The field of terahertz science and technology has been an active and thriving research area for several decades. However, the field has recently experienced an inflection point, as several exciting breakthroughs have enabled new opportunities for both fundamental and applied research. These events are reshaping the field, and will impact research directions for years to come. In this Perspective article, I discuss a few important examples: the development of methods to access nonlinear optical effects in the terahertz range; methods to probe nanoscale phenomena; and, the growing likelihood that terahertz technologies will be a critical player in future wireless networks. Here, a few examples of research in each of these areas are discussed, followed by some speculation about where these exciting breakthroughs may lead in the near future.

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Perspective: Terahertz science and technology

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.

http://science.sciencemag.org/content/sci/360/6390/775.full.pdf

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.

Imaging non-local electron transport via local excess noise

We analyze how a probing particle modifies the infrared electromagnetic near field of a sample. The particle, described by electric and magnetic polarizabilities, represents the tip of an apertureless scanning optical near-field microscope (SNOM). We show that the interaction with the sample can be accounted for by ascribing to the particle dressed polarizabilities that combine the effects of image dipoles with retardation. When calculated from these polarizabilities, the SNOM signal depends only on the fields without the perturbing tip. If the studied surface is not illuminated by an external source but heated instead, the signal is closely related to the projected electromagnetic local density of states (EM-LDOS). Our calculations provide the link between the measured far-field spectra and the sample's optical properties. We also analyze the case where the probing particle is hotter than the sample and evaluate the impact of the dressed polarizabilities on near-field radiative heat transfer. We show that such a heated probe above a surface performs a surface spectroscopy, in the sense that the spectrum of the heat current is closely related to the local electromagnetic density of states. The calculations agree well with available experimental data.

Strong tip-sample coupling in thermal radiation scanning tunneling microscopy

In the wide range of the electromagnetic wave spectrum, the terahertz (THz) frequency region has for a long time been an unexplored region and its technological development has been left behind. Nowadays, however, science and technology based on THz electromagnetic waves have been increasingly progressing and are still growing. In the THz region, both features of ‘wave’ and ‘light’ appear, enabling the manipulation of the THz wave from both approaches of electronics and optics/photonics. From the viewpoint of research targets, THz technology is expected to be the key for unlocking mysteries behind quantum effects in condensed-matter physics, life activities in biology, and the birth of the celestial bodies in astronomy. In addition to such fundamental sciences, THz spectroscopy and imaging can be used as a powerful tool for nondestructive remote inspection in industrial and medical fields. In this article I review cutting-edge technologies of THz sensing, imaging and spectroscopy, and describe ho...

Terahertz waves: a tool for condensed matter, the life sciences and astronomy

Thermal radiation from samples of Au layers patterned on GaAs, SiO2, and SiC at 300 K are studied with a scattering-type scanning near-field optical microscope (wavelength: ~14.5 μm), without applying external illumination. Clear near-field images are obtained with a spatial resolution of ~60 nm. All the near field signals derived from different demodulation procedures decrease rapidly with increasing probe height h with characteristic decay lengths of 40 ~60 nm. Near-field images are free from any signature of in-plane spatial interference. The findings are accounted for by theoretically expected surface evanescent waves, which are thermally excited in the close vicinity of material surfaces. Outside the near-field zone (1 μm < h), signals reappear and vary as a sinusoidal function of h, exhibiting a standing wave-like interference pattern. These far-field signals are ascribed to the effect of weak ambient radiation.

Thermally excited near-field radiation and far-field interference.

We have developed a scattering-type scanning near-field optical microscope in long-wavelength infrared region (wavelength: λ ~ 14.5 μm) with a highly sensitive detector, named charge sensitive infrared phototransistor. A tungsten near-field probe was fabricated via electrochemical etching and the sample-probe distance was precisely controlled in shear-force mode. By vertically modulating the probe independently of lateral oscillation for shear-force mode, we successfully detected thermal evanescent waves from a 3 μm-pitch Au/GaAs grating without any external illumination. The spatial resolution was experimentally estimated to be better than 150 nm (λ/100) and the signal-to-noise ratio was around 4 with an averaging time of 300 ms. The strong near-field signal from Au was suggested to be ascribed to thermally excited surface plasmons generating evanescent waves at the surface.

Passively detecting thermal evanescent waves from room temperature objects

Long wavelength infrared (LWIR) waves contain many important spectra of matters like molecular motions. Thus, probing spontaneous LWIR radiation without external illumination would reveal detailed mesoscopic phenomena that cannot be probed by any other measurement methods. Here we developed a scattering-type scanning near-field optical microscope (s-SNOM) and demonstrated passive near-field microscopy at 14.5 µm wavelength. Our s-SNOM consists of an atomic force microscope and a confocal microscope equipped with a highly sensitive LWIR detector, called a charge-sensitive infrared phototransistor (CSIP). In our s-SNOM, photons scattered by a tungsten probe are collected by an objective of the confocal LWIR microscope and are finally detected by the CSIP. To suppress the far-field background, we vertically modulated the probe and demodulated the signal with a lock-in amplifier. With the s-SNOM, a clear passive image of 3 µm pitch Au/SiC gratings was successfully obtained and the spatial resolution was estimated to be 60 nm (λ/240). The radiation from Au and GaAs was suggested to be due to thermally excited charge/current fluctuations and surface phonons, respectively. This s-SNOM has the potential to observe mesoscopic phenomena such as molecular motions, biomolecular protein interactions and semiconductor conditions in the future.

http://iopscience.iop.org/article/10.1088/0957-0233/22/8/085102/pdf

Probing thermal evanescent waves with a scattering-type near-field microscope*

We have demonstrated ultrasensitive near-field microscopy in the long-wavelength infrared region without any external illumination. A scattering-type scanning near-field optical microscope was developed with a highly sensitive detector (charge sensitive infrared phototransistor: wavelength λ ∼ 14.5 μm) and a thermal evanescent wave was passively obtained from room-temperature objects by vertically modulating a tungsten probe. The spatial resolution of the near-field microscope was estimated to be better than 100 nm (λ/100). The experimental results suggest that thermally excited surface plasmons on Au and surface phonons on SiC could be observed with our microscope. [DOI: 10.1380/ejssnt.2011.173]

Passive Near-Field Microscopy in Long-Wavelength Infrared

We demonstrate probing thermally activated surface waves with a passive near-field microscope (wavelength λ: ∼14.5 μm) without external illumination. A clear passive image of a room temperature object is achieved with a spatial resolution of 60 nm (λ/240). The decay characteristic of thermal evanescent waves on Au is quantitatively consistent with a numerical prediction based on the theories considering the thermal energy localization and the scattering efficiency of the near-field. In addition, the sample-size and temperature dependence of the near-field thermal signals agrees well with the theoretical calculations.

Susumu Komiyama

Papers

Thermally excited near-field radiation and far-field interference.

Passively detecting thermal evanescent waves from room temperature objects

Probing thermal evanescent waves with a scattering-type near-field microscope*

Passive Near-Field Microscopy in Long-Wavelength Infrared

Probing thermal evanescent fields with a near-field microscope