Laser nanostructuring of silicon for photonic and optoelectronic applications  

Irradiation of silicon by a big number of laser pulses in a liquid or gas environment creates quasi-periodic nanostructures or microstructures on the surface of the material, depending on the irradiation conditions and the surrounding medium. This effect was first discovered at the lab of Professor Eric Mazur at Harvard University. Due to the unique electrical, optical, and mechanical properties of the laser-processed areas, microstructured and nanostructured silicon finds applications in the fabrication of optoelectronic devices, as well as a plasmonic substrate for SERS spectroscopy and optical trapping.    

Microstructured and nanostructured silicon surfaces with nanosecond and femtosecond laser systems, respectively.

Plasmonic optical trapping

Optical tweezers are widely employed for contactless manipulation of microparticles and nanoparticles for various applications. However, in the case of nanoparticles, the optical trapping force decreases abruptly with the size of the trapped particle, resulting in inefficient and unstable trapping conditions. Plasmonic optical tweezers yield enhanced electromagnetic near fields and achieve precise and efficient manipulation of nanoparticles at low photon flux. We developed a highly efficient plasmonic optical trap, based on laser-structured silicon samples. Coating the samples with a thin layer of gold or silver results in the spontaneous formation of metallic nanoparticles on the surface. Using the gold/silver-coated nanostructured silicon samples as substrates for plasmonic optical trapping, we were able to trap 400-nm polystyrene beads with an order of magnitude enhancement in the trapping force, compared with conventional tweezers in the absence of the nanostructured substrates. The development of silicon-based plasmonic optical tweezers relies on single-step, maskless, tabletop laser processing of silicon, which is amenable to large-scale fabrication.        

Schematic representation of the optical trap.

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Related publications

D.G. Kotsifaki, M. Kandyla, I. Zergioti, M. Makropoulou, E. Chatzitheodoridis, and A.A. Serafetinides

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M. Shen, J.E. Carey, C.H. Crouch, M. Kandyla, H.A. Stone, and E. Mazur

Nano Letters 8, 2087 (2008)

Nanostructured heterojunctions for blue/UV optoelectronics

We combine laser-nanostructured silicon substrates with thin layers of semiconducting materials, such as ZnO, to form semiconducting heterojunctions with large surface area. ZnO is a promising n-type semiconductor for optoelectronic applications, due to its wide band gap and efficient UV emission. Because of the difficulty in fabricating p-type ZnO, electronic applications based on this material rely on heterojunctions between ZnO and p-type semiconductors. For this purpose, we employed p-type nanostructured silicon substrates to form heterojunctions with ZnO layers. The electric characteristics of the nanostructured heterojunction are improved by orders of magnitude, compared with the characteristics of flat p-Si/n-ZnO junctions, owing to the increased surface area of the nanostructured device. Our fabrication method is scalable and versatile, does not require additional insulating layers, and can be extended to other combinations of materials.            

Schematic representation of a nanostructured p-Si/n-ZnO device.

D. Georgiadou, M. Ulmeanu, M. Kompitsas, P. Argitis, and M. Kandyla

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Video of optically trapped nanoparticle.

D.G. Kotsifaki, M. Kandyla, and P.G. Lagoudakis

Applied Physics Letters 107, 211111 (2015)

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D.G. Kotsifaki, M. Kandyla, and P.G. Lagoudakis

Scientific Reports 6, 26275 (2016)


M. Kanidi, A. Dagkli, N. Kelaidis, D. Palles, S. Aminalragia-Giamini, J. Marquez-Velasco, A. Colli, A. Dimoulas, E. Lidorikis, M. Kandyla, and E.I. Kamitsos

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Surface-enhanced Raman spectroscopy

Integrating 2D materials with plasmonic nanostructures results in multifunctional hybrid systems with enhanced performance for numerous applications. In our work, large areas of femtosecond laser-structured arrays of silicon nanopillars, decorated with gold nanoparticles, are integrated with graphene. We probe graphene for its plasmonic-enhanced Raman spectral signal at four excitation wavelengths, which span the visible range of the electromagnetic spectrum. Broadband enhancement (2–3 orders of magnitude) is observed for all excitation wavelengths, across the entire visible electromagnetic spectrum. Finite-difference time-domain (FDTD) simulations elucidate the advantages of the 3D topography of the substrate. Conformation of graphene to the Au-decorated silicon nanopillars enables graphene to sample near fields from an increased number of nanoparticles. Due to synergistic effects with the nanopillars, different nanoparticles become more active for different wavelengths and locations on the pillars, providing broadband enhancement. 2D materials present the unique ability of being easily combined with 3D substrates, as they can be grown over large areas with low-cost methods such as chemical vapor deposition, unlike bulk semiconductors which require high-quality epitaxial growth.

            

Surface-enhanced Raman spectroscopy of graphene conforming to 3D plasmonic silicon substrate.