We demonstrate photon-number discrimination using a novel semiconductor detector that utilizes a layer of self-assembled
InGaAs quantum dots (QDs) as an optically addressable floating gate in a GaAs/AlGaAs δ-doped field-effect
transistor. When the QDOGFET (quantum dot, optically gated, field-effect transistor) is illuminated, the internal gate
field directs the holes generated in the dedicated absorption layer of the structure to the QDs, where they are trapped.
The positively charged holes are confined to the dots and screen the internal gate field, causing a persistent change in the
channel current that is proportional to the total number of holes trapped in the QD ensemble. We use highly attenuated
laser pulses to characterize the response of the QDOGFET cooled to 4 K. We demonstrate that different photon-number
states produce well resolved changes in the channel current, where the responses of the detector reflect the Poisson
statistics of the laser light. For a mean photon number of 1.1, we show that decision regions can be defined such that
the QDOGFET determines the number (0, 1, 2, or ≥3) of detected photons with a probability of accuracy ≥83 % in a
Since terahertz electric fields can couple strongly to quantum well intersubband transitions we expect interband optical properties of a semiconductor heterostructure to change resonantly under a THz driving field. By driving the excitonic intersubband resonance of an asymmetric quantum well with intense THz electric fields from a free electron laser, we modulate the transmission of a near-IR (NIR) laser beam at terahertz frequencies. This process manifests itself as the emission of optical sidebands on the NIR probe. In previous THz electro-optical studies in semiconductors, only even sidebands of frequency (omega) <SUB>sideband</SUB> equals (omega) <SUB>NIR</SUB> + 2n(omega) <SUB>THz</SUB> had been observed. BY breaking inversion symmetry we are able to generate a comb of even and odd sidebands. The sidebands obey both THz and near-IR polarization selection rules and are enhanced when the NIR energy is resonant with a peak in the excitonic density of states. The ability to generate THz optical sidebands of all orders is important for the future application of THz EO effects in nonlinear spectroscopy and in ultrafast optical phase and amplitude modulation.
We have explored near-infrared (NIR)--far-infrared (FIR) two-color optical experiments in quantum-confined semiconductor systems, using NIR radiation from a tunable cw Ti:Sapphire laser and intense and coherent FIR radiation from the UCSB Free-Electron Lasers. In this paper two recent experiments are discussed, both of which provide new insight into the internal structure and dynamics of confined excitons: (1) We have observed for the first time FIR internal transitions associated with the direct exciton in GaAs/AlGaAs quantum wells. The spectrum of excitations is enriched by the complexities of the valence band and differ significantly from simple reduced-mass, hydrogenic models. We provide a critical test of detailed calculations including the valence-band mixing of Bauer and Ando. (2) We have discovered resonant nonlinear optical mixing of NIR and FIR radiation, which results in strong near-bandgap emission lines, or optical sidebands. The sidebands appear when optically-created excitons are driven strongly by intense FIR fields. The frequencies of the sidebands are (omega) <SUB>NIR</SUB> +/- 2n(omega) <SUB>FIR</SUB>, where (omega) <SUB>NIR</SUB> is the interband exciton-creation frequency, (omega) <SUB>FIR</SUB> is the frequency of the driving field, and n is an integer. The intensity of the sidebands exhibits pronounced resonances as a function of applied magnetic field, which are well- explained in terms of virtual transitions between magnetically-tuned energy levels in the excitons.