Photodetection via Optical Rectification of Terahertz-Modulated Optical Carriers

For fairly fundamental reasons the frequency response of conventional, absorption-based photodetectors decreases with RF modulation frequency, which means they are becoming increasingly hard pressed to meet the need for operation at terahertz (THz) frequencies. Optical rectification (OR) is a non-absorptive, parametric process based on the 2
^nd
-order nonlinear electrical susceptibility, χ
⁽²⁾
. When OR is used to perform photodetection (ORPD), the response time is limited fundamentally by the bandwidth of optical nonlinearity (>200 THz). The differential responsivity of an ORPD increases with RF modulation frequency, which makes it increasingly attractive to consider as the frequency increases into the THz. Additionally, OR photodetectors are amenable to significant optical preamplification without saturating the response, allowing orders of magnitude increase in the output current. We develop a lumped-element model of an ORPD, which is confirmed by experiments, that shows the increase in differential responsivity with frequency, but only gives an order-of-magnitude estimate of the differential responsivity. We then develop a traveling-wave model of an ORPD, which is confirmed by experiments, that more accurately confirms the differential responsivity. We measure world record differential responsivities at 16 GHz and 500 GHz of 5 × 10
⁻⁵
A/W and 3.3 × 10
⁻⁴
A/W, respectively.