Sunmi Kim

Two-color detection with charge sensitive infrared phototransistors

Highly sensitive two-color detection is demonstrated at wavelengths of 9 μm and 14.5 μm by using a charge sensitive infrared phototransistor fabricated in a triple GaAs/AlGaAs quantum well (QW) crystal. Two differently thick QWs (7 nm- and 9 nm-thicknesses) serve as photosensitive floating gates for the respective wavelengths via intersubband excitation: The excitation in the QWs is sensed by a third QW, which works as a conducting source-drain channel in the photosensitive transistor. The two spectral bands of detection are shown to be controlled by front-gate biasing, providing a hint for implementing voltage tunable ultra-highly sensitive detectors.

A high signal-to-noise ratio passive near-field microscope equipped with a helium-free cryostat

We have developed a passive long-wavelength infrared (LWIR) scattering-type scanning near-field optical microscope (s-SNOM) installed in a helium-free mechanically cooled cryostat, which facilitates cooling of an LWIR detector and optical elements to 4.5 K. To reduce mechanical vibration propagation from a compressor unit, we have introduced a metal bellows damper and a helium gas damper. These dampers ensure the performance of the s-SNOM to be free from mechanical vibration. Furthermore, we have introduced a solid immersion lens to improve the confocal microscope performance. To demonstrate the passive s-SNOM capability, we measured thermally excited surface evanescent waves on Au/SiO2 gratings. A near-field signal-to-noise ratio is 4.5 times the improvement with an acquisition time of 1 s/pixel. These improvements have made the passive s-SNOM a more convenient and higher-performance experimental tool with a higher signal-to-noise ratio for a shorter acquisition time of 0.1 s.

Development of a cryogen-free passive near-field microscope

Passive THz s-SNOM (scattering-type scanning near-field optical microscope) is a powerful tool, which can reveal the weak spontaneous radiation on a sample surface (e.g., Au, GaAs, or SiC) without external light source [1, 2]. In our passive s-SNOM, an extremely sensitive CSIP (charge-sensitive infrared phototransistor [3]) detector is necessary to be operated at low temperature less than 10 K. To cool down the CSIP more conveniently and to save the helium resource, we introduced a helium-free cryostat to the passive s-SNOM as shown in Fig. 1(a). Mechanical and helium gas dampers are used to attenuate the vibration from the cold head and the compressor [see Fig. 1(a)]. Furthermore, to get a better SNR for higher scan speed, we improved the confocal microscope as follows: (1) The solid immersion lens (SIL) is introduced to seal the CSIP. It can achieve smaller focusing spot on a sensing area of the CSIP to enlarge the number of the collected photon. (2) The metal-mirror type Cassegrain objective (N.A.: 0.4) whose thermal emission is almost zero. Fig. 1(b) shows that the smooth topography (upper panel) indicated that we have overcome the vibration problem. We have successfully observed the passive near-field signal on a Au stripe (lower panel). The SNR is derived to 5 with a scan speed of 100 ms per step.

Improved Performance of Ultrahigh-Sensitive Charge-Sensitive Infrared Phototransistors (CSIP)

Charge-sensitive infrared phototransistor (CSIP) is a highly sensitive semiconductor terahertz (THz) detector with single-photon sensitivity. Due to the excitation mechanism via intersubband transition in a quantum well, the CSIP requires careful design of the photo-coupler and proper light illumination method to achieve high quantum efficiency. We have improved the quantum efficiency by introducing the radiation from the backside of the CSIP substrate, which leads to efficient surface plasmon excitation in the photo-coupler.