Terahertz waves, a form of electromagnetic radiation with a frequency ranging from a few hundreds to several thousands of Gigahertz, can be generated by illuminating a semiconductor device, called photomixer, with two diode-lasers emitting near-infrared or visible photons. The Terahertz-Photonics team at IEMN Laboratory has demonstrated a novel type of Terahertz photomixer relying instead on mid-infrared diode-lasers [1]. This opens up new perspectives in the field of Terahertz photonic generation.

Terahertz radiation with a frequency spanning from a few 100 GHz up to 1THz is particularly sought after for the next generation of wireless communication, also known as 6G. To this end two types of approaches are presently developed, both based on semiconductor technology. The so-called “electronic approach” exploits the non-linear electrical behavior of semiconductor/metal junctions to generate high order harmonics of a microwave signal in the 10GHz range.

The “photonic approach” exploits instead the optical absorption process in a semiconductor, where an incident photon with a frequency in the near-infrared/visible range (equal to ~3000THz at l = 1mm) excites an electron from the valence to the conduction band, giving rise to an electrical “photo-current”. Since the magnitude of the photo-current is proportional to the square of the electric-field of the incident near-infrared radiation, this process is inherently non-linear. This means, in particular, that if the semiconductor is simultaneously illuminated by two laser beams with frequencies n₁ and n₂, the generated photocurrent will oscillate at n_THz = n₁ – n₂. The value of n_THz can be tuned across the entire Terahertz range by simply tuning n₁ (or n₂), which is easily accomplished by changing the temperature or the drive current of a diode laser. In a photo-mixer device, the oscillating current is then coupled to the electrodes of an antenna, which allows the emission of Terahertz radiation into free-space.

Improving the near-infrared  Terahertz power conversion efficiency is crucial to optimize the performance of a photomixer. This parameter is limited by the maximum value of the photo-current that can be generated for a given incident near-infrared power, which, in turns, is fundamentally limited by the fact that for 1 incident photon absorbed, only 1 electron can be generated. As a result, assuming an equal amount of incident power, it is more efficient to illuminate a photo-mixer with a pair of mid-infrared (f ~ 300THz at l = 10mm), rather than near-infrared diode-lasers. Indeed, in the former case the number of photons (hence of photo-generated electrons) will be larger by a fraction equal to the ratio between the near-infrared and the mid-infrared frequencies, i.e. by approximately a factor 10. This will generate ten times larger photo-current.

To demonstrate a mid-infrared pumped photo-mixer, as an absorbing material we chose to exploit a so-called multi quantum-well (QW) heterostructure, where electrons in the conduction band see a series of 6.5nm-wide potential GaAs QWs, separated by 40nm Al_0.2Ga_0.8As potential barriers. As shown in the top-right inset of the Figure, in this artificial material photon-absorption takes place between two one-dimensional electronic levels. The resulting photo-current flows in the continuum of states above the potential barriers. As shown in the top-left inset, the heterostructure, consisting of 7 QWs, is sandwiched between a bottom gold layer and a top, gold square-patch that are used as electrical contacts. These metal layers also form a so-called “patch-antenna resonator”, which allows to strongly confine the mid-infrared radiation incident perpendicularly to the surface (the electric-field amplitude of the excited electromagnetic mode is shown in color scale). In this way the absorption of the heterostructure, hence the magnitude of the photocurrent, is significantly boosted compared to a conventional photoconductor architecture. The bottom inset shows an electron microscope image of the heart of the photo-mixer, consisting of a matrix of 9 patch-antenna resonators connected in parallel by suspended gold-wires. In turn, the common bottom gold layer and the top gold patches are electrically connected to two the arms of a spiral Terahertz antenna that allows the generated photocurrent to emit Terahertz radiation in free space.

By illuminating the device with a pair of mid-infrared semiconductor lasers operating at 10mm wavelength we have measured emission in free space up to 1THz, demonstrating the first mid-infrared pumped, Terahertz photo-mixer. Although presently the power conversion efficiency is still far from that of well-established near-infrared photo-mixers, however there are many open routes for the optimization of this first prototype, paving the way for the development of a new generation of power-efficient, mid-infrared pumped THz sources.