Optical Rectification Photodetectors

For fundamental reasons the frequency response of conventional absorption-based photodetectors decreases with RF modulation frequency, limiting their usefulness at terahertz (THz) frequencies. Optical rectification (OR) is a non-absorptive, parametric process based on the 2$^{nd}$-order nonlinear electrical susceptibility, $\chi^{(2)}$. When OR is used to perform photodetection (ORPD), the response time is limited fundamentally by the bandwidth of optical nonlinearity ($\gt 200$ THz). The differential responsivity of an ORPD increases with RF modulation frequency, making it increasingly attractive as the frequency increases into the THz. Additionally, ORPD’s are amenable to significant optical preamplification without saturating the response, allowing orders of magnitude increase in output current. We develop a traveling-wave model for ORPD which is confirmed by experiments and accurately predicts the differential responsivity. We measure world record differential responsivities at 16 GHz and 500 GHz of 5 $\times 10^{-5}$ A/W and 3.3$\times 10^{-4}$ A/W, respectively.