Electric-Dipole Contributions to Resonant Far-Infrared Difference-Frequency Mixing in InSb

Calculations have been made of the inversion-asymmetry-induced electric-dipole moment of the spin-resonance conduction-band transition in InSb. This moment leads to a second-order nonlinear polarization which resonates for difference-frequency mixing when the incident radiation frequencies differ by the spin-flip frequency. Comparison with experiments confirms that the dominant contribution to far-infrared difference mixing is from the magnetic-dipole source. However, it is shown that a value for the InSb-band parameter C may, in principle, be determined from the results of mixing experiments. It is also noted that electric fields in excess of 5 V/cm^-1 should induce stronger mixing than given by the preceding two mechanisms. Observations of far-infrared radiation tunable over the range 85-105 cm^-1 and at power levels of up to 10 muW are reported. These are obtained by mixing 10.6-mu m TEA-CO₂-laser radiation with the tunable Stokes radiation produced by spin-flip Raman-laser action in an InSb sample. It is argued that considerable improvement in the power levels should be achieved by modifying the experimental setup.