Volatile Gas Sensing through Terahertz Pipe Waveguide.

Gas sensing to recognize volatile liquids is successfully conducted through pipe-guided terahertz (THz) radiation in a reflective and label-free manner. The hollow core of a pipe waveguide can efficiently deliver the sensing probe of the THz confined waveguide fields to any place where dangerous vapors exist. Target vapors that naturally diffuse from a sample site into the pipe core can be detected based on strong interaction between the probe and analyte. The power variation of the THz reflectance spectrum in response to various types and densities of vapors are characterized experimentally using a glass pipe. The most sensitive THz frequency of the pipe waveguide can recognize vapors with a resolution at a low part-per-million level. The investigation found that the sensitivity of the pipe-waveguide sensing scheme is dependent on the vapor absorption strength, which is strongly related to the molecular amount and properties including the dipole moment and mass of a gas molecule.

Frequency-dependent skin penetration depth of terahertz radiation determined by water sorption-desorption.

A multilayered water-skin model is used to experimentally verify a new sensing method for determining the skin penetration depth of radiation with 0.1-0.9 terahertz (THz) frequencies. A water overlayer is dripped on a skin sample to form a multilayered structure for dynamically measuring the reflected THz-wave amplitude during water desorption. Skin penetration depths can be successfully derived by using the multilayered water-skin model and by considering the measured reflectivity, water dielectric constants, and effective thicknesses of the water overlayer on the skin sample. The maximum penetration depth is approximately 0.3 mm and is obtained with wave frequencies of 0.4-0.6 THz. This penetration depth encompasses the stratum corneum (SC) and part of the epidermis. The high penetration depth of 0.4-0.6 THz waves is also confirmed in the dried and damaged SC.

Terahertz artificial material based on integrated metal-rod-array for phase sensitive fluid detection.

A terahertz artificial material composed of metal rod array is experimentally investigated on its transmission spectral property and successfully incorporated into microfluidics as a miniaturized terahertz waveguide with an extended optical-path-length for label-free fluidic sensing. Theoretical and experimental characterizations of terahertz transmission spectra show that the wave guidance along the metal rod array originates from the resonance of transverse-electric-polarized waves within the metal rod slits. The extended optical path length along three layers of metal-rod-array enables terahertz waves sufficiently overlapping the fluid molecules embedded among the rods, leading to strongly enhanced phase change by approximately one order of magnitude compared with the blank metal-parallel-plate waveguide. Based on the enhanced phase sensitivity, three kinds of colorless liquid analytes, namely, acetone, methanol, and ethanol, with different dipole moments are identified in situ using the metal-rod-array-based microfluidic sensor. The detection limit in molecular amounts of a liquid analyte is experimentally demonstrated to be less than 0.1 mmol, corresponding to 2.7 µmol/mm2. The phase sensitive terahertz metal-rod-array-based sensor potentially has good adaptability in lab-chip technology for various practical applications, such as industrial toxic fluid detection and medical breath inspection.

All-fiber Mach-Zehnder interferometer based on two liquid infiltrations in a photonic crystal fiber.

We propose a novel all-fiber Mach-Zehnder interferometer (MZI) fabricated by infiltrating two separated liquid sections along a PCF. Due to the reduced effective index difference between the core region and the liquid-filled cladding region, the guided field in the liquid sections possesses a larger mode field area and can simultaneously induce the core mode and cladding modes of the empty PCF to form a MZI. The measured results demonstrate that very clear interference spectra can be obtained. By increasing the length of the MZIs, the decreased average fringe spacing can be observed. We have also measured the temperature sensitivity of our proposed PCF-based MZIs. Due to the existence of the two liquid sections, the temperature sensitivities can be enlarged to -0.176nm/°C and -0.53dB/°C.

Broadband Ce/Cr-doped crystal fibers for high axial resolution OCT light source.

The fabrication and characteristics of Ce/Cr-doped crystal fibers employing drawing tower technique are reported. The fluorescence spectrum of the Ce/Cr fibers at the core diameter ranging from 10 to 21 µm exhibited a 200-nm near-Gaussian broadband emission which enabled to provide an axial resolution of 1.8-µm and a power density of 79.1 nW/nm. The proposed broadband Ce/Cr-doped crystal fibers may be provided as a high-resolution light source for the use in optical coherence tomography system as well as industrial inspection and biomedical imaging applications.

Terahertz plasmonic waveguide based on metal rod arrays for nanofilm sensing.

A high-aspect-ratio metallic rod array is demonstrated to generate and propagate highly confined terahertz (THz) surface plasmonic waves under end-fire excitation. The transverse modal power distribution and spectral properties of the bound THz plasmonic wave are characterized in two metallic rod arrays with different periods and in two configurations with and without attaching a subwavelength superstrate. The integrated metallic rod array-based waveguide can be used to sense the various thin films deposited on the polypropylene superstrate based on the phase-sensitive mechanism. The sensor exhibits different phase detection sensitivities depending on the modal power immersed in the air gaps between the metallic rods. Deep-subwavelength SiO(2) and ZnO nanofilms with an optical path difference of 252 nm, which is equivalent to λ/3968 at 0.300 THz, are used as analytes to test the integrated plasmonic waveguide. Analysis of the refractive index and thickness of molecular membranes indicates that the metallic rod array-based THz waveguide can integrate various biochip platforms for minute molecular detection, which is extremely less than the coherent length of THz waves.

Using graphene nano-particle embedded in photonic crystal fiber for evanescent wave mode-locking of fiber laser.

A photonic crystal fiber (PCF) with high-quality graphene nano-particles uniformly dispersed in the hole cladding are demonstrated to passively mode-lock the erbium-doped fiber laser (EDFL) by evanescent-wave interaction. The few-layer graphene nano-particles are obtained by a stabilized electrochemical exfoliation at a threshold bias. These slowly and softly exfoliated graphene nano-particle exhibits an intense 2D band and an almost disappeared D band in the Raman scattering spectrum. The saturable phenomena of the extinction coefficient ß in the cladding provides a loss modulation for the intracavity photon intensity by the evanescent-wave interaction. The evanescent-wave mode-locking scheme effectively enlarges the interaction length of saturable absorption with graphene nano-particle to provide an increasing transmittance ΔT of 5% and modulation depth of 13%. By comparing the core-wave and evanescent-wave mode-locking under the same linear transmittance, the transmittance of the graphene nano-particles on the end-face of SMF only enlarges from 0.54 to 0.578 with ΔT = 3.8% and the modulation depth of 10.8%. The evanescent wave interaction is found to be better than the traditional approach which confines the graphene nano-particles at the interface of two SMF patchcords. When enlarging the intra-cavity gain by simultaneously increasing the pumping current of 980-nm and 1480-nm pumping laser diodes (LDs) to 900 mA, the passively mode-locked EDFL shortens its pulsewidth to 650 fs and broadens its spectral linewidth to 3.92 nm. An extremely low carrier amplitude jitter (CAJ) of 1.2-1.6% is observed to confirm the stable EDFL pulse-train with the cladding graphene nano-particle based evanescent-wave mode-locking.

Subwavelength confined terahertz waves on planar waveguides using metallic gratings.

A terahertz plasmonic waveguide is experimentally demonstrated using a plastic ribbon waveguide integrated with a diffraction metal grating to approach subwavelength-scaled confinement and long-distance delivery. Appropriately adjusting the metal-thickness and the periodical slit width of a grating greatly improves both guiding ability and field confinement in the hybrid waveguide structure. The measured lateral decay length of the bound terahertz surface waves on the hybrid waveguide can be reduced to less than λ/4 after propagating a waveguide of around 50mm-long in length. The subwavelength-confined field is potentially advantageous to biomolecular sensing or membrane detection because of the long interaction length between the THz field and analytes.

Specularly-Reflected Wave Guidance of Terahertz Plasmonic Metamaterial Based on the Metal-Wire-Woven Hole Arrays: Functional Design and Application of Transmission Spectral Dips.

Terahertz (THz) plasmonic metamaterial, based on a metal-wire-woven hole array (MWW-HA), is investigated for the distinct power depletion in the transmittance spectrum of 0.1-2 THz, including the reflected waves from metal holes and woven metal wires. Woven metal wires have four orders of power depletion, which perform sharp dips in a transmittance spectrum. However, only the first-order dip at the metal-hole-reflection band dominates specular reflection with a phase retardation of approximately π. The optical path length and metal surface conductivity are modified to study MWW-HA specular reflection. This experimental modification shows that the first order of MWW-HA power depletion is sustainable and sensitively correlated with a bending angle of the woven metal wire. Specularly reflected THz waves are successfully presented in hollow-core pipe wave guidance specified from MWW-HA pipe wall reflectivity.

Terahertz refractive index sensors using dielectric pipe waveguides.

A dielectric pipe waveguide is successfully demonstrated as a terahertz refractive index sensor for powder and liquid-vapor sensing. Without additional engineered structures, a simple pipe waveguide can act as a terahertz resonator based on anti-resonant reflecting guidance, forming multiple resonant transmission-dips. Loading various powders in the ring-cladding or inserting different vapors into the hollow core of the pipe waveguide leads to a significant shift of resonant frequency, and the spectral shift is related to the refractive-index change. The proven detection limit of molecular density could be reduced to 1.6nano-mole/mm3 and the highest sensitivity is demonstrated at around 22.2GHz/refractive-index-unit (RIU), which is comparable to the best THz molecular sensor [Appl. Phys. Lett. 95, 171113 (2009)].

Highly Transparent and Polarization-Maintained Terahertz Plasmonic Metamaterials Based on Metal-Wire-Woven Hole Arrays: Fundamentals and Characterization of Transmission Spectral Peaks.

Metal-hole-supported terahertz (THz) waves through the structure of a metal-wire-woven hole array (MWW-HA) present high-frequency-passed transmittance spectra of one plasmonic metamaterial with artificial plasmonic frequencies, which are inversely proportional to metal-hole widths. For the transmitted THz waves of MWW-HA, transverse-electric (TE) and transverse-magnetic (TM) waveguide modes mix within a symmetric metal-hole boundary. THz resonance waves transversely crossing the holes of MWW-HA are experimentally characterized with spectral peaks in the frequency range of 0.1-2 THz that are correlated with aperture sizes, unit-cell-hole widths, metal-wire thicknesses, and wire-bending angles. The metal-hole-transported resonance waves of MWW-HA are dominated by TE waveguide modes instead of TM ones because a hole width of MWW-HA is approximate to the half wavelength of a resonance wave. The round metal edges of the woven metal wires can minimize the effective optical length of a thick metal hole to transmit THz resonance waves, thereby resulting the smallest rotation angle of linear polarization and high transmittance up to 0.94. An MWW-HA structure is therefore reliable for supporting metal-hole resonance waves with low resistance, whereas a metal-slab-perforated hole array cannot achieve the same result.

Mode multiplexer for multimode transmission in multimode fibers.

We have numerically demonstrated an efficient mode multiplexer which can tailor the input field patterns by using a phase controller and a mode coupler formed by four single-mode fibers (SMFs). By connecting the mode multiplexer to a multimode fiber (MMF), two orthogonal higher-order modes of the MMF can be simultaneously excited to form two communication channels. The simulated results show that very low modal interference between the two excited modes can be achieved by using the proposed mode multiplexer. We have also discussed the effect of the distance and size of the SMFs in the mode coupler on the performance of the proposed mode multiplexer.

Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers.

A novel light-controllable long-period fiber grating (LPFG) is demonstrated by making use of a PCF infiltrated with a photoresponsive liquid crystal (LC) mixture consisting of nematic LC molecules and light-sensitive 4-methoxyazobenzene (4MAB). With the aid of the photo-induced isomerization of 4MAB, the refractive index of the LC mixture can be modulated and the periodic index perturbation along the fiber can be achieved by exposing the PCF to a blue laser through a mask. The resonance wavelength and dip depth of the LPFG can be controlled by using different blue-laser irradiation time, numbers of period, and 4MAB concentrations. In addition, the photo-induced LPFG is erasable under green-laser illumination.

Birefringent photonic crystal fiber coils and their application to transverse displacement sensing.

We have experimentally investigated the birefringent properties of photonic crystal fiber (PCF) coils in cooperation with a Sagnac loop interferometer. By reducing the bending radius of the PCF coils, very clear interference patterns can be observed for the bending-induced stress effect. Increasing the fiber turns can result in more obvious interference patterns with smaller fringe spacing but has no contribution to the increment of the birefringence value. The fabricated PCF coil is employed in the transverse displacement sensing. Very high sensing sensitivity of 90.4 nm/mm can be achieved due to the large displacement-induced bending radius variations.

Polarization independent Fabry-Pérot filter based on polymer-stabilized blue phase liquid crystals with fast response time.

This work demonstrates a polarization-independent electrically tunable Fabry-Pérot (FP) filter that is based on polymer-stabilized blue phase liquid crystals (PSBPLCs). An external vertical electric field can be applied to modulate the effective refractive index of the PSBPLCs along the optical axis. Therefore, the wavelength-tuning property of the FP filter is completely independent of the polarization state of the incident light. The change in the birefringence in PSBPLCs is governed by Kerr effect-induced isotropic-to-anisotropic transition, and so the PSBPLCs based FP filter has a short response time. The measured tunability and free spectral range of the FP filter are 0.092 nm/ V and 16nm in the visible region, and 0.12nm/ V and 97nm in the NIR region, respectively, and the response time is in sub-millisecond range. The fast-responding polarization-independent electrically tunable FP filter has substantial potential for practical applications.

All-optically controllable dye-doped liquid crystal infiltrated photonic crystal fiber.

A novel demonstration of an all-optically controllable dye-doped liquid crystal infiltrated photonic crystal fiber (DDLCIPCF) is presented. Overall spectral transmittance of the DDLCIPCF can decrease and then increase with a concomitant red-shift of the spectrum curve with increasing irradiation time of one UV beam. Continuing irradiation of one green beam following UV illumination on the DDLCIPCF can cause the transmission spectrum to recover completely. The reversible all-optical controllability of the photonic band structure of the fiber is attributable to the isothermal planar nematic (PN)→scattering (S)→isotropic (I) and I→S→PN state transitions of the LCs via the UV-beam-induced trans→cis and green-beam-induced cis→trans back isomerizations of the azo-dye, respectively, in the cladding of the DDLCIPCF. The photoinduced appearance of the S state and the variation of the index modulation between the core and the cladding of the fiber result in the variation of overall spectral transmittance and the shift of transmission spectrum, respectively.

Subwavelength film sensing based on terahertz anti-resonant reflecting hollow waveguides.

A simple dielectric hollow-tube has been experimentally demonstrated at terahertz range for bio-molecular layer sensing based on the anti-resonant reflecting wave-guidance mechanism. We experimentally study the dependence of thin-film detection sensitivity on the optical geometrical parameters of tubes, different thicknesses and tube wall refractive indices, and on different resonant frequencies. A polypropylene hollow-tube with optimized sensitivity of 0.003 mm/µm is used to sense a subwavelength-thick (λ/225) carboxypolymethylene molecular overlayer on the tube's inner surface, and the minimum detectable quantity of molecules could be down to 1.22 picomole/mm(2). A double-layered Fabry-Pérot model is proposed for calculating the overlayer thicknesses, which agrees well with the experimental results.

Photo and electrical tunable effects in photonic liquid crystal fiber.

This work demonstrates photo alignment and electrical tuning effects in photonic liquid crystal fiber (PLCF). Applying voltages of 0 approximately 130V and 250 approximately 400V shifts the short and long wavelength edges of the transmission bands by about 45 nm and 74 nm toward longer wavelengths, respectively. An electro-tunable notch filter is formed in the PLCF without the use of gratings. The range of tunability of the notch filter is around 180 nm with an applied voltage of 140 approximately 240 V. This photo-induced alignment yields a permanently tilted LC structure in PCF, which reduces the threshold voltage, and can be further modulated by electric fields. The polarization dependent loss and fast response time of photo-aligned PLCF is also demonstrated. The finite-difference frequency-domain method is adopted to analyze the shift of the transmission bandgap, and the simulation results are found to correlate well with experimental data.

Selectively liquid-filled photonic crystal fibers for optical devices.

We have theoretically investigated the propagation properties of two kinds of selectively liquid-filled PCFs. For internally liquid-filled PCFs, the outer air-hole layers function as the second cladding to reduce the penetration of the light field while the inner liquid-hole layers can still induce the tunable PBG effect. The complementary structures, externally liquid-filled PCFs, can be used in long-period fiber gratings to decrease the utilization of the lossy liquids and remain single-mode operation for the existence of the inner air-hole layers. The confinement losses of both selectively liquid-filled PCFs are shown to be efficiently reduced due to the outer or inner air-hole layers, which is quite useful for further applications.

Tunable dual-core liquid-filled photonic crystal fibers for dispersion compensation.

We have theoretically investigated the dispersion characteristics of dispersion compensating fibers based on dual-core liquid-filled PCFs. A very high negative chromatic dispersion value D = -19000 ps/(nm-km) can be achieved at 1.55-microm wavelength by an appropriate design. By varying the geometry of the PCF and the index of the filling liquid, the phase-matching wavelength and dispersion values are shown to be well tuned to desired values. The proposed structure also demonstrates good tunable properties with operation temperature for optical communication systems.