F2 Laser Research Articles

F 2 -laser digital etching of colloidal photonic crystals

We report on digital etching of silica colloidal photonic crystals by employing a pulsed deep-ultraviolet F2 laser at 157nm wavelength. When the laser fluence is above a microsphere-size-dependent threshold and within an appropriate fluence window, colloidal crystals can be etched in a digitized fashion, whereby a single microsphere layer can be removed upon exposure to a single laser pulse. Alternatively, single spheres or lines of spheres can be selectively ejected by patterning the laser beam. The results demonstrate a fast, noncontact, straightforward, and cost-effective approach for engineering extrinsic defects into colloidal photonic crystals.

Reactions of SiCl groups in amorphous SiO2 with mobile interstitial chemical species: Formation of interstitial Cl2 and HCl molecules, and role of interstitial H2O molecules

Reactions of the network-bound chloride (SiCl) groups in amorphous SiO2 (a-SiO2 or SiO2 glass) with mobile interstitial oxygen (O2), water (H2O), and hydrogen (H2) molecules thermally loaded from ambient atmosphere and with mobile radicals created by exposure to F2 laser light (λ=157nm,hν=7.9eV) were investigated. Reactions of the SiCl groups with O2 and H2O produce interstitial chlorine (Cl2) and hydrogen chloride (HCl) molecules, respectively. An infrared-absorption band appearing at ∼2815cm−1 is assigned to the interstitial HCl. The SiCl groups do not react with H2 below 400°C. However, sequential gas loading first with O2, then with H2 leads to the production of interstitial H2O, which decomposes the SiCl groups into HCl. Furthermore, the formation of the interstitial HCl with exposure to F2 laser light, most likely due to the cracking of the Si–Cl bonds with photogenerated hydrogen atoms (H0), was demonstrated. These findings yield a general picture of the reactions of the chlorine-related species in a-SiO2 and demonstrate the significant influence of even minor amounts (<1018cm−3) of interstitial H2O on defect formation and annihilation processes.

Velocity map imaging study of OCS photodissociation followed by S(1S) autoionization at 157 nm

Atomic sulphur ions (S+) were observed directly by crossing a carbonyl sulphide (OCS) molecular beam with a F2 laser. In this study both S+ ion and electron images were measured using the velocity map imaging technique. The results imply that S+ is produced from the well-known photodissociation of OCS at 157 nm leading to the dominant S(1S) + CO(1Σ+) channel, and then the excited S(1S) atom is directly ionized by another 157 nm photon. Correlated vibrationally resolved angular distributions and internal energy distribution of the CO coproducts are reported here and compared with previous studies. This experiment yields strong and sharp S+ images which may be useful for calibrating any imaging or laser ionization apparatus when using a 157 nm laser. A number of technical aspects such as corrections for partial slicing and imperfect laser polarization are described. ion of product angular distributions using both polarized and unpolarized photolysis lasers is also demonstrated using velocity map imaging.

Coherence effects in surface roughness induced by vacuum ultraviolet F_2 laser ablation

We analyze statistical fluctuations in the pulse-to-pulse spatial fluence distribution of a highly multimode F2 laser beam and the influence that coherence effects have on the attainable smoothness of an ablated surface. The magnitude of fluctuations is predicted to be a few percent, with associated roughness increasing as m1/2, where m is the number of ablation pulses. By use of a white-light optical interferometer, measurements have been made of roughness induced on N-BK7 glass surfaces ablated with a 157-nm laser, and reasonable agreement with predictions based on this roughening mechanism has been found.

Gold-coated copper cone detector as a new standard detector for F_2 laser radiation at 157 nm

A new standard detector for high-accuracy measurements of F2 laser radiation at 157 nm is presented. This gold-coated copper cone detector permits the measurement of average powers up to 2 W with an uncertainty of approximately 1%. To the best of our knowledge, this is the first highly accurate standard detector for F2 laser radiation for this power level. It is fully characterized according to Guide to the Expression of Uncertainty in Measurement of the International Organization for Standardization and is connected to the calibration chain for laser radiation established by the German National Metrology Institute.

Microlenses Fabricated on Silicone Rubber Using F2 Laser

Microlenses are fabricated on silicone rubber surfaces employing phenomena in which silicone rubber swells and is modified to SiO2 by F2 laser irradiation at a laser fluence lower than the ablation threshold. In this method, silicone rubber is irradiated using a F2 laser beam through a mask which has circular apertures 10, 20, and 25 µm in diameter. Since silicone rubber swells by laser irradiation, it is necessary to separate the mask from the silicone rubber surface. The swelling is spherical and its surface becomes smooth when the distance between the mask and the silicone rubber surface is very small. The focal lengths of the microlenses are 10–170 µm, which are controlled by adjusting the number of irradiated pulses. Additionally, a 790 nm femtosecond laser beam is focused by the fabricated microlenses, and enables the microdrilling of fluorinated rubber.

Distributed‐feedback color center lasers in wide‐band gap materials fabricated by a pair of chirped femtosecond pulses

A room-temperature-operated, pulsed distributed-feedback (DFB) LiF:F2 color center laser was achieved. A distributed-feedback structure, based on non-erasable periodic gratings of laser-active color centers with fringe spacings as fine as ∼510 nm, was fabricated by a pair of chirped near-IR femtosecond laser pulses. The DFB LiF:F2 laser outputs at 710 nm were obtained with a linewidth of less than ∼1 nm, the slope efficiency ∼10% and beam divergence ∼20 mrad. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of Periodic SiO2 Nanostructures Using a 157 nm F2 Laser

Periodic nanostructures on SiO2 films were fabricated by Fresnel diffraction of a mask with a square aperture using the photochemical deposition method. In this method, an F2 laser beam at a low laser fluence simultaneously irradiates a silicone rubber target and a substrate through the mask. The SiO2 nanostructures are photochemically grown according to the intensity distribution at room temperature. The fringe period of the nanostructure formed on the substrate corresponded to the calculated results, and can be reduced up to ∼200 nm by decreasing the distance from the mask to the substrate.

Effect of Annealing on the Tolerance of LiCaAlF6 Single Crystals against F2 Laser Irradiation

The optical tolerance of annealed LiCaAlF6 single crystals against F2 laser irradiation was investigated. Annealed LiCaAlF6 (t=1.45 mm) shows an improved transmittance at 157 nm, 94.4% after the compensation of the surface reflection loss. Furthermore, after the initial increase by F2 laser cleaning, transmittance at 157 nm remains almost unchanged even after irradiation with a F2 laser beam up to 105 pulses at a fluence of approximately 160 mJ cm-2 pulse-1.

Initial stage of laser ablation of LiCaAlF6 single crystal under F2 laser irradiation

Initial stage of F2 laser ablation of LiCaAlF6 single crystal was investigated for clarifying the possibility of applying this wide bandgap fluoride crystal to vacuum ultraviolet (VUV) optical components. The ablation threshold, determined by the appearance of line emission from ablated species, was approximately 2 J cm-2pulse-1, similar to that of VUV grade CaF2 single crystal. The laser-induced damage on the front surface of LiCaAlF6 was faint, though adhesion of aggregated particulates of several microns was observed.

SiO2 films fabricated by F2 laser-induced chemical deposition using silicone rubber

Silicon dioxide (SiO2) films were selectively fabricated on a Si substrate at room temperature by illuminating both the silicone rubber target and the substrate with an F2 laser (157 nm). The laser fluence was much less than the ablation threshold. Absorption due to absorbed water (H2O) and hydrogen-bonded silanol (SiOH) groups were observed in addition to absorption due to Si-O-Si stretching mode in the Fourier transform infrared spectroscopy (FT-IR) spectra of the films. The illumination with higher laser fluence caused an increase of Si-O-Si/OH peak ratio in the FT-IR spectra, a decrease of etching rate in hydrofluoric acid (HF) solution, and an increase in the refractive index close to the value of a thermal SiO2 film. These results indicate that the quality of the grown SiO2 films was improved. The high photon energy of F2 laser induced photodissociation of main chains and side chains of silicone and oxygen (O2), and bonds between the ejected gaseous molecules including Si and O(1D) to form SiO2 films.

Swelling and modification of silicone surface by F2 laser irradiation

The surface of silicone rubber swelled and was modified by 157-nm F2 laser irradiation at a laser fluence less than the ablation threshold. The irradiated surface swelled to a height of approximately 3 μm. Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy showed that the irradiated surface was modified to SiO2. 193-nm ArF laser irradiation of the silicone rubber induced the surface to swell, but not to modify to SiO2. The IR peaks of end groups of silicone were observed in the FT-IR spectra of the surface. From these results, it is concluded that the main chains (Si-O) of silicone were photodissociated and generation of low molecular weight silicones caused the swelling. In addition, it was observed that methane and carbon dioxide were released from silicone rubber when each laser beam irradiated it. These gases were generated by photodissociation of the side chains (Si-CH3) of silicone. An F2 laser beam can photodissociate the Si-CH3 bonds and the Si-O bonds of silicone and O2 effectively even at a laser fluence less than the ablation threshold, resulting in the modification to SiO2 and the swelling.

Polymer jacket stripping of optical fibres by laser irradiation

Laser jacket stripping of the two-layer polymer jacket coating of Corning SMF-28 silica fibres has been studied as an alternative approach to chemical and mechanical techniques. These polymer outer layers, although chemically similar, become discernable through laser ablation and depth per pulse experiments. Etch rate measurements using nanosecond UV excimer laser sources (F2, ArF, KrF and XeCl lasers) reveal that, as expected, the threshold fluence (energy per unit area) for significant material removal drops as the laser wavelength becomes shorter. For some wavelengths and fluences, spontaneous cone formation has been observed, thus providing additional threshold data through apex angle determination. The possible occurrence of deleterious damage to the silica cladding has been assessed using electron microscopy and optical transmission measurements. A wide fluence range over which damage was not observed characterised all the interactions. Irradiation techniques for producing apertures in the polymer coating or complete jacket removal are demonstrated and discussed. Briefly, polymer jacket stripping using a femtosecond laser source (800 nm) has been demonstrated.

VUV laser ablation of insulators

We present theoretical and experimental investigations of factors that may influence the surface quality of insulators patterned by 157 nm F2 laser ablation. Aspects covered include modelling of beam spatial coherence and diffraction effects for proximity mask exposure, and the role of melt relaxation on surface microstructures. Experimental results for micron-scale patterning of N-BK7 glass and NaCl crystals using proximity masks are presented and discussed within the framework of the modelling.

Laser-induced crystallization of calcium phosphate coatings on polyethylene (PE)

Calcium phosphate (CaP) coatings are used for obtaining a desired biological response. Usually, CaP coatings on metallic substrates are crystallized by annealing at temperatures of at least 400–600°C. For polymeric substrates, this annealing is not possible due to the low melting temperatures. In this work, we present a more suitable method for obtaining crystalline coatings on polymeric substrates, namely laser crystallization. We were successful in obtaining hydroxyapatite coatings on polyethylene. Because of the UV transmission characteristics of the CaP coatings, the use of a low wavelength (157nm) F2 laser was necessary for this. As a result of the laser treatment, the CaP coating broke up into islands. The cracks between the islands became larger and the surface became porous with increasing laser energy. The mechanism behind the formation of this morphology did not become clear. However, the fact that crystalline CaP coatings can be obtained on polymeric substrates in an easy way, possibly allows for the development of new products.

Line Narrowing and Injection Locking of F2 Lasers

To realize an ultranarrow-linewidth and high-output-power F2 laser for next-generation lithography, line narrowing in a F2 laser injection locking system (ILS) that consists of a line-narrowed oscillator and an amplifier with a resonator was investigated. We found out that the time-resolved instantaneous spectrum of the oscillator laser pulse narrows from a start time of discharge and scarcely depends on laser pulse shape. Furthermore, the spectral shape of the ILS is locked to the instantaneous spectral shape of the line-narrowed injection laser when the amplifier discharge starts. We first realized a F2 laser with a linewidth (FWHM) below 0.2 pm and a pulse energy of more than 5 mJ (25 W at 5 kHz repetition rate).

Derivatization of surface-bound peptides for mass spectrometric detection via threshold single photon ionization.

Chemical derivatization of peptides allows efficient F2 laser single photon ionization (SPI) of Fmoc-derivatized peptides covalently bound to surfaces. Laser desorption photoionization mass spectrometry using 337-nm pulses for desorption and 157.6-nm pulses for threshold SPI forms large ions identified as common peptide fragments bound to either Fmoc or the surface linker. Electronic structure calculations indicate the Fmoc label is behaving as an ionization tag for the entire peptide, lowering the ionization potential of the complex below the 7.87-eV photon energy. This method should allow detection of many molecular species covalently or electrostatically bound to surfaces.

Photochemical Modification of Silicone Films Using F2 Laser for Selective Chemical Etching

Silicone ([SiO(CH3)2]n) films were photochemically modified into SiO2 by irradiation with a 157 nm F2 laser. The dissociation of Si–CH3 bonds of silicone simply depended on the photon number of the F2 laser. The quantum yield of the modification was estimated to be approximately 1×10-2. Mechanisms of oxidation and O–H production in the modified films were clarified. The depth of the modification was not limited by the absorption coefficient of silicone because SiO2 of the modified films is transparent for the F2 laser. A fine pattern of the 1.5-µm-thick silicone films could be fabricated by immersing the modified samples in 0.2 wt% hydrogen fluoride solution.

Vacuum-ultraviolet beam array generation by flat micro-optical structures.

Micro-optical structures for VUV laser beam shaping and wave-front sensing were manufactured by thin-film deposition onto CaF2 and transfer by etching. Arrays of Bessel-like F2 laser beams at a wavelength of 157 nm with extremely small conical angles were generated by microaxicon lenses. Beam propagation was studied in simulations and experiments. Apodization by absorbing layers is proposed for beam cleaning.

Correlation between oxygen-deficient center formation and volume compaction in synthetic SiO2 glass upon ArF or F2 excimer-laser irradiation.

Correlations between the refractive-index change and the concentration of an oxygen-deficient center (ODC) induced by thermal treatments and laser irradiation are examined to clarify the origin of laser-induced volume compaction in synthetic SiO2 glasses. A linear correlation between them was clearly observed for thermally induced ODC, whereas no correlation was found for ArF or F2 laser irradiation. The results demonstrate that the dominant origin of laser-induced compaction is not ODC formation. Furthermore, we found that the presence of H2 in SiO2 glass had no influence on volume compaction but enhanced crack formation upon laser irradiation, a phenomenon most likely due to stress corrosion.