Large Cavity Length Research Articles

High-Performance Quality Factor Based Sensor With Diagonal Cylinder Metasurface of the Bound State in the Continuum

High-quality-factor (high-Q-factor) electromagnetic resonance plays an important role in sensor applications. Previously proposed gas refractive index sensors are often limited by the large cavity length or microscale fabrication process in practical applications. Recently, ultra-high Q factor resonance based on the bound state in the continuum (BIC) has provided a feasible approach to solve these problems. In this paper, we propose a metasurface structure consisting of a single size tetramer cylinder. It supports dual band toroidal dipole (TD) resonances driven by BIC. The physical mechanism of double TD resonances is clarified by the multipole decomposition of the metasurface band structure and far-field scattering power. The sensor structure based on this achieves a sensitivity of 518.3 MHz/RIU, and the maximum line width does not exceed 680 kHz. The high-Q-factor electromagnetic resonance has the advantages of polarization independence and simplicity to manufacture. These findings will open up an avenue to develop the ultrasensitive sensor in the gigahertz regime.

High Precision Optical Path Difference Compensation Method Based on Three- Parameter Cosine Fitting Method

For low fineness extrinsic Fabry-Perot inter-ferometer (EFPI) sensors, the discrete gap transform (DGT) algorithm can be usually adopted to acquire the cavity length, and the magnitude of its demodulation error is often larger than several hundred picometer. In this paper, three-parameter cosine fitting (TPCF) method is proposed to compensate for the EFPI cavity length error caused by traditional spectrum truncation, which can improve the demodulation accuracy by 3800 times of that of DGT. This method has a large cavity length demodulation range of 20 μm ∼30000 μm with a maximum error of 65 pm, which was proved by numerical simulation. The experiment shows that when the cavity length is 31 μm and 282.3 μm, the demodulation resolutions with a confidence interval of 68.27% are 15.142 pm and 27.582 pm, respectively, and its actual demodulation time is no more than 0.021 s. Compared with the traditional cavity length demodulation algorithms, the proposed algorithm has broader demodulation range, higher demodulation accuracy and resolution. Moreover, it will be unrestricted by wavelength shift for more than one period and can realize real-time fast demodulation.

Environment-Robust Polarization-Based Phase-Shift Dynamic Demodulation Method for Optical Fiber Acoustic Sensor

In this paper, an environment-robust polarization-based phase-shift dynamic demodulation method for optical fiber acoustic sensor is proposed. Through optical lever effect of polarization low-coherence interference and amplitude scaling differential cross multiplication (AS-DCM) algorithm, acoustic-source induced interference phase of the acoustic sensor can be demodulated in real-time with high precision. The maximum relative phase demodulation error was only 0.694% when the cavity length had an offset ∼4 μm. The maximum cavity length demodulation error was keeping less than 3.988 nm under the optical power reduced ten times, which demonstrated an ultra-stable demodulation for the largest permitted optical power attenuation range, to our best knowledge. The proposed method has a capability of tolerating large optical power attenuation and large cavity length random offset, providing an environment-robust way for optical fiber acoustic sensor working in extreme environments.

A robust hollow metal-organic framework with enhanced diffusion for size selective catalysis.

Single crystalline (SC) hollow metal-organic frameworks (MOFs) are excellent host materials for molecular and nanoparticle catalysts. However, due to synthetic challenges, chemically robust SC hollow MOFs are rare. This work reports the construction of a defect-free and chemically stable SC hollow MOF, MOF-801(h), through templated growth from a unit cell mismatched core, UiO-66. Under the protection of excess MOF-801 ligand, fumaric acid, the MOF-801 shell was perfectly retained while the isoreticular UiO-66 core was selectively and completely etched away by formic acid. The combination of a large cavity, small aperture and short diffusion length allows the Pt nanoparticle encapsulated composite catalyst, Pt⊂MOF-801(h), to perform size selective hydrogenation of nitro compounds at an accelerated speed. Impressively, the catalyst can undergo concentrated HCl or boiling water treatment while maintaining its crystallinity, morphology, catalytic activity, and size selectivity. In addition, Au nanoparticles encapsulated catalyst, Au⊂MOF-801(h), was used for the size selective nucleophilic addition of HCl to terminal alkynes for the first time, which is a harsh reaction involving high concentrations of a strong acid.

Adaptive high-precision demodulation method based on vector matching and cluster-competitive particle swarm optimization for multiplexed FPIs.

Multiplexed fiber optic Fabry-Perot interferometer (FPI) sensors are well known for their precision, simple construction, simpler wiring, and high sensing qualities. However, the limitations on existing demodulation methods degrade the measurement accuracy of multiplexed FPI sensors and necessitate large cavity length differences. In this paper, we propose an adaptive high-precision demodulation method based on vector matching and cluster-competitive particle swarm optimization (CCPSO), which transforms cavity length demodulation into searching for the global extreme. The proposed CCPSO, which uses agglomeration within clusters and competition between clusters simultaneously, enables the improvement of the global extreme search capabilities. The theoretical analysis and experimental results show that the proposed demodulation method decreases the lower limit of the needed cavity length differences to 22 μm, which is reduced by 76.9% compared with the fast Fourier transform-based method. An accuracy of 1.05 nm is achieved with a cavity length difference of 27.5 μm and a signal-to-noise ratio of 36.0 dB for the noise.

НЕОБРАТИМЫЙ ГИСТЕРЕЗИС ТРЕХМЕРНЫХ ТОПОЛОГИЧЕСКИХ ЗАПУТАННЫХ ЛАЗЕРНЫХ СОЛИТОНОВ

In a laser medium with saturable amplification and absorption or in a laser with such a medium and a sufficiently large cavity length, the authors previously demonstrated a class of dissipative three-dimensional topological solitons (Veretenov et al., 2016, 2017). Such solitons have “skeletons” – a set of several closed and unclosed vortex lines, on which the field turns to 0 and the phase of which bypass is incremented by a multiple of 2π. It is significant that the regions of the scheme parameters in which different types of topological solitons exist and are stable, overlap. Accordingly, the question arises about the nature of changes in the structure of solitons and their skeletons with a slow change in the parameters. The answer to this question, mainly in the framework of numerical modeling, is the subject of this report. The initial equation is the generalized Ginzburg-Landau complex equation for a slowly varying envelope of the field, with the longitudinal coordinate along the path of preferential propagation z (Veretenov et al., 2017) serving as an evolution variable. The control parameter is the coefficient of linear (unsaturated) field gain g0. The initial value of this parameter corresponds to the central part of the stability region of the original soliton (for definiteness, the “Solomon soliton”). Then g0 slowly increases, crossing the stability boundary, after which it stabilizes and slowly decreases, eventually returning to its original value. We demonstrate that as a result of such a hysteresis cycle, the soliton does not return to the initial one — its structure is drastically simplified. This is reflected in a decrease in the values of various topological indices, as well as in a decrease in the field energy and an increase in the energy of the active medium (minus its constant background). At the g0 reduction stage, the topological characteristics do not change (two “soliton-apples”). The transient process includes a series of elementary reactions of reconnection of the vortex lines and detachment of closed loops after a strong bend of the “parent” vortex line. In addition, during the transition process, a number of new types of localized topological metastable structures arise. The analytical part of the research was supported by the RAS Presidium Program “Nonlinear Dynamics: Fundamental Problems and Applications”. Numerical modeling was carried out with the support of the grant of the RNF 18-12-00075.

High-sensitivity and large-dynamic-range refractive index sensors employing weak composite Fabry-Perot cavities.

Most sensors face a common trade-off between high sensitivity and a large dynamic range. We demonstrate here an all-fiber refractometer based on a dual-cavity Fabry-Perot interferometer (FPI) that possesses the advantage of both high sensitivity and a large dynamic range. Since the two composite cavities have a large cavity length difference, one can observe both fine and coarse fringes, which correspond to the long cavity and the short cavity, respectively. The short-cavity FPI and the use of an intensity demodulation method mean that the individual fine fringe dips correspond to a series of quasi-continuous highly sensitive zones for refractive index measurement. By calculating the parameters of the composite FPI, we find that the range of the ultra-sensitive zones can be considerably adjusted to suit the end requirements. The experimental trends are in good agreement with the theoretical predictions. The co-existence of high sensitivity and a large dynamic range in a composite FPI is of great significance to practical RI measurements.

Numerical analysis of the unsteady behavior of cloud cavitation around a hydrofoil based on an improved filter-based model

The unsteady cavitation evolution around the Clark-Y hydrofoil is investigated in this paper, by using an improved filter-base model (FBM) with the density correction method (DCM). To improve the prediction accuracy, the filter scale is adjusted based on the grid size. The numerical results show that a small filter scale is crucial for the unsteady simulations of the cavity shedding flow. The hybrid method that combines the FBM and the DCM could help to limit the overprediction of the turbulent viscosity in the cavitation region on the wall of the hydrofoil and in the wake. The large value of the maximum density ratio ρl/ρv,clip promotes the mass transfer rate between the liquid phase and the vapor phase, which results in a large sheet cavity length and the vapor fraction rise inside the cavity. The cavity patterns predicted by the improved method are verified by the experimental visualizations. The time-average lift, the drag coefficient and the primary oscillating frequency St for the cavitation number σ = 0.8, the angle of attack, α = 8°, at a Reynolds number Re = 7 × 105 are 0.735, 0.115 and 0.183, respectively, and the predicted errors are 3.29%, 3.36% and 8.93%. The typical three stages in one revolution are well-captured, including the initiation of the sheet/attached cavity, the growth toward the trailing edge (TE) with the development of the re-entrant jet flow, and the large scale cloud cavity shedding. It is observed that the cloud cavity shedding flow induces the vortex pairs of the TE vortices in the wake and the shedding vortices. The positive vorticity vortex of the re-entrant jet and the TE vortices interacts and merges with the negative vorticity vortex of the leading edge (LE) cavity to produce the shedding flow.

Electrically pumped ultraviolet lasing from ZnO in metal-insulator-semi devices

Electrically pumped ultraviolet random lasing was achieved in metal-insulator-semiconductor (MIS) diodes based on ZnO films at room temperature. The ZnO films were grown by plasma assisted molecular beam epitaxy. Two different kinds of insulator layers, SiOx (0<x≤2) and AlOx (0<x≤1.5) were deposited by electron beam evaporation. X-ray diffraction experiments found these oxide layers were amorphous (or microcrystals), and X-ray photoelectron spectroscopy confirmed the Si and Al were fully oxidized. Compared with devices using SiOx as the insulator layer, diodes with evaporated AlOx layers showed a lower working threshold forward current (∼20 mA to ∼26 mA) and higher emission intensity. Periodic features indicating formation of closed-loop paths were deduced by the power Fourier transform of electroluminescence spectra. The cavity length of both devices increased as forward currents increased, while a larger cavity length was always obtained in the AlOx-involved device under the same working current. The improved performance was attributed to larger hole amount in AlOx layers. These results revealed that evaporated AlOx can serve as good electron blocking and hole supplying layers for hetero-structures.

Analysis of Fabry-Perot optical micro-cavities based on coating-free all-Silicon cylindrical Bragg reflectors

We study the behavior of Fabry-Perot micro-optical resonators based on cylindrical reflectors, optionally combined with cylindrical lenses. The core of the resonator architecture incorporates coating-free, all-silicon, Bragg reflectors of cylindrical shape. The combined effect of high reflectance and light confinement produced by the reflectors curvature allows substantial reduction of the energy loss. The proposed resonator uses curved Bragg reflectors consisting of a stack of silicon-air wall pairs constructed by micromachining. Quality factor Q ~1000 was achieved on rather large cavity length L = 210 microns, which is mainly intended to lab-on-chip analytical experiments, where enough space is required to introduce the analyte inside the resonator. We report on the behavioral analysis of such resonators through analytical modeling along with numerical simulations supported by experimental results. We demonstrate selective excitation of pure longitudinal modes, taking advantage of a proper control of mode matching involved in the process of coupling light from an optical fiber to the resonator. For the sake of comparison, insight on the behavior of Fabry-Perot cavity incorporating a Fiber-Rod-Lens is confirmed by similar numerical simulations.

Compact Passively Q-Switched Yb:YAG Multipass Ring Lasers

An efficient and compact continuous-wave Yb:YAG laser with a slope efficiency of 50.3% with respect to the absorbed power was demonstrated using a multipass ring cavity (MPRC). Large cavity length ranges can be implemented using MPRCs with the same optical elements but different beam configurations. When Cr:YAG was used as the saturable absorber for passive Q-switching, various pulse widths and repetition rates were obtained. When a two-mirror MPRC, which is a compact ring laser cavity was used, the spatial hole-burning effect was reduced, and the spontaneous noise from the gain medium was not directly incident on the saturable absorber, causing less perturbation of the population difference of the saturable absorber than that of a linear cavity. This indicates that MPRCs can reduce the time jitter problem of a passively Q-switched laser. In this study, a Yb:YAG MPRC laser with a 33 ns pulse width is demonstrated, and the timing jitter of the repetition period was restrained to around 11%. The obtained experimental results are in good agreement with simulation results.

Dimensionality considerations of thermal transport mechanisms in a Thermal-Wave Cavity

The dimensionality of the photopyroelectric signal and the importance of radiation heat transfer mechanisms in a thermal-wave cavity (TWC) are closely investigated in the current study. Radiation components were added to the purely conduction model by linearizing the radiation heat transfer at the brass film-air boundary. A 3-D theoretical approach based on the Hankel integral was developed and implemented. Experimental results indicate that a 3-D model is necessary to describe the PPE signal, especially at low frequencies. Radiation is found to be the major contributor of the photopyroelectric (PPE) signal when high frequencies and large cavity lengths are used.

Optimally accurate thermal-wave cavity photopyroelectric measurements of pressure-dependent thermophysical properties of air: Theory and experiments

An experimental technique for the measurement of thermal properties of air at low pressures using a photopyroelectric (PPE) thermal-wave cavity (TWC) was developed. In addition, two theoretical approaches, a conventional one-dimensional thermal-wave model and a three-dimensional theory based on the Hankel integral, were applied to interpret the thermal-wave field in the thermal-wave cavity. The importance of radiation heat transfer mechanisms in a TWC was also investigated. Radiation components were added to the purely conductive model by linearizing the radiation heat transfer component at the cavity boundary. The experimental results indicate that the three-dimensional model is necessary to describe the PPE signal, especially at low frequencies where thermal diffusion length is large and sideways propagation of the thermal-wave field becomes significant. Radiation is found to be the dominant contributor of the PPE signal at high frequencies and large cavity lengths, where heat conduction across the TWC length is relatively weak. The three-dimensional theory and the Downhill Simplex algorithm were used to fit the experimental data and extract the thermal diffusivity of air and the heat transfer coefficient in a wide range of pressures from 760 to 2.6 Torr. It was shown that judicious adjustments of cavity length and computational best fits to frequency-scanned data using three-dimensional photopyroelectric theory lead to optimally accurate value measurements of thermal diffusivity and heat transfer coefficient at various pressures.

Numerical Computations on Heat Transfer Characteristics from the Cavity Bottom in Parallel Plates Swept by Viscoelastic Fluid

A two dimensional numerical study on flow and heat transfer of viscoelastic fluid in a cavity has been performed in order to investigate how effective the heat transfer augmentation technology is by using the Barus effect in the region of low Reynolds and Weissenberg numbers on the bottom wall of the cavity between ribs mounted in the parallel plates. In this paper, the heat transfer characteristics on the bottom of the cavity were focused on. The cavity length and the rib height were changed in three steps, respectively, while the wide flow path width and the solid wall temperature were kept constant. The solvent Reynolds number based on the rib height was set at 100. From the results, it is found that the flow penetration into the cavity effectively occurs just at the inlet of the cavity in the cases of viscoelastic fluids, while a large recirculating region is formed there for the cases of water. The local Nusslt number takes peaks at the position corresponding to the peak positions of velocity gradients on the bottom wall. It is also found that the mean Nusselt number increases with rib height and cavity length. However, the increase rate on the cavity length becomes smaller in a large cavity length region. This indicates there exists the optimum geometric condition and that the present technology has an advantage for making the heat exchanger compact in comparison with that using a water flow.

Coexistence of single-mode and multi-longitudinal mode emission in the ring laser model

A homogeneously broadened unidirectional ring laser can emit in several longitudinal modes for large enough pump and cavity length because of Rabi splitting induced gain. This is the so called Risken–Nummedal–Graham–Haken (RNGH) instability. We investigate numerically the properties of the multi-mode solution. We show that this solution can coexist with the single-mode one, and its stability domain can extend to pump values smaller than the critical pump of the RNGH instability. Moreover, we show that the multi-mode solution for large pump values is affected by two different instabilities: a pitchfork bifurcation, which preserves phase-locking, and a Hopf bifurcation, which destroys it.

Frequency-estimation-based signal-processing algorithm for white-light optical fiber Fabry–Perot interferometers

A novel signal-processing algorithm based on frequency estimation of the spectrogram of single-mode optical fiber Fabry-Perot interferometric sensors under white-light illumination is described. The frequency-estimation approach is based on linear regression of the instantaneous phase of an analytical signal, which can be obtained by preprocessing the original spectrogram with a bandpass filter. This method can be used for a relatively large cavity length without the need for spectrogram normalization to the spectrum of the light source and can be extended directly to a multiplexed sensor system. Experimental results show that the method can yield both absolute measurement with high resolution and a large dynamic range. Performance analysis shows that the method is tolerant of background noise and variations of the source spectrum.

Mutual injection locking of two individual Nd:YVO/sub 4/ lasers

We studied mutual injection locking of two individual continuous-wave Nd:YVO/sub 4/ lasers that were coupled through an external section. The two lasers lased at the same wavelength with a stable output power and interference fringes with high contrast ratio in the combined beam profile showed good mutual coherence of the two lasers. Mutual injection locking was also achieved without active stabilization for individual lasers with a large cavity length difference and low Q-factors. The calculated combined output power under symmetric pumping agreed with the experimental results. Even when output couplings of individual lasers are not optimized, the output coupling of the combining section can be optimized to make the extracted power very close to the sum of maximum powers that can be extracted from individual free running lasers under optimal output coupling.

Highly directional emission from colloidally synthesized nanocrystals in vertical cavities with small mode spacing

The optical properties of light emitting devices based on colloidally synthesized CdSe/CdS core/shell semiconductor nanocrystals embedded in vertical cavities are investigated. The cavities are several micrometers, thick and formed by a metallic mirror and a dielectric Bragg interference mirror deposited on the opposite surfaces of cleaved pristine mica sheets. Due to the large cavity length, up to 30 resonances are found within the Bragg mirror stop band. The corresponding small mode spacing allows one to extract a large portion of the broad nanocrystal luminescence band from the cavity upon optical excitation. The spontaneous emission of these cavities is highly forward directed with a beam divergence smaller than 1.3°. Furthermore, the emission is linearly polarized which is a result of the birefringent properties of the mica sheets.

Unsteady Characteristics of Cavitation on an Oscillating Hydrofoil. 2nd Report. Theoretical Analyses Using Singularity Method.

Two types of two-dimensional analyses using the singurality method were made to clarify the unsteady characteristics of the cavitation on an oscillating hydrofoil. By a linear analysis based on the assumption of infinitesimal disturbances, typical behaviors of cavitation due to the pitching motion of hydrofoil were made clear for the cases with both partial and super cavitations. By a time marching analysis which allows a large cavity length variation, the effect of the pitching motion of hydrofoil on the transitional cavity oscillation was well simulated. Both of the analyses were validated through the comparisons with the experiments described in the previous report.

Cryptands with Large Cavity Lengths Over 16 Å and N8O6Hetero-donors: Template Macrobicyclization and Structure of a Binuclear Copper(I) Cryptate

Hetero-tritopic N8O6 cryptands L1 and L2 were synthesized via 2 + 3 silver(I) template Schiff-base macrobicyclization of tris(2-aminoethyl)amine and tris(3-aminopropyl)amine with 4,4′-methylenedioxy-dibenzaldehyde, respectively. The resulting binuclear silver(I) cryptates [Ag2L1](CIO4)2 (1) and [Ag2L2](CIO4)2 (2) were characterized by various methods (elemental analyses, IR, UV-Vis, 1H and 13C NMR, and FAB mass spectra). The binuclear copper(I) cryptate [Cu2L1I(BF4)2·DMF·EtOH·H2O (3) was synthesized by transmetallation with 1 and studied by spectroscopic and X-ray structural analyses. It crystallizes in the monoclinic, space group P21/n with a = 16.088(5), b = 11.941(4), c = 34.08(1) Å, β = 91.69(2)°, Z = 4. d = 1.41 g cm−3. R 1 = 0.117, wR2 = 0.111 (on 3859 observed with I > 2σ(I)). The bicopper(I) cryptate looks like a twisted elongated ellipsoid, possessing cavity length 16.4Å and Cu(1) Cu(2) separation 12.132(5) Å. The two Cu(I) centers have similarly high stability towards oxidation. The cryptate does not show any reactivity or inclusion towards 4,4′-bipyridine, imidazole, N− 3, Na+, K+ and NH+ 4, mainly due to the highly effective distorted tetrahedron coordination around each Cu(I) enter and twisting among the three bridging-chains in the cryptand.