Time-Resolved Photocurrent Measurements
Already today's technologies heavily rely on materials with unique optoelectronic properties. As these materials will only continue gaining importance for future technologies, active research in advanced materials with tailored optoelectronics is indispensable.The Lake Shore Cryotronics CRX-6.5K Probe Station is the core of our setup enabling time-resolved photocurrent investigations. This way, measurements may be conducted at temperatures between 8.5K and 350K, either under vacuum or in an inert gas atmosphere. To investigate optoelectronic properties, various laser solutions are available.
Nanosecond Photocurrent Set-up
Steady-state photocurrents can be studied using the CW-mode (continuous wave mode) of a 405nm, 636nm, 780nm, or 847nm laser. The utilization of fast on/off switchable lasers furthermore allows for time-resolved photocurrent investigations. Two options, a 635nm and a 1310nm laser are available. These enable photocurrents based on short rectangular pulses. Alternatively, picosecond pulsed laser diodes with 635nm and 780nm may be used. Impulse responses of materials of interest may thus easily be analyzed. This extensive collection of laser diodes allows for analyses ranging from microsecond photocurrents all the way down to short impulse responses lasting merely nanoseconds. Thus, a multitude of possible optoelectronic investigations on diverse materials is reached. As a result, a wide range of time-resolved photocurrents stretching across multiple orders of magnitude can be achieved.
A Lock-In amplifier with Boxcar Averager function (UHFLI, by Zurich Instruments) completes the Probe Station setup. Both, a trans-impedance amplifier (1GHz High Speed Amplifier HSA-Y-1-60, by FEMTO) and two source meters (2634B & 2636B SYSTEM Source Meter, by Keithley), are readily available to be connected to the setup, too. The highest obtainable resolution is 600MHz.
In summary, this setup allows for an easy, broad, and in-depth investigation of important optoelectronic properties of materials that could be used in components of tomorrow's electronic devices.
Group members in charge: Christine Schedel & Fabian Strauss
Two-pulse coincidence technique and ASOPS set-up
To investigate the intrinsic response time of a photodetector material, being the physical limit of the response, the two-pulse coincidence technique is used. Here, the device is illuminated by two ultrashort (fs) laser pulses, separated by a delay time Δt. The corresponding photoresponse signal is detected as a function of the delay time Δt between the two pulses, revealing a dip of the signal at coinciding pulses (Δt = 0), due to a non-linear photoresponse. From the exponential increase of the signal with increasing Δt, the intrinsic response time can be determined. Thus, the first pulse induces a change of the photoactive material, which is probed by the second pulse.
Instead of using a typical mechanical delay stage to generate the delay time between the laser pulses, we use asynchronous optical sampling (ASOPS). Two individual coupled lasers with a slight offset in repetition rate (100 MHz ± Δf) are used. This creates a continuously increasing and thus accumulating delay time Δt, creating sweeps of the entire delay window of 10 ns within milliseconds. Accordingly, the combination of the two-pulse coincidence technique and ASOPS allows response time measurements over a huge delay window of 10 ns with unprecedented measurement speed.
Our set-up consists of two coupled 1560 nm pulsed lasers (~65 fs) with high power of ~90 mW (Optical Sampling Engine OSE, Menlo Systems), incorporated in our low-temperature probe station (Lake Shore Cryotronics CRX-6.5K). Data acquisition is performed by a lock-in amplifier with a periodic waveform analyser (UHFLI, Zurich Instruments). With this, high-speed measurements with a resolution of down to 10 fs can be performed to reveal ultrafast intrinsic response times of novel nanomaterial-based photodetectors.
Group members in charge: Andre Maier & Fabian Strauss