Wave Cavity Research Articles

Enhanced acousto-optic interaction in two-dimensional phoxonic crystals with a line defect

This study presents acousto-optic interaction of optical waves in a two-dimensional phoxonic crystal with a line defect. Because of dual photonic and phononic band gaps generated in the phoxonic crystal, optical waves and acoustic modes can be guided and amplified, respectively, along the line defect that serves simultaneously as an optical waveguide and acoustic wave cavity. By means of finite-element analysis, we show that the confinement of the optical waves and acoustic modes in the same region of space (i.e., in the defect) leads to enhanced modulation of the optical waves by an acoustic cavity mode, and obvious shifts in eigenfrequencies and transmission peaks are observed. Stronger acousto-optic interaction is caused by the amplified acoustic fields and by the long-lifetime interaction of photons and phonons in the line defect.

PH-dependent electronic surface spectra of chromophore species in the charged silica–water interface

The structure of the pH-dependent silica–water interface has been probed with two charged counter-ion chromophores Crystal Violet (CV) and Malachite Green (MG) by evanescent wave cavity ring-down electronic spectroscopy. The electronic spectra of these species in the charged interface have been measured directly over the wavelength range 570–680 nm. The effective CV interfacial counter-ion concentration is 185-fold larger than the bulk with a fully charged silica surface at pH 9, corresponding to a surface concentration of 0.26 molecules nm−2. The chromophore–silica surface association and dissociation kinetics indicate a binding energy at pH 9 >28 kJ mol−1 for each chromophore: a loosely bound molecular layer. The electronic spectrum of CV in the interface shows two maxima: one feature is blue-shifted 2 nm from the solution-phase maximum at 583 nm, and the second feature is red-shifted 24 nm. A Lorentzian deconstruction of the spectra indicates that they are consistent with two molecular environments: one closely associated with the surface sharing delocalised electrons and a second confined to the interface but randomly oriented.

Experimental and modeling study of the oxidation of n-butane in a jet stirred reactor using cw-CRDS measurements.

The gas-phase oxidation of n-butane has been studied in an atmospheric jet-stirred reactor (JSR) at temperatures up to 950 K. For the first time, continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared has been used, together with gas chromatography (GC), to analyze the products formed during its oxidation. In addition to the quantification of formaldehyde and water, which is always difficult by GC, cw-CRDS allowed as well the quantification of hydrogen peroxide (H2O2). A comparison of the obtained mole fraction temperature profiles with simulations using a detailed gas-phase mechanism shows a good agreement at temperatures below 750 K, but an overestimation of the overall reactivity above this temperature. Also, a strong overestimation was found for the H2O2 mole fraction at higher temperatures. In order to improve the agreement between model and experimental results, two modifications have been implemented to the model: (a) the rate constant for the decomposition of H2O2 (+M) ↔ 2OH (+M) has been updated to the value recently proposed by Troe (Combust. Flame, 2011, 158, 594-601) and (b) a temperature dependent heterogeneous destruction of H2O2 on the hot reactor walls with assumed rate parameters has been added. The improvement (a) slows down the overall reactivity at higher temperatures, but has a negligible impact on the maximal H2O2 mole fraction. Improvement (b) has also a small impact on the overall reactivity at higher temperatures, but a large effect on the maximal H2O2 mole fraction. Both modifications lead to an improved agreement between model and experiment for the oxidation of n-butane in a JSR at temperatures above 750 K.

Using cavity ringdown spectroscopy for continuous monitoring of δ13C(CO2) and ƒCO2 in the surface ocean

The role of the global surface ocean as a source and sink for atmospheric carbon dioxide and the flux strengths between the ocean and the atmosphere can be quantified by measuring the fugacity of CO2 (ƒCO2) as well as the dissolved inorganic carbon (DIC) concentration and its isotopic composition in surface seawater. In this work, the potential of continuous wave cavity ringdown spectroscopy (cw‐CRDS) for autonomous underway measurements of ƒCO2 and the stable carbon isotope ratio of DIC [δ13C(DIC)] is explored. For the first time, by using a conventional air‐sea equilibrator setup, both quantities were continuously and simultaneously recorded during a field deployment on two research cruises following meridional transects across the Atlantic Ocean (Bremerhaven, Germany—Punta Arenas, Chile). Data are compared against reference measurements by an established underway CO2 monitoring system and isotope ratio mass spectrometric analysis of individual water samples. Agreement within ΔƒCO2 = 0.35 µatm for atmospheric and ΔƒCO2 = 2.5 µatm and Δδ13C(DIC) = 0.33‰ for seawater measurements have been achieved. Whereas “calibration‐free” ƒCO2 monitoring is feasible, the measurement of accurate isotope ratios relies on running reference standards on a daily basis. Overall, the installed CRDS/equilibrator system was shown to be capable of reliable online monitoring of ƒCO2, equilibrium δ13C(CO2), δ13C(DIC), and pO2 aboard moving research vessels, thus making possible corresponding measurements with high spatial and temporal resolution.

Quantification of hydrogen peroxide during the low-temperature oxidation of alkanes.

The first reliable quantification of hydrogen peroxide (H(2)O(2)) formed during the low-temperature oxidation of an organic compound has been achieved thanks to a new system that couples a jet stirred reactor to a detection by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared. The quantification of this key compound for hydrocarbon low-temperature oxidation regime has been obtained under conditions close to those actually observed before the autoignition. The studied hydrocarbon was n-butane, the smallest alkane which has an oxidation behavior close to that of the species present in gasoline and diesel fuels.

Evanescent Wave Cavity Ring-Down Spectroscopy (EW-CRDS) as a Probe of Macromolecule Adsorption Kinetics at Functionalized Interfaces

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been employed to study the interfacial adsorption kinetics of coumarin-tagged macromolecules onto a range of functionalized planar surfaces. Such studies are valuable in designing polymers for complex systems where the degree of interaction between the polymer and surface needs to be tailored. Three tagged synthetic polymers with different functionalities are examined: poly(acrylic acid) (PAA), poly(3-sulfopropyl methacrylate, potassium salt) (PSPMA), and a mannose-modified glycopolymer. Adsorption transients at the silica/water interface are found to be characteristic for each polymer, and kinetics are deduced from the initial rates. The chemistry of the adsorption interfaces has been varied by, first, manipulation of silica surface chemistry via the bulk pH, followed by surfaces modified by poly(L-glutamic acid) (PGA) and cellulose, giving five chemically different surfaces. Complementary atomic force microscopy (AFM) imaging has been used for additional surface characterization of adsorbed layers and functionalized interfaces to allow adsorption rates to be interpreted more fully. Adsorption rates for PSPMA and the glycopolymer are seen to be highly surface sensitive, with significantly higher rates on cellulose-modified surfaces, whereas PAA shows a much smaller rate dependence on the nature of the adsorption surface.

Multipacting simulation and analysis of a taper quarter wave cavity by using Analyst-PT3P

Since the tapered quarter wave resonator (QWR) cavity is proven to have a much lower peak surface magnetic field in the short plate and a lower peak surface electric field near the beam tube compared with the straight outer conductor QWR, it has been recommended for the separated sector cyclotron linac injector system in the heavy ion research facility in Lanzhou. This paper is focused on the multipacting (MP) analysis for the tapered QWR with a frequency of 80.5 MHz and beta of 0.085. Using the Analyst program, MP bands can be simulated and analyzed with the Particle Tracking module to identify potential problems in the cavity design. This paper will present the simulation results of MP for the tapered QWR cavity.

A rigid, monolithic but still scannable cavity ring-down spectroscopy cell

A novel cell for continuous wave cavity ring-down spectroscopy (cw-CRDS) is described and tested. The cell is monolithic and maintains a rigid alignment of the two cavity mirrors. Two high-resolution and high-force piezoelectric transducers are used to sweep the length of the cell by elastic deformation of the 2.86 cm outer diameter stainless steel tube that makes up the body of the cell. The cavity length is scanned more than 1/2 wavelength of the near-IR light used, which ensures that at least one TEM(00) mode of the cavity will pass through resonance with the laser. This allows the use of a frequency-locked-laser cw-CRDS technique, which increases the precision of the measurements compared to the alternative of sweeping the laser more than one free spectral range of the cavity. The performance of the cell is demonstrated by using it to detect the absorption spectrum of methane (CH(4)) at the wavenumber regions of around 6051.8-6057.7 cm(-1).

Weak turbulence in the magnetosphere: Formation of whistler wave cavity by nonlinear scattering

We consider the weak turbulence of whistler waves in the in low-β inner magnetosphere of the earth. Whistler waves, originating in the ionosphere, propagate radially outward and can trigger nonlinear induced scattering by thermal electrons provided the wave energy density is large enough. Nonlinear scattering can substantially change the direction of the wave vector of whistler waves and hence the direction of energy flux with only a small change in the frequency. A portion of whistler waves return to the ionosphere with a smaller perpendicular wave vector resulting in diminished linear damping and enhanced ability to pitch-angle scatter trapped electrons. In addition, a portion of the scattered wave packets can be reflected near the ionosphere back into the magnetosphere. Through multiple nonlinear scatterings and ionospheric reflections a long-lived wave cavity containing turbulent whistler waves can be formed with the appropriate properties to efficiently pitch-angle scatter trapped electrons. The primary consequence on the earth’s radiation belts is to reduce the lifetime of the trapped electron population.

Evanescent Wave Cavity Ringdown Spectroscopy: A Platform for the Study of Supported Lipid Bilayers

Evanescent wave cavity ringdown spectroscopy (EW-CRDS) is advocated as an approach for monitoring the formation of supported lipid bilayers (SLBs) on quartz substrates in situ and for the quantitative study of fast molecular adsorption kinetics at the resulting modified biomimetic surface. This approach is illustrated using SLBs of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). Complementary atomic force microscopy (AFM) and quartz crystal microbalance with dissipation (QCM-D) measurements confirm the formation of bilayers on quartz. The subsequent interaction of the porphyrin, 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p'',p'''-tetrasulfonic acid tetrasodium hydrate (TPPS) with the cationic bilayer-modified silica surface has been studied using EW-CRDS combined with an impinging-jet to deliver analyte to the surface in a well-defined manner. The adsorption of TPPS to the bilayer was kinetically controlled and the adsorption rate constant was found to be 1.7 (±0.6) × 10(-4) cm s(-1) from finite element modeling of the jet hydrodynamics and associated convective-diffusion equation, coupled to a first-order surface process describing adsorption. These proof-of-concept studies provide a platform for the investigation of molecular processes at biomembranes using EW-CRDS for chemical species showing optical absorbance in the visible and ultraviolet range.

Continuous wave cavity ring down spectroscopy measurements of velocity distribution functions of argon ions in a helicon plasma

We report continuous wave cavity ring down spectroscopy (CW-CRDS) measurements of ion velocity distribution functions (VDFs) in low pressure argon helicon plasma (magnetic field strength of 600 G, T(e) ≈ 4 eV and n ≈ 5 × 10(11) cm(-3)). Laser induced fluorescence (LIF) is routinely used to measure VDFs of argon ions, argon neutrals, helium neutrals, and xenon ions in helicon sources. Here, we describe a CW-CRDS diagnostic based on a narrow line width, tunable diode laser as an alternative technique to measure VDFs in similar regimes but where LIF is inapplicable. Being an ultra-sensitive, cavity enhanced absorption spectroscopic technique; CW-CRDS can also provide a direct quantitative measurement of the absolute metastable state density. The proof of principle CW-CRDS measurements presented here are of the Doppler broadened absorption spectrum of Ar II at 668.6138 nm. Extrapolating from these initial measurements, it is expected that this diagnostic is suitable for neutrals and ions in plasmas ranging in density from 1 × 10(9) cm(-3) to 1 × 10(13) cm(-3) and target species temperatures less than 20 eV.

Development and application of an optical sensor for ethene in ambient air using near infra-red cavity ring down spectroscopy and sample preconcentration

An automated near infra-red (IR) continuous wave cavity ring down spectrometer with sample preconcentration has been developed for the measurement of ethene (C₂H₄) in air. The spectrometer incorporated a distributed feedback diode laser operating at wavelengths λ∼ 1.6 μm and a pre-concentration system containing an adsorbent, molecular sieve 4A (MS4A). An absorption line located at 6148.58 cm⁻¹, and free from spectral overlap with other atmospheric molecules, was used for ethene detection. The spectrometer has a capacity for determination of atmospheric ethene mixing ratios at half hour time intervals, with a detection limit (2 SD above baseline noise) of 280 ppt. Both weekday and weekend measurements were performed in ambient air for periods of up to 30 hours. Average daytime mixing ratios of ethene were observed to be 2 ppbv and 1 ppbv during weekdays and weekends respectively. The mixing ratios of ethene varied from 0.6 ppbv to 1.2 ppbv in Bristol air during the weekend, with influence of meteorological conditions. The observed variations are discussed with consideration of probable sources and various meteorological parameters. A correlation is observed in the mixing ratio of ethene and nitrogen dioxide.

Influence of ring-down cavity parameters on intensity transmission property in trace gas concentration measurement

In this paper, we describe the basic principles and system design of continuous wave cavity ring-down spectroscopy (CWCRDS). We also particularly study the nature and the behavior of a novel method to detune a laser and apply it to a cavity ring-down spectroscopy experiment. Both simulations and experiments are completed on the relation between the transmission characteristic and different reflectivities, as well as scanning speed. Output electric field equation is deduced. It has been investigated that how photons are coupled to the cavity and how to accumulate the intensity and leak out of the cavity as a function of time. It is noted that both accumulation of intensity and decay times decrease, and the oscillation amplitude increases as the reflectivity increases. Relative intensity increases with decreasing scanning velocity. Additionally, the simulations show that a non-detuned cavity displays the transmitted signals which are highly dependent on the mirror reflectivity and piezoelectric translator (PZT) modulation speed. Simulations also display that the laser switching off is different from detuning.

First Cavity Ring-Down Spectroscopy HO2 Measurements in a Large Photoreactor

Abstract The HO2 radical is one of the most important intermediate species in atmospheric chemistry. We report on the development of a new photoreactor with first in-situ measurement of HO2 radical photostationary concentrations using continuous wave cavity ring-down spectrometry (cw-CRDS). Characterization of the actinic photon flux was carried out by NO2 actinometry. Photolysis of Cl2/methanol mixtures in air under UV light allowed the measurement of HO2 photostationary concentrations of a few 1010 molecules cm-3 with an HO2 detection limit of 1.5 × 1010 molecules cm-3 at 6638.207 cm-1. The feasibility of HO2 direct measurement in a reaction chamber is demonstrated through the measurement of the HO2 overall loss at different pressures showing the importance of HO2 diffusion and wall loss in such low pressure quartz reactor. The rate coefficient for the HO2+HO2 reaction has been measured at 6.6, 24 and 118 mbar and found to be in good agreement with the recommended value.

Measurement of Absolute Absorption Cross Sections for Nitrous Acid (HONO) in the Near-Infrared Region by the Continuous Wave Cavity Ring-Down Spectroscopy (cw-CRDS) Technique Coupled to Laser Photolysis

Absolute absorption cross sections for selected lines of the OH stretch overtone 2ν(1) of the cis-isomer of nitrous acid HONO have been measured in the range 6623.6-6645.6 cm(-1) using the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique. HONO has been generated by two different, complementary methods: in the first method, HONO has been produced by pulsed photolysis of H(2)O(2)/NO mixture at 248 nm, and in the second method HONO has been produced in a continuous manner by flowing humidified N(2) over 5.2 M HCl and 0.5 M NaNO(2) solutions. Laser photolysis synchronized with the cw-CRDS technique has been used to measure the absorption spectrum of HONO produced in the first method, and a simple cw-CRDS technique has been used in the second method. The first method, very time-consuming, allows for an absolute calibration of the absorption spectrum by comparison with the well-known HO(2) absorption cross section, while the second method is much faster and leads to a better signal-to-noise ratio. The strongest line in this wavelength range has been found at 6642.51 cm(-1) with σ = (5.8 ± 2.2) × 10(-21) cm(2).

Principal Component Analysis of Observed and Modeled Diurnal Rainfall in the Maritime Continent

Principal component analysis (PCA) is able to diagnose the diurnal rain cycle in the Maritime Continent into two modes that explain most of the diurnal variability in the region. The first mode results from the differential variation in potential instability forced by surface heat flux, insolation, and longwave radiative cooling on land and sea. The second mode is associated with intrinsic mesoscale dynamics of convective systems and its interactions with gravity waves, density currents, and local circulations in coastal regions or mountainous terrain. The spatial phase relation between the two modes determines whether a diurnal signal is propagating or stationary. Thus, validating model simulations of diurnal rainfall using PCA provides insights on the representation of dynamics and physics. In this paper, the main modes of diurnal rain variability in the Maritime Continent from satellite observations are studied and are compared with those from Weather Research and Forecasting (WRF) model simulations. Hovmoeller analyses of the reconstructed rainfall from the first two PCA modes clarify the impact of coastlines and mountains as sources of propagating signals. Wave cavities are identified in the Straits of Malacca, Malay Peninsula, and north Sumatra where stationary signals are produced. WRF reproduces the first two modes but each with a phase lead of about 1–2 h or longer, depending on the satellite rainfall product used for comparison. The basic diurnal forcing in the model seems to be too strong and the model responds too strongly to small islands and small-scale topography. The phase speed of propagating signals over open sea is correctly modeled but that over land is too slow.

Development of a wideband compact leaky wave cavity antenna

As operating frequencies decrease, the aperture size of leaky wave antennas increases significantly – rendering them unsuitable for many sophisticated applications requiring small-sized communication antennas. In this article, a compact leaky wave cavity antenna with wide impedance bandwidth, using tapered cavity with substrate integrated waveguide is proposed. The antenna consists of a substrate and a rectangular metallic cavity fed by an L-shaped feed element. The metallic strips are placed along the sides of the substrate to fabricate a substrate integrated waveguide. The cavity is tapered to steer the beam in the desired direction. The tapering of the cavity causes the placement of the L-shaped feed element near the back wall of the cavity, producing input impedance mismatching. This problem of input impedance mismatching is resolved by the substrate integrated waveguide. It is demonstrated that the proposed antenna is compact (2λ0), and is wideband with voltage standing wave ratio (VSWR) bandwidth of 24% for 2:1 VSWR and a gain of 8.4 dBi. The experimental results for the designed antenna are in close agreement with the simulation results. The proposed antenna can be utilized in different wireless applications such as mobile communications, road vehicle communications and other applications.

Development of Fixed Frequency Beam Steering Wide Band Leaky Wave Cavity Antenna

A complete design of substrate integrated leaky wave cavity antenna for fixed frequency beam steering is presented. The antenna consists of a small rectangular metallic tapered cavity fed by hook-shaped element and covered by the substrate. The steering of the beam in the desired direction is achieved by controlling the taper of the cavity. The beam is scanned at fixed frequency by changing position of the microstrip lines on the substrate. The substrate placed on the cavity resolves the problem of input impedance mismatch. The pertinent features of the antenna are compactness, broadband, and fixed-frequency beam steering capability.

Frequency domain analysis for laser-locked cavity ringdown spectroscopy

In this paper we report on the development of a Fourier-transform based signal processing method for laser-locked Continuous Wave Cavity Ringdown Spectroscopy (CWCRDS). Rather than analysing single ringdowns, as is the norm in traditional methods, we amplitude modulate the incident light, and analyse the entire waveform output of the optical cavity; our method has more in common with Cavity Attenuated Phase Shift Spectroscopy than with traditional data analysis methods. We have compared our method to Levenburg-Marquardt non linear least squares fitting, and have found that, for signals with a noise level typical of that from a locked CWCRDS instrument, our method has a comparable accuracy and comparable or higher precision. Moreover, the analysis time is approximately 500 times faster (normalised to the same number of time domain points). Our method allows us to analyse any number of periods of the ringdown waveform at once: this allows the method to be optimised for speed and precision for a given spectrometer.

High-resolution spectroscopy of the ν8 band of methylene bromide using a quantum cascade laser

A continuous wave cavity ringdown spectrometer with a Fabry-Perot quantum cascade laser has been used to collect a rotationally-resolved infrared spectrum of the ν 8 vibrational band of methylene bromide in a slit nozzle expansion. In our laboratory, previous observations of the vibrational band were limited by spectral coverage to only the P and Q-branches and by the 24 MHz step-size of the laser [1]. The issue of limited spectral coverage has been resolved using a Fresnel rhomb and a wire grid polarizer to protect the laser from the destabilizing effects of back-reflection from the ringdown cavity. The frequency step-size of the spectrometer has been reduced from 24 MHz to 2 MHz. With both of these instrument enhancements, we have been able to record the R-branch of the vibrational band, and can resolve many lines that were previously blended in spectra acquired using a pinhole expansion nozzle. Significant hyperfine splitting was observed for the low- J transitions in the P and R-branches. It was possible to neglect the effects of hyperfine splitting for transitions involving J″ > 2 in the spectral assignment, and simulations using the constants obtained by fitting to Watson’s S-reduced Hamiltonian for CH 2 79Br 81Br, and the A-reduced form for CH 2 79Br 2 and CH 2 81Br 2, provide a good match to experimental spectra. A total of 297 transitions have been assigned for all three isotopologues, with a standard deviation of 0.00024 cm −1(∼7 MHz).