Femtosecond Micromachining Research Articles

A miniature temperature sensor based on upconversion luminescence for real-time GPU temperature monitoring

This paper introduces a miniaturized optical fiber temperature sensor based on Fluorescence Intensity Ratio (FIR) technology for real-time monitoring of graphics processing unit (GPU) temperature. Utilizing a femtosecond micromachining system, miniature rectangular holes are etched on a standard multimode fiber. These holes are then filled with a mixture of Er3+/Yb3+ co-doped TeO2-Na2CO3-ZnO powder and polydimethylsiloxane (PDMS) through capillary action, forming a sandwich structure. When illuminated by a 980 nm light source, upconversion (UC) fluorescence is generated, and a mathematical model correlating the fluorescence intensity ratio of two adjacent energy levels with temperature is established. The fundamental temperature sensing characteristics of the sensor are tested in a temperature-controlled chamber, with a maximum error of ± 1 K. The sensor is also applied to a GPU with real-time temperature variations, demonstrating a maximum error of 0.9 K and a response time of 1.96 s. The sensor not only possesses the advantages of a simple structure, micro size, and convenient fabrication but also exhibits immunity to electromagnetic interference, rapid response, good stability, and excellent repeatability in real-time monitoring of GPU temperature, showing potential for large-scale application.

Femtosecond laser ablated trench array for improving performance of commercial solid oxide cell

The performance of electrode-supported solid oxide cells (SOCs) is limited adversely by gas diffusion impedance in thick and porous support. This work focuses on the improvement of gas transport properties of commercial Ni-YSZ anode-supported SOFC by femtosecond laser-based micromachining where micro-holes of identical depth but different hole separations pitches with minimal heated affected zones were imposed. The polarization resistance calculations and DRT analysis revealed that the presence of the micro-holes improves fuel transport in the anode active zone of commercial SOFC. The presence of the micro-holes resulted in up to 20.8 % and up to 17.2 % reduction in polarization resistance for dry H2 and wet H2 gas-fueled SOFC samples, respectively. Moreover, the decrease in intensity of peaks responsible for fuel diffusion with increasing micro-holed density was observed. Therefore, dense and sparse cells exhibited a performance augmentation of 25 % and 11 % in dry H2 and enhancement of 16 % and 15.6 % in wet H2, respectively. Fs-laser ablation appeared as a unique capability for the post-processing of SOFC elements via imposing different gas channel geometries.

Laser-induced periodic surface structures as optical resonators for organic thin-film distributed feedback lasers

Laser-induced periodic surface structures (LIPSS) have received considerable attention due to their potential for micro-and nanostructuring and surface functionalization of various materials. We present a novel application of LIPSS on a glass substrate as distributed feedback (DFB) laser resonators, capable of providing sufficient positive optical feedback to achieve lasing in thin-film waveguides with organic small molecules as a gain medium. The direct femtosecond micromachining allows for easy variation of the periodicity across a broad range of values, including those required to reach 1st Bragg order DFB operation. We investigate several small molecule organic thin-film systems and observe lasing in strong accordance with the underlying periodicities of all photonic structures involved. These results demonstrate the effectiveness of LIPSS as DFB laser resonators and suggest that they could facilitate the integration of organic thin-film media-based lasers and other photonic devices into various integrated photonic systems.

Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces

Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece’s main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 μm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 μm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., Sa parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average Sa value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.

Antireflective array on zinc sulfide fabricated by femtosecond laser

Surface antireflection of zinc sulfide (ZnS) is crucial to improve the performance of infrared windows. In this study, femtosecond laser Bessel beam micromachining is proposed to fabricate antireflective microstructures (ARMs) on ZnS to suppress surface Fresnel reflection. The ARM consisting of approximately one hundred thousand holes within 30 × 30 mm2 is uniformly spread on the ZnS surface. The Fabry-Perot (FP) and Mie resonances in the multi-resonant surface work together to reduce reflection and are enhanced with increasing ARM depth. The ARM depth is tunable by adjusting the pulse energy and is utilized to facilely regulate the reflectance spectra as well as wettability. Finally, the fabricated ARM with wide-angle antireflection and hydrophobicity proved the effectiveness of the proposed method.

Single-shot femtosecond bulk micromachining of silicon with mid-IR tightly focused beams

Being the second most abundant element on earth after oxygen, silicon remains the working horse for key technologies for the years. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. Mid-IR (beyond 4 µm) ultrafast lasers provide the required control for all these parameters not only on the surface (as in the case of the lithographic techniques), but also inside the bulk of the semiconductor, paving the way to an unprecedented variety of properties that can be encoded via such an excitation. We estimated the deposited energy density as 6 kJ cm−3 inside silicon under tight focusing of mid-IR femtosecond laser radiation, which exceeds the threshold value determined by the specific heat of fusion (~ 4 kJ cm−3). In such a regime, we successfully performed single-pulse silicon microstructuring. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a “ring” with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 × 1021 cm−3. These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon.

Sensitivity-Enhanced Fiber-Optic Sensor Based on a Drilled PDMS Diaphragm for Ultrasound Imaging of Seismic Physical Model

An optical fiber Fabry–Perot interference (FPI) ultrasound sensor, based on a polydimethylsiloxane (PDMS) diaphragm with dual circular holes, is proposed and experimentally demonstrated. Two holes with central axis symmetry were drilled in the 20 μm-thick PDMS diaphragm using femtosecond micro-machining. The dual holes eliminated the pressure difference between the two interfaces of the diaphragm, allowed the strain of the diaphragm vibration to concentrate around the center of the diaphragm, and increased the ultrasound sensing sensitivity. The center distance of dual holes was optimized to enhance high ultrasound response. The ultrasound response peak-to-peak voltage of the PDMS diaphragm with a dual-holes center distance of 45.0 μm increased by 35% compared to that of the PDMS film without drilling. The FPI sensor was packaged using an acoustic plano-concave lens to facilitate the acoustic-to-sensor coupling efficiency and realize underwater ultrasound imaging. Experimental results showed that the proposed sensor realized the ultrasound imaging of an uplifted multilayer physical model.

Coupling scheme for graphene saturable absorber in a linear cavity mode-locked fiber laser.

A saturable absorber based on a graphene layer covered single-mode fiber with inner short waveguides is proposed and demonstrated for a linear cavity Er-doped mode-locked fiber laser. A pair of short waveguides is written in the fiber by using femtosecond micromachining technology, and the propagating light is guided by one short waveguide to the cladding-air interface and interacts with the graphene layer in the form of evanescent waves before being collected back to the core by another short waveguide, and, as a result, the saturable absorption is excited. The designed saturable absorber is used in the passively mode-locked fiber laser to generate traditional soliton mode-locked pulse output with the center wavelength of 1564.9 nm and pulse width of 758 fs at the fundamental frequency of 22.58 MHz. The fabricated saturable absorber device is stable in operation, compact in structure, safe for thermal damage, and can effectively overcome the shortcomings of poor robustness of the saturable absorbers based on a tapered fiber and D-shaped fiber. This provides a new optical coupling scheme for saturable absorbers based on 2D materials such as graphene and has great potential application in the field of ultrashort pulse lasers.

Controlling surface wettability in methacrylic copolymer containing azobenzene by fs-laser microstructuring

Wettability control and optimization are of high significance in various applications given its relevance in fundamental research as well as applications. Controlled wetting behavior can be obtained by femtosecond laser surface micromachining: a single step remarkable process, without contact and use of photomasks to produce micro and nanoscale structures on the surface of various materials. Here, one demonstrates the use of fs-laser micromachining to alter the morphology of the surface of films obtained from a fluorinated acrylate-based copolymer-containing azobenzene, in order to produce hydrophilic surfaces. Specifically, square-shaped pillars were produced on the polymer surface, leading to a maximum decrease of approximately 16ο in the water contact angle, whose wetting behavior could be described by the Cassie-impregnating regime.

Novel fabrication technique for NiTi and TiN micro-structures by femtosecond lasers

NiTi shape memory alloy (SMA) thin films were deposited onto silicon substrate using pulsed DC magnetron sputtering technique. To obtain crystalline NiTi thin films has to be synthesized at higher temperatures (475 - 525) °C. This high temperature requirement restricts the ease in conventional lithographic procedures. The recent advancements in the laser micromachining lend their applications into the fabrication of miniaturized systems. The femtosecond lasers (FSL) allow non-thermal processing of materials by ablation. This work focuses on the deposition and fabrication of NiTi (≈1.5 μm. thick) and titanium nitride (TiN ≈0.3 μm. thick) thin films based miniaturized systems by femtosecond laser bulk micromachining. The NiTi and TiN microstructures were release by bottom silicon etch using reactive ion etching chlorine chemistry (RIE-Cl).

Increase in efficacy of near-infrared femtosecond micromachining in ophthalmic hydrogels with the addition of sodium fluorescein, rose bengal, and riboflavin.

We report on the effect of exogenous doping agents with large two-photon absorption cross sections on the efficacy of near-infrared femtosecond micromachining in ophthalmic hydrogels. Contaflex GM Advance 58 hydrogels were solution doped in low concentrations of sodium fluorescein, rose bengal, and riboflavin dissolved in balanced salt solution prior to femtosecond micromachining with three near-infrared wavelengths: 720, 800, and 1035 nm. Using any of the three doping agents in the concentrations studied produced an increase in the amount of optical phase change induced in the material at all writing wavelengths and reduced the amount of power needed to induce a desired amount of phase change.

Micro-structuring, ablation, and defect generation in graphene with femtosecond pulses

Femtosecond micromachining offers a contact-free and mask-less technique for material patterning. Femtosecond pulse irradiation of single layer graphene by low energy pulses around 12 nJ over a 15 second illumination period is studied. The ablated graphene holes are surrounded by a radially symmetric region characterized by generated defects, whose extension can be modified based on the laser fluence. The femtosecond induced structural modifications can be attractive for graphene or carbon-based device fabrication as well as sensor and transistor applications, where regions of varying carrier concentrations and different electrical, optical, or physical properties are desired.

Femtosecond micro-machining of hydrogels: parametric study and photochemical model including material saturation

We report on the effects of various laser system parameters on the magnitude of phase change induced inside hydrogel-based contact lens materials in the two-photon absorption limit via a laser induced refractive index (LIRIC) technique. In comparison with near infrared writing at 1035 nm where four-photon absorption process dominates, blue writing at 405 nm allows for the achievement of a decent amount of phase change (one wave) with a low power and a fast scan speed due to a more efficient two photon absorption process. Efficacy of the LIRIC process could be further improved by taking advantage of intermediate repetition rate laser pulses instead of high repetition rate (> 60 MHz) pulses or low repetition rate (< 500 KHz) pulses. A generally applicable photochemical model based on multiphoton absorption mechanism and pulse overlapping effect in two dimensions is proposed to predict the scaling behavior of the induced phase change. A modified photochemical model incorporating a saturation factor is developed to account for the behavior at large phase shifts. The modified photochemical model also helps explain the inapparent dependence of the phase change on numerical aperture (NA) at low irradiation doses and the observed sub-linear inverse dependence on scan speed.

Quantitative photochemical scaling model for femtosecond laser micromachining of ophthalmic hydrogel polymers: effect of repetition rate and laser power in the four photon absorption limit

We report on the effect of laser repetition rate on the refractive index (RI) change or phase change induced by the femtosecond micromachining hydrogel-based ophthalmic materials. A repetition rate tunable femtosecond ytterbium (Yb) doped fiber laser was used to study the relation between the laser-induced phase change and the pulse repetition rate in the range of 1 MHz to 60 MHz. We present both the qualitative and quantitative results of the laser-induced phase change obtained in hydrogel-based contact lenses at different repetition rates and discuss the effect of repetition rate on the magnitude of the phase shift, the optical damage threshold and the maximum achievable phase change just below the optically-induced damage threshold. A photochemical model derived in the four photon absorption limit with pulse overlapping is employed to fit the experimental data obtained at four different repetition rates, 5 MHz, 10 MHz, 15 MHz and 60 MHz. This work contributes to the current knowledge of the response of hydrogel polymers to various laser irradiation parameters and the optimization of laser repetition rates enables femtosecond micromachining of ophthalmic materials at a lower average power and a faster writing speed.

Several new directions for ultrafast fiber lasers [Invited

Ultrafast fiber lasers have the potential to make applications of ultrashort pulses widespread - techniques not only for scientists, but also for doctors, manufacturing engineers, and more. Today, this potential is only realized in refractive surgery and some femtosecond micromachining. The existing market for ultrafast lasers remains dominated by solid-state lasers, primarily Ti:sapphire, due to their superior performance. Recent advances show routes to ultrafast fiber sources that provide performance and capabilities equal to, and in some cases beyond, those of Ti:sapphire, in compact, versatile, low-cost devices. In this paper, we discuss the prospects for future ultrafast fiber lasers built on new kinds of pulse generation that capitalize on nonlinear dynamics. We focus primarily on three promising directions: mode-locked oscillators that use nonlinearity to enhance performance; systems that use nonlinear pulse propagation to achieve ultrashort pulses without a mode-locked oscillator; and multimode fiber lasers that exploit nonlinearities in space and time to obtain unparalleled control over an electric field.

Explicit formulation of second and third order optical nonlinearity in the FDTD framework

The finite-difference time-domain (FDTD) method is a flexible and powerful technique for rigorously solving Maxwell’s equations. However, three-dimensional optical nonlinearity in current commercial and research FDTD softwares requires solving iteratively an implicit form of Maxwell’s equations over the entire numerical space and at each time step. Reaching numerical convergence demands significant computational resources and practical implementation often requires major modifications to the core FDTD engine. In this paper, we present an explicit method to include second and third order optical nonlinearity in the FDTD framework based on a nonlinear generalization of the Lorentz dispersion model. A formal derivation of the nonlinear Lorentz dispersion equation is equally provided, starting from the quantum mechanical equations describing nonlinear optics in the two-level approximation. With the proposed approach, numerical integration of optical nonlinearity and dispersion in FDTD is intuitive, transparent, and fully explicit. A strong-field formulation is also proposed, which opens an interesting avenue for FDTD-based modelling of the extreme nonlinear optics phenomena involved in laser filamentation and femtosecond micromachining of dielectrics.

Optofluidic Microlasers based on Femtosecond Micromachining Technology

AbstractWe present the different optofluidic lasers which have been realized using the Femtosecond Micromachining technique to fabricate the monolithic optofluidic structures in glass chips. We show how the great flexibility of this 3D technique allows getting different kind of optical cavities. The most recent devices fabricated by this technique as ring shaped and Fabry-Perot resonators show excellent emission performances.We also point out how the addition of the inkjet printing technique provides further opportunities in realizing optofluidic chips.

Research on the effect of coverage rate on the surface quality in laser direct writing process

Direct writing technique is usually used in femtosecond laser two-photon micromachining. The size of the scanning step is an important factor affecting the surface quality and machining efficiency of micro devices. According to the mechanism of two-photon polymerization, combining the distribution function of light intensity and the free radical concentration theory, we establish the mathematical model of coverage of solidification unit, then analyze the effect of coverage on the machining quality and efficiency. Using the principle of exposure equivalence, we also obtained the analytic expressions of the relationship among the surface quality characteristic parameters of microdevices and the scanning step, and carried out the numerical simulation and experiment. The results show that the scanning step has little influence on the surface quality of the line when it is much smaller than the size of the solidification unit. However, with increasing scanning step, the smoothness of line surface is reduced rapidly, and the surface quality becomes much worse.

Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon

In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser micromachining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study femtosecond (800 nm, 110 fs, 0.1-1 mJ/pulse) micromachining of a material with well-defined initial defect concentrations: doped Si across a range of dopant types and concentrations. The single-pulse ablation threshold (Fth,1) was observed to decrease with increasing dopant concentration, from a maximum of 0.70 J/cm2 (+/-0.02) for undoped Si to 0.51 J/cm2 (+/-0.01) for highly N-type doped Si. The effect was greater for N-type doped Si than for P-type, consistent with the higher carrier mobility of electrons compared to holes. In contrast, the infinite-pulse ablation threshold (Fth,inf) was the same for all doping levels and types. We attribute this asymptotic behaviour to a maximum defect concentration that is independent of the initial defect concentration and type. These results lend insight into the mechanism of multipulse, femtosecond laser ablation.

Fabrication of an integrated high-quality-factor (high-Q) optofluidic sensor by femtosecond laser micromachining.

We report on fabrication of a microtoroid resonator of a high-quality factor (i.e., Q-factor of ~3.24 × 10(6) measured under the critical coupling condition) integrated in a microfluidic channel using femtosecond laser three-dimensional (3D) micromachining. Coupling of light into and out of the microresonator has been realized with a fiber taper that is reliably assembled with the microtoroid. The assembly of the fiber to the microtoroid is achieved by welding the fiber taper onto the sidewall of the microtoroid using CO2 laser irradiation. The integrated microresonator maintains a high Q-factor of 3.21 × 10(5) as measured in air, which should still be sufficient for many sensing applications. We test the functionality of the integrated optofluidic sensor by performing bulk refractive index sensing of purified water doped with tiny amount of salt. It is shown that a detection limit of ~1.2 × 10(-4) refractive index unit can be achieved. Our result showcases the capability of integration of high-Q microresonators with complex microfluidic systems using femtosecond laser 3D micromachining.