Crossing the threshold of ultrafast laser writing in bulk silicon

Abstract

An important challenge in the field of three-dimensional ultrafast laser processing is to achieve permanent modifications in the bulk of silicon and narrow-gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects in the pre-focal region. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating the conditions for an interaction with an extreme numerical aperture near 3 in a perfect spherical sample, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved inside silicon. This opens the door to the direct writing of three-dimensional monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.

Competing Nonlinear Delocalization of Light for Laser Inscription Inside Silicon with a 2- µ m Picosecond Laser

“…We found that permanent modification in the bulk was achievable, but multipulse processing with our longest pulse duration of 10 ps was a requirement [11]. In a previous work, extreme focusing was identified as a spatial optimization to reduce the peak power and prefocal nonlinear interactions for femtosecond pulses [7]. One conclusion with the study in the picosecond regime was that it leads to a similar optimization in the temporal domain for bulk writing.…”

“…Different emerging short-pulse fiber lasers emitting in the short-wave infrared (SWIR) region of the spectrum (1.1-2.5 µm) represent attractive tools to extend the threedimensional (3D) laser writing technologies developed in transparent dielectrics [1] to narrow gap materials as important as silicon (Si) [2][3][4]. Nevertheless, the first femtosecond laser experiments in this prospect reveal that the intrinsic properties of narrow gap materials prevent any permanent material change in the bulk [5,6] unless nonconventional focusing conditions are used [7]. This is caused by a strong clamping of the intensity due to nonlinear effects developing in the prefocal region.…”

Positive- and negative-tone structuring of crystalline silicon by laser-assisted chemical etching

“…Nonlinear processes (especially the plasma defocusing phenomenon) impose severe limits on the laser energy that can be deposited inside crystalline silicon (c-Si) with femtosecond pulses [1], and make material modification with individual pulses extremely challenging [2]. The advent of high-power nanosecond infrared (IR) laser sources has helped to circumvent these limitations [3,4].…”

Origin of Waveguiding in Ultrashort Pulse Structured Silicon

“…Within the last two decades, numerous attempts have been performed to overcome these limitations . Recently, localized modifications in the bulk of silicon using ultrashort laser pulses have been demonstrated . This resulted in the first demonstration of waveguides written with fs laser pulses at a wavelength of 1550 nm by Pavlov et al .…”