Kimani C. Toussaint

Nonlinear Optical Response from Arrays of Au Bowtie Nanoantennas

We demonstrate that the optical response of a single Au bowtie nanoantenna can be favorably modified to increase the local intensity by a factor of 10(3) in the feed gap region when a periodic array of antennas are used. We find that the array periodicity can be used to modulate and shape the spectral emission. An analysis of the emission confirms the presence of second-harmonic generation and two-photon photoluminescence, typical of gold nanostructures, but also reveals a portion of the emitted spectrum that cannot be attributed to a single multiphoton process. Our investigations have important implications for understanding the role of resonant nanostructures in designing optical antennas for next-generation photonic technologies.

Plasmon resonance-based optical trapping of single and multiple Au nanoparticles

The plasmon resonance-based optical trapping (PREBOT) method is used to achieve stable trapping of metallic nanoparticles of different shapes and composition, including Au bipyramids and Au/Ag core/shell nanorods. In all cases the longitudinal plasmon mode of these anisotropic particles is used to enhance the gradient force of an optical trap, thereby increasing the strength of the trap potential. Specifically, the trapping laser is slightly detuned to the long-wavelength side of the longitudinal plasmon resonance where the sign of the real component of the polarizability leads to an attractive gradient force. A second (femtosecond pulsed) laser is used to excite two-photon fluorescence for detection of the trapped nanoparticles. Two-photon fluorescence time trajectories are recorded for up to 20 minutes for single and multiple particles in the trap. In the latter case, a stepwise increase reflects sequential loading of single Au bipyramids. The nonlinearity of the amplitude and noise with step number are interpreted as arising from interactions or enhanced local fields amongst the trapped particles and fluctuations in the arrangements thereof.

Fourier transform-second-harmonic generation imaging of biological tissues.

Fourier transform-second-harmonic generation imaging is employed to obtain quantitative metrics of collagen fibers in biological tissues. In particular, the preferred orientation and maximum spatial frequency of collagen fibers for selected regions of interest in porcine trachea, ear, and cornea are determined. These metrics remain consistent when applied to collagen fibers in the ear, which can be expected from observation. Collagen fibers in the trachea are more random with large standard deviations in orientation, and large variations in maximum spatial frequency. In addition, these metrics are used to investigate structural changes through a 3D stack of the cornea. This technique can be used as a quantitative marker to assess the structure of collagen fibers that may change due to damage from disease or physical injury.

Quantifying collagen structure in breast biopsies using second-harmonic generation imaging

Quantitative second-harmonic generation imaging is employed to assess stromal collagen in normal, hyperplastic, dysplastic, and malignant breast tissues. The cellular scale organization is quantified using Fourier transform-second harmonic generation imaging (FT-SHG), while the molecular scale organization is quantified using polarization-resolved second-harmonic generation measurements (P-SHG). In the case of FT-SHG, we apply a parameter that quantifies the regularity in collagen fiber orientation and find that malignant tissue contains locally aligned fibers compared to other tissue conditions. Alternatively, using P-SHG we calculate the ratio of tensor elements (d15/d31, d22/d31, and d33/d31) of the second-order susceptibility χ2 for collagen fibers in breast biopsies. In particular, d15/d31 shows potential differences across the tissue pathology. We also find that trigonal symmetry (3m) is a more appropriate model to describe collagen fibers in malignant tissues as opposed to the conventionally used hexagonal symmetry (C6). This novel method of targeting collagen fibers using a combination of two quantitative SHG techniques, FT-SHG and P-SHG, holds promise for breast tissue analysis and applications to characterizing cancer in a manner that is compatible with clinical practice.

Quantitative analysis of collagen fiber organization in injured tendons using Fourier transform-second harmonic generation imaging

Fourier transform-second harmonic generation (FT-SHG) imaging is used as a technique for evaluating collagenase-induced injury in horse tendons. The differences in collagen fiber organization between normal and injured tendon are quantified. Results indicate that the organization of collagen fibers is regularly oriented in normal tendons and randomly organized in injured tendons. This is further supported through the use of additional metrics, in particular, the number of dark (no/minimal signal) and isotropic (no preferred fiber orientation) regions in the images, and the ratio of forward-to-backward second-harmonic intensity. FT-SHG microscopy is also compared with the conventional polarized light microscopy and is shown to be more sensitive to assessing injured tendons than the latter. Moreover, sample preparation artifacts that affect the quantitative evaluation of collagen fiber organization can be circumvented by using FT-SHG microscopy. The technique has potential as an assessment tool for evaluating the impact of various injuries that affect collagen fiber organization.

Focusing through dynamic scattering media

We demonstrate steady-state focusing of coherent light through dynamic scattering media. The phase of an incident beam is controlled both spatially and temporally using a reflective, 1020-segment MEMS spatial light modulator, using a coordinate descent optimization technique. We achieve focal intensity enhancement of between 5 and 400 for dynamic media with speckle decorrelation time constants ranging from 0.4 seconds to 20 seconds. We show that this optimization approach combined with a fast spatial light modulator enables focusing through dynamic media. The capacity to enhance focal intensity despite transmission through dynamic scattering media could enable advancement in biological microscopy and imaging through turbid environments.

Generation of optical vector beams with a diffractive optical element interferometer

We present a novel approach to generating radially and azimuthally polarized vector beams that utilize an interferometer constructed from two identical diffractive optical elements. The measured polarization properties of four vector beam states and their phase relationships are in good agreement with theoretical expectations. This interferometer is passively phase stable and robust, making it suitable for linear and nonlinear optical (superresolution) microscopy.

Optical trapping with π-phase cylindrical vector beams

The use of ?-phase radially and azimuthally polarized vector beams in optical trapping is investigated. We find that by tuning the relative phase between the eigenmodes comprising the beams, the optical forces applied to a trapped particle are modified. In particular, axial trapping efficiency is enhanced with increasing z polarization and the lateral trapping efficiency of the vector beams is reduced compared to a Gaussian input beam. In addition, this is the first experimental demonstration of low-power optical trapping in an aqueous environment using vector beams, which may have important applications in biological systems.

Understanding and controlling plasmon-induced convection

The heat generation and fluid convection induced by plasmonic nanostructures is attractive for optofluidic applications. However, previously published theoretical studies predict only nanometre per second fluid velocities that are inadequate for microscale mass transport. Here we show both theoretically and experimentally that an array of plasmonic nanoantennas coupled to an optically absorptive indium-tin-oxide (ITO) substrate can generate >micrometre per second fluid convection. Crucially, the ITO distributes thermal energy created by the nanoantennas generating an order of magnitude increase in convection velocities compared with nanoantennas on a SiO2 base layer. In addition, the plasmonic array alters absorption in the ITO, causing a deviation from Beer-Lambert absorption that results in an optimum ITO thickness for a given system. This work elucidates the role of convection in plasmonic optical trapping and particle assembly, and opens up new avenues for controlling fluid and mass transport on the micro- and nanoscale.

Plasmonic nanotweezers: strong influence of adhesion layer and nanostructure orientation on trapping performance.

Using Au bowtie nanoantennas arrays (BNAs), we demonstrate that the performance and capability of plasmonic nanotweezers is strongly influenced by both the material comprising the thin adhesion layer used to fix Au to a glass substrate and the nanostructure orientation with respect to incident illumination. We find that a Ti adhesion layer provides up to 30% larger trap stiffness and efficiency compared to a Cr layer of equal thickness. Orientation causes the BNAs to operate as either (1) a 2D optical trap capable of efficient trapping and manipulation of particles as small as 300 nm in diameter, or (2) a quasi-3D trap, with the additional capacity for size-dependent particle sorting utilizing axial Rayleigh-Bénard convection currents caused by heat generation. We show that heat generation is not necessarily deleterious to plasmonic nanotweezers and achieve dexterous manipulation of nanoparticles with non-resonant illumination of BNAs.