With an index of refraction approaching 9 at microwave frequencies, water is a high-index dielectric material suitable for studying morphology-dependent resonance (MDR) analogs of metallic plasmonic hotspots. We use experimental, analytical, and computational approaches to study MDRs in aqueous dimers, paying particular attention to formation of evanescent axial hotspots. We use hydrogel beads and thermal imaging, along with FEM simulations to explore polarization, orientation, and size-dependence in MDR hybridization. A novel analytical approach involving vectorial addition of spherical harmonics of monomer MDRs provides geometric insight into which modes most strongly interact to yield strong subwavelength electromagnetic hotspots in aqueous bispheres.
Despite its low index of refraction at visible wavelengths, water exhibits a large complex index of refraction at 2.5 GHz. As such, cm-scale aqueous objects like grapes and hydrogel beads are resonant in microwave radiation, showing the expected sequence of spherical Mie scattering modes. We expand our study to aqueous spheroids, where analytical prediction of resonant sizes and shapes are considerably more difficult to make. We show that 3D standing-wave solutions accurately predict certain fundamental microwave Mie resonances in both oblate and prolate spheroids, and we compare this conceptually and mathematically-simplified approach with results obtained using FEM simulations.
We describe opportunities for integrated testlets as assessments in optics and photonics courses. This new scaffolded multiple-choice approach adopts an answer-until-correct framework that can test complex understanding through either offline or online delivery.
I describe a simple spectroscopic method for measuring birefringence in household films such as adhesive tape and kitchen film. Using an incandescent light source provides students direct appreciation for the wavelength-dependent nature of the phenomenon.
The capabilities of a Coherent anti-Stokes Raman scattering (CARS) microscopy technique that utilizes a commercially available microstructured-fibre for the generation of a Stokes supercontinuum, largely depend on the spectral characteristics of the fibre itself. Such fibres, however, have notoriously limited spectral powers and wavelength coverage, thus limiting the excitation efficiency of fibre-based CARS. We demonstrate that through the combination of time-gating and systematic power-tuning of the input pulse into a supercontinuum-generating fibre—a process we term “spectral surfing”—we are able to extend and bolster the excitation efficiencies of spectral-focusing CARS hypermicroscopy across the vibrational spectrum from ~300 cm-1 to >3500 cm-1.
KEYWORDS: In vivo imaging, Microscopy, Raman spectroscopy, CARS tomography, Absorption, Resonance enhancement, Microscopes, Signal processing, Visualization, In vitro testing
Coherent anti-Stokes Raman scattering (CARS) and four-wave-mixing (FWM) microscopy are a related pair of
powerful nonlinear optical characterization tools. These techniques often yield strong signals from concentrated
samples, but because of their quadratic dependence on concentration, they are not typically employed for imaging or
identifying dilute cellular constituents. We report here that, depending on the excitation wavelengths employed, both
CARS and degenerate-FWM signals from carotenoid accumulations in alga cysts can be exceptionally large, allowing
for low-power imaging of astaxanthin (AXN) deposits in Haematococcus pluvialis microalga. By use of a broadband
laser pulse scheme for CARS and FWM, we are able to simultaneously collect strong intrinsic two-photon-excitation
fluorescence signals from cellular chlorophyll in vivo. We show that CARS signals from astaxanthin (AXN) samples in
vitro strictly follow the expected quadratic dependence on concentration, and we demonstrate the collection of wellresolved
CARS spectra in the fingerprint region with sensitivity below 2mM. We suggest that multimodal nonlinear
optical microscopy is sufficiently sensitive to AXN and chlorophyll concentrations that it will allow for non-invasive
monitoring of carotenogenesis in live H. pluvialis microalgae.
Hyperspectral coherent anti-Stokes Raman scattering (CARS) microscopy has provided an imaging tool for
extraction of 3-dimensional volumetric information, as well as chemically-sensitive spectral information. These
techniques have been used in a variety of different domains including biophysics, geology, and material science.
The measured CARS spectrum results from interference between the Raman response of the sample and a non-resonant
background. We have collected four dimensional data sets (three spatial dimensions, plus spectra)
and extracted Raman response from the CARS spectrum using a Kramers-Kronig transformation. However,
the three dimensional images formed by a CARS microscope are distorted by interference, some of which arises
because of the Gouy phase shift. This type of interference comes from the axial position of the Raman resonant
object in the laser focus. We studied how the Gouy phase manifests itself in the spectral domain by investigating
microscopic diamonds and nitrobenzene droplets in a CARS microscope. Through experimental results and
numerical calculation using finite-diference time-domain (FDTD) methods, we were able to demonstrate the
relationship between the spatial configuration of the sample and the CARS spectral response in three dimensional
space.
Luminal atherosclerosis imaging was demonstrated by multimodal femtosecond CARS
microscopy (MM-CARS). Using a myocardial infarction-prone rabbit model of
atherosclerosis, this study demonstrated the utility of multimodal CARS imaging in
determining atherosclerotic plaque burden through two types of image analysis procedures.
Firstly, multimodal CARS images were evaluated using a signal-intensity parameter based
on intensity changes derived from the multi-channel data (e.g. TPEF, SHG and CARS) to
classify plaque burden within the vessel. Secondly, the SHG images that mainly correspond
to collagen fibrils were evaluated using a texture analysis model based on the first-order
statistical (FOS) parameters of the image histogram. Correlation between FOS parameters of
collagen images with atherosclerosis plaque burden was established. A preliminary study
of using spectroscopic CARS in identifying the different lipid components within the plaque
was also discussed.
We demonstrate the simultaneous collection and separation of femtosecond-laser-based forward-collected coherent anti-Stokes Raman scattering (F-CARS) and two-photon-excitation-induced fluorescence lifetime images (FLIM) using time-correlated single photon counting (TCSPC). We achieve this in a nondescanned geometry using a single multimode fiber without significant loss of light, field of view, and most importantly, TCSPC timing fidelity. In addition to showing the ability to separate CARS images from FLIM images using time gating, we also demonstrate composite multimodal epicollected FLIM imaging with fiber-collected F-CARS imaging in live cells.
Traditional CARS microscopy using picosecond (ps) lasers has been applied to a wide variety of applications;
however, the lasers required are expensive and require an environmentally stable lab. In our work, we demonstrate
CARS microscopy using a single femtosecond (fs) laser combined with a photonic crystal fiber (PCF) and optimal
chirping to achieve similar performance to the ps case with important added advantages: fs-CARS utilizes
versatile source that allows CARS to be combined with other multiphoton techniques (e.g. SHG, TPF) for
multimodal imaging without changing laser sources. This provides an attractive entry point for many researchers
to the field. Furthermore, optimal chirping in fs-CARS also opens the door to the combination and extension
of other techniques used in ps CARS microscopy such as multiplex and FM imaging. The key advantage with
chirped fs pulses is that time delay corresponds to spectral scanning and allows for rapid modulation of the
resonant CARS signal. The combination of a fs oscillator with a PCF leads to a natural extension of the
technology towards an all-fiber source for multimodal multiphoton microscopy. An all-fiber system should be
more robust against environmental fluctuations while being more compact than free-space systems. We have
constructed and demonstrated a proof of concept all-fiber based source that can be used for simultaneous CARS,
TPF and SHG imaging. This system is capable of imaging tissue samples and live cell cultures with 4 μs/pixel
dwell time at low average powers.
We studied the wavelength-, time-, and intensity-dependence of the 3rd-order nonlinear optical response of As2Se3 chalcogenide glass. Bulk samples were characterized using a wavelength-tunable z-scan system, over the range 1200-1600 nm. Thin film samples were characterized using an ultrafast time-resolved differential optical Kerr effect (DOKE) experiment, fed by 125 fs pulses centered at 1425 nm. The z-scans revealed only slight variation in the optical Kerr coefficient n2 over the wavelength range studied. The DOKE experiment confirmed that the nonlinear response is predominately electronic, with response time limited by the experimental setup. For the same beam intensity, DOKE and z-scan measurements were in good agreement. The optical Kerr coefficient extracted from DOKE measurements at varying pump beam intensity showed intensity-dependent behavior, which can be attributed to fifth and higher order nonlinearities.
Differential optical Kerr effect (DOKE) detection is a powerful tool for studying the ultrafast time-resolved dynamics of 3rd-order nonlinear processes. In this study, DOKE was used to measure the nonlinear absorption properties of tetraethynylphenylene (TEP) solutions in THF using 800 nm, 80 fs laser pulses. These two-dimensional, highly-conjugated chromophores (also known as TPEBs) show high instantaneous two-photon absorption (TPA) for relatively small chromophores. The TPA cross section is strongly dependent on the donor-acceptor geometry in these materials: a quadrupolar, all-donor TEP shows the smallest TPA, with a cross-section of σ(2)= 90 ± 15 GM. ortho-TEP, for which the donors (and acceptors) are conjugated via the ortho position across the central phenyl ring, is dipolar and displays the largest cross-section, of σ(2)= 260 ± 30 GM. para-TEP, which is quadrupolar, and meta-TEP, which is dipolar, display similar cross-sections of σ(2)= 160 ± 10 GM and σ(2)= 150 ± 10 GM, respectively. In addition to an instantaneous TPA response, these isomers show unique two-photon assisted excited-state absorption (ESA), with the ortho- and meta-TEP displaying a clear 3-10 ps rise to an ESA peak, and subsequent decay. The differences in the nonlinear absorption behaviour of these materials may be partially explained by selection rules and UV-vis spectroscopy. In addition, the polar geometries, coupled to the various in-plane conjugation paths, may further influence their optical nonlinearities. Understanding these trends impacts both the design of materials with desirable nonlinear absorption properties and our understanding of the electronic landscape in functionalized organic materials.
Carbyne is a hypothetical carbon allotrope that consists of sp-hybridized carbon atoms in an infinitely-long, one-dimensional (1-D) linear chain. Polyynes, the oligomeric cousins of carbyne, with a dense delocalized-electron framework, could offer groundbreaking electronic properties. We have studied the linear and third-order nonlinear optical properties of both triisopropylsilyl end-capped (TIPS-PY) and phenyl end-capped (p-PY) polyynes containing pure sp-hybridized carbon backbones. Analysis of the TIPS-polyyne UV-vis absorption spectra shows that the absorption gap, Eg, in these materials scales very precisely as a power-law with increasing oligomer length, n, with Eg~n-0.379±0.002. The phenylated polyynes show a similar trend of Eg~n-0.36±0.01. Ultrafast molecular second-hyperpolarizabilities, γ, were obtained in solution using 800nm, 100fs pulses in a differential optical Kerr effect (DOKE) setup. Polyyne second-hyperpolarizabilities also scale with a power-law, and, surprisingly, with exponents higher than that of any other reported oligomer system, yielding a behavior of γ~n4.3±0.1 and γ~n3.8±0.1 for the TIPS-polyynes and phenylated-polyynes, respectively. These findings contrast direct theoretical predictions that increases in gamma with increasing conjugation length for polyynes should be considerably lower than those of polyenes and polyenynes. Furthermore, the combined linear and nonlinear optical results agree with recent theoretical studies on ideal 1-D conjugated systems, suggesting that polyynes display true 1-D behavior.
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