Etching‐assisted femtosecond laser modification of hard materials

With high hardness, high thermal and chemical stability and excellent optical performance, hard materials exhibit great potential applications in various fields, especially in harsh conditions. Femtosecond laser ablation has the capability to fabricate three-dimensional micro/nanostructures in hard materials. However, the low efficiency, low precision and high surface roughness are the main stumbling blocks for femtosecond laser processing of hard materials. So far, etching- assisted femtosecond laser modification has demonstrated to be the efficient strategy to solve the above problems when processing hard materials, including wet etching and dry etching. In this review, femtosecond laser modification that would influence the etching selectivity is introduced. The fundamental and recent applications of the two kinds of etching assisted femtosecond laser modification technologies are summarized. In addition, the challenges and application prospects of these technologies are discussed.

Investigation of fluorescence resonance energy transfer ultrafast dynamics in electrostatically repulsed and attracted exciton-plasmon systems

Following the gradual maturation of synthetic techniques for nanomaterials,exciton-plasmon composites have become a research hot-spot due to their controllable energy transfer through electromagnetic fields on the nanoscale.However,most reports ignore fluorescence resonance energy transfer(FRET)under electrostatic repulsion conditions.In this study,the FRET process is investigated in both electrostatic attraction and electrostatic repulsion systems.By changing the Au:quantum dot ratio,local-field induced FRET can be observed with a lifetime of ns and a fast component of hundreds of ps.These results indicate that the intrinsic transfer process can only elucidated by considering both steady and transient state information.

Photon propagation control on laser-written photonic chips enabled by composite waveguides

Femtosecond laser direct writing(Fs LDW)three-dimensional(3D)photonic integrated circuits(PICs)can realize arbitrary arrangement of waveguide arrays and coupling devices.Thus,they are capable of directly constructing arbitrary Hamiltonians and performing specific computing tasks crucial in quantum simulation and computation.However,the propagation constantβis limited to a narrow range in single-mode waveguides by solely changing the processing parameters,which greatly hinders the design of Fs LDW PICs.This study proposes a composite waveguide(CWG)method to increase the range ofβ,where a new single-mode composite waveguide comprises two adjacent circular waveguides.As a result,the photon propagation can be controlled and the variation range ofβcan be efficiently enlarged by approximately two times(Δβ~36 cm-1).With the CWG method,we successfully realize the most compact Fs LDW directional couplers with a 9μm pitch in a straight-line form and achieve the reconstruction of the Hamiltonian of a Hermitian array.Thus,the study represents a step further toward the fine control of the coupling between waveguides and compact integration of Fs LDW PICs.

On-chip single-photon chirality encircling exceptional points

Exceptional points(EPs),which are typically defined as the degener-acy points of a non-Hermitian Hamiltonian,have been investigated in various physical systems such as photonic systems.In particular,the intriguing topological structures around EPs have given rise to novel strategies for manipulating photons and the underlying mechanism is especially useful for on-chip photonic applications.Although some on-chip experiments with the adoption of lasers have been reported,EP-based photonic chips working in the quantum regime largely re-main elusive.In the current work,a single-photon experiment was proposed to dynamically encircle an EP in on-chip photonic waveg-uides possessing passive anti-parity-time symmetry.Photon coinci-dences measurement reveals a chiral feature of transporting single photons,which can act as a building block for on-chip quantum de-vices that require asymmetric transmissions.The findings in the cur-rent work pave the way for on-chip experimental study on the physics of EPs as well as inspiring applications for on-chip non-Hermitian quantum devices.

Holographic laser fabrication of 3D artificial compoundμ-eyes

The demand for fast optical image acquisition without movable optical elements(e.g.,for self-driving car technology)can be met using bioinspired 3D compound eyes.3D laser processing strategies enable designable 3D structuring but suffer from low fabrication efficiency,which significantly limits their applications in producing complex 3D optical devices.Herein,we demonstrate a versatile yet simple wet-etching-assisted holographic laser fabrication method for the development of 3D compound eyes.Artificial compoundμ-eyes can be readily fabricated by programming a 3D spot array for the parallel ablation of a curved fused silica surface,followed by controllable etching in a hydrofluoric(HF)acid solution.A 3D-concave-lens array made on a curved surface over an area of 100μm cross-section with each lenslet of 10μm radius was fabricated with high fidelity and excellent imaging/focusing quality.The resultant 3D-concave-lens can serve as a hard template for the mass production of soft compound eyes through soft lithography.Additionally,using a generative adversarial network(GAN)-based deep learning algorithm,image restoration was conducted for each lenslet,which retained a large field of view and significantly improved image quality.This method provides a simple solution to the requirements of compoundμ-eyes required by Industry 4.0.

Imaging/nonimaging microoptical elements and stereoscopic systems based on femtosecond laser direct writing

The development of modern information technology has led to significant demand for microoptical elements with complex surface profiles and nanoscale surface roughness.Therefore,various micro-and nanoprocessing techniques are used to fabricate microoptical elements and systems.Femtosecond laser direct writing(FsLDW)uses ultrafast pulses and the ultraintense instantaneous energy of a femtosecond laser for micro-nano fabrication.FsLDW exhibits various excellent properties,including nonlinear multiphoton absorption,high-precision processing beyond the diffraction limit,and the universality of processable materials,demonstrating its unique advantages and potential applications in three-dimensional(3D)micro-nano manufacturing.FsLDW has demonstrated its value in the fabrication of various microoptical systems.This study details three typical principles of FsLDW,several design considerations to improve processing performance,processable materials,imaging/nonimaging microoptical elements,and their stereoscopic systems.Finally,a summary and perspective on the future research directions for FsLDW-enabled microoptical elements and stereoscopic systems are provided.

蓝宝石雕刻:干法刻蚀辅助激光加工技术

硬脆材料主要包括金刚石、蓝宝石、光学玻璃、半导体、高硬度合金、陶瓷等.这些材料具有抗腐蚀、耐高低温、抗磨损、硬度高、脆性大等特点,在包括特种空间光学窗口、弹药研制、光电器件、消费电子等在内的诸多领域具有广阔的应用前景.其中,蓝宝石由于其覆盖紫外至中红外的高透过率、超高硬度、良好的热稳定性和化学稳定性等优良的物理、化学和光学性质,已被广泛用于军事、工业、医学和航空航天等领域.硬脆材料具有优异的光电特性以及稳定的机械化学稳定性,在实际应用中发挥了重要的作用.

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.

O-FIB:far-field-induced near-field breakdown for direct nanowriting in an atmospheric environment

Nanoscale surface texturing,drilling,cutting,and spatial sculpturing,which are essential for applications,including thin-film solar cells,photonic chips,antireflection,wettability,and friction drag reduction,require not only high accuracy in material processing,but also the capability of manufacturing in an atmospheric environment.Widely used focused ion beam(FIB)technology offers nanoscale precision,but is limited by the vacuum-working conditions;therefore,it is not applicable to industrial-scale samples such as ship hulls or biomaterials,e.g.,cells and tissues.Here,we report an optical far-field-induced near-field breakdown(O-FIB)approach as an optical version of the conventional FIB technique,which allows direct nanowriting in air.The writing is initiated from nanoholes created by femtosecondlaser-induced multiphoton absorption,and its cutting“knife edge”is sharpened by the far-field-regulated enhancement of the optical near field.A spatial resolution of less than 20 nm(λ/40,withλbeing the light wavelength)is readily achieved.O-FIB is empowered by the utilization of simple polarization control of the incident light to steer the nanogroove writing along the designed pattern.The universality of near-field enhancement and localization makes O-FIB applicable to various materials,and enables a large-area printing mode that is superior to conventional FIB processing.

Protein-based soft micro-optics fabricated by femtosecond laser direct writing

In this work,we report a novel soft diffractive micro-optics,called‘microscale kinoform phase-type lens(micro-KPL)’,which is fabricated by femtosecond laser direct writing(FsLDW)using bovine serum albumin(BSA)as building blocks and flexible polydimethylsiloxane(PDMS)slices as substrates.By carefully optimizing various process parameters of FsLDW(e.g.,average laser power density,scanning step,exposure time on a single point and protein concentration),the as-formed protein micro-KPLs exhibit excellent surface quality,well-defined three-dimensional(3D)geometry and distinctive optical properties,even in relatively harsh operation environments(for instance,in strong acid or base).Laser shaping,imaging and other optical performances can be easily achieved.More importantly,micro-KPLs also have unique flexible and stretchable properties as well as good biocompatibility and biodegradability.Therefore,such protein hydrogel-based micro-optics may have great potential applications,such as in flexible and stretchable photonics and optics,soft integrated optical microsystems and bioimplantable devices.

Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

In this work,the fundamental mechanism of ultrabright fluorescence from surface-modified colloidal silicon quantum dots is investigated in depth using ultrafast spectroscopy.The underlying energy band structure corresponding to such highly efficient direct bandgap-like emissions in our surface-modified silicon quantum dots is unraveled by analyzing the transient optical spectrum,which demonstrates the significant effect of surface molecular engineering.It is observed that special surface modification,which creates novel surface states,is responsible for the different emission wavelengths and the significant improvement in the photoluminescence quantum yields.Following this essential understanding,surface-modified silicon quantum dots with deep blue to orange emission are successfully prepared without changing their sizes.