Self-controlled formation of microlenses by optical breakdown inside wide-band-gap materials

Abstract

By repeatedly illuminating fused silica slabs with focused femtosecond pulses, we permanently decrease the local refractive index without increasing the linear absorption or scattering. This progressively forms a biconvex lens in the prefocal region. With linearly polarized light, the index change reaches several percent and is associated with the formation of an array of planar nanocracks. We analyze the polarization-dependent focusing power of the subwavelength periodic structure. While the detailed material modification changes, spontaneous defocusing lens formation is a common feature of every wide-band-gap transparent materials that we have studied (SiO2, BK7, LiF, sapphire, and mica).

Limitations to laser machining of silicon using femtosecond micro-Bessel beams in the infrared

“…It is today widely demonstrated that material breakdown by intense laser pulse in transparent dielectrics can leave directly a well-defined imprint of the local intensity and also polarization under appropriate conditions. [17][18][19] This relies on the threshold nature of the material response, the strong electric field, and the limited thermal effects in ultrafast regimes. The extremely high-aspect ratio nanostructures fabricated inside glass with femtosecond Bessel beams at fundamental wavelengths (e.g., 800 nm) 10 are a confirmation that it must represent a way to observe the energy distribution of our femtosecond micro-Bessel beam generated at 1300-nm wavelength.…”

High refractive index modification of SiO2created by femtosecond laser nanostructuring

“…However, experiments show that the nanostructured materials exhibit noticeable changes. More precisely, when the polarization of incident light is perpendicular to the produced planar nanostructures, the apparent refractive index is greatly reduced [14,15] because, as we show now, light interacts preferentially with the nanovoid inclusions.…”

“…The experiments are carried out at a 100 kHz repetition rate, but similar results were also obtained at lower rates (0.4 kHz) showing the absence of accumulation of transient effects on a shot-toshot basis in our experiment. The experimental procedure for laser writing inside the medium is similar to that described previously in [14]. As shown in figure 1, these conditions lead to well-defined ordered nanostructuring.…”

“…They therefore fill the general conditions for large index change proposed above. We have recently communicated the formation of a microlens in a-SiO 2 based on this concept [14]. In this paper, we report more detailed experiments on the optical properties of such lenses, accompanied by a detailed analysis that quantifies the refractive index reduction we achieve at (1.8 ± 0.2)%.…”

“…It has been shown that repeated illumination induces birefringence to the material [15,16]. Birefringence, in turn, can serve as a diagnostic of the material modification [10,14]. Nanostructures can also control how etchants interact with the sample [17], allowing nanofluidic channels to be fabricated.…”