optically pumped near the band gap by a tunable parametric oscillator has been studied as a function of several variables. Both n-type high-purity GaAs ( 1.6 \times 10^{14} cm-3 \leq N_{d} + N_{a} \leq 1.6 \times 10^{16} cm-3) and n-type Si-doped GaAs ( N_{d} - N_{a} \simeq 2-5 \times 10^{18} cm-3) have been investigated. The peak of the laser emission at threshold for all samples occurred in the range from 0.88 to 0.89 μm, but the total width of laser emission in the Si-doped samples was significantly broader than in the pure samples. As the pump photon energy E p decreased, the threshold power necessary for lasing increased. This is ascribed to the increasing penetration of the pump radiation and the consequent decrease in excess carrier density, and to the lower degree of population inversion as the pump photon energy approaches the threshold quasi-Fermi level separation necessary for stimulated emission. For the high-purity samples, the lasing behavior did not depend heavily on impurity concentration and was more strongly a function of surface and cleaved-facet quality of individual samples. Typical threshold power densities of 1.75 W/mil (corresponding to approximately 7 \times 10^{5} W/cm2) were measured and at an incident power level of twice threshold, peak power conversion efficiencies up to 4.5 percent were obtained. Measurement of a typical far-field angular mode pattern showed a full-width half-maximum power (FWHM) transverse angle of about 10°, corresponding to an emitting depth perpendicular to the pump face \sim5 \mu m. At higher excitation, stimulated emission was observed over a width of 150 A. Measurements on the Si-doped material showed marked differences from the high-purity n-type samples. At the lower pump energies used on the high-purity samples, the GaAs:Si samples were more transparent, and consequently, higher pump energies had to be used to obtain lasing. Thresholds comparable to those in the pure GaAs were measured, and lasing occurred at roughly the same wavelength as before. However, at a pump power twice threshold, stimulated emission occurred over an increased range from 0.88 to 0.91 μm, a width of 300 A, and the measured FWHM angle decreased to 6°. Peak power conversion efficiencies up to 11 percent have been obtained. This material seems to be quite promising for obtaining efficient bulk optical pumping, with the advantages of lower optical flux densities at the resonator faces and better spatial mode quality.