Ultra-high Numerical Aperture Research Articles

Optical trapping and manipulating with a transmissive and polarization-insensitive metalens

Abstract Trapping and manipulating micro-objects and achieving high-precision measurements of tiny forces and displacements are of paramount importance in both physical and biological research. While conventional optical tweezers rely on tightly focused beams generated by bulky microscope systems, the emergence of flat lenses, particularly metalenses, has revolutionized miniature optical tweezers applications. In contrast to traditional objectives, the metalenses can be seamlessly integrated into sample chambers, facilitating flat-optics-based light manipulation. In this study, we propose an experimentally realized transmissive and polarization-insensitive water-immersion metalens, constructed using adaptive nano-antennas. This metalens boasts an ultra-high numerical aperture of 1.28 and achieves a remarkable focusing efficiency of approximately 50 % at a wavelength of 532 nm. Employing this metalens, we successfully demonstrate stable optical trapping, achieving lateral trapping stiffness exceeding 500 pN/(μm W). This stiffness magnitude aligns with that of conventional objectives and surpasses the performance of previously reported flat lenses. Furthermore, our bead steering experiment showcases a lateral manipulation range exceeding 2 μm, including a region of around 0.5 μm exhibiting minimal changes in stiffness for smoothly optical manipulation. We believe that this metalens paves the way for flat-optics-based optical tweezers, simplifying and enhancing optical trapping and manipulation processes, attributing ease of use, reliability, high performance, and compatibility with prevalent optical tweezers applications, including single-molecule and single-cell experiments.

Ultrathin film optical coatings for all-optical mathematical operations with ultrahigh numerical aperture

Optical analog computation is garnering increasing attention due to its innate parallel processing capabilities, swift computational speeds, and minimal energy requirements. However, traditional optical components employed for such computations are usually bulky. Recently, there has been a substantial shift toward utilizing nanophotonic structures to downsize these bulky optical elements. Nevertheless, these nanophotonic structures are typically realized in planar subwavelength nanostructures, demanding intricate fabrication processes and presenting limitations in their numerical apertures. In this study, we present a three-layer thin-film optical coating different from the conventional Fabry–Pérot nanocavity. Our design functions as a real-time Laplacian operator for spatial differentiation, and it remarkably boasts an ultrahigh numerical aperture of up to 0.7, enabling the detected edges to be sharper and have closely matched intensities. We also experimentally demonstrate its capacity for effective edge detection. This ultracompact and facile-to-fabricate thin-film spatial differentiator holds promising prospects for applications in ultrafast optical processing and biomedical imaging.

Ultra-high NA optical image differentiator based on dielectric metasurfaces

Metasurface-based optical differentiators have recently attracted attention for their potential advantages in real-time and high-throughput image processing technology. However, the existing differentiators suffer from limitations in numerical aperture (NA). Here, we propose a differential metasurface based on silicon clusters covered with polydimethylsiloxane, achieving a remarkable NA of 0.7. We numerically demonstrate the proposed differentiator can perform second-order differentiation on a 1D spatial function and high NA can accurately compute smaller-scale variations. We further demonstrate its ability to achieve edge detection on 2D images. Our results advance the development of ultra-high numerical aperture optical differentiator and provide new opportunities for image processing.

Raman Spectroscopy Fiber Probe Based on Metalens and Multimode Fiber for Malachite Green Determination

The limit of Raman spectroscopy probe based on the traditional objective lens, such as integration difficulties, is the primary motivation to find better alternatives. Due to the potential of vertical integration and significant flexible design, a Raman spectroscopy fiber probe based on metalens at the fiber end facet is demonstrated. Using the "contact and separate" method, we successfully transferred the metalens to the end face of multimode fiber (MMF) to construct the Raman fiber probe with an ultrahigh numerical aperture (NA) of up to 0.66@air. In the case of the surface-enhanced Raman scattering (SERS) substrate composed of Au-plated anodic aluminum oxide (AAO) membrane with the same dry malachite green (MG), compared with the commercial Raman scattering light acquisition system based on an objective lens, our Raman signal intensity increases by at least 2.7 times at the Raman characteristic peak for MG. In the MG aqueous solution, the measurement with a limit of detection (LOD) of 10-9 mol/L@1616 cm-1 is realized. The proposed SERS sensing configuration based on the proposed Raman fiber probe and Au-covered AAO membrane also shows a broad application potential of real-time monitoring for other markers in the fields of food safety and environment detection.

Wavelength tunable dissipative soliton resonant holmium-doped fiber laser beyond [formula omitted

We propose a wavelength tunable dissipative soliton resonant (DSR) mode-locked holmium-doped fiber laser operating at wavelength exceeding 2.1μm. Mode-locking is realized by employing nonlinear amplifying loop mirror technique. Using a piece of long gain fiber to offer enough gain above 2.1μm. A section of ultra-high numerical aperture fiber (UHNAF) with normal dispersion and high nonlinearity is used to reduce mode-locking threshold and increase the intracavity nonlinearity. At the optimized gain fiber length of 9.8 m and UHNAF length of 55 m, 2109 nm DSR pulse with 3-dB spectrum width of 0.46 nm is obtained. Thanks to the birefringence-induced spectral filter effect, the central wavelength of DSR can be tuned from 2102.7 to 2120.5 nm with wavelength range of 17.8 nm.

2.1 m, high-energy dissipative soliton resonance from a holmium-doped fiber laser system

Abstract We propose a 2.1 μm high-energy dissipative soliton resonant (DSR) fiber laser system based on a mode-locked seed laser and dual-stage amplifiers. In the seed laser, the nonlinear amplifying loop mirror technique is employed to realize mode-locking. The utilization of an in-band pump scheme and long gain fiber enables effectively exciting 2.1 μm pulses. A section of ultra-high numerical aperture fiber (UHNAF) with normal dispersion and high nonlinearity and an output coupler with a large coupling ratio are used to achieve a high-energy DSR system. By optimizing the UHNAF length to 55 m, a 2103.7 nm, 88.1 nJ DSR laser with a 3-dB spectral bandwidth of 0.48 nm and a pulse width of 17.1 ns is obtained under a proper intracavity polarization state and pump power. The output power and conversion efficiency are 0.233 W and 4.57%, respectively, both an order of magnitude higher than those of previously reported holmium-doped DSR seed lasers. Thanks to the high output power and nanosecond pulse width of the seed laser, the average power of the DSR laser is linearly scaled up to 50.4 W via a dual-stage master oscillator power amplifier system. The 3-dB spectral bandwidth broadens slightly to 0.52 nm, and no distortion occurs in the amplified pulse waveform. The corresponding pulse energy reaches 19.1 μJ, which is the highest pulse energy in a holmium-doped mode-locked fiber laser system to the best of our knowledge. Such a 2.1 μm, high-energy DSR laser with relatively wide pulse width has prospective applications in mid-infrared nonlinear frequency conversion.

Fabrication of ultra-high numerical aperture GeO2-doped fiber and its use for broadband supercontinuum generation: publisher's note.

This publisher's note serves to correct Appl. Opt.56, 9315 (2017)APOPAI0003-693510.1364/AO.56.009315.

Efficient and scalable edge coupler based on silica planar lightwave circuits and lithium niobate thin films

Recently, lithium niobate on insulator (LNOI) has emerged as a promising photonic integration platform for optical communication due to its outstanding material properties. However, an optical coupler that features high-efficiency is essential to improve the practicability of LN devices. In this work, we proposed a high coupling efficiency and scalable edge coupler, which consists of silica-based planar lightwave circuit (PLC) and LN bilayer taper. The silica-based PLC is positioned on the bottom of LN bilayer taper, which serves as the optical interface connecting the LNOI waveguide to the fiber. The silica-based PLC is interfaced to optical fibers through butt-coupling. Then, LN bilayer taper transfers the light form silica PLC to LN ridge waveguide through adiabatic coupling and converts it gradually into the fundamental mode. When coupled to an ultra-high numerical aperture fiber (UHNA), the simulation results show a low coupling loss of 0.11 dB and 0.128 dB per facet for TE and TM mode, respectively. The scalability of the coupler for large numbers of channels has been proven, which meets the performance requirements of LN devices employed in high-bandwidth, multi-channel systems. These results open a path for lithium niobate photonic integration circuit (PIC) and low-cost optical packaging.

Grazing-Angle Fiber-to-Waveguide Coupler

The silicon photonics market has grown rapidly over recent decades due to the demand for high bandwidth and high data-transfer capabilities. Silicon photonics leverage well-developed semiconductor fabrication technologies to combine various photonic functionalities on the same chip. Complicated silicon photonic integrated circuits require a mass-producible packaging strategy with broadband, high coupling efficiency, and fiber-array fiber-to-chip couplers, which is a big challenge. In this paper, we propose a new approach to fiber-array fiber-to-chip couplers which have a complementary metal-oxide semiconductor-compatible silicon structure. An ultra-high numerical aperture fiber is polished at a grazing angle and positioned on a taper-in silicon waveguide. Our simulation results demonstrate a coupling efficiency of more than 90% over hundreds of nanometers and broad alignment tolerance ranges, supporting the use of a fiber array for the packaging. We anticipate that the proposed approach will be able to be used in commercialized systems and other photonic integrated circuit platforms, including those made from lithium niobate and silicon nitride.

Toward High‐Efficiency Ultrahigh Numerical Aperture Freeform Metalens: From Vector Diffraction Theory to Topology Optimization (Laser Photonics Rev. 16(10)/2022)

Toward High‐Efficiency Ultrahigh Numerical Aperture Freeform Metalens: From Vector Diffraction Theory to Topology Optimization (Laser Photonics Rev. 16(10)/2022)

Experimental analysis for refractive index sensing by using a compact, simple and robust Mach-Zehnder interferometer based on an air gap inside of a fiber

• Fabrication of optical fiber devices with air gap inside optical fiber with a simple procedure. • High sensitivity values for refractive index sensing. • Compact (a few μ m ) and simple optical fiber device for refractive index sensing. Compact and straightforward construction of optical fiber Mach-Zehnder interferometer and its application as a refractive index sensor is presented. The interferometer is composed of an air gap inside an ultra-high Numerical Aperture (NA) fiber. As the light propagates through the air gap, the fundamental mode travels in the air gap. Simultaneously, high-order modes are excited in the section constructed by the air gap and the outside air-cladding interface. The propagating high-order modes are sensitive to the surrounding media because their intensity distribution can extend to the surrounding media. Cargille calibrated oils of different refractive indexes are used to test the refractive index change response of various interferometers with several air gap lengths over refractive index changes in the surrounding media. A maximum sensitivity response of 226.8 nm/RIU within the range from 1.43 to 1.45 for a sample device with 12 min of etching time and inconsistent behavior of the intensity (dBm/RIU) for the different devices are reported.

Toward High‐Efficiency Ultrahigh Numerical Aperture Freeform Metalens: From Vector Diffraction Theory to Topology Optimization

AbstractOptical metasurface is a 2D array of subwavelength meta‐atoms that arbitrarily manipulates amplitude, polarization, and wavefront of incident light, offering great advantages in miniaturization and integration. However, the presuppositions hidden in conventional unit‐cell‐based design approach, including discrete phase sampling, local periodicity approximation, and normal response, imply that the responses and interactions of meta‐atoms are almost impossible to be accurately modeled, thus impeding the effective design of high‐efficiency ultrahigh numerical aperture (NA) metalens. Here, based on vector diffraction theory and plane wave expansion method, theoretical limitation of metalens efficiency is comprehensively investigated. It is identified that for high‐NA metalens, theoretical focusing efficiency is limited by diffraction capability of vector field, while evanescent wave attenuation dominates theoretical efficiency decline for ultrahigh‐NA metalens. It is also shown that the efficiency of conventional metalens has a huge gap from theoretical limitation, owing to imperfect diffractive focusing and impedance mismatch reflections. To fill such efficiency gap, the high‐efficiency high‐NA freeform metalens based on topology optimization is further demonstrated. Particularly, topology geometric constraints are utilized to reduce minimum feature size while keeping high efficiency. The results could shed new light on the understanding and design of ultrahigh‐NA metalens and find promising applications in optical imaging, microscopy, and lithography.

Metalenses with Polarization‐Insensitive Adaptive Nano‐Antennas

AbstractMetalens research has made major advances in recent years. These advances rely on the simple design principle of arranging meta‐atoms in regular arrays to create an arbitrary phase and polarization profile. Unfortunately, the concept of equally spaced meta‐atoms reaches its limit for high deflection angles where the deflection efficiency decreases. The efficiency can be increased using nano‐antennas with multiple elements, but their polarization sensitivity hinders their application in metalenses. Here, it is shown that by designing polarization‐insensitive dimer nano‐antennas and abandoning the principle of equally spaced unit cells, polarization‐insensitive ultrahigh numerical aperture (NA = 1.48) oil‐immersion operation with an efficiency of 42% can be demonstrated. This represents a significant improvement on other polarization‐insensitive designs at visible wavelength. This single layer metalens is used to replace a conventional objective lens and demonstrates the confocal scanning microscopic imaging of a grating with a period of 300 nm at 532 nm operating wavelength. Overall, the results experimentally demonstrate a novel design concept that further improves metalens performance.

Manipulation force analysis of nanoparticles with ultra-high numerical aperture metalens.

Metalens optical tweezers technology has several advantages for manipulating micro-nano particles and high integration. Here, we used particle swarm optimization (PSO) to design a novel metalens tweezer, which can get 3-dimensional trapping of particles. The numerical aperture (NA) of the metalens can reach 0.97 and the average focusing efficiency is 44%. Subsequently, we analyzed the optical force characteristics of SiO2 particles with a radius of 350 nm at the focal point of the achromatic metalens. We found the average maximum force of SiO2 particles in the x-direction and z-direction to be 0.88 pN and 0.72 pN, respectively. Compared with the dispersive metalens, it is beneficial in maintaining the constant of optical force, the motion state of trapped particles, and the stability of the trapping position.

Wavelength switchable and tunable noise-like pulses from a 2 μm figure-eight all-fiber laser

We propose the generation of wavelength switchable and tunable noise-like pulses (NLPs) in 2 μm all-fiber laser mode-locking with nonlinear amplifier loop mirror technique. An properly segment of ultra-high numerical aperture fiber is inserted in the cavity to reduce the pump power required for achieving mode-locking at different wavelengths, and a suitable length of single mode fiber is employed to induce more intracavity birefringence. By increasing the pump power and adjusting the intracavity polarization state, the fiber laser can deliver NLPs with switchable wavelengths between 1979.4 nm and 2003.8 nm. Based on the spectral filter effect induced by fiber birefringence, continuous wavelength tunable NLP operation at these two wavebands can be achieved by adjusting polarization controllers. The center wavelength of NLP can be continuously tuned from 1952.1 nm to 1999.0 nm and from 2002.6 nm to 2014.4 nm, with corresponding spectral tuning ranges of 46.9 nm and 11.8 nm, respectively. This wavelength switchable and tunable NLP fiber laser has great potential in fiber sensing and industrial processing.

Adaptive polymer fiber neural device for drug delivery and enlarged illumination angle for neuromodulation

Objective. Optical fiber devices constitute significant tools for the modulation and interrogation of neuronal circuitry in the mid and deep brain regions. The illuminated brain area during neuromodulation has a direct impact on the spatio-temporal properties of the brain activity and depends solely on the material and geometrical characteristics of the optical fibers. In the present work, we developed two different flexible polymer optical fibers (POFs) with integrated microfluidic channels (MFCs) and an ultra-high numerical aperture (UHNA) for enlarging the illumination angle to achieve efficient neuromodulation. Approach. Three distinct thermoplastic polymers: polysulfone, polycarbonate, and fluorinated ethylene propylene were used to fabricate two step-index UHNA POF neural devices using a scalable thermal drawing process. The POFs were characterized in terms of their illumination map as well as their fluid delivery capability in phantom and adult rat brain slices. Main results. A 100-fold reduced bending stiffness of the proposed fiber devices compared to their commercially available counterparts has been found. The integrated MFCs can controllably deliver dye (trypan blue) on-demand over a wide range of injection rates spanning from 10 nl min−1 to 1000 nl min−1. Compared with commercial silica fibers, the proposed UHNA POFs exhibited an increased illumination area by 17% and 21% under 470 and 650 nm wavelength, respectively. In addition, a fluorescent light recording experiment has been conducted to demonstrate the ability of our UHNA POFs to be used as optical waveguides in fiber photometry. Significance. Our results overcome the current technological limitations of fiber implants that have limited illumination area and we suggest that soft neural fiber devices can be developed using different custom designs for illumination, collection, and photometry applications. We anticipate our work to pave the way towards the development of next-generation functional optical fibers for neuroscience.

Modelling Dispersion Compensation in a Cascaded-Fiber-Feedback Optical Parametric Oscillator

Fiber-feedback optical parametric oscillators (OPOs) incorporate intracavity fibers to provide a compact high-energy wavelength-tunable laser platform; however, dispersive effects can limit operation to the sub-picosecond regime. In this research article, we modeled pulse propagation through systems of cascaded fibers, incorporating SMF-28 and ultra-high numerical aperture (UHNA) fibers with complementary second-order dispersion coefficients. We found that the pulse duration upon exiting the fiber system is dominated by uncompensated third-order effects, with UHNA7 presenting the best opportunity to realise a cascaded-fiber-feedback OPO.

1.7-μm dissipative soliton Tm-doped fiber laser

We report on the dissipative soliton generation in a 1.7-μm net-normal dispersion Tm-doped fiber laser by nonlinear polarization rotation technique. An intra-cavity bandpass filter was employed to suppress the long-wavelength emission, while the cavity dispersion was compensated by a segment of ultrahigh numerical aperture (UHNA4) fiber. The dissipative soliton with a central wavelength of 1746 nm was obtained, covering a spectral range from 1737 nm to 1754 nm. The de-chirped duration and energy of the dissipative soliton were 370 fs and 0.2 nJ, respectively. In addition, the dynamics of multiple dissipative solitons were also investigated. Through optimization of the cavity dispersion, the 50 nm broadband dissipative soliton with de-chirped pulse duration of 230 fs could be achieved. The development of dissipative soliton seed laser represents the first step in achieving the chirped pulse amplification system at the 1.7-μm wave band, which would find potential applications in fields such as biomedical imaging and material processing.

Systematic exploration and characterization on the influence of dispersion to pulse characteristics in Tm-doped NPE mode-locked fiber oscillator

We present a systematic and experimental exploration on the influence of the second-order and third-order dispersion on pulse characteristics in an ultra-high numerical aperture (UHNA) fiber dispersion-managed, Tm-doped and nonlinear polarization evolution (NPE) mode-locked fiber laser cavity; and obtained the detailed regularity of the variation of the key parameters for mode-locked pulse, including pulse duration, pulse spectral profile, and pulse signal-to-noise ratio (SNR) with dispersion. In this process, we find that there are two interesting second-order dispersion points of −0.207 ps2 and 0.09 ps2, in which the pulse SNRs can be optimized to as high as 79 dB and 78 dB, respectively, considerably higher than that in other positions. Besides, the experimental results show that the value of third-order dispersion can not only impact the pulse spectrum but also monotonously affect the quality of pulse SNR. To the best of our knowledge, this is the first report on the detailed analysis of the effects of both the second and the third-order dispersion values on the pulse characteristics in 2-μm mode-locked fiber laser cavity and it will be an important reference to the realization of a Tm-doped mode-locked laser with high pulse contrast.

Ultrahigh numerical aperture meta-fibre for flexible optical trapping

Strong focusing on diffraction-limited spots is essential for many photonic applications and is particularly relevant for optical trapping; however, all currently used approaches fail to simultaneously provide flexible transportation of light, straightforward implementation, compatibility with waveguide circuitry, and strong focusing. Here, we demonstrate the design and 3D nanoprinting of an ultrahigh numerical aperture meta-fibre for highly flexible optical trapping. Taking into account the peculiarities of the fibre environment, we implemented an ultrathin meta-lens on the facet of a modified single-mode optical fibre via direct laser writing, leading to a diffraction-limited focal spot with a record-high numerical aperture of up to NA ≈ 0.9. The unique capabilities of this flexible, cost-effective, bio- and fibre-circuitry-compatible meta-fibre device were demonstrated by optically trapping microbeads and bacteria for the first time with only one single-mode fibre in combination with diffractive optics. Our study highlights the relevance of the unexplored but exciting field of meta-fibre optics to a multitude of fields, such as bioanalytics, quantum technology and life sciences.