Electrical, Transport, and Optical Properties of Multifunctional Graphitic Films Synthesized on Dielectric Surfaces by Nickel Nanolayer-Assisted Pyrolysis.

We demonstrate that predepositing a nanometrically thin nickel film on a dielectric surface is sufficient to transform an amorphous pyrolyzed photoresist film (PPF) into a graphitic film (GRF) enriched with nickel particles. The GRF shows 3 orders of magnitude higher carrier mobility than the amorphous PPF, whereas its electrical conductivity doubles after etching away the nickel remains. The pronounced 2D peak in the Raman spectrum, almost dispersionless absorbance in the spectral range of 750-2000 nm, and the saturable absorption coefficient indicate that GRF possesses a graphene-like band structure. The proposed cost-efficient and scalable synthesis route opens avenues toward fabrication of micron size patterned graphitic structures of any shape directly on a dielectric substrate. Having graphene-like transport and electrical properties at 20 times higher absorbance than the single-layer graphene, GRF is attractive for fabrication of fast modulators for optical radiation, bolometers, and other photonics and optoelectronic devices that require enhanced optical absorption.

Tunable Perfect THz Absorber Based on a Stretchable Ultrathin Carbon-Polymer Bilayer.

By exploring the Salisbury screen approach, we propose and demonstrate a thin film absorber of terahertz (THz) radiation. The absorber is comprised of a less than 100 nm thick layer of pyrolytic carbon deposited on a stretchable polydimethylsiloxane (PDMS) film followed by the metal film. We demonstrate that being overall less than 200 microns thick, such a sandwich structure absorbs resonantly up to 99.9%of the incident THz radiation, and that the absorption resonance is determined by the polymer thickness, which can be adjusted by stretching.

Nonlinear optical components for all-optical probabilistic graphical model.

The probabilistic graphical models (PGMs) are tools that are used to compute probability distributions over large and complex interacting variables. They have applications in social networks, speech recognition, artificial intelligence, machine learning, and many more areas. Here, we present an all-optical implementation of a PGM through the sum-product message passing algorithm (SPMPA) governed by a wavelength multiplexing architecture. As a proof-of-concept, we demonstrate the use of optics to solve a two node graphical model governed by SPMPA and successfully map the message passing algorithm onto photonics operations. The essential mathematical functions required for this algorithm, including multiplication and division, are implemented using nonlinear optics in thin film materials. The multiplication and division are demonstrated through a logarithm-summation-exponentiation operation and a pump-probe saturation process, respectively. The fundamental bottlenecks for the scalability of the presented scheme are discussed as well.

Scalable fabrication of the graphitic substrates for graphene-enhanced Raman spectroscopy.

We propose direct synthesis of ultra-thin graphitic films on a dielectric substrate using sacrificial Ni catalyst layer, which significantly increases the crystallinity of the photoresist pyrolyzed at the temperature of 800 °C and above. A considerable amount of multilayer graphene in the photoresist film pyrolyzed in the presence of the Ni catalyst gives rise to an enhancement of the Raman signal of dye Sudan III molecules deposited on the substrate. We demonstrate comparable enhancement of the Raman signal from Sudan III molecules deposited on the fabricated graphitic substrate and those deposited on graphene, which was conventionally transferred to the silica substrate.

Ultra-Thin Pyrocarbon Films as a Versatile Coating Material.

The properties and synthesis procedures of the nanometrically thin pyrolyzed photoresist films (PPF) and the pyrolytic carbon films (PCF) were compared, and a number of similarities were found. Closer examination showed that the optical properties of these films are almost identical; however, the DC resistance of PPF is about three times higher than that of PCF. Moreover, we observed that the wettability of amorphous PPF and PCF was almost comparable to crystalline graphite. Potential applications executed by utilizing the small difference in the synthesis procedure of these two materials are suggested.

Pyrolytic carbon coated black silicon.

Carbon is the most well-known black material in the history of man. Throughout the centuries, carbon has been used as a black material for paintings, camouflage, and optics. Although, the techniques to make other black surfaces have evolved and become more sophisticated with time, carbon still remains one of the best black materials. Another well-known black surface is black silicon, reflecting less than 0.5% of incident light in visible spectral range but becomes a highly reflecting surface in wavelengths above 1000 nm. On the other hand, carbon absorbs at those and longer wavelengths. Thus, it is possible to combine black silicon with carbon to create an artificial material with very low reflectivity over a wide spectral range. Here we report our results on coating conformally black silicon substrate with amorphous pyrolytic carbon. We present a superior black surface with reflectance of light less than 0.5% in the spectral range of 350 nm to 2000 nm.

Optical characterization of directly deposited graphene on a dielectric substrate.

By using scanning multiphoton microscopy we compare the nonlinear optical properties of the directly deposited and transferred to the dielectric substrate graphene. The direct deposition of graphene on oxidized silicon wafer was done by utilizing sacrificial copper catalyst film. We demonstrate that the directly deposited graphene and bi-layered transferred graphene produce comparable third harmonic signals and have almost the same damage thresholds. Therefore, we believe directly deposited graphene is suitable for the use of e.g. nanofabricated optical setups.

Ultra-thin Graphitic Film: Synthesis and Physical Properties.

A scalable technique of chemical vapor deposition (CVD) growth of ultra-thin graphitic film is proposed. Ultra-thin graphitic films grown by a one-step CVD process on catalytic copper substrate have higher crystallinity than pyrolytic carbon grown on a non-catalytic surface and appear to be more robust than a graphene monolayer. The obtained graphitic material, not thicker than 8 nm, survives during the transfer process from a Cu substrate without a template polymer layer, typically used in the graphene transfer process to protect graphene. This makes the transfer process much more simple and cost-effective. Having electrical and optical properties compatible with what was observed for a few layers of CVD graphene, the proposed ultra-thin graphitic film offers new avenues for implementing 2D materials in real-world devices.

Recognition of multipolar second-order nonlinearities in thin-film samples.

We use two-beam second-harmonic generation to address thin films of silicon nitride (SiN). This technique is able to distinguish between the dipolar and higher-multipolar (magnetic and quadrupolar) contributions to the nonlinearity, as earlier shown for bulk samples. Our results for the SiN films exhibit strong multipolar signatures. Nevertheless, the results can be fully explained by the strong dipolar response of SiN once multiple reflections of the fundamental and second-harmonic fields within the film are properly taken into account. The results show that the recognition of multipolar nonlinearities requires extreme care for samples typically used for the characterization of new materials.

Optical Properties of Pyrolytic Carbon Films Versus Graphite and Graphene.

We report a comparative study of optical properties of 5-20 nm thick pyrolytic carbon (PyC) films, graphite, and graphene. The complex dielectric permittivity of PyC is obtained by measuring polarization-sensitive reflectance and transmittance spectra of the PyC films deposited on silica substrate. The Lorentz-Drude model describes well the general features of the optical properties of PyC from 360 to 1100 nm. By comparing the obtained results with literature data for graphene and highly ordered pyrolytic graphite, we found that in the visible spectral range, the effective dielectric permittivity of the ultrathin PyC films are comparable with those of graphite and graphene.

Graphene-enhanced Raman spectroscopy of thymine adsorbed on single-layer graphene.

Graphene-enhanced Raman scattering (GERS) spectra and coherent anti-Stokes Raman scattering (CARS) of thymine molecules adsorbed on a single-layer graphene were studied. The enhancement factor was shown to depend on the molecular groups of thymine. In the GERS spectra of thymine, the main bands are shifted with respect to those for molecules adsorbed on a glass surface, indicating charge transfer for thymine on graphene. The probable mechanism of the GERS enhancement is discussed. CARS spectra are in accord with the GERS results, which indicates similar benefit from the chemical enhancement.

All-optical control of ultrafast photocurrents in unbiased graphene.

Graphene has recently become a unique playground for studying light-matter interaction effects in low-dimensional electronic systems. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. In particular, strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field effects and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain. The obtained all-optical control of the photocurrents opens new routes towards graphene based high-speed and broadband optoelectronic devices.

Microwave absorption properties of pyrolytic carbon nanofilm.

We analyzed the electromagnetic (EM) shielding effectiveness in the Ka band (26 to 37 GHz) of highly amorphous nanometrically thin pyrolytic carbon (PyC) films with lateral dimensions of 7.2 × 3.4 mm2, which consists of randomly oriented and intertwined graphene flakes with a typical size of a few nanometers. We discovered that the manufactured PyC films, whose thickness is thousand times less than the skin depth of conventional metals, provide a reasonably high EM attenuation. The latter is caused by absorption losses that can be as high as 38% to 20% in the microwave frequency range. Being semi-transparent in visible and infrared spectral ranges and highly conductive at room temperature, PyC films emerge as a promising material for manufacturing ultrathin microwave (e.g., Ka band) filters and shields.

Third-harmonic UV generation in silicon nitride nanostructures.

We report on strong UV third-harmonic generation from silicon nitride films and resonant waveguide gratings. We determine the absolute value of third-order susceptibility of silicon nitride at wavelength of 1064 nm to be χ(³) (-3ω,ω,ω,ω) = (2.8 ± 0.6) × 10⁻²°m²/V², which is two orders of magnitude larger than that of fused silica. The third-harmonic generation is further enhanced by a factor of 2000 by fabricating a resonant waveguide grating onto a silicon nitride film. Our results extend the operating range of CMOS-compatible nonlinear materials to the UV spectral regime.

Self-assembled silver nanoislands formed on glass surface via out-diffusion for multiple usages in SERS applications.

We demonstrate that silver nanoisland film self-assembled on the surface of silver-containing glass in the course of thermal processing in hydrogen is capable to detect 10-7 M concentration of rhodamine 6G in water using surface enhanced Raman spectroscopy (SERS) technique. The film can be multiply restored on the same glass substrate via annealing of the glass in hydrogen. We showed that the film can be self-assembled after as much as ten circles of the substrate cleaning followed by annealing. The proposed technique of the silver nanoisland film formation enables multiple usage of the same glass substrate in SERS experiments.

Efficient second-harmonic generation in silicon nitride resonant waveguide gratings.

We demonstrate a thousand-fold enhancement of second-harmonic generation in an all-dielectric silicon nitride (SiN) resonant waveguide grating compared to a flat SiN film. The strong second-harmonic output measured at 532 nm results from the combination of the enhanced local fields in the nanostructure and the large second-order susceptibility of SiN. The second-harmonic conversion efficiency in the resonant structure is found to be of the order of 10(-8), which is significantly larger than that typically observed from plasmonic metal nanostructures.

Thickness determination of graphene on metal substrate by reflection spectroscopy.

We show that reflectivity measurements enable the determination of the thickness of multilayered graphene on a metal substrate. The developed technique is based on comparison of the substrate reflectance with and without graphene and relies on the strong absorbance of graphene and high refractive index contrast. We demonstrate the technique by measuring the thickness of the CVD graphene film grown on a copper substrate.