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Electrical and Computer Engineering

College of Engineering and Applied Science

Dr. Jon Pikal's Research

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Optoelectronic Materials and Devices

The goal of the optoelectronics research lab is to investigate new materials and structures to improve the performance of optoelectronics devices.  Optoelectronic devices include light emitting devices such as LEDs and lasers as well as photodetectors or solar cells which convert light into electrical energy. Optoelectronic devices are an important class of devices and are used in many different systems. Lasers and photodetectors are used in fiber optic communications systems, optical data storage systems (CD, DVDs, and BlueRay), and numerous sensing applications. LEDs are found in today’s advanced lighting and displays systems while solar cells are one of the many alternative energy sources we are counting on to help reduce pollution and achieve long term energy independence.
University of Wyoming Electrical and Computer Engineering Faculty Dr. Jon Pikal's research projectsIn the past, with the support of the DOD, we have studied charge carrier dynamics in layers of epitaxial grown quantum dots (QDs) to be used as the active gain material in semiconductor lasers. In these studies we measured the charge carrier recombination dynamics, separating the defect, radiative and Auger recombination current contributions to the threshold current of the laser.  We also looked at the role that both free carriers and excitons play in determining the optical gain and lasing characteristics of the laser and how to model these effectively.
More recently we have moved away from semiconductor laser research and into research on novel materials for nanostructured solar cells. This work is supported by the DOE with a primary goal of investigating different materials for use in quantum dot sensitized nanowire solar cells. In these cells, quantum dots are used as the light absorbing material, which are attached to a substructure of nanowires. After the light is absorbed in the quantum dots the excited electrons are separated from the holes by fast transfer to the nanowires. The nanowires then provide an efficient transport medium to carry the electrons to the device contact. This project is a very large multi-investigator grant with collaborators from Physics, Chemistry, and Geology all here at UW. The Quantum dots and nanowires are grown by physics and chemistry faculty while the primary job of our group is to provide optical characterization of these materials. As part of this grant, funds were provided to purchase the parts for an ultrafast pulsed laser measurement system which we have now setup in the Optoelectronics lab in the Department of Electrical and Computer Engineering at UW. The heart of the system is a Coherent Libra regenerative amplifier which generates 100fs, 4mJ pulses at a 1kHz repetition rate and 800nm wavelength. The output of the Libra is fed into a TOPAS Optical Parametric Amplifier (OPA) which maintains the 100fs pulse width but converts these pulses to any wavelength from 290nm to 2.5um. The ultrafast pulses from this laser system are used to excite the quantum dots and nanowires for study. Three basic types of measurements are performed with this system, Photoluminescence (PL), Time Resolved Photoluminescence (TRPL), and pump-probe Transient Absorption (TA).
University of Wyoming Electrical and Computer Engineering Faculty Dr. Jon Pikal's research projectsPL spectral measurements are conducted using the above laser system as a wavelength tunable excitation source and a ½ meter monochromator, UV enhanced Silicon detector, and lock-in amplifier for measurement. These spectra give information on the size and size distribution of the quantum dots, the type and size of the bandgap energy of the nanowires, as well as the optical quality of either material through the observation of any deep level defect or surface states. A similar setup is used for the TRPL measurements except that a very fast photomultiplier tube is substituted for the Si detector and Time Correlated Single Photon Counting is used to count and record the arrival time of single photons. The result is a very accurate map the decay of the photoluminescence from the sample. Our setup has a system response time of ~30ps and thus can be used to study the details of even very fast PL decays. Also due to the single photon counting nature of these measurements even very weak transitions can be studied accurately.
The final type of measurement conducted in our lab is pump-probe transient absorption. Currently we are setup to do degenerate pump-probe (same wavelength for pump and probe beam) but soon will have the capability to do non-degenerate two color pump-probe as well as white light probe TA. In the degenerate pump-probe measurement, the output of the OPA is split into two beams (pump and probe), with the probe being ~10% of the pump power. The pump beam is then synchronously chopped at 500Hz and sent down a programmable optical delay line with 1um resolution, which allows the time relationship between the pump and the probe to be adjusted with ~7fs resolution. The pump and probe beam are then focused on the sample and a simple detector can be used to sample the probe beam but at the pump frequency of 500Hz. The result is a measure of the pumps effect on the probe, and is the differential transmission of the sample at that delay time. The mechanical delay stage is then scanned across the desired time range and the differential absorption as a function of time is recorded with as good as ~140fs time resolution. These measurements are not only useful when the time constants expected are faster than can be resolved with TRPL but also allows us to watch carriers enter and or leave a particular energy state depending on the probe wavelength. In the near future we will be doing non-degenerate as well as white light probe TA. This will allow us to monitor the time evolution of the charge carriers in the material across the entire probe spectrum allowing an even better picture of what is going on inside the material on the sub-picosecond time scale.   

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