Enhancement of laser ablation via high-frequency electromagnetic induction heating.

The efficiency of pulsed laser ablation has always been the focus point of research. A novel high-frequency electromagnetic induction heating-assisted laser ablation scheme is proposed and investigated to enhance the efficiency and improve the surface processing quality during the nanosecond laser ablation of metal substrates. To reduce laser energy required to reach the ablation threshold of metal, this method utilizes the electromagnetic induction to rapidly elevate substrate temperature, making the metal easier to be ablated. The results show that ablation width increases 16% and ablation depth increases 31% with the assistance of electromagnetic induction heating at a laser fluence of 1.32 J/cm2, which increases 90% of the laser-ablated volume. Meanwhile, the surface ablation quality is significantly improved due to the smaller temperature gradient around the ablation region. This new method has great potentials in the laser micromachining at a higher processing efficiency and better laser-processed surface quality.

Approximately 30 nm Nanogroove Formation on Single Crystalline Silicon Surface under Pulsed Nanosecond Laser Irradiation.

Nanogrooves with a minimum feature size down to 30 nm (λ/26) can be formed directly on silicon surface by irradiation from two orthogonal polarized 1064 nm/10 ns fiber laser beams. The creation of such small nanogrooves is attributed to surface thermal stress during resolidification and supercooling with the double laser beams' irradiation. By varying the pulse number and laser fluence, the feature size of narrow grooves on silicon surface can be tuned. The experimental results and numerical calculation of surface thermal behaviors indicated that the high repetition rate of the nanosecond laser leads to the incubation effect and different silicon optical and thermal properties during laser irradiation. Resolution on this scale should be attractive in nanolithography, particularly considering that this method is available in far field and in ambient air.

Bilayer-film-decorated microsphere with suppressed interface reflection for enhanced nano-imaging.

Microspheres as special optical lenses have extensive applications due to their super-focusing ability and outstanding resolving power on imaging. The interface reflection between the microsphere and sample surface significantly affects nano-imaging as exhibited in the form of the Newton's rings pattern in virtual images. In this work, a new scheme of decorating the microsphere with a dielectric bilayer thin film is proposed to suppress the interface reflection and thus enhance the imaging performance. The particle swarm optimization algorithm is performed with a full-wave simulation to refine the bilayer thin film decorated microsphere design, which is successfully realized via a novel fabrication strategy. Experimental imaging results demonstrate that the Newton's rings pattern in virtual images is substantially diminished. Both the imaging contrast and effective field-of-view of the microsphere nano-imaging are improved via this effective light manipulation scheme, which is also applicable to promoting the performance of the microsphere in other optical applications.

Polarization-independent 3D metasurface with complex amplitude modulation.

Metasurfaces, which possess unprecedented capabilities in manipulating electromagnetic wavefronts, are promising for accurate complex amplitude modulation with a compact device. However, current strategy of complex amplitude modulation based on metasurfaces focuses on anisotropic unit design which is intrinsically constrained of polarization states. In this study, we propose a design methodology of polarization-independent metasurface which comprises an array of nanocylinders with various radii and heights. The effectiveness of the proposed scheme is verified using an optical vortex generator and a complex-amplitude hologram device. The straightforward, cost-effective, and polarization-independent design can provide robust and reliable solutions for wavefronts modulation in various optical applications.

Realization of inversely designed metagrating for highly efficient large angle beam deflection.

Directional emission source is one of the key components for multiple-view three-dimensional display. It is hard to achieve high efficiency and large deflection angle direction sources via geometric optics due to the weak confinement of light. The metasurface especially metagrating provides a promising method to control light effectively. However, the conventional forward design methods for metasurface are inherently limited by insufficient control of Bloch modes, which causes a significant efficiency drop at a large deflection angle. Here, we obtained high efficiency large deflection angle metagratings by realizing the constructive interferences among the propagation Bloch modes and enhancing the outcoupling effect at the desired diffraction order. The grating structures that support the coupling of Bloch modes were designed by an inverse design method for different incident wavelengths, and the total phase response of a supercell can be tailored. For a red (620 nm) incident light, the theoretical deflection efficiency of a silicon metagrating can be higher than 80% from 30° to 80°. The experimental deflection efficiency can achieve 86.43% for a 75° deflection metagrating. The matched simulation and experimental results strongly support the reliability of developed algorithm. Our inverse design approach could be extended to the green (530 nm) and blue (460 nm) incident light with titanium dioxide metagratings, with theoretical deflection efficiency of over 80% in a large deflection angle range of 30° to 80°. Considering the multiple visible wavelength deflection capability, the developed algorithm can be potentially applied for full color three-dimensional display, and other functional metagrating devices based on different dielectric materials.

Formation of polarization-dependent optical vortex beams via an engineered microsphere.

In this work, we propose a method that can realize switchable spatial arrangement of the donut-shaped focusing beams through an engineered microsphere, tuned by changing the polarization state of an incident light. In this polarization-dependent light manipulation, the engineered microsphere is designed and fabricated by focused ion beam (FIB). We simulate and experimentally characterize the focus performance of the engineered microsphere. Under the light incidence with radial polarization, multiple focused donut beams are formed along optical axis. By adding an additional linear polarizer with a rotatable relative angle, the pair of donut beams can be re-arranged in the same transverse plane and stay close to each other. Experimental results and numerical simulation are in good agreement. Such tunable polarization-dependent micro-optics can be used for multiplane imaging applications.

Optical nano-imaging via microsphere compound lenses working in non-contact mode.

Microsphere lens for nano-imaging has been widely studied because of its superior resolving power, real-time imaging characteristic, and wide applicability on diverse samples. However, the further development of the microsphere microscope has been restricted by its limited magnification and small field-of-view. In this paper, the microsphere compound lenses (MCL) which allow enlarged magnification and field-of-view simultaneously in non-contact imaging mode have been demonstrated. A theoretical model involving wave-optics effects is established to guide the design of MCL for different magnifications and imaging configurations, which is more precise compared with common geometric optics theory. Experimentally, using MCL to image the specimen with a tunable magnification from 2.8× to 10.3× is realized. Due to the enlarged magnification, a high-resolution target with 137 nm line width can be resolved by a 10× objective. Besides, the field-of-view of MCL is larger than that of a single microsphere and can be further increased through scanning working manner, which has been demonstrated by imaging a sample with ∼76 nm minimum feature size in a large area. Prospectively, the well-designed MCL will become irreplaceable components to improve the imaging performances of microsphere microscope just like the compound lens in the conventional macroscopic imaging system.

Enhancing SERS detection on a biocompatible metallic substrate for diabetes diagnosing.

A method to realize surface-enhanced Raman spectroscopy (SERS) at a titanium alloy substrate for glucose detection has been experimentally demonstrated. A silver-coated laser-induced periodic surface structure (LIPSS) was prepared via femtosecond laser micro-processing. The low detection limit of glucose is 10-7mol/L and, a good linear relationship between the glucose concentration and Raman intensity is found in the range between 1×10-7 and 1×10-3mol/L. Moreover, we investigate SERS detection for glucose sensing in human urine samples, while the results are in good agreement with clinical results. The Letter provides a facile method for producing a structure-controlled SERS substrate to realize glucose detection, which is promising for long-term in vivo diagnostics.

Enhancement of axial resolution and image contrast of a confocal microscope by a microsphere working in noncontact mode.

A new technique, to the best of our knowledge, for improving the axial resolution and imaging contrast of a reflection mode confocal microscope is proposed. A 50 µm silica microsphere is added in front of the objective lens to enhance both the focusing of illumination and the collection of reflected and scattered light from sample surfaces in noncontact mode. An adjustable pinhole is used to compensate the displacement of the focal point in the axial direction. Various samples, including grouped nanolines and nanosteps, are used to demonstrate imaging performance. By comparison to an NA 0.9 commercial confocal microscope, the new setup achieves the axial resolution up to 100 nm and increases the image contrast by 4.56 times. The entire setup offers a cost-effective solution for high imaging performance, which can be applied in many fields from nanotechnology to biology.

Realization of ∼10 nm Features on Semiconductor Surfaces via Femtosecond Laser Direct Patterning in Far Field and in Ambient Air.

Direct fabrication of ∼10 nm features by optical means in far field and in ambient air on semiconductor surfaces is significant for next-generation advances nanomanufacturing. We report here a new method that enables the direct formation of 12 nm (λ/66) features on silicon surfaces. It is processed in far field and in ambient air via the irradiation of orthogonally polarized double femtosecond laser beams. The coupling of orthogonally polarized double femtosecond laser beams and the incubation effect due to multiple femtosecond laser pulses irradiation under high repetition rate enable the 12 nm nanostructures creation parallel to the scanning direction, regardless of scanning path.

High-quality sapphire microprocessing by dual-beam laser induced plasma assisted ablation.

Sapphire is a kind of ultrahard transparent material with good chemical resistance. These great properties also make sapphire functional device fabrication a big challenge. We propose a novel dual-beam laser induced plasma assisted ablation (LIPAA) for high-quality sapphire microprocessing. One laser beam is focused on a sacrificial target for nano-particle generation by LIPAA to assist the sapphire ablation by the other laser beam. The new technology can reduce the ablation threshold of sapphire and the roughness of the fabricated structures. The laser fluence for particle generation is optimized. Furthermore, we demonstrate a sapphire Dammann grating and an OAM generator fabricated by this method. This method can be expanded to arbitrary transparent material precision machining for various applications.

Time delay effect in a microchip pulse laser for the nonlinear photoacoustic signal enhancement.

The Grüneisen relaxation effect has been successfully employed to improve the photoacoustic (PA) imaging contrast. However, complex system design and cost hinder the progress from benchside to bedside, since an additional pre-heating laser source needs to be coupled into the original light path and synchronized with other equipment for conducting the nonlinear effect. To overcome the limitation, we propose a time delay heating PA imaging (TDH-PAI) method based on the time delay effect in a passively Q-switched laser. Experimentally, only one single microchip pulse laser is built and utilized for the nonlinear PA signal enhancement without additional components. The 808 nm pump pulse of the laser diode and the excited 1064 nm pulse are respectively used for pre-heating and acquiring PA signals. The heating effect is optimized by adjusting the input parameters and an enhancement of more than 30% in PA signals is achieved. TDH-PAI reduces the cost and complexity of the nonlinear PA system, which provides an efficient way for achieving a high-contrast PA imaging.

Wavelength-tunable focusing via a Fresnel zone microsphere.

In this Letter, a novel, to the best of our knowledge, structural configuration on a transparent microsphere is proposed to engineer the focusing light field. By patterning a hybrid diffractive Fresnel zone plate structure on a partially milled microsphere using a focused ion beam, wavelength-dependent switching between mono-focal and multi-focal functionalities can be achieved. Generation of on-axis tri-foci and mono-focus light fields under high numerical-aperture (${\rm NA}\gt {0.67}$NA>0.67) conditions at two working wavelengths (405 nm and 808 nm) have been demonstrated both numerically and experimentally.

Extraordinary optical fields in nanostructures: from sub-diffraction-limited optics to sensing and energy conversion.

Along with the rapid development of micro/nanofabrication technology, the past few decades have seen the flourishing emergence of subwavelength-structured materials and interfaces for optical field engineering at the nanoscale. Three remarkable properties associated with these subwavelength-structured materials are the squeezed optical fields beyond the diffraction limit, gradient optical fields in the subwavelength scale, and enhanced optical fields that are orders of magnitude greater than the incident field. These engineered optical fields have inspired fundamental and practical advances in both engineering optics and modern chemistry. The first property is the basis of sub-diffraction-limited imaging, lithography, and dense data storage. The second property has led to the emergence of a couple of thin and planar functional optical devices with a reduced footprint. The third one causes enhanced radiation (e.g., fluorescence), scattering (e.g., Raman scattering), and absorption (e.g., infrared absorption and circular dichroism), offering a unique platform for single-molecule-level biochemical sensing, and high-efficiency chemical reaction and energy conversion. In this review, we summarize recent advances in subwavelength-structured materials that bear extraordinary squeezed, gradient, and enhanced optical fields, with a particular emphasis on their optical and chemical applications. Finally, challenges and outlooks in this promising field are discussed.

All-dielectric planar chiral metasurface with gradient geometric phase.

Planar optical chirality of a metasurface measures its differential response between left and right circularly polarized (CP) lights and governs the asymmetric transmission of CP lights. In 2D ultra-thin plasmonic structures the circular dichroism is limited to 25% in theory and it requires high absorption loss. Here we propose and numerically demonstrate a planar chiral all-dielectric metasurface that exhibits giant circular dichroism and transmission asymmetry over 0.8 for circularly polarized lights with negligible loss, without bringing in bianisotropy or violating reciprocity. The metasurface consists of arrays of high refractive index germanium Z-shape resonators that break the in-plane mirror symmetry and induce cross-polarization conversion. Furthermore, at the transmission peak of one handedness, the transmitted light is efficiently converted into the opposite circular polarization state, with a designated geometric phase depending on the orientation angle of the optical element. In this way, the optical component sets before and after the metasurface to filter the light of certain circular polarization states are not needed and the metasurface can function under any linear polarization, in contrast to the conventional setup for geometry phase based metasurfaces. Anomalous transmission and two-dimensional holography based on the geometric phase chiral metasurface are numerically demonstrate as proofs of concept.

Dynamically tunable multi-lobe laser generation via multifocal curved beam.

Beams with curved properties, represented by Airy beam, have already shown potential applications in various fields. Here we propose a simple method to achieve a multifocal curved beam (MCB). The scheme is based on the ability of microspheres to control the distribution of the light field. Combined with the caustic effect, the dynamic control of the beam curvature and the foci can be realized. The simulation results confirm the mechanism behind this phenomenon. Furthermore, MCB is applied experimentally into the end-pumped microchip laser. This work has verified the theory of MCB and achieved a dynamically tunable multi-lobe laser, which has a wide application prospect.

All-dielectric reflective half-wave plate metasurface based on the anisotropic excitation of electric and magnetic dipole resonances.

We present an all-dielectric metasurface that simultaneously supports electric and magnetic dipole resonances for orthogonal polarizations. At resonances, the metasurface reflects the incident light with nearly perfect efficiency and provides a phase difference of π in the two axes, making a low-loss half-wave plate in reflection mode. The polarization handedness of the incident circularly polarized light is preserved after reflection; this is different from either a pure electric mirror or magnetic mirror. With the features of high reflection and circular polarization conservation, the metamirror is an ideal platform for the geometric phase-based gradient metasurface functioning in reflection mode. Anomalous reflection with the planar meta-mirror is demonstrated as a proof of concept. The proposed meta-mirror can be a good alternative to plasmonic metasurfaces for future compact and high-efficiency metadevices for polarization and phase manipulation in reflection mode.

Orbital angular momentum generation via a spiral phase microsphere.

Vortex beam carrying orbital angular momentum (OAM) attracts much attention in many research fields for its special phase and intensity distributions. In this Letter, a novel design called the spiral phase microsphere (SPMS) is proposed for the first time, to the best of our knowledge, which can convert incident plane wave light into the focused vortex beam that carries OAM with different topological charges l=±1 and ±2. The vortex beam generation is verified by a self-interfered modification of the SPMS. The generation of the vortex beams by the SPMS irradiated by a single-wavelength incident light is studied using the CST MICROWAVE STUDIO simulation. The SPMS provides a new approach to achieve high-efficiency and high-integrated photonic applications related with OAM.

Creation of a longitudinally polarized photonic nanojet via an engineered microsphere.

Dielectric microspheres exhibit the ability to focus an incident beam to a subwavelength spot with strong localized field intensity. In this Letter, a high beam quality of a longitudinally polarized electromagnetic component is created by decorating the surface of the microsphere with engineered structures. By changing the design of the engineered microspheres, the relative contribution of the longitudinal and radial components of a radially polarized incident beam to the photonic nanojet can be modified efficiently, leading to a sharp spot size which exceeds the optical diffraction limit. More importantly, a high conversion efficiency of 0.89 is achieved. At a wavelength of 633 nm, a focal spot of 266 nm (0.42λ) is achieved numerically by illuminating the engineered microsphere with a focusing beam at a numerical aperture of 0.7.

Plasmonic enhancement of cyanine dyes for near-infrared light-triggered photodynamic/photothermal therapy and fluorescent imaging.

Near-infrared (NIR) triggered cyanine dyes have attracted considerable attention in multimodal tumor theranostics. However, NIR cyanine dyes used in tumor treatment often suffer from low fluorescence intensity and weak singlet oxygen generation efficiency, resulting in inadequate diagnostic and therapy efficacy for tumors. It is still a great challenge to improve both the photodynamic therapy (PDT) and fluorescent imaging (FLI) efficacy of cyanine dyes in tumor applications. Herein, a novel multifunctional nanoagent AuNRs@SiO2-IR795 was developed to realize the integrated photothermal/photodynamic therapy (PTT/PDT) and FLI at a very low dosage of IR795 (0.4 µM) based on metal-enhanced fluorescence (MEF) effects. In our design, both the fluorescence intensity and reactive oxygen species of AuNRs@SiO2-IR795 nanocomposites were significantly enhanced up to 51.7 and 6.3 folds compared with free IR795, owing to the localized surface plasmon resonance band of AuNRs overlapping with the absorption or fluorescence emission band of the IR795 dye. Under NIR laser irradiation, the cancer cell inhibition efficiency in vitro with synergetic PDT/PTT was up to 82.3%, compared with 10.3% for free IR795. Moreover, the enhanced fluorescence intensity of our designed nanocomposites was helpful to track their behavior in tumor cells. Therefore, our designed nanoagents highlight the applications of multimodal diagnostics and therapy in tumors based on MEF.