This PDF file contains the front matter associated with SPIE Proceedings Volume 8984 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Hyper Rayleigh Scattering (HRS) is used to determine the absolute first hyperpolarizability of gold nanorods with an aspect ratio of 2.2 and 2.7. Two different long axis lengths are used, namely 25.5 nm and 64 nm. This allows for a discussion of the size effect for these centrosymmetric nanoparticles. A comparison of the first hyperpolarizabilities obtained with that of spherical nanoparticles, also centrosymmetric particles, is then made to discuss the role of the shape of these particles on the origin of the response. For the smallest nanorods, a strong hyperpolarizability normalized per atom is determined underlining the role of the interface and the shape in determining such a large absolute value. For the larger nanorods, the first hyperpolarizability per atom is smaller than that of the smaller nanorods but remains larger than the one obtained for gold nanospheres with a similar volume indicating that the shape and the surface response still continue to play a major role. These results are in agreement with the multipolar theory for the first hyperpolarizability of centrosymmetric nanoparticles but show that for nanorods, the surface regime pertains over a longer size range.

Ultrafast all-optical modulation in Ag/HfO₂/Si/HfO₂/Ag metal-insulator-semiconductor-insulator-metal (MISIM) nanoring resonators through two-photon absorption photogenerated free-carriers is studied using self-consistent 3-D finite difference time domain (FDTD) simulations. The self-consistent FDTD simulations incorporate the two-photon absorption, free carrier absorption, and plasma dispersion effects in silicon. The nanorings are aperture coupled to Ag/HfO₂/Si(100nm)/HfO₂/Ag MISIM waveguides by 300nm wide and 50nm deep apertures. The effects of pump pulse energy, HfO₂ spacer thickness, and device footprint on the modulation characteristics are studied. Nanoring radius is varied between 540nm and 1μm, the HfO₂ spacer thickness is varied between 10nm and 20nm, and the pump pulse energy is explored up to 60pJ. Modulation amplitude, switching time, average generated carrier density, and wavelength resonant shift is studied for each of the device configurations. In a compact device footprint of only 1.4μm2, a 13.1dB modulation amplitude was obtained with a switching time of only 2ps using a modest pump pulse energy of 16.0pJ. The larger bandwidth associated with more compact nanorings and thinner spacer layers is shown to result in increased modulation amplitude.

Future nano-plasmonic devices will most likely be based on active plasmonics, relying on the interplay between the strong intrinsic optical nonlinearities of excitonic nanostructures and the ability of metallic nano-objects to concentrate electromagnetic fields locally. Consequently, the optical properties of hybrid nanostructures comprising active materials, e.g., semiconductors or J-aggregated molecules, and metals are currently attracting considerable attention. In favorable geometries, their properties are governed by a new class of short-lived quasiparticles, exciton - surface plasmon polaritons with hitherto unexplored nonequilibrium dynamics. Since the polariton dynamics in hybrid nanostructures occurs on very short timescale, ultrafast spectroscopy is an essential tool for the investigation of their properties. We demonstrate ultrafast coherent manipulation of the normal mode splitting in metal/molecular-aggregate nanostructures by real-time observation of Rabi oscillations between excitons and surface-plasmon-polaritons. Oscillations in exciton density on a 10-fs timescale control the Rabi splitting.

We propose and study the feasibility of a THz GaN/AlGaN quantum cascade laser (QCL) consisting of only five periods with confinement provided by a spoof surface plasmon (SSP) waveguide for room temperature operation. The QCL design takes advantages of the large optical phonon energy and the ultrafast phonon scattering in GaN that allow for engineering favorable laser state lifetimes, and the SSP waveguide provides the optical confinement for the ultrathin QCL. Our analysis has shown that the waveguide loss is sufficiently low for the QCL to reach its threshold at the injection current density around 6 kA/cm² at room temperature.

Due to the high attenuation in vitreous silica, acoustic attenuations in the THz regime are typically measured by incoherent techniques such as Raman, neutron, and X-ray scattering. Here, we utilized multiple-quantum-well structures to demonstrate acoustic spectroscopy of vitreous silica up to THz regime. The acoustic properties of silica thin films prepared by chemical deposition methods were characterized in the sub-THz regime. This technique may be useful in resolving debated issues relating to Boson peak around 1 THz.

In this paper we present a numerical study of terahertz pulses interacting with crystals of cesium iodide. We model the molecular dynamics of the cesium iodide crystals with the Density Functional Theory software CASTEP, where ultrafast terahertz pulses are implemented to the CASTEP software to interact with molecular crystals. We investigate the molecular dynamics of cesium iodide crystals when interacting with realistic terahertz pulses of field strengths from 0 to 50 MV/cm. We find nonlinearities in the response of the CsI crystals at field strengths higher than 10 MV/cm.

In this paper, we theoretically study optical responses of tightly-coupled terahertz metamaterial. Unlike loosely-coupled one, tightly-coupled metamaterials exhibit resonant optical responses that are highly dependent on the interaction between each unit resonators. In order to predict optical response and spectral behavior of effective parameters of the metamaterials, a model based on coupled-mode theory is applied. Resonant optical response is shown to be highly dependent on the gap-size between the resonators which relates to the inter-unit coupling, and we show that this dependency implies the possibility of miniaturized metamaterials .

Presented here are nanoplasmonic Au/SiO₂/Si metal-insulator-semiconductor waveguides and resonators capable of broadband operation and with the potential for monolithic integration with complementary metal-oxide-semiconductor technology. Bragg reflector resonators, Bragg mirrors, and disk resonators are designed, fabricated, and experimentally characterized. With a compact device footprint as small as 1.5μm2 and quality factors as high as 64.4 at λ=1.545μm, the Bragg reflector resonators have one of the highest quality factor over device footprint figure of merit yet demonstrated for a silicon nanoplasmonic device.

Graphene and carbon nanotubes provide a variety of new opportunities for fundamental and applied research. Here, we describe results of our recent terahertz and ultrafast studies of carriers and phonons in these materials. Time-domain terahertz spectroscopy is a powerful method for determining the basic properties of charge carriers in a non-contact manner. We show how one can modulate the transmission of terahertz waves through graphene by gating and how one can improve the modulation performance by combining graphene with apertures and gratings. In carbon nanotubes, we demonstrate that the terahertz response is dominated by plasmon oscillations, which are enhanced by collective antenna effects when the nanotubes are aligned. Finally, ultrafast spectroscopy of carbon nanotubes allow us to excite and probe coherent phonons, both in the low-energy radial breathing mode and high-energy G-mode, which are strongly coupled with excitonic interband transitions.

We present a microscopic study on the pump fluence dependent carrier relaxation dynamics in optically excited graphene. Our investigation focuses on the particular role of efficient Auger processes on the carrier dynamics in different excitation regimes. It turns out that for low fluence excitations, the carrier generating impact excitation prevails over the Auger recombination resulting in a significant multiplication of optically excited carriers. In contrast, for strong excitations, we show the occurrence of a transient population inversion that quickly decays due to the predominant process of Auger recombination.

Optical multidimensional coherent spectroscopy is a powerful tool for studying structure and dynamics in complex systems, such as semiconductors. In optical two-dimensional coherent spectroscopy (2DCS), where the spectrum is presented in a two-dimensional (2D) plane with two frequency axes, an important advantage is the ability to isolate quantum pathways by unfolding a one-dimensional spectrum onto a 2D plane. For many systems, however, the quantum pathways are only partially separated in a 2D spectrum. In order to completely isolate the quantum pathways, we extend 2DCS into a third dimension to generate three-dimensional (3D) spectra in which the spectrum is further unfolded. A 3D spectrum provides complete and well-isolated information of the third-order optical response of the system. The information can be used to fully characterize the quantum pathways and to determine the system’s Hamiltonian. Quantitative knowledge of the Hamiltonian enables prediction and control of quantum processes. For instance, such information is essential for deterministic control and improved performance of coherent control schemes.

We consider Cherenkov-type radiation from inhomogeneous periodic resonant medium excited by an ultrashort pulses of light. It is shown that if the velocity of the propagating excitation is greater (lower) then the velocity of light in vacuum c, a new Doppler-like frequency appears in the spectrum of the medium response. This frequency depends on the medium density distribution and on the observation angle. Possible applications of the effect are discussed.

Ultrafast optical microscopy (UOM) combines a typical optical microscope and femtosecond (fs) lasers that produce high intensity, ultrashort pulses at high repetition rates over a broad wavelength range. This enables us new types of imaging modalities, including scanning optical pump-probe microscopy, which varies the pump and probe positions relatively on the sample and ultrafast optical wide field microscopy, which is capable of rapidly acquiring wide field images at different time delays, that is measurable nearly any sample in a non-contact manner with high spatial and temporal resolution simultaneously. We directly tracked carriers in space and time throughout a NW by varying the focused position of a strong optical “pump” pulse along the Si core-shell nanowires (NWs) axis while probing the resulting changes in carrier density with a weaker “probe” pulse at one end of the NW. The resulting time-dependent dynamics reveals the influence of oxide layer encapsulation on surface state passivation in core-shell NWs, as well as the presence of strong acoustic phonon oscillations, observed here for the first time in single NWs. Time-resolved wide field images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon nanowire after optical excitation are also captured in real time using a two dimensional smart pixel array detector. Our experiments enable us to extract several fundamental parameters in these samples, including the diffusion current, surface recombination velocity, diffusion coefficients, and diffusion velocities, without the influence of contacts.

Anti-reflection(AR), a well-known technique of reducing unwanted reflections by applying an impedance matching layer, works for a specific wavelength and require the coating layer to be a quarter wavelength thick. A broadband operation of AR, however, is not fully understood except for the trial and error method. Here, we present a systematic analytic method of AR without the restriction of wavelength or thickness, i.e. achieving a perfect AR. Specifically, we find analytic permittivity and permeability profiles that remove any given impedance mismatch at the interface between two different dielectrics in a frequency independent way. Ultra-thin AR coating is also shown to be possible and confirmed experimentally with the l/25-wavelength thick AR coating layer made of metamaterials. We apply the concept of ultrathin double layer AR to the transparent conducting electrode, which we demonstrate by fabricating a low reflective dielectric/metal-layered electrode that provides significant electrical conductivity and light transparency.

Dynamics of exciton-polariton multistability is theoretically investigated. Phase portraits are used as a tool to enlighten the microscopic phenomena which influence spin multistability of a confined polariton field as well as ultrafast reversible spin switching. The formation of a non-radiative reservoir, due to polariton pairing into biexcitons is found to play the lead role in the previously reported spin switching experiments. Ways to tailor this reservoir formation are discussed in order to obtain optimal spin switching reliability.

Transverse patterns in polariton fluids were recently studied as promising candidates for all-optical low-intensity switching. Here, we demonstrate these patterns in a specifically designed double-cavity system. We theoretically and experimentally analyse their formation and optical control. Our detailed theoretical analysis of the coupled nonlinear dynamics of the optical fields inside the double-cavity and the excitonic excitations inside the embedded semiconductor quantum wells is firmly based on a microscopic many-particle theory. Our calculations in the time domain enable us to study both the ultrafast transient dynamics of the patterns and their steady-state behavior under stationary excitation conditions. The patterns we report and analyze go beyond what can be observed and understood in a simple scalar quantum field. We find that polarization-selective excitation of the polaritons leads to a complex interplay between longitudinal-transverse splitting of the cavity modes and the spin-dependent interactions of the polaritons' excitonic component.

Nanotransistors offer great prospect for the development of innovative THz detectors based on the non-linearity of transport characteristics. Semiconductor nanowires are appealing for their one-dimensional nature and intrinsically low capacitance of the devices, while graphene, with its record-high room-temperature mobility, has the potential to exploit plasma wave resonances in the transistor channel to achieve high-responsivity and tuneable detection. First graphene detectors have been recently demonstrated in both monolayer and bilayer field effect devices performances already suitable for first imaging application. Here will discuss the physics and technology of these devices, their operation, as well as first examples of imaging applications.

We detail a new ultrafast scanning tunneling microscopy technique called THz-STM that uses terahertz (THz) pulses coupled to the tip of a scanning tunneling microscope (STM) to directly modulate the STM bias voltage over subpicosecond time scales [1]. In doing so, THz-STM achieves ultrafast time resolution via a mode complementary to normal STM operation, thus providing a general ultrafast probe for stroboscopic pump-probe measurements. We use THz-STM to image ultrafast carrier trapping into a single InAs nanodot and demonstrate simultaneous nanometer (2 nm) spatial resolution and subpicosecond (500 fs) temporal resolution in ambient conditions. Extending THz-STM to vacuum and low temperature operation has the potential to enable studies of a wide variety of subpicosecond dynamics on materials with atomic resolution.

We have recently developed an ultrafast terahertz-pulse-coupled scanning tunneling microscope (THz-STM) that can image nanoscale dynamics with simultaneous 0.5 ps temporal resolution and 2 nm spatial resolution under ambient conditions. Broadband THz pulses that are focused onto the metallic tip of an STM induce sub-picosecond voltage transients across the STM junction, producing a rectified current signal due to the nonlinear tunnel junction currentvoltage (I-V) relationship. We use the Simmons model to simulate a tunnel junction I-V curve whereby a THz pulse induces an ultrafast voltage transient, generating milliamp-level rectified currents over sub-picosecond timescales. The nature of the ultrafast field emission tunneling regime achieved in the THz-STM is discussed.

We discuss two schemes of ultrafast THz imaging, both constituting non-perturbative response of either gas or solidstate media to the THz bias fields and thus offering very sensitive detection of the latter. In the first approach, we utilize air-breakdown plasma for space-time mapping of the THz field. In the second approach, we THz-induce strong electroabsorption response in the multiple quantum-well sample of thickness much smaller than the wavelength of the THz bias. As such, ultrabroadband imaging of the quasi single-cycle THz pulses can be possible.

When measuring optical material parameters with terahertz spectroscopy the accuracy of the material parameters measured depends strongly on knowing and supplying the precise sample thickness when processing the raw terahertz data. It turns out that the terahertz data itself typically does contain accurate information as to the sample thickness. In this study we determine the accuracy with which the exact thickness of nominally 500 μm silicon wafers can be measured using terahertz time domain spectroscopy (THz-TDS). We analyze and compare the resolution of 5 different approaches for determining sample thickness using THz-TDS data including three methods previously proposed in the literature, one novel approach and a refined implementation of an existing approach. The quantitative results and analyses of methods we present will be useful in developing far-infrared optical metrology. Conversely the quantitative results presented in this study can be used to relate uncertainty in sample thickness to uncertainty in the measured terahertz data both in the time domain and frequency domain (phase and amplitude).

Terahertz (THz) electromagnetic radiation is located between the realms of electronics and optics and has successfully been used to probe and even control numerous low-energy excitations including phonons, excitons and Cooper pairs. Here, we show that THz spectroscopy is also a highly useful tool in the field of ultrafast spinbased electronics (spintronics) and consider the resonant manipulation of the magnetization of an antiferromagnet (through the THz Zeeman torque) and the probing of the tailored transport of spin density from a ferromagnetic into a nonmagnetic metal (through the THz inverse spin Hall effect).

The technological demand to push the gigahertz switching speed limit of today’s magnetic memory/logic devices into the terahertz (1THz=1ps⁻¹) regime underlies the entire field of spin–electronics and integrated multi- functional devices. This challenge is met by all–optical magnetic switching based on coherent spin manipulation By analogy to femto–chemistry and photosynthetic dynamics where photo-products of chemical/biochemical re- actions can be influenced by creating suitable superpositions of molecular states, femtosecond (fs) laser–excited coherence between spin/orbital/charge states can switch magnetic orders, by “suddenly” breaking the delicate balance between competing phases of correlated materials, e.g., the colossal magneto–resistive (CMR) manganites suitable for applications. Here we discuss femtosecond (fs) all-optical switching from antiferro- to ferromagnetic ordering via establishment of a magnetization increase within ∼100 fs, while the laser field still interacts with the system. Such non-equilibrium ferromagnetic correlations arise from quantum spin–flip fluctuations corre- lated with coherent superpositions of electronic states. The development of ferromagnetic correlations during the fs laser pulse reveals an initial quantum coherent regime of magnetism, clearly distinguished from the pi- cosecond lattice-heating regime characterized by phase separation. We summarize a microscopic theory based on density matrix equations of motion for composite fermion Hubbard operators, instead of bare electrons, that take into account the strong spin and charge local correlations. Our work merges two fields, femto-magnetism in metals/band insulators and non–equilibrium phase transitions of strongly correlated electrons, where local interactions exceeding the kinetic energy produce a complex balance of competing orders.

Recent experiments have revealed the possibility to optically orient both electron and hole spins in bulk germanium. Here we discuss the wavelength dependence of this spin injection process using time-resolved Faraday rotation. Significant hole spin polarization is found only when addressing indirect optical transitions. In contrast, electron spins can be oriented via both direct and indirect optical transitions and even with excess energies much larger than the spin-orbit coupling energy. For photon energies very close to the indirect bandgap, we find indications that the degree of electron spin polarization is significantly enhanced - a trend in line with theoretical predictions.

Light beams carrying an isolated point singularity with a screw-type phase distribution are called an optical vortex (OV). The fact that in free space the Poynting vector of the beam gives the momentum flow leads to an orbital angular momentum (OAM) of the photons in such a singular beam, independent on the spin angular momentun¹. There are many applications of optical OAM shown in literature that would benefit from the availability of optical vortex beams in all spectral regions. For example it was shown that transitions forbidden by selection rules in dipole approximation appear allowed when using photons with the additional degree of freedom of optical OAM². However, the common techniques of producing new light frequencies by nonlinear optical processes seem problematic in conserving the optical vortex when the nonlinearity becomes large. We show that with the extremely nonlinear process of High Harmonic Generation (HHG) it is possible to transfer OVs from the near-infrared to the extreme ultraviolet (XUV)³ at wavelengths down to ~30 nm. The observed XUV light was examined spatially and spectrally. The spatial profile showed the expected singular behavior, a dark region in the center. A comparison of the far-field fringe pattern caused by a thin wire with corresponding simulations suggests that the XUV vortex beam carries a unit topological charge. A screw-like phase evolution around the profile was also verified by employing a Hartmann type measurement. The generated spectrum revealed that in all Harmonic orders an OV was present. The profile, however, looked the same in all orders, indicating identical topological charge, which runs counterintuitive to the assumption that the phase of exp(–ilφ) is multiplied by the harmonic order in a frequency up-conversion experiment.

We present technical and experimental advances for performing HHG experiments in a range of substituted benzene molecules starting in the liquid phase. We demonstrate sub 3% fluctutaions in the harmonic signal generated in a benzene vapour backed continuous flow gas jet using a mid-IR laser source. The longer drive wavelength is necessary as the target molecules have low ionization potential and exhibit femtosecond timescale dynamics. We present the acquisition of stable and reproducible harmonic spectra from a range of substituted benzene molecules and the dependence of HHG upon the ellipticity of the laser beam was measured for a number of molecules.

This investigation was conducted to study the tunability of the filamentation process using a near infrared laser source at wavelength ranging from 1.6 μm to 2 μm. A Krypton cell filled statically with 4 bar was employed as filamentation medium. A spectral broadening via filamentation was observed over the whole range of wavelengths employed and achieved broadening factor of 2-3. In best experimental conditions about 300nm of bandwidth where generated. The accumulated group velocity dispersion in the filament was compensated by fused silica since the dispersion of fused silica is negative at these wavelengths. It was possible to compress the pulses down to the few-cycle regime with 2-3 cycles for 1.7 μm, 1.8 μm and 1.9 μm. Theses pulses contained about 200 μJ pulse energy. Understanding the phenomenon of filamentaion at theses wavelengths bares significant potential for strong field physics applications such as attosecond science where longer wavelength and few-cycle pulses are of great advantage. As a proof of principle we used these few-cycle pulses to generate high harmonic spectra in several gases such as xenon, krypton, argon and nitrogen.

Optical and infrared antennas provide a promising way to couple photons in and out of nanoscale structures. As counterpart to conventional radio antennas, they are able to increase optical fields in sub-wavelength volumes, to enhance excitation and emission of quantum emitters or to direct light, radiated by quantum emitters. The directed emission of these antennas has been mainly pursued by surface plasmon based devices, e.g. Yagi-Uda like antennas, which are rather complicated due to the coupling of several metallic particles. Also, like all metallic structures in optical or infrared regime, these devices are very sensitive to fabrication tolerances and are affected by strong losses. It has been shown recently, that such directed emission can be accomplished by dielectric materials as well. In this paper we present an optimization of nanoscopic antennas in the near infrared regime starting from a metallic Yagi-Uda structure. The optimization is done via a particle-swarm algorithm, using full time domain finite integration simulations to obtain the characteristics of the investigated structure, also taking into account substrates. Furthermore we present a dielectric antenna, which performs even better, due to the lack of losses by an appropriate choice of the dielectric material. These antennas are robust concerning fabrication tolerances and can be realized with different materials for both the antenna and the substrate, without using high index materials.

Near-infrared (NIR) dyes which absorb and emit light within the range from 700 to 900 nm have several benefits in biological studies. However, because of short fluorescence lifetime in picosecond range, no significant efforts have been made to recognize the theory of these dyes in time-resolved polarization dynamics. For the first time, a set of first-order linear differential equations was developed to model fluorescence polarization dynamics of NIR dye in picoseconds range and verify it by the ultrafast spectra obtained by streak camera.