Quantitative-phase microscopy of nanosecond laser-induced micro-modifications inside silicon

Abstract

Laser-induced permanent modification inside silicon has been recently demonstrated by using tightly focused nanosecond sources at a 1550 nm wavelength. We have developed a quantitative-phase microscope operating in the near-infrared domain to characterize the laser-induced modifications deep into silicon. By varying the number of applied laser pulses and the energy, we observe porous and densified regions in the focal region. The observed changes are associated with refractive index variations |Δn| exceeding 10^-3, enough to envision the laser writing of optical functionalities inside silicon.

Inscribing diffraction gratings in bulk silicon with nanosecond laser pulses

“…The positive sign of this value was additionally confirmed (1) by comparing the relative positions of the occasional jumps in the trace amplitude on a few measurements due to instabilities, and (2) by four-step measurements similar to those in Ref. [14]. According to the length d 187 μm of the line extracted from the NIC image in Fig.…”

“…The underlying mechanisms are the production of a dense plasma induced by two-photon absorption followed by high-temperature hydrodynamic phenomena. Indeed, analyses of the laser-induced modifications have revealed that these pulses are able to produce a wide variety of material structures from voids to densified phases [13][14][15][16]. Therefore, this emerging long pulse duration regime could be suitable for functionalizing c-Si in the volume.…”

“…Ultimately, the refractive index of the lines composing the gratings is measured along the z axis by means of the phase-shift microscope based on IR Mach-Zehnder interferometry (not shown in Fig. 1) detailed in [14]. For cost and compactness considerations, the femtosecond laser source used in the originally developed instrument was replaced by the aforementioned superluminescent diode exhibiting a short coherence length so that the parasitic interferences caused by multiple reflections can be as efficiently avoided.…”

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

“…After laser writing inside c-Si, the modifications are observed by means of a customized phase-shift interferometry microscope described in Ref. [21]. Briefly, this microscope relies on an IR Mach-Zehnder interferometer and provides quantitative information about the refractive index change of the modifications.…”

Inscribing diffraction grating inside silicon substrate using a subnanosecond laser in one photon absorption wavelength

“…The presence of the vertically aligned spots indicates that completely 3D-localized modification was not realized yet, but the modification was localized in two-dimensions thus can be used in two-dimensional applications. Such a series of modified spots have been reported in nanosecond laser processing of silicon 25,26 . In these studies, modified spots were identified as voids.…”