Brian Schulkin

Development of a compact terahertz time-domain spectrometer for the measurement of the optical properties of biological tissues

Terahertz spectrometers and imaging systems are currently being evaluated as biomedical tools for skin burn assessment. These systems show promise, but due to their size and weight, they have restricted portability, and are impractical for military and battlefield settings where space is limited. In this study, we developed and tested the performance of a compact, light, and portable THz time-domain spectroscopy (THz-TDS) device. Optical properties were collected with this system from 0.1 to 1.6 THz for water, ethanol, and several ex vivo porcine tissues (muscle, adipose, skin). For all samples tested, we found that the index of refraction (n) decreases with frequency, while the absorption coefficient (μ(a)) increases with frequency. Muscle, adipose, and frozen/thawed skin samples exhibited comparable n values ranging between 2.5 and 2.0, whereas the n values for freshly harvested skin were roughly 40% lower. Additionally, we found that the freshly harvested samples exhibited higher μ(a) values than the frozen/thawed skin samples. Overall, for all liquids and tissues tested, we found that our system measured optical property values that were consistent with those reported in the literature. These results suggest that our compact THz spectrometer performed comparable to its larger counterparts, and therefore may be a useful and practical tool for skin health assessment.

Non-destructive imaging with compact and portable terahertz systems

This paper illustrates the non-destructive application of a compact and portable Terahertz (THz) system to analyze the structure of an old painting and to measure the layers of composite plastic samples. THz images from the painting reveal features that resemble the signature of the artist that is not visible in the optical or X-ray channels, which support the authenticity of the painting. On the other hand, data from a composite plastic sample is analyzed to measure the thickness of each layer and determine the presence or absence of adhesive bonding between them. The presence and position of the adhesive is clearly visible in the THz images and the measured thickness shows an excellent agreement with nominal thickness. These applications demonstrate the capabilities of THz technology for unique non-destructive inspection applications. Furthermore, available compact and portable THz systems enable to perform these inspections onsite without the need to bring the sample to the laboratory, increasing the u...

Measurement of the optical properties of skin using terahertz time-domain spectroscopic techniques

Terahertz (THz) radiation is increasingly being used in biomedical imaging and spectroscopy applications. These techniques show tremendous promise to provide new sophisticated tools for the improved detection of skin cancer. However, despite recent efforts to develop these applications, few studies have been conducted to characterize the optical properties of skin at THz frequencies. Such information is required to better understand THz-tissue interactions, and is critical for determining the feasibility of proposed applications. In this study, we have developed and tested a THz time-domain spectroscopy system. We used this system to acquire the optical properties for fresh and frozen/thawed excised porcine skin from 0.1 to 2.0 THz. Results show that the index of refraction (n) for both frozen and fresh skin decreases with frequency. For frozen skin, n equals 2.5 at 0.1 THz and 2.0 at 2.0 THz, and for fresh skin equals 2.0 at 0.1 THz and 1.7 at 2.0 THz. Values for the absorption coefficient (μa) increase with frequency for both frozen and fresh skin. Frozen skin exhibits μa values equal to 56 cm-1 at 0.1 THz and 550 cm-1 at 2.0 THz, whereas fresh skin exhibits values of 56 cm-1 at 0.1 THz and 300 cm-1 at 2.0 THz. Assuming the optical penetration depth (δ) is inversely proportional to μa (absorption-dominated interactions), THz radiation has limited δ in skin (200 μm at 0.1 THz to 40 μm at 2.0 THz). These results suggest that applications exploiting THz radiation show the most promise for investigating superficial tissues.

Progress toward handheld THz spectrometry

Development and application of real-time (> 5Hz) first-surface and bulk reflection measurements using the mini-Z THz Time-Domain-Spectrometer (TDS) are presented for standoff detection and identification of organic, pharmaceutical and energetic compounds. A non-iterative baseline correction for scattering is applied to real-time data to expand THz sensing across a wider range of sample preparation techniques, including pressed pellets, powders and other irregular surface geometries. Performance is assessed using selected samples of organic, pharmaceutical and energetic compounds with absorption peak positions in the 0.1–3.5 THz range, with results >99% confidence without the use of HDPE sample references using standard correlation and partial-least-squares (PLS) techniques.

Active Balance in Free-Space Electro-Optic Detection of Terahertz Waves

A method for increasing the stability of terahertz (THz) time-domain spectrometers utilizing electro-optic balanced detection is presented in this paper. The stability of these systems is sensitive to optical alignment and polarization quality; small changes in temperature, pressure, stress, and strain can all affect SNR. High stability is achieved by precise rotation of the quarter-wave plate (QWP) balance position during the operation of the THz spectrometer. Providing feedback via QWP rotation optimizes the noise floor with minimal modulation to the THz signal in a nonideal optical system. Applying active balance control eliminates constant tuning by the operator between scans, after purging with dry nitrogen, applying vacuum, and during system setup.

Progress toward handheld THz spectroscopy and THz Air Photonics

Development and application of real-time (> 5Hz) first-surface and bulk reflection measurements using the mini-Z THz Time-Domain-Spectrometer (TDS) are presented for standoff detection and identification of organic, pharmaceutical and energetic compounds. A non-iterative baseline correction for scattering is applied to real-time data to expand THz sensing across a wider range of sample preparation techniques, including pressed pellets, powders and other irregular surface geometries. Performance is assessed using selected samples of organic, pharmaceutical and energetic compounds with absorption peak positions in the 0.1–3.5 THz range, with results >99% confidence without the use of HDPE sample references using standard correlation and partial-least-squares (PLS) techniques. The current state of THz Air Photonics, where selected gases are used as both THz emitter and detector, and how it relates to real-time identification will also be presented.