Visible Wavelength Laser Research Articles

Two-color 3D printing for reduction in femtosecond laser printing power.

Two-photon polymerization (TPP) has emerged as a favored advanced manufacturing tool for creating complex 3D structures in the sub-micron regime. However, the widescale implementation of this technique is limited partly due to the cost of a high-power femtosecond laser. In this work, a method is proposed to reduce the femtosecond laser 3D printing power by as much as 50% using a combination of two-photon absorption from an 800 nm femtosecond laser and single photon absorption from a 532 nm nanosecond laser. The underlying photochemical process is explained with modeling of the photopolymerization reaction. The results show that incorporating single-photon absorption from a visible wavelength laser efficiently reduces inhibitor concentration, resulting in a decreased requirement for femtosecond laser power. The radical to macroradical conversion is dominated by the reduction in oxygen concentration, while the reduction in photoinitiator concentration limits the threshold power reduction of the femtosecond laser.

Sum rule comparison of narrowband and broadband sum frequency generation spectra and comparison with theory

Earlier sum frequency generation (SFG) experiments involve one infrared and one visible laser, and a measurement of the intensity of the response, yielding data on the surface sensitive properties of the sample. Recently, both the real and imaginary components of the susceptibility were measured in two different sets of experiments. In one set, a broadband infrared laser was used, permitting observations at very short times, while in another set the infrared laser was narrowband, permitting higher spectral resolution. The differences in the spectrum obtained by the two will be most evident in studying narrow absorption bands, e.g., the band due to dangling OH bonds at a water interface. The direct comparisons in the integrated amplitude (sum rule) of the imaginary part of the dangling OH bond region differ by a factor of 3. Due to variations in experimental setup and data processing, corrections were made for the quartz reference, Fresnel factors, and the incident visible laser wavelength. After the corrections, the agreement differs now by the factors of 1.1 within broadband and narrowband groups and the two groups now differ by a factor of 1.5. The 1.5 factor may arise from the extra heating of the more powerful broadband laser system on the water surface. The convolution from the narrowband SFG spectrum to the broadband SFG spectrum is also investigated and it does not affect the sum rule. Theory and narrowband experiments are compared using the sum rule and agree to a factor of 1.3 with no adjustable parameters.

Experimental and numerical study of Si annealed by laser sources of the visible spectrum (red and blue wavelength)

Laser annealing (LA) is an increasingly important process in the semiconductor manufacturing industry. So far, lasers of nonvisible spectrum have been mostly used for semiconductor applications. The study using a visible wavelength laser is not nearly found though it could be a potential tool to expand applications of the LA. In this study, the silicon (Si) deposited on oxide was annealed using laser sources of visible spectrum. LA of Si was performed using a single laser source of either red or blue wavelengths or a dual-laser source using both wavelengths. The surface morphology and microstructure of the annealed Si were analyzed. A numerical simulation was also conducted to predict the temperature and melt pool characteristics of Si during the annealing process. Surface roughness increased with increasing heat input (or power density) of the laser beam, owing to the considerable melting of Si. The dual laser using a relatively high power density produced Si with high polycrystallinity and large grains. However, the Si lattice constant difference induced by different annealing conditions was negligible. Simulation results showed that temperature, melt pool length, and duration increased with the power density of the dual-laser beam. This contributed to the growth of large Si grains.

Enhanced Ablation of Cornea Doped with a Fluorescein Dye Using a Visible Wavelength Laser.

Myopia (nearsightedness) and presbyopia (aging eye) are the most common refractive errors of the human eye. Technology has advanced toward correcting aberration using various surgical procedures, including laser surgery, as well as prescribing corrective lenses. Hence, the effect of various laser wavelengths on the eye has been extensively studied over the last few decades. Usually, excimer lasers are used for this purpose, which increases the cost of the procedure because they are unique and difficult to manufacture and require regular maintenance. Due to the absorption properties, visible wavelengths do not interact with the corneal layers and hence are currently not used for eye surgery. This study presents the first clinical evidence that a 532 nm laser in combination with an eye-safe fluorescein dye that is in wide clinical use in ophthalmology can be utilized for high-precision ablation purposes due to the photochemical reaction that occurs on an ex vivo porcine eye. Our results show the promise of utilizing inexpensive visible wavelength lasers in the ablation of biological tissues, reducing the high costs of ophthalmological surgical procedures, as well as in other applications.

Processability of pure Cu by LPBF using a ns-pulsed green fiber laser

Visible wavelength lasers appear a straightforward solution for improved density in LPBF of Cu. However, only few high power continuous wave (CW) green lasers are industrially available. Nanosecond (ns) pulsed green laser sources can be appealing for a wider industrial use, if adapted towards a melting-based process avoiding ablation based shockwaves. This work reports the use of a novel ns-pulsed green fiber laser with 100 W average power and 30 MHz pulse repetition rate for processing pure Cu. The work shows that free-standing samples without excessive powder ejection from the powder bed and densities of 98.1% can be achieved with processing conditions similar to a conventional CW lasers.

Laser polishing of additive manufactured L-PBF copper parts with visible laser wavelength of 515 nm – Challenges due to high surface roughness

Copper components produced by Laser Powder Bed Fusion (L-PBF) have a high surface roughness of up to Ra = 21.6 µm. The surface has a structure depth of up to 300 µm, as well as porosity. In order to smooth this surface, laser polishing of copper is being investigated. A challenge here is the low absorption of the near-infrared laser beams as well as the high thermal conductivity of the copper material. To increase the absorbed laser radiation, a 1 kW laser source in the visible green wavelength range is used instead of a near-infrared laser source. In the first-time investigations of laser polishing of copper, the laser beam intensity, the line energy as well as the track overlap are varied. Microscope images, cross-sections and tactile roughness measurements are used for the analysis. A laser beam intensity of 4.95 kW/cm² is required to completely remelt the high surface structure depth without porosity. The surface roughness could be reduced by 85.2% from Ra = 21.6 µm to Ra = 3.2 µm.

Advanced Optical Detection through the Use of a Deformably Transferred Nanofilm

Graphene has been extensively investigated in advanced photodetection devices for its broadband absorption, high carrier mobility, and mechanical flexibility. Due to graphene’s low optical absorptivity (2.3%), graphene-based photodetection research so far has focused on hybrid systems to increase photoabsorption. However, such hybrid systems require a complicated integration process and lead to reduced carrier mobility due to heterogeneous interfaces. Crumpled or deformed graphene has previously been reported in electronics and optoelectronics. However, a depth study on the influence of the morphology of nanofilms (e.g., graphite or graphene) related to light absorption in photodetection devices has not been demonstrated yet. Here, we present an interesting study in terms of the effect of the deformable surface and the smooth surface of a nanofilm transferred onto Si through two transfer strategies using isopropanol injection and nitrogen blowing (to form a deformable nanofilm surface) and deionized water injection and van der Waals interaction (to form a smooth nanofilm surface). As a result, optical detection in the case of the deformable nanofilm surface was enhanced significantly (~100%) compared with that of the smooth nanofilm surface in the visible laser wavelength (532 nm). In addition, evidence from the computational simulation also firmly affirms an advancement in the optical detection of deformed nanofilm-surface-based photodetection devices compatible with the experimental results.

Visible Laser Scribing Fabrication of Porous Graphitic Carbon Electrodes: Morphologies, Electrochemical Properties, and Applications as Disposable Sensor Platforms

Porous graphitic carbon electrodes were fabricated by laser scribing of commercial polyimide tape. The process was performed by a simple one-step procedure using visible wavelength laser irradiatio...

A New Family of Ultralow Loss Reversible Phase‐Change Materials for Photonic Integrated Circuits: Sb2S3 and Sb2Se3

AbstractPhase‐change materials (PCMs) are seeing tremendous interest for their use in reconfigurable photonic devices; however, the most common PCMs exhibit a large absorption loss in one or both states. Here, Sb2S3 and Sb2Se3 are demonstrated as a class of low loss, reversible alternatives to the standard commercially available chalcogenide PCMs. A contrast of refractive index of Δn = 0.60 for Sb2S3 and Δn = 0.77 for Sb2Se3 is reported, while maintaining very low losses (k < 10−5) in the telecommunications C‐band at 1550 nm. With a stronger absorption in the visible spectrum, Sb2Se3 allows for reversible optical switching using conventional visible wavelength lasers. Here, a stable switching endurance of better than 4000 cycles is demonstrated. To deal with the essentially zero intrinsic absorption losses, a new figure of merit (FOM) is introduced taking into account the measured waveguide losses when integrating these materials onto a standard silicon photonics platform. The FOM of 29 rad phase shift per dB of loss for Sb2Se3 outperforms Ge2Sb2Te5 by two orders of magnitude and paves the way for on‐chip programmable phase control. These truly low‐loss switchable materials open up new directions in programmable integrated photonic circuits, switchable metasurfaces, and nanophotonic devices.

Freestanding Positionable Microwave-Antenna Device for Magneto-Optical Spectroscopy Experiments

Modern spectroscopic techniques for the investigation of magnetization dynamics in micro- and nano- structures or thin films use typically microwave antennas which are directly fabricated on top of the sample by means of electron-beam-lithography (EBL). Following this approach, every magnetic structure on the sample needs its own antenna, resulting in additional EBL steps and layer deposition processes. We demonstrate a new approach for magnetization excitation that is suitable for optical and non-optical spectroscopy techniques. By patterning the antenna on a separated flexible glass cantilever and insulating it electrically, we solved the before mentioned issues. Since we use flexible transparent glass as a substrate, optical spectroscopy techniques like Brillouin-light-scattering microscopy ({\mu}BLS), time resolved magneto-optical Kerr effect measurements (TRMOKE) or optical detected magnetic resonance (ODMR) measurements can be carried out at visible laser wavelengths. As the antenna is detached from the sample it can be freely positioned in all three dimensions to adress only the desired magnetic sample structures and to achieve effective excitation. We demonstrate the functionality of these antennas using {\mu}BLS and compare coherently and thermally excited magnon spectra to show the enhancement of the signal by a factor of about 400 due to the excitation by the antenna. Moreover, we succeed to characterize yttrium iron garnet thin films with spatial resolution using optical ferromagnetic resonance (FMR) experiments. We analyse the spatial excitation profile of the antenna by measuring the magnetization dynamics in two dimensions. The technique is furthermore applied to investigate injection-locking of spin Hall nano-oscillators.

Visualizing the trade-offs between laser eye protection and laser eye dazzle

Visual detection and color discrimination become more challenging tasks when laser eye protection (LEP) is worn. This is due to the reduced light transmission and wavelength-blocking characteristics of LEP filters. LEP can, however, provide valuable protection against laser eye dazzle—the temporary reduction of vision caused by visible wavelength lasers. To understand this compromise, a model has been developed to allow the visual impacts of LEP to be simulated and balanced against their dazzle protection capabilities. This model is able to use any digital image as a background scene, allowing for real-world visualizations of LEP and dazzle over a broad range of scenarios. The work is built from the CIECAM02 color appearance model and a modified CIE general disability glare equation, together with experimentally validated adjustments to improve the accuracy of this application. The resulting model will help to inform LEP procurement and will serve as an educational tool for LEP users.Visual detection and color discrimination become more challenging tasks when laser eye protection (LEP) is worn. This is due to the reduced light transmission and wavelength-blocking characteristics of LEP filters. LEP can, however, provide valuable protection against laser eye dazzle—the temporary reduction of vision caused by visible wavelength lasers. To understand this compromise, a model has been developed to allow the visual impacts of LEP to be simulated and balanced against their dazzle protection capabilities. This model is able to use any digital image as a background scene, allowing for real-world visualizations of LEP and dazzle over a broad range of scenarios. The work is built from the CIECAM02 color appearance model and a modified CIE general disability glare equation, together with experimentally validated adjustments to improve the accuracy of this application. The resulting model will help to inform LEP procurement and will serve as an educational tool for LEP users.

Femtosecond-laser-ablation induced transformations in the structure and surface properties of diamond-like nanocomposite films

Femtosecond laser ablation processing is applied for surface modification and micropatterning of diamond-like nanocomposite (DLN) films (a-C:H:Si:O films). Using a visible femtosecond laser (wavelength 515 nm, pulse duration 320 fs), microgroove patterns have been fabricated on the DLN films, aimed at further studies of their properties. The studies were focused on (i) structural transformations in the surface layers using Raman spectroscopy and transmission electron microscopy (TEM), (ii) wettability of laser-patterned films, and (iii) nano/microscale friction properties of laser-patterned DLN films using lateral force microscopy. Raman spectroscopy and TEM data showed characteristic features of the surface graphitization during ultrashort-pulse ablation. High resolution TEM study of the microgrooves revealed the formation of cubic SiC nanocrystals (4–8 nm size) on the laser-ablated surface. The water contact angle measurements showed anisotropic wetting behavior of the grooved surfaces (the contact angle was different in the directions parallel and perpendicular to microgrooves), depending on the groove depth (aspect ratio). Lateral force microscopy examination (with micro-sized Si tips) showed that the laser-patterned regions exhibited low friction properties compared to the original surface. The obtained results demonstrate that femtosecond laser processing is an effective technique to generate new properties of hard DLN coatings at the micro and macroscale.

Visible-wavelength two-photon excitation microscopy with multifocus scanning for volumetric live-cell imaging.

.Two-photon excitation microscopy is one of the key techniques used to observe three-dimensional (3-D) structures in biological samples. We utilized a visible-wavelength laser beam for two-photon excitation in a multifocus confocal scanning system to improve the spatial resolution and image contrast in 3-D live-cell imaging. Experimental and numerical analyses revealed that the axial resolution has improved for a wide range of pinhole sizes used for confocal detection. We observed the 3-D movements of the Golgi bodies in living HeLa cells with an imaging speed of 2 s per volume. We also confirmed that the time-lapse observation up to 8 min did not cause significant cell damage in two-photon excitation experiments using wavelengths in the visible light range. These results demonstrate that multifocus, two-photon excitation microscopy with the use of a visible wavelength can constitute a simple technique for 3-D visualization of living cells with high spatial resolution and image contrast.

Visible-Wavelength All-Fiber Vortex Laser

We report the first demonstration of a compact all-fiber visible-wavelength laser directly emitting cylindrical vector or vortex beams. The visible all-fiber cavity is constructed by a few-mode Pr/Yb-codoped ZBLAN fiber pumped by a 850 nm laser diode, a 634.6 nm few-mode fiber Bragg grating, and a visible-wavelength fiber mirror. Direct oscillation of high-order modes (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes) at 634.6 nm wavelength is achieved using core-shifted fiber coupling method. The visible cylindrical vector beams with the radially or azimuthally polarized state can be observed. Furthermore, by adjusting the rotation of the intracavity polarization controller appropriately, two orthogonal modes (i.e., LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">even</sup> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">odd</sup> ) can introduce a π/2 phase difference, and the 634.6 nm vortex beams with ±1-order orbital angular momentum modes are also obtained.

In situ and remote laser diagnostics for material characterization from plasma facing components to Cultural Heritage surfaces

Optical and spectroscopic techniques offer unique possibilities for non destructive or micro-destructive characterization of surfaces, with widespread applications, starting from in-line monitoring of industrial processes. A significant group of industrial applications concerns nuclear grade material characterization and plasma diagnostics relevant to the thermonuclear fusion process. However, technologies and methodologies originally developed for specific in-vessel utilization, can easily find additional in-situ and remote application to environmental and cultural heritage (CH) diagnostics addressed to preventive conservation and restoration of surfaces. At ENEA Frascati different prototypes have been developed and patented to collect reflectance and fluorescence images excited at different ultraviolet and visible laser wavelengths, Raman and LIBS signals. Portable integrated instruments suitable for operation at different distances from a 1.5 to 30 m, have been assembled and operated in laboratory on multilayered samples and in field campaigns for Security and on CH painted surfaces. Significant results relevant to the cross fertilization among different applications will be presented and discussed.

Photothermally Assisted Thinning of Silicon Nitride Membranes for Ultrathin Asymmetric Nanopores.

Sculpting solid-state materials at the nanoscale is an important step in the manufacturing of numerous types of sensor devices, in particular solid-state nanopore sensors. Here we present mechanistic insight into laser-induced thinning of low-stress silicon nitride (SiN x) membranes and films. In a recent study, we observed that focusing a visible wavelength laser beam on a SiN x membrane results in efficient localized heating, and we used this effect to control temperature at a solid-state nanopore sensor. A side-effect of the observed heating was that the pores expand/degrade under prolonged high-power illumination, prompting us to study the mechanism of this etching process. We find that SiN x can be etched under exposure to light of ∼107 W/cm2 average intensity, with etch rates that are influenced by the supporting electrolyte. Combining this controlled etching with dielectric breakdown, an electrokinetic process for making pores, nanopores of arbitrary dimensions as small as 1-2 nm in diameter and thickness can easily be fabricated. Evidence gathered from biomolecule-pore interactions suggests that the pore geometries obtained using this method are more funnel-like, rather than hourglass-shaped. Refined control over pore dimensions can expand the range of applications of solid-state nanopores, for example, biopolymer sequencing and detection of specific biomarkers.

Effect of MRSA Irradiation by 632, 532, and 405 nm (Red, Blue, and Green) Diode Lasers on Antibiotic Susceptibility Tests

Background: Since the discovery of penicillin and till now, antibiotics are considered the most important regime in treating bacterial infections, insipid the fast development of bacterial resistance to many antibiotics over decades. Methicillin resistance Staphylococcus aureus (MRSA) is one of these bacteria types that emerged resistance to all current antibiotic classes. Fortunately, many studies proved phototherapy as a promising supporting to antibiotics in clinical fields.Objectives: The purpose of this study is to evaluate the effect of three lasers wavelengths (red, blue, and green) with 50 mW power density on the susceptibility of clinical MRSA isolates.Materials and methods: 45 clinical identified MRSA isolates are exposed to 632, 532, and 405 nm (red, blue, and green) Diode lasers. The susceptibility tests with 12 antibiotics are determined by disk diffusion method and minimum inhibition concentration (MIC) tests.Results: The results demonstrate an increasing in the clear zone of antibiotics for all the isolates, especially for the red wavelength which was more effective, rather than the blue and green lasers which come next respectively. The effective inhibition doses of the antibiotics are decreased for lasered MRSA. The red wavelength is again more effective than the two others lasers.Conclusion: the visible laser wavelength (red, blue, and green) improved the in vitro action of the antibiotics against MRSA.

Traceable Measurements of CW and Pulse Terahertz Power With Terahertz Radiometer

We report on traceable measurements of continues wave (CW) and pulse terahertz (THz) power with a THz radiometer. A sensitive THz radiometer with a broad bandwidth, high absorbance coating as absorber was fabricated. The absorber has an absorbance of higher than 99% in both 0.1 THz–3.0 THz region and in visible laser wavelengths range, which allows for an absolute measurement of THz radiation power and traceability to the International System of Units (SI). The uncertainty budget of CW THz power measurement with this radiometer is discussed, and the measurement results agreed excellently in the first international THz power comparison. Due to its high sensitivity and flat spectral responsivity, a pulse THz radiation from a THz time-domain spectrometer (THz-TDS) was absolutely measured with this radiometer.

Interferometric Motion Detection in Atomic Layer 2D Nanostructures: Visualizing Signal Transduction Efficiency and Optimization Pathways.

Atomic layer crystals are emerging building blocks for enabling new two-dimensional (2D) nanomechanical systems, whose motions can be coupled to other attractive physical properties in such 2D systems. Optical interferometry has been very effective in reading out the infinitesimal motions of these 2D structures and spatially resolving different modes. To quantitatively understand the detection efficiency and its dependence on the device parameters and interferometric conditions, here we present a systematic study of the intrinsic motion responsivity in 2D nanomechanical systems using a Fresnel-law-based model. We find that in monolayer to 14-layer structures, MoS2 offers the highest responsivity among graphene, h-BN, and MoS2 devices and for the three commonly used visible laser wavelengths (633, 532, and 405 nm). We also find that the vacuum gap resulting from the widely used 300 nm-oxide substrate in making 2D devices, fortunately, leads to close-to-optimal responsivity for a wide range of 2D flakes. Our results elucidate and graphically visualize the dependence of motion transduction responsivity upon 2D material type and number of layers, vacuum gap, oxide thickness, and detecting wavelength, thus providing design guidelines for constructing 2D nanomechanical systems with optimal optical motion readout.

Evaluation of laser cleaning for the restoration of tarnished silver artifacts

In this study we evaluate the laser cleaning of tarnished pure and sterling silver substrates using a nanosecond Q-switched Nd:YAG laser at 1064, 532 and 266nm. To assess the effects associated with cyclic laser cleaning treatments, several cycles of tarnishing followed by laser cleaning were applied on silver coupons that were characterized by gravimetry, colorimetry, scanning electron microscopy, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. According to the obtained results, none of the three wavelengths is recommended for laser cleaning of pure silver objects, while for sterling silver artifacts, the visible laser wavelength of 532nm seems the most appropriate.