This PDF file contains the front matter associated with SPIE Proceedings Volume 9835, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

We develop a computational approach for interaction between strong laser pulse and dielectrics based on time-dependent density functional theory (TDDFT). In this approach, a key ingredient is a solver to simulate electron dynamics in a unit cell of solids under a time-varying electric field that is a time-dependent extension of the static band calculation. This calculation can be regarded as a constitutive relation, providing macroscopic electric current for a given electric field applied to the medium. Combining the solver with Maxwell equations for electromagnetic fields of the laser pulse, we describe propagation of laser pulses in dielectrics without any empirical parameters. An important output from the coupled Maxwell+TDDFT simulation is the energy transfer from the laser pulse to electrons in the medium. We have found an abrupt increase of the energy transfer at certain laser intensity close to damage threshold. We also estimate damage threshold by comparing the transferred energy with melting and cohesive energies. It shows reasonable agreement with measurements.

Two-photon absorption, 2PA, in semiconductors is enhanced by two orders of magnitude due to intermediate-state resonance enhancement, ISRE, for very nondegenerate (ND) photon energies. Associated with this enhancement in loss is enhancement of the nonlinear refractive index, n2. Even larger enhancement of three-photon absorption is calculated and observed. These large nonlinearities have implications for applications including ND two-photon gain and twophoton semiconductor lasers. Calculations for enhancement of ND-2PA in quantum wells is also presented showing another order of magnitude increase in 2PA. Potential devices include room temperature gated infrared detectors for LIDAR and all-optical switches.

Two numerical approaches for determining the charge generated in semiconductors via two-photon absorption (2PA) under conditions relevant for laser-based single-event effects (SEE) experiments are presented. The first approach uses a simple analytical expression incorporating a small number of experimental/material parameters while the second approach employs a comprehensive beam propagation method that accounts for all the complex nonlinear optical (NLO) interactions present. The impact of the excitation conditions, device geometry, and specific NLO interactions on the resulting collected charge in silicon devices is also discussed. These approaches can provide value to the radiation-effects community by predicting the impacts that varying experimental parameters will have on 2PA SEE measurements.

We demonstrate a new possibility of a soliton velocity control at its propagation in a nonlinear layered structure (1D photonic crystal) which is placed in a nonlinear ambient medium. Nonlinear response of the ambient medium, as well as the PhC layers, is cubic. At the initial time moment, a soliton is spread over a few layers of PhC. Then, soliton propagates across the layered structure because of the initial wave-vector direction presence for the laser beam. The soliton reaches the PhC faces and reflects from them or passes through the face. As a nonlinear medium surrounds the PhC, the laser beam obtains additional impulse after interaction with this medium and accelerates (or slows down or stops near the PhC face). Nonlinear response of the ambient medium can be additionally created by another laser beam which shines near the PhC faces.

We use steady-state and time-resolved spectroscopy to evaluate optoelectronic material quality and obtain detailed information about carrier generation, transport, and relaxation in semiconductor devices and test structures. This report focuses on time-resolved and steady-state photoluminescence of III-V reference heterostructures at temperatures between 4K and 300K in order to investigate the mechanisms limiting carrier lifetime and to develop the capability to provide actionable feedback to research-and-development efforts for improvement and optimization of material properties and/or device performance. We combine the results of photoluminescence experiments with model-based analyses and simulations of carrier relaxation to assess the impacts of defects and interface quality on the relaxation dynamics of photo-generated carriers in double heterostructure test vehicles grown by MOCVD and MBE.

The bulk photovoltaic effect describes the generation of photocurrents and photovoltages in bulk materials lacking a center of symmetry. Its principal mechanism is “shift current,” a nonlinear-optical, inherently quantum mechanical process. While most attention has been devoted to its prospects as a means of solar energy capture, shift current bulk photovoltaic effect possesses a number of distinguishing features that make it well-suited to sensing and switching applications: photovoltages substantially exceeding the material’s band gap, response amplitudes and directions that can depend on both photon energy and polarization, and response that occurs on ultrafast timescales.

Ultrafast imaging has been a key enabler to many novel imaging modalities, including looking behind corners and imaging behind scattering layers. With picosecond time resolution and unconventional sensing geometries, ultrafast imaging can fundamentally impact sensing capabilities in industrial and biomedical applications. This paper reviews the fundamentals, recent advances, and the future prospects of ultrafast imaging-based modalities.

Ultrashort laser pulses represent an ideal starting point for frequency conversion of light to almost any wavelength from the THz to x-rays. High-harmonic upconversion (HHG) is a unique process enabled by the combined strong field laser field and the few-cycle pulse duration of a femtosecond laser pulse. HHG makes it possible to generate coherent light in the spectral region from the vacuum-UV into the x-ray region at sub-nm wavelengths. HHG sources are now finding increasingly diverse application for both science and technology, in topics ranging from basic studies of atomic processes, to materials dynamics revealed through time and angle-resolved photoemission. Furthermore, the coherent nature of the HHG process makes possible unprecedented control over light in a new region of the spectrum, making it possible to, for example, control the polarization state and spectral bandwidth, creating the most complex time-domain waveforms ever measured and characterized. Here we review recent work, as well as efforts at commercial implementation of HHG sources.

We report the investigation of the wave-mixing and amplification process with two multiple-cycle pulses with incommensurate frequencies (at 1400 nm and 800 nm). With a non-collinear configuration of the two beams, a different extreme ultraviolet mixing field can be created at low intensity of the 800 nm field. When a very high intensity 800 nm pulse is applied we are able to amplify the coherent extreme ultraviolet radiation in the photon energy range around 80 eV.

In this talk we give an overview of recent advances in the development of high power supercontinuum fiber lasers with powers exceeding 50W and spectral brightness of tens of mW/nm. We also discuss the fundamental limitations of power scaling and spectral broadening and review the existing and emerging applications of this unique light source which combines the broadband properties of a light bulb with the spatial properties of a laser.

This paper summarizes recent improvements of output characteristics of polycrystalline Cr:ZnS/Se master oscillators in Kerr-Lens-Mode-Locked regime: 1.9 W average power at 41 fs pulse duration, 24 nJ pulse energy and 515 kW peak power with efficiency of 19% with regards to 1567 nm pump power from linearly polarized Er-fiber laser. A simple design of mid-IR fs Cr:ZnS MOPA enabled power scaling to 6.8 W at 79 MHz repetition rate. This was accompanied by a 2 fold spectral broadening to 600 nm at -10 dB level, pulse compression from 44 to <30 fs, and overall 25 % optical to optical efficiency. Improved dispersion management of the resonator enabled pulse duration of Cr:ZnS master oscillator approaching 2 optical cycles (<26 fs) and 500 nm (27 THz) bandwidth of the spectrum at half-maximum. Further improvements of the optical coatings will result in octave-spanning polycrystalline Cr²⁺:ZnS/ZnSe lasers. In this work we also report on recent progress in spinning ring gain element technology and show new unprecedented output power levels for Cr:ZnSe laser gain media: ~140 W at 2400-2500 nm spectral range and ~32 W at 2940-2950 nm in CW regime of operation. High gain of the spinning ring Cr:ZnSe power amplifier demonstrated in this work may potentially enable scaling up the femtosecond mid-IR Cr:ZnS MOPA up to 70-100W.

Ultrafast laser sources are one of the main achievements of the past decades. Finding new avenues to obtain higher average powers and pulse energies from these sources is currently a topic of important research efforts both for scientific and industrial applications. SESAM modelocked thin-disk lasers are one of the most promising laser technology to reach this goal from table-top systems: recently, average powers of 275 W and pulse energies of 80 μJ were demonstrated directly from a modelocked oscillators without additional external amplification. In this presentation, we will review the current state-of-the art of such table-top systems and present guidelines for future kilowatt-class systems.

Ultra-short pulse lasers are dominated by solid-state technology, which typically operates in the near-infrared. Efforts to extend this technology to longer wavelengths are meeting with some success, but the trend remains that longer wavelengths correlate with greatly reduced power. The carbon dioxide (CO₂) laser is capable of delivering high energy, 10 micron wavelength pulses, but the gain structure makes operating in the ultra-short pulse regime difficult. The Naval Research Laboratory and Air Force Research Laboratory are developing a novel CO₂ laser designed to deliver ~1 Joule, ~1 picosecond pulses, from a compact gain volume (~2x2x80 cm). The design is based on injection seeding an unstable resonator, in order to achieve high energy extraction efficiency, and to take advantage of power broadening. The unstable resonator is seeded by a solid state front end, pumped by a custom built titanium sapphire laser matched to the CO₂ laser bandwidth. In order to access a broader range of mid infrared wavelengths using CO₂ lasers, one must consider nonlinear frequency multiplication, which is non-trivial due to the bandwidth of the 10 micron radiation.

Computer simulations of ultrafast optical pulses face multiple challenges. This requires one to construct a propagation model to reduce the Maxwell system so that it can be efficiently simulated at the temporal and spatial scales relevant to experiments. The second problem concerns the light-matter interactions, demanding novel approaches for gaseous and condensed media alike. As the nonlinear optics pushes into new regimes, the need to honor the first principles is ever greater, and requires striking a balance between computational complexity and physical fidelity of the model. With the emphasis on the dynamics in intense optical pulses, this paper discusses some recent developments and promising directions in the field of ultrashort pulse modeling.

We report on transient absorption experiments performed at high optical excitation fluences and used to study the ultrafast dynamics in graphene. We employed a degenerated scheme of pump and probe at 800 nm (1.55 eV). The time resolution of our measurements was limited by the pulse duration ~ 100 fs. The samples were prepared by chemical vapor deposition (CVD) as single-layers on silica and, then staked layer-by-layer in order to make a stack of up to 5 graphene monolayers. We observed saturable absorption (SA) and fluence-dependent relaxation times. We see that the ultrafast carrier dynamics is composed by two decay mechanisms, one with response time of about 200 fs and a slower process of about 1 ps. The fast decay, due to both carrier-carrier and carrier-optical phonon scattering, becomes slower when the density of excited carrier was increased. We implemented a theoretical model and found that both the optical phonon rate emission and the optical phonon lifetime are affect by the pump fluence.

Semiconductors that are atomically thin can exhibit novel optical properties beyond those encountered in the bulk compounds. Monolayer transition-metal dichalcogenides (TMDs) are leading examples of such semiconductors that possess remarkable optical properties. They obey unique selection rules where light with different circular polarization can be used for selective photoexcitation at two different valleys in the momentum space. These valleys constitute bandgaps that are normally locked in the same energy. Selectively varying their energies is of great interest for applications because it unlocks the potential to control valley degree of freedom, and offers a new promising way to carry information in next-generation valleytronics. In this proceeding paper, we show that the energy gaps at the two valleys can be shifted relative to each other by means of the optical Stark effect in a controllable valley-selective manner. We discuss the physics of the optical Stark effect, and we describe the mechanism that leads to its valleyselectivity in monolayer TMD tungsten disulfide (WS₂).

Ultrafast bandgap photonics in mid-infrared is an exciting area of nonlinear photonics, which shows different ultrafast damage characteristics of solids compared to that in near-infrared fields. It allows periodic surface nano-structures formation in low bandgap materials like germanium. Ultrafast mid-infrared field interaction at 2 micron wavelength with non-linear photonic crystal results in generation of high efficiency harmonic generation up to sixth harmonic.

Semiconductor microcavities offer a unique way to combine transient all-optical manipulation of GaAs quantum wells with the benefits of structural advantages of microcavities. In these systems, exciton-polaritons have dispersion relations with very small effective masses. This has enabled prominent effects, for example polaritonic Bose condensation, but it can also be exploited for the design of all-optical communication devices. The latter involves non-equilibrium phase transitions in the spatial arrangement of exciton-polaritons. We consider the case of optical pumping with normal incidence, yielding a spatially homogeneous distribution of exciton-polaritons in optical cavities containing the quantum wells. Exciton-exciton interactions can trigger instabilities if certain threshold behavior requirements are met. Such instabilities can lead, for example, to the spontaneous formation of hexagonal polariton lattices (corresponding to six-spot patterns in the far field), or to rolls (corresponding to two-spot far field patterns). The competition among these patterns can be controlled to a certain degree by applying control beams. In this paper, we summarize the theory of pattern formation and election in microcavities and illustrate the switching between patterns via simulation results.

We present a theoretical description of time-resolved photoemission in charge-density-wave insulators that derive their ordering from electron nesting effects. In these pump/probe experiments, a large amplitude (but short duration) pump pulse excites the system into nonequilibrium and then a higher frequency low amplitude probe pulse photoexcites electrons, which are measured at the detector. We describe effects of electron correlations on the photoelectron spectroscopy and provide details for the theoretical techniques used to solve these problems. We also show how the gap fills in as the system is excited, even though the order parameter does not go to zero. The theory is developed for the Falicov-Kimball model, which can be solved exactly with nonequilibrium dynamical mean-field theory.

Recent development of intense terahertz (THz) pulse generation technique has offered novel opportunities to reveal ultrafast phenomena in a variety of materials on tabletop experiments and provided a new pathway toward ultrafast control of quantum phases. Here we present our recent study of nonequilibrium dynamics in metallic superconductors NbN excited by intense THz pulse. Since the superconducting gap energy is located in the THz frequency range, the intense THz pulse excitation makes it possible to instantaneously excite high-density quasiparticles at the gap edge without injecting excess energies. It has also become possible to coherently drive the superconducting ground state without exciting incoherent quasiparticles by tuning the pump frequency below the gap energy. The ultrafast dynamics of the order parameter induced by such an intense low energy excitation is directly probed, and the nature of a collective excitation, namely the Higgs amplitude mode, is revealed. Efficient THz higher-harmonic generation from a superconductor is discovered, manifesting the nonlinear coupling between the THz wave and the Higgs mode. We also report the experimental results in a multi-gap superconductor MgB₂.

Many-body correlation effects in complex quantum systems often lead to phase transitions that bear great technological potential. However, the underlying microscopic driving mechanisms or even the quantum-mechanical properties of the novel ground state often remain elusive. Here we employ phase-locked ultrabroadband terahertz (THz) pulses to disentangle two coexisting orders in the charge density wave phase 1T-TiSe₂ via their individual non-equilibrium multi- THz dynamics. Furthermore, we demonstrate that few-cycle THz pulses can project out the matter part of a transient cold exciton-polariton condensate, providing novel insights into the very nature of this macroscopic quantum state.

New data on Bi₂Sr₂CaCu₂O_8+δ (Bi2212) reveal interesting aspects of photoexcited superconductors. The electrons dynamics show that inelastic scattering by nodal quasiparticles decreases when the temperature is lowered below the critical value of the superconducting phase transition. This drop of electronic dissipation is astonishingly robust and survives to photoexcitation densities much larger than the value sustained by long-range superconductivity. The unconventional behavior of quasiparticle scattering is ascribed to superconducting correlations extending on a length scale comparable to the inelastic mean-free path. Our measurements indicate that strongly driven superconductors enter in a regime without phase coherence but finite pairing amplitude.

This is a very brief discussion of some experimental and theoretical studies of materials responding to fast intense laser pulses, with emphasis on those cases where the electronic response and structural response are both potentially important (and ordinarily coupled). Examples are nonthermal insulator-to-metal transitions and light-induced superconductivity in cuprates, fullerenes, and an organic Mott insulator.

We use intense broadband THz pulses to excite the cuprate superconductors YBCO and NBCO in their underdoped phase, where superconducting and charge density wave ground states compete. We observe pronounced coherent oscillations at attributed to renormalized low-energy phonon modes. These oscillation features are much more prominent than those observed in all-optical pump-probe measurements, suggesting a different excitation mechanism.

From the discovery of sub-picosecond demagnetization over a decade ago [1] to the recent demonstration of magnetization reversal by a single 40 femtosecond laser pulse [2], the manipulation of spins by ultra-short laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation and quantum computation [3]. It was realized that the femtosecond laser induced all-optical switching (AOS) as observed in ferrimagnets exploits the laser induced strongly non-equilibrium dynamics and the antiferromagnetic exchange interaction between their sublattices [4-6]. This opens the way to engineer new magnetic materials for AOS [7,8], though for real applications nanoscale control of inhomogeneities appears to be relevant [9]. Besides the intruiging technological implications of these observations, they broadened remarkably the frontiers of our fundamental knowledge of magnetic phenomena. The laser driven out-of-equilibrium states cannot be described in term of the well-established thermodynamical approach, which is based on the concepts of equilibrium and adiabatic transformations. Theoretical efforts, although in their infancy, have already demonstrated [5,6] that light-induced spin dynamics on the (sub)-picosecond time scale results in phenomena utterly forbidden in a thermodynamical framework. Another challenge is how to bring the optical manipulation of magnetic media to the required nanoscale. This is clearly a key element for the perspectives in terms of magnetic recording. In addition, it would allow to explore a novel regime of spin dynamics, since the investigation of magnets on the femtosecond time-scale and the nanometer length-scale simultaneously is unexplored. One experimental approach which may be successful makes use of wave-shaping techniques [10]. Recent results with engineered hybrid magnetic materials and nanofocusing via a plasmonic antenna showed the practical potential of AOS: the magnetization of domains as small as 50 nm was repeatedly reversed by a single laser pulse [11]. The process was fully deterministic, implying that each laser pulse totally reversed the magnetization of the domain in a reproducible way. Employing antennas provided another significant benefit, by decreasing the threshold laser energy required for the AOS to occur.

Spin-lattice coupling is one of the most prominent interactions mediating response of spin ensemble to ultrafast optical excitation. Here we exploit optically generated coherent and incoherent phonons to drive coherent spin dynamics, i.e. precession, in thin films of magnetostrictive metal Galfenol. We demonstrate unambiguously that coherent phonons, also seen as dynamical strain generated due to picosecond lattice temperature raise, give raise to magnetic anisotropy changes of the optically excited magnetic _lm; and this contribution may be comparable to or even dominate over the contribution from the temperature increase itself, considered as incoherent phonons.

Ultrashort high intensity pulse creates extreme non-equilibrium condition in bandgap material producing dramatic perturbations in electronic structure that, in its turn, leading to changes in electronic, magnetic, and optical states of condensed matter. Interesting experimental results have recently been reported on transient phenomena ranging from ultrafast laser induced detection denial to light induced high temperature superconductivity. While single pulse interaction with bandgap material is well observed, explained and documented, one of the major problem is to maintain meta-stability of such matter states: stability of transient effects that may last well beyond thermalization time. The objective of this paper is to discuss approaches to meta-stability of transient states.