Chi-Kuang Sun

Multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser.

We demonstrate a novel multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser at 1230 nm. By acquiring the whole nonlinear spectrum in the visible and near-NIR region, this novel technique allows a combination of different imaging modalities, including second-harmonic generation, third-harmonic generation, and multiple-photon fluorescence. Combined with the selected excitation wavelength, which is located in the IR transparency window, this microscopic technique can provide high penetration depth with reduced damage and is ideal for studying living cells.

In vivo developmental biology study using noninvasive multi-harmonic generation microscopy

Morphological changes and complex developmental processes inside vertebrate embryos are difficult to observe noninvasively with millimeter-penetration and sub-micrometer-resolution at the same time. By using higher harmonic generation, including second and third harmonics, as the microscopic contrast mechanism, optical noninvasiveness can be achieved due to the virtual-level-transition characteristic. The intrinsic nonlinearity of harmonic generations provides optical sectioning capability while the selected 1230-nm near-infrared light source provides the deeppenetration ability. The complicated development within a ~1.5-mm thick zebrafish (Danio rerio) embryo from initial cell proliferation, gastrulation, to tissue formation can all be observed clearly in vivo without any treatment on the live specimen.

Optical investigations of the dynamic behavior of GaSb/GaAs quantum dots

Time‐resolved radiative recombination measurements on GaSb quantum dots have been performed. The GaSb quantum dots are grown by molecular beam epitaxy on (100) GaAs through a self‐assembly process. Time‐resolved measurements show that, after a rapid hole capture process, the photoluminescence decays with a fast and a slow component. The fast component is shortened significantly with higher excitation intensity while the slow component is roughly constant. The radiative lifetimes are much longer than the lifetimes of ordinary GaSb quantum wells with a straddling band lineup. These results support a staggered band lineup and space charge induced band‐bending model.

Terahertz air-core microstructure fiber

A low-loss terahertz air-core microstructure fiber is demonstrated for terahertz waveguiding. Substantially low attenuation constant less than 0.01cm−1 has been achieved and the guiding wavelength is found to be tunable by linear scaling the fiber size. The experimental results well agree with the simulation based on the finite-difference frequency-domain method, which interprets the guiding mechanism as the antiresonant reflecting waveguiding. The simulated modal pattern shows that most terahertz field is concentrated inside the central hollow air core and is guided without outside interference, which has high potential for guiding intense terahertz waves with minimized loss.

Low-index terahertz pipe waveguides

We propose and demonstrate a simple leaky structure for terahertz (THz) waveguiding. Different from previously reported air-core THz waveguides, in which a high-reflection-coated cladding layer is designed, the proposed structure here is a simple pipe with a large air core and a thin dielectric layer with uniform but low index. Using commercially available Teflon air pipes, we experimentally confirm that THz waves can be successfully guided in the central air core of 3-m-long pipes with excellent mode qualities, high coupling efficiencies, low attenuation constants, and controllable passband width.

Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding

Modal characteristics of the THz pipe waveguide, which is a thin pipe consisting of a large air core and a thin dielectric layer with uniform but low index, are investigated. Modal indices and attenuation constants are calculated for various core diameters, cladding thicknesses, and cladding refractive indices. Numerical results reveal that the guiding mechanism of the leaky core modes, which transmit most of the power in the air-core region, is that of the antiresonant reflecting guiding. Moreover, modal patterns including modal intensity distributions and electric field vector distributions are shown for the fundamental and higher order modes. Experiments using time-domain spectroscopy with PMMA pipes also confirm the antiresonant reflecting guiding mechanism.

Efficient Near-IR Hyperthermia and Intense Nonlinear Optical Imaging Contrast on the Gold Nanorod-in-Shell Nanostructures

New gold nanorod (Au NR)-in-shell nanostructures were developed to be more efficacious than Au NRs in near-IR (NIR) hyperthermia and nonlinear optical imaging contrast. Au NR-in-shell nanostructures are composed of an intact Au NR in a Au/Ag nanoshell. These nanostructures have a broad, intense absorption band that extends from 400 nm to 900 nm in the NIR. They are more efficient and efficacious than Au NRs with respect to in vitro hypothermia performance. Au NR-in-shell-labeled cancer cells were destroyed using continuous-wave NIR radiation with 50% less laser power than needed for Au NRs. Noticeably, the area of the destroyed cells was significantly larger than the size of the laser irradiation beam; in contrast, the destroyed area was usually restricted to the size of the laser beam spot when Au NRs were used. With their extraordinarily broad and strong surface plasmon resonance band, Au NR-in-shell nanostructures efficiently augmented several multiphoton nonlinear processes as well. The multiphoton emission spectrum covered almost the entire visible wavelength. The yield of the multiphoton signals of Au NR-in-shell nanostructures was on average 55 times larger than that of Au NRs. In vitro images of cancer cells targeted by Au NR-in-shell nanostructures revealed a stronger multiphoton contrast than those targeted by Au NRs.

Real-time second-harmonic-generation microscopy based on a 2-GHz repetition rate Ti:sapphire laser.

The problem of weak harmonic generation signal intensity limited by photodamage probability in optical microscopy and spectroscopy could be resolved by increasing the repetition rate of the excitation light source. Here we demonstrate the first photomultiplier-based real-time second-harmonic-generation microscopy taking advantage of the strongly enhanced nonlinear signal from a high-repetition-rate Ti:sapphire laser. We also demonstrate that the photodamage possibility in common biological tissues can be efficiently reduced with this high repetition rate laser at a much higher average power level compared to the commonly used ~80- MHz repetition rate lasers.

Nonlinear bio-photonic crystal effects revealed with multimodal nonlinear microscopy

Highly optically active nonlinear bio‐photonic crystalline and semicrystalline structures in living cells were studied by a novel multimodal nonlinear microscopy. Numerous biological structures, including stacked membranes and aligned protein structures are highly organized on a nanoscale and have been found to exhibit strong optical activities through second‐harmonic generation (SHG) interactions, behaving similarly to man‐made nonlinear photonic crystals. The microscopic technology used in this study is based on a combination of different imaging modes including SHG, third‐harmonic generation, and multiphoton‐induced fluorescence. With no energy release during harmonic generation processes, the nonlinear‐photonic‐crystal‐like SHG activity is useful for investigating the dynamics of structure–function relationships at subcellular levels and is ideal for studying living cells, as minimal or no preparation is required.

Two-photon absorption study of GaN

Two-photon absorption coefficients of GaN for below band gap ultraviolet wavelength and midgap infrared wavelength were measured by using femtosecond pulsewidth autocorrelation and Z-scan techniques. Large two-photon absorption coefficients were obtained. Taking advantage of the large two-photon absorption, we have demonstrated two-photon confocal imaging of a GaN thin film. Direct correlation was found between the yellow luminescence and suppression of bandedge luminescence.