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In this work, we study photoconductive sampling of RF signals using metal semiconductor metal structures with low-temperature grown GaAs (LTG-GaAs) as active media. The sampler is a photoswitch (PS) embedded in a microwave coplanar waveguide (CPW) and it is triggered by ultra short optical pulses. Both the design of the electrodes and the semiconductor (SC) characteristics are optimized to ensure high RF isolation in the dark state [1], large electrical bandwidth and a short response-time to optical exciting pulses. We present here a complete set of results concerning device characterization and demonstrate sampling operation. DC electrical characterization, optoelectronical impulse response and sampling of synchronized RF signals are obtained. From these data, we derive important SC material parameters such as electron mobility and lifetime. Finally we also investigate and explain through a phenomenological model the nonlinearities observed in the optoelectronical response of the sampler.Among the SC material properties, dark conductivity, carrier lifetime and mobility play a major role in optical sampling of mm-wave. For such a purpose it has been demonstrated that LTG-GaAs grown at 250°C with 1 ps response time can be used but at the price of rather low mobility [2]. For our dedicated RF application, i.e. sampling of signal up to 40 GHz, we have chosen higher growth temperatures (Tg 320 or 430°C) that lead to electron lifetime of 7 or 60 ps as determined by optical pump -THz probe experiments. In order to decrease the equivalent serial capacitor of the PS [1] the GaAs wafer is chemically etched at the PS location before growing the LTG-GaAs layer. Finally, using standard photolithographic techniques, Au electrodes are deposited onto the layer to form the CPW with the 50x70 pim2 PS.DC electrical characterization of the devices shows nonlinearity of the I-V curves that is mainly attributed to space-charge limited current (SLC). Due to this effect, the total current density results from both carrier photo-injection and injection at the electrodes, whose behavior is depicted by Poisson's law and photoconduction equation. Comparing experimental data with a phenomenological model of SLC for different CW optical powers (Fig. 1) yields to an electron mobility of 2400cm2/V/s (Tg=320°C) and a defect concentration of Nd=2x1016 cm-3. These values are well consistent with the ones that can be found in the literature for LTG-GaAs grown at such average temperatures. 100 3 D2 < 1 Zy 0 D LU Measurement (Popt=3mW) BIAS VOLTAGE (V) TIME (ps) Fig. 1: I-V curves for different optical powers. Fig. 2: Sampling of 10 and 20 GHz signals Using 100 fs laser pulses, we perform dynamical characterization of the fabricated structures that show transient electrical responses of 33 ps under a 20 V biased voltage. Microwave characterization of the device in the dark shows that only -35 dB of the incident 20 GHz signal is detected at the output of the CPW, confirming the enhanced RF isolation and the small equivalent capacitor of the PS (1m fF...

In a previous presentation we have demonstrated the optoelectronic sampling of RF signals using photoswitches (PSW) made from low-temperature grown (LTG) GaAs [1]. The main characteristics that are required for better sampling performances are high optical sensitivity, high bandwidth, large ON/OFF ratio and electrical linearity response . They are closely linked both to the LTG-GaAs properties and to the geometrical design of the RF waveguide . We tested a large set ofPSW with different configurations and we present here a complete set of experimental and simulation results. Specially, we point out that using the device with a shortest gap length leads to the generation of frequency harmonics while increasing RF input electrical power. Hence, the spurious free dynamic range (SFDR) is reduced when the RF power is increased up to 13 dBm. We suggest that this results from an electrical nonlinear response which is partially due to carrier velocity saturation in LTGGaAs. In accordance with [2, 3], we model it and find an electric field saturation of 14 kV/cm.The tested devices are made of LTG-GaAs layers grown at different temperatures with a gold coplanar waveguide deposited onto. The central electrode is interrupted by a gap that acts as the PSW. This latter is triggered by femtosecond pulses (60 fs FWHM, 75 MHz repetition rate) at A=800 nm delivered by a Ti:Sa laser. The sampling experiment is performed synchronously using RF sources locked in phase with the laser pulse or in a free running sampling manner. For the synchronous experiment, an optical delay line allows us to vary the phase in between the RF source and the pumping optical pulses, leading to a reconstructed RF signal [1].First, we checked the linearity of the optoelectronic sampler by varying the RF input power from -30 dBm to 13 dBm. We used a 10 GHz sinusoidal RF source locked to the laser clock and a set of calibrated RF attenuators .

In this paper we show how the characteristics of semiconductor (SC) material with very short carriers trapping time impact the performances of ultrafast optoelectronic devices used for generation and detection of radiofrequency or TeraHertz signals. The impact of properties such as concentration of deep‐level trapping centres, free carrier drift velocity saturation and long time relaxation of deep‐level trapping centres are studied. For devices based on low‐temperature grown GaAs we observe that working above typical ratings in term of input optical power, laser repetition rate or biasing electrical field lead, to reduction of electrical signal bandwidth and amplitude and linearity of RF signal samplers. All our experimental results are sustained by numerical simulations that allow us to quantify the limitations induced by SC material properties. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)