Propagation of single-cycle terahertz pulses in random media

Influence of temperature and humidity on scattering characteristics of aerosol particles in the terahertz band

The terahertz band is considered to be the next breakthrough point to revolutionize communication technology, attributed to its rich spectrum resources. The study of terahertz atmospheric transmission characteristics is important in guiding the terahertz communication window selection process. In this report, based on the equivalent medium theory, the scattering characteristics of terahertz Gaussian beams by moist media are discussed. Numerical results show that the extinction coefficient of particles is mainly affected by the humidity, and the scattering efficiency is affected by both temperature and humidity. When the temperature is over 273 K and the humidity is 0.5, the extinction efficiency shows a trend of increasing initially and decreasing afterwards. Hence, the appropriate temperature is beneficial to minimizing the attenuation coefficient.

Nonlinear field-control of terahertz waves in random media for spatiotemporal focusing

Controlling the transmission of broadband optical pulses in scattering media is a critical open challenge in photonics. To date, wavefront shaping techniques at optical frequencies have been successfully applied to control the spatial properties of multiple-scattered light. However, a fundamental restriction in achieving an equivalent degree of control over the temporal properties of a broadband pulse is the limited availability of experimental techniques to detect the coherent properties (i.e., the spectral amplitude and absolute phase) of the transmitted field. Terahertz experimental frameworks, on the contrary, enable measuring the field dynamics of broadband pulses at ultrafast (sub-cycle) time scales directly. In this work, we provide a theoretical/numerical demonstration that, within this context, complex scattering can be used to achieve spatio-temporal control of instantaneous fields and manipulate the temporal properties of single-cycle pulses by solely acting on spatial degrees of freedom of the illuminating field. As direct application scenarios, we demonstrate spatio-temporal focusing, chirp compensation, and control of the carrier-envelope-phase (CEP) of a CP-stable, transform-limited THz pulse.

Nonlinear field-control of terahertz waves in random media for spatiotemporal focusing

Controlling the transmission of broadband optical pulses in scattering media is a critical open challenge in photonics. To date, wavefront shaping techniques at optical frequencies have been successfully applied to control the spatial properties of multiple-scattered light. However, a fundamental restriction in achieving an equivalent degree of control over the temporal properties of a broadband pulse is the limited availability of experimental techniques to detect the coherent properties (i.e., the spectral amplitude and absolute phase) of the transmitted field. Terahertz experimental frameworks, on the contrary, enable measuring the field dynamics of broadband pulses at ultrafast (sub-cycle) time scales directly. In this work, we provide a theoretical/numerical demonstration that, within this context, complex scattering can be used to achieve spatio-temporal control of instantaneous fields and manipulate the temporal properties of single-cycle pulses by solely acting on spatial degrees of freedom of the illuminating field. As direct application scenarios, we demonstrate spatio-temporal focusing, chirp compensation, and control of the carrier-envelope-phase (CEP) of a CP-stable, transform-limited THz pulse.

Multiresolution spectrally-encoded terahertz reflection imaging through a highly diffusive cloak

Turbid media, made of wavelength-scale inhomogeneous particles, can give rise to many significant imaging and spectroscopy challenges. The random variation of the refractive index within such media distorts the spherical wavefronts, resulting in smeared and speckly images. The scattering-induced artifacts can obscure the characteristic spectral fingerprints of the chemicals in a sample. This in turn prevents accurate chemical imaging and characterization of the materials cloaked with a diffusive medium. In this work, we present a novel computational technique for creating spatially- and spectrally-resolved chemical maps through a diffusive cloak using terahertz time-domain spectroscopy. We use the maximal overlap discrete wavelet transform to obtain a multiresolution spectral decomposition of THz extinction coefficients. We define a new spectroscopic concept dubbed the "bimodality coefficient spectrum" using the skewness and kurtosis of the spectral images. We demonstrate that broadband wavelet-based reconstruction of the bimodality coefficient spectrum can resolve the signature resonant frequencies through the scattering layers. Additionally, we show that our approach can achieve spectral images with diffraction-limited resolution. This technique can be used for stand-off characterization of materials and spectral imaging in nondestructive testing and biological applications.

Deterministic Terahertz Wave Control in Scattering Media

Scattering-assisted synthesis of broadband optical pulses is recognized to have a cross-disciplinary fundamental and application importance. Achieving full-waveform synthesis generally requires means for assessing the instantaneous electric field, i.e., the absolute electromagnetic phase. These are generally not accessible to established methodologies for scattering-assisted pulse envelope and phase shaping. The lack of field sensitivity also results in complex indirect approaches to evaluate the scattering space-time properties. The terahertz frequency domain potentially offers some distinctive new possibilities, thanks to the availability of methods to perform absolute measurements of the scattered electric field, as opposed to optical intensity-based diagnostics. An interesting conceptual question is whether this additional degree of freedom can lead to different types of methodologies toward wave shaping and direct field-waveform control. In this work, we theoretically investigate a deterministic scheme to achieve broadband, spatiotemporal waveform control of terahertz fields mediated by a scattering medium. Direct field access via time-domain spectroscopy enables a process in which the field and scattering matrix of the medium are assessed with minimal experimental efforts. Then, illumination conditions for an arbitrary targeted output field waveform are deterministically retrieved through numerical inversion. In addition, complete field knowledge enables reconstructing field distributions with complex phase profiles, as in the case of phase-only masks and optical vortices, a significantly challenging task for traditional implementations at optical frequencies based on intensity measurements aided with interferometric techniques.

Diffuse terahertz spectroscopy in turbid media using a wavelet-based bimodality spectral analysis

Current terahertz (THz) spectroscopy techniques only use the coherent light beam for spectral imaging. In the presence of electromagnetic scattering, however, the scattering-mitigated incoherent beams allow for flexible emitter-detector geometries, which enable applications such as seeing through turbid media. Despite this potential, THz spectroscopy using diffuse waves has not been demonstrated. The main obstacles are the very poor signal to noise ratios of the diffused fields and the resonance-like spectral artifacts due to multiple Mie scattering events that obscure the material absorption signatures. In this work, we demonstrate diffuse THz spectroscopy of a heterogeneous sample through turbid media using a novel technique based on the wavelet multiresolution analysis and the bimodality coefficient spectrum, which we define here for the first time using the skewness and kurtosis of the spectral images. The proposed method yields broadband and simultaneous material characterization at detection angles as high as 90° with respect to the incident beam. We determined the accuracy of the wavelet-based diffuse spectroscopy at oblique detection angles, by evaluating the area under the receiver operating characteristic curves, to be higher than 95%. This technique is agnostic to any a priori information on the spectral signatures of the sample materials or the characteristics of the scattering medium, and can be expanded for other broadband spectroscopic modalities.

Monitoring stress changes in carbon fiber reinforced polymer composites with GHz radiation

We performed proof of concept experiments to demonstrate that the reflected power of GHz illumination from the surface of carbon fiber reinforced polymer (CFRP) composites is linearly related to the stress in the material. We introduce a stress coefficient to describe the change in normalized power with applied stress, analogous to the stress-optic coefficient, because the effect is attributed to changes in the refractive index of the effective medium comprising the polymer matrix and carbon fibers. Stress coefficients of -0.549±0.134/GPa and -0.154±0.024/GPa were measured for two different composite materials, both linear in the measurement range of 40 MPa and 100 Mpa, respectively. This technique opens up the possibility of non-destructive evaluation of stresses in CFRP components for quality assurance in manufacturing and in structural health monitoring of in-service aerospace and automotive parts.

Analysis of granular packing structure by scattering of THz radiation

Scattering methods are widely used to characterize the structure and constituents of matter on small length scales. This motivates this introductory text on identifying prospective approaches to scattering-based methods for granular media. A survey to light scattering by particles and particle ensembles is given. It is elaborated why the established scattering methods using X-rays and visible light cannot in general be transferred to granular media. Spectroscopic measurements using terahertz radiation are highlighted as they probe the scattering properties of granular media, which are sensitive to the packing structure. Experimental details to optimize a spectrometer for measurements on granular media are discussed. We perform transmission measurements on static and agitated granular media using Fourier transform spectroscopy at the THz beamline of the Bessy II storage ring. The measurements demonstrate the potential to evaluate degrees of order in the media and to track transient structural states in agitated bulk granular media.

Research on the Terahertz Absorption Spectra of Histidine Enantiomer (L) and its Racemic Compound (DL)

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption spectra of polycrystalline L- and DL-histidine in the frequency range of 10-100 cm^-1. The spectra exhibit distinct differences in peak frequencies between the enantiomer (L-histidine) and racemic compound (DL-histidine). The observed spectral differences are attributed to the intermolecular interactions. With the density function theory (DFT) method, the frequencies of vibrational modes of L-histidine and DL-histidine in the THz range are calculated and well assigned according to the measured spectra. The origin of the observed vibrational modes is found to be non-localized and of a collective (phonon-like) nature, which points to the lattice and skeleton vibrations mediated by the hydrogen bond. Furthermore, we propose and demonstrate a method for determining the composition ratio of histidine mixtures based on the THz absorption spectra.

Influence of absorption on stability of terahertz difference frequency generation

This work presents numerical studies of the stability feature of terahertz difference frequency generation (THz-DFG) with a ZnGeP(2) crystal using two pump wavelengths. We found that the maximum output of a THz wave is located in the unstable output region because of the competitive equilibrium between the absorption and the gain. Furthermore, the output stability is dependent on the pump stability. Different from the results at the pump wavelength of 9.588 μm, there is neither an appropriate stable output region nor gain saturation region at the pump wavelength of 1.064 μm for a larger absorption coefficient. This work demonstrates that the stable output region of the THz wave is difficult to obtain when the pump absorption is excessively large in DFG.

Terahertz time-domain spectroscopy of glucose and uric Acid

We report the use ofterahertz time-domain spectroscopy for thestudy of two therapeutic bio-molecules:glucose and uric acid. Terahertztransmission spectra of crystalline samplesof both molecules were measured between 0.1-3.0 THz using an evacuated spectroscopysystem. We propose that the stereo-isomersof glucose show spectral featuresoriginating from intermolecular vibrationalmodes, as do uric acid and its derivativemolecule, allantoin. In addition, wepresent a full temperature dependence ofthe terahertz absorption of L-glucose.

T-Ray Sensing and Imaging

Terahertz (THz) radiation occupies part of the electromagnetic spectrum between the infrared and microwave bands. Until recently, technology at THz frequencies was under-developed compared to the rest of the electromagnetic spectrum, leaving a gap between millimeter waves and the far-infrared (FIR). In the past decade, interest in the THz gap has been increased by the development of ultrafast laser-based T-ray systems and their demonstration of diffraction-limited spatial resolution, picosecond temporal resolution, DC-THz spectral bandwidth and signal-to-noise ratios above 104.

This chapter reviews the development, the state of the art and the applications of T-ray spectrometers. Continuous-wave (CW) THz-frequency sources and detectors are briefly introduced in comparison to ultrafast pulsed THz systems. An emphasis is placed on experimental applications of T-rays to sensing and imaging, with a view to the continuing advance of technologies and applications in the THz band.

Spectral artifacts from non-uniform samples analyzed by terahertz time-domain spectroscopy

We report the impact of the spatial coherence distortion on the measured absorption spectra and the identification of materials analyzed by terahertz time-domain spectroscopy. It is shown that the deformation of the terahertz beam wave front can result into the overestimation of the electromagnetic absorption, the generation of artificial absorption peaks and even to the disappearance of characteristic absorption peaks. Obtaining clear absorption spectra without artifacts is crucial for applications based on terahertz imaging and spectroscopy.

Nondestructive evaluation of aircraft composites using transmissive terahertz time domain spectroscopy

Terahertz time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Damage to glass fiber was studied including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive index or absorption coefficient noticeably; however, material blistering was detected. Voids were located by TDS transmissive imaging using amplitude and phase techniques. The depth of delaminations was measured via the timing of Fabry-Perot reflections after the main pulse. Evidence of bending stress damage and simulated hidden cracks was also detected with terahertz imaging.

Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy

Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6).

Bayesian approach to non-Gaussian field statistics for diffusive broadband terahertz pulses

We develop a closed-form expression for the probability distribution function for the field components of a diffusive broadband wave propagating through a random medium. We consider each spectral component to provide an individual observation of a random variable, the configurationally averaged spectral intensity. Since the intensity determines the variance of the field distribution at each frequency, this random variable serves as the Bayesian prior that determines the form of the non-Gaussian field statistics. This model agrees well with experimental results.

Coherent plasmonic enhanced terahertz transmission through random metallic media

We experimentally show coherent, enhanced terahertz transmission through dense 3D random metallic media having subwavelength heterogeneity. Preservation of the incident polarization state and strong dispersion of the transmitted radiation indicate that the enhanced transmission is due to delocalized plasmonic propagation over distances 5 orders of magnitude greater than the skin depth. The experimental observations are supported by numerical finite difference time-domain simulations.

Spectral shifts as a signature of the onset of diffusion of broadband terahertz pulses

We describe measurements of polarization dynamics as a probe of multiple scattering of photons in a random medium by use of single-cycle terahertz pulses. We measure the degree of polarization and correlate it directly with the single-scattering regime in the time domain. We also measure the evolution of the temporal phase of the radiation and show that the average spectral content depends on the state of polarization. In the case of broadband radiation, this effect can be used to distinguish photons that have been scattered a few times from those that are propagating diffusively.

Propagation of terahertz pulses in random media

We describe measurements of single-cycle terahertz pulse propagation in a random medium. The unique capabilities of terahertz time-domain spectroscopy permit the characterization of a multiply scattered field with unprecedented spatial and temporal resolution. With these results, we can develop a framework for understanding the statistics of broadband laser speckle. Also, the ability to extract information on the phase of the field opens up new possibilities for characterizing multiply scattered waves. We illustrate this with a simple example, which involves computing a time-windowed temporal correlation between fields measured at different spatial locations. This enables the identification of individual scattering events, and could lead to a new method for imaging in random media.

Statistics of multiply scattered broadband terahertz pulses

We describe the first measurements of the diffusion of broadband single-cycle optical pulses through a highly scattering medium. Using terahertz time-domain spectroscopy, we measure the electric field of a multiply scattered wave with a time resolution shorter than one optical cycle. This time-domain measurement provides information on the statistics of both the amplitude and phase distributions of the diffusive wave. We develop a theoretical description, suitable for broadband radiation, which adequately describes the experimental results.

Terahertz wave imaging: horizons and hurdles

Terahertz (THz) science will profoundly impact biotechnology. It has tremendous potential for applications in imaging, medical diagnosis, health monitoring, environmental control and chemical and biological identification. THz research will become one of the most promising research areas in the 21st century for transformational advances in imaging, as well as in other interdisciplinary fields. However, terahertz wave (T-ray) imaging is still in its infancy. This paper discusses the uniqueness and limitations of T-ray imaging, identifies the major challenges impeding T-ray imaging and proposes solutions and opportunities in this field. It also concentrates on the generation, propagation and detection of T-rays by the use of femtosecond optics.

Scale model experimentation: using terahertz pulses to study light scattering

We describe a new class of experiments involving applications of terahertz radiation to problems in biomedical imaging and diagnosis. These involve scale model measurements, in which information can be gained about pulse propagation in scattering media. Because of the scale invariance of Maxwell's equations, these experiments can provide insight for researchers working on similar problems at shorter wavelengths. As a first demonstration, we measure the propagation constants for pulses in a dense collection of spherical scatterers, and compare with the predictions of the quasi-crystalline approximation. Even though the fractional volume in our measurements exceeds the limit of validity of this model, we find that it still predicts certain features of the propagation with reasonable accuracy.