Nanotextured surfaces which have surface features spanning 10-100 nm in length and height scales are among the most promising for surface enhanced Raman scattering/spectroscopy (SERS). Randomness of the feature sizes and surface morphology of such sensors brings an added benefit of spectrally broadband action and, consequently, augmented SERS intensity. Surfaces which are most promising for high sensitivity yet cost efficient for large scale production are overviewed with black CuO, which is made by chemical oxidation of Cu foil, as a representative example. Application potential and challenges to establishing quantitative SERS measurements are outlined.
Pyramidal silicon nanospikes, termed black-Si (b-Si), with controlled height of 0.2 to 1 μm, were fabricated by plasma etching over 3-in wafers and were shown to act as variable density filters in a wide range of the IR spectrum 2.5 to 20 μm, with transmission and its spectral gradient dependent on the height of the spikes. Such variable density IR filters can be utilized for imaging and monitoring applications. Narrow IR notch filters were realized with gold mesh arrays on Si wafers prospective for applications in surface-enhanced IR absorption sensing and “cold materials” for heat radiation into atmospheric IR transmission window. Both types of filters for IR: spectrally variable and notch are made by simple fabrication methods.
Photo-thermal - to - electrical converter is demonstrated by using a commercial Peltier Bi-Te element with a hot contact made out of nanotextured Si (black-Si). Black-Si with colloidal Au nanoparticles is shown to further increase the efficiency of thermal-to-electrical conversion. Peculiarities of heat harvesting using black-Si with plasmonic Au nanoparticles at different gold densities are analyzed. Solar radiation absorption and electric field enhancement in plain and Au nanoparticle decorated black-Si was simulated using finite difference time domain (FDTD) method. Thermal conduction in nanotextured black-Si was explained using phonon Monte-Carlo simulations at the nanoscale. Strategies for creating larger thermal gradient on Peltier element using nanotextured light absorbers is discussed.
Plasmonics and nanoscale antennas have been intensively investigated for sensors, metasurfaces and optical trapping where light control at the nanoscale enables new functionalities. To confine and manipulate the light in tiny spaces sub-wavelength antennas should be used with dimensions from micro- to nano-meters and are still challenging to make. Direct fabrication/modification of nanostructures using focused ion beam (FIB) milling is demonstrated for several types of antennas. Arrays of identical nanoparticles were fabricated in a single step by (i) milling gold films or (ii) by modifying structures which were already defined by electron beam or mask projection lithography. Direct FIB writing enables to exclude resist processing steps, thus making fabrication faster and simpler. Sensor areas of 25x25 μm<sup>2</sup> of densely packed nanoparticles separated by tens-of-nanometers were fabricated in half an hour (10<sup>3</sup> μm2/h throughput at 90 nm resolution). Patterns of chiral nanoparticles by groove inscription is demonstrated. The processing speed and capability to mill complex 3D surfaces due to depth of focus not compromised over micrometers length, makes it possible to reach sub-50 nm resolution of direct write. FIB technology is practical for emerging applications in nano-fabrication/photonic/fluidic/magnetic applications.
Rapid and cost effective fabrication of nano-textured surfaces of CuO and Cu<sub>2</sub>O by chemical bath process was used to fabricated large surface areas with cross sections in centimeters. Through chemical etching and oxidation induced nano-texturation Cu foils are rendered black and their surface area is increased by two orders of magnitude. Magnetronic Au sputtering was used to coat the nano-textured Cu<sub>x</sub>O features with nano-granular metal films which were found to be conformal for the range of 5-50 nm layer thicknesses. The Au coated substrates of Cu<sub>x</sub>O were tested for surface enhanced Raman scattering (SERS) performance and showed one of the best sensitivity enhancements when compared with other nano-textured surfaces. Application potential of the black-Cu<sub>2</sub>O for SERS sensing and for solar cell applications is discussed.
Electron and ion beam lithographies were used to fabricate and/or functionalize large scale - millimetre footprint - micro-optical elements: coupled waveguide-resonator structures on silicon-on-insulator (SOI) and THz antennas on low temperature grown LT-GaAs. Waveguide elements on SOI were made without stitching errors using a fixed beam moving stage approach. THz antennas were created using a three-step litography process. First, gold THz antennas defined by standard mask projection lithography were annealed to make an ohmic contact on LT-GaAs and post-processing with Ga-ion beam was used to define nano-gaps and inter digitised contacts for better charge collection. These approaches show the possibility to fabricate large footprint patterns with nanoscale precision features and overlay accuracy. Emerging 3D nanofabrication trends are discussed.
The nano-textured surface of black silicon can be used as a surface-enhanced Raman scattering (SERS) substrate. Sputtered gold films showed increasing SERS sensitivity for thicknesses from 10 up to 300 nm, with sensitivity growing nonlinearly from around 50 nm until saturation at 500 nm. At 50 nm, a cross over from a discontinuous to a fully percolated film occurs as revealed by morphological and electrical measurements. The roughness of the Au coating increases due to formation of nanocrystallites of gold. Structural characterization of the black- Si needles and their surfaces revealed presence of silicon oxide and fluoride. The sharpest nano-needles had a tip curvature radius of ~10 nm. SERS recognition of analyte using molecular imprinted gels with tetracycline molecules of two different kinds is demonstrated.
Recently, new types of silica polarization converters fabricated by femtosecond lasers have been introduced. These devices use spatially arranged nanogratings found under certain femtosecond laser exposure conditions in fused silica to create arbitrary polarization states by shaping spatially and locally the retardance of an incoming beam. Using this principle, radial and azimuthal polarization converters were demonstrated. These devices make use of a large density of femtosecond laser spots, introducing localized defects, affecting the performance of the converter. To optimize the writing and the post-processing annealing step of these kind of devices, here we introduce a novel fluorescence lifetime imaging microscope (FLIM) working with deep UV (240-280 nm) wavelength excitations. Specifically, we demonstrate the potential of this technique and more generally, how it can be used for characterizing a variety of femtosecond laser induced modifications in fused silica. This UV-FLIM can be used with micro-fluidic and bio-samples to characterize temporal characteristics of fluorescence.
GaN light emitting diodes (LEDs) on sapphire substrates can be improved by micro-patterning substrate to perform epitaxial over-growth which drastically reduces defects' density in the light emitting region. We patterned Al<sub>2</sub>O<sub>3</sub> with focused ion beam and show a successful overgrowth of GaN. The exact shape of pattern milled into Al<sub>2</sub>O<sub>3</sub> was replicated into a 0.4-mm-thick shim of Ni by electroplating. The surface roughness of Ni was ~5:5 ±2 nm and is applicable for the most demanding replication of nano-rough surfaces. This technique can be used to replicate at micro-optical elements Fresnel-axicons defined by electron beam lithography made on sub-1 mm areas without stitching errors (Raith EBL). Shimming of macro-optical elements such as car back- reflectors is also demonstrated. Ni-shimming opens possibility to make replicas of nano-textured small and large area patterns and use them for thermal embossing and molding of optically-functionalized micro-fluidic chips and macro-optical elements.
A terahertz (THz or T-rays) photomixer consisting of a meander type antenna with integrated nanoelectrodes on a low temperature grown GaAs (LT-GaAs) is demonstrated. The antenna was designed for molecular fingerprinting and sensing applications within a spectral range of 0.3-0.4 THz. A combination of electron beam lithography (EBL) and focused ion beam (FIB) milling was used to fabricate the T-ray emitter. Antenna and nanoelectrodes were fabricated by standard EBL and lift-off steps. Then a 40-nm-wide gap in an active photomixer area separating the nanoelectrodes was milled by a FIB. The integrated nano-contacts with nano-gaps enhance the illuminated light and THz electric fields as well as contribute to a better collection of photo-generated electrons. T-ray emission power from the fabricated photomixer chips were few hundreds of nanowatts at around 0.15 THz and tens of nanowatts in the 0.3-0.4 THz range.
Chiral patterns are created by focused ion-beam milling nano-grooves with sub-15 nm resolution on thin metal films and arrays of nanoparticles, scattering and absorbing light selectively for left and right circularly polarized light, with high fidelity over fields up to 100 x 100 μm<sup>2</sup> without positioning errors. This allows to carry out numerical simulations to estimate light enhancement and circular dichroism both on ideal and realistic particles taken from SEM images, showing doubling of scattering cross-section and enhancement changes up to 5 times controlled by dichroism, with localized field enhancements up to 20000.
3D plasmonic structures extending out of a gold film plane are created by dry etching of the film in the openings of a resist mask defined by electron beam lithography. Conical vertical protrusions (nano-wells) are left, and their optical properties are numerically simulated, showing easily reachable out-of-plane trapping of both dielectric and metal plasmonic nano-spheres, with trapping forces up to 20 pN/W/μm<sup>2</sup>.
Wideband refractive index spectra in 3D-FDTD are correctly represented by overcoming the polynomial approximations to give accurate field and force/torque results for generalized artificial materials.