Ultrafast laser photoinscription for 3D optical systems

Today's micro- and nanotechnologies rely entirely on tools capable of structuring materials with maximum precision and minimum collateral effects. Laser radiation is one of the most widely used and studied tools, from macroscopic applications (welding, cutting, drilling) to nanoscopic applications (two-photon photopolymerization, UV lithography for microelectronics). To further increase achievable precision, ultra-short femto-picosecond laser pulses (10 ^–15 to 10 ^–12 s) are being studied in particular. Ultra-short laser micromachining has become a high-performance technology capable of meeting new challenges in material structuring down to the nanometric scale. During ultra-short irradiation, the non-linear nature of the excitation and the limited effects of thermal diffusion enable photonic energy to be concentrated to unprecedented levels, paving the way for unique applications.

Efforts to miniaturize and integrate multiple functions on a single substrate are directly impacted by these possibilities. The field of application par excellence is naturally that of photonics, in particular data transmission and processing. The concept of a photonic circuit combining several optical functions on a monolithic substrate is at the heart of application development. The performance of such a device depends directly on the ability to realize a three-dimensional design within a material of optical interest.

The idea of three-dimensional modification of transparent materials is intrinsically linked to irradiation with short and ultra-short pulses in a wavelength range for which the material itself is transparent. This makes it possible to bring laser irradiation to the desired point of impact within the volume without absorption losses. Focusing also makes it possible to concentrate irradiation only in the area to be modified. This concentration of light energy makes it possible to reach extreme intensities (of the order of TW · cm ^–2 ), and only in a zone corresponding approximately to the confocal distance given by the focusing element (lens, curved mirror, etc.); the material is thus locally ionized by non-linear absorption. The energy accumulated by the electrons then relaxes towards the material's molecular matrix, modifying its structure and thus its optical properties. As a result, a highly localized change in refractive index can be achieved. This modification constitutes the building block that can be reproduced indefinitely by moving the laser beam in three dimensions to realize optical functions. This provides a relevant and robust technique for fabricating optical elements and functions within a material. Although extremely precise...