The third-order susceptibility can couple an optical field and a DC electrical field to give an output at the second-harmonic frequency of the optical field.

This effect is known as electric-field induced second-harmonic generation(EFISHG).

The diagram below (a) shows a planar silicon photodiode structure and illustrates the practicality of EFISHG in three possible scenarios:

2. 

   (b) Normal incidence requires tensor elements that are necessarily zero in silicon and yields no EFISHG.
   

3. (c) p-polarised incident light can be coupled to the EFISHG field by both non-zero elements of the third-order susceptibility tensor.
   

4. (d) s-polarised light can only be coupled to the EFISHG field by the weaker off-diagonal component of the third-order susceptibility tensor. 
   

The figures above show the results from our EFISHG experiments.  The raw data recorded on a lock-in amplifier is shown in (a) where the horizontal axis is increasing reverse bias voltage in steps of 0.2V. The decrease in the signal above ~1V corresponds to Zener breakdown in the junction which reduces the field across the junction and causes the EFISHG signal to fall.  (b) shows a quadratic fit to the EFISHG signal data for voltages in the range below breakdown.  We chopped the current to the junction at 400Hz and monitored the EFISHG signal when the bias voltage was turned off (0V) and when it was 1V.  The EFISHG signal (c) shows the chopped signal clearly after 512 averages on a digitizing oscilloscope.