THz Measurement Facility

The NEAR-Lab measurement facility is located in suite 25-00 at 1900 SW 4th Ave, Portland OR 97207.

The measurement lab is also setup to design and build underwater sensing equipment for use in the field. You can go here for more complete details on all NEAR-Lab projects. Please contact Prof. Lisa Zurk for more information.




Terahertz

Terahertz (THz) radiation is a portion of the electromagnetic spectrum between 300 GHz (300 x 10^9 Hz) and 30 THz (30 x 10^12 Hz). As with every other part of the spectrum, there are applications and challenges. Terahertz is known for its ability to pierce through many materials (such as clothing and packaging material) and also for showing unique spectral features in different chemicals (much like we see colors in visible light). This allows us to image someone that could potentially holding a weapon, or scan for dangerous chemicals or explosives. Millimter waves, the younger brother to the terahertz regime, have been investegated by TSA in order to scan passagners for potential concealed objects.

The NEAR-Lab measurement facility at PSU is equipped with two terahertz systems; the Picometrix T-Ray 4000 time-domain spectroscopy system and a Rhode & Schwarz ZVA-40 vector network analyzer equipped with Virginia Diodes ZVA extender systems.















(a) The T-Ray 4000 terahertz system emits and detects short picosecond pulses.
(b) Basic configuration for angle-dependent reflection measurements.

The Picometrix T-Ray 4000 (shown above), emits a short burst radiation, which is focused by a polyethylene lens into a collimated beam. The beam travels through air and is then detected and processed for frequency content. The system can be configured in reflection mode (beam scatters off object, shown on right) or transmission (transmits through material of interest, shown on left). Our reflection mode setup includes a motorized arm that automatically scans across angle with precision of 0.001 degrees. With one minute of averaging, the detected signal contains frequencies out to 4.0 THz. Some specifications for the system are:

• Laser frequency: 780-850 nm
• Pulse repition rate: 50 MHz
• Detector time-resolution: 78 picoseconds
• SNR: 80 dB at 0.5 THz
• Labview-based operating system

The Virginia Diodes ZVA extender systems operate by taking the 40 GHz signal from the Rhode & Schwartz VNA and running it though a modular series of frequency doublers, triplers, subharmonic mixers, and amplifiers. Six configurations are possible, allowing narrow band, continuous wave measurements that span 75 to 750 GHz.















(a) The Virginia Diodes THz continuous wave system
(b) NEAR-Lab students conduct P-type Silicon transmission measurement.

Currently, we are working on three main projects. In the first project, we are characterizing materials for index of refraction and absorbtion. (there are many out there that have yet to be measured!) We hope to find materials that can be used as phantoms in place of dangerous chemicals or explosives. Using these “fake” samples, we can optimize our signal processing techniques to identify specific materials for imaging systems.

In our second project, we are measuring angle-dependent scattering from a random rough surface. Sandpaper, which contains a randomly varying surface profile, is coated with gold (assumed to be a perfect reflector). The sandpaper is then mounted horizontally in front of the transmitter at some angle of incidence, see photo above. As shown in the photo, a movable arm rotates around the stationary sandpaper sample allowing us to fully characterize the reflectance of the surface. Such a profile is reffered to as a BRDF (bidirectional reflection distribution function).

In our third project, we can image actual objects. The mechanical rails (shown to left) move the transmitter/receiver pair back and forth while recording data. The imaging system is capable of:

• Live streaming images
• x-y scan’s with full frequency spectrum
• 3-d tomography
• Scan resolution: 125 microns


Each pixel that is imaged will record a time domain waveform. Using the time of each pulse arival, an image would represent a contour of a surface. If that pulse was Fourier transformed with an appropriate color code, an image could be made according to some unique reflection spectra. Eventually we hope to image rough material at an arbitrary angle, and obtain a usable image. As noted above, we can potentially image with sub-millimeter resolution. Although this may sound small, it isn’t as sharp as an typical photograph as we are accustomed to seeing. This is because we are limited by the wavelength of radiation (~300 microns). Please check back as we will upload some pictures soon.