Standing-wave Resonator Research Articles

Theoretical analysis on cavity-enhanced laser cooling of Er3+-doped glasses

In recent years, Er3+ doped CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2 (CNBZN) glass has become one of the new materials in the field of laser cooling of solids. In this paper, using the theory of laser output and standing wave resonance, intracavity-and extracavity-enhanced laser cooling of Er3+-doped CNBZN glass are theoretically analyzed. Calculated results show that enhancement factor can achieve tens to hundreds of times. Moreover, two schemes are compared with each other, and the results show that for low material absorption, especially when the sample length is less than 0.3 mm, intracavity configuration has the advantage of high pumping power and high absorption. However, for high material absorption, especially when the sample length is longer than 3 mm, the extracavity configuration becomes a more efficient means for laser cooling. Finally, according to the operating wavelength and power requirements of Er3+-doped material, cavity enhancement can be realized experimentally using semiconductor diode laser.

Time- and space-modulated Raman signals in graphene-based optical cavities

We present fabrication and optical characterization of micro-cavities made of multilayer graphene (MLG) cantilevers clamped by metallic electrodes and suspended over Si/SiO2 substrates. Graphene cantilevers act as semi-transparent mirrors closing air wedge optical cavities. This simple geometry implements a standing-wave optical resonator along with a mechanical one. Equal thickness interference fringes are observed in both Raman and Rayleigh backscattered signals, with interfringe given by their specific wavelength. Chromatic dispersion within the cavity makes possible the spatial modulation of graphene Raman lines and selective rejection of the silicon background signal. Electrostatic actuation of the multilayer graphene cantilever by a gate voltage tunes the cavity length and induces space and time modulation of the backscattered light, including the Raman lines. We demonstrate the potential of these systems for high-sensitivity Raman measurements of generic molecular species grafted on a multilayer graphene surface. The Raman signal of the molecular layer can be modulated both in time and space in a similar fashion and shows enhancement with respect to a collapsed membrane.

Cooling of atomic ensembles in optical cavities: Semiclassical limit

The semiclassical dynamics of atoms are theoretically studied, when the atoms are confined inside a standing-wave high-finesse resonator. The atoms are cooled by scattering processes in which the photons of a transverse laser are coherently scattered into the cavity mode. We derive a Fokker-Planck equation for the atomic center-of-mass variables which allows us to determine the equations of motion in the semiclassical limit for any value of the intensity of the laser field. We extract its prediction for the dynamics when the resonator is essentially in the vacuum state and the atoms are cooled by scattering photons into the cavity mode, which then decays. Its predictions for the stationary atomic distribution are compared with the ones of the Fokker-Planck equation in [P. Domokos, P. Horak, and H. Ritsch, J. Phys. B 34, 187 (2001)], which has been derived under different assumptions. We find full agreement in the considered parameter regime.

Investigation of the Velocity Profiles in a Ninety-Degree Curved Standing Wave Resonator with Particle Image Velocimetry

Travelling wave thermoacoustic heat engines have been reported to have a higher efficiency than the standing wave ones. The former are generally large systems which consist of toroidal shape resonators. While standing wave heat engines are inherently smaller, a reduction in size could be considered which may involve curvatures as compared to the straight tube conventional systems. However, as with the streaming losses in the travelling wave resonators, losses due to the curvature may be generated. This study involves preliminary experimental measurements using the Particle Image Velocimetry (PIV) method to analyze the velocity profiles in a standing wave resonator before and after a ninety degree curvature. This design can reduce the space generally occupied by the straight standing wave resonator. The overall length of the resonator fits a quarter wavelength wave based on the straight closed-end tube type. The working gas is air at 1 atmospheric pressure. Results have shown that the velocity profiles after the stack but before the curvature exhibit clear straight paths up just as reported elsewhere. Signs of disordered motion could be observed just before the bend and the pattern continues until after the curvature. The results are obtained before one periodic cycle and before the acoustic wave front hit the tube end. The trend is expected to affect the overall thermoacoustic performance of the engine as returning gas particles interact with the oncoming particles that pass by the curvature.

Configurations for short period rf undulators

Several configurations for rf undulators energized at millimeter wavelengths and designed to produce coherent nanometer radiation from sub-GeV electron beams are analyzed and compared with one another. These configurations include a traveling-wave resonant ring, a standing wave resonator, and a resonator operating close to cutoff.

Computational fluid dynamics simulation of Rayleigh streaming in a vibrating resonator

Rayleigh streaming is a time-averaged flow that can exist in the thermal buffer tubes of thermoacoustic prime movers and refrigerators and is driven by the viscous stresses close to the solid boundaries. This mean flow leads to mean convective heat transport, which can have large impact on the performance of thermoacoustic devices. Rayleigh streaming in a standing wave resonator is simulated using a commercially available computational fluid dynamics (CFD) code and is compared to existing analytical models of Hamilton et al. (2003). A test case is developed, and a standing wave is generated by applying a harmonic volume force to the domain. Both the inner and outer streaming vortices are well described for a range of radii from R/δν = 3…20 and the magnitude of the streaming velocity matches analytical values. This paper shows the possibility of using available as-is CFD software for the simulation of streaming in a standing wave resonator. The presented results pave the way for the simulation of more complex geometries and studies to reduce the negative effects Rayleigh streaming can have on thermo-acoustic prime mover and refrigerator efficiency.

Direct numerical simulations of acoustic streaming in standing wave tubes using the Lattice Boltzmann Method

One important factor in the efficiency of thermoacoustic engines is acoustic streaming, which causes convective heat transfer between high and low temperature reservoirs. Most experimental and numerical studies performed so far have focused on Rayleigh streaming. Less work has been done on acoustic streaming due to the stack. Most numerical studies of Rayleigh streaming were performed using Navier-Stokes based numerical methods. In this study, large eddy numerical simulations were performed using schemes based on the lattice Boltzmann method (LBM). The model considered a simplified thermoacoustic refrigerator made of a rectangular standing wave resonator with a flat plate spoiler. Low-amplitude results obtained for Rayleigh streaming velocity magnitudes were compared with linear acoustic theory for verification. High amplitude recirculated streaming flow structures around the edges of the flat plate spoiler were identified. These are likely to contribute to heat transfer much more than Rayleigh streaming. Parametric studies were performed to investigate the effects of Strouhal number and spoiler edge shape. The results confirm that vertical edge streaming flows play a significant role in thermoacoustic heat transport.

Universal scaling of plasmonic refractive index sensors

We establish experimental and numerical evidence that the refractive index sensitivities of various subwavelength plasmonic sensors obey a simple universal scaling relation that the sensitivities linearly increase with λm/neff (where λm is the resonant wavelengths and neff is the effective refractive index of the environment) and exhibit a slope equal to 1 instead of 2 predicted theoretically. The universal scaling relation is independent of the geometrical structures or contributions of multipolar resonances of individual metal structures (i.e. plasmonic atoms). It is also independent of spatial distributions or field-enhancements of electromagnetic hot spots in coupled metal structures (i.e. plasmonic molecules). The universal scaling relation reveals the fundamental standing wave resonances for all plasmonic atoms and the predominant near-field electric couplings for most plasmonic molecules. The established universal relation also helps to exclude some magnetically coupled plasmonic molecules for practical applications due to their reduced sensitivities.

Standing-Wave Plasmonic Resonance in Terahertz Extraordinary Transmission

The effect of total circumference on the terahertz transmission response of coaxial H-shaped annular aperture arrays punctured in a thin metal film on a silicon substrate is investigated. Unexpectedly, the resonance frequencies are not sensitive to the details of the shape for the coaxial annular aperture, but intensely depend on the total circumference in a unit cell. Furthermore, a clear inverse proportion relationship between the resonance frequencies and the total circumference is revealed and confirmed experimentally, which is interpreted by the standing-wave plasmonic resonance mechanism. This characteristic paves an avenue to the quick and accurate construction of the desired operating frequencies for filtering, biosensing, and ultrafast switching applications.

Anomalous sound absorption of finite amplitude sound in liquid 4He

We report the nonlinear and unsteady response of the finite amplitude pressure wave in superfluid 4He, using a classical vibrating liquid column standing wave resonator. The frequency was kept at the resonance frequency of the liquid column, so that the maximum of the pressure field always came to the surface of the driver. As a result, in the low driving-amplitude regime, I/O relations were linear. However, in the high driving-amplitude regime, anomalous responses were found; The amplitude of the signals suddenly reduced and gradually recovered; they were repeated with random intervals. Moreover, in the highly suppressed region in the waveform, we found a generation of new type of turbulence. These absorptions are expected to be the result of the vapor bubble ejection from the surface of the driver transducer.

Energy harvesting from a convection-driven Rijke-Zhao thermoacoustic engine

A convection-driven Rijke-Zhao thermoacoustic engine is developed. It can produce intensive oscillations at two different temperatures. Furthermore, it does not involve any heat exchanger and stack/regenerator, which play critical roles in conduction-driven standing- or travelling-wave engines. Thus, the Rijke-Zhao engine is much simpler in design and lower cost in fabrication. To demonstrate its potential of energy-harvesting, a design for the conversion of heat into electricity via sound is proposed by integrating Rijke-Zhao engine with a piezoelectric generator. The preliminary experimental results are presented. And it is found that 60% more power is generated than that from conduction-driven standing-wave thermoacoustic-piezoelectric resonator [Smoker et al., J. Appl. Phys. 111, 104901, (2012)]. In order to gain insights on the generation mechanism of the thermoacoustic oscillations in the present energy-harvesting system, 2D numerical simulations are conducted. Comparing the numerical results with the experimental one reveals that good quantitative agreement is obtained.

Ultrathin multi-band planar metamaterial absorber based on standing wave resonances

We present a planar waveguide model and a mechanism based on standing wave resonances to interpret the unity absorptions of ultrathin planar metamaterial absorbers. The analytical model predicts that the available absorption peaks of the absorber are corresponding to the fundamental mode and only its odd harmonic modes of the standing wave. The model is in good agreement with numerical simulation and can explain the main features observed in typical ultrathin planar metamaterial absorbers. Based on this model, ultrathin planar metamaterial absorbers with multi-band absorptions at desired frequencies can be easily designed.

High order standing-wave plasmon resonances in silver u-shaped nanowires

Optical measurements of the transmission spectra through nanofabricated planar arrays of silver u-shaped nanowires on a silicon substrate resonating at infrared frequencies are performed. Good agreement with the numerically simulated surface plasmon standing wave resonances supported by the structures is found. Such resonances exhibit field enhancement and are able to provide magnetic and electric responses when used as the unit cell of a metamaterial. The magnetic excitation of the resonators using oblique incidence is shown to be drastically reduced by the existence of a high index substrate such as silicon.

Coupling light into and out from the surface plasmon polaritons of a nanometer-thin metal film with a metal nanostrip

Scattering of light by a metal nanostrip placed near a nanometer-thin metal film in a homogeneous dielectric environment is considered. For a plane wave incident on the geometry from outside we study the scattering cross sections governing the excitation of symmetric and antisymmetric surface plasmon polariton (SPP) waves and out-of-plane (OUP) propagating waves. For a symmetric (long-range) or antisymmetric (short-range) SPP propagating along the metal film and being incident on the nanostrip, we study the fraction of power coupled into forward and backward propagating symmetric and antisymmetric SPP waves, and into OUP propagating waves, respectively. Resonance peaks in the scattering cross sections, or dips in the relative power going into all scattering channels due to resonant absorption, are related to standing-wave resonances of counterpropagating gap SPP waves in the gap between the metal nanostrip and the metal film. The resonances are studied with respect to the width of the metal nanostrip and the size of the gap between the metal film and nanostrip. For a 15-nm-thick film it is found that coupling of light in and out of the symmetric SPP is weak compared with coupling to and from OUP and antisymmetric SPP waves. Improving the in and out coupling of power in the symmetric SPP waves with thicker films (50 nm) and larger scattering objects is considered.

Theoretical and experimental investigation of desalting and dehydration of crude oil by assistance of ultrasonic irradiation

In this paper desalting/dehydration process of crude oil by ultrasonic irradiation in a novel batch standing-wave resonator reactor is studied both theoretically and experimentally. The effect of main parameters including ultrasonic irradiation parameters, namely irradiation input power and irradiation time, and also operating parameters, such as temperature and injected water, on the removal efficiencies of salt and water is examined. The obtained results demonstrate that finding the optimum values of the above mentioned parameters is important to prevent a significant decrease in the removal efficiencies of water and especially salt. Thus, crude oil was subjected to optimal ultrasonic irradiation with an input power of 57.7W, and irradiation time of 6.2min at temperature of 100°C. The injected water to dissolve the salt of crude oil was 7vol.%. Also, the applied settling time and dosage of chemical demulsifier were 60min and 2ppm, respectively. Under these optimum conditions the removal efficiencies of the desalting/dehydration process were 84% and 99.8%, respectively, which are suitable for refineries.Also, based on the optimal experimental data, two inferential estimators are developed to obtain the relationships between the salt and water removal efficiencies, and input energy density. These empirical relationships can offer a proper estimation for the salt and water removal efficiencies with irradiation input energy.

Terahertz detector with transmission-line type superconducting tunnel junctions

We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.

Optical standing-wave artifacts in reflection-absorption FTIR microspectroscopy of biological materials

Reflection-absorption spectra obtained with an infrared microscope should yield the same absorption coefficients as direct micro-transmission measurements as long as the correct effective sample thickness is used, but in practice, severe optical artifacts can complicate the spectra. Using deposited protein gel fdms as a homogenous model for biological cell-like samples, we demonstrate the effect of standing-wave interference of the IR beam at the reflective substrate surface which dramatically and systematically alters the absorbance intensity across the spectrum as a function of sample thickness. To explain the observed spectral artifacts, we simulate the optical standing-wave for the focussed IR beam, and insert the parameters into an existing standing-wave absorption theory. By introducing an additional term to the theory representing a component of the standing-wave resonant with the film thickness, the data are accurately reproduced, and the relative band intensities can be corrected to the direct transmission values. This approach may be generally applicable in reflection-absorption experiments to obtain reliable absorbance spectra of homogenous samples even when the sample thickness is larger than the IR wavelength.

Metal-dielectric-metal surface plasmon-polariton resonators

A theoretical study of standing-wave resonances of surface plasmon polaritons (SPPs) in finite-length metal-dielectric-metal cavities is presented. A Fabry-P\'erot model is constructed to describe the cavity resonances, and the associated optical parameters are calculated analytically. One key parameter is the phase acquired by resonating SPPs upon reflection from cavity end faces. This phase pickup is associated with the near-field energy storage at these end faces, and the imaginary part of the reflection coefficient is shown to be approximately proportional to the stored energy. Using the Fabry-P\'erot model, we also calculate the transmission cross section, peak position, as well as the $Q$ factor of the cavity, and we find good agreement with full-field numerical simulations for a wide range of wavelengths and device dimensions.

Design of mM-W Fully Integrated CMOS Standing-Wave VCOs Using Low-Loss CPW Resonators

The design of two fully integrated 43-GHz voltage-controlled oscillators (VCOs) implemented in a 90-nm CMOS process is presented. Both use standing-wave transmission line resonators instead of a lumped tank to provide extended output frequency ranges and relatively low phase noise (PN). Coplanar waveguide structures with and without slow-wave features were employed in the two VCO circuits. The test results of the manufactured chips were compared. The circuit with the slow-wave feature showed a lower PN (-102.7 dBc/Hz at 1 MHz), whereas the one without the slow-wave structure showed a wider tuning range (1.96 GHz). The two VCOs were developed for use in licensed E-band transceiver systems.

Theory and experiments of Bragg cavity modes in passive and active metallic nanoslit array devices

Metallic nanoslit arrays exhibit several unique, surprising, and useful properties, such as resonant enhanced transmission and resonant local field enhancements. Here we present both a theoretical study of these static properties, as well as experiments showing the utilization of these features combined with active optical media. We develop an approximated, simple closed-form model for predicting and explaining the general emergence of enhanced transmission resonances through metallic gratings, in various configurations and polarizations. This model is based on an effective index approximation and it unifies in a simple way the underlying mechanism of all forms of enhanced transmission in such structures as emerging from standing wave resonances of the different diffraction orders of periodic structures. The known excitation of surface plasmon polaritons or slit cavity modes emerges as a limiting case of a more general condition. We also use this understanding of the resonant behavior of nanoslit arrays to design and fabricate such structures with embedded nanocrystal quantum dots, and show beaming of nonclassical light to a narrow angle of less than 4 deg, as well as an enhancement of the two-photon upconversion fluorescence process by a factor of ∼400.