H. Sakuma

A highly sensitive scanning far-infrared microscope with quantum Hall detectors

We develop a highly sensitive scanning far-infrared (FIR) microscope, which consists of a silicon solid immersion lens that probes FIR and a condenser lens that focuses the FIR onto a small quantum Hall detector (400 μm×400 μm). The solid immersion lens is in contact with the backside of a Hall bar sample, which is moved with a mechanical XY stage. The quantum Hall detector, which function as a narrow band FIR detector (bandwidth of about 2%), is a Hall bar with a large length-to-width ratio in integer quantum Hall effect regimes. The microscope is successfully applied to image extremely weak cyclotron emissions from quantum Hall devices with a spatial resolution of about 50 μm and a signal-to-noise ratio improved by a factor 18 compared to a previous system.

An improved far-infrared microscope with quantum Hall detectors

Abstract A highly-sensitive scanning far-infrared (FIR) microscope is developed. The microscope consists of a silicon solid immersion lens that probes FIR and a condenser lens that focuses the FIR onto a small and highly-sensitive quantum Hall detector. The solid immersion lens is in contact with a sample, which is moved with a mechanical stage. The microscope is successfully applied to image extremely weak cyclotron emission from quantum Hall devices with a spatial resolution about 50 μm and a signal-to-noise ratio improved by a factor 18 compared to a previous system.

Photon-counting THz imaging with quantum dot detectors

A GaAs/AlGaAs quantum dot THz detector is incorporated in a confocal scanning THz microscope, realizing photon counting THz imaging of extremely weak cyclotron emission in quantum Hall devices.

Imaging non‐equilibrium edge states in quantum Hall conductors

Non‐equilibrium edge states in a transition region between quantum Hall states (filling factor 2<v<3) have been visualized through cyclotron emission measured by using a recent developed scanning terahertz microscope. Cyclotron emission occurs in a narrow region along one sample boundary at large currents of more than 20 μA even though characteristic of adiabatic transport completely diminishes in resistance measurements. A striking feature of non‐equilibrium edge states on the lower potential side (with positive Hall voltage) is first found as significant difference of luminous efficiency with the opposite side, suggesting radiative recombination due to an anomalous non‐equilibrium population formed between the edge and the bulk states.

Cyclotron emission imaging of quantum Hall devices

The current state of highly sensitive terahertz imaging microscopes is reported in the context of applications for cyclotron emission studies of quantum Hall devices. Without external excitation other than the transport current, the microscope maps the distribution of nonequilibrium electrons generated in quantum devices by detecting weak cyclotron emission. Firstly, we present the imaging results to discuss the origin of nonequilibrium electrons. Finally, we describe our attempts towards photon counting terahertz microscope with quantum-dot photon detectors utilizing single electron transistors.