A novel scheme for ultrashort terahertz pulse generation over a gapless wide spectral range: Raman-resonance-enhanced four-wave mixing.

Ultrashort energetic terahertz (THz) pulses have created an exciting new area of research on light interactions with matter. For material studies in small laboratories, widely tunable femtosecond THz pulses with peak field strength close to MV cm-1 are desired. Currently, they can be largely acquired by optical rectification and difference frequency generation in crystals without inversion symmetry. We describe in this paper a novel scheme of THz pulse generation with no frequency tuning gap based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, particularly diamond. We show that we could generate highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles and carrier frequency tunable from 5 to >20 THz. They had a stable and controllable carrier-envelop phase and carried ~15 nJ energy per pulse at 10 THz (with a peak field strength of ~1 MV cm-1 at focus) from a 0.5-mm-thick diamond. The measured THz pulse characteristics agreed well with theoretical predictions. Other merits of the scheme are discussed, including the possibility of improving the THz output energy to a much higher level.

Elastic ice microfibers.

Ice is known to be a rigid and brittle crystal that fractures when deformed. We demonstrate that ice grown as single-crystal ice microfibers (IMFs) with diameters ranging from 10 micrometers to less than 800 nanometers is highly elastic. Under cryotemperature, we could reversibly bend the IMFs up to a maximum strain of 10.9%, which approaches the theoretical elastic limit. We also observed a pressure-induced phase transition of ice from Ih to II on the compressive side of sharply bent IMFs. The high optical quality allows for low-loss optical waveguiding and whispering-gallery-mode resonance in our IMFs. The discovery of these flexible ice fibers opens opportunities for exploring ice physics and ice-related technology on micro- and nanometer scales.

Nucleation and dissociation of methane clathrate embryo at the gas-water interface.

Among natural energy resources, methane clathrate has attracted tremendous attention because of its strong relevance to current energy and environment issues. Yet little is known about how the clathrate starts to nucleate and disintegrate at the molecular level, because such microscopic processes are difficult to probe experimentally. Using surface-specific sum-frequency vibrational spectroscopy, we have studied in situ the nucleation and disintegration of methane clathrate embryos at the methane-gas-water interface under high pressure and different temperatures. Before appearance of macroscopic methane clathrate, the interfacial structure undergoes 3 stages as temperature varies, namely, dissolution of methane molecules into water interface, formation of cage-like methane-water complexes, and appearance of microscopic methane clathrate, while the bulk water structure remains unchanged. We find spectral features associated with methane-water complexes emerging in the induction time. The complexes are present over a wide temperature window and act as nuclei for clathrate growth. Their existence in the melt of clathrates explains why melted clathrates can be more readily recrystallized at higher temperature, the so-called "memory effect." Our findings here on the nucleation mechanism of clathrates could provide guidance for rational control of formation and disintegration of clathrates.

Gate-Controlled Sum-Frequency Vibrational Spectroscopy for Probing Charged Oxide/Water Interfaces.

The rich chemistry of oxide/aqueous interfaces originates from the interfacial layer formed by surface charges and adjoining water molecules. Yet not much is clear about such layers, because they are difficult to access, and measurements unavoidably collect signals from the diffuse layer nearby, which is perturbed by the surface potential extending into the bulk water. Here we show that gating of a semiconductor/oxide/water junction can effectively vary the surface charge density at the oxide/water interface but keep the surface potential low and barely varying, allowing effective removal of the diffuse layer contribution. With sum-frequency vibrational spectroscopy on a silicon/silica/deionized-water model junction, the variation of the bonded layer water structure in response to surface charging can be readily detected. This new scheme is generally applicable to all oxide/water interfaces, providing opportunities for future investigations at a deeper molecular level.

Mapping Dynamical Magnetic Responses of Ultrathin Micron-Size Superconducting Films Using Nitrogen-Vacancy Centers in Diamond.

Two-dimensional superconductors have attracted growing interest because of their scientific novelty, structural tunability, and useful properties. Studies of their magnetic responses, however, are often hampered by difficulties to grow large-size samples of high quality and uniformity. We report here an imaging method that employed NV- centers in diamond as a sensor capable of mapping out the microwave magnetic field distribution on an ultrathin superconducting film of micron size. Measurements on a 33 nm thick film and a 125 nm thick bulklike film of Bi2Sr2CaCu2O8+δ revealed that the alternating current (ac) Meissner effect (or repulsion of ac magnetic field) set in at 78 and 91 K, respectively; the latter was the superconducting transition temperature of both films. The unusual ac magnetic response of the thin film presumably was due to thermally excited vortex-antivortex diffusive motion in the film. Spatial resolution of our ac magnetometer was limited by optical diffraction and the noise level was at 14 µT/Hz1/2. The technique could be extended with better detection sensitivity to extract local ac conductivity/susceptibility of ultrathin or monolayer superconducting samples as well as ac magnetic responses of other two-dimensional exotic thin films of limited lateral size.

Phase-sensitive sum frequency vibrational spectroscopic study of air/water interfaces: H2O, D2O, and diluted isotopic mixtures.

Using phase-sensitive sum-frequency vibrational spectroscopy with a carefully chosen phase reference, we revisited the vibrational spectra of vapor/water interfaces of neat H2O and D2O as well as HDO in diluted isotopic mixtures. Using z-cut quartz as the phase reference, with proper frequency scaling, the gross features of the OH and OD stretching spectra from H2O and D2O and from HDO in two isotopic mixtures look similar and agree with those reported earlier, but differences are also apparent. In particular, a weak positive band at low frequencies, which has been asserted by molecular dynamic simulations but not detectable in the experimental OH spectrum with pure H2O, is now visible in the OD spectrum. The differences must arise from the change of intermolecular interaction of water molecules with their surrounding molecules upon exchange of the isotopes.

Facet-specific interaction between methanol and TiO2 probed by sum-frequency vibrational spectroscopy.

The facet-specific interaction between molecules and crystalline catalysts, such as titanium dioxides (TiO2), has attracted much attention due to possible facet-dependent reactivity. Using surface-sensitive sum-frequency vibrational spectroscopy, we have studied how methanol interacts with different common facets of crystalline TiO2, including rutile(110), (001), (100), and anatase(101), under ambient temperature and pressure. We found that methanol adsorbs predominantly in the molecular form on all of the four surfaces, while spontaneous dissociation into methoxy occurs preferentially when these surfaces become defective. Extraction of Fermi resonance coupling between stretch and bending modes of the methyl group in analyzing adsorbed methanol spectra allows determination of the methanol adsorption isotherm. The isotherms obtained for the four surfaces are nearly the same, yielding two adsorbed Gibbs free energies associated with two different adsorption configurations singled out by ab initio calculations. They are (i) ∼-20 kJ/mol for methanol with its oxygen attached to a low-coordinated surface titanium, and (ii) ∼-5 kJ/mol for methanol hydrogen-bonded to a surface oxygen and a neighboring methanol molecule. Despite similar adsorption energetics, the Fermi resonance coupling strength for adsorbed methanol appears to depend sensitively on the surface facet and coverage.

Structural definition of the BIL and DL: a new universal methodology to rationalize non-linear χ(2)(ω) SFG signals at charged interfaces, including χ(3)(ω) contributions.

This work provides unambiguous definitions from theoretical simulations of the two interfacial regions named the BIL (binding interfacial layer) and DL (diffuse layer) at charged solid/water and air/water interfaces. The BIL and DL nomenclature follows the pioneering work of Wen et al. [Phys. Rev. Lett. 2016, 116, 016101]. Our definitions are based on the intrinsic structural properties of water only. Knowing the BIL and DL interfacial regions, one is then able to deconvolve the χ(2)(ω) non-linear SFG (sum frequency generation) response into χ(ω) and χ(ω) contributions, thus providing a detailed molecular interpretation of these signals and of the measured total SFG. We furthermore show that the χ(ω) spectrum arises from the χ(3)(ω) non-linear third order contribution of bulk liquid water, here calculated for several charged interfaces and shown to be universal. The χ(ω) contribution therefore has the same origin in terms of molecular normal modes at any charged interface. The molecular interpretation of χ(ω) is hence at the heart of the unambiguous molecular comprehension and interpretation of the measured total SFG signal at any charged interface.

Phase reference in phase-sensitive sum-frequency vibrational spectroscopy.

Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) has been established as a powerful technique for surface characterization, but for it to generate a reliable spectrum, accurate phase measurement with a well-defined phase reference is most important. Incorrect phase measurement can lead to significant distortion of a spectrum, as recently seen in the case for the air/water interface. In this work, we show theoretically and experimentally that a transparent, highly nonlinear crystal, such as quartz and barium borate, can be a good phase reference if the surface is clean and unstrained and the crystal is properly oriented to yield a strong SF output. In such cases, the reflected SF signal is dominated by the bulk electric dipole contribution and its phase is either +90° or -90°. On the other hand, materials with inversion symmetry, such as water, fused quartz, and CaF2 are not good phase references due to the quadrupole contribution and phase dispersion at the interface. Using a proper phase reference in PS-SFVS, we have found the most reliable OH stretching spectrum for the air/water interface. The positive band at low frequencies in the imaginary component of the spectrum, which has garnered much interest and been interpreted by many to be due to strongly hydrogen-bonded water species, is no longer present. A weak positive feature however still exists. Its magnitude approximately equals to that of air/D2O away from resonances, suggesting that this positive feature is unrelated to surface resonance of water.

Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy.

A sum-frequency spectroscopy scheme is developed that allows the measurement of vibrational spectra of the interfacial molecular structure of charged water interfaces. The application of this scheme to a prototype lipid-aqueous interface as a demonstration reveals an interfacial hydrogen-bonding water layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. This novel technique provides unique opportunities to search for better understanding of electrochemistry and biological aqueous interfaces at a deeper molecular level.

Surface sum-frequency vibrational spectroscopy of nonpolar media.

Sum-frequency generation spectroscopy is surface specific only if the bulk contribution to the signal is negligible. Negligible bulk contribution is, however, not necessarily true, even for media with inversion symmetry. The inevitable challenge is to find the surface spectrum in the presence of bulk contribution, part of which has been believed to be inseparable from the surface contribution. Here, we show that, for nonpolar media, it is possible to separately deduce surface and bulk spectra from combined phase-sensitive sum-frequency vibrational spectroscopic measurements in reflection and transmission. The study of benzene interfaces is presented as an example.

Structure of the submonolayer of ethanol adsorption on a vapor/fused silica interface studied with sum frequency vibrational spectroscopy.

Sum-frequency vibrational spectroscopy in the CH and OH stretch region was used to study ethanol adsorption on fused silica from vapor of different ethanol partial pressures. It was found that the adsorbed ethanol molecules were oriented with their methyl group tilted away from the surface normal by an average angle of ∼45° at low ethanol vapor pressures and ∼39° when approaching saturated vapor pressure. The spectral change with ethanol vapor pressure and the deduced adsorption isotherm show that ethanol molecules have two distinct adsorption sites on silica: One is the silanol group site to which an ethanol molecule can be strongly hydrogen-bonded, and the other is the siloxane (Si-O-Si) group site to which an ethanol molecule can be weakly bonded. The presence of water in vapor significantly reduced the surface coverage of ethanol on silica due to competitive adsorption between ethanol and water.

Single nanowire optical correlator.

Integration of miniaturized elements has been a major driving force behind modern photonics. Nanowires have emerged as potential building blocks for compact photonic circuits and devices in nanophotonics. We demonstrate here a single nanowire optical correlator (SNOC) for ultrafast pulse characterization based on imaging of the second harmonic (SH) generated from a cadmium sulfide (CdS) nanowire by counterpropagating guided pulses. The SH spatial image can be readily converted to the temporal profile of the pulses, and only an overall pulse energy of 8 µJ is needed to acquire a clear image of 200 fs pulses. Such a correlator should be easily incorporated into a photonic circuit for future use of on-chip ultrafast optical technology.

In situ sum-frequency vibrational spectroscopy of electrochemical interfaces with surface plasmon resonance.

Electrochemical (EC) reactions are crucial in many applications, yet most EC analytical tools lack the sensitivity to access molecular-level information of reactants and products. By combining sum-frequency vibrational spectroscopy and surface plasmon resonance at EC interfaces, we demonstrate the feasibility of measuring in situ and real-time vibrational spectra during EC reactions at noble metal electrodes. Application of the technique to EC reactions at a gold surface helps in understanding how the surface in a basic solution is oxidized and reduced during a cyclic voltammetry cycle. Study of desorption of a thiol self-assembled monolayer from gold through EC reactions in a basic solution shows that the desorbed thiols by reductive reaction remain as an ordered layer near the gold interface, but do diffuse away if they are desorbed oxidatively from gold.

Ultrafast all-optical graphene modulator.

Graphene is an optical material of unusual characteristics because of its linearly dispersive conduction and valence bands and the strong interband transitions. It allows broadband light-matter interactions with ultrafast responses and can be readily pasted to surfaces of functional structures for photonic and optoelectronic applications. Recently, graphene-based optical modulators have been demonstrated with electrical tuning of the Fermi level of graphene. Their operation bandwidth, however, was limited to about 1 GHz by the response of the driving electrical circuit. Clearly, this can be improved by an all-optical approach. Here, we show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼ 2.2 ps, limited only by the intrinsic carrier relaxation time of graphene. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.

Hot phonon dynamics in graphene.

The dynamics of hot phonons in supported, suspended, and gated monolayer graphene was studied by using time-resolved anti-Stokes Raman spectroscopy. We found that the hot phonon relaxation is dominated by phonon-phonon interaction in graphene, and strongly affected by the interaction between graphene and the substrate. Relaxation via carrier-phonon coupling, known as Landau damping, is ineffective for hot phonons which are in thermal equilibrium with excited carriers. Our findings provide a basis for better management of energy dissipation in graphene devices.

Nonlinear responses in optical metamaterials: theory and experiment.

We employed both theoretical calculations and experiments to study the nonlinear responses in optical metamaterials. The spectra of second-harmonic generations measured on a fishnet metamaterial are in quantitative agreements with calculations based on full-wave numerical simulations combined with field integrations, both exhibiting ~80 times enhancements at the magnetic resonance frequency. Our calculations explained several interesting features observed experimentally, and suggested an optimal metamaterial structure to yield the strongest nonlinear signals.

Graphene plasmonics for tunable terahertz metamaterials.

Plasmons describe collective oscillations of electrons. They have a fundamental role in the dynamic responses of electron systems and form the basis of research into optical metamaterials. Plasmons of two-dimensional massless electrons, as present in graphene, show unusual behaviour that enables new tunable plasmonic metamaterials and, potentially, optoelectronic applications in the terahertz frequency range. Here we explore plasmon excitations in engineered graphene micro-ribbon arrays. We demonstrate that graphene plasmon resonances can be tuned over a broad terahertz frequency range by changing micro-ribbon width and in situ electrostatic doping. The ribbon width and carrier doping dependences of graphene plasmon frequency demonstrate power-law behaviour characteristic of two-dimensional massless Dirac electrons. The plasmon resonances have remarkably large oscillator strengths, resulting in prominent room-temperature optical absorption peaks. In comparison, plasmon absorption in a conventional two-dimensional electron gas was observed only at 4.2 K (refs 13, 14). The results represent a first look at light-plasmon coupling in graphene and point to potential graphene-based terahertz metamaterials.