Dru Morrish

Enhanced photothermal therapy assisted with gold nanorods using a radially polarized beam

We report on the use of a radially polarized beam for photothermal therapy of cancer cells labeled with gold nanorods. Due to a three-dimensionally distributed electromagnetic field in the focal volume, the radially polarized beam is proven to be a highly efficient laser mode to excite gold nanorods randomly oriented in cancer cells. As a result, the energy fluence for effective cancer cell damage is reduced to one fifth of that required for a linearly polarized beam, which is only 9.3% of the medical safety level.

Three-dimensional trapping of Mie metallic particles by the use of obstructed laser beams

In this article, we propose a method for three-dimensional optical trapping of metallic Mie particles using an obstructed laser beam. It is found from the ray-optics model that Mie gold, silver, and copper particles can be trapped against gravity in the focal region of a high numerical-aperture objective illuminated by a centrally obstructed Gaussian (TEM00-mode) beam. The axial trapping force of the three types of metallic particles is maximized in the near-infrared wavelength region. The maximum axial trapping efficiency increases with the size of the center obstruction and the aperture angle of an objective. Axial trapping force on Mie metallic particles is enhanced by a factor of two if an obstructed doughnut (TEM01*-mode) beam is employed. The experimental condition for achieving three-dimensional trapping is also discussed.

Enhancement of transverse trapping efficiency for a metallic particle using an obstructed laser beam

We report that the transverse trapping efficiency for a metallic particle can be enhanced by use of a laser beam obstructed by a circular opaque disk. In the case of gold particles, the enhancement factor for a p- or s-polarized trapping beam is at least 1.7 or 2.5, respectively. The dependence of the transverse trapping efficiency for gold particles (diameter=2 μm) on the size of the obstruction is measured and agrees with the theoretical prediction based on the ray-optics model.

Morphology-dependent resonance induced by two-photon excitation in a micro-sphere trapped by a femtosecond pulsed laser.

We report on the measurement of morphology-dependent resonance within a laser-trapped micro-sphere excited under two-photon absorption. Both trapping and two-photon excitation are simultaneously achieved by a single femtosecond pulsed laser beam. The effect of the laser power as well as the pulse width on the transverse trapping force is first investigated. The dependence of two-photon-induced morphology-dependent resonance on the scanning velocity of a trapped particle is then experimentally determined.

Observation of orthogonally polarized transverse electric and transverse magnetic oscillation modes in a microcavity excited by localized two-photon absorption

We report on the observation of orthogonally polarized transverse electric (TE) and transverse magnetic (TM) oscillation modes in a microcavity excited by localized two-photon absorption. The polarization-dependent features of morphology-dependent resonance (MDR) effects in a microsphere under two-photon fluorescence excitation are quantitatively investigated. In addition to a clear separation of excitation and resonance wavelengths under two-photon excitation, the fluorescence emission can be tightly controlled in three-dimensional space within a microsphere. The experimental results demonstrate not only the orthogonal polarization nature of TE and TM oscillation modes but also the dependence of the strength and the polarization properties of MDR peaks on excitation locations in a microsphere.

Near-field optical trapping with an ultrashort pulsed laser beam

We report the focused evanescent optical trapping of nonfluorescent and fluorescent dielectric microspheres using a femtosecond laser. The experiment confirms that the trapping efficiency increases with the size of the particles. As a result, a pulsed laser has been used to trap particles in the Mie regime and to excite whispering gallery modes in them. The excitation of whispering gallery modes in a near-field femtosecond trap shows a significant suppression of the two-photon fluorescence background with an improvement of the photon storage factor by 46%, as compared to far-field two-photon excitation.

Super-resolution imaging and statistical analysis of CdSe/CdS Core/Shell semiconductor nanocrystals.

Here we present a multifunctional algorithm. Firstly a super-resolution method is presented for optically imaging the spatial distribution of semiconductor nanocrystals with nanometre localisation. Secondly highly resolved multiple photoluminescence trajectories of hundreds of single semiconductor nanocrystals are obtained simultaneously.

Scanning particle trapped optical microscopy based on two-photon-induced morphology-dependent resonance in a trapped microsphere

We report on scanning particle trapped optical microscopy based on two-photon-induced morphology-dependent resonance (MDR) in a trapped microsphere. In this imaging mode, a femtosecond pulsed laser is adopted for simultaneous laser trapping and two-photon excitation. Due to the localized excitation, MDR is significantly enhanced. As a result, an image contrast enhancement of 9.3% is achieved by the spectral detection of a single on-resonance MDR mode compared with that obtained by the off-resonance MDR mode without compromise in transverse resolution.

Enhanced coupling to whispering gallery modes by two-photon absorption induced by a highly focused field

The authors report on enhanced coupling to whispering gallery modes in fluorescent polystyrene microspheres using two-photon absorption induced by a highly focused field. Due to the highly confined excitation nature under focused evanescent illumination achieved by a circularly obstructed beam, the whispering gallery modes can be excited within a small volume near the perimeter of the microsphere. As a result, the visibility, the Q factor, and the degree of polarization of the fluorescence spectra induced in the microsphere are enhanced by 60%, 37%, and five times, respectively.

Black-Si as a platform for sensing

The nano-textured surface of black silicon can be used as a surface-enhanced Raman scattering (SERS) substrate. Sputtered gold films showed increasing SERS sensitivity for thicknesses from 10 up to 300 nm, with sensitivity growing nonlinearly from around 50 nm until saturation at 500 nm. At 50 nm, a cross over from a discontinuous to a fully percolated film occurs as revealed by morphological and electrical measurements. The roughness of the Au coating increases due to formation of nanocrystallites of gold. Structural characterization of the black- Si needles and their surfaces revealed presence of silicon oxide and fluoride. The sharpest nano-needles had a tip curvature radius of ~10 nm. SERS recognition of analyte using molecular imprinted gels with tetracycline molecules of two different kinds is demonstrated.