This PDF file contains the front matter associated with SPIE Proceedings Volume 9584 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

We present a technical review of a beamline built to perform pump-probe experiments with a temporal resolution of < 200 attosecond. This is designed specifically to use the technique of attosecond transient absorption spectroscopy (ATAS) to resolve ultra-fast electron dynamics in atoms and molecules. A non-collinear, interferometrically stable geometry is adopted to allow us to individually control the characteristics of each of the pump and probe arms independent from each other. With the use of an auxiliary interferometer to correct for long-term drifts between the pump and probe arms we measure better than 150as resolution for our time-corrected delay despite having separated beam paths of over 4m in length. In our first experiment we have focused on the time dependence of the electronic states of an atom in a strong laser field. An extreme ultra-violet (XUV) attosecond pulse train (APT) and a precisely synchronized 30fs IR pulse are used in this work. Delay-dependent absorption modulations are observed at even multiples (2 and 4) of the IR dressing field frequency as the pump-probe delay is scanned. We investigate the dependence of the 2ω order absorption modulation amplitude from the transient absorption of laser-dressed helium as the IR dressing field ellipticity is varied, and we discuss the issues in obtaining such results. We present qualitative data indicating a clear anisotropy in the response of the atom to an IR dressing field, and discuss how we will improve this measurement in future experiments.

In recent years, femtosecond electron pulses have emerged as a powerful tool to probe ultrafast dynamics in matter. They have been used in ultrafast diffraction and imaging to reveal the atomic-detail structural dynamics in real time, covering a wide range of applications in physics, materials science, chemistry and biology. In this study, we report direct and real-time measurements of the ejected-charge dynamics surrounding laser-produced warm dense matter using femtosecond electron pulses. Our study reveals a two-step dynamical process of ejected electrons: an initial emission and accumulation of electrons outside the pumped surface followed by the formation of escaping hemispherical clouds of electrons into the vacuum at an isotropic and nearly constant velocity. Based on these observations, we also developed a model of the escaping charge distribution that not only reproduces the main features of the observed charge expansion dynamics but also allows us to extract the number of ejected electrons remaining in the cloud. The perspective of the field is also reviewed.

Bond models, also known as polarizable-point or mechanical models, have a long history in optics, starting with the Clausius-Mossotti relation but more accurately originating with Ewald’s largely forgotten work in 1912. These models describe macroscopic phenomena such as dielectric functions and nonlinear-optical (NLO) susceptibilities in terms of the physics that takes place in real space, in real time, on the atomic scale. Their strengths lie in the insights that they provide and the questions that they raise, aspects that are often obscured by quantum-mechanical treatments. Statics versions were used extensively in the late 1960’s and early 1970’s to correlate NLO susceptibilities among bulk materials. Interest in NLO applications revived with the 2002 work of Powell et al., who showed that a fully anisotropic version reduced by more than a factor of 2 the relatively large number of parameters necessary to describe secondharmonic- generation (SHG) data for Si(111)/SiO₂ interfaces. Attention now is focused on the exact physical meaning of these parameters, and to the extent that they represent actual physical quantities.

The transient absorption response of melanin is a promising optically-accessible biomarker for distinguishing malignant melanoma from benign pigmented lesions, as demonstrated by earlier experiments on thin sections from biopsied tissue. The technique has also been demonstrated in vivo, but the higher optical intensity required for detecting these signals from backscattered light introduces higher-order nonlinearities in the transient response of melanin. These components that are higher than linear with respect to the pump or the probe introduce intensity-dependent changes to the overall response that complicate data analysis. However, our data also suggest these nonlinearities might be advantageous to in vivo imaging, in that different types of melanins have different nonlinear responses. Therefore, methods to separate linear from nonlinear components in transient absorption measurements might provide additional information to aid in the diagnosis of melanoma.

We will discuss numerical methods for analyzing the various nonlinear contributions to pump-probe signals, with the ultimate objective of real time analysis using digital signal processing techniques. To that end, we have replaced the lock-in amplifier in our pump-probe microscope with a high-speed data acquisition board, and reprogrammed the coprocessor field-programmable gate array (FPGA) to perform lock-in detection. The FPGA lock-in offers better performance than the commercial instrument, in terms of both signal to noise ratio and speed. In addition, the flexibility of the digital signal processing approach enables demodulation of more complicated waveforms, such as spread-spectrum sequences, which has the potential to accelerate microscopy methods that rely on slow relaxation phenomena, such as photo-thermal and phosphorescence lifetime imaging.

Photothermal imaging in the mid-infrared enables highly sensitive, label-free microscopy by relying on bond-specific characterization of functional groups within the samples. In a pump-probe configuration, the mid-infrared (mid-IR) pump laser is tuned to characteristic vibrational modes and through localized absorption thermal changes in the refractive index are induced. The shorter wavelength probe scatter can be detected with lock-in technology, utilizing highly sensitive detectors at telecommunication wavelengths. This mitigates the need of complex detector technology as required for traditional infrared spectroscopy/Fourier Transform Infrared Spectroscopy.

The presented photothermal system integrates a high brightness quantum cascade laser that can be tuned continuously over a spectral range of interest with a fiber probe laser.

Fiber laser technology features a compact footprint and offers robust performance metrics and reduced sensitivity to environmental perturbations compared to free-space laser configurations. In systematic spectroscopy studies where the probe laser parameters were modified, we demonstrate that the signal-to-noise ratio can be significantly enhanced by utilizing a mode-locked laser compared to a continuous-wave laser. With a raster-scanning approach, photothermal spectroscopy can be extended to hyperspectral label-free mid-infrared imaging to combine spectral content with localized sample details. By tuning the pump laser to the amide-I absorption band around 1650 cm^-1 in biological tissue samples, the spectral characteristics can provide insight into the secondary structure of proteins (e.g. amyloid plaques; alpha-helix, beta-sheet). We present the versatility of our mid-IR photothermal system by analyzing histopathological tissue sections of cancerous tissue in a non-contact, non-destructive approach with good sensitivity.

Photothermal treatment is a valuable part of cancer therapies, in which the temperature of the heated region must rise to at least 40-45°C for protein destruction to occur[1, 2]. In practice, heating temperature distributions are typically non-uniform, resulting in incomplete kill of cancer cells. Gold nanorods (AuNRs) show strong absorption in the near infrared which leads to a strong plasmonic photothermal (PPT) effect. However, basic scientific understanding of AuNR local temperature and heat transfer to local surroundings has not been investigated in detail. In our study, the near infrared (NIR) excited Upconversion nanoparticle (UCNP)-AuNR nanostructure combines the powerful diagnostic and thermal sensing capacity of UCNPs, with the known therapeutic property of AuNRs. We show enhanced upconverted emission with AuNRs coupling, improving diagnostic capacity of the construct. We demonstrate mapping of the temperature profile within tumor tissue phantom medium, at high spatial and temporal sensitivity.

Living cells are crowded with macromolecules and organelles. Yet, it is not fully understood how macromolecular crowding affects the myriad of biochemical reactions, transport and the structural stability of biomolecules that are essential to cellular function and survival. These molecular processes, with or without electrostatic interactions, in living cells are therefore expected to be distinct from those carried out in test tube in dilute solutions where excluded volumes are absent. Thus there is an urgent need to understand the macromolecular crowding effects on cellular and molecular biophysics towards quantitative cell biology. In this report, we investigated how biomimetic crowding affects both the rotational and translation diffusion of a small probe (rhodamine green, RhG). For biomimetic crowding agents, we used Ficoll-70 (synthetic polymer), bovine serum albumin and ovalbumin (proteins) at various concentrations in a buffer at room temperature. As a control, we carried out similar measurements on glycerolenriched buffer as an environment with homogeneous viscosity as a function of glycerol concentration. The corresponding bulk viscosity was measured independently to test the validity of the Stokes-Einstein model of a diffusing species undergoing a random walk. For rotational diffusion (ps–ns time scale), we used time-resolved anisotropy measurements to examine potential binding of RhG as a function of the crowding agents (surface structure and size). For translational diffusion (μs–s time scale), we used fluorescence correlation spectroscopy for single-molecule fluctuation analysis. Our results allow us to examine the diffusion model of a molecular probe in crowded environments as a function of concentration, length scale, homogeneous versus heterogeneous viscosity, size and surface structures. These biomimetic crowding studies, using non-invasive fluorescence spectroscopy methods, represent an important step towards understanding cellular biophysics and quantitative cell biology.

We experimentally demonstrate adaptive control of linearly polarized (LP) modes based on a fiber Bragg grating (FBG), where feedback for the adaptive algorithm is provided by the optical signal transmitted through and reflected by the FBG. In this approach, by maximizing the ratio of optical power transmitted and reflected by the FBG, we can selectively excite the LP₀₁ and the LP₁₁ modes at an optical wavelength associated with the appropriate FBG reflection peak. Combining this method with time-division-multiplexing, it should be possible to adaptively control guided modes at any desired location within a multimode fiber network.

Multimode optical fibers potentially allow the transmission of larger amounts of information than their single mode counterparts because of their high number of supported modes. However, propagation of a light pulse through a multimode fiber suffers from spatial distortions due to the superposition of the various exited modes and from time broadening due to modal dispersion.

We present a method based on digital phase conjugation to selectively excite in a multimode fiber specific optical fiber modes that follow similar optical paths as they travel through the fiber. The excited modes interfere constructively at the fiber output generating an ultrashort spatially focused pulse. The excitation of a limited number of modes following similar optical paths limits modal dispersion, allowing the transmission of the ultrashort pulse. We have experimentally demonstrated the delivery of a focused spot of pulse width equal to 500 fs through a 30 cm, 200 micrometer core step index multimode fiber. The results of this study show that two-photon imaging capability can be added to ultra-thin lensless endoscopy using commercial multimode fibers.

High-quality whispering-gallery-mode optical resonators have garnered interest in particle sensing for a variety of applications. Here, we further explore the idea of using microresonators to enhance single-particle detection and identification by monitoring the Raman scattering from a particle adhered to a silica micro-sphere. A tunable diode laser is critically coupled into a resonant mode of the micro-sphere resonator, allowing circulating power to build up within the cavity for enhanced interaction with the attached particle. Experimental results of single particle Raman scattering in microsphere resonators are presented.

We have measured UV resonance Raman near and at the resonance phonon-allowed absorption lines of several liquid species. Resonance absorption with excitation on the symmetry-forbidden but strongly phonon coupled bands in the 230- 290 nm spectral band present enhancement 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. Large resonance gains (~3500x) reflect the narrower vapor phase lines. At the low laser fluence used, bubble formation is observed when the excitation energy corresponds to the maximum in Raman signal generation, not at the wavelength of maximum absorption in the liquid sample, which is several nanometers away.

Nonlinear and non-equilibrium properties of low-dimensional quantum materials are fundamental in nanoscale science yet transformative in nonlinear imaging/photonic technology today. These have been poorly addressed in many nano-materials despite of their well-established equilibrium optical and transport properties. The development of ultrafast terahertz (THz) sources and nonlinear spectroscopy tools facilitates understanding these issues and reveals a wide range of novel nonlinear and quantum phenomena that are not expected in bulk solids or atoms. In this paper, we discuss our recent discoveries in two model photonic and electronic nanostructures to solve two outstanding questions: (1) how to create nonlinear broadband terahertz emitters using deeply subwavelength nanoscale meta-atom resonators? (2) How to access one-dimensional (1D) dark excitons and their non-equilibrium correlated states in single-walled carbon nanotubes (SWMTs)?

The results of optical modeling of biological tissues polycrystalline multilayer networks have been presented. Algorithms of reconstruction of parameter distributions were determined that describe the linear and circular birefringence. For the separation of the manifestations of these mechanisms we propose a method of space-frequency filtering. Criteria for differentiation of benign and malignant tissues of the women reproductive sphere were found.

The work consists of investigation results of diagnostic efficiency of a new azimuthally stable Muellermatrix method of analysis of laser autofluorescence coordinate distributions of biological liquid layers. A new model of generalized optical anisotropy of biological tissues protein networks is proposed in order to define the processes of laser autofluorescence. The influence of complex mechanisms of both phase anisotropy (linear birefringence and optical activity) and linear (circular) dichroism is taken into account. The interconnections between the azimuthally stable Mueller-matrix elements characterizing laser autofluorescence and different mechanisms of optical anisotropy are determined. The statistic analysis of coordinate distributions of such Mueller-matrix rotation invariants is proposed. Thereupon the quantitative criteria (statistic moments of the 1st to the 4th order) of differentiation of human urine polycrystalline layers for the sake of diagnosing and differentiating cholelithiasis with underlying chronic cholecystitis (group 1) and diabetes mellitus of degree II (group 2) are estimated.

In this work the results of cell-tissue grafts research with a complex of optical methods – confocal fluorescent microscopy and Raman spectroscopy are presented. It was established that coefficient M scatter is related to irregularity of demineralization process. It was microscopically shown that the quantity of integrated cells into these types of transplants amounts to 20% of its surface.

The presence of natural radionuclides in raw materials used in cement manufacturing was determined by using analytical methods. The used Raw materials are limestone, clay, slag, and gypsum, which be used with different concentrations in cement production. Different analytical techniques such as Laser Induced Breakdown Spectroscopy (LIBS) technique, Gamma spectroscopy, Inductively Coupled Plasma (ICP) spectroscopy, X-ray fluorescence spectroscopy (XRF), in addition to X-Ray Diffraction (XRD) for phase identification of a crystalline material. The obtained data show that there is no significant radiological hazards arising from using the present cement components in the different applications. XRD data shows that there is no crystalline structures in the raw materials.