Thomas Boggess Research

Optical Sources and Techniques for Time-Resolved Mid-Infrared Spectroscopy:

Many of the important interactions involving charge carriers (electrons and holes) in semiconductors occur on subnanosecond and even subpicosecond time scales. For example, electrons high in the conduction band of a semiconductor will tend to relax to the band edge by emitting optical phonons, i.e., by launching a specific form of mechanical vibration in the lattice. The characteristic time for this emission process is typically less than a picosecond. At high carrier densities, electrons injected with a specific energy into a semiconductor will thermalize to a Fermi-Dirac distribution by electron-electron scattering typically in less than 100 femtoseconds. Such processes are far too fast to be characterized by purely electronic techniques. On the other hand, recent developments in laser technology have made optical pulses with duration of 100 fs or less available routinely. These pulses can be used to time resolve ultrafast processes. The techniques involved can be likened to ultrafast stopwatches or cameras with ultrafast shutters and film. A simple example of these techniques is the pump-probe or differential transmission measurement. A single ultrashort optical pulse is split into two using a beam splitter to produce one very intense pulse (pump) and a much weaker pulse (probe). The two pulses traverse separate paths, one a fixed path and the other variable path (e.g., by reflecting the pulse from a retroreflector mounted on a precision translation stage). The two pulses are then brought back together on the semiconductor sample, and the probe transmission is measured as a function of delay between the pump and probe. The intense pump pulse changes the optical properties of the sample, e.g., by promoting electrons from the valence band to the conduction band, thus changing the transmission of the probe (which by itself is too weak to influence the sample). By measuring the transmission of the probe as the delay is varied, the recovery of the sample to equilibrium can be measured. For sufficiently thin samples (usually not a problem for semiconductor heterostructures, which are typically on the order of a micron), the temporal resolution of this measurement is limited only by the pulse duration. A resolution of 100 fs is obtained easily, while state-of-the-art systems can provide a resolution of 10 fs or less. More sophisticated techniques have yielded temporal resolution of less than 1 fs!

One relatively unique aspect of our work is that, because of our interest in ultrafast processes in narrow band gap structures, we must perform ultrafast measurements at mid-infrared wavelengths. While ultrafast sources and techniques are by now quite common at near-infrared and visible wavelengths, such sources and techniques are not common in the mid-infrared. Recent work on THz radiators and detectors has led to ultrafast time-resolved measurements in the far-infrared. Ultrafast measurements in the mid-infrared are usually based on the nonlinear optical processes of sum and difference frequency mixing. We have implemented techniques involving both of these processes in our laboratories.

Specifically, in our laboratories we have constructed a synchronously-pumped optical parametric oscillator (OPO) for ultrafast mid-IR spectroscopy. This system uses a potassium titanyl arsenate (KTA) crystal to split photons in an intense 100 fs, 800 nm pulse from a mode-locked Ti:sapphire laser into two longer wavelength photons, one in the near-IR and one in the mid-IR. The near-IR photons are trapped in a resonant optical cavity that is carefully matched to the cavity length of the Ti:sapphire laser. After making one round trip in the cavity, the near-IR photons arrive back in the KTA crystal just in time to interact with a subsequent pulse from the Ti:sapphire laser. This process provides gain for the near-IR pulse and, provided the gain can compensate for the cavity losses, oscillation occurs. This system produces pulse of less than 200 fs duration at wavelengths from 1 - 1.2 and to 2.6 - 4.5 microns. More recently, we have constructed a synchronously-pumped OPO that utilizes quasi phase matching in periodically-poled lithium niobate (PPLN) for the nonlinear interaction. This system offers lower threshold, easier alignment, and more robust operation compared to the critically-phase-matched KTA system. The tunability of the system is demonstrated in the figures below. The visible output is either the non-phase-matched second harmonic of the oscillating signal beam or sum-frequency generation between the signal and the pump. Tuning is achieved by simply changing the cavity length on a micron scale.

A second mid-IR spectroscopic system in our laboratories uses sum-frequency generation or frequency upconversion. In these measurements, a semiconductor sample is irradiated by an intense 100 fs pulse from the Ti:sapphire laser. This pulse creates in the sample electrons and holes that eventually recombine with one another. Some of these recombination events are radiative, i.e., light is emitted at a wavelength equal to the sum of the electron, hole, and band gap energies. We collect this photoluminescence and combine it with a second intense pulse from the Ti:sapphire laser in a nonlinear optical crystal. The crystal is configured to produce the sum frequency of the Ti:sapphire signal and some portion of the photoluminescence spectrum. The sum frequency process is essentially instantaneous, i.e., the sum frequency is generated only so long as both the Ti:sapphire laser pulse and the photoluminescence are overlapped in the crystal. Hence, by varying the relative delay between the pulse that excites that luminescence and the pulse that interacts with the luminescence in the nonlinear optical crystal, we can time resolve the photoluminescence. The early phases of the photoluminescence emission reflect carrier thermalization and cooling by optical phonon emission, while the latter stages reveal the slower carrier recombination processes.