1.9 picosecond high-sensitivity sampling optical temporal analyzer

The rapid development of ultrafast sciences places an increasing demand on high-speed, high-dynamic range measurement techniques. An optical correlation technique based on photoconductivity for measuring ultrafast laser pulses was first demonstrated by Austin et aL1 However, due to relatively low efficiency of.both signal detection and sampling, such a system was not able to measure weak optical signals. Recently, Li et aL2 showed that a Schottky photodiode with the integration of a microwave detector can measure weak optical laser pulses with picosecond resolution. In this letter, we report an all solid-state circuit that we believe is the best in terms of speed and dynamic range for performing optical correlation measurements. Our device structure is based on Ketchen et aZ.‘s3 original work with incorporation of a highspeed, high-responsivity metal-semiconductor-metal (MSM) interdigitated photodetector/switch developed in our laboratory. As the result, we obtained a novel sampling optical temporal analyzer (SOTA) with high sensitivity. We were able to measure extremely weak signals with picosecond resolution. We reported earlier4 that an interdigitated MSM photoconductive detector grown on low-temperature (LT) GaAs can function as either a high-sensitivity photodetector or photoswitch. As a detector, operating at a low input optical power level of 4 ,!.i,W (0.04 pJ/pulse), the device has a response time of 1.2 ps and responsivity of 0.1 A/w. As input optical power level increases to >l mW (10 pJ/pulse), the same device functions as high-sensitivity photoconductive switch. With an input power of 2.2 mW (22 pJ/pulse), the gap resistance is driven from lo7 to 30 !I& while the time resolution increases to only 1.5 ps. Our experiment takes advantage of this unique functionality for picosecond measurement of low-intensity optical signals. Figure 1 shows the schematic diagram of the SOTA experiment. The interdigitated structure bridging between the coplanar transmission lines functions as the photodetector. We placed interdigitated detector between coplanar transmission lines instead of using photoconductive gap switching to assure good coupling of the generated electrical pulse to the propagating mode. The second interdigitated structure, connected between the ground electrode of the transmission lines and the sampling line, functions as photoswitch to