A method for single particle tracking through a multimode fiber.

Multimode optical fiber (MMF) endoscopes have recently gained widespread attention as a novel tool for imaging deep within tissue using light microscopy. We here present a method for particle tracking through the MMF, which overcomes the lack of a fast enough wide-field fluorescence imaging modality for this type of endoscope, namely a discrete implementation of orbital particle tracking. We achieve biologically relevant tracking speeds (up to 1.2 µm/s) despite using a slow SLM for the wavefront shaping. We demonstrate a tracking accuracy of λ/50 for a 0.3 NA fiber and show tracking of a pinhole moving to mimic Brownian motion with diffusion rates of up to 0.3 µm2/s.

Suppression of the non-linear background in a multimode fibre CARS endoscope.

Multimode fibres show great potential for use as miniature endoscopes for imaging deep in tissue with minimal damage. When used for coherent anti-Stokes Raman scattering (CARS) microscopy with femtosecond excitation sources, a high band-width probe is required to efficiently focus the broadband laser pulses at the sample plane. Although graded-index (GRIN) fibres have a large bandwidth, it is accompanied by a strong background signal from four-wave mixing and other non-linear processes occurring inside the fibre. We demonstrate that using a composite probe consisting of a GRIN fibre with a spliced on step-index fibre reduces the intensity of the non-linear background by more than one order of magnitude without significantly decreasing the focusing performance of the probe. Using this composite probe we acquire CARS images of biologically relevant tissue such as myelinated axons in the brain with good contrast.

Label-free CARS microscopy through a multimode fiber endoscope.

Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection.

Wavelength dependent characterization of a multimode fibre endoscope.

Multimode fibres have recently shown promise as miniature endoscopic probes. When used for non-linear microscopy, the bandwidth of the imaging system limits the ability to focus light from broadband pulsed lasers as well as the possibility of wavelength tuning during the imaging. We demonstrate that the bandwidth is limited by the dispersion of the off-axis hologram displayed on the SLM, which can be corrected for, and by the limited bandwidth of the fibre itself. The selection of the fibre is therefore crucial for these experiments. In addition, we show that a standard prism pulse compressor is sufficient for material dispersion compensation for multi-photon imaging with a fibre endoscope.

A novel optical microscope for imaging large embryos and tissue volumes with sub-cellular resolution throughout.

Current optical microscope objectives of low magnification have low numerical aperture and therefore have too little depth resolution and discrimination to perform well in confocal and nonlinear microscopy. This is a serious limitation in important areas, including the phenotypic screening of human genes in transgenic mice by study of embryos undergoing advanced organogenesis. We have built an optical lens system for 3D imaging of objects up to 6 mm wide and 3 mm thick with depth resolution of only a few microns instead of the tens of microns currently attained, allowing sub-cellular detail to be resolved throughout the volume. We present this lens, called the Mesolens, with performance data and images from biological specimens including confocal images of whole fixed and intact fluorescently-stained 12.5-day old mouse embryos.

Two-Color, Two-Photon Imaging at Long Excitation Wavelengths Using a Diamond Raman Laser.

We demonstrate that the second-Stokes output from a diamond Raman laser, pumped by a femtosecond Ti:Sapphire laser, can be used to efficiently excite red-emitting dyes by two-photon excitation at 1,080 nm and beyond. We image HeLa cells expressing red fluorescent protein, as well as dyes such as Texas Red and Mitotracker Red. We demonstrate the potential for simultaneous two-color, two-photon imaging with this laser by using the residual pump beam for excitation of a green-emitting dye. We demonstrate this for the combination of Alexa Fluor 488 and Alexa Fluor 568. Because the Raman laser extends the wavelength range of the Ti:Sapphire laser, resulting in a laser system tunable to 680-1,200 nm, it can be used for two-photon excitation of a large variety and combination of dyes.

Ultrafast second-Stokes diamond Raman laser.

We report a synchronously-pumped femtosecond diamond Raman laser operating with a tunable second-Stokes output. Pumped using a mode-locked Ti:sapphire laser at 840-910 nm with a duration of 165 fs, the second-Stokes wavelength was tuneable from 1082 - 1200 nm with sub-picosecond duration. Our results demonstrate potential for cascaded Raman conversion to extend the wavelength coverage of standard laser sources to new regions.

Widefield Two-Photon Excitation without Scanning: Live Cell Microscopy with High Time Resolution and Low Photo-Bleaching.

We demonstrate fluorescence imaging by two-photon excitation without scanning in biological specimens as previously described by Hwang and co-workers, but with an increased field size and with framing rates of up to 100 Hz. During recordings of synaptically-driven Ca(2+) events in primary rat hippocampal neurone cultures loaded with the fluorescent Ca(2+) indicator Fluo-4 AM, we have observed greatly reduced photo-bleaching in comparison with single-photon excitation. This method, which requires no costly additions to the microscope, promises to be useful for work where high time-resolution is required.

Label-free imaging of thick tissue at 1550 nm using a femtosecond optical parametric generator.

We have developed a simple wavelength-tunable optical parametric generator (OPG), emitting broadband ultrashort pulses with peak wavelengths at 1530-1790 nm, for nonlinear label-free microscopy. The OPG consists of a periodically poled lithium niobate crystal, pumped at 1064 nm by a ultrafast Yb:fiber laser with high pulse energy. We demonstrate that this OPG can be used for label-free imaging, by third-harmonic generation, of nuclei of brain cells and blood vessels in a >150 µm thick brain tissue section, with very little decay of intensity with imaging depth and no visible damage to the tissue at an incident average power of 15 mW.

Strong Schottky barrier reduction at Au-catalyst/GaAs-nanowire interfaces by electric dipole formation and Fermi-level unpinning.

Nanoscale contacts between metals and semiconductors are critical for further downscaling of electronic and optoelectronic devices. However, realizing nanocontacts poses significant challenges since conventional approaches to achieve ohmic contacts through Schottky barrier suppression are often inadequate. Here we report the realization and characterization of low n-type Schottky barriers (~0.35 eV) formed at epitaxial contacts between Au-In alloy catalytic particles and GaAs-nanowires. In comparison to previous studies, our detailed characterization, employing selective electrical contacts defined by high-precision electron beam lithography, reveals the barrier to occur directly and solely at the abrupt interface between the catalyst and nanowire. We attribute this lowest-to-date-reported Schottky barrier to a reduced density of pinning states (~10(17) m(-2)) and the formation of an electric dipole layer at the epitaxial contacts. The insight into the physical mechanisms behind the observed low-energy Schottky barrier may guide future efforts to engineer abrupt nanoscale electrical contacts with tailored electrical properties.

Combining near-field scanning optical microscopy with spectral interferometry for local characterization of the optical electric field in photonic structures.

We show how a combination of near-field scanning optical microscopy with crossed beam spectral interferometry allows a local measurement of the spectral phase and amplitude of light propagating in photonic structures. The method only requires measurement at the single point of interest and at a reference point, to correct for the relative phase of the interferometer branches, to retrieve the dispersion properties of the sample. Furthermore, since the measurement is performed in the spectral domain, the spectral phase and amplitude could be retrieved from a single camera frame, here in 70 ms for a signal power of less than 100 pW limited by the dynamic range of the 8-bit camera. The method is substantially faster than most previous time-resolved NSOM methods that are based on time-domain interferometry, which also reduced problems with drift. We demonstrate how the method can be used to measure the refractive index and group velocity in a waveguide structure.

Optical properties of rotationally twinned InP nanowire heterostructures.

We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.

Monolithic GaAs/InGaP nanowire light emitting diodes on silicon.

Vertical light emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates, have been fabricated. The devices can be fabricated over large areas and can be precisely positioned on the substrates, by the use of standard lithography techniques, enabling applications such as on-chip optical communication. LED functionality was established on both kinds of substrate, and the devices were evaluated in terms of temperature-dependent photoluminescence and electroluminescence.

Growth and optical properties of strained GaAs-GaxIn 1-x P core-shell nanowires.

We have synthesized GaAs-Ga(x)In(1-x)P (0.34 < x < 0.69) core-shell nanowires by metal-organic vapor phase epitaxy. The nanowire core was grown Au-catalyzed at a low temperature (450 degrees C) where only little growth takes place on the side facets. The shell was added by growth at a higher temperature (600 degrees C), where the kinetic hindrance of the side facet growth is overcome. Photoluminescence measurements on individual nanowires at 5 K showed that the emission efficiency increased by 2 to 3 orders of magnitude compared to uncapped samples. Strain effects on the band gap of lattice mismatched core-shell nanowires were studied and confirmed by calculations based on deformation potential theory.