Publisher's Note: This oral presentation, originally published on 5 October 2023, was replaced with a corrected/revised version of the presentation slides on 19 April 2024. No change was made to the published manuscript accompanying this video presentation.
The optically driven mechanics of a 2.17 μm-diameter water droplet subjected to a linearly-polarized, zeroth-order, tightly-focused, continuous-wave, 532 nm wavelength, Hermite-Gaussian laser beam are simulated in the Kirchoff-Fresnel diffraction region. Coupled electrodynamic and weighted orthogonal multi-relaxation kinetic lattice-Boltzmann methods evaluate Maxwell and Navier-Stokes equations, and a central-difference analysis at each location in space and instant in time evaluates the momentum continuity postulated by seven electrodynamic formalisms. Morphology of the 2.17 μm diameter water droplet is unique for each electrodynamic formalism, electric field polarization, focal displacement, and beam divergence of the incident Hermite-Gaussian beam. Unique droplet morphology predicted by each electrodynamic formalism in a focused Hermite-Gaussian beam also results in distinct electromagnetic mode confinement and scattering patterns measurable from the far field. Therefore, an electrodynamic theory may be experimentally deduced from the irradiance, polarization, and phase of the far-field angular light scattering patterns when compared against numerical analysis and standard near-to- far field transformation. Probing water droplets in the Kirchoff-Fresnel diffraction region may experimentally disprove long-standing electrodynamic theories, or suggest an appropriate electrodynamic theory for predicting the nonlinear deformation of light-scattering droplets.
We develop a method to detect surface plasmon polaritons (SPP) launched by a sub-wavelength slit structure
using optical microscopy. The mechanism relies on an ultra-thin layer of polymer, whose thickness is varied with
nano-scale precision to enable matching between the momentum of incident light and that of SPPs on the metal
surface adjacent to the slit exit. At an optimal layer thickness, the SPP coupling efficiency is enhanced about
six times relative to that without the layer. The enhanced efficiency results in distinctive and bright signatures
visible under a far-field optical microscope. We show how this method can be used for parallel measurement of
SPPs through a simple experiment in which the SPP propagation distance is extracted from a single microscope
image. We also use optical microscopy to image SPPs focussed by a curved array of holes, obtaining results that
are consistent with previous measurements using near-field optical microscopy.
Surface plasmon resonance (SPR) sensors exploit optical coupling to surface plasmons, light waves bound to a
metal surface. In the most common configuration, a SPR sensor is used with an external light source, optical
components to polarize incident light and guide light to and from a metal surface, a coupling device to convert
free-space light into surface plasmons and back into free-space light, and a light detector. The light source,
the optical components, and the light detector are external to the SPR device, and the coupling structure is
often integrated directly with the surface-plasmon-sustaining metal surface. The requirement of several external
components restricts the miniaturization of SPR devices and prohibits low-cost implementation. To address
these limitations, we design, fabricate, and test a new SPR device chip that is fibre-addressable, does not require
a discrete coupling structure, and integrates light delivery, light polarization control, surface plasmon coupling
onto a thin, flexible substrate. Our SPR chip is constructed from a thin gold layer deposited on top of a clear
plastic sheet, which is then optically connected from the bottom surface onto a plastic linear polarizer sheet.
Two cleaved fibres, one to input light and the other to collect reflected light, are then optically attached to SPR
device. We experimentally characterize the SPR device and find good agreement between our measurements and
a theoretical model based on transfer matrix formalism.
We propose and design a broadband optical resonator fashioned from two types of waveguides: one sustaining a
positive-index mode with a positive phase velocity and another sustaining a negative-index mode with a negative
phase velocity. The former has a wavelength that decreases as a function of frequency, while the latter has a
wavelength that increases as a function of frequency. Because frequency-dependent wavelength increases in one
waveguide component of the resonator are compensated by wavelength decreases in the other component, the
resonator can potentially support a continuum of standing wave patterns that all satisfy the same resonance
condition, effectively widening the frequency range over which resonance is achievable. We have tailored the
geometry of the resonator so that the net phase accrued within the resonator is nearly constant over a large
portion of the visible frequency range, and, as shown through FDTD simulations and analytical calculations, a
broadband optical resonance is achieved.
Water droplets are an attractive medium to realize visible-frequency optical elements. The smoothness of a
droplet surface mitigates losses due to light scattering, the shape of a water droplet is reconfigurable by either
applying pressure or a potential, water is nearly transparent over the visible frequency range, and water is highly
abundant. Here, we explore a simple methodology to dispense and shape water droplets for application as the
magnifying element in a microscope using either reflection-mode or transmission-mode illumination. A water
droplet is created at the end of a syringe and then coated with a thin layer of silicone oil to mitigate evaporation.
By applying mechanical pressure to the water droplet using a metal tip, the shape of the droplet is tuned to
yield focusing properties amenable for microscopy. Images captured using the microscope demonstrate micron-scale
resolution, variable magnification, and imaging quality comparable to that obtained by a conventional,
laboratory-grade microscope.
Optical disc drives are inexpensive, readily available, and use highly sophisticated optoelectronic components which can
be adapted for sensing. One limitation of using compact discs (CDs) and optical disk drives for sensing of analytes
placed on a CD is the fluctuations in the voltage signal from the disk drive generated while reading the data on the CD.
In this study, we develop a simple, low-cost strategy for sensing and identification using CDs and optical disk drives that
spectrally separates contributions to the voltage signal caused by an analyte intentionally placed onto the CD and that
caused by the underlying data on the CD. Analytes are printed onto a CD surface with fixed spatial periodicity. As the
laser beam in an optical disk drive scans over the section of the CD containing the analyte pattern, the intensity of the
laser beam incident onto the photodiode integrated into the disk drive is modulated at a frequency dependent on the
spatial periodicity of the analyte pattern and the speed of the optical disk drive motor. Fourier transformation of the
voltage signal from the optical disk drive yields peaks in the frequency spectrum with amplitudes and locations that
enable analyte sensing and identification, respectively. We study the influence of analyte area coverage, pattern
periodicity, and CD rotational frequency on the peaks in the frequency spectrum associated with the patterned analyte.
We apply this technique to discriminate differently-coloured analytes, perform trigger-free detection of multiple analytes
distributed on a single CD, and detect at least two different, overlapped analyte patterns on a single CD. The extension
of this technique for sensing and identification of colorimetric chemical reagents is discussed. Future work will focus on
adapting this technique to perform measurements at multiple wavelengths and streamlining the data collection and
processing.
We propose a high efficiency coupling scheme of surface plasmon polaritons (SPP) in a metal-dielectric slit structure. The design includes a narrrow silver-slit structure, which is immersed in a dielectric with refractive index 1.5. We map the dependence of SPP coupling efficiency on the dielectric layer thickness (d). By varying d the dispersion behavior of the SPP mode is tuned enabling minimized wavevector mismatch between the light exiting the slit and the SPP mode. A dielectric layer of thickness in the range 50 nm < d < 150 nm yields coupling efficiencies of approximately 80%, representing a nearly four-fold enhancement relative to the coupling efficiency without a dielectric layer.
The emergence of nanotechnology now enables the controlled fabrication of nanometer scale structures capable
of steering and confining light waves over small distances. To realize complex nanoscale light guiding structures, it will
be necessary to develop methods to guide light around tight bends and corners with high efficiency. Achieving high
efficiency waveguide bends, however, is generally difficult because of radiation losses at the bend. To achieve tight
waveguide bends several approaches have been put forward including the use of dielectric photonic crystals and
resonators. One recent and promising method to limit the amount of loss over a bend is to restrict the path of light by
encasing a bend in an opaque medium, such as metal. Such a bend can be conceptualized by joining the two metal-dielectric-
metal (MDM) waveguides such that their dielectric cores are connected to each other at 90° as shown in
Fig. 1. When the thickness of the dielectric cores is subwavelength, only the lowest order surface plasmon polariton
(SPP) mode is sustained by the bend. Further, when the metal walls are constructed from a low-loss metal such as Ag,
the SPP mode can propagate over the bend.
Ferromagnetic particle collections typically possess anisotropic terahertz (THz) transmission properties that are
sensitive to the orientation of an applied external magnetic field. Here, we show that the particle surface morphology
can have a large and dominant effect on the magnetic field orientation dependence of the THz transmission. In
particular, the THz transmission through highly porous ferromagnetic Ni particles shows isotropic dependence on the
external magnetic field orientation. This isotropic magnetic phenomenon suggests the possibility of innovative photonic
materials with tailored magnetic properties.
In this work, we explore the interaction of terahertz electromagnetic pulses with chiral metallic
mesostructures. In contrast to conventional continuous wave experiments conducted in the visible and
microwave regimes, time-domain THz spectroscopy enables direct measurement of the electric field and
polarization dynamics of electromagnetic waves propagated through the chiral structure. With this
experimental methodology, we discover significant polarization circularization of the radiation scattered
from a sub-wavelength sized helix in the axial configuration. Numerical simulations are in excellent
quantitative agreement with the experimental results. Understanding light behaviour in a helical structure
is not only of fundamental significance, but could potentially lead to the development of entirely new
materials to improve communication and information technology.
We investigate on-axis terahertz transmission through random collections of sub-wavelength sized metallic particles. Despite both the inherent opacity of the metallic particles at terahertz frequencies and the absence of straight-line photon trajectories through the samples, we observe significant, polarized terahertz transmission through dense metallic particle collections, which are five orders of magnitude thicker than the radiation skin depth. The effects of sample length, particle size, particle shape, and conductivity on the enhanced transmission are explored experimentally and numerically. Our findings show that the polarized terahertz transmission is mediated by coherent near-field coupling of surface electromagnetic waves across the ensemble.
We experimentally investigate terahertz pulse transmission through dense, random ensembles of sub-wavelength ferromagnetic Co particles. Due to the magnetoresistance inherent to the ferromagnetic metal, the terahertz optical properties of the Co particle ensembles are shown to be strongly dependent on the strength and orientation of an external magnetic field.
We study free-space terahertz pulse propagation through samples of densely packed Cu microparticles that are coated with Au nano-layers. By coating the Cu particles with Au nano-layers, the terahertz transmission is dramatically attenuated. The substantial attenuation cannot be reconciled by the inherent resistivities of the Cu and Au metals. The experimental results strongly show that the transmission attenuation arises from contact resistance between the Au and Cu metals.
We present a model of THz emission enhancement from femtosecond pulse excited n-GaAs and InAs surfaces with the application of a dc magnetic field. The far-field THz emission at different optical excitation densities, magnetic field strengths, and magnetic field orientations is determined. The model accurately describes the power dependence of THz emission from n-GaAs and InAs surfaces for magnetic field strengths up to ±10 T and ±6 T, respectively. THz emission saturation in both semiconductors for optical excitation densities from 40 nJ/cm2 to 2.2 μJ/cm2 are in accordance with previously reported experimental data. The model provides a useful tool for the description of ultra-fast processes in semiconductors.
We report on experimental and numerical studies of free space terahertz (THz) propagation in strongly scattering random dielectric media. The on-axis ballistic and small angle scattered transmission is measured through media of varying thickness. The experimental variations of the terahertz pulse group delay and scattering-induced effects such as temporal pulse distortion, spectral decay, and power attenuation as a function of sample thickness are well described by a Monte Carlo photon migration model. The transmitted pulses are analysed using the classical Bruggemann effective medium approximation (EMA). It is found that the effective medium approximation underestimates the accumulated pulse phase acquired by the high frequencies during pulse propagation. An empirically modified EMA provides accurate description of the random dielectric medium.
Laser micromachining may be used for a variety of applications including drilling holes or creating trenches in dielectric materials. Cracking around the ablated features can be a significant problem for many applications, particularly when micromachining glass. One possible method for crack reduction, investigated here, involves heating of the substrate during ablation. This leads to a more ductile material that is more able to withstand the thermal shock of the ablation process. In order to increase the ductility, the glass targets are heated by physical contact with an electric heating element. The results of micromachining are analyzed using an optical microscope. The amount of cracking is quantified in terms of the number of visible radial cracks. For nanosecond micromachining, a reduction in the number of cracks and an improvement in the quality of the holes are observed as the glass is heated. The relative improvement using heated substrates and nanosecond pulses is also compared to femtosecond ablation of room temperature substrates.
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