Terahertz Radiation

Daniel Mittleman's research involves the generation and detection of sub-picosecond pulses of far-infrared radiation, with central frequencies in the vicinity of 1 terahertz (THz). These pulses are generated by gating an optoelectronic switch with an ultrafast (visible or near-IR) laser pulse. The THz radiation generated in this fashion consists of a single cycle of electromagnetic radiation, and as a result has an extremely broad bandwidth, extending (typically) from below 100 GHz to over 3 THz. Although the energy in each THz pulse is quite low, the coherent detection, in which the detector is gated with a second visible ultrafast pulse, permits extremely high signal-to-noise spectroscopic measurments. Because the THz pulses are measured using photoconductive sampling to map out the time-domain waveform, this technique has been called terahertz time-domain spectroscopy, or THz-TDS.

This new technique has proven to be valuable both as a spectroscopic tool for probing the properties of materials in the far infrared range and as a new imaging system. The lower right figure on the cover shows an image of a packaged integrated circuit, in which the leads and the silicon die are visible through the plastic packaging. Numerous applications of this technology are being explored, and new imaging modes are being developed to exploit the unique aspects of this sensing technology.

Mittleman's research also focuses on the study of the far infrared properties of materials. One project involves the measurement of the THz dielectric properties of nanometer-sized pools of water confined within inverse micelles. Such nanometer-scale liquid systems can exhibit unusual absorption resonances, resulting from the confinement. The extent to which their spectra differ from that of bulk liquid water is an important measure of the mesoscopic structuring of the liquid.

Free electron lasers (FELs) can generate intense, coherent, and tunable THz radiation and provide nonlinear spectroscopic methods that are complimentary to the above-described THz-TDS. Junichiro Kono has been an active user of FELs, currently maintaining a user laboratory at the Stanford FEL Center dedicated to his study of THz dynamics in quantum semiconductor structures. His group's research has shown that the intense THz radiation from the FEL can strongly perturb the dynamics and energy levels of quantum-confined carriers, which in turn results in drastic ultrafast modifications in the interband optical properties of the driven semiconductor system. This type of strong-field phenomena, or THz electro-optics, require understanding of both field-like (classical) and photon-like (quantum) aspects of the strong electromagnetic radiation and can lead to important device concepts for future THz solid state technologies that combine electronics (< 100 GHz) and photonics (> 3 THz).