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A microfabrication process using ultrafast laser pulses in glass was investigated. We investigated the formation of semiconductors by the irradiation of glasses with femtosecond laser pulses. ZnS- or PbS-doped SiO2-Al2O3-B2O3-CaO-ZnO-Na2O-K2O glasses were prepared by a melting method and irradiated by femtosecond laser pulses. Periodic structures in the sample glasses with a high refractive index difference were produced by femtosecond laser pulses. The maximum relative refractive index difference between the irradiated area and the nonirradiated areas was 20%. Diffraction gratings were also fabricated inside the ZnS- or PbS-doped silicate glasses. The diffraction efficiency of these gratings was approximately 90% in the infrared region.
©2004 Optical Society of America
1. Introduction
Recently, microprocessing by irradiation of transparent materials with ultrashort laser pulses has been attracting interest [1–3]. One advantage of focused ultrashort-pulsed laser microprocessing is its very large electric-field intensity, which is generally in the range of several GW/cm2. Ultrashort laser pulses can also induce nonlinear optical effects, so they can be used with transparent materials in very small areas of the order of several microns in size.
Microprocessing by femtosecond laser pulses irradiation of glass has been investigated extensively. Various optical devices, such as optical waveguides [1], three-dimensional optical memory [4], and micro-diffraction optical elements [5] have been studied by using the femtosecond laser pulses. Hirao et al. reported that three-dimensional microprocessing is possible by using femtosecond laser pulses in glasses [1]. Although, glasses processed by using femtosecond laser pulses have mainly been applied as optical waveguides, there have been a few reports of integration of optical devices. The most important factor for achieving a three-dimensional optical circuit is the ability to create very high refractive-index differences in glass.
Previously, the thermal effects in microprocessing by irradiation with the femtosecond laser pulses have largely been ignored. We found that an intriguing phase separation occurred on local heating as a result of irradiation of a multi-component silicate glass with femtosecond laser pulses. Furthermore, we demonstrated a thermal process involving rapidly repeating femtosecond laser pulses and reported the precipitation of silver particles without heat treatment as a result of local heating by rapidly repeating femtosecond laser pulses [6,7].
Consequently, we attempted to form ZnS or PbS particles in glass by means of a thermal process involving femtosecond laser pulses. Semiconductors have higher refractive indexes than do oxide glasses. Additionally, it is possible to create optical control devices, such as optical switches, by using the highly nonlinear optical properties of semiconductors.
In this report, we describe the fabrication of periodic structures with a high refractive-index difference by using the local heating effect of femtosecond laser pulse in ZnS- or PbS-doped silicate glasses.
2. Experimental
ZnS- and PbS-doped silicate glasses were used in this study. Samples were prepared from reagent-grade SiO2, Al2O3, B2O3, CaCO3, ZnO, Na2CO3, K2CO3, and ZnS or PbS powder. A mixture of the raw materials was melted in an alumina crucible at 1350 ▫for 2 h. The melt was poured onto a carbon plate and annealed at 450 ▫ for 60 min. The resulting glass was cut and polished into a plate 1 mm thick.
The glass samples were irradiated with femtosecond laser pulses. We used a regenerative amplified Ti-sapphire laser. The pulse duration was adjusted to 200 fs by tuning a pulse compressor. The repetition rate was 250 kHz. The average maximum power was 750 mW. A 6-mm-diameter laser beam was focused into a spot a few microns diameter by using a 40x objective [Numerical Aperture (NA)=0.55]. The beam was focused at a depth of 150 µm below the sample’s surface. The average power on the sample was adjusted in the range 100–500 mW by means of an attenuator that was inserted before the objective. By using an air-bearing XY stage, the sample was translated within an area of 1.5×1.5 mm at a speed of 1000 µm/sec. After irradiation, the glass samples were examined with a microscope. The optical absorption of the sample was measured by a conventional spectrometer (U-4100, HITACHI, Japan). The refractive index of the irradiated area was measured at 675 nm by beam-profile reflectometry (BPR; Opti Probe-2000, TORAY Research Center, Japan).
3. Results and discussion
Figure 1 shows photographs of the glass samples after they were treated by laser irradiation. The irradiated area of the ZnS-doped glass changed from colorless to brown. The irradiated area of the PbS-doped glass also changed from colorless to mid-brown. Additive-free and the PbO-doped glasses showed no color change in the irradiated areas; this confirmed that the color change was a consequence of the formation of ZnS or PbS particles.
Fig. 1. Photographs of the samples after irradiation.(a) ZnS-doped glass (b) PbS-doped glass (c) additive-free (d) PbO-doped glass
Figure 2 shows microscopic images of plane views of the laser-irradiated area, which is the color change area in Fig. 1, in the ZnS-doped glass. A clear structure was observed in the transmitted and reflected images, and areas of high reflection were observed in the reflected image. These areas of high reflection formed a periodic structure with a 10-µm period. Areas of high reflection were also observed outside the irradiated area, as shown in Fig. 3. The width of this highly reflective area was approximately 3 µm in a 10-µm period grating. The width of this highly reflective area changed with the pulse energy, the stage scan speed, and the NA of the objective. We infer that this change of width occur by changing of heat effect depended on laser irradiation conditions.
Fig.2 . Microscopic images of the ZnS-doped glass in plane view (a) Transmitted image (b) Reflected image
Figure 4 shows the absorbance spectra of fabricated gratings with periods of 3 µm to 15 µ m in the PbS-doped glass and the absorbance spectra of the parallel and total light that measured using by an integrating sphere.
Fig. 4. The absorbance spectra for the fabricated grating in ZnS- and PbS-doped glass. (a) Absorbance spectra of fabricated gratings that had periods from 3µm to 15µm in PbS-doped glass (b) Absorbance spectra of a collimating light and a diffusion light in ZnS-doped glass (c) Absorbance spectra of a collimating light and a diffusion light in PbS-doped glass
Absorption peaks with a maximum attenuation of 25 dB were observed after laser irradiation. The positions of these peaks were shifted to the long wavelength direction with increasing line period. These peaks were not observed in the absorbance spectra of total light when using the integrating sphere. In PbS-doped glass, an absorption peak of PbS nanoparticles was observed at around 800 nm: the absorption peak in the infrared region was therefore not due to absorption. We concluded that this attenuation was caused by diffraction occurred by the periodic structure with a high refractive index, as shown in Fig.2.
Figure 5 shows refractive index profiles measured by the BPR method at 675 nm in the PbS-doped glass.
The maximum refractive index of the high-reflection area was 1.9, and the refractive index of the nonirradiated area was approximately 1.56, corresponding to a relative refractive index difference of about 20%. Also, the Z-direction lengths of the high refractive-index region changed on increasing the grating period. The peak wavelength of a grating depends on the grating length, and the diffraction efficiency depends on the refractive index. We therefore concluded that the peak wavelengths of diffraction efficiency shifted with changes of the Z-direction length and that the high diffraction efficiency was observed as a result of the creation of a very high refractive-index difference.
Figure 6 shows electron probe microanalysis (EPMA) images of the laser-irradiated area in the PbS-doped glass. Phase separation was observed. In particular, the concentrations of Zn, Si, Al, and O were changed markedly. Silicon, aluminum, and oxygen concentrations were decreased outside the laser-irradiated area. This result agrees with the result of refractive-index measurement. The refractive index outside the laser-irradiated area was increased in the result of the refractive index measurement. Silicon dioxide and alumina have strong bond strengths. It would therefore appear that the bond strength outside the laser-irradiated area was weakened, and ZnS or PbS nanoparticles were prone to form in this area. Therefore, we infer that the refractive index was increased outside the laser-irradiated area.
These results open up the prospect for the formation of the functional materials and functional optical devices in glasses.
4. Conclusions
We succeeded in the fabrication of periodic structures with a high refractive-index difference in ZnS- or PbS-doped glass by using femtosecond laser pulses. The maximum relative refractive index difference between an irradiated area and a nonirradiated area was 20%. The diffraction gratings produced had a high diffraction efficiency of 25 dB in the infrared region.
These results and techniques will be useful for the production of functional optical devices and three-dimensional optical circuits.
Acknowledgments
This work was carried out in the Nanotechnology Glass Project as part of the Nanotechnology Materials Program supported by the New Energy and Industrial Technology Development Organization (NEDO).
References and links
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