Observation of two-photon emission from semiconductors

Two-photon emission is a process in which electron transition between quantum levels occurs through the simultaneous emission of two photons. This phenomenon is important for astrophysics and atomic physics, and semiconductor twophoton emission was recently proposed as a compact source of entangled photons, essential for practical quantum information processing, and three orders of magnitude more efficient than the existing down-conversion schemes. Two-photon absorption in semiconductors has been extensively investigated; however, spontaneous semiconductor two-photon emission has not been observed, nor has it been fully analysed theoretically so far. We report the first experimental observations of two-photon emission from semiconductors and develop a corresponding theory. Spontaneous two-photon emission is demonstrated in optically pumped bulk GaAs and in electrically driven GaInP/AlGaInP quantum wells. Singly stimulated two-photon emission measurements demonstrate the theoretically predicted twophoton optical gain in semiconductors—a necessary ingredient for any realizations of future two-photon semiconductor lasers. A photon-coincidence experiment is presented to validate the simultaneity of the electrically driven GaInP/AlGaInP two-photon emission, limited only by the detector’s temporal resolution. Two-photon transitions are much weaker than the related firstorder processes, so observations of multiphoton spontaneous decays have so far been restricted to a few atomic transition cases. In these instances the lowest-order transition is forbidden by selection rules or suppressed by a cavity, and the twophoton spectrum is continuous and centred at about half the one-photon transition frequency. Semiconductors can be injected with very high charge-carrier densities, making even the weak second-order spontaneous processes measurable, and their two-photon emission (TPE) spectrum is expected to be determined by the photonic state density as well as by the carrier energy distribution. Here we report the first experimental observation of spontaneous TPE from room-temperature semiconductors, excited both optically in bulk GaAs and electrically in GaInP/AlGaInP quantum wells (QWs) grown on a GaAs substrate. We develop a theoretical model for spontaneous TPE from semiconductors, validating the measured results. In additional experiments, the optically pumped bulk GaAs was singly stimulated to emit specific photon pairs by launching photons at a prescribed wavelength, yielding the co-emission of complementary wavelength photons with the suppression of the rest of the spectrum. Similar results are shown for GaInP/AlGaInP QWs, and a photon-coincidence experiment verifies the simultaneity of the TPE. In our first experiment, we optically pumped GaAs with a 100-mW continuous-wave 514-nm argon laser (photon energy, vpump 1.73Egap where Egap is the energy gap between the electron levels), focused with a spot size of 30 mm (after filtering out spontaneous infrared (IR) emission) onto a 200-mm-thick GaAs sample (much thicker than the laser penetration depth), inducing a local carrier concentration of 1.2 10 cm, and the pump-induced local sample heating was estimated to be 330 K. The pump laser was chopped at 236 Hz, and the collected emission from the sample in a transmission configuration was detected by a New-Focus IR femtowatt photoreceiver by means of a lock-in amplifier. The spectrum was obtained using an ActonResearch monochromator using a 1,600-nm blazed grating and 10-nm spectral resolution. As expected, the measured spectrum for the TPE was very wide (Fig. 1a), complying with the theory described below, and the overall collected optical power was 3 nW. To validate unambiguously that the observed luminescence is indeed TPE, and to dismiss any possibility of inhomogeneously broadened emission from mid-gap levels, we measured singly stimulated TPE by launching a specific wavelength into the optically pumped GaAs. As a result of this excitation, a complementary TPE-wavelength peak appeared. In all the stimulated TPE experiments only the carrier density was modulated for lock-in detection; the stimulating lasers were not modulated, and the observed signals were proportional to the pump power. Stimulation with a 0.2-mW 1,630-nm (0.761 eV) laser resulted in a peak at 1,451 nm (0.854 eV), whereas a 1,600-nm (0.775 eV) laser yielded a peak at 1,476 nm (0.84 eV) (Fig. 1a). In both cases the photon energies are complementary about the centre of the spontaneous emission at 0.81 eV within an error of 3 meV, as expected owing to energy conservation. Changing the stimulation wavelength to 1,570 nm (0.79 eV) caused the stimulating and complementary peaks to merge, approaching the degenerate case. The peaks appeared to merge for any stimulation energy closer than 30 meV to the spontaneous TPE centre. Furthermore, during this excitation, the rest of the spontaneous emission spectrum was suppressed (Fig. 1), clearly demonstrating that the wideband emission is homogeneously broadened, and the strongest effect of the stimulation on the TPE spectrum was observed in the direction collinear with the stimulation. In TPE, the wideband LETTERS