This PDF file contains the front matter associated with SPIE Proceedings Volume 8623, including the Title Page, Copyright Information, Table of Contents, Introduction and Conference Committee listing.

The ultrafast dynamics of water and aqueous solutions are probed by the optical Kerr effect (OKE). The translational origin of water dynamics observed by the OKE is described. The ultrafast response can be described by H-bond stretching and bending modes. These modes are persistent to unexpectedly high solute concentrations, where most water molecules are in a solvation shell. In some cases this suggests the formation of water clusters. All solutes slow the dynamics observed by OKE. Hydrophilic solutes give rise to a larger retardation of water dynamics than hydrophobic ones. These trends persist in molecular solutes, peptides and proteins.

We describe the use of a range of modern spectroscopic techniques—from terahertz time-domain spectroscopy (THz- TDS) to high dynamic-range femtosecond optical Kerr-effect (OKE) spectroscopy—to study the interaction of proteins, peptides, and other biomolecules with the aqueous solvent. Chemical reactivity in proteins requires fast picosecond fluctuations to reach the transition state, to dissipate energy, and (possibly) to reduce the width and height of energy barriers along the reaction coordinate. Such motions are linked with the structure and dynamics of the aqueous solvent making hydration critical to function. These dynamics take place over a huge range of timescales: from the nanosecond timescale of diffusion of water molecules in the first solvation shell of proteins, picosecond motions of amino-acid side chains, and sub-picosecond librational and phonon-like motions of water. It is shown that a large range of frequencies from MHz to THz is accessible directly using OKE resulting in the reduced anisotropic Raman spectrum and by using a combination of techniques including THz-TDS resulting in the dielectric spectrum. Using these techniques, we can now observe very significant differences in the spectra of proteins in aqueous solvent in the 3-30 THz range and more subtle differences at lower frequencies (10 GHz-3 THz).

Using Terahertz near field microscopy we find orientation dependent narrow band absorption features for lysozyme crystals. Here we discuss identification of protein collective modes associated with the observed features. Using normal mode calculations we find good agreement with several of the measured features, suggesting that the modes arise from internal molecular motions and not crystal phonons. Such internal modes have been associated with protein function.

We studied low-frequency spectra of hydration water molecules around the hydrophobic probe in an aqueous solution by using tetraalkylammonium cation as a probe and terahertz time-domain spectroscopic technique. The phenomenon, called dynamical transition, has been known to be universally observed among proteins and polypeptides. In this work we investigated temperature and hydration dependence of low-frequency dynamics to clarify relationships between the dynamical transition and protein structures, and its functional states. We also mention general behaviors of the lowfrequency spectra of globular proteins.

Ultrashort pulses in the terahertz (THz) spectral range allow us to study and control spin dynamics on time scales faster than a single oscillation cycle of light. In a first set of experiments, we harness an optically triggered coherent lattice vibration to induce a transient spin-density wave in BaFe₂As₂, the parent compound of pnictide superconductors. The time-dependent multi-THz response of the non-equilibrium phases shows that the ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. The results suggest important implications for unconventional superconductivity. In a second step, we utilize the magnetic field component of intense THz transients to directly switch on and off coherent spin waves in the antiferromagnetic nickel oxide NiO. A femtosecond optical probe traces the magnetic dynamics in the time domain and verifies that the THz field addresses spins selectively via Zeeman interaction. This concept provides a universal ultrafast handle on magnetic excitations in the electronic ground state.

We discuss the mid-infrared optical response of a charge and spin-ordered nickelate in the ultrafast time domain. A strong photo-induced modulation of the optical reflectivity is observed on the sub-picosecond timescale, indicating the transient filling and recovery of the pseudogap in the mid-infrared charge transport. A variational Kramers-Kronig analysis of equilibrium reflectivity data is extended to time-resolved experiments, allowing us to extract the optical conductivity despite a comparatively limited frequency range of tunable femtosecond parametric sources. The fast dynamics of the spectral weight transfer supports an electronic origin of the mid-infrared pseudogap in nickelates.

We report experimental results on the electron spin relaxation length during vertical transport in spin lightemitting diodes (LEDs). Our devices are GaAs based LEDs with InAs quantum dots in the active region, an MgO tunnel barrier and an Fe/Tb multilayer spin injector with perpendicular magnetic anisotropy, i.e. remanent out-of-plane magnetization, enabling efficient electrical spin injection in magnetic remanence. Additionally, our devices can be operated at room temperature. A series of samples with different injection path lengths allows us to experimentally determine the spin relaxation length in our devices. In combination with operation in magnetic remanence, we are able to determine the spin relaxation length without the influence of external magnetic fields and at room temperature and find it to be 27 nm. Applying an additional external magnetic field, we find that at a field strength of 2 T, this relaxation length almost doubles, which is in good agreement with spin relaxation times in GaAs. Temperature control of our samples allows us to measure the temperature dependence of the spin relaxation length. At 200 K, the spin relaxation length doubles to 50 nm and reaches 80 nm at 30 K, in good agreement with theoretic calculations. Our results show that polarization values obtained with spin-LEDs inside strong magnetic fields and at low temperatures are not comparable to those in remanence and at room temperature. However, the transfer of efficient spintronic devices to such applicationenabling settings is absolutely necessary and will be a major challenge considering the enormous differences in spin relaxation.

We present a systematic study of electron spin relaxation in wurtzite GaN. Fast relaxation is caused by a Rashba effective magnetic field that linearly depends on the electron momentum k. The field prevents spin lifetimes to exceed 50 ps at room temperature and is the origin of an anisotropic spin relaxation tensor that we evidence by magnetic field dependent magneto-optical pump-probe measurements. In addition, the spin lifetime depends - as compared to GaAs - weaker on temperature and doping density. We give a fully analytical description of both effects based on D'yakonov-Perel' theory that describes our results quantitatively without any fitting parameter.

Ultrafast dynamics of carriers and phonons in topological insulators Cu_xBi₂Se_3-y (x=0, 0.1, 0.125, y=0, 1) was studied using femtosecond optical pump-probe spectroscopy. One damped oscillation was clearly observed in the transient reflectivity changes (ΔR/R), which is assigned to the coherent optical phonon (A_1g ¹). According to the red shift of A_1g¹ phonon frequency, the Cu atoms in Cu_xBi₂Se₃ crystals may predominantly intercalated between pair of the quintuple layers. Moreover, the carrier dynamics in the Dirac-cone surface state is significantly different from that in bulk state, which was investigated using optical pump mid-infrared (mid-IR) probe spectroscopy. The rising time and decay time of the negative component in ΔR/R, which is assigned to carrier relaxation in Dirac cone, is 1.62 ps and 20.5 ps, respectively.

Emitters based on semiconductor quantum dots are promising sources for both single photons and pairs of polarization-entangled photons. Typically, single photons are generated through a photon emission event bringing the electronic system from a single-exciton state back to the ground state. In the case of entangled photons two photons are generated through a cascaded emission from the biexciton to either one of the single-exciton states and then back to the ground state. Alternatively, emission of two photons can also be achieved through a higher-order two-photon process bringing the quantum dot directly from the biexciton state (through a virtual intermediate state) back to the ground state. Here we discuss in particular how this two-photon process can lead to certain conceptual advantages in both the generation of polarization-entangled photon pairs and generation of single photons.

Previous experimental measurements and numerical simulations give evidence of strong electric and magnetic field interaction between split-ring resonators in dense arrays. One can expect that such interactions have an influence on the second harmonic generation. We apply the Discontinuous Galerkin Time Domain method and the hydrodynamic Maxwell-Vlasov model to simulate the linear and nonlinear optical response from SRR arrays. The simulations show that dense placement of the constituent building blocks appears not always optimal and collective effects can lead to a significant suppression of the near fields at the fundamental frequency and, consequently, to the decrease of the SHG intensity. We demonstrate also the great role of the symmetry degree of the array layout which results in the variation of the SHG efficiency in range of two orders of magnitude.

In this proceedings we describe our recent results on semiconductor nonlinear optics, investigated using single-cycle THz pulses. We demonstrate the nonlinear absorption and self-phase modulation of strong-field THz pulses in doped semiconductors, using n-GaAs as a model system. The THz nonlinearity in doped semiconductors originates from the near-instantaneous heating of free electrons in the ponderomotive potential created by electric field of the THz pulse, leading to ultrafast increase of electron effective mass by intervalley scattering. Modification of effective mass in turn leads to a decrease of plasma frequency in semiconductor and produces a substantial modification of THz-range material dielectric function, described by the Drude model. As a result, the nonlinearity of both absorption coefficient and refractive index of the semiconductor is observed. In particular we demonstrate the nonlinear THz pulse compression and broadening in n-GaAs, as well as an intriguing effect of coexisting positive and negative refractive index nonlinearity within the broad spectrum of a single-cycle THz pulse. Based on Drude analysis we demonstrate that the spectral position of zero index nonlinearity is determined by (but not equal to) the electron momentum relaxation rate. Single cycle pulses of light, irrespective of the frequency range to which they belong, inherently have an ultrabroadband spectrum covering many octaves of frequencies. Unlike the single-cycle pulses in optical domain, the THz pulses can be easily sampled with sub-cycle resolution using conventional femtosecond lasers. This makes the THz pulses accessible model tools for direct observation of general nonlinear optical phenomena occurring in the single-cycle regime.

We report the observation of strong terahertz-induced electroabsoption (EA) modulation in in multiple double quantum well (MDQW) structures Broadband terahertz generated by two-color laser induced plasma is focused (~1MV/cm) onto the MQW and spectro-temporal response is probed in transmission geometry, where up to 60% modulation signals are observed. EAS signal is attributed to several mechanisms, including observed qualitative agreement with the two-dimensional Franz-Keldysh response. Utilizing strong EA signals, we present a simple THz imaging scheme using conventional imagers.

The interaction of large-area single-layer CVD-graphene with a metasurface constituted by THz split-ring resonators was studied via THz Time-Domain Spectroscopy in the frequency range 250 GHz÷2.75 THz. Transmission measurements showed that the presence of the graphene shifts the resonances of the THz-metasurface towards lower energies and increases the transmittance, mainly at resonance. A comparison between two possible configuration is here presented revealing a much stronger interaction for the case of split-ring resonators evaporated directly onto the CVD-graphene layer with respect to the opposite configuration. From the recent literature the presented system is a good candidate for THz modulators with possible use also in cavity-QED experiments.

The spontaneous emission of a quantum emitter depends on its environment. This fundamental effect of quantum electrodynamics has become a cornerstone of nano-optics, with the objective to control light absorption and emission at the nanometer scale. At the heart of the effect lies the emitter-cavity coupling. An important figure of merit is the famous Q/V ratio introduced by Purcell in 1946 and largely used by the photonic-crystal community over the last decennia, with Q the quality factor of the cavity and V the mode volume. Here we revisit the classical problem of field coupling between quantum emitters and cavities to encompass the important case of metallic nanoresonators. We propose a generalized Purcell formula, which substantially differs from the classical one and which is capable of accurately handling cavity modes with strong radiative leakage, absorption and material dispersion. Fully-vectorial numerical calculations obtained for distinct nanocavity constructs representative of modern studies in nanophotonics provide a strong support to our theory.

Photoemission from nanostructures offers sub-wavelength field localization and enhancement. Excited by ultrashort pulses, electron emission can be confined and controlled in both time and space. Studies with metallic nanotips have examined the transition to strong-field conditions in photoemission. Reaching deeply into this regime with ultrashort mid-infrared pulses, we generate photoelectrons up to hundreds of electron volts and observe dynamics in which electrons are ejected from the field-enhanced region in less than an optical half cycle. Moreover, single cycle terahertz pulses are shown to yield a novel means of control over the photoemission process.

We discuss experiments that address the ultrafast dynamics inherent to the photoemission process in condensed matter. In our experimental approach, an extreme ultraviolet attosecond light pulse launches photoelectron wave packets inside a solid. The subsequent emission dynamics of these photoelectrons is probed with the light field of a phase-stabilized near-infrared laser pulse. This technique is capable of resolving subtle emission delays of only a few attoseconds between electron wave packets that are released from different energy levels of the crystal. For simple metals, we show that these time shifts may be interpreted as the real-time observation of photoelectrons propagating through the crystal lattice prior to their escape into vacuum. The impact of adsorbates on the observed emission dynamics is also investigated.

Attosecond physics, centering on the control of electronic matter waves within a single cycle of the optical laser’s driving field, has led to tremendously successful experiments with atoms and molecules in the gas phase. We show that pivotal phenomena such as elastic electron rescattering at the parent matter, a strong carrier-evenlope phase sensitivity and electronic matter wave intereference also show up in few-cycle laser driven electron emission from nanometric sharp metal tips. Furthermore, we utilize spectral signatures to measure the enhanced near-field with a spatial resolution of 1nm.

In our recent work, we have generated the shortest wavelengths from periodically-poled lithium niobate (PPLN) in the vicinity of 62.5 microns at the poling period of 7.1 microns. We have demonstrated large enhancements in the output powers from three gratings. For the long-poling periods, we have generated THz radiation with output wavelengths in the range of 126-1382 μm and output powers as high as 432 μW, corresponding to the photon conversion efficiency of 29%. We have also efficiently generated far-infrared radiation at the wavelengths centered at 20.8 microns in the vicinity of one of the polariton resonances of lithium niobate. Such an efficient nonlinear conversion is made possible by exploiting phase-matching for difference-frequency generation in lithium niobate. The highest peak power reached 233 W. These wavelengths correspond to the shortest wavelength in the terahertz region and longest wavelength in the mid-infrared/far-infrared regions from lithium niobate, respectively.

Nonlinear optics and, in general, the interaction of light with matter is usually studied in the context of a stationary medium that does not change in time or changes slowly on the time scale of the optical frequency cycle. However, there are a number of interesting effects that arise when the medium is allowed to change at very rapid rates. We first discuss the recent experimental discovery of a new kind of resonant radiation emission from laser pulse shock fronts and consider its main features in terms of a possible route to investigate light scattering from a relativistically moving scatterer. As a second example we consider the case of a periodically changing medium: such a situation bears a close resemblance to the dynamical Casimir effect.

We demonstrate simultaneous terahertz (THz) and high-order harmonic generation (HHG) in a gas plasma generated by co-polarized two-color excitation using femtosecond pulses centered at 800 nm and its second harmonic at 400 nm. We identify the strong correlation between the XUV and THz radiation under coherent control of the relative phase between the two-color excitation fields. Further investigation into this correlation is underway regarding various experimental parameters. These experiments will allow for the more fundamental understanding of the generation processes of XUV and THz radiation using coherently controlled hybrid fields.

The excitation of exciton resonances in semiconductors with certain polarization-shaped optical pulses reveals the existence of new photocurrents. For resonant excitation of excitons with an optical pulse whose linear polarization is slowly rotating versus time we observe an antisymmetric shift current linked to a second-order nonlinear tensor that is antisymmetric in its last two Cartesian indices. Moreover, for non-resonant excitation of excitons with an elliptically polarized optical pulse whose major and minor axis slowly decreases and increases in time and become minor and major axis, respectively, we observe an additional symmetric shift current being associated with a second-order nonlinear tensor that is symmetric in its last two Cartesian indices. Both of these currents vanish for continuous-wave excitation and, thus, differ from previously known shift currents.

We report the optical properties and exciton dynamics of undoped and hole-doped single-walled carbon nanotubes (SWCNTs). In the one-dimensional structures of SWCNTs, an electron and a hole form an exciton with a huge exciton binding energy. Stable excitons govern the optical responses of SWCNTs even at room temperature. With hole doping of SWCNTs, new peaks due to positive trions (positively charged excitons) appear below the E₁₁ exciton peaks in the absorption and photoluminescence spectra. Trions are also stable at room temperature. Using femtosecond pump-probe transient absorption spectroscopy, we revealed that the exciton decay dynamics depends on the number of holes in SWCNTs. The exciton lifetime of hole-doped SWCNTs is much shorter than that of undoped SWCNTs. Fast decay components with lifetimes of a few picoseconds are attributed to trion formation and exciton–hole scattering in holedoped SWCNTs.

In the present work plasmonic properties of metal nanocrystals, monocrystalline gold or silver nanocubes, nanorods, or nanocages deposited on planar substrates or on tilted fiber Bragg grating (TFBG) sensors have been fine tuned to enhance the performance of such novel sensing platforms. Superior refractive index sensitivities of nanocrystal/substrate or nanocrystal/TFBG have been observed and correlated with their plasmonic properties. Surface enhancement of Raman signal using nanocrystal coated TFBG was detected. The study proposes a novel fiber based sensing platform utilizing localized surface plasmon resonances.

We experimentally demonstrate ultrafast control over reciprocal light paths in random media. The combination of multiple scattering and coherence of light gives rise to strong interference contributions in light transport. An important interference correction to diffusion theory is the coherent backscattering effect, the constructive interference of reciprocal light paths in the backscattering direction. Our experiments show that the phase coherence between these paths can be suppressed by introducing dynamics faster than the photon dwell time. This adiabatic dephasing is of interest for its potential for controlling weak and strong localization and adiabatic storage and release of photonic information.

THz metamaterials have been shown to be a promising candidate for CQED experiments, i.e. Ultrastrong coupling has been demonstrated.¹ Modes of split ring resonators (SRRs) have been coupled via the AC-electric field to inter landau level transitions in two dimensional electron gases (2DEGs). SRR typically exhibit two distinct electromagnetic modes.² One mode consists of an electric field confined in the slit of the resonator, while current in the resonator ring stores the magnetic field. This mode is often called LC-mode, in analogy to the lumped circuit representation, and has a nearly Lorentzian line shape. The other mode is a dipole mode localized physically on the resonator edges. The line shape of this resonance is more sensitive to the actual resonator geometry. We studied extensively the influence of the resonator geometry on the coupling strength. The resonances studied span the frequency range from 250 GHz to 1.3 THz for the LC type and 0.8 THz to 2.5 THz for the dipole resonance. The number and position of gaps in the SRR has been varied, putting them both in series as well as in parallel. These modifications influence the mode volume of the LC-mode which changes the coupling strength in two opposite ways. The electric field magnitude increases and thus the coupling. However, the number of participating electrons in the 2DEG is reduced which reduces the √N coupling enhancement. The largest coupling we measured so far is =Ω/ω₀ = 0.58.¹

Surface plasmon polaritons have become a contender for next-generation optical computing with their superior subwavelength modal confinement and nonlinearity over conventional photonics. Gap plasmon waveguides, also known as metal-insulator-metal (MIM) waveguides, have been shown to have one of the best balances in the inherent trade-off between confinement (<200nm) and propagation length (several microns) among plasmonic waveguide geometries. There is great interest in introducing gain into these plasmonic systems to compensate for their innate short propagation lengths. To this end, we present an electrically pumped Ag/HfO₂/In_0.485Ga_0.515As/HfO₂/Ag metal-insulatorsemiconductor-insulator-metal (MISIM) amplifier design for loss-compensation in nanoplasmonic interconnects at thetelecommunication wavelength of 1.55 μm. Finite difference time domain simulations utilizing the full rate equations were used to study the signal gain experienced upon transmission through the device. The direct bandgap semiconductorgain medium In_0.485Ga_0.515As was modeled as a four level laser system with homogeneous broadening. The effect of varying critical amplifier dimensions, namely the HfO₂ spacer layer thickness and the width of the In_0.485Ga_0.515As core, on the amplifier’s performance was studied. A 3 μm long linear amplifier is shown to be capable of restoring a 500 GHz, 500 fs FWHM pulse train after 150 μm of propagation through a nanoplasmonic interconnect network without significant pulse distortion at a pump current density of 36.6 kA/cm^2,or 1 mA total current. This pump current is shown to cause acceptable levels of device heating. A periodic arrangement of such devices could therefore be used to indefinitely increase the effective propagation length of a signal.

In this paper, we present our recent measurement of second harmonic generation (SHG) from silicon nanowires which are vertically aligned. The SHG shows a great enhancement due to the increase of the surface area which breaks the symmetry of silicon lattice and increase the surface SHG. A high SHG is also obtained in counter polarization for both S and P polarization excitation. An enhancement of 80 times is also observed. This huge enhancement opens the door for novel applications including frequency mixing and frequency generation for various novel nonlinear application of silicon based devices.

Metal-insulator-metal plasmonic waveguides (plasmonic slot waveguides, PSW) are known to offer high propagation lengths and confinement factors and have recently been gaining increasing attention in the literature. We analytically study the interplay between group velocity dispersion and self-phase modulation on ultrafast surface plasmon-polariton (SPP) pulse-reshaping for plasmonic-slot waveguides with nonlinear dielectric core. The analytic investigation of the role of the core nonlinearity on pulse propagation has, to our knowledge, not been investigated in the literature. We correlate our analytical results with numerical FDTD simulations.

We have developed a new system to generate GHz repetition rate tunable pulses in the picosecond regime at any wavelength by using self-phase-locked stimulated Brillouin scattering (SBS). The phase-locked comb at any required wavelength is generated using a single length of fiber in a ring cavity, seeded by an amplified single frequency CW pump laser. We demonstrate a coherent phase relationship between multiple cascaded Stokes waves in the cavity, which directly leads to a highly stable pulse regime. This scheme does not require the matching of frequency components to the cavity, as the frequency components are generated by SBS and not oscillating modes. The coherent pulses in the time domain are in the order of ~10 ps. The nature of the fiber leads to a stable SBS frequency shift, which is directly correlated to the repetition frequency and which is in the order of tens of GHz. Since the process is governed by SBS, it is self-starting and has a linear dependence with temperature (1 MHz/°C), which could be used for fine adjustments. Such a laser is therefore suitable for high-speed optical clocks and optical communication system, amongst other applications. This system allows the ultra-short pulses to be generated at any wavelength by simply tuning the wavelength of the seed laser. The pump power allows the pulse width to be tuned in steps, by generating additional Stokes orders. The repetition rate is altered by the choice of fiber cavity or by the choice of the orders of the SBS shifted frequencies.

Detection of the electric field induced second harmonic (EFISH) signal has been used in gas plasma for measurement of the symmetry-breaking THz transients. Previously, detection linearity with the THz field was accomplished by either mixing EFISH signal with a second harmonic component of an octave-spanning plasma supercontinuum or by addition of a high voltage DC bias across the plasma. Here we report a new method where controlled injection of the second harmonic signal provides the necessary bias for the coherent signal detection. This is accomplished simply by insertion of the BBO crystal in an optical path. The absence of high intensity or high voltage makes the detection scheme more viable for remote sensing.