This PDF file contains the front matter associated with SPIE Proceedings Volume 11825, including the Title Page, Copyright Information, and Table of Contents.

The near field and dynamics of surface plasmons are probed by ultrafast photoemission electron microscopy from multiple domains. In particular, we reveal the plasmon interaction in the strong coupling region and observed the ultrafast dynamics of plasmons in the coupled plasmonic systems.

Coherent excitation of materials via ultrafast laser pulses can have interesting, observable dynamics in time-resolved photoemission measurements. The broad spectral width of ultrafast pump pulses can coherently excite exciton energy levels. When such coherently excited states are probed by means of photoemission spectroscopy, interference between the polarization of different levels can lead to observable coherent beats. Here, we present a theoretical formalism for evaluating the Time- and Angle- Resolved Photoemission Spectra (tr-ARPES) arising from two different kinds of excitations in the material. First, we consider coherently excited exciton states. We apply our formalism to a simple model example of hydrogenic exciton energy levels to identify the dependencies that control the quantum beats. Our findings indicate that the most pronounced effect of coherent quantum excitonic beats is seen midway between the excited exciton energy levels and the central energy of the pump pulse provides tunability of this effect. Second, we discuss the potential creation and measurement of coherences in both dispersive solids and qubit-like single levels using current generation time- and angle-resolved photoemission technology. We show that in both cases, when both the pump and the probe overlap energetically with the coherent levels, and when the probe preferentially measures one level as compared to the other, that the time-resolved photoemission signal shows a beating pattern at the energy difference between the levels. In the case of dispersive bands, this leads to momentum-dependent oscillations, which may be used to map out small energy scales in the band structure. We further develop the two-sided Feynman diagrams for time-resolved photoemission, and discuss the measurement of decoherence to gain insight into the characteristics of qubit and dispersive bands.

We investigate the terahertz and optical driven ultrafast dynamics in platinum and gold thin films by pump-probe reflectivity measurements with an optical probe. Platinum shows similar reflectivity change for both pump excitations, whereas gold shows a negligible reflectivity change for the terahertz pump. By employing a two-temperature model, we can explain the dynamics in platinum and determine electron-phonon coupling constant for the two pump excitations. We estimate a 20% larger electron-phonon coupling for terahertz-driven dynamics compared to the optical one, which we attribute to a concomitant nonthermal regime for both phonons and electrons. In the case of gold, we explain the negligible response as a terahertz-driven field emission of electrons via Fowler-Nordheim tunneling.

Cellular metabolism and cell behavior are believed to be significantly different in two-dimensional (2D) cultures from that in vivo. Here, we investigate the environmental effects of the metabolic state of murine breast cancer cells line (4T1) in 2D monolayer and three-dimensional (3D) collagen matrix cultures using integrated two-photon (2P) micro-spectroscopy (FLIM) of intrinsic NAD(P)H autofluorescence. In addition, we examined the metabolic responses to two novel compounds, MD1 and TPPBr, that target cellular metabolism by disrupting monocarboxylate transporters (MCTs) or oxidative phosphorylation, respectively, using 2P-FLIM of intracellular NAD(P)H in 2D and 3D cultures. Integrating nonlinear microscopy and spectroscopy of intrinsic NAD(P)H with refined 3D tumor-matrix in vitro models is a promising approach towards in-depth understanding of the roles of metabolism and metabolic plasticity in tumor growth and metastatic behavior.

We have developed Second Harmonic Generation (SHG) microscope tools to selectively and specifically probe changes in collagen organization in diseased tissues. Using a novel form of 3D machine learning, we successfully classified six types of ovarian tumors based on the observed collagen fiber morphology. We also exploit the SHG coherence to extract sub-resolution fibril assembly (size and packing) by analyzing the emission directionality in conjunction with measured optical scattering properties and Monte Carlo simulations. This approach further classified a spectrum of ovarian tumors. We developed polarization sensitive SHG methods to extract collagen macro/supramolecular structural aspects and found significant differences between normal and malignant ovarian tissues. We also used this set of SHG analyses to probe structural changes in idiopathic pulmonary fibrosis (IPF) compared to normal lung tissues, and found comparable collagen changes over normal tissues.

The first detection of surface-bound free rotors with exceptionally long rotational and vibrational coherence lifetimes was detected for both CO and CH4 adsorption to Au at near-ambient pressures. Hybrid femtosecond/picosecond sum-frequency generation vibrational spectroscopy was used to detect rovibrational coherence recurrences with coherence lifetimes lasting several 10’s of picoseconds. The observations challenge our understanding of weak adsorption of molecules to metal surfaces.

We present a new category of computational ultrafast imaging technique, light field tomography (LIFT), which can perform 3D snapshot transient (time-resolved) imaging at an unprecedented frame rate with full-fledged light field imaging capabilities including depth retrieval, post-capture refocusing, and extended depth of field. As a niche application, we demonstrated real-time non-line-of-sight imaging of fast-moving hidden objects, which is previously impossible without the presented technique. Moreover, we showcased 3D imaging of fiber-guided light propagation along a twisted path and the capability of resolving extended 3D objects.

2D layered materials have been intensively studied as candidates for next-generation nanometric optoelectronic devices due to their strong light-matter interactions resulting from 2D quantum confinement. Thus, metalenses based on 2D materials have demonstrated attractive properties, such as nanometer thickness, high focusing resolution and efficiency, high mechanical strength and flexibility, and fast and low-cost fabrication process, and can be applied in harsh environments for different applications. Here we demonstrate the recent advance in 2D materials metalenses, which can achieve diffraction-limited imaging, nanoparticle tracking with nanometer precision, and broadband in-situ zoom imaging with varifocal graphene metalenses.

Mid-infrared photothermal microscopy has demonstrated unique capabilities in the field of chemical imaging including sub-diffraction limited resolution and sub cellular imaging. Vibrational Infrared Photothermal and Phase Signal (VIPPS) introduces an additional contrast mechanism based on different thermal properties. This enables high contrast imaging of features with overlapping absorption profiles but different thermal diffusion characteristics. Our approach paves the way for high contrast sub-diffraction limited imaging of secondary protein conformations in fibroblast cells grown in a protein rich collagen matrix at the subcellular level.

Chemical imaging based on mid-infrared (MIR) spectroscopic contrast is an important technique with a myriad of applications, including biomedical imaging and environmental monitoring. Current MIR cameras, however, lack in performance and are much less affordable compared to mature Si-based devices, which operate in the visible and near-infrared. We demonstrate fast MIR imaging through non-degenerate two-photon absorption (NTA) in a scientific CMOS camera. We show that wide-field MIR images can be obtained at more than 100 frames/s with energies of only a few fJ per pixel. This on-chip approach does not rely on phase-matching, it is alignment-free and it does not necessitate complex post-processing of the images. We show chemically selective MIR imaging of polymers and biological samples, including MIR videos of live organisms and complete 3D volumetric images of objects acquired at rates as fast as 1 volume/s.

Multidimensional, multicontrast, spatial frequency modulation imaging with built-in femtosecond pulse characterization and dispersion compensation is presented. We present wavelength domain imaging which produces one-dimensional imaging with single element detection, and a multiplexed spatial and wavelength domain system that produces two-dimensional images and is compatible with single element detection imaging.

Following the coupled motion of electrons and nuclei in molecules is difficult if one uses time-resolved approaches that only provide direct information on one or the other. We combine two complementary measurements, Time- Resolved Photoelectron/PhotoIon Spectroscopy (TRPES and TRPIS) and Ultrafast Electron Diffraction, to follow the electronic and nuclear dynamics of gas phase CH2I2 when exposed to UV light. In order to interpret the measurement, trajectory surface hopping calculations are carried out and all the measurement observables are simulated and directly compared with the measurement signals. Our measurements highlight the coupled electron-nucleus dynamics that allow for electronic potential energy to be converted into nuclear kinetic energy as well as complicated structural rearrangements of the molecule that involve symmetry breaking, dissociation, rotation, and non-local wave-packet dynamics.

The conversion of light into chemical and mechanical energy mediates many important processes in nature, e.g. vision, photosynthesis and DNA photodamage. To understand the structure-function relationships regulating such processes one must strive to study them in their natural environment, i.e. in the liquid-phase. This presentation reports on the design of a novel Ultrafast Electron Diffraction instrument capable of resolving structural dynamics in liquid samples. The capabilities of this instrument are showcased in the study of water, where its structure was resolved up to the 3rd hydration shell with 0.6 Å spatial resolution, and dynamics were resolved with 200 fs resolution.

We investigated the ultrafast photochemical ring-opening in the molecule α-phellandrene by a combination of megaelecronvolt ultrafast electron diffraction and excited state ab initio multiple spawning wavepacket simulations. α- Phellandrene exhibits a number of different conformers which produce different ring-opening photoproducts according to the Woodward-Hoffmann rules. In our study we image the conversion of a specific conformer of α-phellandrene in the Woodward-Hoffmann predicted photoproduct in real time and space.

The conversion of light into mechanical and chemical energy, at the level of single molecules, drives many processes in nature such as vision and photosynthesis, and is important for solar energy conversion and storage applications. These changes take place at the atomic level, on femtosecond timescales. We use ultrafast electron diffraction, which probes changes in molecular structure with atomic (sub-Angstrom) resolution in space and femtosecond resolution in time. Here we show that we can retrieve, with a high level of detail, the structural dynamics that take place after photoexcitation of complex molecules.

We propose and demonstrate the use of multimode fibers (MMF) inside a laser cavity as a new path to generate spatiotemporal modelocked pulses with high beam quality and high energy. Prior to our work, MMFs in optical cavities resulted in the generation of low-quality output beam profiles by spatiotemporal mode-locking. Here we present a versatile approach to reach high energy per pulse directly in the mode-locked MMF oscillator with a near single-mode output beam profile. Our approach relies on spatial beam self-cleaning via the nonlinear Kerr effect inside the cavity achieved by controlling spatiotemporal pulse propagation with a dispersion-managed design. We demonstrate the versatility of our approach with Yb-doped and Er-doped multimode laser cavities which generate pulse energies of 24 nJ and 16 nJ, respectively. The high peak power reached in the MMF within the cavity induced a Kerr self-beam cleaning which produced a near Gaussian mode output (M2<1.13).

We present a robust, multi-line, tunable and power scalable ultrashort optical source suitable for nonlinear optical imaging. A systematic characterization of the seed oscillator, pulse stretcher, pre-amp, and power-amp along with the methods of pulse compression, pulse division and the phenomenon of soliton self frequency shift through a photonic crystal fiber arepresented to make a tunable and multi-colored pulsed source capable of dual polarization and power-scalable operation.

Kerr resonators are simple and compact devices that enable ultrashort pulse and frequency comb generation over a wide range of wavelength and pulse parameters that are difficult to access with traditional mode-locked laser sources. Pulse generation in these systems derives from the formation of stable optical solitons. The pulse performance can be enhanced by exploiting novel classes of optical solitons. This talk will examine recently discovered cavity solitons in fiber Kerr resonators, including stretched-pulse and chirped pulse solitons. Stretched-pulse solitons in dispersion-managed systems enable record short pulses from Kerr resonators and chirped pulsed solitons in normal-dispersion cavities have the potential to stabilize much higher pulse energies. This talk will also examine the most recent results for pulse performance enhancement in stretched-pulse systems and the remarkable tolerance for dissipation of chirped-pulse Kerr resonator solitons.

A diode-pumped femtosecond laser oscillator producing <100 fs pulses with high pulse energy was demonstrated. The laser was based on the Yb:CALGO crystal and produced 91 fs pulses with 750 mW of average output power. The laser operated at 27 MHz and produced pulses with 28 nJ of energy corresponding to >300 kW of peak power. Such a high peak power performance with <100 fs pulses make this source an excellent platform for fluorescence lifetime imaging, supercontinuum generation, multiphoton imaging or seeding of amplifiers.

Superfluorescence (SF) is a unique optical phenomenon that consists of an ensemble of emitters coupling collectively to produce a short but extremely intense burst of light. SF has also only been realized in extreme conditions (at low temperatures of around 6 K). Moreover, no anti-Stokes shift SF has been discovered in either an ensemble of nanoparticles or at bulky crystal levels. We report on a new lanthanidedoped upconversion nanoparticles (UCNPs) as a medium to achieve cavity free anti-Stokes shifted SF at room temperature, culminating in rapid, intense, and narrow spectral peaks of upconverted SF. This is the first time that SF has been discovered in a single nanocrystal regime and is the smallest-ever SF media. We observed the resultant UCNP SF with an extremely narrow spectral width at single nanocrystal-level (full-width at half-maximum, FWHM = 2 nm), and to have a significantly shortened lifetime (τ = 46 ns, 10,000-fold accelerated radiative decay, when compared to the lifetime of τ = 455.8 μs of normal upconversion luminescence (UCL). The significantly upspeeded upconverted SF lifetimes at tens of nanoseconds scale should break through the key limitation in normal UCL. This will open up the opportunity to carry out high speed bioimaging using upconversion nanoparticles without compromising the imaging quality. In addition, our ultrafast upconverted SF will achieve fine temporal resolution control of highly dynamic physiological processes that have been constrained by normal UCL.

Cells respond to forces, and their quantification can potentially inform on the role of mechanics in cell development, differentiation, tissue repair and homeostasis. Other force sensitive processes include cancer cell metastasis, heart development in embryos driven by fluid forces, and individual cell response to tension by enhancing microtubule growth and connections. Development of current mechano-sensing approaches has not yielded many options, especially in directional force measurement. We present a sharpened fiber-based approach for uniaxial forces. An upconversion nanoparticle (UCNP) is mounted on the tip of the fiber and optically accessed through the fiber, which is manipulated as a probe. In UCNPs, the modification of the crystal field via mechanical forces result in changes in emission intensity, spectral shifts, upconversion luminescence (UCL) lifetime and ratiometric UCL response. We report on a discernably large peak shift of between 5-10 nm, and an apparent phase transition, with increasing amount of applied force in the micro Newton regime, in a single direction. Moreover, the peak shift is linear to the applied compression force. We investigate the influence of the UCNP force sensing process using Raman spectroscopy.

The combination of far-field and near-field scanning optical microscopy (NSOM) with a position (line) and wavelength dependent detection allow us to observe the fluorescence form points away from the excitation spot. This contrasts with confocal illumination/detection and with fluorescent imaging of uniform illumination, the two commonly used fluorescence measurement modes. We discuss the origins of the nonlocal emission and argue that the results can be used to measure the exciton diffusion length in these two-dimensional materials. In particular, we study transition metal dichalcogenides. We use near-field second harmonic generation (SHG) with NSOM detection of the mode-locked femtosecond laser pulse generated signals.

Owing to its many outstanding properties, polydimethylsiloxane (PDMS) has been an important engineering elastomer. An integration of compact, low-loss, three-dimensional (3D) optical waveguides with fluidic functionalities will greatly enhance the capabilities of PDMS-based devices and enable heretofore unimaginable applications. Recently, we have demonstrated the fabrication of compact polymer waveguides in PDMS through multiphoton laser direct writing both with and without a photoinitiator. While the photoinitiator-free process enjoyed a high refractive index contrast and a very low optical loss, it was subject to excessive defects due to material damage during the high-intensity optical irradiation. Our process using a photoinitiator achieved a defect-free fabrication despite a lower refractive index contrast and a higher loss. We show that the defects are a result of uncontrolled optical beam collapse through self-focusing and discuss potential solutions to mitigate the defects.