This paper is a review of the history of the measurement of coherent optical frequencies. Since coherent optical frequency implies a laser device, this is therefore a review of laser frequency measurement. The development of frequency measurement from the Cs frequency standard to the visible is traced. Two related aspects of optical frequency measurements, the speed of light and the redefinition of the meter, are also discussed.

Optical frequency measurements

Video detection experiments from 0.7 to 30 THz and mixing experiments at 30 THz have been performed with GaAs Schottky‐barrier diodes and W‐Ni metal‐insulator‐metal (MIM) diodes. Above approximately 12 THz the MIM diode is the more sensitive video detector (a factor of 5–10 at 30 THz). Difference frequencies up to 34 GHz were generated by mixing the radiation of two adjacent CO2 laser lines and a microwave source in a Schottky‐barrier diode. The dependence of the mixing signal on different diode parameters (doping density, plasma frequency, cutoff frequency) and on bias current was measured. Compared with MIM diodes at 30 THz the Schottky‐barrier diodes are less efficient for mixing (a factor of 104). The results suggest that thermionic emission is the dominant physical mechanism responsible for video detection and mixing at 30 THz.

Video detection and mixing performance of GaAs Schottky‐barrier diodes at 30 THz and comparison with metal‐insulator‐metal diodes

The frequency response and the frequency‐mixing properties of the tunneling junction of a scanning tunneling microscope (STM) were investigated using the radiation of two CO2 lasers. Difference frequencies of up to 90 MHz were detected over a wide range of bias voltages and tunneling currents. At tunneling currents typical of STM operation the beat signal decreases as the bias voltage is increased. However, this dependence is reversed at higher tunneling currents, where the STM begins to resemble a point‐contact metal‐insulator‐metal diode. For low tunneling currents the generation of the beat signal is attributed mainly to thermally assisted tunneling.

Laser‐frequency mixing in the junction of a scanning tunneling microscope

Metal/insulator/metal (MIM) diodes work on the inherently fast mechanism of tunneling and have been used in a number of high-frequency applications. This makes them a promising candidate as the rectifying element in rectenna solar cells. In this chapter we describe the operating mechanism of these diodes and review the work done on using them in rectennas. We also provide a simulation methodology to accurately model low-barrier MIM diodes that are used in rectennas. Analytical models based on the WKB method for probability of tunneling are not well suited for analyzing such diodes. We simulate single-insulator (MIM) diodes with varying asymmetry to point out their limited nonlinearity. We also simulate double-insulator (MIIM) diodes that have improved nonlinearity as compared to MIM diodes providing a path for designing more efficient multi-insulator diodes.

http://ecee.colorado.edu/~moddel/QEL/Papers/Grover13c.pdf

Metal Single-Insulator and Multi-Insulator Diodes for Rectenna Solar Cells

A nanoparticle separated from a metallic surface by a few-nanometer thick polymer layer forms a nanoscale junction, or nanogap. Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. The four-wave mixing signal generated by this illumination of the nanogap provides a simultaneous monitoring of the collapse, increasing dramatically upon gap closure. Collapse is irreversible, occurring with simultaneous ablation of the dielectric from the junction.

Optically-driven collapse of a plasmonic nanogap self-monitored by optical frequency mixing.

Point-contact diodes based either on the mechanism of electronic tunnelling through an insulating barrier or on the thermoelectric effect of hot carriers in heavily doped semiconductors are described. The requirements necessary for these devices to be used as harmonic generators and mixers in the 2–200 THz region are emphasized, and some new developments are reported. Outstanding characteristics of the devices which use either the semimetal antimony or the semiconductors SnTe or Bi2Te3 as a diode base are high detection sensitivity and high mechanical stability over long periods of use either as harmonic generators or as mixers.

Point-contact diodes

A number of different point-contact diodes were made by mechanically contacting various metals, semimetals and semiconductors with different metallic whiskers. The diodes have been tested as frequency mixers and harmonic generators using CO2 and CO lasers. The results suggest that two mechanisms, electron tunnelling through a potential barrier and another slower process are responsible for the operation of the diodes. Some of the new devices are electrically and mechanically more stable than the commonly used tungsten-nickel diode and show better responsivity.

H. D. Riccius

Papers

Point-contact diodes

Experiments with point-contact diodes in the 30–130 THz frequency region