Indirect laser etching of fused silica: Towards high etching rate processing

The indirect laser processing approach (LIBWE) laser-induced backside wet etching allows defined microstructuring of transparent materials at low laser fluences with high quality. The optical and the thermal properties of the solid/liquid interface determine the temperatures and therefore the etching mechanism in conjunction with the dynamic processes at the interface due to the fast heating/cooling rates. The exploration of organic liquid solvents and solutions such as 0.5 M pyrene/toluene results in low etch rates (∼20 nm/pulse). By means of liquid metals as absorber here, demonstrated for gallium (Ga), etch rates up to 600 nm/pulse can be achieved. Regardless of the high etch rates a still smooth surface similar to etching with organic liquid solutions can be observed. A comparative study of the two kinds of absorbing liquids, organic and metallic, investigates the etch rates regarding the fluence and pulse quantity. Thereby, the effect of incubation processes as result of surface modification on the etching is discussed. In contrast to pyrene/toluene solution the metallic absorber cannot decompose and consequently no decomposition products can alter the solid/liquid interface to enhance the absorption for the laser radiation. Hence, incubation can be neglected in the case of the silica/gallium interface so that this system is a suitable model to investigate the primary processes of LIBWE. To prove the proposed thermal etch mechanism an analytical temperature model based on a solution of the heat equation is derived for laser absorption at the silica/gallium interface.     

Wavelength-tunable multicolored femtosecond laser pulse generation in a fused silica glass plate

We obtained an array of multicolored femtosecond laser pulses with as many as 17 different colors that are spatially isolated. The mechanism of generation was proved to be cascaded four-wave mixing and with the following procedure. The output beam from a femtosecond laser was split into two. One of the two beams was pulse-compressed with a hollow core fiber and the intensity of the other was reduced. The two beams were synchronized and combined with a small crossing angle in a plate of fused silica glass plate. The wavelengths of the sidebands are continuously tunable from near-ultraviolet to near-infrared. The pulse duration, spatial mode, spectrum, and energy stability of the sidebands were studied. As many as fifteen spectral up-shifted pulses and two spectral downshifted pulses were obtained with spectral bandwidths broader than 1.8 octaves. Properties such as pulse energy as high as 1 μmJ, 45 fs pulse duration, smaller than 1.1 times of the diffraction limit Gaussian spatial profile, and better than 2% RMS power stability of the generated sidebands make it can be used in various experiments. The characterization showed that the sidebands have sufficiently good qualities to enable application to for various multicolor femtosecond laser experiments, for example, a multicolor pump-probe experiment.     

Polarization-selective etching in femtosecond laser-assisted microfluidic channel fabrication in fused silica

We fabricate microchannels in fused silica by femtosecond laser irradiation followed by etching in diluted hydrofluoric acid. We show a dramatic dependence of the etch rate on the laser polarization, spanning 2 orders of magnitude. We establish the existence of an energy-per-pulse threshold at which etching of the laser-modified zones becomes highly polarization selective. The enhanced selective etching is due to long-range, periodic, polarization-dependent nanostructures formed in the laser-modified material.     

Ultrafast time-resolved photography of femtosecond laser induced modifications in BK7 glass and fused silica

The dynamics of absorption after excitation of fused silica and BK7 glass with femtosecond laser radiation are visualized by transient absorption spectroscopy. Focusing laser radiation with pulse durations in the picosecond time regime in BK7 glass generates free electrons with relaxation by emission of radiation or by formation of defects. The temporal and spatial emission characteristics are observed by high-speed photography in the streak mode. The beam waist moves within the pulse duration towards the incoming laser radiation by self-focusing and with the laser radiation absorbed by multi-photon processes. The dynamics of the long lasting stress formation is visualized by time-resolved Nomarski-Photography. The modification of the glass is investigated during and after irradiation with ultra-short pulsed laser radiation (100 fs<t_p<3 ps) at the wavelength =810 nm. The formation of a sound wave in fused silica and BK7 glass is observed and the mechanical stress, depending on the excitation pulse duration, is measured. PACS 06.60.Jn; 42.50.Md; 78.47.-p; 81.16.-c; 82.53.-k     

Deep subsurface optical waveguides produced by direct writing with femtosecond laser pulses in fused silica and phosphate glass

In the present work, we have analyzed the use of elliptical beam shaping along with low numerical aperture focusing optics in order to produce circular cross-section waveguides in different materials at large processing depths by direct femtosecond laser writing (100 fs, 800 nm, 1 kHz). A variable slit located before the focusing optics allows to generate a nearly elliptical beam shape and also to reduce the effective numerical aperture of the beam along the shat axis of the ellipse. The focusing optics allows to focus the beam deep inside the sample, which is translated at a constant speed transversely to the writing beam direction. The influence of several experimental parameters (energy per pulse, slit width, processing depth) on the properties of the produced waveguides has been analyzed. The influence of the intrinsic properties of the material (refractive index, composition) has been analyzed by comparing results obtained in fused silica and Er:Yb co-doped phosphate glass. The results obtained show that this approach leads to the successful production of deep subsurface (up to 7 mm) waveguides with circular cross-sections. Preliminary results using chirped pulses in the phosphate glass suggest that temporal pulse shaping can be used as an additional parameter to optimize the guided mode symmetry.     

High power supercontinuum generation by dual-color femtosecond laser pulses in fused silica

High power supercontinuum (SC) is generated by focusing 800 nm and 400 nm femtosecond laser pulses in fused silica with a microlens array. It is found that the spectrum of the SC is getting broader compared with the case of a single laser pulse, and the spectral energy density between the two fundamental laser wavelengths is getting significantly higher by optimizing the phase matching angle of the BBO. It exceeds μJ/nm over 490 nm range which is from 380 nm to 870 nm, overcoming the disadvantage of relative lower power in the ranges far from the fundamental wavelength.     

Backside laser etching of fused silica using liquid gallium

Laser-induced backside wet etching (LIBWE) that is regularly performed with hydrocarbon solutions is demonstrated with the liquid metal gallium as a new class of absorbers for the first time. Well-contoured square etch pits in fused silica with smooth bottoms and well-defined edges were achieved already with the first pulse from a 248 nm excimer laser. The etching is characterized by a threshold fluence of 1.3 J/cm² and a straight proportional etch rate growth with the fluence up to 8.2 J/cm². In addition, the etch depth increases linearly for onward pulsed laser irradiation and gives evidence for an only marginal incubation effect. The high fluences necessary for etching originate from the high reflection losses as well as the high thermal conductivity of the metallic absorber. The suggested etch mechanism comprises the heating of the fused silica up to or beyond the fused silica melting point by the laser heated gallium and the removing of the softened or molten fraction of the material by mechanical forces from shock waves, bubbles, high pressures, or stress fields. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

Nanosecond and femtosecond excimer laser ablation of fused silica

Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 m/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm²) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.The results lead to the conclusion that two different ablation mechanisms are involved: the first is based on two photon bulk absorption, the second on controlled surface damage in relation with (partially laser induced) singularity conditions at the surface.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

Backside etching of fused silica with Nd:YAG laser

The laser-induced backside etching of fused silica with gallium as highly absorbing backside absorber using pulsed infrared Nd:YAG laser radiation is demonstrated for the first time. The influence of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography was studied. The comparable high threshold fluences of about 3 and 7 J/cm² for 18 and 73 ns pulses, respectively, are caused by the high reflectivity of the fused silica-gallium interface and the high thermal conductivity of gallium. For the 18 and 73 ns long pulses the etch rate rises almost linearly with the laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. Incubation processes are almost absent because etching is already observed with the first laser pulse at all etch conditions and the etch rate is constant up to 30 pulses.The etched grooves are Gaussian-curved and show well-defined edges and a smooth bottom. The roughness measured by interference microscopy was 1.5 nm rms at an etch depth of 0.6 μm. The laser-induced backside etching with gallium is a promising approach for the industrial application of the backside etching technique with IR Nd:YAG laser.     

Control of femtosecond laser written waveguides in silica glass

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. PACS 42.65.Jx; 42.70.Ce; 42.79.Gn     

Using IR laser radiation for backside etching of fused silica

Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about 3 J/cm² and 7 J/cm² for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom. A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm. The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

The polarization-dependence of femtosecond laser damage threshold inside fused silica

The laser damage threshold inside fused silica is dependent not only on the numerical aperture (NA) of the focusing objective, but also on the polarization of the incident femtosecond laser pulses. The damage threshold for circularly polarized beams is higher than that for linearly polarized beams when NA>0.4, but the former was lower than the latter when NA<0.4. The reverse might be due to different damage processes: laser induced damage at high NA and the self-focusing induced breakdown at low NA.     

Optical and ultrasonic monitoring of femtosecond laser filamentation in fused silica

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica (FS) by single focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy E_opt = 5 μJ corresponding to a few megawatt power levels pulses collapse due to self-focusing, producing channels filled by electron-hole plasma and luminescent defects, and exhibits predominantly compressive pressure transients. Analysis of the optical and ultrasonic response versus the laser pulse energy suggests that filamentary pulse propagation in the channels occurs with considerable dissipation of about ∼10 cm⁻¹. The predominant ionization mechanism is most likely associated with avalanche ionization, while the main mechanism of optical absorption is free-carrier absorption via inverse Bremsstrahlung interaction with the polar lattice.     

Spatio-temporal evolution scenarios of femtosecond laser pulse filamentation in fused silica

We investigated the evolution of femtosecond laser pulses at different wavelengths corresponding to normal, zero, and anomalous regimes of group velocity dispersion (GVD) in fused silica. The laser pulse filamentation in different GVD regimes under the same similarity parameters was first considered. It was established numerically that the scenario of the pulse filamentation depends both on temporal factors, which are determined by pulse GVD and self-phase modulation, and spatial factors associated with Kerr self-focusing and plasma defocusing. In presence of strong normal GVD the dispersive stretching causes, a pulse power decrease followed by lowering of the intensity in filament, electron density reduction in plasma channel, and suppressing of the refocusing. For zero GVD the multipeak regime of radiation propagation is realized in the filament as a result of recurring self-focusings of powerful pulse tail, which was defocused in laser plasma. When GVD is anomalous a sequence of ??light bullets?? with duration about 10 fs forms in the filament. And the peak intensity in ??light bullet?? stays the same ?? 5 × 10¹³ W/cm². In the regime of anomalous GVD power is transferred from the pulse edges to its center, where the repeated self-focusings occur and form a ??light bullet?? sequence.     

Thermal process of silica glass microchannels fabricated by femtosecond laser ablation

In order to improve the morphology of microchannels fabricated by femtosecond laser ablation, the thermal process was introduced into the post-treatment processing. It was found that the thermal process cannot only decrease the roughness but also the width and depth of the microchannel. The change rates of width, depth, and roughness of the microchannel increase with processing temperature. When we prolong the time of constant temperature, the change rate of the width decreases at the beginning, and then it tends to be stable. However,the change rates of depth and roughness increase, and then they tend to be stable. In this Letter, we discuss the reasons of the above phenomena.     

Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses

The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for structuring transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF₂, MgF₂), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond (t_p = 10 ps) lasers at UV wavelengths (λ₁ = 355 nm and λ₂ = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with different slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a λ₁ = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths (λ₁ = 266 nm and λ₂ = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm², respectively.     

Improvement of laser damage thresholds of fused silica by ultrasonic-assisted hydrofluoric acid etching

Polished fused silica samples were etched for different durations by using hydrofluoric(HF) acid solution with HF concentrations in an ultrasonic field. Surface and subsurface polishing residues and molecular structure parameters before and after the etching process were characterized by using a fluorescence microscope and infrared(IR) spectrometer, respectively. The laser induced damage thresholds(LIDTs) of the samples were measured by using pulsed nanosecond laser with wavelength of 355 nm. The results showed that surface and subsurface polishing residues can be effectively reduced by the acid etching process, and the LIDTs of fused silica are significantly improved. The etching effects increased with the increase of the HF concentration from 5 wt.% to 40 wt.%. The amount of polishing residues decreased with the increase of the etching duration and then kept stable. Simultaneously, with the increase of the etching time, the mechanical strength and molecular structure were improved.