We have realized a high-detection-efficiency photon number resolving detector at an operating wavelength of about 850 nm. The detector consists of a titanium superconducting transition edge sensor in an optical cavity, which is directly coupled to an optical fiber using an approximately 300-nm gap. The gap reduces the sensitive area and heat capacity of the device, leading to high photon number resolution of 0.42 eV without sacrificing detection efficiency or signal response speed. Wavelength dependent efficiency in fiber-coupled devices, which is due to optical interference between the fiber and the device, is also decreased to less than 1% in this configuration. The overall system detection efficiency is 98%±1% at wavelengths of around 850 nm, which is the highest value ever reported in this wavelength range.

We fabricated a large-aperture photoconductive terahertz (THz) emitter array on a semi-insulating GaAs substrate. The device was composed of seven 1 cm2 photoconductive antenna units having microstructured interdigitated electrodes with 10 µm lines and spaces. By illuminating it with amplified femtosecond optical pulses, a large THz field comparable to that obtained from conventional large-aperture photoconductive antennas was obtained at a bias voltage as low as 30 V. The coherent superposition of the output of the seven units was observed.

Metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated on the (111)A surface of In 0:53 Ga 0:47 As for the first time. Al 2 O 3 gate dielectrics were formed by atomic layer deposition on sulfur-stabilized InGaAs surfaces. The MISFET on (111)A demonstrated channel mobility higher than that on (100), achieving more than 100% improvement with respect to Si even at a high surface carrier concentration.

Photon number resolving detectors based on titanium-transition edge sensors with high speed and high quantum efficiency have been developed for quantum sensors in the fields of quantum information and quantum radiometry. The two devices optimized at wavelengths of interest showed 81% and 64% system detection efficiencies at 850 nm and 1550 nm, respectively. The response speed of the device optimized for a high counting operation is 190 ns, which corresponds to a counting rate over 1 MHz.

Channel mobility μ eff for InGaAs MISFETs is improved by using the (111)A surface orientation and (NH 4 ) 2 S treatment. These μ eff improvements are associated with negative shifts in V th and V fb . We propose that carrier scattering by fluctuated dipoles at the MIS interfaces contributes to μ eff for the III-V MISFETs. For the InP MISFETs, the effects of the interface dipoles are not apparent due to their inferior interface quality.