Albert Redo-Sanchez

Identification and classification of chemicals using terahertz reflective spectroscopic focal-plane imaging system

We present terahertz (THz) reflective spectroscopic focal-plane imaging of four explosive and bio-chemical materials (2, 4-DNT, Theophylline, RDX and Glutamic Acid) at a standoff imaging distance of 0.4 m. The 2 dimension (2-D) nature of this technique enables a fast acquisition time and is very close to a camera-like operation, compared to the most commonly used point emission-detection and raster scanning configuration. The samples are identified by their absorption peaks extracted from the negative derivative of the reflection coefficient respect to the frequency (-dr/dv) of each pixel. Classification of the samples is achieved by using minimum distance classifier and neural network methods with a rate of accuracy above 80% and a false alarm rate below 8%. This result supports the future application of THz time-domain spectroscopy (TDS) in standoff distance sensing, imaging, and identification.

Terahertz Science and Technology Trends

The recent progress in terahertz science and technology (THz-S&T) opens up a range of potential research opportunities. Historically, THz technologies were mainly used by the astronomy community for searching far-infrared radiation (cosmic background), and by the laser fusion community for the diagnostics of plasmas. Since the first demonstration of THz wave time-domain spectroscopy in the late 1980s, there has been a series of significant advances (particularly in recent years) as intense THz sources and more sensitive detectors provide new opportunities for understanding the basic science in the THz frequency range. THz radiation can penetrate through many nonpolar dielectric materials and can be used for nondestructive/noninvasive sensing and imaging of targets under nonpolar, nonmetallic covers or containers. An immediate application of THz wave technology is in nondestructive testing or inspection. Short-term applications (within three to five years) are expected in spectroscopic sensing and imaging for homeland security. Biomedical applications are expected in the long term (five to ten years). By comparing the publication record trend of THz-S&T related papers with the publication record of proxy fields (laser, microwave, Raman, and infrared), it is possible to anticipate that the number of publications in the THz-S&T arena will increase and also the impact in other research areas. We compare the publication pattern (number of papers versus time) with searching keywords in title or abstract. We found that all the publication trends share a common pattern with four periods defined by discovery, acceptance, adoption, and maturity. From this pattern trend, THz-S&T seems to be in an acceptance period. The unique properties of THz-S&T suggest that its applications will grow.

Assessment of terahertz spectroscopy to detect antibiotic residues in food and feed matrices

We report the use of terahertz (THz) spectroscopy to explore the spectral properties of eleven antibiotics commonly used in livestock production. Eight of the eleven antibiotics showed specific fingerprints in the frequency range between 0.1 and 2 THz. The main spectral features of two antibiotics (doxycycline and sulfapyridine) were still detectable when they were mixed with three food matrices (feed, milk, and egg powder). These preliminary results indicate that THz spectroscopy could be suitable for screening applications to detect the presence of antibiotic residues in the food industry, with the prospect to allow inspections directly on the production lines. THz spectroscopy is a non-destructive, non-contact, and real-time technique that requires very little sample preparation. Moreover, THz radiation can penetrate plastic and paper, which enables the detection of antibiotics in packaged food.

STANDOFF SENSING AND IMAGING OF EXPLOSIVE RELATED CHEMICAL AND BIO-CHEMICAL MATERIALS USING THz-TDS

We report the sensing and imaging of explosive related chemical and bio-chemical materials by using terahertz time domain spectroscopy (THz-TDS) at standoff distance. The 0.82 THz absorption peak of RDX is observed at a distance up to 30 m away from the emitter and receiver. Multiple absorption features of RDX, 2,4-DNT and Glutamic Acid are identified by using a large scale 2-D imaging system. These results support the feasibility of using THz-TDS technique in remote sensing and detection of chemical materials.

Nanostructured porous silicon films for terahertz optics

A simple technique is reported to create 31 and 45 μm thick, graded-index Si films in the form of nanospirals on a Si substrate using a dynamic, oblique angle deposition technique. We show that the success in producing such a thick, nanostructured film without delamination from the Si substrate is primarily due to the nano-porous nature of the film which effectively eliminates the stress generated during growth. Effective refractive indices of 1.9 and 2.1 were extracted from the terahertz time-domain reflectivity data, which correspond to 57% and 51% porosity for the 31 and 45 μm thick films, respectively. The gradient of porosity through the film was modeled to describe quantitatively the terahertz reflectance data in the 0.2-2.0 THz regime.

Self-referenced method for terahertz wave time-domain spectroscopy

The method allows retrieval of the absorbance of a sample without the need for a reference measurement. The method measures the dynamic variation of frequency resolution as the waveform is being acquired. In terahertz wave time-domain spectroscopy, the frequency resolution increases as the temporal window increases. Therefore, narrow absorption peaks will appear in the spectrum when the temporal window is long enough to resolve the peak. By measuring the dynamic values of each frequency component at specific points in time, a reference value and a peak value are extracted and, hence, the self-referenced is achieved. In addition, the method provides a mechanism to remove the effects of echoes, which enables arbitrary temporal window length and, thus, achieves high-resolution frequency. Examples of extraction of the water vapor lines and resonant features in gas and semiconductors are demonstrated in transmission and reflection geometries.

2-D Acoustic Phase Imaging With Millimeter-Wave Radiation

We report on 2-D acoustic phase imaging with millimeter-wave radiation for locating concealed objects. Active adaptation of the imaging interferometer provides enhanced images. We demonstrate that the method allows for finding metallic as well as dielectric materials. Even objects having similar optical properties as the background can be located.

Bubble Detector in Polyurethane Applications Based on a Microwave System

This paper discusses a device that detects bubbles in glue depositions on a windshield glass, which may cause water leakages inside the cockpit. This device is inexpensive, more reliable than other existing solutions, and implements a specific signal treatment and a bubble size calculation model. The device is based on microwave radiation system in the X-band and has been developed and implemented in real production in the final assembly area of the automotive industry

High speed imaging with CW THz for security

Continuous THz wave (CW THz) has been widely used in imaging field. But for security screening such as inspection at the airport, the speed of the imaging calls for an improvement since the former CW image systems which scan point to point could not satisfy. To increase the image speed, we proposed a fast CW THz image system in which a galvanometer is introduced for the first time. The galvanometer makes the coming beam reflected in different angles by vibrating at a certain frequency which can significantly decrease the image acquisition time compare to point scan THz imaging. A big hyperbolic polyethylene lens is also used in the system to collect all the beams on to the target. A Gunn oscillator and a corresponding Schottky diode are the source and detector respectively. The image we get has ideal resolution. And after image processing, the images looked not only clear but also realistic. The system has more practicality because it is designed in reflection geometry instead of transmission geometry. Moreover, the source and detector in our system do not as ponderous as gas laser which has been used in many THz imaging system previously. Example of measurements of weapons concealed behind the cloth and box are presented and discussed. A compact high speed THz imaging system is expectable which will have a widely application in security field.