1 6 A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 2 THz WAVES ARE TRANSPARENT TO MANY DIELECTRIC MATERIALS INCLUDING GLASSES, CERAMICS, VISIBLY OPAQUE MATERIALS, CLOTHING, AND PACKAGING MATERIALS, ENABLING NONDESTRUCTIVE IMAGING FOR VARIOUS APPLICATIONS... Terahertz (THz) radiation is on the order of ~1012 Hz or 1 THz, positioned at 0.1-10 THz (100 GHz -10 THz, λ = 3-0.03 mm, and photon energies on the order of 0.41-41 meV). With long wavelengths, these waves are nondestructive and nonionizing, making them ideal for noncontact spectroscopy of materials, specifically ceramics, glasses, and composites. THz spectroscopy measurements probe low-energy interactions within matter and materials, describing molecular rotations and vibrational spectra, crystalline phonon vibrations, low frequency bond vibrations, molecular rotations and vibrations of gases, hydrogen-bonding stretching and torsions of liquids, and charge transport[1-3]. Terahertz time-domain spectros- copy (THz-TDS) enables material exam- ination through transmissionand reflection geometries. Reference and sample scans must be performed under pure nitrogen atmosphere due to absorption of THz waves by water vapor[4-6]. THz-TDS sample requirements include having flat and pristine front and back surfaces, as incident THz waves will interact with the material and the phenomena of transmission and reflections can be observed. To extract optical and dielectric constants from THz-TDS measurements, additional sample constraints include using a homogenous representative portion of the sample with a known thickness. In terms of sample geometry, too thin or thick of a sample results in superposition of THz pulses and multiple reflections, as the THz waves have a wavelength on the order of the sample thickness[5,7]. THz-TDS uses femtosecond laser systems, e.g., Ti:sapphire laser, to produce optical pulses with femtosecond durations (e.g., < 90 fs). A beam spitter splits the beam into pump and probe components. The pump component is transformed into a THz pulse (e.g., < 500 fs) using a photoconductive antenna (PCA) and passes through a sample of known thickness, while the probe component is used to detect the beam using a PCA[1,6,8]. A temporal delay between pump and probe components is performed by increasing the path length of a beam and is critical for detection of THz waves. The reference or probe pulse and the sample or pump pulse time profile are not identical, allowing for comparison and therefore determination of optical and dielectric properties. THz-TDS measures the THz wave temporal electric field (not the intensity), while a reference pulse determines the electric field where the transmitted THz pulse height and phase (peak position) is shifted by the absorption and refractive index of the samples, respectively[5,9]. The transmitted sample time-domain waveform is transformed into the frequency domain, through a Fourier transformation, and the resulting THz wave formamplitude and phase change results in determination of the absorption coefficient and refractive index, respectively. THz-TDS examination of materials allows for determination of optical and dielectric constants or identification of materials through absorption signatures, termed fingerprints, at THz frequencies[1]. Extraction of optic and dielectric constants including a complex refractive index does not require a Kramers-Kronig transformation. Real and imaginary parts of the refractive index are associated with sample thickness, absorption, and conductivity of the materials. THz waves are transparent to many dielectric materials including glasses, ceramics, visibly opaque materials, clothing, and packaging materials, enabling nondestructive imaging for various applications including general identification and quantification of compositional information for use in quality control of packaged materials[10-14]. Other uses include biomedical detection and diagnosis of diseases[15-17] such as skin cancer using reflection imaging, and in the pharmaceutical industry[18-21] to study the crystal structure of drug molecules as well as to determine tablet coating thickness, uniformity, porosity, and defects. Further applications include security and defense[17,22] for use in crowd control, in remote sensing to identify chemical and biological substances or detect prohibited items including drugs, weapons, or explosives, and in communication for high-capacity wireless data transmission at short distances[17]. This article presents a brief overview of THz-TDS as a nondestructive analytical characterization technique used to examine materials, particularly ceramics, glasses, and composites. Recent progress at the Terahertz Waves Science and Technology Laboratory (T-Lab) at Alfred University is also discussed. For more details on THz-TDS, see references 1, 2, 3, and 8. THz-TDS AT ALFRED UNIVERSITY The THz-TDS nondestructive examination and characterization carried out at Alfred University’s T-Lab uses a TeraView Spectra 3000 (TeraView, U.K.) operating in transmission mode under pure nitrogen conditions (Fig. 1). THz spectra are generally collected and averaged five times from 0.2-1.0 THz (material and sample thickness dependent). A mode-locked Ti:sapphire laser centered at 800 nm with a repetition rate of 80 MHz and pulse duration of 100 fs produces incident THz radiation that is divided into pump and probe detection beams. A comparison between reference and sample time delay of the THz pulse is essential to determining optical and dielectric properties. The authors have studied optical and dielectric properties including refractive index, absorption coefficient, and real dielectric constant of ceramics, glasses, and composites at THz frequencies using THz-TDS, particularly changes in structure and properties across a compositional space. Complementary structural studies of materials
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