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Picosecond (~10 ps) pulsed laser irradiation at 532 nm led to the efficient and scalable fabrication of dichroic areas in glass with spherical silver nanoparticles of ~30 – 40 nm in diameter embedded in a surface layer of thickness ~20 μm. The observed dichroism is due to the uniform and permanent shape transformation of the nanoparticles - from spherical to spheroidal shapes - throughout the irradiated areas and along the laser polarization direction, paving the way for affordable manufacture of polarization-selective diffractive optical elements. The shape modification threshold and the dichroism as a result of Surface Plasmon Resonance band separation were identified. The process was then studied as a function of the laser polarization, repetition rate and the number of pulses fired per spot.
© 2013 Optical Society of America
1. Introduction
Dielectrics containing metal nanoparticles are of interest due to their unique linear and nonlinear optical properties. These properties are dominated by the strong surface plasmon resonances (SPRs) of the metal nanoparticles. The spectral position and shape of the SPRs can be designed within a wide spectral range throughout the visible and near-infrared spectra by choice of the metal and the dielectric matrix [1], plus manipulation of nanoparticle size [2], shape [3], and spatial distribution [4]. Therefore, these nanocomposites comprise a promising class of materials for many applications in optoelectronics [5–8].
The material under consideration here is glass with embedded spherical silver nanoparticles. Recently it was discovered that femtosecond (fs) pulsed laser irradiation at near the SPR of this class of metal-glass nanocomposites leads to the production of dichroic areas [7, 9, 10]. The nanoscopic origin of this effect was identified as laser-induced permanent deformation of the particles from spherical to spheroidal shapes and along the polarization direction of the laser beam. It was identified that trapping of the directionally emitted electrons during the reshaping process takes approximately 10 to 100 ps [11].
Our aim here is to investigate this finding and study the changes in the optical properties of glass containing spherical silver nanoparticles induced by repetitive irradiation with picosecond (~10 ps) laser pulses at 532 nm. The nanoparticle shape modification threshold was identified to be close to that of reported for fs irradiation. The effect was studied as a function of laser polarization, number of applied pulses and laser fluence. A spectral gap of ~240 nm for the SPR band separation was achieved for the laser repetition rate of 200 kHz and after 1000 pulses per spot were fired into the nanocomposite. These effects are of particular interest for technical applications given that ps pulsed laser sources are industrially friendly and that methods of producing pulses in this regime are robust and more affordable as compared with fs laser pulses.
2. Experimental methods
Glass with embedded nanoparticles has traditional been fabricated using ion-exchange techniques [12]. Here, the silver-doped nanocomposite glass samples were prepared from a 1 mm thick soda-lime float glass (comprising in wt.-%: 72.5 SiO2, 14.4 Na2O, 6.1 CaO, 0.7 K2O, 4.0 MgO, 1.5 Al2O3, 0.1 Fe2O3, 0.1 MnO, 0.4 SO3) by Ag+-Na+ ion exchange and subsequent annealing in a H2 reduction atmosphere [13]. This resulted in the formation of spherical silver nanoparticles of 30-40 nm mean diameter in a thin surface layer of ~20-25 μm on both sides of the glass substrate. The extinction spectrum of the original sample and an image of the sample are shown in Figs. 1(a) and 1(b).
Fig. 1 a) Extinction spectrum measured for a soda-lime glass sample containing spherical silver nanoparticles of ~30 – 40 nm in diameter. The Surface Plasmon Resonance band has its maximum at ~430 nm. b) Image of the original sample – Top view. c) Cross section of the sample showing the layer of silver nanoparticles embedded in the glass. The black arrow indicates the sample surface. The characterizations of the sample were performed using a JASCO V-670 UV/VIS/NIR Spectrophotometer and KEYENCE Digital Microscope VHX-1000.
The original sample shows a strong and broad SPR band at around 430 nm corresponding to the embedded spherical silver nanoparticles of ~30 to 40 nm in diameter with high volume fill factor. The nanoparticle-containing layers were formed some 30 nm beneath the surface of the glass. Single-sided samples were used in our experiments, made by removing the nanoparticle containing layer from one side of the samples by etching in 12% HF acid.
In order to visualize the depth profile of the silver particle-containing layer, one of the samples was cut and a thin slice was prepared – Fig. 1(c). For this, we embedded the sample in an epoxy resin (Specifix-20, Struers Limited) to prevent chipping of the glass and to make it physically manageable for grinding, polishing etc. The resin cures at room temperature. The ~30 μm thick section was polished on both sides.
The sample was irradiated using a linearly polarized 10 ps laser beam at 532 nm (Coherent Talisker Ultra System). Figure 2 shows the experimental setup used for the irradiation. The laser beam had a Gaussian intensity profile (M2 < 1.3) and was focused on to the target surface using a flat field scanning lens system with a focal length of 100 mm - a specialized lens system in which the focal plane of the deflected laser beam is a flat surface. The flat field scanning lens systems are commonly used in laser processing applications to offset the off-axis deflection of the beam through the focusing lens system.
Fig. 2 Experimental setup used for laser irradiation. The maximum average power of the laser at 532 nm is ~8 W. The linearly polarized laser beam was focused on the sample with a 100 mm focal length lens, which resulted in a ~12 μm diameter beam in air. A fully automatized system allowed us to scan different patterns on the sample.
The beam spot diameter on the target was measured to be ~12 μm. The beam was raster scanned over the surface of the target irradiating 5 mm × 5 mm squares at different velocities using a computer-controlled scanner system. The modification threshold of the material was measured to be ~20 mJ/cm2. Measurements were performed for single pulse per spot irradiation and care was taken to only modify the nanoparticle layer such that the thin glass surface layer stayed unaffected. It is worth pointing out that the modified spot diameter on the sample shows a high dependence on laser fluence, so that it increases for higher laser fluence values. In order to achieve uniform and homogenous dichroic areas a laser fluence of ~88 mJ/cm2 was required. After the irradiation, samples were annealed at 200 °C for 1 hour, to remove color centers and other defects in glass that could influence the quality of the optical measurements.
3. Results and discussion
A number of experiments were performed in order to prove the ability for uniform reshaping of spherical nanoparticles upon ps pulsed laser irradiation: changing the irradiation density by varying the applied number of pulses per spot, changing the laser repetition rate and fluence. In each case dichroism was observed, as a result of elongation of the originally spherical silver nanoclusters along the polarization direction of the laser beam.
As can be seen from Fig. 3(a), linearly polarized laser irradiation (1000 pulses per spot at the laser fluence of 88 mJ/cm2) resulted in the separation of the SPR band for light polarized parallel and perpendicular to the polarization of the laser beam. The SPR band separation is due to the nanoparticle shape modification. Spheroids exhibit separation of their SPR band - one for each of their geometrical axes [14]. Knowing the separation distance between the peaks of the long (p- polarization) and short (s- polarization) axes is of paramount importance when estimating the nanoparticle aspect ratio. The latter can be tuned by changing the laser fluence values. This is shown in Fig. 3(b) where the change in the laser fluence values, from just above the modification threshold (22 mJ/cm2) to approximately 88 mJ/cm2, is shown.
Fig. 3 Extinction spectra as a function wavelength for: a) Sample irradiated with 1000 pulses per spot, laser fluence of 88 mJ/cm2 and repetition rate of 200 kHz. S- polarization / P- polarizations are the polarization of light in the spectrophotometer, that is perpendicular / parallel to the polarization of the laser beam, respectively. Separation of the SPR band is a result of shape transformation of silver nanoparticles from spherical to spheroidal along the polarization the laser beam. For the p- polarization spectrum there is still the remnant of the spectrum from spherical nanoparticles, this is due to the fact that the employed modification wavelength of 532 nm is far from the SPR peak of 430 nm and that not all nanoparticles were being elongated in the focal volume. This issue could be overcome by using a sample with thinner nanoparticle-containing layer. b) Sample irradiated with different laser fluence values starting just above the modification threshold value (which is 18 mJ/cm2) and ending on a value that gives the most uniform dichroism (88 mJ/cm2). Irradiation was performed with the laser repetition rate of 200 kHz and 1000 pulses per spot were fired into the target. The employed laser intensities are (from lowest to highest): 2.21 GW/cm2, 2.65 GW/cm2, 3.54 GW/cm2, 6.19 GW/cm2 and 8.84 GW/cm2.
As can be seen, the SPR band (especially pronounced for the p- polarized light) experiences a shift towards the longer wavelengths and at the same time exhibits higher extinctions as the fluence increases. Peaks of the red shifted bands are at (from lowest fluence to the highest): 590 nm, 596 nm, 607 nm, 639 nm, and 660 nm. Furthermore, as the laser fluence increases more nanoparticles are being uniformly reshaped. This exhibits itself as an increase in the peak of the absorption band.
We have also observed that the SPR band separation and hence dichroism occurs for different regimes of irradiation. Figure 4(a) shows the change in the SPR gap between the red shifted p- polarized and s- polarized bands when changing the applied number of pulses per spot value. Figure 4(b) demonstrates that changing the laser repetition rate affects the spectrum in a similar manner.
Fig. 4 (a) Surface Plasmon Resonance band separation distance as a function of number of pulses per spot. Irradiation was performed for 88 mJ/cm2 (8.84 GW/cm2) and a 200 kHz laser repetition rate. (b) Surface Plasmon Resonance band separation distance as a function of laser repetition rate. Irradiation performed with 88 mJ/cm2 (8.84 GW/cm2) and 1000 pulses per spot. Both graphs represent the degree of elongation that the nanoparticle experiences. They also show a similar relation, that the SPR gap separation rises to its maximum value of ~240 nm.
These observations are in agreement with the temperature model presented for fs pulsed laser modification in [15]. According to the model each pulse during irradiation accumulates the effect of elongation by heating up the nanoparticle (Two Temperature Model) and then spreading its heat to the glass matrix by heat diffusion (Three Temperature Model). The overall effect of reshaping increases for each pulse because the system relaxes to a slightly higher temperature than it had before. Approaching high temperatures results in dissolution of the nanoparticles in the glass matrix due to the increase mobility of the emitted silver ions and their diffusion into the glass matrix during the irradiation [16, 17]. Therefore high repetition rates or firing large number of pulses per spot can lead to undesirable effects.
Here, the maximum achieved SPR gap is about 240 nm. At this value the temperature in the close vicinity of the nanoparticles is approaching its maximum value for silver ions coupling with the electrons on the poles of the nanoparticles. Higher temperatures would lead to diffusion of silver ions further away from the original silver lattice and electrons, leading to the dissolution of the nanoparticles into the glass matrix. The results of the large area picosecond irradiation of glass with embedded spherical silver nanoparticles at 532 nm at 200 kHz are presented in Fig. 5. The laser fluence is 88 mJ/cm2 and the number of pulses fired per spot was varied. As can be seen, the irradiated areas and the observed dichroism are homogeneous (island-free). These results are very similar to the ones observed in fs pulsed laser irradiation of glass with embedded silver nanoparticles. However, for 150 fs pulsed laser irradiation at 400 nm the dissolution was reported to occur for the laser operating at just below 100 kHz repetition rate and the modification threshold was reported to be ~0.2 TW/cm2 (~30 mJ/cm2) [16]. These values are fairly close to our measured values for the 532 nm ps pulsed laser modification threshold of ~20 mJ/cm2. It is worth stressing the fact that here the irradiation wavelength is far from the SPR peak absorption. For the ps pulsed laser irradiation at 532 nm most of the experiments have been performed using a laser intensity of ~9 GW/cm2 (~88 mJ/cm2). The fact that pulses of ~10 ps duration can produce dichroism as efficiently as fs pulses confirms our previous conclusions [15–17] about the characteristic time scale of the processes of particle deformation and that the emitted electrons during the process are penetrating and settling down in the matrix within 10-20 ps.
Fig. 5 Top view images of the 5 × 5 mm squares irradiated at laser fluence of 88 mJ/cm2 at 200 kHz. Number of pulses per spot in the irradiation areas on the left hand side are (from left to right): 500, 300, 100, 200 (top row) and 400, 200, 100, 100 (bottom row). The right hand side image shows the same as the one on the left but flipped 90° clockwise. The black arrow represents the polarization of the light penetrating the samples from the back of the image (perpendicular to the paper). The red arrow represents the polarization direction of the laser beam for irradiation of the areas grouped within the red dash-lines. The green arrow represents the polarization direction of the laser beam for irradiation of the areas grouped within the green dash-lines. The dichroic effect is easily observable, with higher extinction for the nanoparticles elongated along the polarization direction of the penetrating light.
4. Conclusion
Picosecond pulsed laser irradiation of glass containing spherical silver nanoparticles led to the homogenous deformation of the nanoparticles without destruction of the sample. This effect, which is nanoscopically caused by permanent deformation of the initially spherical particles to non-spherical shapes along the polarization direction of the laser beam was studied as a function of laser polarization, repetition rate and number of the applied pulses using a 10 ps, Nd:YVO4 laser at 532 nm. A spectral gap of ~240 nm for the SPR band was achieved for a repetition rate of 200 kHz after 1000 pulses per spot were fired into the sample.
Previously, and in order to achieve an optimum dichroism via fs pulsed laser irradiation of the nanocomposites in ambient temperature [16, 17], a laser repetition rate of 10 kHz was suggested. This hindered the operation where high processing speeds are required. The experimentally applied values here can be tailored depending on the requirements. This should lead to an industrially affordable fabrication of new optical elements such as wavelength- and polarization-selective diffraction gratings based on glasses containing metallic nanoparticles.
Acknowledgments
This work was conducted under the aegis of the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (EP/I004173/1), and has been supported by the CLIC Project under CERN collaboration agreement KE1865/TE. We are very grateful to CODIXX AG of Barleben/Germany for providing the samples for this study. MAT is a Marie Curie Fellow within the LA3NET Network (Grant Agreement Number 289191). AA is currently an EPSRC Career Acceleration Fellow at the University of Dundee.
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