We discuss studies of laser damage inside various transparent materials (glasses, polymers, sapphire, diamond) caused by femtosecond lasers at 515, 800, and 1030 nm, with nJ to mJ pulse energies, single-shot to 2 MHz repetition rates, single and 10 ns burst-mode pulses, chirped pulses, and linear, circular, and radial beam polarizations. Experiments have created high-aspect damage features and voids using aberration-controlled focusing, axicon-formed Bessel beams with <1 μm diameter central lobes extending hundreds of microns through the materials, and tightly focused lines (~1 μm × <100 μm). Mechanisms include self-focusing, filamentation, and material expansion/compaction and expulsion.
Femtosecond (fs) laser beams may be shaped into Bessel beam (BB) profiles by spatial light modulators or axicon lenses. Temporally reshaping laser pulses by chirping and stretching further alters the spatio-temporal intensity pattern within the elongated focal volume. Such beam shaping applied to high-power pulses is useful for numerous materials processing applications, enabling fabrication of very high aspect ratio columns in optically transparent materials. We report the development of a compact, adaptable microscope turret-mounted assembly containing an axicon and a high numeric aperture aspheric lens imaging system, and the use of temporal reshaping studies in various fs laser machining applications. Depending on axicon angle, lens separations, and the refractive index of the substrate, the central lobe diameter of the BB may be less than 1 μm but extending over 500 μm long, effectively forming a narrow, long cylindrical column. Moreover, because an entire column can be machined with a single, energetic pulse, high processing rates are possible. Materials such as fused silica and polymers are found to be good candidates for directly formed voids. Microchannels in silica can be used in single-molecule recycling experiments, while permeable membranes in thin plastics are sought for cellular studies, passing nutrients but not cells. In rigid crystalline structures like diamond and sapphire, the substrate material is transformed in place. In particular, a BB column machined through electrically insulating diamond can become conductive graphite, which is of interest for developing radiation-hard detectors of highenergy particles.
Scalable methods must be developed for fabricating high-density arrays of conductive microchannels through ~0.5 mmthick synthetic diamonds in order to form radiation-hard 3D particle tracking detectors for use in future high-energy particle physics experiments, such as those beyond the scheduled 2022 high-luminosity upgrade of the Large Hadron Collider. Prototype detectors with small-area arrays of graphitic columns, each written by slowly translating a femtosecond laser beam focus through the diamond, have established proof of concept, but much faster procedures are needed to manufacture large arrays of electrodes with <100 micron spacing over diameters of ~10 cm. We have used a Bessel beam, formed using a 10° axicon and 0.68 NA aspheric lens, to very quickly write micron-diameter columns through ~0.5 mm-thick electronic grade CVD diamonds without axially translating the diamond with respect to the beam. We employ an optical microscope to visualize columns, Raman spectroscopy to ascertain the degree of graphitization, and cat-whisker probes to test overall conductivity. Bessel focusing enables formation of a complete column with just a few femtosecond laser pulses, and so provides a scalable manufacturing method. However, reduction in the electrode resistivity is desired. To this end, expulsion of material from the column is probably needed, as carbon plasma will otherwise condense back into diamond, due to the disparate densities of graphite and diamond. We describe the use of several different femtosecond laser systems to evaluate a range of pulse parameters with the goal of increasing the level of graphitization and improving the conductivity of the electrodes.
Reducing reagent needs and costs while increasing throughput constitute important needs for assays in pharmaceutical
drug discovery. We are developing an ultrasensitive, fluorescence-based detection system in highly parallel microfluidic
channels with kHz readout rates in each channel. Prototype microfluidic devices with an array of >150 microchannels
have been fabricated by direct femtosecond laser machining of fused silica substrates. A device is placed in a custombuilt,
wide-field microscope where a line-generating red diode laser provides uniform epi-illumination just a few
microns high across a 500 micron field of view. Single-molecule levels in the probe volumes can be attained by flowing
suitably dilute aqueous solutions (~pM) of fluorescently labeled biomolecules through the microchannels. Fluorescence
is detected with an electron-multiplying CCD camera allowing readout rates up to 7 kHz for each microchannel. Rapid
initial assessment of detected fluorescence signals is performed through digital filtering derived from simulations based
on experimental parameters. Fluorescence correlation spectroscopy can then provide more detailed analysis of the
sample within each microchannel. Optimized microfluidic devices could be mass-produced in low-cost polymers using
imprint lithography.
We discuss the capabilities for sub-diffraction, single-nanoparticle position determination in a confocal one- or twophoton
microscope with four-focus pulse-interleaved excitation and time-gated single-photon counting. As the technique
is scalable to multiple detectors for multi-color observations, it can be used to find the separations of differently colored
molecules over a distance range that is complementary to that achievable by FRET. Also, there is a possibility for
improved spatial localization by using the nonlinearity of saturation of the excitation or by using the technique together
with imaging of the point spread function. Applications of two experimental set-ups for four-focus fluorescence
excitation for studies of quantum dots and single-particle manipulation and trapping are also discussed.
We describe the fabrication of sub-100-nanometer-sized channels in a fused silica lab-on-a-chip device and experiments
that demonstrate detection of single fluorescently labeled proteins in buffer solution within the device with high signal
and low background. The fluorescent biomolecules are transported along the length of the nanochannels by
electrophoresis and/or electro-osmosis until they pass into a two-focus laser irradiation zone. Pulse-interleaved excitation
and time-resolved single-photon detection with maximum-likelihood analysis enables the location of the biomolecule to
be determined. Diffusional transport of the molecules is found to be slowed within the nanochannel, and this facilitates
fluidic trapping and/or prolonged measurements on individual biomolecules. Our goal is to actively control the fluidic
transport to achieve rapid delivery of each new biomolecule to the sensing zone, following the completion of
measurements, or the photobleaching of the prior molecule. We have used computer simulations that include
photophysical effects such as triplet crossing and photobleaching of the labels to design control algorithms, which are
being implemented in a custom field-programmable-gate-array circuit for the active fluidic control.
We show that local fields associated both with overall structural features and with unintended defects can be important in
the second-order nonlinear response of metal nanostructures. We first consider noncentrosymmetric T-shaped gold
nanodimers with nanogaps of varying size. The reflection symmetry of the T-shape is broken by a small slant in the
mutual orientations of the horizontal and vertical bars, which makes the sample chiral and gives rise to a different
nonlinear response for left- and right-hand circularly-polarized fundamental light. Measurements of achiral and chiral
second-harmonic signals as well as the circular-difference response exhibit a nontrivial dependence on the gap size. All
results are explained by considering the distribution of the resonant fundamental field in the structure and its interaction
with the surface nonlinearity of the metal. We also prepared arrays of ideally centrosymmetric circular nanodots.
Second- and third-harmonic generation microscopies at normal incidence were used to address polarization-dependent
responses of individual dots. Both signals exhibit large differences between individual dots. This is expected for second-harmonic
generation, which must arise from symmetry-breaking defects. However, similar results for third-harmonic
generation suggest that both nonlinear responses are dominated by strongly localized fields at defects.
We present a comprehensive multipolar tensor analysis to investigate the roles of dipolar and higher-order
multipoles to second-harmonic radiation from a regular array of noncentrosymmetric L-shaped gold nanoparticles.
We find the nonlinear response to be dominated by a tensor component which is associated with chiral symmetry
breaking and has strong multipolar character. These findings substantiate our interpretation that one of the
major contributors to the optical response of the present sample are structural defects, which break the symmetry
and make multipolar contributions to the SH response important.
The mutual non-orthogonal orientations of its horizontal and vertical bars make T-shaped gold nanodimers chiral.
Because of the broken symmetry second-harmonic generation from the structure has different efficiencies for left- and
right-hand circularly-polarized fundamental light. The chiral signature arises from the coupling between the bars. One
would therefore assume that the chiral signature is largest when the gap size is very small, because then the coupling is
presumably the strongest. Counter-intuitively, the measurement results show a very small chiral signature for the
smallest gap. To explain the results, one needs to consider the distribution of the local field in the unit cell of the
structure.
A freely diffusing single fluorescent molecule may be scrutinized for an extended duration within a confocal microscope
by actively trapping it within the femtoliter probe region. We present results from computational models and ongoing
experiments that research the use of multi-focal pulse-interleaved excitation with time-gated single photon counting and
maximum-likelihood estimation of the position for active control of the electrophoretic and/or electro-osmotic motion
that re-centers the molecule and compensates for diffusion. The molecule is held within a region with approximately
constant irradiance until it photobleaches and/or is replaced by the next molecule. The same photons used for
determining the position within the trap are also available for performing spectroscopic measurements, for applications
such as the study of conformational changes of single proteins. Generalization of the trap to multi-wavelength excitation
and to spectrally-resolved emission is being developed. Also, the effectiveness of the maximum-likelihood position
estimates and semi-empirical algorithms for trap control is discussed.
We demonstrate both experimentally and computationally that SHG from arrays of T-shaped gold nanodimers with differing nanogap sizes results from asymmetry in the local field distribution rather than strict dependence on the nanogap size. Normal-incidence SHG measurements reveal that the SHG responses depend non-trivially
on the nanogap size. Calculations show that strong orthogonal polarization components, which are not present
in the exciting field, are induced, and that these induced components yield the dominant SHG response. The
calculations also reveal that field enhancement is roughly independent of nanogap size and persists even for
large nanogap sizes. A simple phenomenological model wherein the local surface susceptibility of the nanodimer interacts with the local field distribution along the nanodimer perimeter qualitatively explains the experimental results with good agreement.
Nanoscale variations in the local fields and material properties can enable higher-multipole (magnetic-dipole and
electric-quadrupole) contributions to the nonlinear response in addition to electric-dipole contributions. Moreover,
the local-field distribution in the structure is important to achieve favorable interaction with the locally varying
nonlinearity. Local-field enhancement is particularly important for nonlinear optical effects. Extremely
small features of a few nm, such as nanogaps between two particles, are expected to be particularly beneficial
for field localization and enhancement. Here, we provide evidence of multipole interference in polarized secondharmonic
generation from arrays of L-shaped gold nanoparticles. We also prepare T-shaped gold nanodimers
and vary the size of the nanogap between their vertical and horizontal bars. Surprisingly, the second-harmonic
signals do not decrease trivially with increasing gap size, because the gap region is nearly centrosymmetric,
thereby forbidding second-order effects. Instead, asymmetric local fundamental field distributions along the
dimer perimeter are favorable, in accordance with the symmetry rule.
Great progress has been achieved in fabricating arbitrary metal nanoparticle shapes and geometries in order to control their linear optical properties. However, their nonlinear optical properties, particularly their second-order response, are frequently overlooked. Exploiting the nonlinear responses of metal nanoparticles opens another exciting avenue for developing nanoscale photonics applications. Second-harmonic generation (SHG) from metal nanoparticles is typically attributed to electric dipole excitations at their surfaces, but nonlinearities involving higher multipole effects, such as magnetic dipole interactions, electric quadrupoles, etc., may also be significant due to strong nanoscale gradients in the local material properties and fields. The nanoscale nonlinear optical processes in metal nanoparticles are not well-understood at present, and determining the sources of the SHG response can be arduous. In order to study the role of higher multipoles in the second-order response of gold nanoparticle arrays, we propose SHG measurements employed in both transmission and reflection geometries. Due to different radiative properties of the various multipoles in the forward and backward directions, the presence of multipoles should lead to opposing interference effects in the two directions. Strong polarization dependence of the response can modify the relative strengths of the interfering terms, thereby allowing electric-dipole and higher-multipole contributions to the overall SHG response to be distinguished. Analysis of the measured polarization dependencies would thus provide further knowledge of the mechanisms underlying the nanoscale SHG process in gold nanoparticles.
Sensitive second-harmonic polarization measurements can yield important information about the symmetry properties of thin films. When the linewidth of the probe laser is relatively narrow, as in the case of a nanosecond pulse from a Nd:YAG laser, the polarization-sensitive optical components, waveplates in particular, behave
according to specifications. However, the question arises, does this case also apply to a broadband source, such as a femtosecond laser pulse, where the linewidth is much greater than that of the waveplate? We show that the case does indeed hold by comparing measurements using both sources. However, special attention must
be focused on the performance of the optics themselves, as manufacturer's specifications may not be accurate for each individual piece. The agreement of the measurements opens the door for determining the symmetry properties of completely new types of low-symmetry samples, such as gold nanoparticle arrays, which must be
studied with a femtosecond source to avoid damaging the particles.
The physical processes underlying the complex nanoscale optical responses of metal nanoparticles must be understood
both experimentally and theoretically if they are to be developed for use in photonic devices. While
many linear optical measurements have been performed on gold nanoparticle arrays, only a handful of nonlinear
measurements have been reported. Here, we discuss a collection of experiments of both types on arrays of gold
nanoparticles. However, on nanoscale-rough metal surfaces, such as nanoparticles with small-scale defects, local
electric fields may vary rapidly and strong field gradients can induce significant multipolar contributions, making
a theoretical description of second-harmonic generation (SHG) from nanoparticle arrays infeasible at present.
A macroscopic nonlinear response tensor approach based on the input and output fields to the system avoids
with these complications. Contributions from higher multipoles are implicitly included, and electric-dipole-type
selection rules can be applied to address symmetry issues. While the experimental geometry constrains the
formalism, additional insight into the underlying physical processes is expected from experimental variations.
Good agreement with direct SHG tensor measurements validates the formalism, providing the framework for a
deeper understanding of the nanoscale optical responses of metal nanoparticles.
Considerable attention is devoted to determining and refining the optical properties of metal nanoparticle arrays. The evolution of nanofabrication techniques towards miniaturizing optoelectronic devices naturally suggests the possibility of using such arrays in nanoscale optical components. However, small-scale defects (tens of nanometers or less) in individual particles themselves may exert a significant influence on the overall optical responses of the array, especially when the particles (and/or arrays) appear symmetric on the scale of the particle (and/or array). We have observed strong linear and nonlinear chiral responses from regular arrays of lithographically-designed, low-symmetry, L-shaped gold nanoparticles (~ 200 nm arm lengths) through polarization azimuth rotation and circular difference measurements. Second-harmonic generation measurements exhibit much larger circular difference responses, being more sensitive to symmetry. Comparisons between arrays of symmetric and asymmetric particles imply that the small defects may be the primary source of broken symmetry and hence chirality.
Metallic nanostructures can have strong effects on the polarization state of light and present significant polarization sensitivity. However, quite often these phenomena have only negligible effects thus passing our attention without careful analysis. We show that these effects can be enhanced by using resonance effects arising from waveguide modes propagating along the surface. This enables the use of metallic nanostructures as artificial media components modulating the polarization state of light.
Recent interest in the study of metal nanoparticles and related structures has greatly increased. Technologies such as electron beam lithography facilitate the fabrication of such subwavelength structures. Much research has focused on the linear optical properties of high-symmetry particles, such as ellipsoids and spheroids. However, we focus on both the linear and nonlinear optical responses of low-symmetry L-shaped nanoparticles. We show that these nanoparticle arrays are exceptionally sensitive to polarization. Small asymmetries in the particle shapes lead to large deviations in the primary extinction directions from expected locations. The structural asymmetries may also induce optical activity. We present results of detailed polarization analysis through second-harmonic generation experiments that are based on symmetry arguments regarding the second-order susceptibility tensor. The results confirm that the structural deviations from the ideal shape lead to further breakdown in the symmetry properties of the arrays.
Optical phase conjugation (PC) by non-resonant degenerate four-wave mixing (DFWM) in thick media of poly(methyl methacrylate) (PMMA) with doped disperse red 1 (DR1) is reported. With vertically polarized counterpropagating pump waves, PC reflectivities of 43% and 37% were achieved respectively for a horizontally and vertically polarized probe wave, which is more than 50 times higher than the value reported on resonance. Reflectivities over 30% were achieved over a wide range of intensity for both polarization configurations. Photoinduced modulation of ordering of the DR1 chromophore is the main mechanism of the PC wave generation. Other mechanisms involved in the configuration of all vertical polarization waves are also examined. Influence of the squeezing process in making volume samples on the PC wave efficiency is significant.
Techniques to characterize the molecular excited states of nonlinear chromophores are presented. These techniques include linear absorption, fluorescence, and quadratic electroabsorption. The experiments are described and results presented. A new approach, electrofluorescence, where the fluorescence signal is modulated by an applied voltage, is also discussed. Preliminary electrofluorescence results indicate that this approach has merit as a valuable characterization technique.
The second-order nonlinear properties of step-index polymer electrooptic (EO) fiber are discussed. The fiber is fabricated in the Nonlinear Optics Laboratory at WSU. Use of a Mach-Zehnder interferometer to determine the linear electrooptic coefficient of the EO fiber and quadratic electrooptic coefficient for a thin film is discussed. The data thus obtained is shown to be reproducible and of correct magnitude.
We have made single mode polymer optical fiber with embedded electrodes. The electrodes can be used to pole the dye-doped core and electro-optic phase modulation of light in the waveguide has been demonstrated. The method of lead attachment to the electrodes is discussed and the optical/electrical properties of device structures characterized.
We report a method for determining the refractive index profile of polymer optical fiber preforms by direct beam deflection measurements. The method is simple to use, compact, and has good resolution. The profile is obtained from the deflection data by numerically integrating the differential ray equation for a radial refractive index gradient. Refractive index profiles of both graded-index (GRIN) and step-index fiber preforms are reported.
We have observed light coupling in dual-core dye-doped polymer optical fibers. The experimentally observed coupling length is consistent with the standard coupled-mode theory for single-mode--singlemode coupling between step-index waveguides [1. We have also observed intensity-dependent coupling in a fixed-length dual-core fiber.
Single mode polymer optical fibers are promising candidates for all-optical devices because of fabrication flexibility, ability to tailor materials to meet a given application, and ease of fiber fabrication. In this paper, we discuss the fabrication process that is used to make single-mode polymer fibers and more complex fiber structures such as dual-core fibers. We also report on linear characterization studies of these fibers. In particular, we discuss refractive index profile measurements in both graded index and step index fiber preforms, dye concentration profiles, and waveguiding studies in dual-core optical fibers. Such linear-optical characterization is an essential input into the design of all-optical devices.
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