This PDF file contains the front matter associated with SPIE Proceedings Volume 9198 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Heart and cardiovascular diseases are one of the most common in the world, in particular – arthrosclerosis. The aim of the research is to distinguish pathological and healthy tissue regions in biological samples, in this case – to distinguish collagen and lipid rich regions within the arterial wall. In the work a specific combination of such methods are used: FLIM and SHG in order to evaluate the biological tissue morphology and functionality, so that this research could give a contribution for creating a new biological tissue imaging standard in the closest future. During the study the most appropriate parameter for fluorescence lifetime decay was chosen in order to evaluate lifetime decay parameters and the isotropy of the arterial wall and deposition, using statistical methods FFT and GLCM. The research gives a contribution or the future investigations for evaluating lipid properties when it can de-attach from the arterial wall and cause clotting in the blood vessel or even a stroke.

Carcinogenic and toxic contaminants in food and feed products are nowadays mostly detected by destructive, time-consuming chemical analyses, like HPLC and LC-MS/MS methods. However, as a consequence of the severe and growing regulations on food products by the European Union, there arose an increased demand for the ultra-fast, high-sensitive and non-destructive detection of contaminants in food and feed products. Therefore, we have investigated fluorescence spectroscopy for the characterization of carcinogenic aflatoxins. With the use of a tunable titanium-sapphire laser in combination with second and third harmonic wavelength generation, both one- and two-photon induced fluorescence excitation wavelengths could be generated using the same setup. We characterized and compared the one- and two-photon induced fluorescence spectra of pure aflatoxin powder, after excitation with 365nm and 730nm respectively. Moreover, we investigated the absolute fluorescence intensity as function of the excitation power density. Afterwards, we applied our characterization setup to the detection of aflatoxins in maize grains. The fluorescence spectra of both healthy and contaminated maize samples were experimentally characterized. In addition to the fluorescence spectrum of the pure aflatoxin, we observed an unwanted influence of the intrinsic fluorescence of the maize. Depending on the excitation wavelength, a varying contrast between the fluorescence spectra of the healthy and contaminated samples was obtained. After a comparison of the measured fluorescence signals, a detection criterion for the optical identification of the contaminated maize samples could be defined. As a result, this illustrates the use of fluorescence spectroscopy as a valuable tool for the non-destructive, real-time and high-sensitive detection of aflatoxins in maize.

The fluorescence and Raman spectroscopy (RS) combined method of in vivo detection of malignant human skin cancer was demonstrated. The fluorescence analysis was used for detection of abnormalities during fast scanning of large tissue areas. In suspected cases of malignancy the Raman spectrum analysis of biological tissue was performed to determine the type of neoplasm. A special RS phase method was proposed for in vivo identification of skin tumor. Quadratic Discriminant Analysis was used for tumor type classification on phase planes. It was shown that the application of phase method provides a diagnosis of malignant melanoma with a sensitivity of 89% and a specificity of 87%.

Many applications require GHz femtosecond pulses with the center wavelength tunable over a broad range. Recently we have demonstrated a 3-GHz, femtosecond Yb-fiber laser system with >12-W average power using the pre-chirp management method. Nonlinear wavelength conversion based on this powerful laser source has enabled us to derive 3- GHz femtosecond sources at other useful wavelengths: using stimulated Raman scattering inside photonic crystal fibers, we have implemented a femtosecond Raman soliton source tunable between 1.06-1.35 μm; using fiber-optic Cherenkov radiation, we have demonstrated a 14-fs source centered at 850 nm with >300-mW average power.

We report on a new phase noise detection technique for radio frequency (RF) dissemination based on transferring mode locked laser pulses via optical fiber. The proposed approach is insusceptible to optical fiber interconnection reflection by combining optical frequency comb (OFC) expansion generated by four wave mixing (FWM) in dispersion shifted fiber (DSF) and wavelength division multiplexing (WDM) technique. An experimental system based on a fiber link of 100km was demonstrated. The measured fractional stability was 1.5×10^-13 at 1s and 1.7×10^-16 at 1000s.

A mid-IR photothermal imaging system is presented that features an integrated ultrafast erbium-doped fiber probe laser for the first time. With a mid-IR tunable quantum cascade laser (QCL) as the pump laser, vibrational molecular modes are excited and the thermally-induced changes in the refractive index are measured with a probe laser. The custom-built, all-fiber ultrafast probe laser at telecommunication wavelengths is compact, robust and thus an attractive source compared to bulky and alignment sensitive Ti:sapphire probe lasers. We present photothermal spectra and images with good contrast for a liquid crystal sample, demonstrating highly sensitive, label-free photothermal microscopy with a mode-locked fiber probe laser.

We introduce a novel method for characterizing the spatio-temporal evolution of ultrashort optical field by recording the spectral hologram of frequency resolved optical gating (FROG) trace. We show that FROG holography enables the measurement of phase (up to an overall constant) and group delay of the pulse which cannot be measured by conventional FROG method. To illustrate our method, we perform numerical simulation to generate holographic collinear FROG (cFROG) trace of a chirped optical pulse and retrieve its complex profile at multiple locations as it propagates through a hypothetical dispersive medium. Further, we experimentally demonstrate our method by retrieving a 67 fs pulse at three axial locations in the vicinity of focus of an objective lens and compute its group delay.

We study systems in which the resonance Raman process is fast due to the requirement for phonon involvement in the absorption. The resonance enhancement is found to track the isolated molecule, or vapor phase, absorption since the molecule does not have time to exchange energy with its neighbors. This corroborates with studies of pre-resonance, where Heisenberg’s uncertainty principle enforces a rapid process, but differs from resonance on electronically allowed transitions, where the resonance allows a relatively prolonged interaction. High resolution excitation spectroscopy reveals large gains and narrow features usually associated with the isolated molecule. Vibration energies shift as the resonance is approached and the excited state vibration levels are probed. Several multiplets and overtone modes are enhanced along with the strongly coupled ring-breathing mode in aromatic molecules.

Darkfield microscopy is an extremely sensitive imaging and sensing modality due to its very low background. Metal nanoparticles as small as 20nm can been detected by darkfield imaging setups. However, traditional darkfield microscopes are bulky and require special illumination condensers, which limits their application in point-of-care biosensing. In this paper, we present a miniaturized darkfield microscope based on liquid metallic on-chip condensers and imaging lenses. This microscope is fully compatible with PDMS microfluidics and can be attached to a smartphone camera to build a complete handheld biosensing system with very high sensitivity and low cost.

Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley-dependent carrier spin polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit unusually strong many body effects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. Not much is known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS₂ and WS₂. We found that excitonic many body interactions significantly contribute to the relaxation process. Biexciton formation reveals hole valley/spin relaxation time in MoS₂. Our results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley polarization for holes through an electron-hole spin exchange mechanism in both MoS₂ and WS₂.

Two-Dimensional (2D) layered materials have garnered interest due to their novel optical and electronic properties. In this work, we investigate Second Harmonic Generation (SHG) in Tungsten Disulfide (WS₂) monolayers grown on SiO₂/Si substrates and suspended on a transmission electron microscopy grid; we find an unusually large second order susceptibility, which is nearly three orders of magnitude larger than common nonlinear crystals. We have also developed a Green’s function based formalism to model the harmonic generation from a 2D layer .

We investigate the valley related carrier dynamics in monolayer MoS₂ using helicity resolved non-degenerate ultrafast pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer MoS₂ shows remarkable transient reflection signals, in stark contrast to bilayer and bulk MoS₂ due to the enhancement of many-body effect at reduced dimensionality. The helicity resolved ultrafast time-resolved result shows that the valley polarization is preserved for only several ps before scattering process makes it undistinguishable. We suggest that the dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the exfoliated MoS₂ samples.

A two-step algorithm is developed that can reconstruct the full 3-D molecular structure from diffraction patterns of partially aligned molecules in gas phase. This method is applicable to asymmetric-top molecules that do not need to have any specific symmetry. This method will be important for studying dynamical processes that involve transient structures where symmetries, if any, can possibly be broken. A new setup for the diffraction experiments that can provide enough time resolution as well as high currents suitable for gas phase experiments is reported. Time resolution is obtained by longitudinal compression of electron pulses by time-varying electric fields synchronized to the motion of electron pulses.

Continuing technical advances in the creation of (sub-) femtosecond VUV and X-ray pulses with Free-Electron Lasers and laser-based high-harmonic-generation sources have created new opportunities for studying ultrafast dynamics during chemical reactions. Here, we present an approach to image the geometric structure of gas-phase molecules with fewfemtosecond temporal and sub-Ångström spatial resolution using femtosecond photoelectron diffraction. This technique allows imaging the molecules “from within” by analyzing the diffraction of inner-shell photoelectrons that are created by femtosecond VUV and X-ray pulses. Using pump-probe schemes, ultrafast structural changes during photochemical reactions can thus be directly visualized with a temporal resolution that is only limited by the pulse durations of the pump and the probe pulse and the synchronization of the two light pulses. Here, we illustrate the principle of photoelectron diffraction using a simple, geometric scattering model and present results from photoelectron diffraction experiments on laser-aligned molecules using X-ray pulses from a Free-Electron Laser.

The coexistence of various electronic and structural phases that are close in free-energy is a hallmark in strongly correlated electron systems with emergent properties, such as metal-insulator transition, colossal magnetoresistance, and high-temperature superconductivity. The cooperative phase transitions from one functional state to another can involve entanglements between the electronically and structurally ordered states, hence deciphering the fundamental mechanisms is generally difficult and remains very active in condensed matter physics and functional materials research. We outline the recent ultrafast characterizations of 2D charge-density wave materials, including the nonequilibrium electron dynamics unveiled by ultrafast optical spectroscopy-based techniques sensitive to the electronic order parameter. We also describe the most recent findings from ultrafast electron crystallography, which provide structural aspects to correlate lattice dynamics with electronic evolutions to address the two sides of a coin in the ultrafast switching of a cooperative state. Combining these results brings forth new perspectives and a fuller picture in understanding lightmatter interactions and various switching mechanisms in cooperative systems with many potential applications. We also discuss the prospects of implementing new ultrafast electron imaging as a local probe incorporated with femtosecond select-area diffraction, imaging and spectroscopy to provide a full scope of resolution to tackle the more challenging complex phase transitions on the femtosecond-nanometer scale all at once based on a recent understanding of the spacespace- charge-driven emittance limitation on the ultimate performance of these devices. The projection shows promising parameter space for conducting ultrafast electron micordiffraction at close to single-shot level, which is supported by the latest experimental characterization of such a system.

The advances made in femtosecond electron sources over the last thirty years have been remarkable. In particular, the development of ultrabright femtosecond electron sources has made possible the observation of molecular motion in labile organic materials and it is paving the way towards the study of complex protein systems. The principle of radio frequency (RF) rebunching cavities for the compression of ultrabright electron pulses is presented, alongside with a recent demonstration of its capabilities in capturing the relevant photoinduced dynamics in weakly scattering organic systems. Organic and biological systems can easily decompose or lose crystallinity as a consequence of cumulative heating effects or the formation of side reaction photoproducts. Hence, source brightness plays a crucial role in achieving sufficient signal-to-noise ratio before degradation effects become noticeable on the structural properties of the material. The current brightness of electron sources in addition to the high scattering cross section of keV-MeV electrons have made femtosecond electron diffraction a powerful tool for the study of materials composed by low-Z elements.

In this paper we review the present status of MeV electron sources for ultrafast diffraction and microscopy applications and trace the path forward to improve the spatio-temporal resolution of electron scattering probes.

Native coenzymes such as the reduced nicotinamide adenine dinucleotide (NADH) and oxidized flavin adenine dinucleotide play pivotal roles in energy metabolism and a myriad of biochemical reactions in living cells/tissues. These coenzymes are naturally fluorescent and, therefore, have the potential to serve as intrinsic biomarkers for mitochondrial activities, programmed cell death (apoptosis), oxidative stress, aging, and neurodegenerative disease. In this contribution, we employ two-photon fluorescence lifetime imaging microscopy (FLIM) and time-resolved anisotropy imaging of intracellular NADH for quantitative, non-invasive biochemistry on living cells in response to hydrogenperoxide- induced oxidative stress. In contrast with steady-state one-photon, UV-excited autofluorescence, two-photon FLIM is sensitive to both molecular conformation and stimuli-induced changes in the local environment in living cells with minimum photodamage and inherently enhanced spatial resolution. On the other hand, time-resolved, two-photon anisotropy imaging of cellular autofluorescence allows for quantitative assessment of binding state and environmental restrictions on the tumbling mobility of intrinsic NADH. Our measurements reveal that free and enzyme-bound NADH exist at equilibrium, with a dominant autofluorescence contribution of the bound fraction in living cells. Parallel studies on NADH-enzyme binding in controlled environments serve as a point of reference in analyzing autofluorescence in living cells. These autofluorescence-based approaches complement the conventional analytical biochemistry methods that require the destruction of cells/tissues, while serving as an important step towards establishing intracellular NADH as a natural biomarker for monitoring changes in energy metabolism and redox state of living cells in response to environmental hazards.

In this presentation we report a new 3D scanned DSLM. The system combined 1) two-photon excitation, 2) scanning along the illumination axis (x-axis) using tunable acoustic gradient lens (TAG) to stretch the Rayleigh range [5], 3) scanning vertically to the illumination axis (y-axis) by one galvo mirror to create light sheet. 4) scanning along Z-axis to do fast 3D imaging by another galvo mirror. The image plane was kept aligned with the fast z-axis scanned light sheet plane by an electric tunable lens (ETL) as described in ref. 6. The light sheet can be tailored to any shape between 50×50 μm2 and more than 500×500 μm2 with constant thickness limited by diffraction and fast imaging rates limited by the detector. The tailorable illumination area allows multi-scale field of view (FOV), and is consequently capable of imaging cells, tissue and live animals in one setup.

Our recent work on optical two-dimensional coherent spectroscopy (2DCS) of semiconductor materials is reviewed. We present and compare two approaches that are appropriate for the study of semiconductor nanostructures. The first one is based on a non-collinear geometry, where the Four-Wave-Mixing (FWM) signal is detected in the form of a radiated optical field. This approach works for samples with translational symmetry, such as Quantum Wells (QWs), or large and dense ensembles of Quantum Dots (QDs). The second method is based on a collinear geometry, where the FWM is detected in the form of a photocurrent. This second approach enables 2DCS of samples where translational symmetry is broken, such as single QDs, nanowires, or nanotubes, and small ensembles thereof. For each method, we provide an example of experimental results obtained on semiconductor QWs. In particular, it is shown how 2DCS can reveal coherent excitonic coupling between adjacent QWs.

We review our previous result: backward difference-frequency generation can be exploited to achieve phase conjugation in a second-order nonlinear medium. The backward configuration can be utilized to achieve broadband quasi-phasematching, compared with the forward counterpart. Our calculation shows that a nonlinear reflectivity of close to 100% is achievable from a laser emitting an output power of ≈ 1 mW. Such an efficient phase conjugator is made feasible by placing the nonlinear medium inside a pump laser cavity. In addition, a Fabry-Perot cavity at the input frequency is used to significantly improve the nonlinear reflectivity. In our previous experimental result, we demonstrated that broadband and polarization-insensitive phase conjugation, achieved based on difference-frequency generation in a second-order nonlinear composite consisting of stacked KTP plates, was exploited to restore blurred images due to phase distortion as a novel scheme. Due to the quasi-phase matching in the stacked KTP crystals, our result reveals that the image restoration is insensitive to the polarization direction and wavelength of the input beam.

Multimode optical waveguides, including optical fibers, are traditionally regarded as unsuitable for a wide range of applications such as communications and sensing. A main challenge in using multimode waveguides for aforementioned applications is how to control the spatial profile of optical waves propagating within the waveguide. In this paper, we present experimental studies that demonstrate the feasibility of using adaptive optics (AO) and waveguide devices such as directional couplers to control the form of guided wave in a multimode waveguide.

In recent years it has become possible to generate terahertz-frequency (THz) fields that are strong enough to induce nonlinear responses in ordinary molecules and materials. Part of the development of THz technology and nonlinear spectroscopy has relied on optical imaging of THz field profiles and their time and position-dependent evolution. A THz "polaritonics" platform enables extensive control over THz fields that are generated; integration of functional elements such as bandgap structures and metamaterial devices; optical imaging of the THz near and far fields with subcycle temporal and subwavelength spatial resolution; and exploitation of the results for nonlinear spectroscopy.

A perfect lens with unlimited resolution has always posed a challenge to both theoretical and experimental physicists. Recent developments in optical meta-materials promise an attractive approach towards perfect lenses using negative refraction to overcome the diffraction limit, improving resolution. However, those artificially engineered meta-materials usually company by high losses from metals and are extremely difficult to fabricate. An alternative proposal on using negative refraction by four-wave mixing has attracted much interests recently, though most of existing experiments still require metals and none of them has been implemented for an optical lens. Here we experimentally demonstrate a metalfree flat lens for the first time using negative refraction by degenerate four-wave mixing with a simple thin glass slide. We realize optical lensing utilizing a nonlinear refraction law, which may have potential applications in infrared microscopy and super-resolution imaging.

A supercontinuum generation system is developed, which consists of an erbium-doped fiber ring laser, an erbium-doped fiber amplifier, and a 100-m highly nonlinear fiber. Through nonlinear polarization rotation, the fiber ring laser generates a train of noise-like pulses in the form of repetitive picosecond pulse packets consisting of femtosecond noise-like fine temporal structures. The noise-like pulses are amplified before being sent into the highly nonlinear fiber. As a result, an octave-spanning supercontinuum from 1177 nm to 2449 nm is obtained, which has a 20-dB spectral width of 980 nm. Because of the nonlinearity of the fiber amplifier, the duration of the noise-like pulses is shortened while their average power is enhanced. However, the enhanced pulse energy makes the key contribution to the spectral broadening of the resulting spuercontinuum in this study since the highly nonlinear fiber is so long that the effect of the pulse compression on supercontinuum generation is weak.

The results of experimental studies of donor bone samples (rat, rabbit and human) with varying degrees of mineralisation by Raman spectroscopy were presented. Raman spectra were obtained for the Raman bands 950-962 (РО₄)^3-, 1065-1070 (СО₃)^2- and 1665 cm-1 (Amide I). In demineralized bone a sharp decline (to 98 %) in the range of 950-962 cm-1 (РО₄)^3- and 1065 - 1070 cm^-1 was observed. This decrease was accompanied by the emergence of the 1079-1090 cm-1 band corresponding to the hydrated state СО₃ ^2-.