Demand for simple and robust nanoprocessing technologies for next-generation smart optical, electro-optical, electronic, mechanical, and biological devices is increasing. Laser nanofabrication of subwavelength structures on a variety of substrates makes possible more sophisticated near-field devices, micro- and nanoscale sensors, color displays, and encryption and anti-counterfeiting systems. Technologies such as plasmonic light harvesting for solar cells and high light intensity GaN LEDs are now becoming possible, as are new analytical techniques such as surface-enhanced Raman scattering (SERS).
We have shown that two complementary patterning techniques—femtosecond laser ablation, and plasmonic scattered fields—can both be exploited by making a simple change to a single experimental setup. The distance between the laser and the substrate is the key factor. When the laser source is held in the near-field of the substrate—the region close to an object where exponentially-decaying evanescent waves dominate— irradiating structures such as nano- and microspheres placed on the substrate causes an optical field enhancement underneath each structure. Ablation patterns are formed on the surface that have feature dimensions smaller than the Abbe diffraction limit, which in classical far-field optics prevents light being focused to areas smaller than roughly half its wavelength. When a laser is positioned further from the surface, structures such as nano- and microspheres create scattered fields of radial intensity that can also be harnessed to carry out ablation, enabling high-precision high-throughput processing on various kinds of materials.
Figure 1. Complementary techniques of near-field and far-field scattering: (a) nanocrater fabrication with near-field Mie scattering ablation; and (b) circular ripples formed from interference between a far-field laser pulse and a coherent surface plasmon wave.
Figure 1(a) illustrates the near-field method we used to create nanocraters on silicon. We irradiated nanosphere arrays with a near-infrared femtosecond laser held in the near-field of the substrate (typically <100nm). This initiated a nonlinear ablation process, forming nanocraters with diameters less than 100nm: much smaller than the wavelength of the light used.1 The nature of the near-field defines a fast decay of intensity as a function of the distance from the nanostructure surface, enabling material processing in the sub-micron to nanoscale regimes.
Figure 2. Scanning electron microscopy images of a silicon surface following irradiation with a single femtosecond laser pulse through polystyrene nanospheres with (a) 800nm- and (b) 400nm- wavelength respectively. Scanning electron microscopy images of ordered hexagonal monolayer arrays on the silicon substrate prior to laser irradiation are shown in (c) and (d).
To demonstrate our method, we made nanocrater patterns on silicon by irradiating hexagonal close-packed arrays of polystyrene spheres with a single femtosecond laser pulse. In one case—see Figure 2(a)—we used a wavelength of 800nm, and in another, 2(b), 400nm. Patterns can be formed over large areas, making this a high-throughput process. The physical basis for near-field optical enhancement underneath the spheres, shown in Figure 2(c) and (d) derives from a shift away from a microlens effect to a Mie scattering effect: this is a phenomenon that follows from Maxwell's equation that applies in the special case of spheres. Feature size is determined mainly by the size of the spheres and independently of excitation wavelength.2 Patterning using this method is fast and simple, due to the ease of arranging spheres and the lack of capacitance coupling between neighboring polystyrene nanospheres.
Metallic spheres also generate intense near-field plasmonic scattering when irradiated by laser light, but the underlying mechanism for near-field generation by metallic spheres is different from that by dielectric spheres. The electromagnetic coupling between the incident laser and the collective electron oscillation within the spheres—i.e. plasmon resonance—plays a crucial role in the generation of an intense near-field optical enhancement.3 We can control ablation feature size with our metallic spheres by manipulating the incident angle of the laser.
We have also fabricated concentric circle ripple structures on substrates, which can be formed through the interference of an incident femtosecond pulse from a far-field laser source and a coherent surface plasmon wave. This method is illustrated in Figure 1(b). To achieve this we deposited a gold nanosphere onto a silicon surface and irradiated it with multiple femtosecond laser pulses. This produced a pattern over a large surface area.
Figure 3. (a) Circular ripples on the silicon surface and (b) simulated result of the field intensity distribution around two gold particles on silicon.
In addition, we have shown using simulations4 that different interference patterns can be formed by different arrangements of scattering structures. Figure 3(a) shows concentric circular ripple patterns fabricated on a silicon surface, and the simulated result of the optical field intensity distribution around a pair of gold particles placed on the silicon substrate. The simulated result shown in Figure 3(b) is in agreement with the experimental result shown in Figure 3(a). The observed spherical ripple wave in Figure 3(b) indicates the interference pattern between the plasmonic far field and the incident wave. If the peak intensity of the ripples exceeds the ablation threshold or phase transition threshold, the substrate surface is periodically ablated or modified morphologically at the initial stages of the surface ripple growth process.
In summary, we have demonstrated near-field and far-field surface ablation patterning, employing Mie resonance scattering and plasmonic scattered fields to make features smaller than the diffraction limit. We have shown that we can accurately model wave patterns from multiple structures, providing a means to design intricate patterns on a surface. Although the study of photon scattering can be traced back to Rayleigh in 1871 and Mie in 1908, it is only recently that scattering near-field and far-field have recently been made viable and revived as an innovative nanoprocessing technology. We plan to further investigate the underlying physics of near- and far-field nanoablation in detail and, in doing so, extend optical science by creating a new class of nano-processed photonics and electronics.
Mitsuhiro Terakawa, Minoru Obara
Department of Electrical Engineering and Electronics
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