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Materials with massless Dirac fermions can possess exceptionally strong and widely tunable optical nonlinearities. Experiments on graphene monolayer have indeed found very large third-order nonlinear responses, but the reported variation of the nonlinear optical coefficient by orders of magnitude is not yet understood. A large part of the difficulty is the lack of information on how doping or chemical potential affects the different nonlinear optical processes. Here we report the first experimental study, in corroboration with theory, on third harmonic generation (THG) and four-wave mixing (FWM) in graphene that has its chemical potential tuned by ion-gel gating. THG was seen to have enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM appeared just the opposite. The latter was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant third-order nonlinearity. These truly amazing characteristics of graphene come from the possibility to gate-control the chemical potential, which selectively switches on and off one- and multi-photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices., Comment: submitted for publication on August 14th, 2017

Layer-stacking domain walls in bilayer graphene are emerging as a fascinating one-dimensional system that features stacking solitons structurally and quantum valley Hall boundary states electronically. The interactions between electrons in the 2D graphene domains and the one-dimensional domain-wall solitons can lead to further new quantum phenomena. Domain-wall solitons of varied local structures exist along different crystallographic orientations, which can exhibit distinct electrical, mechanical and optical properties. Here we report soliton-dependent 2D graphene plasmon reflection at different 1D domain-wall solitons in bilayer graphene using near-field infrared nanoscopy. We observe various domain-wall structures in mechanically exfoliated graphene bilayers, including network-forming triangular lattices, individual straight or bent lines, and even closed circles. The near-field infrared contrast of domain-wall solitons arises from plasmon reflection at domain walls, and exhibits markedly different behaviours at the tensile- and shear-type domain-wall solitons. In addition, the plasmon reflection at domain walls exhibits a peculiar dependence on electrostatic gating. Our study demonstrates the unusual and tunable coupling between 2D graphene plasmons and domain-wall solitons.

The authors report the observation of plasmons that exhibit quantized velocities in carbon nanotubes.

Artificial structures made of stacked two-dimensional crystals have recently been the focus of intense research activity. As in twisted or stacked graphene layers, these structures can show unusual behaviours and new phenomena. Among the various layered compounds that can be exfoliated, transition-metal dichalcogenides exhibit interesting properties governed by their structural symmetry and interlayer coupling, which are highly susceptible to stacking. Here, we obtain-by folding exfoliated MoS2 monolayers-MoS2 bilayers with different stacking orders, as monitored by second harmonic generation and photoluminescence. Appropriate folding can break the inversion symmetry and suppress interlayer hopping, evoking strong valley and spin polarizations that are not achieved in natural MoS2 bilayers of Bernal stacking. It can also enlarge the indirect bandgap by more than 100 meV through a decrease in the interlayer coupling. Our work provides an effective and versatile means to engineer transition-metal dichalcogenide materials with desirable electronic and optical properties.

The authors of this Article have become aware of the following recent paper that reports the electrically tunable degenerate four-wave mixing in graphene-covered SiN waveguides: “Alexander, K., Savostianova, N. A., Mikhailov, S. A., Kuyken, B. & Van Thourhout, D. Electrically tunable optical nonlinearities in graphene-covered SiN waveguides characterized by four-wave mixing. ACS Photon. 4, 3039–3044 (2017).” This reference has been added as ref. 33 in the new sentence “Very recently, degenerate four-wave mixing in graphene-covered SiN waveguides with ion-gel gating was experimentally studied33.” Accordingly, the previous sentence has been amended to read as “Few such experiments have been reported, although they have been suggested in theoretical work28–32” and all subsequent references have been renumbered. These changes have been made in the online versions.

Vibrational spectroscopy via infrared-visible sum-frequency generation was used to probe interfaces of hexagonal ice(0001) with vapor, a hydrophobic solid, and a hydrophilic solid. The OH stretch vibrational modes for hydrogen-bonded and free dangling OH bonds were measured at various temperatures below bulk melting. At the vapor/ice interface, the dangling OH mode provided information about surface melting. Surface structural disordering appeared to set in at ∼200 K and increased dramatically with temperature.

Sum frequency generation (SFG), using non-linear laser optics, detects vibrational spectra of submonolayer amounts of adsorbates with excellent energy and time resolution. Scanning tunneling spectroscopy (STM) is sensitive to the atomic surface structure; readily imaging defects, steps and kinks as well as stationary adsorbed species. Both of these techniques can be used during reactions at high pressures and temperatures to obtain molecular information in situ. We report studies of propylene hydrogenation over Pt(111) crystal surfaces at atmospheric pressures and 300 K using SFG and STM. Four surface species (2-propyl, π-bonded propylene, di σ-bonded propylene, and propylidyne) were identified; the first two being implicated as reaction intermediates. The platinum surface structure remains unchanged during the reaction, consistent with the structure insensitive nature of olefin hydrogenation. Propylene decomposition induced substantial surface reconstruction.

We discuss the possibility of observing photon echoes in the infrared range from a surface monolayer. Three different detection schemes are considered: direct infrared detection, external up-conversion, and in-situ up-conversion. The external up-conversion scheme appears most promising, while the in-situ up-conversion scheme has the advantage of being highly surface specific. Dephasing relaxations of surface vibrational excitations should be measurable.

Graphene-doped polymer nanofibers are fabricated by taper drawing of solvated polyvinyl alcohol doped with liquid-phase exfoliated graphene flakes. Nanofibers drawn this way typically have diameters measured in hundreds of nanometers and lengths in tens of millimeters; they show excellent uniformity and surface smoothness for optical waveguiding. Owing to their tightly confined waveguiding behavior, light–matter interaction in these subwavelength-diameter nanofibers is significantly enhanced. Using approximately 1350-nm-wavelength femto-second pulses, we demonstrate saturable absorption behavior in these nanofibers with a saturation threshold down to 0.25 pJ pulse−1 (peak power ∼1.3 W). Additionally, using 1064-nm-wavelength nanosecond pulses as switching light, we show all-optical modulation of a 1550-nm-wavelength signal light guided along a single nanofiber with a switching peak power of ∼3.2 W. Graphene-doped polymer nanofibers have been used to perform low-threshold nonlinear switching and modulation of light. Chao Meng and co-workers fabricated the nanofibers by drawing a solvated polymer containing liquid-phase exfoliated graphene flakes. The resulting nanofibers typically had diameters of several hundred nanometers and lengths of tens of millimetres. The nanofibers’ tight confinement of light on the subwavelength scale enhances light–matter interactions and allows nonlinear effects such as saturable absorption to occur at low optical powers. Using 1064-nm-wavelength, nanosecond control pulses as a switch, a 1550-nm signal was all-optically modulated in the nanofiber with a switching peak power of about 3.2 W. The researchers say that the nanofibers feature excellent surface smoothness and uniformity and are promising for use in nanophotonic circuits and devices.

A brief overview on second-harmonic generation and sum-frequency generation as surface analytical tools is given with a discussion on the intrinsic limitations of the techniques often encountered in the applications. The possibly of using nonlinear optical processes for surface studies has attracted increasing attention in recent years. Optical Second-Harmonic Generation (SHG) and Sum-Frequency Generation (SFG), in particular, have been well accepted as viable surface probes [1]. They have many advantages over the conventional techniques. By nature, they are highly surface-specific and has a submonolayer sensitivity. As coherent optical processes, they are capable of in-situ probing of surface in hostile environment as well as applicable to all interfaces accessible by light. With ultrafast pump-laser pulses, they can be employed to study surface-dynamic processes with a subpicosecond time resolution. These advantages have opened the door to many exciting research opportunities in surface science and technology. This paper gives a brief overview of this fast-growing new area of research.

The surface vibrational spectrum of atactic polypropylene was measured using IR+Visible Sum Frequency Generation (SFG) vibrational spectroscopy, in the temperature range 23 to -50 °C. A sharp rise in the intensity of the C-H symmetric stretch of CH2 groups is observed in the temperature range 0 to -20 °C. The elastic modulus and friction of the same polymer were measured with Atomic Force Microscopy (AFM) in the same temperature range i.e. 23 to -50 °C. There is a sharp rise in the modulus and a sharp decrease in the friction coefficient, at temperatures between 23 and 0 °C. These changes are attributed to the glass transition in atactic polypropylene which is expected to occur in the temperature range 10 to -10 °C. The temperature at which the glass transition is observed, from changes detected in AFM experiments, is higher than that observed by SFG experiments and that expected from bulk measurements. This elevation in the glass transition temperature has been attributed to the high pressure applied to the polymer under the AFM tip.

We report the observation of UV photoluminescence (PL) from wurtzite GaN by multiphoton excitation. The dependence of the PL intensity on excitation intensity as well as PL excitation measurements with sub-bandgap photon energies indicate the existence of deep defect states centered at about 1.0 eV above the top of the valence band. This result was confirmed by a sum-frequency excitation spectrum. We correlate these measurements with the omnipresent yellow luminescence in GaN. Our two-photon PL excitation spectrum yields a two-photon absorption coefficient that agrees very well with theoretical predictions.

Multiphonon resonant Raman scattering up to four phonons in GaSe and one- and two-phonon resonant Raman scattering in the mixed GaSxSe1−x crystals withx≤0.23 have been investigated. The results can be explained by a simple theory in which the dispersion of the resonance behavior is mainly dominated by resonances with the 1s direct exciton state.

Optical second-harmonic generation and the related technique of infrared – visible light sum-frequency generation are extremely versatile tools for studies of many kinds of surfaces and interfaces. With the help of ultra-short laser pulses, they can be used to monitor surface dynamics and reactions with sub-picosecond time resolution.

We have studied experimentally self-focusing of light in the liquid crystalline mediump-ethoxybenzyldine-p-butylaniline (EBBA). By variation of temperature, the response time of the field-induced refractive index was adjusted from larger than to smaller than theQ-switched laser pulsewidth. Accordingly, self-focusing varied from transient to quasi-steady-state. The results agree well with existing theoretical predictions and can be qualitatively understood from a simple physical description. The induced stimulated Brillouin scattering was also measured and was shown to be responsible for the limiting diameter of the self-focused beam in the present case.

Four-wave mixing with two circularly polarized counter-propagating waves inducing a helical birefringence structure can be used to measure a number of material parameters of a liquid crystalline medium. Application to a cholesteric substance is particularly interesting. A resonant effect is expected when the induced helix has a pitch coinciding with the natural helical pitch of the medium towards the isotropic → cholesteric transition. Field-induced shift in the transition temperature is also expected.

We have measured the optical properties of HfS2, SnS2 and SnSe2 at 5 K and 300 K in the range (2÷6) eV. We report a large number of new structures and discuss the main transitions in the light of recent band structure calculations. We have also investigated the indirect-absorption edge of the 3 compounds by means of additional piezotransmission measurements at 80 K.

The importance of interfaces for material science and electronic devices has stimulated great interest in the development of surface analytical tools. Among them, modern optical techniques using lasers have attracted the most attention in recent years. They have the advantage of being applicable to all interfaces accessible by light, and the high temporal, spatial, and spectral resolutions offer unique opportunities for studying ultrafast molecular dynamics and other transient phenomena at interfaces. Optical second harmonic generation (SHG) and sum-frequency generation (SFG) are particularly being noticed because of the many recent successful demonstrations of their versatility. This article briefly introduces these newly developed surface probes, first outlining the basic principles behind surface SHG and SFG, and then illustrating the power of the techniques with selected examples. A more complete treatment of the theory can be found in References 4–6. An overview of the earlier applications can be found in Reference 3.SHG arises from the nonlinear polarization P(2)(2ω) induced in a medium by an incident laser field E(ω). In the electric dipole approximation, P is given by:where is a second-order nonlinear susceptibility. For a medium with inversion symmetry, it follows directly from Eq. 1 that = 0. However, at an interface the surface nonlinear susceptibility is nonvanishing because there the inversion symmetry is necessarily broken.

Optical second harmonic generation (SHG) is applied to investigate the structure of the interfacial layer of soluble surfactants adsorbed to the air/water interface.