Chirped pulses speed T-ray imaging

Figure 1. T-ray CT system images a hollow dielectric sphere attached to a plastic tube, which is rotated by the rotation stage. The sphere was scanned with a 1 mm step size, and the THz image was obtained for 18 different projection angles. (RPI)

Nestled between far-infrared (IR) and millimeter waves, the emerging field of terahertz wave (or T-ray) imaging promises to combine spatial imaging with spectroscopy to reveal details about cancerous tissues, the health of teeth, and defects on semiconductors, among other things. More than just the added utility of spectroscopic information, T-rays offer submillimeter spatial resolution and are nonionizing, therefore making them safer for patients when compared to more common imaging modalities such as x-rays.

In recent years, researchers have used turnkey ultrafast lasers such as titanium-doped sapphire (Ti:sapphire) systems and gallium arsenide (GaAs) photoconductive antennas or zinc telluride (ZnTe) nonlinear electro-optic (EO) crystals to create less expensive terahertz transceivers. This has energized efforts to commercialize T-ray imaging systems. However, the data acquisition time from these systems is still slow compared to MRI and x-rays because the T-ray radiation covers a broad spectrum between the far-IR (30 µm) and the millimeter wave spectral regions. Detecting the various frequency bands for each pixel in an image can take hours.

By chirping the ultrafast probe pulse in a nonlinear crystal to 'read' simultaneously broad portions of the terahertz spectrum between 0.1 THz to 3 THz, X.-C. Zhang and colleagues at the Rensselaer Polytechnic Institute (Troy, NY) have developed a method to potentially image a sample in minutes.

Zhang's recent work using T-rays for computed tomography (CT) emphasizes the need for faster detection because a single 3-D image may require dozens or hundreds of images of the sample taken from different angles. In his T-ray CT experiment, Zhang fires approximately 150 fs pulses of 800 nm light from a Ti:sapphire laser at a photoconductive GaAs antenna to create a broadband T-ray pulse. The T-ray pulse, which is generally collinear to the original pump beam, passes through the sample. The difference of the composition of the sample gives selective absorption in the transmitted T-ray spectrum.

A ZnTe nonlinear crystal illuminated by a chirped Ti:sapphire mode-locked pulsed laser acts as a detector for the T-rays. Inside the crystal, T-rays impose their amplitude and phase characteristics on the chirped near-IR probe pulses from the Ti:sapphire. By analyzing the polarization and amplitude of the various near-IR frequencies transmitted by the crystal along with the spatial position of each T-ray frequency, Zhang is able to determine simultaneously the index of refraction and chemical properties for each point along the sample.

Don Arnone, CEO of Teraview (Cambridge, UK), has commercialized a 2-D terahertz imaging system that uses reflected T-rays in living tissues. "The application [for Zhang] is that you can take images in real time. We still have several minutes, but he can do images at almost video frame rates. The disadvantage is that it's still in the lab and the signal-to-noise (SNR) is not great."

Zhang agrees. "If the sample is a millimeter or a centimeter, use a chirp technique. But for larger samples, it doesn't work as well because the chirped pulse doesn't have a long temporal window to cover the signal. We're pushing the theoretical particle limit with chirping." Zhang is currently developing a time delay method for reading the T-rays in conjunction with diffraction CT, which should boost the SNR and increase image contrast.

Like many new technologies, T-ray imaging offers benefits that outshine the technical hurdles that remain. "[Terahertz imaging] has the potential to provide a whole new series of contrast element--particularly with terahertz spectroscopy--that can't be obtained with any other method," explains Bruce Tromberg, director and associate professor of surgery and physiology/biophysics at the Beckman Laser Institute (Irvine, CA). "This can provide insight into whether a tissue is normal or malignant in a wide field of view. Typically, if you want to have high resolution in any imaging method, you're stuck looking at a very small spot. For people with multiple potential lesions, it's not easy to biopsy hundreds of lesions or to chart changes in these lesions. So, while [T-ray imaging] does not offer super high spatial resolution, it does offer a potentially high degree of functional resolution."