Yusuke Kajihara

Tip size dependence of passive near-field microscopy.

We improve the spatial resolution and investigate the tip-sample coupling in a passive scattering-type scanning near-field optical microscope (s-SNOM), which probes thermally excited surface waves without any external light source. We study the spatial resolution, the intensity, and the decay behavior of the thermally excited near-field signals with different radii of curvatures of tungsten-tip apexes. We also study the tip size dependence of the interference pattern in the far-field region. The spatial resolution is closely related to the tip size, but the decay behavior of the near field is unrelated. These results suggest that the strength of the tip-sample coupling is unrelated to the tip size in the passive s-SNOM. We propose a theoretical model able to interpret the experimental data for the passive s-SNOM.

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.

Terahertz single-photon detectors based on quantum wells

Semiconductor charge-sensitive infrared phototransistors (CSIPs) based on quantum wells are described. They are the only detectors that are able to count single photons in the terahertz region at present. In terms of the noise equivalent power (NEP), the detectors show experimental values of 7 × 10−20 W/Hz1/2, while theoretically expected values are even much lower. These NEP values are by several orders of magnitude lower than any other state-of-the-art highly sensitive detectors. In addition to the outstanding sensitivity, the detectors are featured by strong advantage of huge current responsivity (>1 × 105 A/W) and low output impedance (<10 kΩ). This excellent performance in the above has been obtained for λ = 12–28 μm. By introducing a modified scheme of detection (called “lateral-escape”) along with an improved coupler structure (bowtie antenna), we have achieved similar excellent performance for 45 μm. The CSIP provides extremely promising detectors for a variety of applications covering a wide spectral range of 12–100 μm.

Imaging of nonlocal hot-electron energy dissipation via shot noise

Noise is usually a hindrance to signal detection. As stressed by Landauer, however, noise can be an invaluable signal that reveals kinetics of charge particles. Understanding local non-equilibrium electron kinetics at nano-scale is of decisive importance for the development of miniaturized electronic devices, optical nano-devices, and heat management devices. In non-equilibrium conditions electrons cause current fluctuation (excess noise) that contains fingerprint-like information about the electron kinetics. A crucial challenge is hence a local detection of excess noise and its real-space mapping. However, the challenge has not been tackled in existing noise measurements because the noise studied was the spatially integrated one. Here we report the experiment in which the excess noise at ultra-high-frequency(21.3THz), generated on GaAs/AlGaAs quantum well (QW) devices with a nano-scale constriction, is locally detected and mapped for the first time. We use a sharp tungsten tip as a movable, contact-free and noninvasive probe of the local noise, and achieved nano-scale spatial resolution (~50nm). Local profile of electron heating and hot-electron kinetics at nano-scales are thereby visualized for the first time, disclosing remarkable non-local nature of the transport, stemming from the velocity overshoot and the intervalley hot electron transfer. While we demonstrate the usefulness of our experimental method by applying to mesoscopic conductors, we emphasize that the method is applicable to a variety of different materials beyond the conductor, and term our instrument a scanning noise microscope (SNoiM):In general non-equilibrium current fluctuations are generated in any materials including dielectrics, metals and molecular systems. The fluctuations, in turn, excite fluctuating electric and magnetic evanescent fields on the material surface, which can be detected and imaged by our SNoiM.Taking the temperature of hot electrons As electronic chips become smaller, efficient heat dissipation becomes a greater challenge. Electrons in such devices quickly accelerate over small distances, becoming “hot”—that is, out of equilibrium with the rest of the system. Weng et al. designed a thermometry probe that measures the effective temperature of hot electrons with a spatial resolution of about 50 nanometers. The method is based on the optical measurement of current noise and provides a glimpse into where heat is naturally dissipated in a working device. Science, this issue p. 775 A scanning probe maps the temperature of electrons going through a constriction in a GaAs/AlGaAs device. In modern microelectronic devices, hot electrons accelerate, scatter, and dissipate energy in nanoscale dimensions. Despite recent progress in nanothermometry, direct real-space mapping of hot-electron energy dissipation is challenging because existing techniques are restricted to probing the lattice rather than the electrons. We realize electronic nanothermometry by measuring local current fluctuations, or shot noise, associated with ultrafast hot-electron kinetic processes (~21 terahertz). Exploiting a scanning and contact-free tungsten tip as a local noise probe, we directly visualize hot-electron distributions before their thermal equilibration with the host gallium arsenide/aluminium gallium arsenide crystal lattice. With nanoconstriction devices, we reveal unexpected nonlocal energy dissipation at room temperature, which is reminiscent of ballistic transport of low-temperature quantum conductors.

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.

Near-field microscopy of spontaneous evanescent waves

All material surfaces are covered with strong infrared/terahertz (IR/THz) evanescent waves since all materials contain positive and negative charges and the charge movement generates local electromagnetic waves at finite temperature. Previous theoretical analyses suggest that the thermal evanescent waves on metals and dielectrics are strongly localized within 100 nm from the surface, the energy density is more than 10000 times higher than the one of Planck’s radiation and their spectra lie in THz regions (wavelength: 8～20 μm). Probing such spontaneous evanescent waves with nanoscale spatial resolution can visualize the local dynamics of thermal equilibrium and non-equilibrium phenomena. Recently we have developed a passive scanning near-field optical microscope (SNOM), which probes surface evanescent waves without any external illumination. The microscope consists of a confocal infrared microscope, and an ultra-highly sensitive detector, named the charge-sensitive infrared phototransistor (CSIP, wavelength range is 14.5 ± 0.7 μm). In this presentation, we first describe the development of the passive SNOM. Then we show the results obtained with the SNOM; thermal evanescent waves on metals and dielectrics, and nanoscale distribution of evanescent waves derived from non-equilibrium phenomena in two dimensional electron gas. Our SNOM should open up a new way of investigating material surfaces in general and provide a novel technique of probing local temperature/electro-magnetic field distribution.

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.

Ultra-highly sensitive passive near-field microscopy of electromagnetic evanescent waves

Electromagnetic (EM) evanescent waves generated on material surfaces without external illumination are of particular interest. If materials are in thermal equilibrium, thermal fluctuation of current and/or charge is induced by stochastic thermal motion of conduction electrons (in metals) or by thermally-activated lattice vibrations (in polar dielectrics), which generates intense thermal EM evanescent waves [1]. If materials are driven out of equilibrium, say, by electric-field-induced currents or by chemical reactions, even stronger EM evanescent waves are induced because the fluctuation of current and/or charge is promoted by activated motion of charge carriers or electric dipoles in non-equilibrium states. Hence, a sensitive passive microscopy of EM evanescent waves with nanometer resolution is expected to provide us with a unique and powerful probe to the local kinetics of material phenomena in equilibrium and non-equilibrium conditions. We have developed such microscopes as described in the above, by incorporating ultra-highly sensitive detectors, called CSIP [2], into passive terahertz (THz) scattering-type scanning near-field optical microscopes (s-SNOMs) [3]. Owing to the high sensitivity, detailed nature of the thermal EM evanescent waves has been clarified through quantitative analysis of imaging of both metals [4, 5] and dielectrics. The spatial resolution reaches a value about 20nm (λ/750) [6]. We also demonstrate the potentiality and powerfulness of our system for probing non-equilibrium kinetics: We show that current-induced hot-electron distribution in narrow metal wires at room temperature is clearly visualized with nanometer resolution. These results show that our systems will find broad application areas in the near future.

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.

Passive THz Near-Field Imaging and its Applications for Engineering

We have recently developed a THz near-field microscope with an ultrahighly sensitive detector, CSIP (charge-sensitive infrared phototransistor). The microscope probes spontaneous evanescent field on samples derived from local phenomena and the signal origin from metals was previously revealed to be thermal charge/current fluctuations. The intensity of passive near-field signal is very well consistent with Bose-Einstein distribution, which corresponds to the sample temperature. In this study, we demonstrate nano-thermometry with the microscope by monitoring passive near-field signals on a biased NiCr pattern. The obtained signals correspond to the local temperature and the result shows that the inner side of the line curve is much brighter than outer side. It can be easily interpreted by Kirchhoff’s law. The spatial resolution is 60 nm, which cannot be experimentally achieved by any other optical thermometry. This demonstration strongly suggests that our microscope is very well suited for real-time temperature mapping of complicated circuit patterns, and others like bio-samples.