We image individual particles flying from a thin metal target following nanosecond laser ablation from the transparent substrate side. Time-resolved imaging of the plume is performed using intensified CCD camera registering either spectrally-integrated thermal emission or Mie scattering signal from a synchronized illumination laser. Individual particles appear as streaks on the image, from which particle velocity distribution is derived. All particles are confirmed to leave the target at the same time. Simultaneous single-shot Mie/Thermal imaging allows to correlate plume structure with particle content.
Various contrast mechanisms in imaging applications in biological and material sciences are of great importance for multimodal sample visualization. Nonlinear-optical interactions in the sample provide multitude of possibilities to imaging with different contrasts, including sensitivity to chemically-specific vibrational signatures. Cubic order nonlinearity is present in all materials since it does not require broken inversion symmetry. Cubic non- linearity offers several useful interaction modalities, including vibrationally resonant ones, such as third-order sum-frequency generation (TSFG) and four-wave mixing (FWM), which we explore in this work using femtosecond lasers in a laser-scanning all-reflective microscope. We observe strong dependence of image contrast on delay between interacting pulses and the frequency of the mid-IR laser relative to the CH vibrational mode of the sample. Images of oil-water interfaces demonstrate striking visual contrast and impressive signal-to-noise ratio in our system. Pathways to expand TSFG and FWM imaging onto biological samples are explored.
We demonstrate all-optical sensing and imaging of quasi-DC electric fields using vibrationally-resonant electric field-induced sum-frequency generation (VR-EFSFG). Two femtosecond laser pulses at fixed 1035 and tunable 3500 nm are mixed in a cubically-nonlinear CH-rich sample to which an external field is applied by means of metallic electrodes defined via electron-beam lithography. The 2D images are acquired by raster scanning the lasers in an all-reflective laser scanning microscope. Photon-counting detection is implemented in transmission mode. Volt-level potentials across sub-micrometer gaps are imaged at a rate of approximately 0.5 frames per second. Signal enhancement of up to 50 times due to CH vibrational resonance is typically obtained, as verified by wavelength tuning of the mid-IR laser in the range 2400-3400 wavenumbers. Nearly ideal quadratic dependence of the signal on quasi-DC field amplitude is obtained confirming purely cubic nonlinear interaction in the sample. Using numerical modeling we establish the connection to imaging of transmembrane potential on neuronal axons and derive sensitivity limits of the method.
A partially coherent beam (PCB) is modulated at 1 Gbps with pseudorandom bit sequence data stream and propagated through laboratory turbulence. Eye diagrams are measured and compared to those resulting from a fully coherent beam propagated through the same turbulence. Reduced scintillations of the PCB, as measured separately, expectedly result in a higher quality eye as compared to that of a fully coherent beam. Experimental data is supported by numerical modeling. This work demonstrates the feasibility and simplicity of using PCBs for Gbps data rate free-space optical communication through turbulent atmosphere.
A partially coherent beam generated by coupling the output of a superluminescent diode to a multimode optical fiber is propagated through a stationary laboratory turbulence. Statistical quantities are measured as a function of propagation distance and coherence radius of the beam and are compared to existing theories in the regime of weak fluctuations.
Scintillation indices, probability distribution functions and signal spectra are measured simultaneously for spatially
coherent and partially coherent optical beams propagating through various distances in an open atmosphere.
The partially coherent beam is produced by coupling the broadband output of a superluminescent diode to a
multimode optical fiber. A simple system to adjust the coherence radius by controlling the numerical aperture
at the fiber output is implemented. Substantial reduction of the scintillation index and signal fade probability
as compared to a slightly diverging coherent Gaussian beam are observed at all propagation distances studied
up to 6.5 km.
We present the formation of a singularity in k-space from a periodic metal-dielectric nanostructure. The singularity
originates from the balance between alternating normal and anomalous coupling. By employing the formalism of Dirac
dynamics for relativistic quantum particles, we theoretically describe propagation dynamics of surface plasmon
polaritons and demonstrate a strong diffraction anomaly (conical-like diffraction) near the singular point.
Optical waveguides provide rich environment for various nonlinear
processes thanks to the long interaction lengths, sustained high
intensities and diverse dispersion regimes. Nonlinear and dispersion
properties of fibers and waveguides can be widely controlled through
microstructuring resulting in a broad family of photonic crystal and
bandgap waveguides. This flexibility can be used to realize
previously impossible nonlinear interaction regimes for solitons and
quasi-continuous waves. The dynamics of femtosecond optical pulses
in such dispersive and nonlinear materials provide a truly
challenging measurement task, but reward us with most spectacular
images of nonlinear wave interactions. We visualized the dynamics of
solitons and continua in several such structures using cross- correlation frequency-resolved optical gating, the technique which provides experimentally the most complete information about an optical pulse. These detailed time and frequency-resolved measurements infinitely surpass the simple spectral measurements or
even the time axis-symmetric FROG spectrograms. Soliton dynamics in
the vicinity of the second zero-dispersion point of a silica PC
fiber, Cherenkov continuum generation, stabilization against the
Raman self-frequency shift and other resonant interactions as well
as the supercontinuum generation in soft-glass fibers were characterized. Recent theoretical studies were brought about to
develop a fundamental understanding of these resonant interactions
and excellent agreement was found.
Fiber optics provide a convenient way to deliver light to remote locations otherwise optically inaccessible. Arguably, optical fibers represent the most effective means of delivering optical radiation through nearly arbitrary paths. The delivery of ultrashort pulses through fibers is a topic of interest for a variety of applications yet constitutes a challenge due to the physical interaction between the intense light fields and the bulk constituents of the fiber. The nonlinear optical effects that take place induce a variety of distortion on the pulses which alter the features of the optical field at the output of the fiber. The ability to control these nonlinearities and use them as means to passively control the pulse characteristics at the output end of the fiber would be of extreme practical use. Phase sensitive pulse measurement techniques are a crucial element needed for the characterization of the pulses and to control their shape so that fibers and femtosecond phenomena may coexist effectively and advantageously.
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