Research Interests

  Terahertz spectroscopy and imaging

Our current 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. Our first emitter setup looks like this. 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 measurements. 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. Here's a photo of the first T-ray pulses generated in Texas! And the people responsible for it.

Until relatively recently, the technology required to produce ultrafast laser pulses was restricted to a laboratory environment. However, recent developments have led to the availability of lasers which are compact, portable, and (relatively) inexpensive, and which reliably produce light pulses as short as 100 femtoseconds in duration. These advances have enabled the use of these lasers in a wide range of applications; in particular, it is now possible to build a terahertz spectrometer for use in "real-world" manufacturing or processing environments.

In parallel with these advances in laser science, significant progress has been achieved in the THz-TDS technology. Pioneered in the late 1980's by groups at Bell Laboratories and IBM, these techniques have been refined and improved in several significant ways. It is now possible, for example, to measure the entire waveform of the THz pulse in only a few milliseconds, rather than the few minutes which had been required. Such advances as these have enabled the use of THz-TDS for imaging. Far-IR images of many common objects have been generated, both in transmission and reflection. "T-Ray" imaging, described in a recent press release from AT&T Bell Labs, has potential applications in a wide range of fields, including package inspection, manufacturing process control, environmental monitoring, and non-destructive materials evaluation. Additional applications are being explored in biomedical engineering research, real-time plant monitoring for agricultural applications, semiconductor wafer characterization, and plasma physics.

The recent extension of this technique to a reflection geometry allows one to construct full three-dimensional images of many objects, since the short pulse duration provides an accurate means of measuring the distance to reflecting surfaces, as in  tomography. This figure shows the THz waveforms incident on (a) and reflected from (b) a 3.5" floppy disk. Each of the interfaces (air-to-plastic or plastic-to-air) generates a reflected pulse, visible in the reflected pulse train. Curve (c) represents the reflected waveform, after the instrument response (represented by curve (a)) has been deconvolved. The front and rear surfaces of the thin (~100 micron) magnetic recording material are now clearly resolved.

In addition, because this range of the electromagnetic spectrum has been so difficult to access until relatively recently, the THz spectroscopic properties of many materials remain poorly characterized. Both linear and non-linear optical properties of semiconductors, superconductors, and, more recently, condensed phase chemical systems are being studied with THz-TDS. We are actively engaged in performing spectroscopy on novel condensed matter systems, using techniques such as terahertz emission spectroscopy.  We also have recently begun studying a new form of waveguiding technique, which relies on the propagation of terahertz pulses on bare metal wires.  This has proven to be an extremely interesting terahertz waveguide, with many possible applications in sensing and imaging.  A calculation showing a terahertz wave propagating on a wire is shown here.

 For more information, or to register any comments or point out errors or omissions on this page, feel free to e-mail. Or click here for a list of recent publications from our group.

We also maintain a list of other groups working in the field of THz-TDS.  Please help us keep this list updated!


  Photonic crystals and strongly scattering media

Photonic crystals are materials that strongly diffract light, due to an extended spatial dielectric periodicity.  Our research in this area began as a collaborative effort with the Colvin Group, from the Chemistry Department.  This collaboration was initially directed towards understanding the transmissive optical properties of thin films of colloidal crystals, close-packed arrays of spherical silica colloids.  These films exhibit striking diffractive optical properties, resulting from the periodicity of the dielectric function in the medium.  Peng Jiang, a student in the Colvin group, has recently invented a method for fabricating colloidal crystals which produces large (1 square centimeter) single crystal films, and which permits us to control the film thickness.  Films grown with different colloid sizes appear colored in different hues due to the sensitivity of the diffraction process to the spacing between adjacent layers of spheres.  Using this novel fabrication method, we were the first to study the evolution of the optical properties of a photonic crystal as a function of its thickness.

Subsequently, we have become involved in several different aspects of photonic crystals research.  We have explored the optical properties of a crystal of hollow shells, for which a complete photonic band gap can be roughly two times larger than for a more conventional inverse opal structure.  We have also begun to explore the importance of crystalline defects, and their influence on the properties of these films.  Recently, we have also begun to extend this work into the THz spectral range. This is appealing because one can fabricate essentially perfect crystals for millimeter-wave radiation, and more systematically explore the role of defects or imperfections.  Also, the THz system permits us to measure not only the amplitude but also the phase of the radiation transmitted through a photonic crystal, which is a lot of additional information.

More information on these and other photonic band gap materials can be found here. Our publications on this subject are listed here.


Group Members

Faculty Fellow: Graduate students: Undergraduate researchers:             Group photo, December 2005
           (most of the) group photo, April 2009
           CLEO 2009 - Research group goes nautical
           Marx's post-defense celebration
           Jingbo's defense
           Group photo, March 2014

  Group Alumni

  Group Phone List


  Weekly Group Meeting Schedule (Fall 2011)

Every Friday unless otherwise notified
9:30 - 10:00
 
10:00 - 10:30
 
10:30 - 11:00
 
11:00 - 11:30
Daniel
11:30 - noon
 
noon - 1:30
Lunch
1:30 - 2:00 Bobby
2:00 - 2:30
Kim
2:30 - 3:00
Nick
3:00 - 3:30
 Rajind
3:30 - 4:00
 


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