Virtual photoconductivity

We show that a semiconductor photodiode, or photoconductor, will exhibit a strong reactive response to optical radiation tuned to the transparent region just below the band gap. This response is due to the excitation of virtual electron-hole gas readily polarized by the dc electric field which can contribute a major change (>1) in the low-frequency dielectric constant. For ultrahigh speed signals, this nondissipative photoresponse can become similar to conventional dissipative photoresponse by real carriers excited above the band gap. The influence of this reactive response on zero-point electromagnetic fluctuations may aid the laboratory detection of Unruh-Davies-Fulling-DeWitt radiation.