Near-Field Radiative Nanothermal Imaging of Nonuniform Joule Heating in Narrow Metal Wires.

Probing spatial variation of temperature at the nanoscale provides key information for exploring diverse areas of modern science and technology. Despite significant progress in the development of contact thermometers with high spatial resolution, one inherent disadvantage is that the quantitative analysis of temperature can be complicated by the direct thermal contact. On the other hand, noncontact infrared radiation thermometer is free from such contact-induced disturbance, but suffers from insufficient spatial resolution stemming from diffraction-limit in the micrometer range. Combining a home-built sensitive infrared microscope with a noncontact scattering probe, we detected fluctuating electromagnetic evanescent fields on locally heated material surface, and thereby mapped temperature distribution in subwavelength scales. We visualize nanoscale Joule heating on current-carrying metal wires and find localized "hot-spots" developing along sharp corners of bended wires in the temperature mapping. Simulation calculations give quantitative account of the nanoscale temperature distribution, definitely indicating that the observed effect is caused by the nonuniform energy dissipation due to the current-crowding effect. The equipment in this work is a near-field version of infrared radiation thermometer with a spatial resolution far below the detection wavelength (<100 nm, or λ/140) in which local temperature distribution of operating nanoscale devices can be noninvasively mapped with a temperature resolution ∼2 K at room-temperature.

Room-Temperature Single-Photon Detector Based on Single Nanowire.

Single-photon detectors that can resolve photon number play a key role in advanced quantum information technologies. Despite significant progress in improving conventional photon-counting detectors and developing novel device concepts, single-photon detectors that are capable of distinguishing incident photon number at room temperature are still very limited. We demonstrate a room-temperature photon-number-resolving detector by integrating a field-effect transistor configuration with core/shell-like nanowires. The shell serves as a photosensitive gate, shielding negative back-gated voltage, and leads to a persistent photocurrent. At room temperature, our detector is demonstrated to identify 1, 2, and 3 photon-number states with a confidence of >82%. The detection efficiency is determined to be 23%, and the dark count rate is 1.87 × 10-3 Hz. Importantly, benefiting from the anisotropic nature of 1D nanowires, the detector shows an intrinsic photon-polarization selection, which distinguishes itself from existing intensity single-photon detectors. The unique performance for the single-photon detectors based on single nanowire demonstrates the great potential for future single-photon detection applications.

Nanoscale thermal imaging of hot electrons by cryogenic terahertz scanning noise microscopy.

Nanoscale thermal imaging and temperature detection are of fundamental importance in diverse scientific and technological realms. Most nanoscale thermometry techniques focus on probing the temperature of lattice or phonons and are insensitive to nonequilibrium electrons, commonly referred to as "hot electrons." While terahertz scanning noise microscopy (SNoiM) has been demonstrated to be powerful in the thermal imaging of hot electrons, prior studies have been limited to room temperature. In this work, we report the development of a cryogenic SNoiM (Cryo-SNoiM) tailored for quantitative hot electron temperature detection at low temperatures. The microscope features a special two-chamber design where the sensitive terahertz detector, housed in a vacuum chamber, is efficiently cooled to ∼5 K using a pulse tube cryocooler. In a separate chamber, the atomic force microscope and the sample can be maintained at room temperature under ambient/vacuum conditions or cooled to ∼110 K via liquid nitrogen. This unique dual-chamber cooling system design enhances the efficacy of SNoiM measurements at low temperatures. It not only facilitates the pre-selection of tips at room temperature before cooling but also enables the quantitative derivation of local electron temperature without reliance on any adjustable parameters. The performance of Cryo-SNoiM is demonstrated through imaging the distribution of hot electrons in a cold, self-heated narrow metal wire. This instrumental innovation holds great promise for applications in imaging low-temperature hot electron dynamics and nonequilibrium transport phenomena across various material systems.

Development of a cryogenic passive-scattering-type near-field optical microscopy system.

Passive scattering-type, scanning near-field optical microscopy (s-SNOM) has been employed to study localized, long-wavelength infrared (LWIR) surface waves without external illumination. Here, we develop a cryogenic passive s-SNOM instrument in a vacuum chamber with 4 K liquid-helium cooling. Notably, the extremely low-temperature environment inside the chamber enables the realization of passive near-field detection with low background thermal noise. The technique mainly utilizes a highly sensitive LWIR confocal optical system and a tuning fork-based atomic force microscope, and the near-field detection was performed at a wavelength of 10.2 ± 0.9 µm. In this paper, we discuss the cryogenic s-SNOM implementation in detail and report the investigation of thermally excited surface electromagnetic fields on a self-heated NiCr wire deposited on SiO2 at a temperature of 5 K. The origin of the surface electromagnetic fields was established to be the thermally excited fluctuating charges of the conduction electrons. The cryogenic s-SNOM method presented herein shows significant promise for application in a variety of spheres, including hot-carrier dissipation in ballistic conductors.

A comprehensive model of electrical noise in AlGaAs/GaAs long-wavelength quantum well infrared photodetectors.

We present the results of a comprehensive model for the electric noise simulation of a kind of nano-optoelectronics device: AIGaAs/GaAs long-wavelength quantum well infrared photodetectors (LW-QWIPs) in dark conditions by assuming a three-dimensional carrier transport in the barriers where the electrical field are obtained in a self-consistent way. This model takes into account all the fundamental mechanisms involved in the device detection process. The electrical field distribution, dark currents, electrical noise are carefully calculated and analyzed. The numerical results also explain well our experimental observations.

Quasiadiabatic electron transport in room temperature nanoelectronic devices induced by hot-phonon bottleneck.

Since the invention of transistors, the flow of electrons has become controllable in solid-state electronics. The flow of energy, however, remains elusive, and energy is readily dissipated to lattice via electron-phonon interactions. Hence, minimizing the energy dissipation has long been sought by eliminating phonon-emission process. Here, we report a different scenario for facilitating energy transmission at room temperature that electrons exert diffusive but quasiadiabatic transport, free from substantial energy loss. Direct nanothermometric mapping of electrons and lattice in current-carrying GaAs/AlGaAs devices exhibit remarkable discrepancies, indicating unexpected thermal isolation between the two subsystems. This surprising effect arises from the overpopulated hot longitudinal-optical (LO) phonons generated through frequent emission by hot electrons, which induce equally frequent LO-phonon reabsorption ("hot-phonon bottleneck") cancelling the net energy loss. Our work sheds light on energy manipulation in nanoelectronics and power-electronics and provides important hints to energy-harvesting in optoelectronics (such as hot-carrier solar-cells).

Imaging of nonlocal hot-electron energy dissipation via shot noise.

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.

Quantum dot single-photon switches of resonant tunneling current for discriminating-photon-number detection.

Low-noise single-photon detectors that can resolve photon numbers are used to monitor the operation of quantum gates in linear-optical quantum computation. Exactly 0, 1 or 2 photons registered in a detector should be distinguished especially in long-distance quantum communication and quantum computation. Here we demonstrate a photon-number-resolving detector based on quantum dot coupled resonant tunneling diodes (QD-cRTD). Individual quantum-dots (QDs) coupled closely with adjacent quantum well (QW) of resonant tunneling diode operate as photon-gated switches- which turn on (off) the RTD tunneling current when they trap photon-generated holes (recombine with injected electrons). Proposed electron-injecting operation fills electrons into coupled QDs which turn "photon-switches" to "OFF" state and make the detector ready for multiple-photons detection. With proper decision regions defined, 1-photon and 2-photon states are resolved in 4.2 K with excellent propabilities of accuracy of 90% and 98% respectively. Further, by identifying step-like photon responses, the photon-number-resolving capability is sustained to 77 K, making the detector a promising candidate for advanced quantum information applications where photon-number-states should be accurately distinguished.