Terahertz radiation mixer
View Patent ↗A terahertz radiation mixer comprises a heterodyned field-effect transistor (FET) having a high electron mobility heterostructure that provides a gatable two-dimensional electron gas in the channel region of the FET. The mixer can operate in either a broadband pinch-off mode or a narrowband resonant plasmon mode by changing a grating gate bias of the FET. The mixer can beat an RF signal frequency against a local oscillator frequency to generate an intermediate frequency difference signal in the microwave region. The mixer can have a low local oscillator power requirement and a large intermediate frequency bandwidth. The terahertz radiation mixer is particularly useful for terahertz applications requiring high resolution.
1. A terahertz radiation mixer for detecting an electromagnetic input signal having a radio frequency, comprising:
a field-effect transistor formed in a semiconductor substrate, comprising a heterostructure that provides a two-dimensional electron gas in the channel region between the source and the drain of the field-effect transistor, and a periodic grating gate comprising a plurality of fingers on a front surface above the channel region to modulate the electron density in the two-dimensional electron gas;
means for applying a gate voltage to the periodic grating gate;
a local oscillator signal, incident on the front surface, that couples to the two-dimensional electron gas in the channel region and has a frequency detuned from the radio frequency of the electromagnetic input signal, incident on the front surface, thereby providing an output signal having a difference intermediate frequency; and
means for detecting the output signal.
2. The terahertz radiation mixer of claim 1 , wherein the gate voltage is tuned to modulate the electron density under the grating gate so that the local oscillator signal resonates with a spatial frequency of a standing plasmon resonance of the two-dimensional electron gas in the channel region.
3. The terahertz radiation mixer of claim 2 , wherein the intermediate frequency has a bandwidth greater than 1 GHz.
4. The terahertz radiation mixer of claim 3 , wherein the intermediate frequency has a bandwidth greater than 10 GHz.
5. The terahertz radiation mixer of claim 1 , wherein the gate voltage is greater than the pinch-off voltage of the channel.
6. The terahertz radiation mixer of claim 1 , wherein the heterostructure comprises one or more quantum wells.
7. The terahertz radiation mixer of claim 6 , wherein the heterostructure comprises two quantum wells.
8. The terahertz radiation mixer of claim 1 , wherein the heterostructure comprises dissimilar III-V compound semiconductors, II-VI compound semiconductors, or Ge—Si alloys.
9. The terahertz radiation mixer of claim 8 , wherein the dissimilar III-V compound semiconductors comprise GaAs and AlGaAs.
10. The terahertz radiation mixer of claim 1 , wherein the electromagnetic input signal has a radio frequency of greater than 100 GHz.
11. The terahertz radiation mixer of claim 1 , wherein the detecting means comprises measuring the photoconductive response of the mixer to the local oscillator signal and the electromagnetic input signal.
12. The terahertz radiation mixer of claim 1 , wherein the detecting means comprises measuring the photovoltaic response of the mixer to the local oscillator signal and the electromagnetic input signal.
13. The terahertz radiation mixer of claim 1 , wherein the local oscillator signal is tunable.
14. The terahertz radiation mixer of claim 1 , wherein the power of the local oscillator signal is less than 3 mW.
15. The terahertz radiation mixer of claim 1 , wherein the local oscillator signal is provided by a Gunn oscillator, a solid-state multiplier, a backward-wave oscillator, a far-infrared gas laser, or a quantum cascade laser.
16. The terahertz radiation mixer of claim 1 , further comprising a back gate on the opposite side of the channel region from the grating gate.
17. The terahertz radiation mixer of claim 1 , further comprising a transparent gate on the front surface above the channel region.
18. The terahertz radiation mixer of claim 17 , wherein the transparent gate material comprises titanium.
19. The terahertz radiation mixer of claim 1 , further comprising a resistive shunt that connects the fingers of the grating gate in parallel and means for applying a shunt voltage to the resistive shunt.
20. The terahertz radiation mixer of claim 19 , wherein the shunt voltage is equal to the drain voltage.
21. The terahertz radiation mixer of claim 1 , wherein at least one of the fingers of the grating gate of the grating gate is individually biased.
22. The terahertz radiation mixer of claim 21 , wherein the at least one finger is biased to greater than the pinch-off voltage of the channel.