Femtosecond Laser Writing Research Articles

High Sensitivity Fiber Bragg Grating Humidity Sensors Made With Through-the-Coating Femtosecond Laser Writing and Polyimide Coating Thickening

High Sensitivity Fiber Bragg Grating Humidity Sensors Made With Through-the-Coating Femtosecond Laser Writing and Polyimide Coating Thickening

Magnetic‐Driven Micro‐Gear Pair Fabricated by Femtosecond Laser Writing

Magnetic‐Driven Micro‐Gear Pair Fabricated by Femtosecond Laser Writing

High-fidelity and polarization-insensitive universal photonic processors fabricated by femtosecond laser writing

Abstract Universal photonic processors (UPPs) are fully programmable photonic integrated circuits that are key components in quantum photonics. With this work, we present a novel platform for the realization of low-loss, low-power, and high-fidelity UPPs based on femtosecond laser writing (FLW) and compatible with a large wavelength spectrum. In fact, we demonstrate different UPPs, tailored for operation at 785 nm and 1550 nm, providing similar high-level performances. Moreover, we show that standard calibration techniques applied to FLW-UPPs result in Haar random polarization-insensitive photonic transformations implemented with average amplitude fidelity as high as 0.9979 at 785 nm (0.9970 at 1550 nm), with the possibility of increasing the fidelity over 0.9990 thanks to novel optimization algorithms. Besides being the first demonstrations of polarization-insensitive UPPs, these devices show the highest level of control and reconfigurability ever reported for a FLW circuit. These qualities will be greatly beneficial to applications in quantum information processing.

A Brief Review on Nonlinear Photonic Crystals Induced by Direct Femtosecond Laser Writing

Direct femtosecond laser writing or inscription is a useful technique, and it has been employed to engineer various materials in many applications including nonlinear photonic crystals, which are of periodically patterned second-order nonlinearity to get and control the coherent light at new frequencies. By manipulation of second-order nonlinearity, either erased or poled, quasi-phase matching has been achieved in several crystals, especially three-dimensional nonlinear photonic crystals have been originally proposed and proved to be truly three-dimensional. Here we shortly review on the recent advances in the research field of nonlinear photonic crystals inscribed by femtosecond laser, as well as look into the future in this field.

Multiplexing Linear and Nonlinear Bragg Diffractions through Volume Gratings Fabricated by Femtosecond Laser Writing in Lithium Niobate Crystal

The femtosecond-laser-writing technique provides a flexible method for fabrication of nonlinear photonic crystals in three dimensions, providing structures that enable efficient complex nonlinear wave interactions and modulation for applications including nonlinear holography, nonlinear beam shaping, and waveguide-integrated wavelength conversion. However, the tightly focused laser pulse inevitably causes structural modification and then changes the local refractive index, resulting in additional linear modulation of the interacting waves. Here, we use the same periodic distributions of the refractive index and the second-order nonlinear coefficient for grating arrays engineered in lithium niobate crystals by femtosecond laser writing to achieve polarization-dependent linear and nonlinear Bragg diffractions simultaneously. The experimental results show that the linear and nonlinear diffraction efficiencies range up to 31% and 2.9 × 10−5, respectively, for grating arrays with dimensions of 100 μm (x) × 100 μm (y) × 100 μm (z). This work paves the way toward the realization of the multiplexing of linear and nonlinear optical modulations in a single structure for potential applications that include multidimensional optical data storage and optical coding.

Highly Efficient Refractive Index Sensor Based on a Dual-Side Polished SMS Fiber Enabled by Femtosecond Laser Writing.

Fiber-optic refractive index (RI) sensors based on wavelength-shift-based interrogation continue to present a challenge in achieving high sensitivity for a wide detection range. In this paper, we propose a sensor for determining the RI of liquids based on femtosecond laser (fs-laser) writing of a dual-side polished singlemode-multimode-singlemode (SMS) fiber. The proposed sensor can determine the RI value of a surrounding liquid by detecting the dip wavelength in the transmission spectrum of the light propagating through the sensing area. The high RI sensitivity is attributed to the increased interaction area established by the fs-laser, which creates hydrophilic surfaces and maintains the wide detection range of the SMS structure. The results of the wavelength-shift-based interrogation reveal that the fabricated device exhibited a high sensitivity of 161.40 nm per refractive index unit (RIU) over a wide RI detection range of 0.062 RIU. The proposed device has high processing accuracy and a simple manufacturing process. Hence, it has the potential to be used as a lab-on-fiber sensing platform in chemical and biotechnological applications.

Si-Cr Nano-Alloys Fabricated by Direct Femtosecond Laser Writing.

Ultra-short 230 fs laser pulses of 515 nm wavelength were tightly focused into 700 nm focal spots and utilised in opening ∼400 nm nano-holes in a Cr etch mask that was tens-of-nm thick. The ablation threshold was found to be 2.3 nJ/pulse, double that of plain silicon. Nano-holes irradiated with pulse energies below this threshold produced nano-disks, while higher energies produced nano-rings. Both these structures were not removed by either Cr or Si etch solutions. Subtle sub-1 nJ pulse energy control was harnessed to pattern large surface areas with controlled nano-alloying of Si and Cr. This work demonstrates vacuum-free large area patterning of nanolayers by alloying them at distinct locations with sub-diffraction resolution. Such metal masks with nano-hole opening can be used for formation of random patterns of nano-needles with sub-100 nm separation when applied to dry etching of Si.

Guided-Wave Up-Conversion Luminescence in Er3+/Yb3+ Co-doped Phosphate Glass Waveguide Produced by Direct Femtosecond Laser Writing

We report on guided-wave up-conversion luminescence in femtosecond laser writing cladding waveguides in Er3+/Yb3+ co-doped phosphate glass. The waveguides were fabricated with 30-μm- and 100-μm-diameter of the guiding cores. The guiding properties of waveguides have been investigated at 633 nm by end face coupling of free space light and physical contact of fibers. The experimental and calculated results of propagating modal profiles and losses have proved favorable performances suitable to Gaussian mode field and multi-mode applications. Under the optical pump laser at 980 nm, the guided-wave up-conversion luminescence at visible light range has been realized through the waveguides.

Improvement in Measuring Losses by Interferometric Technique for Glass Waveguides Produced by Femtosecond Laser Writing

Improvement in Measuring Losses by Interferometric Technique for Glass Waveguides Produced by Femtosecond Laser Writing

Hollow Channel Formation inside Sodium Aluminoborate Glass by Femtosecond Laser Writing and Distilled Water Etching.

Recently, the effect of nanograting formation was demonstrated for binary sodium borate glass with the possibility of data storage with an enhanced level of security. The obvious disadvantage of such glass is poor chemical stability, which limits real applications. In this paper, we show that the introduction of Al2O3 allows preserving the possibility of nanograting formation with a significant increase of chemical resistance and thus to preserve optical memory applications. On the other hand, the possibility of selective etching of laser-written tracks by means of distilled water is revealed, which was not demonstrated for other types of glasses. The dependence of retardance of nanogratings form birefringence on laser writing parameters is established and discussed. Structural features of laser-modified microdomains are studied via Raman spectroscopy which revealed an increase of three-coordinated boron content. A possible mechanism of selective etching is discussed.

Beam Splitters Fabricated by Nonlinear Focusing of Femtosecond Laser Writing in Pure YAG Crystal

We report on the fabrication of waveguides and beam splitters in pure YAG crystals by femtosecond laser direct writing (FLDW). During the femtosecond laser writing process, a positive refractive index is induced through the nonlinear focusing above the focus position, resulting in an unusual guiding cross-sectional configuration. The supported guiding modes at 632.8 nm are measured and analyzed using the end–face coupling system. The propagation loss can be as low as 1.9 dB/cm for the single-line waveguide. Different from the geometry of the traditional fs-laser modified area, this novel structure might offer a new approach in the quest toward integrated photonics.

A simple strategy for increasing optical waveguide performance using spherical aberration

Femtosecond Laser Writing (FLW) has some advantages over competing techniques; it is a direct, highly flexible, and maskless process that can inscribe waveguides in almost any type of transparent material. FLW has proven to produce photonic integrated circuits with innovative design in a deterministic way for several applications, including quantum photonics and microfluidics. Although direct laser writing of waveguides has been extensively studied, there is still an intense search for methods to produce waveguides with lower losses and higher optical quality, because improvements in this sense will impact photonic research and technology. In this work, straight waveguides were directly inscribed into Eagle XG glass samples in a single process step at fabrication depths up to 370 µm. The propagation loss and the mode profile for single-mode waveguides were obtained. The laser pulse energy threshold that triggers the optical breakdown as a function of the depth was measured and used to study its relationship with the resulting waveguide optical properties. A trend between the waveguide losses and the observed pulse energy threshold for optical breakdown was found, which is connected with the microscope objective spherical aberration. It can be useful for finding the best conditions for producing low-loss waveguides. Compared to the waveguides written with the corrected pulses, propagation losses up to 2.5 times smaller were observed in our experiments when applying the strategy presented here. Therefore, by taking advantage of the inherent spherical aberration, the FLW technique can be made capable of much more at even higher intensities than previously possible.

Femtosecond Laser Writing of Spin Defects in Hexagonal Boron Nitride

Optically active spin defects in wide-bandgap materials have many potential applications in quantum information and quantum sensing. Spin defects in two-dimensional layered van der Waals materials are just emerging to be investigated. Here we demonstrate that optically-addressable spin ensembles in hexagonal boron nitride (hBN) can be generated by femtosecond laser irradiation. We observe optically detected magnetic resonance (ODMR) of hBN spin defects created by laser irradiation. We show that the creation of spin defects in hBN is strongly affected by the pulse energy of the femtosecond laser. When the laser pulse number is less than a few thousand, the pulse number only affects the density of defects but not the type of defects. With proper laser parameters, spin defects can be generated with a high probability of success. Our work provides a convenient way to create spin defects in hBN by femtosecond laser writing, which shows promising prospects for quantum technologies.

Color Centers Enabled by Direct Femto-Second Laser Writing in Wide Bandgap Semiconductors.

Color centers in silicon carbide are relevant for applications in quantum technologies as they can produce single photon sources or can be used as spin qubits and in quantum sensing applications. Here, we have applied femtosecond laser writing in silicon carbide and gallium nitride to generate vacancy-related color centers, giving rise to photoluminescence from the visible to the infrared. Using a 515 nm wavelength 230 fs pulsed laser, we produce large arrays of silicon vacancy defects in silicon carbide with a high localization within the confocal diffraction limit of 500 nm and with minimal material damage. The number of color centers formed exhibited power-law scaling with the laser fabrication energy indicating that the color centers are created by photoinduced ionization. This work highlights the simplicity and flexibility of laser fabrication of color center arrays in relevant materials for quantum applications.

Femtosecond Laser Writing: Low Power Reconfigurability and Reduced Crosstalk in Integrated Photonic Circuits Fabricated by Femtosecond Laser Micromachining (Laser Photonics Rev. 14(10)/2020)

Femtosecond Laser Writing: Low Power Reconfigurability and Reduced Crosstalk in Integrated Photonic Circuits Fabricated by Femtosecond Laser Micromachining (Laser Photonics Rev. 14(10)/2020)

Three-Dimensional Multifunctional Magnetically Responsive Liquid Manipulator Fabricated by Femtosecond Laser Writing and Soft Transfer.

Nature-inspired magnetically responsive intelligent topography surfaces have attracted considerable attention owing to their controllable droplet manipulation abilities. However, it is still challenging for magnetically responsive surfaces to realize three-dimensional (3D) droplet/multidroplet transport in both horizontal and vertical directions. Additionally, the droplet horizontal propulsion speed needs to be improved. In this work, a 3D droplet/multidroplet transport strategy based on magnetically responsive microplates array (MMA) actuated by a spatially varying and periodic magnetic field is proposed. The modified superhydrophobic surface can transport droplets rapidly both in horizontal and vertical directions, and it can even realize against-gravity upslope propulsion. The rapid horizontal droplet propulsion (∼58.6 mm/s) is ascribed to the abrupt inversion of the modified surface induced by the specific magnetic field. Furthermore, the nonmagnetically responsive microplates (NMMs)/MMA composite surface is constructed to realize 3D multidroplet manipulation. The implementations of MMA in manipulation of continuous fluids and liquid metal are further demonstrated, providing a valuable platform for microfluidic applications.

Mach-Zehnder Interferometer-Based Evanescent Refractometer Inscribed at the Surface of Eagle2000 by Femtosecond Laser Writing

The potential of evanescent Mach-Zehnder interferometers, embedded in Eagle2000 substrates, as refractive index sensors was assessed. For that, femtosecond laser direct writing and wet etching were used to fabricate and expose the sensing arm at the surface of the glass substrate, while keeping the reference arm buried. From the analysis of the structures' spectral response, we found that the wavelength shift of the different order peaks increased greatly for refractive indices nearing that of the glass, indicating a greater overlap between the guided mode's evanescent field and the external medium. Therefore, a maximum sensitivity of 10271 nm/RIU was obtained at a refractive index of 1.491. The sensitivity in the refractive index range of water-based solutions was, on the other hand, limited to 446 ± 39 nm/RIU. Due to the geometry of the device, applications with films deposited at the surface of the substrate and PDMS based microfluidic channels can be explored.

Visible and Infrared Photoluminescence in Hexagonal Silicon Carbide by Direct Femtosecond Laser Writing

Optically active color centers in silicon carbide have attracted considerable attention in the past few years as candidates for quantum technologies such as single-photon sources, nanomagnetic resonance imaging, and spintronic devices. Control over defect position and their placement at the desired location within a chip, necessary to integrate them within optical and electronic devices, is still a challenge. Recently, laser writing emerged as a new tool to generate vacancies in crystals as a starting point for the formation of color centers. In this work, a laser writing method has been used to produce color centers in 4H and 6H bulk silicon carbide by using a femtosecond laser. An array of color centers was fabricated by different pulse laser energies in sites of square grids at varying depths (from the surface to 10 µm below the surface). We optically characterized the fabricated color centers using confocal imaging, photoluminescence, and Raman spectroscopy. We show that the technique can produce specifically vacancy color centers with a relevant emission in the visible (peak around 700 nm) and near-infrared (peak at 900 nm) with the latter identified as the silicon-vacancy. This method can be adapted to engineer color centers in silicon carbide at different depths in the material, for the above-mentioned applications, in addition to the fabrication of light-emitting diodes.

飞秒激光加工凹陷包层波导及其应用

飞秒激光加工凹陷包层波导及其应用

Mass Producible Low-Loss Broadband Optical Waveguides in Eagle2000 by Femtosecond Laser Writing

Optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass, by femtosecond laser direct writing, with varying pulse energy and scan velocity. A spectral characterization, from 500 nm to 1700 nm, was made in order to determine their losses and understand its dependence on the processing parameters. Three major loss mechanisms were identified. At longer wavelengths, loss is mainly due to weak coupling. On the other hand, the behavior at shorter wavelengths is governed by propagation loss due to Rayleigh scattering, which was shown to be practically eliminated ( $\cdot$ cm $^{-1} {\cdot }\,\,\mu \text{m}^{4}$ ) at higher scan velocities. Bulk absorption was also found to have an influence in the propagation losses at higher wavelengths. The combination of intermediate pulse energies (between 125–250 nJ) and high scan velocities (above 6 cm/s) allowed the fabrication of optical waveguides offering low losses across the entire range of wavelengths tested, facilitating applications that require larger wavelength working bands. Furthermore, since optimal fabrication conditions are achieved at higher scanning velocities, mass production with reduced fabrication times can be achieved.