Controlling ultrafast laser writing in silica glass by pulse temporal contrast.

We demonstrate that the temporal contrast of femtosecond light pulses is a critical parameter in laser writing inside transparent dielectrics, allowing different material modifications. In particular, anisotropic nanopores in silica glass are produced by high-contrast of 107 femtosecond Yb:KGW laser pulses rather than low-contrast of 103 Yb fiber laser pulses. The difference originates in the fiber laser storing a third of its energy in a post-pulse of up to 200 ps duration. The absorption of this low-intensity fraction of the pulse by laser-induced transient defects with relatively long lifetime and low excitation energy, such as self-trapped holes, drastically changes the kinetics of energy deposition and the type of material modification. We also demonstrate that low-contrast pulses are effective in creating lamellar birefringent structures, possibly driven by a quadrupole nonlinear current.

Ultra-broadband absorbance of nanometer-thin pyrolyzed-carbon film on silicon nitride membrane.

Fifty percents absorption by thin film, with thickness is much smaller than the skin depth and optical thickness much smaller than the wavelength, is a well-known concept of classical electrodynamics. This is a valuable feature that has been numerously widely explored for metal films, while chemically inert nanomembranes are a real fabrication challenge. Here we report the 20 nm thin pyrolyzed carbon film (PyC) placed on 300 nm thick silicon nitride (Si3N4) membrane demonstrating an efficient broadband absorption in the terahertz and near infrared ranges. While the bare Si3N4membrane is completely transparent in the THz range, the 20 nm thick PyC layer increases the absorption of the PyC coated Si3N4membrane to 40%. The reflection and transmission spectra in the near infrared region reveal that the PyC film absorption persists to a level of at least 10% of the incident power. Such a broadband absorption of the PyC film opens new pathways toward broadband bolometric radiation detectors.

Nanodiamond surface as a photoluminescent pH sensor.

A systematic spectroscopic characterization of highly homogeneous water suspensions of 'buckydiamonds' comprising sp3cubic nanodiamond (ND) core covered with disordered sp2shell densely decorated with oxygen-containing groups demonstrates the excitation-wavelength-dependent photoluminescence (PL) given by at least four types of specific structures on the ND surface (hydroxyl, C=O containing ketones, carboxylic anhydrides, and carboxyl groups). PL properties of NDs suspensions possess concentration-dependent behavior revealing tendency of NDs to agglomerate. PL of NDs has been found to be strongly sensitive to pH of the environment in wide range of pH values, i.e. 2-11. We disclosed the mechanisms of pH sensitivity of the 'buckydiamond' and proved that it can serve as all-optical sensor of tiny pH variations suitable for further exploitation for pH sensing locally in the area where NDs have been delivered for any purpose, e.g. bioimaging or therapeutic needs.

Fragmented graphene synthesized on a dielectric substrate for THz applications.

Fragmented multi-layered graphene films were directly synthesized via chemical vapor deposition (CVD) on dielectric substrates with a pre-deposited copper catalyst. We demonstrate that the thickness of the sacrificial copper film, process temperature, and growth time essentially influence the integrity, quality, and disorder of the synthesized graphene. Atomic force microscopy and Kelvin probe force microscopy measurements revealed the presence of nano-agglomerates and charge puddles. The potential gradients measured over the sample surface confirmed that the deposited graphene film possessed a multilayered structure, which was modelled as an ensemble of randomly oriented conductive prolate ellipsoids. THz time domain spectroscopy measurements gave theacconductivity of the graphene flakes and homogenized graphitic films as being around 1200 S cm-1and 1000 S cm-1, respectively. Our approach offers a scalable fabrication of graphene structures composed of graphene flakes, which have effective conductivity sufficient for a wide variety of THz applications.

Interaction of polarization-sensitive surface photocurrents in semitransparent CuSe/Se film.

We demonstrate that the transverse polarization-sensitive photoresponse of the CuSe/Se nanocomposite film deposited on a transparent substrate depends on whether the film is irradiated from the air side or substrate side. In particular, the nanosecond photocurrent pulse is either bipolar or unipolar pulse depending on which interface beam hits first. The observed phenomenon can be described in terms of the interplay between counter-propagating photocurrents generated at the air/nanocomposite and substrate/nanocomposite interfaces due to the surface photogalvanic effect. Our experimental findings can be employed to control the amplitude and temporal profile of the photoresponse by changing the polarization of the excitation laser beam.

Rapid and delayed effects of single-walled carbon nanotubes in glioma cells.

Single-walled carbon nanotubes (SWCNTs) demonstrate a strong potential as an optically activated theranostic nano-agent. However, using SWCNTs in theranostics still requires revealing mechanisms of the SWCNT-mediated effects on cellular functions. Even though rapid and delayed cellular responses can differ significantly and may lead to undesirable consequences, understanding of these mechanisms is still incomplete. We demonstrate that introducing short (150-250 nm) SWCNTs into C6 rat glioma cells leads to SWCNT-driven effects that show pronounced time dependence. Accumulation of SWCNTs is carried out due to endocytosis with modification of the actin cytoskeleton but not accompanied with autophagy. Its initial stage launches a rapid cellular response via significantly heightened mitochondrial membrane potential and superoxide anion radical production, satisfying the cell demand of energy for SWCNT transfer inside the cytoplasm. In the long term, SWCNTs agglomerate to micron-sized structures surrounded by highly active mitochondria having parameters return to control values. SWCNTs postponed effects are also manifested themselves in the suppression of the cell proliferative activity with further restoration after five passages. These results demonstrate relative cellular inertness and safety of SWCNTs eliminating possible side effects caused by optically activated theranostic applications.

Electrical impedance sensing of organic pollutants with ultrathin graphitic membranes.

In this paper we propose an original approach for the real-time detection of industrial organic pollutants in water. It is based on the monitoring of the time evolution of the electrical impedance of low-cost graphitic nanomembranes. The developed approach exploits the high sensitivity of the impedance of 2D graphene-related materials to the adsorbents. We examined sensitivity of the nanomembranes based on pyrolyzed photoresist, pyrolytic carbon (PyC), and multilayer graphene films. In order to realize a prototype of a sensor capable of monitoring the pollutants in water, the membranes were integrated into an ad hoc printed circuit board. We demonstrated the correlation between the sensitivity of the electric impedance to adsorbents and the structure of the nanomembranes, and revealed that the amorphous PyC, being most homogeneous and adhesive to the SiO2substrate, is the most promising in terms of integration into industrial pollutants sensors.

Visualizing hypochlorous acid production by human neutrophils with fluorescent graphene quantum dots.

In living organisms, redox reactions play a crucial role in the progression of disorders accompanied by the overproduction of reactive oxygen and reactive chlorine species, such as hydrogen peroxide and hypochlorous acid, respectively. We demonstrate that green fluorescence graphene quantum dots (GQDs) can be employed for revealing the presence of the hypochlorous acid in aqueous solutions and cellular systems. Hypochlorous acid modifies the oxygen-containing groups of the GQD, predominantly opens epoxide ring C-O-C, forms excessive C=O bonds and damages the carbonic core of GQDs. These changes, which depend on the concentration of the hypochlorous acid and exposure time, manifest themselves in the absorbance and fluorescence spectra of the GQD, and in the fluorescence lifetime. We also show that the GQD fluorescence is not affected by hydrogen peroxide. This finding makes GQDs a promising sensing agent for selective detecting reactive chlorine species produced by neutrophils. Neutrophils actively accumulate GQDs allowing to visualize cells and to examine the redox processes via GQDs fluorescence. At high concentrations GQDs induce neutrophil activation and myeloperoxidase release, leading to the disruption of GQD structure by the produced hypochlorous acid. This makes the GQDs a biodegradable material suitable for various biomedical applications.

Macro-, Micro- and Nano-Roughness of Carbon-Based Interface with the Living Cells: Towards a Versatile Bio-Sensing Platform.

Integration of living cells with nonbiological surfaces (substrates) of sensors, scaffolds, and implants implies severe restrictions on the interface quality and properties, which broadly cover all elements of the interaction between the living and artificial systems (materials, surface modifications, drug-eluting coatings, etc.). Substrate materials must support cellular viability, preserve sterility, and at the same time allow real-time analysis and control of cellular activity. We have compared new substrates based on graphene and pyrolytic carbon (PyC) for the cultivation of living cells. These are PyC films of nanometer thickness deposited on SiO2 and black silicon and graphene nanowall films composed of graphene flakes oriented perpendicular to the Si substrate. The structure, morphology, and interface properties of these substrates are analyzed in terms of their biocompatibility. The PyC demonstrates interface biocompatibility, promising for controlling cell proliferation and directional intercellular contact formation while as-grown graphene walls possess high hydrophobicity and poor biocompatibility. By performing experiments with C6 glioma cells we discovered that PyC is a cell-friendly coating that can be used without poly-l-lysine or other biopolymers for controlling cell adhesion. Thus, the opportunity to easily control the physical/chemical properties and nanotopography makes the PyC films a perfect candidate for the development of biosensors and 3D bioscaffolds.

Thermal stability of volume Bragg gratings in chloride photo-thermo-refractive glass after femtosecond laser bleaching.

We demonstrate that the Joule heating of the volume Bragg grating recorded in chloride photo-thermo-refractive glass can be suppressed by bleaching the silver nanoparticles with intense ultrashort laser pulses. Measurement of the bleached grating angular selectivity showed that, at the signal wavelength at 972 nm, the spectral drift is 0.5 nm at the CW laser diode beam intensity as high as 145 W/cm2. Thus, the bleaching of silver nanoparticles results in the improved thermal stability of transmission gratings, allowing one to employ them to control the powerful CW laser radiation.

Fluorescent clusters in chloride photo-thermo-refractive glass by femtosecond laser bleaching of Ag nanoparticles.

We report photoluminescence in bulk chloride photo-thermo-refractive glass under irradiation with femtosecond laser pulses. The fluorescence originates from the bleaching of silver nanoparticles precipitating in the glass. Similarly to the conventional process of the femtosecond re-shaping of metal inclusions with diameter tens of nanometers, irradiation of the smaller nanoparticles results in a fast shrinking size with an ellipsoidal shape via photofragmentation. Under UV excitation, remaining sub-nanometer silver molecular clusters show visible and near IR fluorescence, which increases with chlorine concentration. The observed bleaching of silver nanoparticles in bulk glass-metal nanocomposite can find applications in data storage and bleaching of volume Bragg gratings.

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.

Form birefringence in Kerr media: analytical formulation and rigorous theory.

Employing the first-order effective medium theory, we develop an analytical model that governs light propagation inside a form birefringent medium with isotropic dielectric Kerr nonlinear material. This analytical model is found to be in excellent agreement with the recently developed rigorous Fourier modal method for Kerr nonlinear material [J. Opt. Soc. Am. B31, 2371 (2014)JOSAAH0030-394110.1364/JOSAB.31.002371]. Theoretical results demonstrate that form birefringent linear gratings with Kerr nonlinear materials behave like uniaxial crystals. However, the magnitude of birefringence can be tuned with a change of the incident light intensity. This paves the way toward all-optical control of form birefringence by exploiting optical nonlinearities in subwavelength structures.

Near-IR nonlinear optical filter for optical communication window.

We report on a high performance nonlinear optical filter for the telecommunication window that employs detonation nanodiamonds (NDs). The nanosecond Z-scan experiments revealed that the heavy water ND suspensions enable strong optical limiting in the wavelength range of 1400-1675 nm. We observed an enhancement of the optical limiting performance in the blue part of the communication window. In particular, at the wavelength of 1400 nm the transmittance of the 2 mm thick sample with 4.5 wt. % ND concentration is suppressed by 45% for the input fluence of 3.8 J/cm(2). The proposed nonlinear optical filter employs the phenomena of the nonlinear absorption and the nonlinear light scattering in ND suspensions.

Size effect on the optical limiting in suspensions of detonation nanodiamond clusters.

We report on the optical limiting (OL) in stable aqueous suspensions of detonation nanodiamond (ND) clusters with average size of 50, 110, and 320 nm. The nanosecond Z-scan measurements at wavelength of 532 nm revealed that the larger the cluster size, the better the OL performance and the higher the ray stability of the ND suspension. Our analysis showed that the nonlinear scattering and the nonlinear absorption are the dominant mechanisms of OL in aqueous ND suspensions.

Over Two-Fold Photoluminescence Enhancement from Single-Walled Carbon Nanotubes Induced by Oxygen Doping.

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) is a promising way to improve their photoluminescent (PL) brightness and thus make them applicable as a base material for infrared light emitters. We report as high as over two-fold enhancement of the SWCNT PL brightness by using oxygen doping via the UV photodissociation of hypochlorite ions. By analyzing the temporal evolution of the PL and Raman spectra of SWCNTs in the course of the doping process, we conclude that the enhancement of SWCNTs PL brightness depends on the homogeneity of induced quantum defects distribution over the SWCNT surface.

Black Silicon: Breaking through the Everlasting Cost vs. Effectivity Trade-Off for SERS Substrates.

Black silicon (bSi) is a highly absorptive material in the UV-vis and NIR spectral range. Photon trapping ability makes noble metal plated bSi attractive for fabrication of surface enhanced Raman spectroscopy (SERS) substrates. By using a cost-effective room temperature reactive ion etching method, we designed and fabricated the bSi surface profile, which provides the maximum Raman signal enhancement under NIR excitation when a nanometrically-thin gold layer is deposited. The proposed bSi substrates are reliable, uniform, low cost and effective for SERS-based detection of analytes, making these materials essential for medicine, forensics and environmental monitoring. Numerical simulation revealed that painting bSi with a defected gold layer resulted in an increase in the plasmonic hot spots, and a substantial increase in the absorption cross-section in the NIR range.

Efficient ultrafast laser writing with elliptical polarization.

Photosensitivity in nature is commonly associated with stronger light absorption. It is also believed that artificial optical anisotropy to be the strongest when created by light with linear polarization. Contrary to intuition, ultrafast laser direct writing with elliptical polarization in silica glass, while nonlinear absorption is about 2.5 times weaker, results in form birefringence about twice that of linearly polarized light. Moreover, a larger concentration of anisotropic nanopores created by elliptically polarized light pulses is observed. The phenomenon is interpreted in terms of enhanced interaction of circularly polarized light with a network of randomly oriented bonds and hole polarons in silica glass, as well as efficient tunneling ionization produced by circular polarization. Applications to multiplexed optical data storage and birefringence patterning in silica glass are demonstrated.

Stable and Reusable Lace-like Black Silicon Nanostructures Coated with Nanometer-Thick Gold Films for SERS-Based Sensing.

We propose a simple, fast, and low-cost method for producing Au-coated black Si-based SERS-active substrates with a proven enhancement factor of 106. Room temperature reactive ion etching of silicon wafer followed by nanometer-thin gold sputtering allows the formation of a highly developed lace-type Si surface covered with homogeneously distributed gold islands. The mosaic structure of deposited gold allows the use of Au-uncovered Si domains for Raman peak intensity normalization. The fabricated SERS substrates have prominent uniformity (with less than 6% SERS signal variations over large areas, 100 × 100 µm2). It has been found that the storage of SERS-active substrates in an ambient environment reduces the SERS signal by less than 3% in 1 month and not more than 40% in 20 months. We showed that Au-coated black Si-based SERS-active substrates can be reused after oxygen plasma cleaning and developed relevant protocols for removing covalently bonded and electrostatically attached molecules. Experiments revealed that the Raman signal of 4-MBA molecules covalently bonded to the Au coating measured after the 10th cycle was just 4 times lower than that observed for the virgin substrate. A case study of the reusability of the black Si-based substrate was conducted for the subsequent detection of 10-5 M doxorubicin, a widely used anticancer drug, after the reuse cycle. The obtained SERS spectra of doxorubicin were highly reproducible. We demonstrated that the fabricated substrate permits not only qualitative but also quantitative monitoring of analytes and is suitable for the determination of concentrations of doxorubicin in the range of 10-9-10-4 M. Reusable, stable, reliable, durable, low-cost Au-coated black Si-based SERS-active substrates are promising tools for routine laboratory research in different areas of science and healthcare.

Surface-plasmon enhanced optical activity in two-dimensional metal chiral networks.

We investigated the optical properties of a novel chiral metamaterial; two-dimensional metal chiral networks formed from metal ribbons deposited on a dielectric substrate. For zeroth-order transmitted light, sharp optical resonances were observed at spectral positions, which are determined by the surface plasmon resonance frequencies of the periodic metal structures. The experimental results are in excellent agreement with numerical calculations.