Modification of Glass by FS Laser for Optical/Memory Applications Kazuyuki Hirao and Kiyotaka Miura Department of Material Chemistry Kyoto University International Workshop on Scientific Challenges of New Functionalities in Glass
Review of the interaction effect between glass and laser Characteristics of the femtosecond laser in glass processing Mechanism of the refractive index change Application of photo-induced refractive index changes Other structural changes Precipitation of gold nano-particles Valence state change of samarium ions Writing of nano-grating Growth of non-linear optical crystal Precipitation and growth of silicon in the glass
Resonance wavelength (e.g., UV laser or light) Nonresonance wavelength (e.g., CW, nanosecond laser or visible light) Nonresonance wavelength (femtosecond laser) high peak power Reaction area Non-reaction Reaction area Glass The surface Non-reaction (one-photon process) The interior When a transparent material like glass is irradiated by a tightly focused femtosecond laser, photo-induced reactions should occur only near the focused part of the laser beam. Femtosecond laser can be used to modify transparent materials like glass microscopically and three-dimensionally.
alkali silicate glass (A) (B) (C) 10μm 100μm 100μm 0.5 μj, f 300, 1 khz 1 μj, x 50, single pulse 3 μj, x 50, 200 khz Low Energy density High (A) Coloring line due to defect formation (B) Refractive index changes due to local densification by a single pulse (C) Melting due to heat accumulation The structural changes induced by a femtosecond laser take various forms depending on the condition of laser irradiation.
Pump Beam (775 nm; 500 fs) Probe beam (388 nm) Harmonic Separator Laser of the second harmonic from part of the pump laser Transient lens method 20X Objective Al: Silicate glass f=30 mm Blue filter The change in the spatial pattern of the probe beam was monitored to obtain the refractive-index distribution. We found that a shock wave forms at the focal point inside the glass of the beam created by a femtosecond laser and propagates to the surrounding area. Iris Prism CCD Camera Optical setup used to observe the shock wave
Δφ (r) phase change 8 6 4 2 0 0 2 The center of the focusing laser beam 4 6 r/μm 8 10 12 5000 ps 2000 ps 1600 ps 1400 ps 1200 ps 1000 ps 800 ps 600 ps 400 ps 300 ps 200 ps 100 ps 1 ps Fig. The phase distribution obtained by a phase retrieval calculation of the spatial pattern. 14 Density The density at the center decreases and the highdensity area due to pressure-wave generation rapidly expands into the surrounding area. fs laser Shock wave The positive peak due to the pressure wave propagates outward with a constant velocity. ~6.2 μm/ns The temperature and pressure of this region rises very rapidly and dramatically. >3000 K, ~1 GPs
One pulse The irradiation of a single pulse Temperature increase and thermo-elastic stress should be induced in a very limited volume. The relaxation of the thermo-elastic stress produces the force driving the shock wave. The shock wave propagates outward. The structure of the glass is extended outside. After pulse irradiation Compressive stress moves back toward the center. A graduated high-density region is formed in the laser-focusing area. After the thermal diffusion from the irradiated region, the structural change becomes a permanent refractive-index change. If laser pulses are repeatedly irradiated, the high-temperature area rapidly expands, and melting regions can be formed at arbitrary sites within the glass.
Array of refractive index changes inside silica glass (a) (b) 250 khz Laser Beam Microscope Objective Waveguide (a) (b) Refractive index change By using a femtosecond laser with a high repetition rate, refractive index changes are continuously induced along a path traversed by the focal point. We can write waveguides in glass. 20μm 20μm Fig. Photo-written waveguides were written by translating the sample (a) parallel or (b) perpendicular to the axis of the laser beam Similar waveguides can be written inside various types of glass, such as silica, borosilicate, fluoride and chalcogenide glass.
Refractive Index 1.485 1.480 1.475 1.470 1.465 1.460 1.455-10 -5 0 5 10 Radial Distance (μm) Objective power (NA) (mw) Ⅰ 0.25 425 Ⅱ 0.13 425 Ⅲ 0.13 350 Common Writing Condition Wavelength : 800 nm Repetition rate : 250 khz Pulse width : 270 fs Scanning rate : 50 μm/s Fig. The refractive index profiles of the core of waveguides written in silica glass at various numerical apertures and average powers. The characteristics of a writing waveguide can be controlled by adjusting the writing conditions Table Guide mode and mode field diameter calculated from the refractive-index profile I II III Refractive index reference[%] 1.32 0.53 0.27 Change diameter[μm] 20 18 11 MFD@1.3 μm 6.0 9.0 12.0 MFD@1.55 μm 6.7 10.3 13.5 Calculated guide mode@1.3μm LP01, LP02, LP11 LP01, LP11 LP01 Calculated guide mode@1.55μm LP01, LP11 LP01, LP11 LP01
2.5 x 3.75 x 1 mm Characterization This device has matrix-arrayed multichannel optical I/Os. The optical path are redirected by 90 degrees. This structure and size enable optical interconnects to be integrated in a board-to-board or board-tobackplane communication structure more densely. Optical circui LD or PD PCB LSI Optical-path redirected waveguide It's not easy to fabricate an optical-path redirected waveguide using the conventional method, so this is an example where the laser writing technique has a clear advantage.
Grating & Binary lens (microscopic view) 1024 μm Δ=46.5 μm CCD f=9 mm Beam profile at the focal plane 254 μm 5 mm Beam diameter = 12-13 μm 10 mm 5 mm λ=633 nm Split beam power ~ 27 % of input beam
Laser intensity: (a) < (b) < (c) TEM (a) (c) Absorbance [a.u.] (c) (a) (b) Blue shift 300 400 500 600 700 800 Wavelength [nm] (b) 600 550 for 1 hr 550 Peak position [nm] 500 450 400 100 nm Particle size: Small 10 14 10 15 10 16 Laser intensity [W/cm 2 ]
Applications may also include developments in techniques such as surface plasmon resonance and near-field scanning optical microscopy.
LASER BEAM Wavelength: 800 nm Pulse width: 50 fs Repetition rate: 250 khz Average power: 100 mw Ovjective: 50x, NA=0.6 Emission intensity (a.u.) Irradiated area Sm 3+ : 4 G5/2-6 HJ J=5/2 7/2 9/2 5 D 0 7 F 0 Sm 2+ 5 D 0 7 F 1 5 D 0 7 F 2 Photo-reduction Sm 3+ Sm 2+ GLASS SmF3 : AlF3-YF3-BaF2-SrF2-CaF2-MgF2 500 550 600 650 700 750 800 Wavelength (nm) Fig. Photoluminescence spectra for glass excited at 488 nm : red line shows spectrum for laserirradiated areas; yellow line shows spectrum for non-irradiated area. Laser-irradiated areas (photo-reduced areas) recorded inside glass can be detected only by emissions at 680, 700 or 725 nm.
(a) Ⅰ Ⅱ 200 nm Photoluminescence image before the erasure. Photoluminescence images of alphabetic characters recorded by using the photoreduction of samarium ion on different layers. Alphabetical character comprised 300 to 500 photo-reduction bits. The bits had a diameter of 200 nm. The spacing between each characters was 1 μm. Recording : Sm 3+ Sm 2+ Femtosecond laser 100 nj/pulse at 800nm Readout : Ar + laser 1 mw at 488 nm Erasure : Sm 2+ Sm 3+ Ar + laser 10 mw at 488 nm (b) (c) Ⅰ Ⅱ Sm 2+ Sm 3+ Image after an Ar+ laser irradiation to photoreduction bit I and II Image after a femtosecond laser irradiation to areas I and II. Three-dimensional optical memory with rewrite capability is achievable.
SiO 2 glass longitudinal plasma wave E Polish Hot plasma 200 khz, 150 fs, 600 mw Laser Polarization BEI Oxygen defect SiO 2-x (x ~ 0.4) Δn ~ -0.1 0.1 The refractive index of nano lines is lower than that of the circumference, and periodic nano lines function as a grating.
Laser Beam 1 sec. 30 min. SEM g 30 μm Melting region BaO-Al 2 O 3 -B 2 O 3 glass LASER Wavelength: 80 0nm Pulse width: 50 fs Repetition rate: 250 khz Average power: 400 mw Objective: 100x, NA=0.9 Intensity X 線強度 ( (a.u.) 任意 ) Ba Ba Al Al 1 1 sec. ps 0 20 40 60 80 100 Scanning 走査距離 distance (μm) (μm) 0 20 40 60 80 100 Scanning 走査距離 distance (μm) (μm) Elemental distributions of Ba and Al at around the focal point of the laser. Intensity (a.u.) X 線強度 ( 任意 ) Ba Ba Al Al 40 min 30 min. The concentration of Al increased at the center of the irradiated area, and Ba increased outside the center with the passage of time. Measurement of the x-ray diffraction patterns confirmed that only beta-bbo crystals grew in the focused area.
Metallic clusters or particles can be precipitated. Specific ion can be reduced or oxidized. Oxygen-deficiency centers ( Si-Si, etc. ) can be formed. Specific ions can be diffused at the micrometer order. Silicon cluster or particle can be extracted from silica or silicate glass. General oxide glasses can not trap superfluous oxygen ions generated by cutting the Si-O bond and growth of the silicon in a closed space like the glass inside it. We tried to deposit silicon from silicate glass prepared using metallic aluminum as the starting material.
Silicon as a photonic medium has unique advantages for photonic integrated circuits Past Present Future ~10cm ~ 10 cm Refractive indices of core and cladding layers are approximately 1.50 and 1.45, respectively. ~10cm ~ 1 cm Refractive indices of silicon and conventional oxide glass are approximately 3.5 and 1.5, respectively. ~1mm ~ 1 mm Three-dimensional integration of different optical components in the glass In order to achieve compact and integrated devices, high refractive index difference is required. The high refractive index contrast between the silicon waveguide core and a surrounding glass cladding enables the fabrication of small optical structures. If silicon can be deposited in the glass at three dimensions, this leads to further compactness and higher integration of optical devices.
Si-Na Al -O Si - Ca Al - O Si-Ca-Na Al -O Si-Ca-B Al -O Metallic aluminum : ~20 mol% in air Melting Temp. : 1400-1500 Crucible : Al 2 O 3 1 Absorbance 0.8 0.6 0.4 SiO 2 - CaCO 3 - Al SiO 2 - CaCO 3 -Al 2 O 3 0.2 0 500 1000 1500 Wavelength (nm)
Oxygen deficiency defect : O 3 Si-Si O 3 Emission intensity(arb.units) PL(ex:256 nm) PLE(em:310 nm) PLE(em:480 nm) 200 300 400 500 600 700 800 Wavelength(nm) SiO 2 CaCO 3 (CaO) Al ( 20 mol%)
LASER Pulse width: 130 fs Pulse energy: 3 μj Repetition rate: 200kHz Objective: 50x, NA=0.85 Irradiation Time: 5 sec. Si - Ca Al O glass Al Sample glass Polish Irradiation of fs laser Si 20 μm Microscope Ca O 20 10 μm μm Backscattered electron EPMA
Al Si Ca O Si
Al: Silicate glass fs laser Si O Al Heat treatment Si particle Thermite reaction Oxygen deficiency centers Si-Si etc. + Aluminum rich structures Silicon rich structure Si-Si + Si-Si-Si : more Silicon cluster Silicon rich structure grows into a particle because of the thermite reaction promoted by the heat treatment Glass prepared using metallic aluminum as the source material has oxygen deficiency centers Si-O bonds are continuously broken and re-formed with numerous unbound atoms Si and Al-rich structures are formed at the center of the irradiated area. Si rich structure grows into a particle because of the thermite-like reaction.