Eric J. Gansen

Single-photon detection using a quantum dot optically gated field-effect transistor with high internal quantum efficiency

We investigate the operation of a quantum dot, optically gated, field-effect transistor as a photon detector. The detector exhibits time-gated, single-shot, single-photon sensitivity, a linear response, and an internal quantum efficiency of up to (68±18)% at 4K. Given the noise of the detector system, they find that a particular discriminator level can be chosen so the device operates with an internal quantum efficiency of (53±11)% and dark counts of 0.003 counts per shot.

Submicrometer photoresponse mapping of nanowire superconducting single-photon detectors

We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10×10μm2). Using a confocal microscope configuration and solid immersion lens, we achieve a spot size of 320nm full width at half maximum onto the device at 470nm wavelength. We compare the response maps of two devices: The higher detection efficiency device gives a uniform response, whereas the lower detection efficiency device is limited by a single defect or constriction.

Femtosecond all-optical polarization switching based on the virtual excitation of spin-polarized excitons in quantum wells

An optically addressed nonlinear polarization switch based on the near-resonant excitation of a spin-polarized population of virtual excitons is demonstrated in a multiple quantum well. Pulse-width-limited switching (∼400 fs full width at half maximum) and high-contrast performance (194:1) are achieved in a thin (40 well) sample at 100 K. Differential absorption measurements identify the dominant switching mechanisms.

Operational Analysis of a Quantum Dot Optically Gated Field-Effect Transistor as a Single-Photon Detector

We report on the operation of a novel single-photon detector, where a layer of self-assembled quantum dots (QDs) is used as an optically addressable floating gate in a GaAs/Al0.2Ga0.8As delta-doped field-effect transistor. Photogenerated holes charge the QDs, and subsequently, change the amount of current flowing through the channel by screening the internal gate field. The photoconductive gain associated with this process makes the structure extremely sensitive to light of the appropriate wavelength. We investigate the charge storage and resulting persistent photoconductivity by performing time-resolved measurements of the channel current and of the photoluminescence emitted from the QDs under laser illumination. In addition, we characterize the response of the detector, and investigate sources of photogenerated signals by using the Poisson statistics of laser light. The device exhibits time-gated, single-shot, single-photon sensitivity at a temperature of 4 K. It also exhibits a linear response, and detects photons absorbed in its dedicated absorption layer with an internal quantum efficiency (IQE) of up to (68 plusmn18)%. Given the noise of the detection system, the device is shown to operate with an IQE of (53 plusmn 11)% and dark counts of 0.003 counts per shot for a particular discriminator level.

Ultrafast polarization modulation induced by the “virtual excitation” of spin-polarized excitons in quantum wells: Application to all-optical switching

The operation of an all-optical coherent polarization switch that makes use of spin-polarized virtual excitons in unstrained quantum wells is thoroughly investigated experimentally over a wide range of excitation intensities. The device is shown to exhibit a 415 fs switching time and a contrast ratio of >300:1 at ∼100 K in a thin (40 well) sample. The switching mechanisms are discussed in terms of the circular optical selection rules and the virtual excitation is studied by performing differential measurements for various input polarizations. The polarization state changes induced by the spin-polarized virtual population are measured and their contributions to the switch signal separated using a combination of time-averaged, time-resolved, and spectrally resolved ellipsometric techniques.

Analysis of photoconductive gain as it applies to single-photon detection

We detail a mathematical framework for photoconductive gain applied to the detection of single photons. Because photoconductive gain is derived from the ability to measure current change for an extended period, its magnitude is reduced as detection speed is increased. We theoretically show that high-speed detection is still possible as long as the noise spectrum of the device is 1/f in nature. Using signal analysis techniques, we develop tools to apply to device noise spectra to determine the performance of single-photon detectors that utilize photoconductive gain. We show that there is no speed penalty when one considers the signal-to-noise ratio for the fundamental 1/f noise typical of high electron mobility transistors. We outline a technique for quickly characterizing a detector’s sensitivity and speed through purely electrical measurements of the device’s noise spectra. Consequently, the performance of the detector can be determined and optimized without conducting optical measurements. Finally, we e...

A near-room-temperature all-optical polarization switch based on the excitation of spin-polarized “virtual” carriers in quantum wells

Room temperature operation of a polarization switch based on virtual excitation of spin-polarized carriers in quantum wells is demonstrated, which exhibits a contrast ratio of /spl sim/27:1 and a switching time of /spl sim/575 fs.

Temperature dependence of the single-photon sensitivity of a quantum dot, optically gated, field-effect transistor

We present a systematic study of the temperature dependence of the electrical noise in a quantum dot, optically gated, field-effect transistor (QDOGFET) and detail how the noise influences the sensitivity of these novel single-photon detectors. Previous studies have shown that when cooled to 4 K, QDOGFETs exhibit single-photon sensitivity and photon-number-resolving capabilities; however, there has been no systematic study of how operating temperature affects their performance. Here, we measure the noise spectra of a device for a range of sample temperatures between 7 K and 60 K. We use the noise data to determine the signal-to-noise ratio of the optical responses of the devices for various temperatures and detection rates. Our analysis indicates that QDOGFETs can operate over a broad range of temperatures, where increased operating temperature can be traded for decreased sensitivity.

Designing high electron mobility transistor heterostructures with quantum dots for efficient, number-resolving photon detection

We describe the design of the epitaxial layers for an efficient, photon-number-determining detector that utilizes a layer of self-assembled quantum dots as an optically addressable gate in a field-effect transistor. Our design features a dedicated absorption layer where photoexcited holes are produced and directed with tailored electric fields to the quantum dot layer. A barrier layer ensures that the quantum dot layer is located at a two-dimensional potential minimum of the structure for the efficient collection of holes. Using quantum dots as charge traps allows us to contain the photoexcited holes in a well-defined plane. We derive an equation for a uniform size of the photon signal based on this precise geometry. Finally, we show corroborating data with well-resolved signals corresponding to different numbers of photons.

Coherent all-optical polarization switching based on exciton–exciton interactions in quantum wells

A coherent all-optical nonlinear polarization switch based on exciton–exciton correlations is demonstrated in a multiple-quantum-well semiconductor structure. A contrast ratio of 8:1 and a relaxation time of less than a picosecond are reported at 80 K using only ten wells. The results are compared to a simple phenomenological model to demonstrate that many-body effects are solely responsible for the switching action and that the turn-on and turn-off times are determined by the dephasing time.