The editorial introduces a special section of the Journal of Biomedical Optics on Advances in Terahertz Biomedical Science and Applications. This special section includes one review and ten research papers addressing the complex challenges of terahertz biophotonics and related areas of biomedical optics.
We perform a comprehensive analysis of uniform-velocity bilayer spacetime crystals, combining concepts of conventional photonic crystals and special relativity. Given that a spacetime crystal consists of a sequence of spacetime discontinuities, we do this by solving the following sequence of problems: (1) the spacetime interface, (2) the double spacetime interface, or spacetime slab, (3) the unbounded crystal, and (4) the truncated crystal. For these problems, we present the following results: (1) an extension of the Stokes principle to spacetime interfaces, (2) an interference-based analysis of the interference phenomenology, (3) a quick linear approximation of the dispersion diagrams, a description of simultaneous wavenumber and frequency bandgaps, and (4) the explanation of the effects of different types of spacetime crystal truncations and the corresponding scattering coefficients. This work may constitute the foundation for a virtually unlimited number of novel canonical spacetime media and metamaterial problems.
We report an all-polymer flexible piezoelectric fiber that uses both judiciously chosen geometry and advanced materials in order to enhance fiber piezoelectric response. The microstructured/nanostructured fiber features a soft hollow polycarbonate core surrounded with a spiral multilayer cladding consisting of alternating layers of piezoelectric nanocomposites (polyvinylidene enhanced with BaTiO3, PZT or CNT) and conductive polymer (carbon filled polyethylene). The conductive polymer layers serve as two electrodes and they also form two spatially offset electric connectors on the fiber surface designed for the ease of connectorization. Kilometer-long piezoelectric fibers of submilimeter diameters are thermally drawn from a macroscopic preform. The fibers exhibit high output voltage of up to 6V under moderate bending, and they show excellent mechanical and electrical durability in a cyclic bend-release test. The micron/nano-size multilayer structure enhances in-fiber poling efficiency thanks to the small distance between the conducting electrodes sandwiching the piezoelectric composite layers. Additionally, spiral structure greatly increases the active area of the piezoelectric composite, thus promoting higher voltage generation and resulting in 10-100 higher power generation efficiency over the existing piezoelectric cables. Finally, we weave the fabricated piezoelectric fibers into technical textiles and demonstrate their potential applications in power generation when used as a sound detector and a wearable textiles
We present a theoretical formulation and an experimental demonstration of a fast compression-less terahertz imaging technique based on broadband Fourier optics. The technique exploits k-vector/frequency duality in Fourier optics which allows to use a single-pixel detector to perform angular scan along a circular path, while the broadband spectrum is used to scan along the radial dimension in Fourier domain. The proposed compression-less image reconstruction technique (hybrid inverse transform) requires only a small number of measurements that scales linearly with the image linear size, thus promising real-time acquisition of high-resolution THz images. We develop an algorithm based on a polar formulation of the Fourier transform to reconstruct the image. First, we show how the equations are transformed when passing from a spatial integral to a frequency integral. Second, we analytically demonstrate that, in the case of binary amplitude objects and phase objects, the reconstructed image from our formulation is proportional to the original object. Third, we experimentally demonstrate the image reconstruction method in the two above-mentioned cases: we use a metal aperture for the binary object and an engraving in a polymer sample for the phase object. A detailed analysis of the novel technique advantages and limitations is presented, and its place among other existing THz imaging techniques is clearly identified.
We present a semi-analytical solution for the design of a high-speed rotary optical delay line that use a combination of two rotating curvilinear reflectors. We demonstrate that it is possible to design an infinite variety of the optical delay lines featuring linear dependence of the optical delay on the rotation angle. This is achieved via shape optimization of the rotating reflector surfaces. Moreover, a convenient spatial separation of the incoming and outgoing beams is possible. For the sake of example, we present blades that fit into a circle of 10cm diameter. Finally, a prototype of a rotary delay line is fabricated using CNC machining, and its optical properties are characterized.
We demonstrate detection of liquid analyte refractive index by using a hollow-core photonic Bragg fiber. We apply this fiber sensor to monitor concentrations of commercial cooling oil. The sensor operates on a spectral modality. Variation in the analyte refractive index modifies the bandgap guidance of a fiber, leading to spectral shifts in the fiber transmission spectrum. The sensitivity of the sensor to changes in the analyte refractive index filling in the fiber core is found to be 1460nm/Refractive index unit (RIU). By using the spectral modality and effective medium theory, we determine the concentrations of commercial fluid from the measured refractive indices with an accuracy of ~0.42%. The presented fiber sensor can be used for on-line monitoring of concentration of many industrial fluids and dilutions with sub-1%v accuracy.
Fabrication, characterization, and applications of a fast rotary linear optical delay line (FRLODL) for THz time-domain spectroscopy are presented. The FRLODL features two reflective surfaces with spatially separated incoming and outgoing beams. It has been manufactured using CNC machining. A linear dependence of the optical delay on the rotation angle allows a straightforward extraction of the conversion factor between the acquisition time (in ms) and the terahertz pulse time (in ps). The FRLODL has been tested using rotation speeds of up to 48 Hz, corresponding to an acquisition rate of up to 192 Hz with four blades incorporated on the same disk. At high speeds we observe a decrease of the bandwidth due to the limitations of the electronics, in particular, the transimpedance amplifier. An error analysis is performed by experimentally evaluating the signal-to-noise ratio and the dynamic range. With regard to the applications of the FRLODL, we first present observation of the evaporation of liquids, namely water, acetone and methanol. We then demonstrate monitoring of the spray painting process. Finally, detection of fast moving objects at 1 m/s and their thickness characterization are presented.
Integration of optical functionalities such as light emission, processing and collection into flexible woven matrices of
fabric have grabbed a lot of attention in the last few years. Photonic textiles frequently involve optical fibers as they can
be easily processed together with supporting fabric fibers. This technology finds uses in various fields of application
such as interactive clothing, signage, wearable health monitoring sensors and mechanical strain and deformation
detectors. Recent development in the field of Photonic Band Gap optical fibers (PBG) could potentially lead to novel
photonic textiles applications and techniques. Particularly, plastic PBG Bragg fibers fabricated in our group have strong
potential in the field of photonic textiles as they offer many advantages over standard silica fibers at the same low cost.
Among many unusual properties of PBG textiles we mention that they are highly reflective, PBG textiles are colored
without using any colorants, PBG textiles can change their color by controlling the relative intensities of guided and
reflected light, and finally, PBG textiles can change their colors when stretched. Some of the many experimental
realization of photonic bandgap fiber textiles and their potential applications in wearable displays are discussed.
The origin and the behavior of the birefringence of solid-core
air-silica microstructured fibers is described with
the help of a simple approximate model. The first two modes of three different types of fibers are studied.
Numerical results, obtained from both finite element and boundary integral methods calculations, are presented
to support the validity of the model and to delineate its limits.
Biodegradable microstructured polymer optical fibers have been created using synthetic biomaterials such as poly(L-lactic acid), poly(-caprolactone), and cellulose derivatives. Original processing techniques were utilized to fabricate a variety of biofriendly microstructured fibers that hold potential for in vivo light delivery, sensing, and controlled drug-release.
We propose various designs of porous polymer fibers for guiding terahertz radiation. Numerical simulations are
presented for three fiber geometries: a Bragg fiber consisting of periodic multilayers of ferroelectric polyvinylidene
fluoride (PVDF) and polycarbonate (PC), a sub-wavelength waveguide containing multiple sub-wavelength holes, as
well as a cobweb-like porous Bragg fiber consisting of solid film layers suspended by a network of bridges. Various
properties of these fibers are presented. Emphasis is put on the optimization of the geometries to increase the fraction of
power guided in the air, thereby alleviating the effects of material absorption. Losses of about 10 dB/m, 7.8 dB/m, and 1.7 dB/m at 1 THz are respectively predicted for these three structures.
By formulating Maxwell's equations in perturbation matched curvilinear coordinates, we have derived the rigorous perturbation
theory (PT) and coupled mode theory (CMT) expansions that are applicable in the case of generic non-uniform dielectric profile
perturbations in high index-contrast waveguides, including photonic band gap fibers, 2D and 3D waveguides. PT is particularly useful in the optimization stage of a component design process where fast evaluation of an optimized property with changing controlling variables is crucial. We demonstrate our method by studying radiation scattering due to common geometric variations in planar 2D photonic crystals waveguides. We conclude the paper by statistical analysis of experimental images of 2D planar PCs to characterize common imperfections in such structures.
We characterize coupling between two identical collinear hollow core Bragg fibers, assuming TE01 launching condition. Using multipole method and finite element method we investigate dependence of the beat length between supermodes of the coupled fibers and supermode radiation losses as a function of the inter-fiber separation, fiber core radius and index of the cladding. We established that coupling is maximal when fibers are touching each other decreasing dramatically during the first tens of nanometers of separation. However, residual coupling with the strength proportional to the fiber radiation loss is very long range decreasing as an inverse square root of the inter-fiber separation, and exhibiting periodic variation with inter-fiber separation. Finally, coupling between the TE01 modes is considered in a view of designing a directional coupler. We find
that for fibers with large enough core radii one can identify broad frequency ranges where inter-modal coupling strength exceeds super-mode radiation losses by an order of magnitude, thus opening a possibility of building a directional coupler. We attribute such
unusually strong inter-mode coupling both to the resonant effects
in the inter-mirror cavity as well as a proximity interaction between the leaky modes localized in the mirror.
We characterize coupling between two identical collinear hollow
core Bragg fibers, assuming T01 launching condition. Using multipole method and finite element method we investigate dependence of the beat length between supermodes of the coupled fibers and supermode radiation losses as a function of the
inter-fiber separation, fiber core radius and index of the
cladding. We established that coupling is maximal when fibers are
touching each other decreasing dramatically during the first tens
of nanometers of separation. However residual coupling with the
strength proportional to the fiber radiation loss is very long
range decreasing as an inverse square root of the inter-fiber
separation, and exhibiting periodic variation with inter-fiber
separation. Finally, coupling between the T01 modes is considered in a view of designing a directional coupler. We find that for fibers with large enough core radii one can identify
broad frequency ranges where inter-modal coupling strength exceeds
super-mode radiation losses by an order of magnitude, thus opening
a possibility of building a directional coupler. We attribute such
unusually strong inter-mode coupling both to the resonant effects
in the inter-mirror cavity as well as a proximity interaction
between the leaky modes localized in the mirror.
Standard perturbation theory (PT) and coupled mode theory (CMT)
formulations fail or exhibit very slow convergence when applied to
the analysis of geometrical variations in high index-contrast
optical components such as Bragg fibers and photonic crystals
waveguides. By formulating Maxwell's equations in perturbation
matched curvilinear coordinates, we have derived several rigorous
PT and CMT expansions that are applicable in the case of generic
non-uniform dielectric profile perturbations in high
index-contrast waveguides. In strong fiber tapers and fiber Bragg
gratings we demonstrate that our formulation is accurate and
rapidly converges to an exact result when used in a CMT framework
even in the high index-contrast regime. We then apply our method
to investigate the impact of hollow Bragg fiber ellipticity on its
Polarization Mode Dispersion (PMD) characteristics for telecom
applications. Correct PT expansions allowed us to design an
efficient optimization code which we successfully applied to the
design of dispersion compensating hollow Bragg fiber with
optimized low PMD and very large dispersion parameter. We have
also successfully extended this methodology to treat radiation
scattering due to common geometric variations in generic photonic
crystals. As an example, scattering analysis in strong 2D photonic
crystal tapers is demonstrated.
We argue that OmniGuide fibers, which guide light within a hollow core by concentric multilayer films having the property of omnidirectional reflection, have the potential to lift several physical limitations of silica fibers. We show how the strong confinement in OmniGuide fibers greatly suppresses the properties of the cladding materials: even if highly lossy and nonlinear materials are employed, both the intrinsic losses and nonlinearities of silica fibers can be surpassed by orders of magnitude. This feat, impossible to duplicate in an index-guided fiber with existing materials, would open up new regimes for long-distance propagation and dense wavelength-division multiplexing (DWDM). The OmniGuide-fiber modes bear a strong analogy to those of hollow metallic waveguides; from this analogy, we are able to derive several general scaling laws with core radius. Moreover, there is strong loss discrimination between guided modes, depending upon their degree of confinement in the hollow core: this allows large, ostensibly multi-mode cores to be used, with the lowest-loss TE01 mode propagating in an effectively single-mode fashion. Finally, because this TE01 mode is a cylindrically symmetrical ('azimuthally' polarized) singlet state, it is immune to polarization-mode dispersion (PMD), unlike the doubly-degenerate linearly-polarized modes in silica fibers that are vulnerable to birefringence.
New type of fiber optic sensor of electrostatic field is considered. Using the interferometric detection technique the sensitivity of about 0.2 (V/m)/(root)Hz was achieved. Theoretical evaluation yields the thermal fluctuation limit of sensitivity of about 2.5 X 10-4 (V/m)/(root)Hz.