Effect of CHF3/Ar Gas Flow Ratio on Self-masking Subwavelength Structures Prepared on Fused Silica Surface

Based on an advanced technology, randomly-aligned subwavelength structures（SWSs） were obtained by a metal-nanodot-induced one-step self-masking reactive-ion-etching process on a fused silica surface. Metal-fluoride（mainly ferrous-fluoride） nanodots induce and gather stable fluorocarbon polymer etching inhibitors in the reactive-ion-etching polymers as masks. Metal fluoride（mainly ferrous fluoride） is produced by the sputtering of argon plasma and the ion-enhanced chemical reaction of metal atoms. With an increase in CHF_3/Ar gas flow ratio, the average height of the SWSs increases, the number of SWSs per specific area increases and then decreases, and the optical transmittance of visible light increases and then decreases. The optimum CHF_3/Ar gas flow ratio for preparing SWSs is 1：5.

Novel Even Beam Splitters Based on Subwavelength Binary Simple Periodic Rectangular Structure

In this paper, a novel method of a subwavelength binary simple periodic rectangular structure is presented to realize even beam splitting by combining the rigorous couple-wave analysis with the genetic algorithm. Several even splitters in the terahertz region were designed and one of the silicon-based beam splitters designed to separate one incident beam into four emergent beams has total efficiency up to 92.23 %. Zero-order diffraction efficiency was reduced to less than 0.192 % and the error of uniformity decreased to 6.51 9 10-6. These results break the limitation of even beam splitting based on the traditional scalar theory. In addition, the effects of the incident angle, wavelength, as well as the polarizing angle on the diffraction efficiency and uniformity were also investigated.

Heat accumulation effects in femtosecond laser-induced subwavelength periodic surface structures on silicon

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for femtosecond laser-induced subwavelength periodic surface structures on silicon.Femtosecond laser micromachining is no longer a nonthermal process,as long as the repetition rate reaches up to 100 kHz due to heat accumulation.Moreover,a higher repetition rate generates much better defined ripple structures on the silicon surface,based on the fact that accumulated heat raises lattice temperature to the melting point of silicon(1687 K),with more intense surface plasmons excited simultaneously.Comparison of the surface morphology on repetition rate and on the overlapping rate confirms that repetition rate and pulse overlapping rate are two competing factors that are responsible for the period of ripple structures.Ripple period drifts longer because of a higher repetition rate due to increasing electron density;however,the period of laser structured surface is significantly reduced with the pulse overlapping rate.The Maxwell–Garnett effect is confirmed to account for the ripple period-decreasing trend with the pulse overlapping rate.

Microtrap on a concave grating reflector for atom trapping

We propose a novel scheme of optical confinement for atoms by using a concave grating reflector.The two-dimension grating structure with a concave surface shape exhibits strong focusing ability under radially polarized illumination.Especially,the light intensity at the focal point is about 100 times higher than that of the incident light.Such a focusing optical field reflected from the curved grating structure can provide a deep potential to trap cold atoms.We discuss the feasibility of the structure serving as an optical dipole trap.Our results are as follows.（i） Van der Waals attraction potential to the surface of the structure has a low effect on trapped atoms,（ⅱ） The maximum trapping potential is ~1.14 mK in the optical trap,which is high enough to trap cold ^87Rb atoms from a standard magneto-optical trap with a temperature of 120 μK,and the maximum photon scattering rate is lower than 1/s.（ⅲ） Such a microtrap array can also manipulate and control cold molecules,or microscopic particles.

High sensitivity refractive index gas sensing enhanced by surface plasmon resonance with nano-cavity antenna array

The surface plasmon resonance gas sensor is presented for refractive index detection using nano-cavity antenna array. The gas sensor monitors the changes of the refractive index by measuring the spectral shift of the resonance dip, for modulating the wavelength of incident light. It is demonstrated that minute changes in the refractive index of a medium close to the surface of a metal film, owing to a shift in the resonance dip of the wavelength, can be detected. The average detection sensitivity is about 3200 nm/RIU （refractive index units）, which is more than twice that of a metal grating-based gas sensor. The reflectivity of the surface plasmon resonance dip is only - 0.03%, and the full widths at half maximum （FWHMs） of bandwidth of the angle and wavelength are - 0.20° and 4.71 nm, respectively.

Sensitivity Analysis of a Bioinspired Refractive Index Based Gas Sensor

It was found out that the change of refractive index of ambient gas can lead to obvious change of the color of Morpho butterfly＇s wing. Such phenomenon has been employed as a sensing principle for detecting gas. In the present study, Rigorous Coupled-Wave Analysis （RCWA） was described briefly, and the partial derivative of optical reflection efficiency with respect to the refractive index of ambient gas, i.e., sensitivity of the sensor, was derived based on RCWA. A bioinspired grating model was constructed by mimicking the nanostructure on the ground scale of Morpho didius butterfly＇s wing. The analytical sensitivity was verified and the effect of the grating shape on the reflection spectra and its sensitivity were discussed. The results show that by tuning shape parameters of the grating, we can obtain desired reflection spectra and sensitivity, which can be applied to the design of the bioinspired refractive index based gas sensor.

Dynamic control of the terahertz rainbow trapping effect based on a silicon-filled graded grating

We theoretically propose a scheme to realize the dynamic control of the properties of the terahertz（THz） rainbow trapping effect（RTE） based on a silicon-filled graded grating（SFGG） in a relatively broad band via optical pumping.Through the theoretical analysis and finite-element method simulations, it is conceptually demonstrated that the band of the RTE can be dynamically tuned in a range of -0.06 THz. Furthermore, the SFGG can also be optically switched between a device for the RTE and a waveguide for releasing the trapped waves. The results obtained here may imply applications for the tunable THz plasmonic devices, such as on-chip optical buffers, broad band slow-light systems, and integrated optical filters.

Ultra-thin Glass Film Coated with Graphene: A New Material for Spontaneous Emission Enhancement of Quantum Emitter

We propose an ultra-thin glass film coated with graphene as a new kind of surrounding material which can greatly enhance spontaneous emission rate(SER) of dipole emitter embedded in it. With properly designed parameters,numerical results show that SER-enhanced factors as high as 1.286 9 106 can be achieved. The influences of glass film thickness and chemical potential/doping level of graphene on spontaneous emission enhancement are also studied in this paper. A comparison is made between graphene and other coating materials such as gold and silver to see their performances in SER enhancement.

Physical manipulation of ultrathin-film optical interference for super absorption and two-dimensional heterojunction photoconversion

Ultrathin optical interference in a system composed of absorbing material and metal reflector has attracted extensive attention due to its potential application in realizing highly efficient optical absorption by using extremely thin semiconductor material. In this paper, we study the physics behind the high absorption of ultrathin film from the viewpoint of destructive interference and admittance matching, particularly addressing the phase evolution by light propagation and interface reflection. The physical manipulations of the ultrathin interference effect by controlling the substrate material and semiconductor material/thickness are examined. We introduce typical two-dimensional materials — i.e., MoS2and WSe2— as the absorbing layer with thickness below 10 nm, which exhibits ～ 90% absorption in a large range of incident angle(0°～70°). According to the ultrathin interference mechanism, we propose the ultrathin(< 20 nm) MoS2/WSe2heterojunction for photovoltaic application and carefully examine the detailed optoelectronic responses by coupled multiphysics simulation. By comparing the same cells on SiO2substrate, both the short-circuit current density(up to 20 mA/cm~2) and the photoelectric conversion efficiency(up to 9.5%) are found to be increased by ～200%.

Focusing performance of small f-number metallic lens with depth-modulated slits

This paper studies a small f-number metallic lens with depth-modulated slits. Slits filled with dielectric between silver plates are designed to produce desired optical phase retardations based on the particular propagation properties of surface plasmon polaritons in nanostructures. Numerical simulation of this structure is performed through the finite- difference time-domain method. Different from the conventional dielectric lens, the metallic lens can be used as a pure phase element without energy loss brought by the light refraction at curved surfaces and total internal reflection. The focusing performance is consequently improved, with larger diffraction efficiency than that of the same shaped dielectric lens.

Terahertz broadband polarizer using bilayer subwavelength metal wire-grid structure on polyimide film

A tcrahcrtz （THz） broadband polarizer using bilayer subwavelength metal wire-grid structure on both sides of polyimide fihn is simulated by the finite-difference time-domain method. We amdyze tile effect of film thickness, material loss, and lateral shift between two metallic gratings on the performance of the THz polarizer. Bilayer wire-grid polarizers are fabricated by a simple way of laser induced and non-electrolytic plating with copper. The THz time-domain spectroscopy measurements show that in 0.2 1.6 THz frequency range, the extinction ratio is better than 45 dB, the average extinction ratio reaches 53 dB, and the transmittance exceeds 67%, which shows great advantage over conventional single wire-grid THz polarizer.

Plasmonic nano-printing: large-area nanoscale energy deposition for efficient surface texturing

The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plasmonic nano-printing.Irreversible nanoscale modifications are delivered by surface plasmon polariton(SPP)using:(i)fast scan and(ii)cylindrical focusing of femtosecond laser pulses for a high patterning throughput.The mechanism of ripple formation on ZnS dielectric is experimentally proven to occur via surface wave at the substrate–plasma interface.The line focusing increase the ordering quality of ripples and facilitates fabrication over wafer-sized areas within a practical time span.Nanoprinting using SPP is expected to open new applications in photo-catalysis,tribology,and solar light harvesting via localized energy deposition rather scattering used in photonic and sensing applications based on re-scattering of SPP modes into far-field modes.

Birefringent response of graphene oxide film structurized via femtosecond laser

In-plane birefringent materials present an effective modulation of the optical properties and more degrees of freedom for the signal detection in low dimension,and thus remain a hot topic in realizing the integrated,miniature,and flexible devices for multiple applications.Here,the artificial in-plane birefringence properties have been successfully achieved on a graphene oxide film by a novel femtosecond laser lithography method,which provides a high-speed,large-area,and regular subwavelength gratings(~380 nm)fabrication and photoreduction.The obtained sample manifests an evident optical birefringence(~0.18)and anisotropic photoresponse(~1.21)in the visible range,both of which can be significantly modulated by either the structural morphology or the degree of oxide reduction.Based on the analysis of effective-medium theory and measurements of angle-resolved polarized Raman spectroscopy,the artificial in-plane birefringence is originated from various optical responses of the periodic subwavelength structures for the incident light with different polarization states.This technique shows great advantages for the fabrication of integrated in-plane polarization-dependent devices,which is expected to solve the problems in this field,such as the deficient selection of materials,complex design of micro/nanostructure,and inflexible processing technology.

Wide range bilayer aluminum nanowire grating sensors with robust reflective peaks

A grating-coupled surface plasmon resonance sensor based on bilayer aluminum nanowire arrays is fabricated by laser interference lithography. The device presents impressive reflective sharp peaks by lateral surface plasmon resonances even for aluminum thicknesses of merely several nanometers. Distinct reflective peaks and dramatic color shifts under different analytes are observed within a wide range of incident angle, metal thickness, and refractive index. The sensitivity of 307 nm per refractive index unit is experimentally obtained.The reflective-peak-type surface plasmon resonance sensors are suitable for practical applications because of easy fabrication, low cost, wide range, and high signal visibility.

Laser printing based on curvature-driven shape transition of aluminum nanodiscs[Invited]

Plasmonic structural colors have plenty of advantages over traditional colors based on colorants.The pulsed laser provides an important method generating plasmonic structural colors with high efficiency and low cost.Here,we present plasmonic color printing Al nanodisc structures through curvature-driven shape transition.We systematically study the mechanism of morphologic evolution of the Al nanodisc below the thermal melting threshold.A multi-pulse-induced accumulated photothermal effect and subsequent curvature-driven surface atom diffusion model are adopted to explain the controllable shape transition.The shape transition and corresponding plasmonic resonances of the nanodisc can be independently and precisely modulated by controlled irradiations.This method opens new ways towards high-fidelity color prints in a highly efficient and facile laser writing fashion.

Multifrequency superscattering pattern shaping

Multifrequency superscattering is a phenomenon in which the scattering cross section from a subwavelength object simultaneously exceeds the single-channel limit at multiple frequency regimes.Here,we achieve simultaneously,within a graphene-coated subwavelength structure,multifrequency superscattering and superscattering shaping with different engineered scattering patterns.It is shown that multimode degenerate resonances at multiple frequency regimes appearing in a graphene composite structure due to the peculiar dispersion can be employed to resonantly overlap electric and magnetic multipoles of various orders,and,as a result,effective multifrequency superscattering with different engineered angular patterns can be obtained.Moreover,the phenomena of multifrequency superscattering have a high tolerance to material losses and some structural variations.Our work should anticipate extensive applications ranging from emission enhancing,energy harvesting,and antenna design with improved sensitivity and accuracy due to multifrequency operation.

Misalignments among stacked layers of metamaterial terahertz absorbers

Misalignment among stacked layers of absor- bers is inevitable in practice. Adverse effects induced by this undesired factor was investigated and analyzed in this paper. The absorption responses of thin terahertz metama- terial （MM） absorber with different degree of misalign- ment were simulated by finite-difference time-domain （FDTD） method under both transverse magnetic （TM） and transverse electric （TE） polarization. Results show that slight misalignment deteriorates absorption response due to the decreased spatial resolution. The analyses are given in terms of the magnetic field distribution in the cross section. In addition, the depravation is changed with polarization, which depends on the direction of excursion.

A star-like photodetector for angle-based light sensing in 3D space

The development of three-dimensional(3D)space light angle detection is vital in optical technology for applications such as 3D imaging,computer vision,and augmented reality.Current methods involve advanced sensors and algorithms,including time-offlight cameras,which need multiple cameras and light sources to improve accuracy.However,it is a great challenge to integrate these complex components into compact devices.Subwavelength semiconductor structures offer optical resonance characteristics,enabling precise light–matter interaction regulation.A 3D star-like photodetector,fabricated using a template assistant printing strategy,demonstrates optical resonances of the subwavelength facade and the shielding effect of spatial arrangement.It achieves light angle detection with the resolution of 10°in vertical space and the resolution of 36°in horizontal space,making it a promising prototype for various applications.

Feature size below 100 nm realized by UVLEDbased microscope projection photolithography

The demand for miniaturization and integration of optical elements has fostered the development of various micro-and nanofabrication technologies.In this work,we developed a low-cost UV-LED-based microscope projection photolithography system for rapid and high-resolution fabrication.This system can be easily implemented using off-the-shelf components.It allows for micro-and nanostructuring within seconds.By optimizing the process,a minimum feature size down to approximately 85 nm was successfully realized.In addition,investigations on fabrication of the same structures using both costly and economic microscope objectives were performed.Feature sizes below 100 nm can be stably achieved.The demonstrated approach extends the technology capabilities and may find applications in fields such as nanophotonics,biophotonics sensing and material science.