Photomixing
Encyclopedia
Photomixing is the generation of continuous wave terahertz radiation
from two lasers. The beams are mixed together and focussed onto a photomixer device which generates the terahertz radiation.
It is technologically significant because there are few sources capable of providing radiation in this waveband, others include frequency multiplied electronic/microwave sources, quantum cascade laser
and ultrashort pulse
d lasers with photoconductive switches as used in terahertz time-domain spectroscopy. The advantages of this technique are that it is continuously tunable over the frequency range from 300 GHz to 3 THz (10 cm-1 to 100 cm-1) (1 mm to 0.1 mm), and spectral resolutions in the order of 1 MHz can be achieved. However, the achievable power is on the order of 10-8 W.
lasers with identical polarisation are required, the lasers with frequency
ω1 and ω2 are spatially overlapped to generate a terahertz beatnote. The co-linear lasers are then used to illuminate an ultra fast semiconductor material such as GaAs
. The photonic absorption and the short charge carrier lifetime results in the modulation of the conductivity at the desired terahertz frequency ωTHz = ω1 - ω2. An applied electric field allows the conductivity variation to be converted into a current which is radiated by a pair of antenna. A typical photoconductive device or 'photomixer' is made from low temperature GaAs
with a patterned metalized layer which is used to form an electrode array and radiating antenna.
The instrument can be divided into the following functional units:
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
from two lasers. The beams are mixed together and focussed onto a photomixer device which generates the terahertz radiation.
It is technologically significant because there are few sources capable of providing radiation in this waveband, others include frequency multiplied electronic/microwave sources, quantum cascade laser
Quantum cascade laser
Quantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.Unlike typical...
and ultrashort pulse
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...
d lasers with photoconductive switches as used in terahertz time-domain spectroscopy. The advantages of this technique are that it is continuously tunable over the frequency range from 300 GHz to 3 THz (10 cm-1 to 100 cm-1) (1 mm to 0.1 mm), and spectral resolutions in the order of 1 MHz can be achieved. However, the achievable power is on the order of 10-8 W.
Principle
Two continuous waveContinuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
lasers with identical polarisation are required, the lasers with frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
ω1 and ω2 are spatially overlapped to generate a terahertz beatnote. The co-linear lasers are then used to illuminate an ultra fast semiconductor material such as GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
. The photonic absorption and the short charge carrier lifetime results in the modulation of the conductivity at the desired terahertz frequency ωTHz = ω1 - ω2. An applied electric field allows the conductivity variation to be converted into a current which is radiated by a pair of antenna. A typical photoconductive device or 'photomixer' is made from low temperature GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
with a patterned metalized layer which is used to form an electrode array and radiating antenna.
High resolution spectrometer
The photomixing source can then form the basis of a laser spectrometer which can be used to examine the THz signature of various subjects such as gases, liquids or solid materials.The instrument can be divided into the following functional units:
- Laser sources which provide a THz beatnote in the optical domain. These are usually two near infrared lasers and maybe an optical amplifier.
- The photomixer device converts the beatnote into THz radiation, often emitted into free space by an integrated antenna.
- A THz propagation path, depending on the application suitable focusing elements are used to collimateCollimated lightCollimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...
the THz beam and allow it to pass through the sample under study.
- Detector, with the relatively low levels of available power, in the order of 1µW, a sensitive detector is required to ensure a reasonable signal to noise ratio. Si bolometers provide a solution for in-coherent instruments. Alternatively a second photomixer device can be used as a detector and has the advantage of allowing coherent detection.