Optical antennas are devices that convert freely propagating optical radiation into localized energy, and vice versa. They enable the control and manipulation of optical fields at the nanometre scale, and hold promise for enhancing the performance and efficiency of photodetection, light emission and sensing. Although many of the properties and parameters of optical antennas are similar to their radiowave and microwave counterparts, they have important differences resulting from their small size and the resonant properties of metal nanostructures. This Review summarizes the physical properties of optical antennas, provides a summary of some of the most important recent developments in the field, discusses the potential applications and identifies the future challenges and opportunities.

Antennas for light

1. Introduction 2. Quantum confined systems 3. Transmission in nanostructures 4. Quantum dots and single electron phenomena 5. Interference in diffusive transport 6. Temperature decay of fluctuations 7. Non-equilibrium transport and nanodevices.

Transport in Nanostructures

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

1 Introduction 2 Quantum confined systems 3 Transmission in nanostructures 4 Quantum dots and single electron phenomena 5 Interference in diffusive transport 6 Temperature decay of fluctuations 7 Non-equilibrium transport and nanodevices

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Transport in nanostructures

Resonant tunneling through a single quantum well of GaAs has been observed. The current singularity and negative resistance region are dramatically improved over previous results, and detecting and mixing have been carried out at frequencies as high as 2.5 THz. Resonant tunneling features are visible in the conductance‐voltage curve at room temperature and become quite pronounced in the I‐V curves at low temperature. The high‐frequency results, measured with far IR lasers, prove that the charge transport is faster than about 10−13 s. It may now be possible to construct practical nonlinear devices using quantum wells at millimeter and submillimeter wavelengths.

Resonant tunneling through quantum wells at frequencies up to 2.5 THz

Oscillations have been observed for the first time from double barrier resonant tunneling structures. By eliminating impurities from the wells, we have been able to increase the tunneling current density by a factor of nearly 100. With the attendant increase in gain and improved impedance match to the resonant circuit, the devices oscillated readily in the negative resistance region. Oscillator output power of 5 μW and frequencies up to 18 GHz have been achieved with a dc to rf efficiency of 2.4% at temperatures as high as 200 K. It is shown that higher frequencies and higher powers can be expected.

Quantum well oscillators

We have made heterodyne radiometric measurements with GaAs Schottky diode mixers, mounted in a corner‐reflector configuration, over the spectral range 170 μm to 1 mm. At 400 μm, system noise temperatures of 9700 K DSB (NEP=1.4×10−19 W/Hz) and mixer noise temperatures of 5900 K have been achieved. This same quasioptical mixer has also been used to generate 10−7 W of tunable radiation suitable for spectroscopic applications.

Far‐ir heterodyne radiometric measurements with quasioptical Schottky diode mixers

Schottky diodes have been used for the first time as harmonic mixers in the 0.1–1.0‐mm wavelength region. Beat notes between the 33rd harmonic of a 74‐GHz V‐band klystron and 118.8‐μ laser radiation are observed directly without the need of narrow‐band synchronous detection. The demonstrated performance of these room‐temperature diodes as wide‐band or heterodyne detectors of submillimeter radiation and their rugged construction make them superior to current point contact devices.

Submillimeter detection and mixing using Schottky diodes

Tunable narrow‐band cw generation of far‐infrared radiation has been achieved for the first time in the 70‐μm to 1‐mm wavelength region by noncollinear difference‐frequency mixing of two single‐mode CO2 lasers in GaAs at 80 K. Using a Schottky diode as a heterodyne harmonic mixer with a carcinotron local oscillator, the far‐infrared signal is shown to have a linewidth of less than 100 kHz and a fine tuning capability in excess of 50 MHz.

P. E. Tannenwald

Papers

Resonant tunneling through quantum wells at frequencies up to 2.5 THz

Quantum well oscillators

Far‐ir heterodyne radiometric measurements with quasioptical Schottky diode mixers

Submillimeter detection and mixing using Schottky diodes

cw generation of tunable narrow-band far-infrared radiation