Tantalum pentoxide: a new material platform for high-performance dielectric metasurface optics in the ultraviolet and visible region.

Dielectric metasurfaces, composed of planar arrays of subwavelength dielectric structures that collectively mimic the operation of conventional bulk optical elements, have revolutionized the field of optics by their potential in constructing high-efficiency and multi-functional optoelectronic systems on chip. The performance of a dielectric metasurface is largely determined by its constituent material, which is highly desired to have a high refractive index, low optical loss and wide bandgap, and at the same time, be fabrication friendly. Here, we present a new material platform based on tantalum pentoxide (Ta2O5) for implementing high-performance dielectric metasurface optics over the ultraviolet and visible spectral region. This wide-bandgap dielectric, exhibiting a high refractive index exceeding 2.1 and negligible extinction coefficient across a broad spectrum, can be easily deposited over large areas with good quality using straightforward physical vapor deposition, and patterned into high-aspect-ratio subwavelength nanostructures through commonly-available fluorine-gas-based reactive ion etching. We implement a series of high-efficiency ultraviolet and visible metasurfaces with representative light-field modulation functionalities including polarization-independent high-numerical-aperture lensing, spin-selective hologram projection, and vivid structural color generation, and the devices exhibit operational efficiencies up to 80%. Our work overcomes limitations faced by scalability of commonly-employed metasurface dielectrics and their operation into the visible and ultraviolet spectral range, and provides a novel route towards realization of high-performance, robust and foundry-manufacturable metasurface optics.

Optimization of process parameters in micro-scale pneumatic aerosol jet printing for high-yield precise electrodes.

Aerosol jet printing (AJP) is a new non-contact direct writing technique designed to achieve precise and intricate patterns on various substrates. Specifically, the pneumatic AJP process breaks down the ink into fine particles, significantly reducing the risk of nozzle clogging and rendering it highly advantageous for industrial applications. This paper focuses on the optimization of the line electrode formation process using soluble silver clusters as the conductive ink, along with the aerosol formation procedure. The main parameters of the AJP process, namely sheath flow rate, atomizer flow rate, and dispensing speed, were identified and examined for their influence on line width and resistivity. Through this analysis, an operability window, including optimized conditions for printing high-quality lines using the AJP process, was established, along with a regression equation enabling the statistical estimation of line width. In summary, the outcomes of this investigation underscore the feasibility of an integrated printing system capable of precision control over line width, achieved through the optimization of AJP process parameters. Furthermore, it was established that pneumatic AJP offers robust process stability. The practical applicability of the proposed optimization techniques was assessed, highlighting their potential utilization in electrode formation processes within the electronic and display industry.

Disordered metasurface enabled single-shot full-Stokes polarization imaging leveraging weak dichroism.

Polarization, one of the fundamental properties of light, is critical for certain imaging applications because it captures information from the scene that cannot directly be recorded by traditional intensity cameras. Currently, mainstream approaches for polarization imaging rely on strong dichroism of birefringent crystals or artificially fabricated structures that exhibit a high diattenuation typically exceeding 99%, which corresponds to a polarization extinction ratio (PER) >~100. This not only limits the transmission efficiency of light, but also makes them either offer narrow operational bandwidth or be non-responsive to the circular polarization. Here, we demonstrate a single-shot full-Stokes polarization camera incorporating a disordered metasurface array with weak dichroism. The diattenuation of the metasurface array is ~65%, which corresponds to a PER of ~2. Within the framework of compressed sensing, the proposed disordered metasurface array serves as an efficient sensing matrix. By incorporating a mask-aware reconstruction algorithm, the signal can be accurately recovered with a high probability. In our experiments, the proposed approach exhibits high-accuracy full-Stokes polarimetry and high-resolution real-time polarization imaging. Our demonstration highlights the potential of combining meta-optics with reconstruction algorithms as a promising approach for advanced imaging applications.

Dual-Modal Nanoplasmonic Light Upconversion through Anti-Stokes Photoluminescence and Second-Harmonic Generation from Broadband Multiresonant Metal Nanocavities.

Metal nanocavities can generate plasmon-enhanced light upconversion signals under ultrashort pulse excitations through anti-Stokes photoluminescence (ASPL) or nonlinear harmonic generation processes, offering various applications in bioimaging, sensing, interfacial science, nanothermometry, and integrated photonics. However, achieving broadband multiresonant enhancement of both ASPL and harmonic generation processes within the same metal nanocavities remains challenging, impeding applications based on dual-modal or wavelength-multiplexed operations. Here, we report a combined experimental and theoretical study on dual-modal plasmon-enhanced light upconversion through both ASPL and second-harmonic generation (SHG) from broadband multiresonant metal nanocavities in two-tier Ag/SiO2/Ag nanolaminate plasmonic crystals (NLPCs) that can support multiple hybridized plasmons with high spatial mode overlaps. Our measurements reveal the distinctions and correlations between the plasmon-enhanced ASPL and SHG processes under different modal and ultrashort pulsed laser excitation conditions, including incident fluence, wavelength, and polarization. To analyze the observed effects of the excitation and modal conditions on the ASPL and SHG emissions, we developed a time-domain modeling framework that simultaneously captures the mode coupling-enhancement characteristics, quantum excitation-emission transitions, and hot carrier population statistical mechanics. Notably, ASPL and SHG from the same metal nanocavities exhibit distinct plasmon-enhanced emission behaviors due to the intrinsic differences between the incoherent hot carrier-mediated ASPL sources with temporally evolving energy and spatial distributions and instantaneous SHG emitters. Mechanistic understanding of ASPL and SHG emissions from broadband multiresonant plasmonic nanocavities marks a milestone toward creating multimodal or wavelength-multiplexed upconversion nanoplasmonic devices for bioimaging, sensing, interfacial monitoring, and integrated photonics applications.

Voltage Modulation of Nanoplasmonic Metal Luminescence from Nano-Optoelectrodes in Electrolytes.

Metallic nanostructures supporting surface plasmon modes can concentrate optical fields and enhance luminescence processes from the metal surface at plasmonic hotspots. Such nanoplasmonic metal luminescence contributes to the spectral background in surface-enhanced Raman spectroscopy (SERS) measurements and is helpful in bioimaging, nanothermometry and chemical reaction monitoring applications. Although there is growing interest in nanoplasmonic metal luminescence, its dependence on voltage modulation has received limited attention in research investigations. Also, the hyphenated electrochemical surface-enhanced Raman spectroscopy (EC-SERS) technique typically ignores voltage-dependent spectral background information associated with nanoplasmonic metal luminescence due to limited mechanistic understanding and poor measurement reproducibility. Here, we report a combined experiment and theory study on dynamic voltage-modulated nanoplasmonic metal luminescence from hotspots at the electrode-electrolyte interface using multiresonant nanolaminate nano-optoelectrode arrays. Our EC-SERS measurements under 785 nm continuous wavelength laser excitation demonstrate that short-wavenumber nanoplasmonic metal luminescence associated with plasmon-enhanced electronic Raman scattering (PE-ERS) exhibits a negative voltage modulation slope (up to ≈30% V-1) in physiological ionic solutions. Furthermore, we have developed a phenomenological model to intuitively capture the plasmonic, electronic, and ionic characteristics at the metal-electrolyte interface to understand the observed dependence of the PE-ERS voltage modulation slope on voltage polarization and ionic strength. The current work represents a critical step toward the general application of nanoplasmonic metal luminescence signals in optical voltage biosensing, hybrid optical-electrical signal transduction, and interfacial electrochemical monitoring.

Dicke-Cooperativity-Assisted Ultrastrong Coupling Enhancement in Terahertz Metasurfaces.

A system of N two-level atoms cooperatively interacting with a photonic field can be described as a single giant atom coupled to the field with interaction strength ∝N. This enhancement, known as Dicke cooperativity in quantum optics, has recently become an indispensable element in quantum information technology. Here, we extend the coupling beyond the standard light-matter interaction paradigm, enhancing Dicke cooperativity in a terahertz metasurface with N meta-atoms. The cooperative enhancement is manifested through the hybridization of the localized surface plasmon resonance in individual meta-atoms and surface lattice resonance due to the periodic array. Furthermore, through engineering of the capacitive split-gap in the meta-atoms, we were able to enhance the coupling rate into the ultrastrong coupling regime by a factor of N. Our strategy can serve as a new platform for demonstrating effective control of fermionic systems by weak pumping, superradiant emission, and ultrasensitive sensing of molecules.

Scalable two-tier protruding micro-/nano-optoelectrode arrays with hybrid optical-electrical modalities by hierarchical modular design.

In situ spatiotemporal characterization of correlated bioelectrical and biochemical processes in living multicellular systems remains a formidable challenge but can offer crucial opportunities in biology and medicine. A promising approach is to develop bio-interfaced multifunctional micro-/nano-sensor arrays with complementary biophotonic-bioelectronic modalities and biomimetic topology to achieve combined bioelectrical and biochemical detection and tight device-cell coupling. However, a system-level engineering strategy is still missing to create multifunctional micro-/nano-sensor arrays that meet the multifaceted design requirements for in situ spatiotemporal characterizations of living systems. Here, we demonstrate a hierarchical modular design and fabrication approach to develop scalable two-tier protruding micro-/nano-optoelectrode arrays that extend the design space of biomimetic micro-/nano-pillar topology, plasmonic nanoantenna-based biophotonic function in surface-enhanced Raman spectroscopy (SERS), and micro-/nano-electrode-based bioelectronics function in electrochemical impedance spectroscopy (EIS). Notably, two-tier protruding micro-/nano-optoelectrode arrays composed of nanolaminate nanoantenna arrays on top of micropillar electrode arrays can support plasmonic nanocavity modes with high SERS enhancement factors (≈106) and large surface-to-volume ratio with significantly reduced interfacial impedance in EIS measurements. We envision that scalable two-tier protruding micro-/nano-optoelectrode arrays can potentially serve as bio-interfaced multifunctional micro-/nano-sensor arrays for in situ correlated spatiotemporal bioelectrical-biochemical measurements of living multicellular systems such as neuronal network cultures, cancerous organoids, and microbial biofilms.

Synthesizing ultrafast optical pulses with arbitrary spatiotemporal control.

The ability to control the instantaneous state of light, from high-energy pulses down to the single-photon level, is an indispensable requirement in photonics. This has, for example, facilitated spatiotemporal probing and coherent control of ultrafast light-matter interactions, and enabled capabilities such as generation of exotic states of light with complexity, or at wavelengths, that are not easily accessible. Here, by leveraging the multifunctional control of light at the nanoscale offered by metasurfaces embedded in a Fourier transform setup, we present a versatile approach to synthesize ultrafast optical transients with arbitrary control over its complete spatiotemporal evolution. Our approach, supporting an ultrawide bandwidth with simultaneously high spectral and spatial resolution, enables ready synthesis of complex states of structured space-time wave packets. We expect our results to offer unique capabilities in coherent ultrafast light-matter interactions and facilitate applications in microscopy, communications, and nonlinear optics.

Continuous Patterning of Silver Nanowire-Polyvinylpyrrolidone Composite Transparent Conductive Film by a Roll-to-Roll Selective Calendering Process.

The roll-to-roll (R2R) continuous patterning of silver nanowire-polyvinylpyrrolidone (Ag NW-PVP) composite transparent conductive film (cTCF) is demonstrated in this work by means of slot-die coating followed by selective calendering. The Ag NWs were synthesized by the polyol method, and adequately washed to leave an appropriate amount of PVP to act as a capping agent and dispersant. The as-coated Ag NW-PVP composite film had low electronic conductivity due to the lack of percolation path, which was greatly improved by the calendering process. Moreover, the dispersion of Ag NWs was analyzed with addition of PVP in terms of density and molecular weight. The excellent dispersion led to uniform distribution of Ag NWs in a cTCF. The continuous patterning was conducted using an embossed pattern roll to perform selective calendering. To evaluate the capability of the calendering process, various line widths and spacing patterns were investigated. The minimum pattern dimensions achievable were determined to be a line width of 0.1 mm and a line spacing of 1 mm. Finally, continuous patterning using selective calendering was applied to the fabrication of a flexible heater and a resistive touch sensing panel as flexible electronic devices to demonstrate its versatility.

Nano-optoelectrodes Integrated with Flexible Multifunctional Fiber Probes by High-Throughput Scalable Fabrication.

Metallic nano-optoelectrode arrays can simultaneously serve as nanoelectrodes to increase the electrochemical surface-to-volume ratio for high-performance electrical recording and optical nanoantennas to achieve nanoscale light concentrations for ultrasensitive optical sensing. However, it remains a challenge to integrate nano-optoelectrodes with a miniaturized multifunctional probing system for combined electrical recording and optical biosensing in vivo. Here, we report that flexible nano-optoelectrode-integrated multifunctional fiber probes can have hybrid optical-electrical sensing multimodalities, including optical refractive index sensing, surface-enhanced Raman spectroscopy, and electrophysiological recording. By physical vapor deposition of thin metal films through free-standing masks of nanohole arrays, we exploit a scalable nanofabrication process to create nano-optoelectrode arrays on the tips of flexible multifunctional fiber probes. We envision that the development of flexible nano-optoelectrode-integrated multifunctional fiber probes can open significant opportunities by allowing for multimodal monitoring of brain activities with combined capabilities for simultaneous electrical neural recording and optical biochemical sensing at the single-cell level.

High-quality large-scale electron-beam-written resonant filters for the long-wave infrared region.

We present new, to the best of our knowledge, large-scale, high-quality spectral filters operating in the long-wave infrared (LWIR) spectral region. We employ high-spatial resolution nanofabrication techniques to achieve large-area (12mm×12mm) spectrally tunable notch filters. Filter operation is based on the guided-mode resonance effect. The device structure consists of a germanium waveguide grating on top of a zinc selenide substrate. The filters reflect the incident broadband light at one (or more) narrow spectral bands while fully transmitting the rest. We tune the reflected wavelength by tilting the filter. Filters based on one-dimensional gratings are polarization sensitive. We fabricate prototype filters and characterize their polarization dependence and spectral tuning performance using a tunable quantum cascade laser system that spans the ∼7-13µm spectral band. We obtain an excellent agreement between the theoretical and experimental results.

Plasmonic Electronic Raman Scattering as Internal Standard for Spatial and Temporal Calibration in Quantitative Surface-Enhanced Raman Spectroscopy.

Ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in quantitative analysis because of its susceptibility to local optical field variations at plasmonic hotspots in metallo-dielectric nanostructures. Current SERS calibration approaches using Raman tags have inherent limitations due to spatial occupation competition with analyte molecules, spectral interference with analyte Raman peaks, and photodegradation. Herein, we report that plasmon-enhanced electronic Raman scattering (ERS) signals from metal can serve as an internal standard for spatial and temporal calibration of molecular Raman scattering (MRS) signals from analyte molecules at the same hotspots, enabling rigorous quantitative SERS analysis. We observe a linear dependence between ERS and MRS signal intensities upon spatial and temporal variations of excitation optical fields, manifesting the |E|4 enhancements for both ERS and MRS processes at the same hotspots in agreement with our theoretical prediction. Furthermore, we find that the ERS calibration's performance limit can result from orientation variations of analyte molecules at hotspots.

Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures.

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 107. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

Sub-10 nm Nanolaminated Al2O3/HfO2 Coatings for Long-Term Stability of Cu Plasmonic Nanodisks in Physiological Environments.

By supporting localized plasmon modes, metal-based plasmonic nanostructures can confine optical fields at deep-subwavelength scale in various applications, such as biological and chemical sensing, nanoscale light emission, and solar energy harvesting. While Cu is a low-cost complementary metal oxide semiconductor (CMOS) compatible material, its poor chemical stability limits the use of Cu plasmonic nanodevices in corrosive biochemical aqueous environments. In this paper, we demonstrate that sub-10 nm Al2O3/HfO2 nanolaminated coatings can significantly extend the lifetime of Cu nanodisk arrays from ∼5 h to ∼180 days in the physiological environment of 1× phosphate-buffered saline (PBS) at 37 °C. Cu nanodisk arrays are fabricated using freestanding Au nanohole array films as the physical vapor deposition masks and sub-10 nm nanolaminated coatings composed of alternating Al2O3 and HfO2 nanolayers are grown on Cu nanodisk arrays by atomic layer deposition (ALD). Time-dependent optical extinction measurements of Cu nanodisk arrays are conducted in 1× solutions at 37 °C to investigate the anticorrosion performance for different pure and nanolaminated ALD coatings. We observe a linear relationship between the lifetime of Cu nanodisk arrays in 1× PBS at 37 °C and the nanolaminated coating thickness, and ∼1.3 nm nanolaminated coatings of ∼10 ALD cycles can extend the lifetime of Cu plasmonics up to ∼20 days. Furthermore, we find that the anticorrosion performance of Al2O3/HfO2 nanolaminated ALD coatings strongly depends on the processing and the geometric parameters, such as the annealing temperature and the nanolaminated backbone unit size.

Real-Time Monitoring of Ligand Exchange Kinetics on Gold Nanoparticle Surfaces Enabled by Hot Spot-Normalized Surface-Enhanced Raman Scattering.

Nanoparticle surface coatings dictate their fate, transport, and bioavailability. We used a gold nanoparticle-bacterial cellulose substrate and "hot spot"-normalized surface-enhanced Raman scattering (HSNSERS) to achieve in situ and real-time monitoring of ligand exchange reactions on the gold surface. This approach enables semiquantitative determination of citrate surface coverage. Following exposure of the citrate-coated nanoparticles to a suite of guest ligands (thiolates, amines, carboxylates, inorganic ions, and proteins), the guest ligand signal exhibited first-order growth kinetics, while the desorption mediated decay of the citrate signal followed a first-order model. Guest ligand functional group chemistry dictated the kinetics of citrate desorption, while the guest ligand concentration played only a minor role. Thiolates and BSA were more efficient at ligand exchange than amine-containing chemicals, carboxylate-containing chemicals, and inorganic salts due to their higher binding energies with the AuNP surface. Amine-containing molecules overcoated rather than displaced the citrate layer via electrostatic interaction. Citrate exhibited low resistance to replacement at high surface coverages, but higher resistance at lower coverage, thus suggesting a transformation of the citrate-binding mode during desorption. High resistance to replacement in streamwater suggests that the role of surface-adsorbed citrate in nanomaterial fate and transport must be better understood.

Multiresonant Composite Optical Nanoantennas by Out-of-plane Plasmonic Engineering.

Optical nanoantennas can concentrate light and enhance light-matter interactions in subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional optical nanoantennas operating at a single wavelength band are not suitable for multiband applications. Here, we propose and exploit an out-of-plane plasmonic engineering strategy to design and create composite optical nanoantennas that can support multiple nanolocalized modes at different resonant wavelengths. These multiresonant composite nanoantennas are composed of vertically stacked building blocks of metal-insulator-metal loop nanoantennas. Studies of multiresonant composite nanoantennas demonstrate that the number of supported modes depends on the number of vertically stacked building blocks and the resonant wavelengths of individual modes are tunable by controlling the out-of-plane geometries of their building blocks. In addition, numerical studies show that the resonant wavelengths of individual modes in composite nanoantennas can deviate from the optical response of building blocks due to hybridization of magnetic modes in neighboring building blocks. Using Au nanohole arrays as deposition masks to fabricate arrays of multilayered composite nanoantennas, we experimentally demonstrate their multiresonant optical properties in good agreement with theory predictions. These studies show that out-of-plane engineered multiresonant composite nanoantennas can provide new opportunities for fundamental nanophotonics research and practical applications involving optical multiband operations, such as multiphoton process, broadband solar energy conversion, and wavelength-multiplexed optical system.

Improved Quantitative SERS Enabled by Surface Plasmon Enhanced Elastic Light Scattering.

The application of surface-enhanced Raman spectroscopy (SERS) for everyday quantitative analysis is hindered by the point-to-point variability of SERS substrates that arises due to the heterogeneous distribution of localized electromagnetic fields across a suite of plasmonic nanostructures. Herein, we adopt surface-enhanced elastic scattering as a SERS internal standard. Both elastic and inelastic (i.e., Raman) scattering are simultaneously enhanced by a given "hot spot", and thus, the surface-enhanced elastic scattering signal provides a localized intrinsic internal standard that scales across all of the plasmon-enhanced electromagnetic fields within a substrate. Elastically scattered light originates from the amplified spontaneous emission (ASE) of the commercial laser, leading to the formation of a low-wavenumber pseudo band that arises from the interaction of the ASE and the edge filter. A theoretical model was developed to illustrate the underlying mechanism supporting this normalization approach. The normalized Raman signals are independent of the incident laser intensity and the density of "hot spots" for numerous SERS substrates. Following "hot-spot" (HS) normalization, the coefficient of variation for the tested SERS substrates decreases from 10 to 60% to 2%-7%. This approach significantly improves SERS quantitation of four chloroanilines and enables collection of highly reproducible analyte adsorption results under both static and dynamic imaging conditions. Overall, this approach provides a simple means to improve SERS reproducibility without the need to use additional chemicals as internal standards.

Quantitative SERS by hot spot normalization - surface enhanced Rayleigh band intensity as an alternative evaluation parameter for SERS substrate performance.

The performance of surface-enhanced Raman spectroscopy (SERS) substrates is typically evaluated by calculating an enhancement factor (EF). However, it is challenging to accurately calculate EF values since the calculation often requires the use of model analytes and requires assumptions about the number of analyte molecules within the laser excitation volume. Furthermore, the measured EF values are target analyte dependent and thus it is challenging to compare substrates with EF values obtained using different analytes. In this study, we propose an alternative evaluation parameter for SERS substrate performance that is based on the intensity of the surface plasmon enhanced Rayleigh band (IRayleigh) that originates from the amplified spontaneous emission (ASE) of the laser. Compared to the EF, IRayleigh reflects the enhancing capability of the substrate itself, is easy to measure without the use of any analytes, and is universally applicable for the comparison of SERS substrates. Six SERS substrates with different states (solid, suspended in liquid, and hydrogel), different plasmonic nanoparticle identities (silver and gold), as well as different nanoparticle sizes and shapes were used to support our hypothesis. The results show that there are excellent correlations between the measured SERS intensities and IRayleigh as well as between the SERS homogeneity and the variation of IRayleigh acquired with the six SERS substrates. These results suggest that IRayleigh can be used as an evaluation parameter for both SERS substrate efficiency and reproducibility.

Design of highly uniform spool and bar horns for ultrasonic bonding.

Although the groove and slot have been widely utilized for horn design to achieve high uniformity, their effects on uniformity have not been analyzed thoroughly. In this work, spool and bar horns for ultrasonic bonding are designed in a systematic way using the design of experiments (DOE) to achieve high amplitude uniformity of the horn. Three-dimensional modal analysis is conducted to predict the natural frequency, amplitude, and stress of the horns, and the DOE is employed to analyze the effects of the groove and slot on the amplitude uniformity. The design equations are formulated to determine the optimum dimensions of the groove and slot, and the uniformity is found to be influenced most significantly by the groove depth and slot width. Displacements of the spool and bar horns were measured using a laser Doppler vibrometer (LDV), and the predicted results are in good agreement with the experimental data.