Laser System


Our research group is applying ultrafast optical techniques to the study of rapid processes in semiconductor materials of interest for a variety of applications, including solar cells, photonic and spintronic devices, and next generation computing technologies that would exploit quantum mechanical effects. Our experiments all utilize special lasers that produce short pulses of light with a duration of about 100 femtoseconds. In order to put this time scale into perspective, comparing 100 femtoseconds to 1 second is equivalent to comparing 1 centimeter to the distance between the earth and the sun. These so-called ultrafast laser pulses may be used to study extremely rapid events, such as the motion of electrons in solids. Our recent research projects have focused on charge and spin dynamics in perovskite photovoltaic materials, quantum state manipulation in semiconductor quantum dots though pulse engineering, and the study and control of magnetic behavior in diluted magnetic semiconductors. See our most recent papers in each of these areas on Research Gate.

Ultrafast Optical Studies of Perovskite Photovoltaic Materials

The organic-inorganic perovskite class of semiconductors has recently gained interest for the development of low-cost solar cells. The perovskite absorber material CH3NH3PbI3 has led to the fastest rise of solar cell efficiencies of any technology in history, reaching par with conventional silicon solar cells (that present-day store-bought solar panels are made from) already a few years ago. This has paved the way for the exploration of other perovskite materials such as 2D heterostructures and engineered alloys for optimizing solar cell characteristics. Unlike silicon solar cells, perovskite cells can be solution processed, dramatically reducing the cost of fabrication, and tandem cells involving silicon and/or solar integrated building materials may be possible. This class of materials is also of interest for the development of low-cost optoelectronics (e.g. photosensors, lasers) and spintronic devices as they possess good charge transport properties and a large spin-orbit interaction. Considerable research is needed to understand the photophysical properties in order to realize these applications. Our research group is currently applying a variety of ultrafast techniques to studying these materials, including femtosecond four-wave mixing spectroscopy and spin-dependent pump probe spectroscopy.

Quantum State Engineering

Optical pulse shaping provides a versatile approach to tailoring the Hamiltonian governing the interaction of light with matter. The central idea is to manipulate in an arbitrary way the phase of the control laser pulse to achieve a desired final quantum state of the system. Together with powerful adaptive feedback algorithms, this approach is now used routinely in the control of a variety of physical processes. For example, it has been used to target specific pathways in chemical reactions and energy transfer channels in light-harvesting complexes, to amplify the optical response of specific molecules for low level detection, and for the optimization of high harmonic generation. Quantum information science provides a natural arena for applications of optimal quantum control. Pulse engineering in this case provides a way to optimize the speed and fidelity of elementary quantum gates, may enable parallel quantum processing, and provides approaches for computational architectures based on complex instruction set quantum computing. Our research group is pursuing quantum state engineering in semiconductor quantum dots using femtosecond pulse shaping as a control optimization strategy. Semiconductor quantum dots are tiny (nanometer-sized) regions of one type of semiconductor inside a 3D matrix of another semiconductor. These tiny regions (the dots) may be used to trap individual electrons or holes (or excitons, corresponding to an electron-hole pair that may be generated optically). As a result, quantum dots may play the role of a “quantum bit” in a solid state quantum computer, promising advances in high power computing, cryptography, software validation and verification, and other important areas. Our focus is to realize optically-mediated quantum gates and other optimal control strategies on extremely short time scales, allowing computations to be carried out before decoherence (which destroys the quantum information) sets in.

Ultrafast Spectroscopy of Spintronic Materials

Our research group is applying femtosecond spectroscopy techniques to study a class of materials referred to as diluted magnetic semiconductors, which are traditional semiconductors doped with magnetic atoms and have the potential to change the way we build computers. In particular, it may become possible to integrate logic and memory functions and to incorporate optical interconnects and switching, leading to computers with lower power consumption and higher speed. The high level of interest in these materials stems from their ability to combine the non-volatility of ferromagnetic order as a basis for storing information into a traditional semiconductor device platform. Such an integration is in principle seamless because it involves only semiconductor materials. As an added benefit over the metallic ferromagnetic materials at the heart of commercial magnetic random access memory, these ferromagnetic semiconductors also offer the ability to tune the magnetic characteristics using electrical gates and/or light. This combination of features leads to a whole host of possible directions for magneto-sensitive (so-called spintronic) devices, including spin-field-effect transistors, spin resonant tunnel diodes, integrated optical isolators, and non-thermal approaches to magneto-optical memory, to name a few. Clearly in order to realize applications like these, the fundamental properties of these materials must be well understood so that we are armed with the tools needed to consider a variety of material combinations and to engineer the magnetic, electronic and optical properties for specific device applications. Achieving this essential understanding is a formidable task because of the complex interplay of disorder and exchange coupling. Our research group is bringing nonlinear (femtosecond) optical tools and techniques to bear on the problem, allowing us to shed new light on the electronic structure and the origins of ferromagnetic order.