Photomixing is a process that uses two laser beams with slightly different optical frequencies to generate radiation at the difference frequency. Depending on the photomixing process and method, the frequency of the radiation produced by this method can vary from near DC to several THz. An advantage of this technique is the ability to produce high spectral purity radiation over a very large range of frequencies. A disadvantage is the typically low power levels of less than 10−8 W.
Principle Photomixing is the generation of continuous-wave radiation through a process in which two frequency-offset lasers with aligned polarization illuminate a photomixer. The total electrical field of the exciting beam in the photomixing material may be written as:
E ( t ) = ∑ i = 1 2 A i cos ( ω i t + ϕ i ) e ^ i {\displaystyle E(t)=\sum _{i=1}^{2}A_{i}\cos \!{\big (}\omega _{i}t+\phi _{i}{\big )}\,{\hat {e}}_{i}}
where ω i {\displaystyle \omega _{i}} is the angular frequency of the laser beam i {\displaystyle i} , A i {\displaystyle A_{i}} its amplitude, e ^ i {\displaystyle {\hat {e}}_{i}} the unit vector along the direction of polarization of the laser beam i {\displaystyle i} , and ϕ i {\displaystyle \phi _{i}} the phase difference between the two beams. Depending on the material employed, photomixing can occur via two different processes: a non-linear optical process where the laser beams generate radiation at the difference frequency or a linear optical process where the laser beams modulate the conductance of an antenna integrated into a semiconductor material. Non-linear optical process. In nonlinear dielectric crystals, photo-mixing is a second-order nonlinear process that produces a nonlinear field
P ( 2 ) ( t ) = χ ( 2 ) : E ( t ) E ( t ) {\displaystyle P^{(2)}(t)={\boldsymbol {\chi }}^{(2)}:E(t)E(t)}
where χ ( 2 ) {\displaystyle {\boldsymbol {\chi }}^{(2)}} is a conversion tensor, and “:” denotes the tensorial product. The radiation generation by the non-linear process results from the beating term (frequency difference) Δ ω = | ω 1 − ω 2 | {\displaystyle \Delta \omega =\left|\omega _{1}-\omega _{2}\right|} . As the two laser beams and the generated radiation propagate in the crystal the intensity of the generated radiation is:
I ( Ω ) ∝ I 1 I 2 n ( Ω ) d e f f 2 sin 2 ( Δ k z / 2 ) ( Δ k / 2 ) 2 {\displaystyle I(\Omega )\propto I_{1}I_{2}\,n(\Omega )\,d_{\mathrm {eff} }^{\,2}\,{\frac {\sin ^{2}(\Delta k\,z/2)}{(\Delta k/2)^{2}}}}
… excerpt ends here. Continue reading the full article.
