TERAHERTZ IMAGING: 2.8 THz QCL and IR camera capture real-time images

Short enough to provide submillimeter resolution capability, yet long enough to penetrate most nonmetallic materials, terahertz waves in the 0.3 to 10 THz spectral range are being exploited for detection of concealed objects and even for medical applications in distinguishing cancer from normal tissue, thanks to the nonionizing impact of this radiation. In the quest to develop cost-effective and commercially viable terahertz sources and detectors, collaborating researchers from the Naval Postgraduate School (Monterey, CA), Agilent Laboratories (Santa Clara, CA), and the University of Neuchâtel (Neuchâtel, Switzerland) have developed a real-time imaging system capable of distinguishing between plastic and metal that uses a commercially available uncooled infrared (IR) microbolometer camera and a milliwatt-scale 2.8 THz quantum-cascade laser (QCL).¹

Sensitivity of the 160 × 120-pixel focal-plane-array (FPA) camera was first assessed theoretically to determine whether the camera was capable of satisfactorily detecting terahertz radiation without an external source of illumination. Evaluation of the noise-equivalent temperature-difference (NETD) capability of the camera revealed that the total background power density incident upon the FPA in the 1 to 5 THz region was only 12 W/m² at 300 K—significantly below the 170 W/m² in the 8 to 14 µm wavelength range for which the camera was designed. This yields an NETD value for the terahertz regime that is at least a full order of magnitude greater than that for the infrared.

Not only did the analysis confirm the need for external illumination when using the microbolometer camera to image terahertz radiation, but several optical modifications to the camera system were necessary to maximize the amount of terahertz radiation received by the FPA. Namely, the original antireflection-coated germanium lens on the camera—which attenuated terahertz radiation—was replaced by a biconvex lens made of Picarin material from Microtech Instruments (Eugene, OR). Picarin, with a transmission value of 0.65 at 2.8 THz, was also used for the FPA’s cryostat window.

The QCL used in the imaging system was fabricated by molecular-beam epitaxy and consisted of a multiple-quantum-well active region on a semi-insulating gallium arsenide substrate. The 14 × 200 µm rectangular active facet produces an elliptical beam that passes through a cryostatic cooler and is collimated by two off-axis parabolic mirrors.

Image averaging

For imaging experiments, various metal objects were concealed within plastic, cloth, and paper, and placed midway between the parabolic mirrors to ensure that a focused image formed on the microbolometer FPA. The laser was operated at a 300 kHz repetition range for single-frame imaging; however, image-averaging techniques were applied to multiple still images to reduce noise and improve overall image quality (see figure).

Despite the presence of diffraction effects in the images due to QCL rectangular aperture diffraction and etalon effects between the Picarin lens and cryostat window, metallic objects obscured by opaque materials are easily imaged by this technique.

“This imaging scheme has proven to be a very versatile method of obtaining real-time imaging with an uncooled detector—two features that have been difficult to achieve, simultaneously, in the terahertz regime,” said Barry N. Behnken, researcher and Ph.D. candidate at the Naval Postgraduate School. “More recently, we’ve expanded our experiments to include the use of a 3.6 THz QCL; operating at a shorter wavelength range has allowed us to image with better resolution and much longer persistence times. Interestingly, it has also provided sufficient contrast to successfully image many types of nonmetallic materials—including some surprising security features embedded within foreign currency notes.”

REFERENCE

1. B.N. Behnken et al., Optics Lett. 33(5) 440 (2008).