Metallic On-Chip Light Concentrators Fabricated by In Situ Plasmonic Etching Technique

One-dimensional tapered metallic nanostructures are highly interesting for nanophotonic applications because of their plasmonic waveguiding and field-focusing properties. Here, we developed an in situ etching technique for unique tapered crystallized silver nanowire fabrication. Under the focused laser spot, plasmon-induced charge separation of chemically synthesized nanowires is excited, which triggers the uniaxial etching of silver nanowires along the radial direction with decreasing rate, forming tapered structures several micrometers long and with diameter attenuating from hundreds to tens of nanometers. These tapered metallic nanowires have smooth surfaces showing excellent performance for plasmonic waveguiding, and can be good candidates for nanocircuits and remote-excitation sources.

Feasibility of X-ray beam nanofocusing with compound refractive lenses

A more general analytical theory of X-ray beam propagation through compound refractive lenses (CRLs) than the earlier study by Kohn [(2003). JETP, 97, 204-215] is presented. The problem of nanofocusing with CRLs is examined in detail. For a CRL with a relatively large aperture the focusing efficiency is limited by the radiation absorption in the lens material. The aperture does not affect the focusing process and it is replaced by the effective aperture. The X-ray transverse beam size at the focus is then by a factor of γ = β/δ times smaller than the transverse beam size just behind the CRL. Here, δ and β are the real and imaginary parts of the CRL material refractive index n = 1 - δ + iβ. In this instance, to improve focusing efficiency, it is advantageous to decrease the CRL aperture and increase the photon energy E. However, with increasing photon energy, the material absorption decreases, which results in the CRL aperture impact on the transverse beam size. The latter leads to the fact that with a proper CRL length the beam size is independent of both the aperture and photon energy but depends only on the CRL material electron density and is approximately equal to w_c = λ/(8δ)^1/2, where λ denotes the radiation wavelength, as predicted by Bergemann et al. [(2003). Phys. Rev. Lett, 91, 204801].

Light Engineering in Nanometer Space

Significant advances have been made in photonic integrated circuit technology, similar to the development of electronic integrated circuits. However, the miniaturization of cavity resonators, which are the essential components of photonic circuits, still requires considerable improvement. Over the past decades, various optical cavities have been utilized to implement next-generation light sources in photonic circuits with low energy, high data traffic, and integrable physical sizes. Nevertheless, it has been difficult to reduce the size of most commercialized cavities beyond the diffraction limit while maintaining high performance. Herein, recent advancements in subwavelength metallic cavities that can improve performance, even with the use of lossy plasmonic modes, are reviewed. The discussion is divided in three parts according to light engineering methods: subwavelength metal-clad cavities engineered using intermediate dielectric cladding; implementation of plasmonic cavities and waveguides using plasmonic crystals; and development of deep-subwavelength plasmonic waveguides and cavities using geometric engineering. A direction for further developments in photonic integrated circuit technology is also discussed, along with its practical application.

Metamaterial Lensing Devices

In recent years, the development of metamaterials and metasurfaces has drawn great attention, enabling many important practical applications. Focusing and lensing components are of extreme importance because of their significant potential practical applications in biological imaging, display, and nanolithography fabrication. Metafocusing devices using ultrathin structures (also known as metasurfaces) with superlensing performance are key building blocks for developing integrated optical components with ultrasmall dimensions. In this article, we review the metamaterial superlensing devices working in transmission mode from the perfect lens to two-dimensional metasurfaces and present their working principles. Then we summarize important practical applications of metasurfaces, such as plasmonic lithography, holography, and imaging. Different typical designs and their focusing performance are also discussed in detail.

X-ray mirror surface figure correction with nanometre precision controlled by layer stresses simulated by FEA

Hard X-rays with energies higher than several kiloelectronvolts can be focused to spot sizes below 10 nm with the present synchrotron beamlines, offering unique advantages for the chemical, elemental and structure analysis of matter. Nevertheless, a surface precision on the nanometre scale for the focusing optics is required and remains the main hurdle limiting X-ray analytical techniques with single-nanometre spatial resolution. On the other hand, to preserve the wavefront properties of coherent X-ray beams, precise control of the reflective mirror surface quality at the nanometre scale is demanded for X-ray free-electron laser applications. In this work, the surface shape of a multilayer-coated X-ray mirror is controlled by layer stresses. The desired surface profile of the mirror is differentiated to its second order to obtain its corresponding curvature profile. With a step size of 1 mm along the mirror length, different coating thicknesses are applied to create different layer thermal stresses from uniform temperature change. The mirror surface profile can be obtained by integrating the curvature profile to its second order and further corrected by moving constant values for the slope and height. The technical process is simulated by finite element analysis (FEA). A case study showed that the residual slope error and the residual height error between the desired shape and the FEA result are 0.22 µrad (r.m.s.) and 1.42 nm (r.m.s.), respectively.

Selective Coherent Anti-Stokes Raman Scattering Microscopy Employing Dual-Wavelength Nanofocused Ultrafast Plasmon Pulses

Ultrafast surface plasmon polariton (SPP) nanofocusing on a plasmonic metal tapered tip with femtosecond laser pulses enables background-free localized excitation beyond the diffraction limit. We demonstrate simultaneous nanofocusing of ultrafast SPP pulses at 440 and 800 nm, which were coupled with a common diffraction grating structure fabricated on an aluminum (Al) tapered tip, to the tip apex with a radius of ∼35 nm. We achieved selective coherent anti-Stokes Raman scattering (CARS) microscopy that combined an 800 nm (ω) SPP pump pulse, which achieves selective vibrational excitation by spectral focusing, and a 440 nm (2ω) SPP probe pulse. Raman intensity of this novel 2ω-CARS increased by a factor of 3.96 at the G-band and 4.00 at the 2D-band compared with that with ω-CARS for the monolayer graphene. The 2ω-CARS imaging method was applied for imaging a multiwalled carbon nanotube at the D-, G-, and 2D-bands. This dual-wavelength nanofocusing will open up new nanoscale microspectroscopy and optical excitation at the tip apex, such as sum frequency mixing, two-photon excitation.

Aberration-free aspherical lens shape for shortening the focal distance of an already convergent beam

The shapes of single lens surfaces capable of focusing divergent and collimated beams without aberration have already been calculated. However, nanofocusing compound refractive lenses (CRLs) require many consecutive lens surfaces. Here a theoretical example of an X-ray nanofocusing CRL with 48 consecutive surfaces is studied. The surfaces on the downstream end of this CRL accept X-rays that are already converging toward a focus, and refract them toward a new focal point that is closer to the surface. This case, so far missing from the literature, is treated here. The ideal surface for aberration-free focusing of a convergent incident beam is found by analytical computation and by ray tracing to be one sheet of a Cartesian oval. An `X-ray approximation' of the Cartesian oval is worked out for the case of small change in index of refraction across the lens surface. The paraxial approximation of this surface is described. These results will assist the development of large-aperture CRLs for nanofocusing.

Broadband Ultra-Deep Sub-Diffraction-Limit Optical Focusing by Metallic Graded-Index (MGRIN) Lenses

The development of techniques for efficiently confining energy in the visible and infrared spectral regions to the deep subwavelength spatial scale with dimensions as small as a few nanometers would have great significance for scientific research and engineering practices. Such an ability to manipulate light is impossible for conventional dielectric lenses due to the diffraction limit. Here, we propose a metallic graded-index (MGRIN) lens formed by an array of coupled metallic waveguides with identical nanoscale widths embedded by index-varying dielectrics to enable the optical nanofocusing. The focusing mechanism of the MGRIN lens is theoretically investigated based on Hamiltonian optics, which are verified by the finite-difference time-domain (FDTD) method. Numerical results reveal that an ultra-deep subwavelength focus of 8 nm (λ/500) with a long focal depth (1.93λ) and enhanced field intensity can be achieved. Moreover, the nanofocusing capability of the MGRIN lens without redesigning the structure can be well kept when the incident wavelength changes over a broad range from visible to infrared. Our design of optical nanofocusing shows great potential for use in nano-optics and nanotechnology.

Size-Reduction Template Stripping of Smooth Curved Metallic Tips for Adiabatic Nanofocusing of Surface Plasmons

We present a new technique to engineer metallic interfaces to produce sharp tips with smooth curved surfaces and variable tip angles, as well as ridges with arbitrary contour shapes, all of which can be integrated with grating couplers for applications in plasmonics and nanophotonics. We combine template stripping, a nanofabrication scheme, with atomic layer deposition (ALD) to produce the ultrasharp nanoscale tips and wedges using only conventional photolithography. Conformal ALD coating of insulators over silicon trench molds of various shapes reduces their widths to make nanoscale features without high-resolution lithography. Along with a metal deposition and template stripping, this size-reduction scheme can mass-produce narrow and ultrasharp (<10 nm radius of curvature) metallic wedges and tips over an entire 4 in. wafer. This size-reduction scheme can create metallic tips out of arbitrary trench patterns that have smooth curved surfaces to facilitate efficient adiabatic nanofocusing which will benefit applications in near-field optical spectroscopy, plasmonic waveguides, particle trapping, hot-electron plasmonics, and nonlinear optics.

Boosting Local Field Enhancement by on-Chip Nanofocusing and Impedance-Matched Plasmonic Antennas

Strongly confined surface plasmon-polariton modes can be used for efficiently delivering the electromagnetic energy to nanosized volumes by reducing the cross sections of propagating modes far beyond the diffraction limit, that is, by nanofocusing. This process results in significant local-field enhancement that can advantageously be exploited in modern optical nanotechnologies, including signal processing, biochemical sensing, imaging, and spectroscopy. Here, we propose, analyze, and experimentally demonstrate on-chip nanofocusing followed by impedance-matched nanowire antenna excitation in the end-fire geometry at telecom wavelengths. Numerical and experimental evidence of the efficient excitation of dipole and quadrupole (dark) antenna modes are provided, revealing underlying physical mechanisms and analogies with the operation of plane-wave Fabry-Pérot interferometers. The unique combination of efficient nanofocusing and nanoantenna resonant excitation realized in our experiments offers a major boost to the field intensity enhancement up to ∼12000, with the enhanced field being evenly distributed over the gap volume of 30 × 30 × 10 nm(3), and promises thereby a variety of useful on-chip functionalities within sensing, nonlinear spectroscopy and signal processing.

Nanofocusing of X-ray free-electron lasers by grazing-incidence reflective optics

Total-reflection mirror devices for X-ray free-electron laser focusing are discussed in terms of optical design, mirror-fabrication technology, a wavefront diagnosis method and radiation-damage testing, as a review of the present status of the focusing optics at the SPring-8 angstrom compact free-electron laser (SACLA). Designed beam sizes of 1 µm and 50 nm, and spot sizes almost matching prediction have been achieved and used to explore topics at the forefront of natural science. The feasibility of these devices is determined to be sufficient for long-term and stable operation at SACLA by investigating the radiation-damage threshold and achievable accuracies in the mirror figure and alignment.

On-chip hybrid photonic-plasmonic light concentrator for nanofocusing in an integrated silicon photonics platform

The enhancement and confinement of electromagnetic radiation to nanometer scale have improved the performances and decreased the dimensions of optical sources and detectors for several applications including spectroscopy, medical applications, and quantum information. Realization of on-chip nanofocusing devices compatible with silicon photonics platform adds a key functionality and provides opportunities for sensing, trapping, on-chip signal processing, and communications. Here, we discuss the design, fabrication, and experimental demonstration of light nanofocusing in a hybrid plasmonic-photonic nanotaper structure. We discuss the physical mechanisms behind the operation of this device, the coupling mechanisms, and how to engineer the energy transfer from a propagating guided mode to a trapped plasmonic mode at the apex of the plasmonic nanotaper with minimal radiation loss. Optical near-field measurements and Fourier modal analysis carried out using a near-field scanning optical microscope (NSOM) show a tight nanofocusing of light in this structure to an extremely small spot of 0.00563(λ/(2n(rmax)))(3) confined in 3D and an exquisite power input conversion of 92%. Our experiments also verify the mode selectivity of the device (low transmission of a TM-like input mode and high transmission of a TE-like input mode). A large field concentration factor (FCF) of about 4.9 is estimated from our NSOM measurement with a radius of curvature of about 20 nm at the apex of the nanotaper. The agreement between our theory and experimental results reveals helpful insights about the operation mechanism of the device, the interplay of the modes, and the gradual power transfer to the nanotaper apex.

Vertically oriented, three-dimensionally tapered deep-subwavelength metallic nanohole arrays developed by photofluidization lithography

The field-gradient, superficial photo fluidization of azomaterials allows a specific 3D nano-silhouette to be shaped over a large area, so as to get easy access to a 3D-tapered, deep sub-wavelength Au nanohole (20 nm spatial size) array. The squeezing of visible light into the deep sub-wavelength point and the relevant extraordinary optical transmission (EOT) are achieved using this 3D-tapered, 20 nm Au nanohole.

Sub-100 nm focusing of short wavelength plasmons in homogeneous 2D space

We present a direct measurement of short-wavelength plasmons focused into a sub-100 nm spot in homogeneous (translation invariant) 2D space. The short-wavelength (SW) surface plasmon polaritons (SPP) are achieved in metal-insulator-insulator (MII) platform consisting of silver, silicon nitride, and air. This platform is homogeneous in two spatial directions and supports SPP at wavelength more than two times shorter than that in free space yet interacts with the outer world through the evanescent tail in air. We use an apertureless (scattering) near-field scanning optical microscope (NSOM) to map directly the amplitude and phase of these SW-SPP and show they can be focused to under 70 nm without structurally assisted confinement such as nanoantennas or nanofocusing. This, along with the use of visible light at 532 nm which is suitable for optical microscopy, can open new directions in direct biological and medical imaging at the sub-100 nm resolution regime.