This PDF file contains the front matter associated with SPIE Proceedings Volume 10638, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

High intensity ultrashort pulse causes dramatic perturbations in electronic structure of condensed matter. In the same time energy in high intensity single pulse may not be sufficient to disrupt sample thermal equilibrium. Interesting experimental results in ultrashort pulse photo-excited solids have been reported recently on transient athermal phenomena induced by ultrashort high intensity low energy pulse – phenomena related to both athermal phase transitions and athermal state changes. Athermal non-equilibrium of electronic system – and induced changes in magnetic and optical states, may exist only for a period of time comparable to excited carriers’ relaxation time. That time is not sufficient for emerging application ranging from light induced superconductivity to infrared countermeasures. While single pulse interaction with condensed matter leading to transit state appearance is well observed, documented, and, to some extends, explained, one of the major problem is to maintain meta-stability of such transient states. Metastability of athermal non-equilibrium that could last well beyond electronic system relaxation time. The objective of this paper is to discuss some issues and approaches to meta-stability of transient states induced by ultrashort pulses in condensed matter

The excitonic insulator (EI) is an intriguing phase of condensed excitons undergoing a Bose-Einstein-Condensation (BEC)-type transition. A prominent candidate has been identified in Ta₂NiSe₅. Ultrafast spectroscopy allows tracing the coherent response of the EI condensate directly in the time domain. Probing the collective electronic response we can identify fingerprints for the Higgs-amplitude equivalent mode of the condensate. In addition we find a peculiar coupling of the EI phase to a low frequency phonon mode. We will discuss the transient response on multiple energies scales ranging from the exciton dynamics to the coherent THz response of the gap.

We develop the theory for nonresonant Raman scattering of strongly correlated electrons in time-resolved pump- probe experiments. The electrons are initially pumped with an intense pulse of light and then a probe pulse measures the Raman response function after an adjustable time delay. We describe how the width of the probe pulse and the strength of the electron correlations affect the Raman cross section. The theory is developed for the case of B_1g symmetry, where incident and scattered light are polarized perpendicular to each other. We illustrate the exact solution with the Falicov-Kimball model, which is solved via nonequilibrium dynamical mean-field theory.

The growing attention to perovskite nanocrystals is connected with their unusual and potentially useful electronic and optical properties. I will discuss the bulk energy band structure of CsPbX3 (X = I, Cl, and Br) perovskites and show that all of them have the band edge at R-point of the Brillouin zone. To describe electronic and optical properties of perovskite nanocrystals we have derived the four band effective mass Hamiltonian, which describes the electronic properties of electron and holes near the band edge. Using this Hamiltonian we calculate the lowest quantum confined levels of electrons and holes and the spectra of the allowed optical transitions. The calculations takes into account the cubic shape of the perovskite nanocrystals, that results into inhomogeneous electric field of emitted and absorb photons. The symmetry of the ground exciton state has been analyzed and the radiative decay time has been calculated. The results of our theoretical calculations have explained 200 ps radiative decay time and polarization properties measured in single CsPb(BrCl2) quantum dot experiments .

Recently, careful stacking of 2D materials and heterostructures has shown that new interlayer electronics state can have lifetimes that are orders of magnitude longer than present in the monolayer. Using time-space resolved ultrafast microscopy on individual 2D crystal grains, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or by twisting the layer stacking orientation. Considering first twisted bilayer graphene (tBLG), we discovered how stacking-angle tunable absorption resonances form a strongly-bound exciton state as a consequence of the symmetrized rehybridization of constrained interlayer 2p orbitals. Using two-photon photoluminescence and intraband-transient absorption microscopies, we have recently imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly-bound (450-550 meV), stable interlayer exciton states. Unlike stacked graphene semiconducting 2D transition metal dichalcogenides (TMDCs) has diffuse interlayer d-orbital overlap that restricts interlayer mobility and exciton transport. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer directed E-field, inducing electron-hole dissociation. We show that stacked WSe2 TMD devices can have both IQE >50% and fast (<50 ps) picosecond electron escape times. Using a first-principle kinetic master equation, our methods analytically extracts both the E-field-dependent interlayer escape velocity and rate-limiting exciton dissociation time. Remarkably our photocurrent response function produces the same E-field-dependent electronic escape and dissociation rates for both the optical and PC addressed ultrafast measurements. As confirmation, the resulting ratio of the electronic rates accurately matches our overall WSe2 device IQE in the intensity limit of zero Auger recombination. Thus through time-space resolved microscopy, we now obtain a timeline selective to the interlayer electronic dynamics of TMDCs and tBLG van der Waals materials. We show how this novel scanning microscopy approach, combines ultrafast photocurrent and transient absorption to identify new long-lived and metastable interlayer electronic states in emerging twisted and stacked 2D materials and devices.

Functional surfaces find application in a number of areas, such as designing flexible electronic devices and integrating electronic systems with biological ones. However, the preparation of functional surfaces entails processing that is destructive to fragile polymer or biological substrates. A benign transfer method is thus needed to move pre-functionalized surfaces from a stable substrate to a fragile one. Chemical hydrogenation of graphene weakens the adhesion force between the graphene and its substrate. We exploit this phenomenon to construct a method for transferring graphene with pre-formed chemical, physical, and electronic functionalities from a heat-, vacuum-, and chemical-stable substrate such as silicon to several less robust ones, including polymers and living cells. We also discuss reversibility of graphene hydrogenation and the implications for re-adhering graphene securely to new substrates.

Since the discovery of monolayer graphene in 2004 many other layered materials have been reduced to monolayer thickness. Strong confinement and reduced screening in two-dimensional materials result in novel electronic properties, different from which of their 3D counterpart. Recently, transition metal dichalcogenides (TMDs) have moved under the spotlight. Group VI TMDs are indirect bandgap semiconductors. In the monolayer limit the band gap becomes direct with giant exciton binding energies due to strong electronic correlations. Also, monolayer group VI TMDs exhibit broken inversion symmetry that, in combination with strong spin-orbit interaction, leads to a sizable spin-splitting of the band structure of up to several hundreds of meV. Stacking different 2D materials to form van der Waals (vdW) heterostructures with new functionalities is a long-sought goal that now starts to turn into reality. Such heterostructures can be used to tailor screening for bandgap engineering as well as charge and spin transfer across the layers. We have synthesized WS2/graphene vdW heterostructures via chemical vapor deposition (CVD) [1] on epitaxial graphene on SiC(0001) [2] and characterized the electronic properties with high-resolution angle-resolved photoemission spectroscopy (ARPES) [3]. These measurements demonstrate perfect epitaxial alignment of WS2 on graphene and provide evidence for significant static charge transfer between the two layers. Further, we excited these heterostructures with femtosecond laser pulses resonant to the exciton in WS2 and probed the resulting carrier dynamics with time-resolved ARPES, providing evidence for ultrafast charge transfer between WS2 and graphene. [1] Avsar et al., Nat. Comm. 5, 4875 (2014) [2] Rossi et al., 2D Mat 3, 031013 (2016) [3] Forti et al., Nanoscale 9, 16412 (2017)

In systems of coupled III-V semiconductor nanostructures of mixed dimensionality, the different classes like quantum wells, quantum dots, and sub{monolayers form new mixed states when they are combined. To address the complexity of such systems over a wide energy range, we present a white-light approach for multidimensional coherent spectroscopy. As a proof of principle we determine the homogeneous linewidth of InAs quantum dots in an inhomogeneously broadened ensemble.

The lead halide hybrid perovskites have gained considerable attention in recent years due to their stellar performance as absorber layers in solution-processed solar cells, with efficiencies recently reaching over 22 percent [1]. Owing to their large spin-orbit coupling, these materials are also of interest for spintronic applications, in which the presence of lead may be less of an impediment to their adoption [2]. Measurements of spin dynamics in bulk CH3NH3PbI3-xClx have been reported in recent years [3,4,5], the spin-dependent optical Stark effect was demonstrated in 4F-PEPI [6], and a large Rashba effect has been predicted in both bulk and 2D perovskites [2], highlighting the need for further studies of the spin-related properties of these materials. Here we report spin-dependent measurements of carrier kinetics in butylammonium methylammonium lead iodide 2D perovskite and measurements of the coherent carrier response in 3D CH3NH3PbI3. Both experiments provide direct evidence of the impact of Rashba on the carrier kinetics in these systems, further supporting the potential for developing spin-optoelectronic devices using these materials. [1] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg. [2] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017). [3] D. Giovanni et al. Nano Lett. 15, 1553 (2015). [4] C. Zhang et al. Nat. Phys. 11, 427 (2015). [5] P. Odenthal et al. Nat. Phys. 13, 894 (2017). [6] D. Giovanni et al. Science Advances 2, e1600477 (2016).

Since the first reports of their successful synthesis in 2015, all-inorganic perovskite quantum dots (PQDs), emerged as promising material for lighting applications. Despite their unique optical properties, to date, PQDs based LEDs and lasing media operation is still far from ideal. One of the limiting factors is the strong Coulomb interaction, which contributes to nonradiative decay processes. Here, we discuss in detail the multiexciton interactions in PQDs based on their size dependence. We expand on their nonlinear optical properties, focusing on the two-photon absorption characteristics, showing results for two-photon pumped amplified stimulated emission, suggesting that these materials may be promising for two-photon pumped lasers.

The advances of ultrafast imaging and spectroscopy technologies in the last decades have enabled a new perspective in understanding the fundamental processes at far from equilibrium. Ranging from molecular systems to long-range-ordered electronic crystals, the photo-driven transient decoupling between constituents, e.g. electronic, spin and vibrational degrees of freedom, allows the interaction hierarchy to be observed. These photoinduced nonequilibrium dynamics are not only central for understanding the complex interactions, but also may yield new ways for controlling the processes with many technical implications. We will discuss a surprising generality of self-organizing behavior emerging at nonequilibrium in driven systems involving many-body metastable phases in electronic crystals visualized by the ultrafast electron imaging techniques. We will focus on photoinduced phase transitions in charge-ordering systems [1,2] while drawing their analog observed in vanadium dioxide nanocrystals [3,4] to highlight this generality and the potential control. We will also discuss methods to combine diffraction, imaging, and spectroscopy in a single setup to follow many key degrees of freedom at once as a next step to resolve complex dynamics in the development of high-intensity ultrafast electron microscopy systems [5,6]. References: [1] T.R. T. Han et al., Phys. Rev. B 86, 075145 (2012). [2] T.R. T. Han et al., Sci. Adv. 1, e1400173 (2015). [3] Z. Tao et al., Phys. Rev. Lett. 109, 166406 (2012). [4] Z. Tao et al., Sci. Rep. 6, 38514 (2016). [5] F. Zhou, J. Williams, C.-Y. Ruan, Chem. Phys. Lett. 683, 488 (2017). [6] J. Williams et al., Struc. Dyn. 4, 044035 (2017).

The refinement of materials to facilitate their use in a broad range of applications is dependent on a detailed characterization and understanding of their interaction with light. This is especially true for the properties of materials’ surfaces and interfacial regions where deviations from the bulk structure significantly impact the flow of energy. Adding to the complexity of this problem is the fact that these regions contain an overall small number of reporters resulting in undetectable signal buried under the massive bulk response. To directly overcome these challenges, electronic sum frequency generation (eSFG) can selectively probe interfacial species, defects, and ordering. The sensitivity of this technique arises from the requirement that second order nonlinear signals originate from noncentrosymmetry that is inherent at surfaces and interfaces. Further, the enhancement of eSFG signal due to resonance of material transitions with any one of the three electric fields involved generates a spectrum analogous to linear absorption but originating solely from these regions of interest. Here we present our instrumental implementation of this technique which centers around the use of supercontinuum from a photonic crystal fiber for broadband spectral analysis and a microscopic apparatus to limit, and eventually probe, sample heterogeneity. Finally our application of this instrument to multiple crystalline materials provides new information to inform future design directions.

Since 2012, heterostructures formed by two-dimensional materials have drawn considerable attention due to the potentials of combining their novel properties. In developing these materials, one key issue is to understand and control charge transfer process. In this presentation, latest progress on experimental studies of ultrafast electron transport in van der Waals heterostructures, including both vertical and lateral heterostructures, will be discussed. First, previous studies by several groups on electron transfer in van der Waals vertical hetero-bilayers, such as MoS2/MoSe2 and WS2/graphene, will be reviewed. Secondly, latest results on van der Waals bilayers will be presented, including the demonstration of a structure with type-I band alignment and study of electron transfer in homo-bilayers. Next, new experiments on electron transport in van der Waals multilayers, such as 3 layers and 4 layers, will be presented. The coherent nature of transport and the systematic control of the carrier dynamics by thickness will be demonstrated. Attempts of using band-alignment engineering to control flow of carriers will be discussed. As an example, unipolar optical doping of graphene with significantly extended photocarrier lifetimes were achieved. Finally, latest ultrafast measurements of charge transfer in lateral junctions will be discussed, including a heterojunction formed by connecting monolayers of MoS2 and MoSe2 and a homojunction by varying the bandgap of a region of a MoSe2 monolayer by a dielectric top-layer. In both cases, transient absorption measurements with high temporal and spatial resolution revealed efficient photocarrier transfer across the lateral junctions.

Atomically thin layers of transition metal dichalcogenides (TMDs) have unique electronic and optical properties, offering the possibility of novel applications in electronics, optoelectronics and valleytronics. These applications require a fundamental understanding of the valley dynamics involving the carrier radiative and non-radiative recombination, valley polarization, and valley coherence. We will present our study of valley dynamics in monolayer TMD by using optical two-dimensional coherent spectroscopy (2DCS). Compared to conventional one-dimensional spectroscopic techniques, optical 2DCS has many advantages such as separating homogeneous and inhomogeneous linewidths, isolating relaxation pathways, detecting valley coherence and measuring coherence dephasing time. In rephasing 2D spectra, homogeneous and inhomogeneous linewidths are associated with line shape in the cross-diagonal and diagonal directions, respectively. The homogeneous linewidth can be extracted from a cross-diagonal fit to give the coherence dephasing time. The measurement is repeated with various excitation intensities and sample temperatures to extract the intrinsic dephasing time at zero power and temperature. By using various combinations of excitation pulse helicities, our experiment can selectively excite and detect a particular valley population and coherence. This allows to isolate and measure valley exchange and coherence between the two valleys.

Interest in atomically-thin transition metal dichalcogenide (TMD) semiconductors such as MoS2 and WSe2 has exploded in the last few years, driven by the new physics of coupled spin/valley degrees of freedom and their potential for new spintronic and valleytronic devices. Although robust spin and valley degrees of freedom have been inferred from polarized photoluminescence (PL) studies of excitons, PL timescales are necessarily constrained by short-lived (1-30 ps) recombination timescales of excitons. Direct probes of spin and valley dynamics of the resident electrons and holes in n-type or p-type doped TMD monolayers, which may persist long after recombination ceases, are still at a relatively early stage. In this work, we directly measure the coupled spin-valley dynamics of resident electrons and resident holes in n-type and p-type monolayer TMD semiconductors using time-resolved Kerr rotation. Very long relaxation timescales in the nanosecond to microsecond range are observed at low temperatures - orders of magnitude longer than typical exciton lifetimes. In contrast with III-V or II-VI semiconductors, electron spin relaxation in monolayer MoS2 is found to accelerate rapidly in small transverse magnetic fields. This indicates a novel mechanism of electron spin dephasing in monolayer TMDs that is driven by rapidly-fluctuating internal spin-orbit fields that, in turn, are due to fast electron scattering between the K and K' conduction bands [1]. More recent studies of gated TMD monolayers also allow observation of very long spin/valley relaxation of resident holes, a consequence of spin-valley locking [2]. These studies provide direct insight into the physics underpinning the spin and valley dynamics of resident electrons and holes in 2D TMD semiconductors. [1] L. Yang et al., Nature Physics 11, 830 (2015). [2] P. Dey et al., Phys. Rev. Lett. 119, 137401 (2017).

We use a non-equilibrium many-body theory that engages the elements of transient coherence, correlation, and nonlinearity to describe changes in the magnetic and electronic phases of strongly correlated systems induced by femtosecond nonlinear photoexcitation. Using a generalized tight–binding mean field approach based on Hubbard operators and including the coupling of the laser field, we describe a mechanism for simultaneous insulator–to–metal and anti- to ferro–magnetic transition to a transient state triggered by non–thermal ultrafast spin and charge coupled excitations. We demontrate, in particular, that photoexcitation of composite fermion quasiparticles induces quasi-instantaneous spin canting that quenches the energy gap of the antiferromagnetic insulator and acts as a nonadiabatic “initial condition” that triggers non-thermal lattice dynamics leading to an insulator to metal and antiferromagnetic (AFM) to ferromagnetic (FM) transitions. Our theoretical predictions are consistent with recent ultrafast pump-probe spectroscopy experiments that revealed a magnetic phase transition during 100fs laser pulse photoexcitation of the CE–type AFM insulating phase of colossal magnetoresistive manganites. In particular, experiment observes two distinct charge relaxation components, fs and ps, with non- linear threshold dependence at a pump fluence threshold that coincides with that for femtosecond magnetization photoexcitation. Our theory attributes the correlation between femtosecond spin and charge nonlinearity leading to transition in the magnetic and electronic state to spin/charge/lattice coupling and laser-induced quantum spin canting that accompanies the driven population inversion between two quasi–particle bands with different properties: a mostly occupied polaronic band and a mostly empty metallic band, whose dispersion is determined by quantum spin canting.

We have preformed femtosecond time-resolved and nanometer spatially resolved measurements of the photo-excited insulator to metal transition (IMT) in Vanadium Dioxide (VO2). This work was made possible by several technical advances made by the authors including the development of a nano-imaging technique that is compatible with pulsed laser sources, which is guaranteed to be artifact free [1]. Additionally the authors have combined the Pharos Yb:kGW solid state laser system, which operates at relatively low repetition rates (750 kHz) with a commercial microscope from Neaspecc. This laser system provides the intense pumping that is required to photo-excite the IMT in VO2 and provides extremely broad spectral coverage for probing the IMT (660-20,000 nm). Using these technological advances the authors have obtained femtosecond time-resolved nano-imaging data on VO2, which are guaranteed to be artifact free. our findings expose that the non-equilibrium photo-induced IMT is highly inhomogeneous. The authors are able to extract the length scale of emergent metallic domains as a function of time-delay between the pump and probe channels to provide insight into the mechanisms of growth. Furthermore, by monitoring the monoclinic phonon with nanometer spatial resolution and femtosecond temporal resolution the authors are able to provide insight into the role that the monoclinic to rutile structural transition plays in the IMT. Our advances pave a pathway to study a wide range of systems with nanoscopic spatial, and ultrafast temporal resolution. [1] A. Sternbach et al., "Artifact free time resolved near-field spectroscopy" Optics Express 25 (23), 28589-28611 (2017)

Nondegenerate nonlinear refraction (ND-NLR) in semiconductors is greatly enhanced over the degenerate case and exhibits strong nonlinear dispersion, which provides the potential to greatly modify the refractive dispersion. This nondegenerate enhancement arises from the resonance of the small photon energy with the intraband self-transition, and the larger photon energy with the interband transition. Our earlier theory predicts the dispersion of ND-NLR in semiconductors from a Kramers-Kronig transformation of a nonlinear absorption spectrum, which considers nondegenerate two-photon absorption (ND-2PA), electronic Raman, and quadratic Stark effect. Experimentally, the dispersion of ND-NLR is measured using our Beam-Deflection technique, and the coefficient, n2, is determined over a broad spectral range with various degrees of nondegeneracy. In the extremely nondegenerate case, n2 is greatly enhanced near the onset of ND-2PA, which rapidly switches sign to negative near the bandgap over a very narrow wavelength range. The data suggests larger figure-of-merits by using ND-NLR for all-optical switching. Near and within the ND-2PA regime, strongly dispersive ND-NLR can significantly alter the dispersion of a material, and provides the possibility to optically modify the group index, and group velocity dispersion (GVD) properties. From their irradiance dependence, the nonlinear group index, n2,g, and the nonlinear GVD factor, D2, are calculated from the first and second order derivatives of n2, which show even greater nondegenerate enhancement. Also the nonlinear group index is maximized where there is no two-photon absorption. Potential applications, including nondegenerate all-optical switching based on n2,g, and ultrafast all-optical pulse shaping based on D2, are discussed.

Materials exhibiting near zero refractive index are shown to have interesting nonlinear optical properties such as enhanced second and third harmonic generation, and large nonlinear refraction (NLR) due to their unique interplay between linear and nonlinear optical features. In particular, the NLR of highly doped semiconductors such as Indium Tin Oxide and Aluminum doped Zinc Oxide is enhanced in the near-infrared spectral regions, where the real part of the permittivity crosses zero with the advantage of having a tunable zero crossover frequency by controlling the doping level. This is also known as the epsilon near zero (ENZ) regime, where the refractive index is very small. We have used the Beam-Deflection (BD) method to directly characterize the temporal dynamics and polarization dependence of the nondegenerate (ND) NLR of doped semiconductors at ENZ. The origin of the nonlinear optical response of these materials is different than for the case of bound electronic nonlinearities which depend upon the third-order susceptibility. The ND BD technique has the potential to study the dependence on relative polarization of excitation and probe waves to accurately determine the instantaneous electronic nonlinearities separately from the non-instantaneous mechanisms such as carrier redistribution effects, however, the carrier nonlinearities are dominant in such materials. This method also reveals the effect of tuning the wavelength of excitation or probe waves through ENZ separately. BD has sensitivity to induced optical path length as small as 1/20,000 of a wavelength, which enables the possibility to resolve NLR in the presence of large nonlinear absorption backgrounds.

Non-instantaneous nonlinear medium response is a widespread case for a propagation of laser pulse with femtosecond duration. It occurs if a femtosecond pulse propagates in fused silica or in air at sufficient high incident intensity, or in a medium, containing nanoparticles, for example. The last propagation case is very important for practical application at developing of all-optical multi-dimensional devices for data storage. As a rule, such femtosecond laser pulse propagation is accompanied by nonlinear non-stationary absorption. In the media with nonlinear non-stationary absorption a self-similar mode of a femtosecond pulse propagation is of great interest because of its spectrum unchanging at the distances up to several dispersion lengths at least. In our report we predict and investigate a novel type of solitons (or similariton) - nonlinear chirped solitons - formation if a nonlinear absorption and/or nonlinear refraction of the laser pulse occurs. This type of solitons is characterized by a nonlinear complicated frequency chirp and can possess an asymmetric pulse shape.

Artificial conversion of sunlight to chemical fuels has attracted attention for several decades as a potential source of clean, renewable energy. For example, in light-driven proton reduction to molecular hydrogen, a light-absorbing molecule (the photosensitizer) rapidly transfers a photoexcited electron to a catalyst for reducing protons. We recently found that CdSe quantum dots (QDs) and simple aqueous Ni2+ salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen. To understand why this system has such extraordinary catalytic behavior, ultrafast transient absorption (TA) spectroscopy studies of electron transfer (ET) processes from the QDs to the Ni catalysts were performed. CdSe QDs transfer photoexcited electrons to a Ni-dihydrolipoic acid (Ni-DHLA) catalyst complex extremely fast and with high efficiency: the amplitude-weighted average ET lifetime is 69 ± 2 ps, and ~90% of ultrafast TA signal is assigned to ET processes. Interestingly, under high fluence, sufficient to create on average almost 2 excitons per QD, the relative fraction of TA signal due to ET remains well over 80%, and depopulation from exciton-exciton annihilation is minimal (6%). We also found that increasing QD size and/or shelling the core CdSe QDs with a shell of CdS slowed the ET rate, in agreement with the relative efficiency of photochemical H2 generation. The extremely fast ET provides a fundamental explanation for the exceptional photocatalytic H2 activity of the CdSe QD/Ni-DHLA system and guides new directions for further improvements.

We investigate ultrafast magnetization dynamics due to electron-phonon interaction in a ferromagnetic model sys- tem including spin-orbit coupling. By computing the reduced spin-density matrix and Boltzmann-type scattering integrals, we identify the microscopic mechanism with which the electronic spin is dissipated by electron-phonon scattering processes in ferromagnets. We present in some detail the numerical approach used to compute the scattering dynamics and discuss problems that arise in treating these dynamics.

Due to its high charge carrier mobility, broadband light absorption, and ultrafast carrier dynamics, graphene is a promising material for the development of high-performance photodetectors. Graphene-based photodetectors have been demonstrated to date using monolayer graphene operating in conjunction with either metals or semiconductors. Most graphene devices are fabricated on doped Si substrates with SiO₂ dielectric used for back gating. Here, we demonstrate photodetection in graphene field effect phototransistors fabricated on undoped semiconductor (SiC) substrates. The photodetection mechanism relies on the high sensitivity of the graphene conductivity to the local change in the electric field that can result from the photo-excited charge carriers produced in the back-gated semiconductor substrate. We also modeled the device and simulated its operation using the finite element method to validate the existence of the field-induced photoresponse mechanism and study its properties. Our graphene phototransistor possesses a room-temperature photoresponsivity as high as ~7.4 A/W, which is higher than the required photoresponsivity (1 A/W) in most practical applications. The light power-dependent photocurrent and photoresponsivity can be tuned by the source-drain bias voltage and back-gate voltage. Graphene phototransistors based on this simple and generic architecture can be fabricated by depositing graphene on a variety of undoped substrates, and are attractive for many applications in which photodetection or radiation detection is sought.

We search for efficient schemes of second and terahertz harmonic generation in nanocomposites consisted of metal-oxide semiconductor quantum dots incorporated into a dielectric matrix, when the quantum dots are in resonance and the dielectric matrix is out of resonance with femtosecond light pulse. It’s established that large efficiency of frequency up-conversion is possible to attain, which may be for the optimal quantum dot concentration in above mentioned nanocomposites by 70% higher than in pure nonlinear dielectric matrix.

Optical parametric oscillators (OPOs), pumped by Ti:sapphire and Yb-doped femtosecond lasers, provide unique capabilities to address a broad range of parameters of interest to precision spectroscopy. We review here a variety of OPOs under development that offer tuning from 1.5 to 13 μm, repetition rates from 100 MHz to 10 GHz and pulse durations from < 25 fs to a few picoseconds. Spectroscopic techniques revealing the individual frequency comb modes are discussed, along with dual-comb spectroscopy at 3 μm and from 6-8 μm.

Progress in the performance of high-power ultrafast lasers continues to give momentum to many fields of science and technology. Nowadays, ultrafast laser systems delivering hundreds of watts of average power with pulse energies ranging from hundreds of microjoules to hundreds of millijoules start to be even commercially available. In particular, disk lasers have consistently been at the forefront of this progress: their geometry is particularly well-suited for power and energy scaling of ultrashort pulses. Among these laser systems based on the disk technology, one particular technology is particularly attractive as a potential path to achieve the desired level from a simple, one-box, multi-MHz repetition rate oscillator: mode-locked thin-disk oscillators can reach hundreds of watts of average power with femtosecond pulses at multi-MHz repetition rate. Exponential progress in the achievable levels is only an illustration of their enormous potential. So far, these oscillators reach up to 275 W average power, and pulse energies up to 80 μJ, both based on Yb:YAG thin-disk lasers. This talk will review latest progress achieved with this technology and next steps and challenges towards further scaling, as well as their prospect as compact driving sources for the generation of high-power THz radiation.

High-order harmonic generation (HHG), resulting from the interaction of an intense laser field with an atomic or molecular gas, has been of great importance to the study of ultrafast dynamics for more than two decades. In the last several years, HHG has been observed in condensed matter systems driven by intense mid-infrared lasers. Investigations of HHG from solids can offer new capabilities for studying electronic structure and ultrafast carrier dynamics in photo-excited materials. However, HHG from solids is not yet well-understood, and even the generation mechanism cannot be uniquely determined in many systems. In this paper, we experimentally investigate HHG driven in solids by a high-power femtosecond optical parametric amplifier, producing mid-IR driving pulses with tunable central wavelength and >10 μJ pulse energy. We generate coherent high order harmonic radiation in ZnO and Si crystals, and characterize the dependence of the harmonic spectrum on the 3D crystal orientation. We further compress the driving pulse duration to below three optical cycles and investigate the resulting high-order harmonic spectrum. Moreover, we investigate the potential to generate harmonics in novel materials with the goal of probing the ultrafast dynamics arising from strong-field photo-excitation in such materials.

We have identified major paradigm shifts relative to near-IR filamentation when high power multiple terawatt laser pulses are propagated at mid-IR and long-IR wavelengths within key atmospheric transmission windows. Individual filaments at near-IR (800 nm) wavelengths typically persist only over tens of centimeters, despite the whole beam supporting them being sustained over about a Rayleigh range. In the important mid-IR atmospheric window (3.2 - 4 μm) optical carrier wave self-steepening (carrier shocks) tend to dominate and modify the onset of long range filaments. These shocks generate bursts of higher harmonic dispersive waves that constrain the intensity growth of the filament to well below the traditional ionization limit, making long range low loss propagation possible. For long wavelength pulses in the 8-12 μm atmospheric transmission window, many-electron dephasing collisions from separate gas species act to dynamically suppress the traditional Kerr self-focusing lens and leads to a new type of whole beam self-trapping over multiple Rayleigh ranges. This prediction is key, since strong linear diffraction at these wavelengths are the major limitation and normally requires large launch beam apertures. We will present simulation results that predict multiple Rayleigh range propagation paths for whole beam self-trapping and will also discuss some recent efforts to extend the HITRAN linear atmospheric transmission/refractive index database to include nonlinear responses of important atmospheric molecular constituents.

The effect of laser noise on the atmospheric propagation of high-power CW lasers and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. High-power CW laser systems have relatively large fractional levels of intensity noise and frequency noise. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. A paraxial wave equation, containing dispersion and nonlinear effects, is expanded in terms of fluctuations in the intensity and phase. Longitudinal and transverse intensity noise and frequency noise are considered. The laser propagation model includes group velocity dispersion, Kerr, delayed Raman response, and optical self-steepening effects. A set of coupled linearized equations are derived for the evolution of the laser intensity and frequency fluctuations. In certain limits these equations can be solved analytically. We find, for example, that in a dispersive medium, frequency noise can couple to and induce intensity noise, and vice versa. At high intensities the Kerr effect can reduce this intensity noise. In addition, significant spectral modification can occur if the initial intensity noise level is sufficiently high. Finally, our model is used to study the transverse and longitudinal modulational instabilities. We also present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm, 1 μm, and 10.6 μm.

Possible scenarios of high-intense vortex (and Gaussian) pulsed beam propagation in Kerr media and light bullet (LB) formation conditions are considered. The system of modified nonlinear Schroedinger equation for the complex envelope of the electric field and kinetic equation for the electron plasma density is exploited. Two-scale variational analysis is combined with direct numerical simulations based on finite-difference methods. Hamiltonian approach allows to reveal LB formation conditions. It is shown that the LB parameters correspond to minimum of potential energy when the whole balance of competing processes occurs. Numerical experiment confirms the results obtained on the base of variational analysis, demonstrating at the same time softer conditions for LB formation. It is emphasized that the linear and nonlinear dynamics of spatial and temporal radii obey the coupled oscillator theory.

In this paper, we present a model to account for the absorption of ultrafast laser pulse on a metallic tip and to calculate its induced hot carriers (or electrons) emitted from the surface over a wide range of operating condition from low (multiphoton absorption) to high (optical tunneling) laser field. The model self-consistently include the non-equilibrium heating of the electrons with time-dependent electron energy distribution and multiple-energy time-dependent tunneling process. A universal critical Keldysh parameter at the transition between the multiphoton absorption and optical tunneling regimes is obtained. The effect of the plasmonic enhancement at the tip at sub-10 fs laser pulses are also studied. It is found that at very short laser pulse (a few cycles), the classical photo-electric effect is not valid. For the plasmonic effect, there is a time delay between the plasmonic field and laser field, and thus the emission of electrons are lapsed than the laser field. The enhancement of electron emission due to the ultrafast laser induced plasmonic field at low field regime is also discussed.

High-average power, ultra-broadband, mid-IR radiation can be generated in a nonlinear medium by illuminating it with a multi-line laser radiation. Propagation of a multi-line CO₂ laser beam in a nonlinear medium, e.g. gallium arsenide or chalcogenide, will generate directed, broadband, IR radiation in the atmospheric window (2-13 μm). A 3-D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward propagating waves. In addition, we include transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed; in particular, grading the refractive index radially will tend to guide the CO₂ radiation and extend the interaction distance, allowing for enhanced spectral broadening. Finally, we show that the laser phase noise associated with the finite CO₂ linewidths can significantly enhance the spectral broadening. In a dispersive medium laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism.

Semiconductor lasers are a promising technology to make optical comb systems more accessible and cost-efficient. We stabilized the carrier-envelope offset (CEO) frequency of a semiconductor disk laser. The laser was modelocked by a SESAM and generates pulses at a wavelength of 1034 nm. It operates at a repetition frequency of 1.8 GHz. The 270-fs pulses are amplified to 3 W and compressed to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a highly nonlinear fiber. Using a standard f-to-2f interferometer, we detect the CEO beat with a signal-to-noise ratio of ~30 dB. By applying a feedback signal to the pump current, the CEO frequency is phase-locked to an external reference.

In this work we investigate the overall stability of tunable, broadband electro-optic frequency combs created in an optoelectronic oscillator filtered with an ultra-high finesse Fabry-Perot etalon. Fractional frequency stability measurements show Allan deviations as low as 3E-13 for the comb central frequency of 193THz, and as low as 5E-10 on the 10.5GHz repetition frequency. At timescales of τ = 0.1s, the total frequency fluctuation of the outermost comb lines is calculated to be on the order of 388Hz. Individual comb line amplitude stability is also investigated, and the average amplitude fluctuation amongst all comb lines within 10dB amplitude deviation is measured to be ± 0.22dB standard deviation.

Using dual optical frequency comb (OFC) spectroscopy in the longwave infrared (LWIR), we demonstrate standoff detection of trace amounts of target compounds on diffusely scattering surfaces. The OFC is based on quantum cascade lasers (QCL) that emit ~1 Watt of optical power under cw operation at room temperature over coherent comb bandwidths approaching 100 cm^-1. We overlap two nearly identical 1250 cm^-1 QCL OFC sources so that the two interfering optical combs create via heterodyne a single comb in the radio frequency (rf) that represents the entire optical spectrum in a single acquisition. In a laboratory scale demonstration we show detection of two spectrally distinct fluorinated silicone oils, poly(methyl-3,3,3-trifluoropropylsiloxane) and Krytox™, that act as LWIR simulants for security relevant compounds whose room temperature vapor pressure is too low to be detected in the gas phase. These target compounds are applied at mass loadings of 0.3 to 90 μg/cm² to sanded aluminum surfaces. Only the diffusely scattered light is collected by a primary collection optic and focused onto a high speed (0.5 GHz bandwidth) thermoelectrically cooled mercury cadmium telluride (MCT) detector. At standoff distances of both 0.3 and 1 meter, we demonstrate 3 μg/cm² and 1 μg/cm2 detection limits against poly(methyl-3,3,3-trifluoropropylsiloxane) and Krytox™, respectively.