Ultrafast Phenomena

Quantum Interference and Generation of THz Radiation

Optical excitations involve the promotion of electrons from low- to high-energy states in a material. These excitations are driven by photons of light. Different excitation pathways, corresponding to different physical processes, can link these two states. When two distinct excitation processes terminate in the same final state, the possibility of quantum interference exists. This quantum mechanical interference can manifest itself in a variety of macroscopic effects that can be measured in the laboratory. We are studying the emission of far-infrared electromagnetic radiation (i.e. terahertz frequencies) that can result from quantum interference of optical excitations in solids. These coherences are extremely short-lived (of the order of a picosecond or less) and require the techniques of ultrafast laser spectroscopy.

One area of research is quantum interference of one- and two-photon absorption in semiconductors. Here, initial and final states (in the valence and conduction band, respectively) are linked by two distinctly different absorption processes. Two ultrashort laser pulses, with frequencies differing exactly by a factor of 2, excite the material. Depending on the phase and polarization of the two different light fields, a directional photocurrent can be generated. The acceleration of charges associated with this current will lead to the emission of electromagnetic radiation. This effect has been analyzed theoretically for a variety semiconductors and material scaling laws have been derived (Phys. Rev. B, R11247, 2002).

A second way to obtain electromagnetic radiation via quantum interference is to exploit symmetry properties of certain crystals. Specifically, Raman excitations in different classes of crystals have a space-angle dependence that can be observed with polarized light. Scattering of light by these fundamental modes is described by the so-called Raman tensor. The Raman excitations of interest are plasmons (collective motion of the electronic system) and phonons (the quanta of vibrational excitation). In properly chosen experimental conditions, these excitations can be driven by different Raman processes and interference may take place. We have observed far-infrared electromagnetic radiation emitted by plasmons and phonons via Raman scattering of femtosecond laser pulses. These collective and vibrational modes are excited impulsively, i.e. the laser pulse duration is much shorter than the period of oscillation. In addition, the plasmon and phonon oscillations are coherent, which leads to the generation of a macroscopic radiation field. This excitation process is known as impulsive stimulated Raman scattering (ISRS). We have used ISRS to generate THz light via coherent plasmons and phonons in the semiconductor InSb. In these experiments, a single ultrashort laser pulse is used; quantum interference of Raman scattering is obtained by rotating the azimuthal angle of the crystal about the fixed linear polarization of the laser. This work was presented at the 2003 Quantum Electronics and Laser Science Conference in Baltimore, MD. A short write-up can be found here.

We have also studied how the oscillatory motion of coherent plasmons and phonons can couple as the result of Coulomb forces. The frequency of the plasmon is adjusted by changing the background doping density of the semiconductor sample; we used bulk InAs in our experiments. When the plasma frequency and bare phonon frequency approach each other, coupling occurs and new `hybrid' modes emerge. Mode hybridization in semiconductors has been studied for decades using Raman spectroscopy, but our experiments demonstrated for the first time that such excitations can directly radiate light (Phys. Rev. B, 233203, 2002).

MOSAIC: Modified Spectrum Auto-Interferometric Correlation

Our group has developed a simple, efficient algorithm for monitoring the chirp of an ultrashort laser pulse in real time.   `Chirp' refers to the frequency drift that occurs in the temporal envelope of a laser pulse. This means that the center wavelength of the pulse changes slightly as a function of time. Chirp generally occurs when the ultrashort pulse passes through optical elements such as lenses, prisms, waveplates, and windows as the result of group velocity dispersion.

The widely used laboratory technique of interferometric autocorrelation (IAC) can determine the pulse temporal profile and duration, but it is not very sensitive to the presence of chirp. When you are tuning an ultrashort laser system for minimum pulse duration as determined by IAC, you are not guaranteed to have minimum chirp. It is often surprising that an otherwise clean IAC trace may be hiding a significant amount of chirp. This can be a critical issue in experiments where very short and very coherent pulses are needed. While methods such as frequency-resolved optical gating (FROG) can characterize chirp, additional optical components are required. Furthermore, these algorithms tend to be computationally intensive, which is not conducive to real-time adjustments and monitoring of the laser system during an experiment.

MOSAIC overcomes these problems by performing a simple spectral filtering operation on the IAC waveform. If your laboratory can generate real-time IAC traces on a digital oscilloscope and you have access to a PC, you already have all the hardware needed to implement MOSAIC! By connecting the scope to the PC, you can monitor chirp and adjust the laser system in real-time. We have written MOSAIC programs compatible with a variety of Tektronix digital oscilloscopes. These virtual instruments (VI's) run on the National Instruments LabVIEW platform and can be downloaded free of charge from our MOSAIC website.

Our ultrafast research is supported by the National Science Foundation.