Deep-subwavelength Structures Research Articles

Plasmonic lithography fast imaging model based on least square fitting for periodic patterns

As a new and alternative lithography technology, plasmonic lithography can break through the diffraction limit of traditional lithography by exciting the surface plasmon polaritons to make the evanescent wave at the mask participate in imaging. Plasmonic lithography is capable of fabricating deep subwavelength structures for nanophotonics, metasurfaces, and various other applications, and it is expected to be applied to integrated circuit manufacturing. The photoresist aerial image distribution of different mask patterns can be calculated by establishing an imaging model, which is the basis for understanding and further optimizing imaging. Based on the idea of machine learning and least square fitting, a fast imaging model for plasmonic lithography is established, including a one-dimensional line/space periodic pattern and a two-dimensional square hole pattern, which can be used as a supplement to the previous model developed by Ding et al. [Opt. Express 31, 192 (2023)OPEXFF1094-408710.1364/OE.476825]. Compared with the rigorous numerical method, the fast imaging model can greatly improve the calculation speed with high accuracy. Under the same hardware conditions, the calculation speed of the 1D fast imaging model is improved by two orders of magnitude, and the 2D fast imaging model is improved by about 25 times, which creates conditions for the development of computational lithography technology.

Flat-band Friedrich-Wintgen bound states in the continuum based on borophene metamaterials.

Many applications involve the phenomenon of a material absorbing electromagnetic radiation. By exploiting wave interference, the efficiency of absorption can be significantly enhanced. Here, we propose Friedrich-Wintgen bound states in the continuum (F-W BICs) based on borophene metamaterials to realize coherent perfect absorption with a dual-band absorption peak in commercially important communication bands. Metamaterials consist of borophene gratings and a borophene sheet that can simultaneously support a Fabry-Perot plasmon resonance and a guided plasmon mode. The formation and dynamic modulation of the F-W BIC can be achieved by adjusting the width or carrier density of the borophene grating, while the strong coupling leads to the anti-crossover behavior of the absorption spectrum. Due to the weak angular dispersion originating from the intrinsic flat-band characteristic of the deep sub-wavelength periodic structure, the proposed plasmonic system exhibits almost no change in wavelength and absorption at large incident angles (within 70 degrees). In addition, we employ the temporal coupled-mode theory including near- and far-field coupling to obtain strong critical coupling, successfully achieve coherent perfect absorption, and can realize the absorption switch by changing the phase difference between the two coherent beams. Our findings can offer theoretical support for absorber design and all-optical tuning.

Sub-70 nm surface structures on femtosecond laser irradiated GaAs in distilled water and sensing application.

This study reveals the possibility of distinct ablation mechanisms at different radial positions of the ablated track on GaAs when ablated with femtosecond pulses in distilled water. From the center to the edges of the ablated track, fascinating features such as micron-sized cones, nano-pores, and nano-ripple trenches (average size of 60-70 nm) were observed. The requirement for simulations incorporating the variations in a Gaussian beam fluence and dynamics of the melt flow/surrounding media is discussed. Deep-subwavelength structures, i.e., nano-ripple trenches with a ripple size of ∼λ/11 are achieved on the GaAs surface in this study. Further, these GaAs surface structures acted as excellent hybrid surface-enhanced Raman spectroscopy platforms upon gold coating.

Labyrinth acoustic metamaterials with fractal structure based on Hilbert curve

Acoustic metamaterials can break the law of mass and manipulate acoustic wave propagation with a deep subwavelength structure. We designed a labyrinthine acoustic metamaterial with ultra-high refractive index fractal structure based on the construction method of Hilbert curve, which is capable of acoustic wave control in the deep subwavelength range. Simulation by finite element software reveals that the band gap moves to lower frequencies as the fractal level increases. The transmission characteristics analysis shows that stable acoustic isolation performance can be obtained at the frequency points of monopole resonance (800 Hz) and dipole resonance (1520 Hz), and the isolation performance at this point shows strong stability to very small filling rates or small displacements. In addition, the designed second-order acoustic metamaterial can achieve sound cloaking and sound supertunneling.

Nanoscale control of non-reciprocal ripple writing.

Femtosecond laser-induced deep-subwavelength structures have attracted much attention as a nanoscale surface texturization technique. A better understanding of the formation conditions and period control is required. Herein, we report a method of non-reciprocal writing via a tailored optical far-field exposure, where the period of ripples varies along different scanning directions, and achieve a continuous manipulation of the period from 47 to 112 nm (±4 nm) for a 100-nm-thick indium tin oxide (ITO) on glass. A full electromagnetic model was developed to demonstrate the redistributed localized near-field at different stages of ablation with nanoscale precision. It explains the formation of ripples and the asymmetry of the focal spot determines the non-reciprocity of ripple writing. Combined with beam shaping techniques, we achieved non-reciprocal writing (regarding scanning direction) using an aperture-shaped beam. The non-reciprocal writing is expected to open new paths for precise and controllable nanoscale surface texturing.

Topological surface plasmon resonance in deep subwavelength structure

We propose a plasmonic topological junction structure that combines topological robustness and subwavelength localization properties. The proposed structure is a junction of two topologically distinguished metal-insulator-metal waveguide gratings. Assuming Au–SiO2–Au waveguide gratings, we theoretically demonstrate a plasmonic Jackiw-Rebbi-state resonance which has substantially small effective mode-field area in the order of 0.1 μm2 at 1560 nm wavelength and requires only 10 periods for fully sustaining the ideal resonance Q-factor of the infinitely large array. Therefore, our proposed approach provides an efficient scheme for creating various plasmonic leaky-mode resonance components that can take advantages of spectral robustness and strong localization properties.

Control of localization and optical properties with deep-subwavelength engineered disorder.

The effect of deep subwavelength disorder in one-dimensional dichromic multilayer films on the optical transmission, localization length, and Goos-Hänchen shift around the critical angle is analyzed using sets of disordered multilayer films with different degrees of order metric τ. For each Gaussian-perturbed multilayer film designed by a Metropolis algorithm targeting the predetermined order metric τ, the numerically obtained localization length and transmission show excellent agreement with the recent theoretical analysis developed for disordered multilayer films, further revealing τ-dependence of the Goos-Hänchen shift across the critical angle. Emphasizing the role of deep subwavelength structures in disorder-induced transmission enhancement, our result thus paves the way toward the inverse design of a deep subwavelength disordered structural landscape for the targeted order metric τ or abnormal optical responses - including the Goos-Hänchen shift.

Broadband low-frequency acoustic absorber based on metaporous composite

Broadband absorption of low-frequency sound waves via a deep subwavelength structure is of great and ongoing interest in research and engineering. Here, we numerically and experimentally present a design of a broadband low-frequency absorber based on an acoustic metaporous composite (AMC). The AMC absorber is constructed by embedding a single metamaterial resonator into a porous layer. The finite element simulations show that a high absorption (absorptance A > 0.8) can be achieved within a broad frequency range (from 290 Hz to 1074 Hz), while the thickness of AMC is 1/13 of the corresponding wavelength at 290 Hz. The broadband and high-efficiency performances of the absorber are attributed to the coupling between the two resonant absorptions and the trapped mode. The numerical simulations and experimental results are obtained to be in good agreement with each other. Moreover, the high broadband absorption can be maintained under random incident acoustic waves. The proposed absorber provides potential applications in low-frequency noise reduction especially when limited space is demanded.

Laser Fabrication of Nanoholes on Silica through Surface Window Assisted Nano-Drilling (SWAN).

Nano-structures have significant applications in many fields such as chip fabrications, nanorobotics, and solar cells. However, realizing nanoscale structures on hard and brittle materials is still challenging. In this paper, when processing the silica surface with a tightly focused Bessel beam, the smallest nanohole with ~20 nm diameter has been realized by precisely controlling the interior and superficial interaction of the silica material. An effective surface window assisted nano-drilling (SWAN) mechanism is proposed to explain the generation of such a deep subwavelength structure, which is supported by the simulation results of energy depositions.

Subwavelength optical localization with toroidal excitations in plasmonic and Mie metamaterials

AbstractSince the performance of electronic circuits is becoming rather limited in face of intensively increasing of amount of information and related operations, all‐optical processing offers a promising strategy for future information system. It would benefit a great deal if the all‐optical processing could be implemented within the developed electronic chips of nanoscale structures. In that it is highly desirable to break the diffraction limit of light for achieving effective light manipulations with deep subwavelength structures compatible with the state‐of‐the‐art nanofabrication processes. It is of fundamental importance to get subwavelength optical localization, that is, squeeze light wave into subwavelength space for achieving freely manipulating of light fields. This review summarizes the development in realizing subwavelength optical localization by exciting toroidal mode in photonic metamaterials. The toroidal excitations in plasmonic metamaterials and Mie resonant metamaterials, in 3D structures and planar metamaterials, with single or few layers in spectral regime from microwave to optical frequencies are surveyed. Based on the discussion on the configurations of toroidal excitations, the recent development on toroidal‐related optical scattering control actively manipulates the toroidal excitations, and promising applications are further investigated and highlighted.image

Low-Frequency Broadband Acoustic Metasurface Absorbing Panels

A broadband sound absorption attained by a deep-subwavelength structure is of great interest to the noise control community especially for extremely low frequencies (20-100 Hz) in room acoustics. In this paper, we report on an analytical, numerical and experimental study of a low-frequency broadband (50-63 Hz, one third octave band), high absorption (average absorption coefficient ≈93%), near-omnidirectional (0° - 75°) acoustic metasurface absorber at a thickness of 15.4 cm (1/45 of the wavelength at 50 Hz). To further broaden the bandwidth (50-100 Hz, one octave band), another design is analytically studied to achieve an average absorption coefficient of 85% for a wide angle range (0°-75°) at a thickness of 20 cm (1/34 of wavelength at 50 Hz). The proposed design methodology may solve the long-standing issue for low frequency absorption in room acoustics.

Opportunities for enhanced omnidirectional broadband absorption of the solar radiation using deep subwavelength structures

Omnidirectional absorption is important to non-tracking systems designed for the harvesting of solar energy. Presently, we examine both experimentally and numerically the broadband absorption under oblique illumination driven by deep sidewall subwavelength structures (DSSS) in silicon nanopillar arrays (DSSS arrays). Specifically, we target DSSS geometries that are a built-in side effect of the top-down cyclic Bosch dry etch process employed to realize high aspect ratio silicon nanopillar (NP) arrays. We numerically compare the DSSS array with an optically-optimized straight-sidewall nanopillar array (SSNP array) under oblique illumination. We show how the presence of DSSS generates a higher absorptivity particularly for the spectral range >600 nm, induces the formation of absorptivity peaks at the near-infrared spectral range, and overall provides an enhanced omnidirectional broadband absorption. We experimentally show that the specular reflectivity and the total reflectivity of silicon DSSS arrays that were fabricated using a top-down Bosch dry etch process is significantly lower compared with that of the corresponding SSNP arrays. Specifically, we show a decrement in the broadband specular reflection of up to 30% for certain angles of illumination, and about 13% decrement in the total reflectivity.

An efficient and deterministic photon management using deep subwavelength features

Light trapping and the broadband absorption of the solar radiation are significant to a plethora of absorption-based photonic devices. Specifically, efficient broadband absorption was recently demonstrated with arrays of subwavelength structures. The current study examines both numerically and experimentally light trapping driven by deep sidewall subwavelength structures (DSSS) in silicon nanopillar (NP) arrays (DSSS arrays). Particularly, the focus is on DSSS geometries that are an inherent outcome of the top-down dry etch approach used in arrays of high aspect ratio NPs due to the periodical operationality of the Bosch dry etch method. ~10% enhancement in the broadband absorption of DSSS arrays compared with NP arrays is demonstrated numerically, as well as the generation of near-IR absorptivity peaks of ~25% for DSSS arrays. Importantly, it is shown that the introduction of DSSS systematically blue-shifts the absorptivity peaks of the NP arrays and in this manner a deterministic light trapping is possible. Finally, decrements of ~40% in direct reflectivity and ~7% in diffused reflectivity in DSSS arrays realized on silicon wafers is demonstrated experimentally.

Efficient mode converter to deep-subwavelength region with photonic-crystal waveguide platform for terahertz applications.

Metallic deep-subwavelength features can aid in integration of microscopic components or strong light-matter interaction with a low-loss dielectric waveguide platform. A mode converter or coupler is required to integrate the devices. However, there is a vast difference in the physical scale and modal distribution between the deep-subwavelength structures and the dielectric waveguide platform. Here, we employ a tapered-slot mode converter to facilitate the electromagnetic wave transition from a gap width smaller than 1/100 of a wavelength (λ) to a larger-scale mode that is amenable to a terahertz (THz) silicon photonic-crystal waveguide. The mode converter is metallic, and fabricated on top of indium phosphide substrate, leading to incongruity with the modal field distribution of the silicon photonic-crystal waveguide. To mitigate this, a sandwiched structure is developed to match the symmetry of the mode of photonic-crystal waveguide, thereby facilitating efficient transfer of energy. For a proof of concept, we integrate a resonant tunneling diode (< 2 µm) as a THz detector in a photonic-crystal waveguide platform in the 0.3-THz band (λ ∼ 1 mm). The coupling efficiency is close to unity (∼90%) with broadband operation (∼50 GHz) in experiments. Thereafter, we employ the developed integrated device as a receiver in a THz communication experiment. In this manner, we successfully achieve real-time error-free data transmission at 32 Gbit/s, and demonstrate wireless transmission of uncompressed 4K high-definition video.

Complex frequency analysis of the scattering matrix for the design of perfect absorbers in reflection and transmission problems

The design of deep-subwavelength structures for low frequency sound perfect absorption is challenging. Subwavelength perfect absorption implies increasing of the density of states at low frequency while maintaining impedance matching to the surrounding medium. Therefore, the study of the eigenvalues and eigenvectors of the scattering matrix in the complex frequency plane appears extremely powerful to analyze such systems and derive the critical coupling condition. The latter consists in exactly compensating the leakage of the structure by using the intrinsic losses. Two simple structures, the thicknesses of which are 88 times and 40 times smaller than the perfectly absorbed wavelengths, are presented for reflexion and transmission problems, respectively.

(Invited) Raman Stress Measurements at the Nanoscale

In semiconductor technology, mechanical stress is one of the principal performance boosters available to device engineering for the development of state-of-the-art logic transistors. Currently available strain characterization techniques are either destructive or have low spatial resolution because they use broad beams (Raman, X-ray diffraction). Fast, accurate and non-invasive stress measurements beyond the diffraction limit remain a challenge. However, it was recently found that the geometry of the structure itself can be used as an enhancement tool to bring out the relevant Raman signals from the region of interest. The theoretical understanding of the effect was fine-tuned such that the concept can be applied on the materials and structures of choice. Recently a strategy was developed to extract detailed and quantitative stress values of deep sub-wavelength semiconductor structures. The approach enables the quick and non-invasive quantitative characterization of stress inside CMOS devices of the next generation.

Pantoscopic and temperature-controlled dual-band perfect absorber based on strontium titanate material

A nearly perfect dual-band deep sub-wavelength scale and temperature-controlled terahertz absorber is theoretically investigated. The absorber is composed of a periodic metal-dielectric stack array placed on a metallic substrate. Simulation results show that there are two distinct absorption peaks located at frequencies of about 0.140 THz and 0.192 THz in the spectra line under room temperature T = 300 K, and their maximum absorption rates are respectively 99.133% and 98.251%. It is also found that the period of the unit cell is less than 8.97% of the minimum resonant wavelength, which means the deep sub-wavelength absorber structure. Furthermore, it is shown that the proposed absorber is polarization-insensitive, and has good tolerance to the incident angle for both TE and TM wave, i.e., it is pantoscopic for the incident light. Meanwhile, when the temperature changes from 200 K to 400 K, the absorption of the two peaks did not change significantly within the considered frequency range, but the resonant frequency of the two absorption peaks will have a clear blueshift. The effect of structural parameters on the absorbing performance have also been discussed. This work provides a new idea for the design of frequency-agile deep sub-wavelength scale perfect THz absorbers, and the design scheme may be easily extended to the near ultraviolet, optical, infrared, terahertz and millimeter-wave regions.

Terahertz wave interaction with metallic nanostructures

AbstractUnderstanding light interaction with metallic structures provides opportunities of manipulation of light, and is at the core of various research areas including terahertz (THz) optics from which diverse applications are now emerging. For instance, THz waves take full advantage of the interaction to have strong field enhancement that compensates their relatively low photon energy. As the THz field enhancement have boosted THz nonlinear studies and relevant applications, further understanding of light interaction with metallic structures is essential for advanced manipulation of light that will bring about subsequent development of THz optics. In this review, we discuss THz wave interaction with deep sub-wavelength nano structures. With focusing on the THz field enhancement by nano structures, we review fundamentals of giant field enhancement that emerges from non-resonant and resonant interactions of THz waves with nano structures in both sub- and super- skin-depth thicknesses. From that, we introduce surprisingly simple description of the field enhancement valid over many orders of magnitudes of conductivity of metal as well as many orders of magnitudes of the metal thickness. We also discuss THz interaction with structures in angstrom scale, by reviewing plasmonic quantum effect and electron tunneling with consequent nonlinear behaviors. Finally, as applications of THz interaction with nano structures, we introduce new types of THz molecule sensors, exhibiting ultrasensitive and highly selective functionalities.

Formation of Deep-Subwavelength Structures on Organic Materials by Femtosecond Laser Ablation

Femtosecond (fs-) laser induced subwavelength and deep-subwavelength periodic structures have attracted attention due to their sub-diffraction feature size and rich physics of laser-matter interaction. However, the formation mechanisms of fs-laser induced deep-subwavelength structures on organic materials have not been reported nor systematically studied. Herein, based on the degree of laser induced free-carrier excitation, the formation of deep-subwavelength structures by fs-laser ablation was systematically studied on popular materials: negative tone resists SU8 and conductive polymer poly-3, 4-ethylenedioxythiophene: poly-(styrenesulfonate) (PEDOT: PSS). A weak propensity to form deep-subwavelength structures was found on poly-methylmethacrylate and polyvinyl alcohol. The photo-excited electrons forming the surface plasmonic wave contributed to energy localization and absorption, which led to the imprinting of deep-subwavelength structures on polymers. It has been found that materials with $\pi $ -bonds and benzene rings were most susceptible to deep-subwavelength structure formation. Based on a laser-induced surface plasmonic model, the period of regular patterns was estimated and was in a good accordance with experimental results. The proposed formation mechanism of the deep-subwavelength structures on organic materials extends an application field of fs-laser micro/nanomachining of polymers.

Design of angle-resolved illumination optics using nonimaging bi-telecentricity for 193 nm scatterfield microscopy

Accurate optics-based dimensional measurements of features sized well-below the diffraction limit require a thorough understanding of the illumination within the optical column and of the three-dimensional scattered fields that contain the information required for quantitative metrology. Scatterfield microscopy can pair simulations with angle-resolved tool characterization to improve agreement between the experiment and calculated libraries, yielding sub-nanometer parametric uncertainties. Optimized angle-resolved illumination requires bi-telecentric optics in which a telecentric sample plane defined by a Köhler illumination configuration and a telecentric conjugate back focal plane (CBFP) of the objective lens; scanning an aperture or an aperture source at the CBFP allows control of the illumination beam angle at the sample plane with minimal distortion. A bi-telecentric illumination optics have been designed enabling angle-resolved illumination for both aperture and source scanning modes while yielding low distortion and chief ray parallelism. The optimized design features a maximum chief ray angle at the CBFP of 0.002° and maximum wavefront deviations of less than 0.06 λ for angle-resolved illumination beams at the sample plane, holding promise for high quality angle-resolved illumination for improved measurements of deep-subwavelength structures using deep-ultraviolet light.