Laser micro/nano fabrication in glass with tunable-focus particle lens array

Dielectric optical waveguide fabricated on a transparent substrate

Transparent glass substrates are routinely used in the fabrication of metasurfaces, augmented reality (AR), virtual reality (VR), and holographic devices. While readily compatible with photolithographic patterning methods, when electron beam (E-Beam) techniques are used, field distortion and stitching errors can result due to the buildup of charge. A common approach to overcome this issue is to deposit a thin conductive polymer layer (E-Spacer). However, if high-voltage E-Beam is used to achieve nano-features, the polymer conductivity is not sufficient. We have shown that by using chromium (Cr) as an overcoating conductive layer on the resist, we can achieve accurate and seamless patterning in multiple writing fields and used the method to fabricate on-chip Si3N4 waveguides on SiO2. This technique has the potential to enable the fabrication of large-scale integrated photonic systems on transparent or dielectric substrates.

A planar ultraviolet objective lens for optical axis free imaging nanolithography by employing optical negative refraction

Lithography is one of the most key technologies for integrated circuit (IC) manufacturing and micro/nano-functional device fabrication, while the imaging objective lens plays one important role. Due to the curved surface of the conventional objective lens, the imaging field of view is limited and the objective lens system is complex. In this paper, a planar objective lens based on the optical negative refraction principle is demonstrated for achieving optical axis free and long depth of focus imaging nanolithography. Through employing a hyperbolic metamaterial composed of silver/titanium dioxide multilayers, plasmonic waveguide modes could be generated in multilayers, which results in optical negative refraction and then flat imaging at ultraviolet wavelength. The corresponding imaging characteristics are investigated in simulation and experiment. At the I-line wavelength of 365 nm, the highest imaging resolution of 165 nm could be realized in the 100 nm photoresist layer under the working gap of 100 nm between the objective lens and substrate. Moreover, this planar objective lens has good ability for cross-scale and two-dimensional imaging lithography, and is similar to a conventional projection objective lens. It is believed that this kind of planar objective lens will provide a promising avenue for low-cost nanofabrication scenarios in the near future.

Interference surface patterning using colloidal particle lens arrays

Surface patterns of complex morphology can be made by combining the near-field colloidal lithography and the multiple-beam interference of the incident laser light. Our calculation shows that patterns made of bright and dim photonic jets can be formed beneath the dielectric spheres within the close-packed colloidal monolayer. An algorithm to find the propagation directions, amplitudes, and phases of the incident beams needed to make the desired photonic jet pattern is proposed. The field contrast in those patterns is studied.

Modifying the Morphology of Silicon Surfaces by Laser Induced Liquid Assisted Colloidal Lithography

Single, or isolated small arrays of, spherical silica colloidal particles (with refractive index n_colloid = 1.47 and radius R = 350 nm or 1.5 μm) were placed on a silicon substrate and immersed in carbon tetrachloride (n_liquid = 1.48) or toluene (n_liquid = 1.52). Areas of the sample were then exposed to a single laser pulse (8 ps duration, wavelength λ = 355 nm), and the spatial intensity modulation of the near field in the vicinity of the particles revealed via the resulting patterning of the substrate surface. In this regime, n_colloid < n_liquid and the near-field optical intensification is concentrated at and beyond the edge of the particle. Detailed experimental characterization of the irradiated Si surface using atomic force microscopy reveals contrasting topographies. The same optical behavior is observed with both liquids, i.e., the incident laser light diverges on interaction with the colloidal particle, but the resulting interaction with the substrate is liquid dependent. Topographic analysis indicates localized ablation and patterning of the Si substrate when using toluene, whereas the patterning induced under carbon tetrachloride is on a larger scale and extends well below the original substrate surface-hinting at a laser induced photochemical contribution to the surface patterning.

Overstepping the upper refractive index limit to form ultra-narrow photonic nanojets

In general, photonic nanojets (PNJs) occur only when the refractive index (Ri) difference between the microparticle and background media is less than 2. The minimum full width at half-maximum (FWHM) of the PNJ is ~130 nm (approximately one-third of the illumination wavelength λ = 400 nm) formed within the evanescent field region. This paper proposes and studies a method to overstep the Ri upper bound and generate ultra-narrow PNJs. Finite element method based numerical investigations and ray-optics theoretical analyses have realized ultra-narrow PNJs with FWHM as small as 114.7 nm (0.287 λ) obtained from an edge-cut, length-reduced and parabolic-profiled microparticle with Ri = 2.5 beyond evanescent decay length. Using simple strain or compression operations, sub-diffraction-limited PNJs can be flexibly tuned on the order of several wavelengths. Such ultra-narrow PNJs offer great prospects for optical nonlinearity enhancements of greater enhancing effect, optical nanoscopy of higher spatial resolution, optical microprobes of smaller measurement accuracy, nano/micro-sized sample detections of higher sensing sensitivity, nanoscale objects of more accurate control, advanced manufactures of smaller processing size, optical-disk storage of larger data capacity and all-optical switching of lower energy consumption.

Substrate surface patterning by optical near field modulation around colloidal particles immersed in a liquid

Optical near field enhancements in the vicinity of particles illuminated by laser light are increasingly recognized as a powerful tool for nanopatterning applications, but achieving sub-wavelength details from the near-field distribution remains a challenge. Here we present a quantitative analysis of the spatial modulation of the near optical fields generated using single 8 ps, 355 nm (and 532 nm) laser pulses around individual colloidal particles and small close packed arrays of such particles on silicon substrates. The analysis is presented for particles in air and, for the first time, when immersed in a range of liquid media. Immersion in a liquid allows detailed exploration of the effects on the near field of changing not just the magnitude but also the sign of the refractive index difference between the particle and the host medium. The level of agreement between the results of ray tracing and Mie scattering simulations, and the experimentally observed patterns on solid surfaces, should encourage further modelling, predictions and demonstrations of the rich palette of sub-wavelength surface profiles that can be achieved using colloidal particles immersed in liquids.

3-D patterning of silicon by laser-initiated, liquid-assisted colloidal (LILAC) lithography

We report a comprehensive study of laser-initiated, liquid-assisted colloidal (LILAC) lithography, and illustrate its utility in patterning silicon substrates. The method combines single shot laser irradiation (frequency doubled Ti-sapphire laser, 50fs pulse duration, 400nm wavelength) and medium-tuned optical near-field effects around arrays of silica colloidal particles to achieve 3-D surface patterning of silicon. A monolayer (or multilayers) of hexagonal close packed silica colloidal particles act as a mask and offer a route to liquid-tuned optical near field enhancement effects. The resulting patterns are shown to depend on the difference in refractive index of the colloidal particles (ncolloid) and the liquid (nliquid) in which they are immersed. Two different topographies are demonstrated experimentally: (a) arrays of bumps, centred beneath the original colloidal particles, when using liquids with nliquid<ncolloid, and (b) a combination of holes, created in the interstices between the colloidal particles, and bumps when using liquids with nliquid>ncolloid - and explained with the aid of complementary Mie scattering simulations. The LILAC lithography technique has potential for rapid, large area, organized 3-D patterning of silicon (and related) substrates.

Super-long photonic nanojet generated from liquid-filled hollow microcylinder

Photonic nanojet (PNJ) from liquid-filled hollow microcylinder (LFHM) under a liquid immersion condition is numerically investigated based on the finite element method and physically analyzed with ray optics. Simulation and analysis results show that, by simultaneously introducing the immersed liquid and filled liquid, the propagation beam is greatly flattened, and super-long PNJs with decay length more than 100 times the illumination wavelengths are obtained in the outer near-field region of the LFHM. With the variation of the refractive index contrast between the filled and immersed-liquids, the properties of the PNJs, such as the focal distance, decay length, full width at half-maximum, and maximum light intensity can be flexibly tuned.

High-throughput nanofabrication of infra-red and chiral metamaterials using nanospherical-lens lithography

Various infra-red and planar chiral metamaterials were fabricated using the modified Nanospherical-Lens Lithography. By replacing the light source with a hand-held ultraviolet lamp, its asymmetric light emission pattern produces the elliptical-shaped photoresist holes after passing through the spheres. The long axis of the ellipse is parallel to the lamp direction. The fabricated ellipse arrays exhibit localized surface plasmon resonance in mid-infra-red and are ideal platforms for surface enhanced infra-red absorption (SEIRA). We also demonstrate a way to design and fabricate complicated patterns by tuning parameters in each exposure step. This method is both high-throughput and low-cost, which is a powerful tool for future infra-red metamaterials applications.

Detection of embedded ultra-subwavelength-thin dielectric features using elongated photonic nanojets

Photonic nanojets have been previously shown (both theoretically and experimentally) to be highly sensitive to the presence of an ultra-subwavelength nanoscale particle within the nanojet. In the present work, photonic nanojets elongated by almost an order of magnitude (relative to the latest previously published work) are found to possess another key characteristic: they are sensitive to the presence of ultra-subwavelength nanoscale thin features embedded within a dielectric object. This additional characteristic of photonic nanojets is demonstrated through comparisons between fundamentally different 3-D and corresponding 1-D full Maxwell's equations finite-difference time-domain (FDTD) models.