This PDF file contains the front matter associated with SPIE Proceedings Volume 8845 including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Three-dimensional imaging of molecules in the gas phase has been an important but challenging task, since the randomly oriented molecules only provide one-dimensional structural information. In this work, we show that a three-dimensional structure can be reconstructed from ultrafast electron diffraction from impulsively laser-aligned molecules. The diffraction pattern is taken at the maximum degree of alignment, around two picoseconds after the excitation of the laser. An iterative retrieval algorithm is developed to resolve the problem generated by imperfect alignment and a holographic algorithm is used to reconstruct molecular structure.

We investigate theoretically the direct imaging of coherent electronic motion in atoms and molecules using attosecond electron pulses. The theories of time-resolved ultrafast electron diffraction and (e, 2e) momentum spectroscopy as well as the requisite conditions for carrying out time-resolved measurements to obtain timedependent images are discussed. Results of simulations showing images of the motions of coherent superposition states in both the hydrogen atom and the hydrogen molecular ion are shown, thus indicating the capability of ultrafast electron pulses to investigate time-dependent electron dynamics.

Ultrafast electron microscopy in the space and time domains utilizes a pulsed electron probe to directly map structural dynamics of nanomaterials initiated by an optical pump pulse, in imaging, di raction, spectroscopy, and their combinations. It has demonstrated its capability in the studies of phase transitions, mechanical vibrations, and chemical reactions. Moreover, electrons can directly interact with photons via the near eld component of light scattering by nanostructures, and either gain or lose light quanta discretely in energy. By energetically selecting those electrons that exchanged photon energies, we can map this photon-electron interaction, and the technique is termed photon-induced near eld electron microscopy (PINEM). Here, we give an account of the theoretical understanding of PINEM. Experimentally, nanostructures such as a sphere, cylinder, strip, and triangle have been investigated. Theoretically, time-dependent Schrodinger and Dirac equations for an electron under light are directly solved to obtain analytical solutions. The interaction probability is expressed by the mechanical work done by an optical wave on a traveling electron, which can be evaluated analytically by the near eld components of the Rayleigh scattering for small spheres and thin cylinders, and numerically by the discrete dipole approximation for other geometries. Application in visualization of plasmon elds is discussed.

With ultrafast transmission electron microscopy (UTEM), access can be gained to the spatiotemporal scales required to directly visualize rapid, non-equilibrium structural dynamics of materials. This is achieved by operating a transmission electron microscope (TEM) in a stroboscopic pump-probe fashion by photoelectrically generating coherent, well-timed electron packets in the gun region of the TEM. These probe photoelectrons are accelerated down the TEM column where they travel through the specimen before reaching a standard, commercially-available CCD detector. A second laser pulse is used to excite (pump) the specimen in situ. Structural changes are visualized by varying the arrival time of the pump laser pulse relative to the probe electron packet at the specimen. Here, we discuss how ultrafast nanoscale motions of crystalline materials can be visualized and precisely quantified using diffraction contrast in UTEM. Because diffraction contrast sensitively depends upon both crystal lattice orientation as well as incoming electron wavevector, minor spatial/directional variations in either will produce dynamic and often complex patterns in real-space images. This is because sections of the crystalline material that satisfy the Laue conditions may be heterogeneously distributed such that electron scattering vectors vary over nanoscale regions. Thus, minor changes in either crystal grain orientation, as occurs during specimen tilting, warping, or anisotropic expansion, or in the electron wavevector result in dramatic changes in the observed diffraction contrast. In this way, dynamic contrast patterns observed in UTEM images can be used as sensitive indicators of ultrafast specimen motion. Further, these motions can be spatiotemporally mapped such that direction and amplitude can be determined.

The noncentrosymmetry requirement of sum frequency generation (SFG) spectroscopy allows selective detection of crystalline cellulose in plant cell walls and lignocellulose biomass without spectral interferences from hemicelluloses and lignin. In addition, the phase synchronization requirement of the SFG process allows noninvasive investigation of spatial arrangement of crystalline cellulose microfibrils in the sample. This paper reviews how these principles are applied to reveal structural information of crystalline cellulose in plant cell walls and biomass.

Because of its optical property of photostability, Nitrogen-Vacancy center (NV center) is desired to be applied for biomedical staining for super-resolution microscopy. In this paper, we report the sub-diffraction imaging of NV centers in nano-diamond and bulk materials. The resolution of ~65nm is achieved in the FND sample with our home built CW STED system.

As an amorphous material with full inversion symmetry, silica-based microstructures cannot possess significant secondorder nonlinearity. We recently developed a method that can potentially overcome this deficiency by coating a silica fiber taper with layers of radially aligned nonlinear molecules. The coating process can be accomplished through layerby- layer self-assembly, where the alignment of the nonlinear molecules is maintained through electrostatic interaction. As a result, the nonlinear fiber structures are thermodynamically stable and can generate significant second-order nonlinear responses despite their full rotational symmetry. This prediction has been experimentally confirmed through SHG measurements. To further enhance the overall second-order nonlinearity, we have developed an UV-ablation-based approach that can generate second-order nonlinearity that is spatially periodic along the fiber taper. Our preliminary experiments suggest that SHG intensity can be enhanced by such quasi-phase-matching configurations.

We can also use the self-assembly approach to construct tunable plasmonic systems. As a proof-of-concept study, we assembled swellable polymer films over a planar Au substrate through layer-by-layer assembly and covered the swellable polymer with a monolayer of quantum dots. After immersing the swellable plasmonic structure in solution and adjusting its pH value, we used a fluorescence lifetime based approach to demonstrate that the thickness of the swellable polymers can be modified by almost 400%. The fluorescence lifetime measurements also confirmed that the plasmonic resonance can be significantly modified by the swellable polymers.

In this paper, we extend the spectrography method to visualize 3D structures of complex samples from only one spectral view. Utilizing a weighted difference map and the Fourier central slice theorem, a number of Fourier planes are reconstructed, which go through the origin of the 3D Fourier space and interact with a region formed by the Ewald spheres. Thus, the complex x-ray wave fronts can be recovered at small tilting angles from the incident x-ray beam. Patterns from various computed projections can generate perception of 3D structure features inside the sample. To demonstrate the feasibility of the proposed spectrographic imaging method, numerical simulations are performed and analyzed. The results suggest that spectrography is an effective method for 3D structure studies by a single spectral exposure.

We perform numerical study of Compressive Multi-heterodyne Optical Spectroscopy [CMOS], which is based on multiple heterodyne measurements using a dynamically encoded frequency comb. Compressive sensing enables us to utilize sparsity in typical optical spectra of interest to reduce the number of heterodyne measurements. Numerical results are presented to demonstrate retrieval of coherent and incoherent sparse hypothetical Lorentzian spectra over a 42 nmwide bandwidth, sampled every 100 MHz (~0.2 pm), by using as few as 25% measurements.

This paper presents a liquid metal based on-chip optofluidic grating spectrograph integrated with polydimethylsiloxane (PDMS) microfluidics for biomedical applications such as handheld fluorescence-actuated cell sorting, fluorescence immunosensing and Raman spectroscopy. We designed a Czerny-Turner spectrograph with 1.4nm spectral resolution, 300nm FSR and a footprint of 1cm by 2cm. The spectrograph structure was fabricated in PDMS using conventional replica molding soft lithography, and then filled with room temperature liquid metal, Gallium-Indium-Tin Alloy. The material and fabrication method is fully compatible with PDMS microfluidics, which allowed us to integrate a sheath flow based microfluidic cell focusing system with the spectrograph on the same substrate. This integration represents an important step towards a handheld flow cytometer. In addition, as a new class of on-chip optofluidic components, liquid metallic optical elements such as mirrors, gratings and pinholes will likely find other applications for building miniaturized optofluidic systems.

This work analyzes ultrafast carrier dynamics in GaP under intense photoexcitation. The dynamics are initially dominated by hot electron scattering from the central Γ valley to the X₇ sidevalley over 700 fs and X₆ sidevalley over 4 ps. Subsequent pump-fluence-dependent relaxation is observed over 30 to 52 ps for as pump fluence increases. This prolonged energy relaxation is ascribed to impeded phonon decay. Experimental and theoretical results are shown to provide evidence for a hot phonon bottleneck at the high fluences. The implications of these ultrafast carrier dynamics are discussed for emerging GaP applications.

We have measured UV resonance Raman scattering at and near the resonance absorption lines of liquid benzene and toluene. Resonance occurs for excitation on the symmetry-forbidden but strongly phonon coupled states in the ¹B_2u band, ~230-270 nm, resulting in enhancements corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. The resonance gains (~1000x) are larger than expected due to the narrower vapor phase lines. Several multiplet and overtone modes are enhanced along with the strongly coupled ring-breathing mode. A contrasting case of resonance Raman of ice is also discussed; in this case resonance is observed for excitation energy corresponding to absorptions that depend upon the final state shielding by the neighbors, and corresponds with the solid phase absorption. This typifies the more common, slow, time dependence of the resonance Raman process.

We investigate Raman spectroscopic sensing using whispering gallery microresonators as a label-free method toward single particle detection. Whispering gallery mode microresonators are used as platforms to perform sensitive particle detection by exploiting the strong, evanescent field of a resonant mode exposed on the surface of the microresonator. Particles adhered to the microresonator surface interact with the field and scatter photons circulating within the resonator. In particular, Raman scattered photons are detected, providing molecular-specific "fingerprint" information regarding the adhered particles. The exploitation of a resonant mode allows for enhancement of generated Raman signal over traditional methods of spontaneous Raman scattering. Preliminary proof-of-concept experimental results are shown.

This study presents the portable multispectral imaging system that can acquire the image of specific spectrum in vivo for oral cancer diagnosis. According to the research literature, the autofluorescence of cells and tissue have been widely applied to diagnose oral cancer. The spectral distribution is difference for lesions of epithelial cells and normal cells after excited fluorescence. We have been developed the hyperspectral and multispectral techniques for oral cancer diagnosis in three generations. This research is the third generation. The excited and emission spectrum for the diagnosis are acquired from the research of first generation. The portable system for detection of oral cancer is modified for existing handheld microscope. The UV LED is used to illuminate the surface of oral cavity and excite the cells to produce fluorescent. The image passes through the central channel and filters out unwanted spectrum by the selection of filter, and focused by the focus lens on the image sensor. Therefore, we can achieve the specific wavelength image via fluorescence reaction. The specificity and sensitivity of the system are 85% and 90%, respectively.

We developed a novel addressable multiregional multiphoton microscope that employs a fast one-dimensional discrete-line scanning approach based on a spatial light modulator (SLM). The phase-only SLM shapes an incoming mode-locked, near-infrared Ti:sapphire laser beam into multiple specific discrete-lines, which are designed according to the sizes and locations of the target samples. Only the target-sample areas of are scanned one-dimensionally, resulting in an efficient use of the laser’s power. Compared with conventional multiphoton microscopies, this technique shortens scanning time and minimizes photodamage by concentrating scanning energy and dwell time on the areas of interest. Additionally, our discrete-line-focus design eliminates the cross-talk that occurs in conventional one-dimensional line-scanning multiphoton microscopes, thus enhancing the lateral and axial resolutions of the line-scanning imaging system.

We present experimental data obtained during investigation of synchronously pumped optical parametric oscillators (SPOPO’s) pumped by fundamental (1030 nm) and second harmonic (515 nm) radiation of mode-locked Yb:KGW laser, providing 105 fs pulses at 76 MHz repetition rate with an average power of 4 W. Different nonlinear crystals such as beta barium borate (BBO), and periodically poled lithium niobate (PPLN) and MgO doped PPLN (MgO:PPLN) were tested to estimate wavelength tuning capabilities and SPOPO’s efficiency. Rotation of BBO nonlinear crystal and SPOPO’s cavity length variation and, in the case of SPOPO based on PPLN, change of grating period and cavity length allowed signal wavelength tuning in 630 – 1030 nm and 1350 – 1700 nm spectral ranges, respectively. Parametric light conversion from pump power to signal power efficiency was as high as 25 %. Including the idler pulses the tuning ranges were from 630 to 2400 nm and from 1350 to 4000 nm in case of BBO and PPLN crystals, respectively. SPOPO based on BBO wsithout intracavity group velocity dispersion (GVD) compensation generates longer than transform limited pulses, so SPOPO based on BBO with dispersive prisms were investigated.

Visible light communication (VLC) technology has attained its attention in both academic and industry lately. It is determined by the development of light emitting diode (LED) technology for solid-state lighting (SSL).It has great potential to gradually replace radio frequency (RF) wireless technology because it offers unregulated and unlicensed bandwidth to withstand future demand of indoor wireless access to real–time bandwidth-demanding applications. However, it was found to provide intrusive uplink channel that give rise to unpleasant irradiance from the user device which could interfere with the downlink channel of VLC and hence limit mobility to users as a result of small coverage (field of view of VLC).To address this potential problem, a Hybrid VLC system which integrates VLC (for downlink) and RF (for uplink) technology is proposed. It offers a non-intrusive RF back channel that provides high throughput VLC and maintains durability with conventional RF devices. To deploy Hybrid VLC system in the market, it must be energy and cost saving to attain its equivalent economical advantage by comparing to existing architecture that employs fluorescent or LED lights with RF technology. In this paper, performance evaluation on the proposed hybrid system was carried out in terms of device cost and power consumption against data throughput. Based on our simulation, Hybrid VLC system was found to reduce device cost by 3% and power consumption by 68% when compares to fluorescent lights with RF technology. Nevertheless, when it is compared to LED lights with RF technology, our proposed hybrid system is found to achieve device cost saving as high as 47% and reduced power consumption by 49%. Such promising results have demonstrated that Hybrid VLC system is a feasible solution and has paved the way for greater cost saving and energy efficient compares with the current RF architecture even with the increasing requirement of indoor area coverage.

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition (CVD) graphene monolayer transferred onto a SiO₂/Si(001) substrate is enhanced about 3 times by the flow of direct (dc) electric current in graphene. We also find that optical SHG in reflection from a 4-layer-graphene film epitaxially grown on a vicinal SiC(0001) substrate is enhanced 25% by the flow of dc electric current in graphene. Measurements of rotationalanisotropy SHG from both samples revealed that the current-induced SHG varies strongly with the measurement location on graphene along the current flow direction. The enhancement of SHG from the graphene/SiO₂/Si(001) sample is due to current-associated charge trapping at the graphene/SiO₂ interface, which introduces a vertical electric field across the SiO₂/Si interface that produces electric field-induced SHG. The enhancement of SHG from the graphene/vicinal-SiC(0001) sample is due to the current-associated electric field at the graphene/SiC interface that produces electric field-induced SHG. The functions of the current-induced SHG varying with the measurement location are different for the CVD graphene/SiO₂/Si(001) sample and the epitaxial graphene/vicinal-SiC(0001) sample.