When a crystal is a semiconductor – p-doped InAs in this case – the ‘surface states’ created by the left-over bonds that are inevitable at the surface 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.”
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 engineer this process, the UCLA team built a nano-antenna array on the surface of the InAs.
Incoming photons, 1550nm infra-red in picosecond pulses in the experiment, excite the antenna array to couple photo-excited surface plasmons to the surface region where the built-in electric field is maximized – 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. Coupled to the antennas by the built-in electric field, the resonant energy couples into the antennas and it radiated away, in this case as a pules with a spectrum spanning up to 4THz – wavelengths from 100μm to 1mm.
To make this effective the antenna geometry and semiconductor structure are chosen to maximise the spatial overlap between the built-in electric field and photo-absorption profiles
“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, a prototype crystal was bonded across the face of a cleaved optical fibre, with no intervening precision optics, 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.
Above is a simplification. The work is covered in depth in the Nature Communications paper ‘Wavelength conversion through plasmon-coupled surface states‘ – viewable in full without payment.