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We present novel photodetectors based on nitrogen-implanted GaAs. Compared to low-temperature-grown GaAs devices, our ion-implanted GaAs detectors exhibit better sensitivity without sacrificing the ultrafast photoresponse time.

We present an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) material, as a transducer in our electro-optic-sampling system. Compared to LiTaO3, DAST has the lower dielectric constant and significantly higher electro-optic coefficient, resulting in much improved performance of the system.

Polycrystalline silicon films, deposited on fused silica substrates, have been implemented as fast-response-time photoconductive switches. The switches were illuminated with 800-nm, 100-fs optical pulses from a Ti:sapphire mode-locked laser, and the photoresponse was observed using both a 34-GHz sampling oscilloscope and a subpicosecond electro-optic (EO) sampling system. We observed a 3-ps transient with the EO sampler, which is the fastest signal ever reported for this type of material. The switch also responded to 1.55-µm femtosecond laser pulses with transient signals as short at 36 ps, limited by the switch geometry.

The single-picosecond photoresponse of a YBa2Cu3O7−x photodetector has been measured using electro-optic sampling and modeled by the Rothwarf-Taylor equations describing nonequilibrium conditions in the superconductor.

This communication describes an interferometric and a polarimetric ultrafast electro-optic imager, each capable of imaging the voltage distribution over a rectangular region of an (opto)electronic device. The detector is a commercial 2-dimensional (2-D) charge-coupled-device (CCD) array having a limited dynamic range. Our analysis focuses on techniques that take advantage of the speed and convenience of a CCD measurement system and overcome the dynamic-range limitations of CCD’s. Experimental results obtained with a non-optimized polarimeter having a voltage sensitivity of −1.8 V are presented. Theoretical voltage sensitivity for an interferometric system is 270 mV.

Ultrafast electrical pulses are beginning to be used for performing broadband frequency domain characterization of circuits and for test and failure analysis of digital circuits.3 With frequency components in the terahertz regime, these pulses have been used for the characterization of transmission lines for high-speed interconnections on integrated circuits,1 and for measuring microwave S-parameters.2 Generation of ultrafast electrical pulses has been developed using pulsed lasers on semiconductors.4-6 Some of these techniques require special materials or geometries, which makes it difficult to use these methods as an analysis tool as in Ref. 2. However, edge illumination requires no special designs other than a coplanar transmission line.

Since the discovery of high-temperature superconductors (HTS), there have been intensive investigations of their transient response under ultrashort laser-pulse excitation. Recently, using the femtosecond pump-probe technique, Han et al.[1] observed a very fast (several picoseconds) optical response on YBa2Cu3Ox (YBCO) in the superconducting state under weak excitation and attributed it to the generation and recombination of quasi- particles. As the ambient temperature drops below the superconductor’s critical temperature (Tc), the transient reflectivity signal abruptly changes both its sign and temporal behavior—this striking behavior appears to be correlated to the onset of superconductivity. However, the physical significance of such an ultrafast response is still not clear, since the position of the Fermi level (EF) and the density of states near EF are not well known. Independently, a series of transient photovoltage measurements have also been performed on YBCO. Recent experimental work [2] demonstrated that for ultra-thin films the relative magnitude of the nonthermal component was enhanced and could be as fast as tens of picoseconds. The fact that the electrical response is at least one order of magnitude slower than the optical response suggests that more in-depth studies are necessary in order to find the ultimate speed of optoelectronic devices made of HTS.