A simpler source of THz radiation
Author: EIS Release Date: Aug 9, 2021
Plasmon-coupled semiconductor surface states can down-convert 1550nm optical wavelengths to terahertz frequencies four-orders of magnitude more efficiently than non-linear optical methods, according to UCLA.
When a crystal is a semiconductor – p-doped InAs in this case – the ‘surface states’ created by the left-over bonds that are inevitable on the outside of a crystal lattice can create high gradient electric fields as they interact with the semiconductor. In turn, incident photons can interact with this field.
“Incoming light can hit the electrons in the semiconductor lattice and move them to a higher energy state, at which point they are free to jump around within the lattice,” according to UCLA. “The electric field created across the surface of the semiconductor further accelerates these photo-excited high-energy electrons, which then unload the extra energy they gained by radiating it at different optical wavelengths, thus converting the wavelengths.”
UCLA InAs lattice THz nanoantenna converter
A titanium-gold nano-antenna (brown) on the surface of an InAs crystal – red loops are surface plasmons, blue ovals are dangling surface bonds, spots and circles are electrons and holes
To enhance this process, the UCLA team built a nano-antenna array on the surface of the InAs.
“The nano-antennas have two major functions: one is the efficient excitation of surface plasmon waves and the other is the efficient and broadband radiation of terahertz waves,” UCLA engineering professor Mona Jarrahi told Electronics Weekly.
Picosecond pulses of 1550nm infra-red provide photons for the experiment – with the short pulse duration spreading the spectrum. These photos stimulate the antenna array to couple photo-excited surface plasmons to the surface region where the built-in electric field is maximised – it is described as a “shallow but giant built-in electric field across the semiconductor surface” by the researchers.
The absorbed photons generate an electron gas under the antenna contacts, that resonates at beat frequencies from the mixing of different input pulse frequencies.
“Whenever two monochromatic optical waves [two different wavelength components of the 1550nm pulse in this case] have a spatial overlap along their propagation path, the superposition of their fields provides an effective electric field envelope with a beat frequency equal to their frequency difference,” explained Jarrahi.
The semiconductor structure and part of the antenna geometry are optimised to maximise the spatial overlap between the built-in electric field and photo-absorption profiles.
Coupled to the antennas by the built-in electric field, the resonant energy is radiated away, in this case as a pules with a spectrum spanning up to 4THz – wavelengths from 100μm to 1mm.
“Through this new framework, wavelength conversion happens easily and without any extra added source of energy as the incoming light crosses the field,” said research engineer Deniz Turan.
In an application demonstration, with no intervening precision optics, a prototype crystal was bonded across the face of a cleaved optical fibre to create the source for an endoscope-like THz analyser.
“Without this wavelength conversion, it would have required 100 times the optical power level to achieve the same terahertz waves, which the thin optical fibres used in the endoscopy probe cannot support,” according to UCLA.
The technique is also applicable to other conversions, spanning microwaves to far-infra-red wavelengths, according to the researchers.
The work is covered in depth in the Nature Communications paper ‘Wavelength conversion through plasmon-coupled surface states‘ – viewable in full without payment.