Raman, hyper-Raman, hyper-Rayleigh, two-photon luminescence and morphology-dependent resonance modes in a single optical tweezers system

Longitudinal position dependence of the second-harmonic generation of optically trapped silica microspheres

We report the design of a setup combining the simultaneous and independent optical trapping and second-harmonic generation (SHG) of 1 µm diameter silica microspheres with two independent laser beams. Optical trapping is achieved with a tightly focused continuous wave infrared laser beam whereas the SHG intensity from the trapped microparticles is obtained with a 810 nm femtosecond pulsed laser. The silica microparticles are dispersed in an aqueous solution, and a microfluidic channel flow is used to remove untrapped microparticles. We show by the perpendicular displacement of the optical trap from the microfluidic channel wall that it is possible to control the contribution of the channel wall/solution interface to the overall SHG intensity. Stable trapping and SHG detection of two microparticles is also demonstrated. Combining the independent trapping of centrosymmetrical silica microparticles with SHG offers new avenues for analytical studies like surface sensing or all-optical devices where the SHG intensity is controlled by the trapping beam.

Expansion of arbitrary electromagnetic fields in terms of vector spherical wave functions

Since 1908, when Mie reported analytical expressions for the fields scattered by a spherical particle upon incidence of plane-waves, generalizing his analysis for the case of an arbitrary incident wave has been an open question because of the cancellation of the prefactor radial spherical Bessel function. This cancellation was obtained before by our own group for a highly focused beam centered in the objective. In this work, however, we show for the first time how these terms can be canceled out for any arbitrary incident field that satisfies Maxwells equations, and obtain analytical expressions for the beam shape coefficients. We show several examples on how to use our method to obtain analytical beam shape coefficients for: Bessel beams, general hollow waveguide modes and specific geometries such as cylindrical and rectangular. Our method uses the vector potential, which shows the interesting characteristic of being gauge invariant. These results are highly relevant for speeding up numerical calculation of light scattering applications such as the radiation forces acting on spherical particles placed in an arbitrary electromagnetic field, as in an optical tweezers system.

Revealing silent vibration modes of nanomaterials by detecting anti-Stokes hyper-Raman scattering with femtosecond laser pulses

We proposed a scheme in which normal Raman scattering is coupled with hyper-Raman scattering for generating a strong anti-Stokes hyper-Raman scattering in nanomaterials by using femtosecond laser pulses. The proposal was experimentally demonstrated by using a single-layer MoS2 on a SiO2/Si substrate, a 17 nm-thick MoS2 on an Au/SiO2 substrate and a 9 nm-thick MoS2 on a SiO2-SnO2/Ag/SiO2 substrate which were confirmed to be highly efficient for second harmonic generation. A strong anti-Stokes hyper-Raman scattering was also observed in other nanomaterials possessing large second-order susceptibilities, such as silicon quantum dots self-assembled into "coffee" rings and tubular Cu-doped ZnO nanorods. In all the cases, many Raman inactive vibration modes were clearly revealed in the anti-Stokes hyper-Raman scattering. Apart from the strong anti-Stokes hyper-Raman scattering, Stokes hyper-Raman scattering with small Raman shifts was detected during the ablation process of thick MoS2 layers. It was also observed by slightly defocusing the excitation light. The detection of anti-Stokes hyper-Raman scattering may serve as a new technique for studying the Raman inactive vibration modes in nanomaterials.

Forces due to pulsed beams in optical tweezers: linear effects

We present a method for the precise calculation of optical forces due to a tightly-focused pulsed laser beam using generalized Lorenz-Mie theory or the T-matrix method. This method can be used to obtain the fields as a function of position and time, allowing the approximate calculation of weak non-linear effects, and provides a reference calculation for validation of calculations including non-linear effects. We calculate forces for femtosecond pulses of various widths, and compare with forces due to a continuous wave (CW) beam. The forces are similar enough so that the continuous beam case provides a useful approximation for the pulsed case, with trap parameters such as the radial spring constant usually differing by less than 1% for pulses of 100 fs or longer. For large high-index (e.g., polystyrene, with n = 1.59) particles, the difference can be as large as 3% for 100 fs pulses, and up to 8% for 25 fs pulses. A weighted average of CW forces for individual spectral components of the pulsed beam provides a simple improved approximation, which we use to illustrate the physical principles responsible for the differences between pulsed and CW beams.

Effect of pulse temporal shape on optical trapping and impulse transfer using ultrashort pulsed lasers

We investigate the effects of pulse duration on optical trapping with high repetition rate ultrashort pulsed lasers, through Lorentz-Mie theory, numerical simulation, and experiment. Optical trapping experiments use a 12 femtosecond duration infrared pulsed laser, with the trapping microscope's temporal dispersive effects measured and corrected using the Multiphoton Intrapulse Interference Phase Scan method. We apply pulse shaping to reproducibly stretch pulse duration by 1.5 orders of magnitude and find no material-independent effects of pulse temporal profile on optical trapping of 780nm silica particles, in agreement with our theory and simulation. Using pulse shaping, we control two-photon fluorescence in trapped fluorescent particles, opening the door to other coherent control applications with trapped particles.

Multi-focus excitation coherent anti-Stokes Raman scattering (CARS) microscopy and its applications for real-time imaging

We developed a multi-focus excitation coherent anti-Stokes Raman scattering (CARS) microscope using a microlens array scanner for real-time molecular imaging. Parallel exposure of a specimen with light from two highly controlled picosecond mode-locked lasers (jitter of 30 fs through an electronic low-pass filter with 150 Hz bandwidth, point-by-point wavelength scan within 300 ms) and parallel detection with an image sensor enabled real-time imaging. We demonstrated real-time CARS imaging of polystyrene beads (frame rate of 30 fps), a giant multi-lamellar vesicle of dipalmitoylphosphatidylcholine (frame rate of 10 fps), and living HeLa cells (frame rate of 10 fps).