Micro-modification in Borosilicate Glass Using Femtosecond Laser

Advances in Optoelectronic Materials (AOM) Volume 3, 2015

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doi: 10.14355/aom.2015.03.001

Micro-modification in Borosilicate Glass Using Femtosecond Laser Sunita Kedia1*, M. N. Deo2, Sucharita Sinha1 Laser and Plasma Technology Division, 2 High Pressure & Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India 1

*

skedia@barc.gov.in; ssinha@barc.gov.in

Abstract Direct laser writing technique has gained popularity on account of its application potential for achieving micro-modification inside various glasses, crystals and transparent media. Such modification is an important requirement for efficient guidance of waves for modern communication technology and integrated optical devices. When a focused ultrashort laser beam gets absorbed in a transparent material via nonlinear absorption, it will lead to permanent modification in the optical properties of the solid within the focused volume. This phenomenon has been used to modify the optical property of borosilicate glass using 45 femtosecond pulse laser beam at 3 kHz repetition rate. Refractive index of the material changed along the beam path enabling writing of an optical waveguide. Structural modification in the laser treated area is confirmed on the basis of the results obtained from Raman and Fourier Transform Infrared spectra. Written waveguide structure supported guidance of HeNe laser beam at 633 nm and its transmitted profile were used to calculate the change in refractive index of the laser exposed area. An increased refractive index by 0.5 x 10-4 in the laser treated region was measured compared with surrounding glass.

Keywords Femto-Second Laser Writing; Micro-Modification; Waveguides; Laser Material Processing; Borosilicate Glass

Introduction Femtosecond laser writing is a well-known technique for 3D micro-machining within bulk of transparent optical materials, such as glasses [1], crystals [2] and photopolymers [3]. For most materials, the electron-phonon coupling time lies between a picosecond to a nanosecond with typical heat-diffusion times ranging from a nanosecond to a microsecond. At high peak intensities achievable with femtosecond laser pulses interaction with material can occur via multiphoton nonlinear interactions instead of conventional linear absorption. Therefore, with ultrashort laser pulses delivering high peak power when tightly focused inside the solid, bound and free electrons acquire energy from the incident pulse by multi-photon absorption. The absorbed energy gets coupled to sample lattice resulting in bond breaking and material expansion, thereby leading to a permanent modification of the optical properties in the laser treated zone [4]. Self-focusing arising from the third order nonlinearity is an additional effect that can be exploited to induce long, narrow three-dimensional modification traces in the bulk of wide band-gap materials using femtosecond lasers. Ultrashort laser-matter interactions are characterized by negligible heat-affected zones and possibility of achieving sub-wavelength structure sizes in the bulk of a solid. With femtosecond short pulses not only is energy deposition efficient, rapid and localized but deformation and ablation thresholds are also welldefined. This non-thermal nature of interaction and efficient nonlinear absorption of these laser pulses in the medium minimizes damage of surrounding mass make ultrashort laser systems a unique tool for high precision material processing that allows machining of geometries and shapes not possible using conventional methods.

Structural modification in the focal volume of the laser treated area can either be in the form of air-channels [5] or a permanent change in the refractive index of the material [6]. Both these changes, mainly depend upon the amount of laser energy accumulated in the focal volume inside the sample. Parameters, which affect the quality of modified area, include laser fluence, laser wavelength, pulse duration, pulse repetition rate, laser polarization, and sample scan rate [7]. This modification in the refractive index can be shaped into waveguides [8], gratings [9], couplers [10], or micro-photonic devices [11,12].

Optical waveguides have been fabricated by several techniques, such as electron beam lithography, UV direct writing, and epitaxy [13, 14-15]. A significant advantage associated with the femtosecond laser writing process

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over other techniques is the possibility of micro-modification within the bulk of transparent solid without affecting its surrounding surface. This single-step writing technique is relatively easier, faster and less-expensive because the sample does not require any pre/post-treatment or mask fabrication as is necessary in other techniques [15]. Also, this technique can be used on large variety of samples ranging from glasses to photopolymers and modification can be done over an unrestricted length within the sample. Other than femtosecond lasers, micro-machining has also been tried with nanosecond [16] and picoseconds [17] laser pulses. However, femtosecond lasers yield better results and with higher reproducibility.

In this work, micro-modification has been performed within Borosilicate glass (BK-7) samples leading to formation of waveguides. BK7 (Na2O–B2O3–SiO2) is a widely used silicate glass whose main compositions are silica, boron oxide, and sodium oxide. Other than these, potassium oxide, titanium oxide, calcium oxide and impurities are often present in small percentage. With this composition, BK7 has lower softening temperature and higher expansion coefficient than fused silica [7]. Dimension of waveguide being written can be controlled more precisely in BK7 because it exhibits controlled growth within the laser treated zone in comparison to fused silica [18].

A sample slab of BK7 was directly exposed to femtosecond pulses and at pulse energy ~33 µ J optical waveguides were achieved. The morphology of the modified region was observed using an optical microscope (OM). The associated modification of the laser treated region was confimed by comparing Fourier Transform Raman (FTRaman) and Fourier Transform Infrared (FTIR) spectra of laser treated and un-treated regions of the sample. A shift in Raman line and a significant change in the IR absorption was observed in the waveguide region compared to bulk of BK7. Fabricated waveguide supported the guidance of He-Ne laser at 633 nm. The change in refractive index of the material was estimated from the He-Ne laser beam profile transmitted through the waveguide [6, 19].

Experimental A Ti: Sapphire pulsed laser which emits at 800 nm, delivering energy 0.8 mJ with pulse duration 45 fs and repetition rate 3 kHz is employed to write waveguides. The experimental set-up is illustrated in Fig. 1a. The laser beam is focused on the sample using a 5 cm focal length lens. The sample is translated in transverse direction in a plane perpendicular to the direction of incident beam. This provides freedom of writing long structures in the samples. Pulse energy at the sample varied between 20 µ J to 70 µ J using suitable neutral density filters. The scanning speed varied from 10 µ m/s to 100 µ m/s using a motorized translational stage.

NDF Ti : Sapphire Laser

M

MO L

A

S

D

Laser

Power Meter

S (b)

(a) XYZ-Stage

FIG. 1 (A) EXPERIMENTAL SET-UP USED TO WRITE WAVEGUIDES IN BK7 GLASS, NDF IS NEUTRAL DENSITY FILTER, M IS MIRROR, L IS 5 CM FOCAL LENGTH LENS, AND S IS SAMPLE, AND (B) SET-UP USED TO DETERMINE THE NUMERICAL APERTURE AND REFRACTIVE INDEX OF THE WAVEGUIDE, MO IS 10X MICROSCOPE OBJECTIVE, S IS SAMPLE, A IS APERTURE, AND D IS PHOTODETECTOR

Subsequent to laser treatment the modified regions were observed under an inverted optical microscope (Leica, CMS GmbH). For FT-Raman spectra (Bruker’s ‘Multiram’) the sample was illuminated at 1064 nm using a Nd:YAG laser. The Raman lines were obtained by collecting backscattered light in the spectral range 300-1500 cm-1 from the sample using a Germanium detector and an edge filter. For FTIR, ‘Bruker IFS 125HR FTS’ system equipped with

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Globar IR source and liquid-N2 cooled HgCdTe (MCT) detector was employed. Absolute IR-spectrum were obtained in Attenuated Total Reflection (ATR) mode using Zinc Selenide (ZnSe) accessory.

To measure guidance ability of generated waveguide, the sample was mounted on a xyz-translational stage with suitable θ – φ tilt facility and He-Ne laser beam at 633 nm was coupled into its end face using 10X objective having a numerical aperture (NA) 0.25, as shown in Fig. 1b. Guided intensity profile was obtained by scanning an aperture-photodetector combination along the diameter of the output beam.

Results and Discussion The writing parameters were optimized and good quality waveguides were obtained at pulse energy of 33 µ J and scan speed 40 µ m/s. The OM images of the waveguides are shown in Fig 2, in which laser exposed region of BK7 is clearly distiguisiable from unexposed part of the sample. The laser induced modification was found to be homogenous along the length and breadth of the waveguide, can be seen in Fig. 2a. Fig. 2b shows one end of the waveguide, light was coupled to the waveguide after polishing this end. The modification was done over a long length of about 10 mm. In Fig. 2c, modifcation in a larger legth scale is shown.

FIG. 2 OM IMAGE OF WAVEGUIDE WRITTEN AT 33 µ J PULSE ENERGY AND 40 µ M/S SCAN SPEED WITH SCALE BAR (a) 20 µ m, (b) 20 µ m (SHOWING ON OF THE END FACE OF WAVEGUIDE) AND (c) 100 µ m.

FIG. 3 DOTTED LINE CORRESPONDS TO UNTREATED BK7 SAMPLE AND SOLID LINE REPRESENTS RESULT OBTAINED FROM WAVEGUIDE WRITTEN AT 33.3 µ J PULSE ENERGY AND 40 µ m/s SCAN SPEED (a) RAMAN SPECTRA, INSET SHOWS GRAPH IN 1200 cm-1 TO 1800 cm-1 RANGE, (b) DIFFERENTIAL RAMAN SPECTRA, INSET IS MAGNIFIED IMAGE, AND (c) IR-ABSORPTION SPECTRA, INSET SHOWS GRAPH FROM 1100 cm-1 TO 1700 cm-1.

The dotted line in Fig. 3a is the Raman spectrum of BK7 glass. The spectrum has sharp peaks centred at 513 cm-1, 628 cm-1, and 1078 cm-1 along with a broad band centred at 1480 cm-1. With SiO2 (70%) being the main constituent of BK7, Raman lines at 513 cm-1 result because of bending of Si-O-Si linkage in the sample. Peak at 628 cm-1

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corresponds to the breathing mode of boroslicate rings. Raman lines centred at 1078 cm-1 were due to stretching of Si-O tetrahedron bonds (Q3 unit) with different ratios of bridge and non-bridge oxygen atoms per silicon. The band at higher frequency region at 1480 cm-1 was associated with stretching vibrations of B-O unit attached to metaborate groups [20]. The Raman intensity of the waveguide (solid line in Fig 3a) decreased in comparison to intensity of untreated BK7 in the spectral region ranging from 300 cm-1 to 1242 cm-1. At 1242 cm-1, Raman intensity obtained from laser treated region exceeded intensity from un-treated BK7, as can be seen in the inset of the figure.

The increment was about 100 % at 1480 cm-1. This confirms modification in the optical property such as refractive index of BK7 when treated with ultra short laser pulses. This result is similar to the result reported by Little et al in Ref-21, where such increment in Raman intensity was considered as a change in refractive index of the material by (5.0 ± 0.2) x 10-3. The intensity of the high frequency band at 1480 cm-1 was enhanced when bridging oxygen atoms in the glass are converted into non-bridging atoms by bond breaking. This results in the formation of more ionic bonds in glass network and energy band-gap of laser treated part is expected to reduce. This has been suggested as the main mechanism responsible for refractive index modification in BK7 sample [21]. Differential Raman spectra for un-treated BK7 and laser treated BK7 are shown in Fig 3b using dotted and solid lines respectively. Differential Raman intensity for laser induced waveguide is found to be raised at 565 cm-1, 648 cm-1 and 1132 cm-1 in comparison to differential Raman intensity of pristine BK7. This indicates disordering of lattice structure of BK7 glass after laser exposure [20]. A clear Raman shift for waveguide part of BK7 can be observe in the inset of figure (marked with arrow). The FTIR spectra of the un-exposed BK7 and waveguide region are shown with dotted and solid lines in Fig 3c, respectively. A significant reduction of IR-absorbance for laser treated region was observed in the spectral range from 630 cm-1 to 1270 cm-1. However, IR absorbance for laser treated region i.e. waveguide, showed an increase beyond 1270 cm-1 (inset in the figure). This behaviour is similar to the behaviour as seen in Raman spectrum for waveguide and this confirms the modification in the optical properties of sample. A shift in IR-absorption spectra by a few wave numbers from 764 cm-1, 847 cm-1 and 1066 cm-1 (for BK7) to 767 cm-1, 852 cm-1, and 1078 cm-1 (for waveguide) respectively were observed. Since the expected change in refractive index is small, a shift of only 2 to 3 wave numbers as observed appears to be reasonable in this case.

FIG. 4 (A) PHOTOGRAPH SHOWS LASER BEAM GUIDING THROUGH WAVEGUIDE WRITTEN AT 33.3 µ J PULSE ENERGY AND 40 µ m/s SCAN SPEED, (B) INTENSITY PROFILE OF THE GUIDED BEAM (CIRCLES) FITTED WITH GAUSSIAN PROFILE (SOLID LINE), INSET IS THE PHOTOGRAPH OF THE BEAM EMERGING FROM THE WAVEGUIDE.

He-Ne laser beam at 633 nm was successfully launched into the polished face of the waveguide using a 10X microscope objective. Propagation of laser beam through waveguide can be seen in figure 4a. To calculate refractive index change of the modified part in BK-7 (core of the waveguide), the guided beam was projected onto a screen placed at a distance D (~ 25 cm) from end face of the waveguide. Spatial profile of the beam emerging from the waveguide was recorded using a photodiode-pinhole combination. Intensity profile of the output guided beam is plotted in figure 6b (circles). The intensity profile is well fitted with a Gaussian profile, solid line in the

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figure. A photograph of the output beam on the screen is shown as an inset of figure 4b. The NA of the core was estimated by analyzing light transmitted through waveguide in far field. (

)

(1)

Here, r is half of the distance between radial positions within the Gaussian profile where intensity falls to 5% of the peak value (figure 4b). The NA was found as 0.012. The refractive index of the core (Ncore) of the waveguide was calculated using the following equation √

(2)

and found to have increased by 5 x 10-3 in comparison to the refractive index of the cladding (Nclad = 1.51) i.e, the bulk of surrounding BK-7. Value of refractive index obtained in present case is same as the value estimated from the increase in the Raman intenisty of the waveguide region by Little et al [21].

Conclusion In conclusion, 3-dimensional waveguide structure was fabricated in the bulk of BK-7 glass by direct femtosecond laser writing method. Two parameters, laser pulse energy and sample scan speed varied during the experiment and both parameters were observed to play a significant role in achieving good quality structures. Smooth and continuous photo-induced waveguides were formed in the bulk of BK-7 with appropriate combination of writing parameters. A significant change in the Raman intensity and IR-absorbance were observed in the waveguide region in comparison to bulk of BK7. Bond breaking and presence of non-bridging oxygen atoms in the laser treated area appeared to be the main mechanism for refractive index modification. He-Ne laser beam was sucessfully guided through the fabricated waveguide. NA for the waveguide was estimated as 0.012 and the refractive index of the core was found to have increased by 5 x 10-3 in comparison to the cladding of the waveguide.

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