Sub-surface modifications in silicon with ultra-short pulsed lasers above 2  µm

Abstract

Nonlinear optical phenomena in silicon such as self-focusing and multi-photon absorption are strongly dependent on the wavelength, energy, and duration of the exciting pulse, especially for wavelengths >2 µm. We investigate the sub-surface modification of silicon using ultra-short pulsed lasers at wavelengths in the range of 1950-2400 nm, at a pulse duration between 2 and 10 ps and pulse energy varying from 1 µJ to 1 mJ. We perform numerical simulations and experiments using fiber-based lasers built in-house that operate in this wavelength range for the surface and sub-surface processing of Si-wafers. The results are compared to the literature data at 1550 nm. Due to a dip in the nonlinear absorption spectrum and a peak in the spectrum of the third-order nonlinearity, the wavelengths between 2000 and 2200 nm prove to be more favorable for creating sub-surface modifications in silicon. This is the case even though those wavelengths do not allow as tight focusing as those at 1550 nm. This is compensated for by an increased self-focusing due to the nonlinear Kerr-effect around 2100 nm at high light intensities, characteristic for ultra-short pulses.

In‐Volume Laser Direct Writing of Silicon—Challenges and Opportunities

“…confirmed the specificity of reaching the permanent modification regime at 2‐µm wavelength with picosecond pulses by comparing the results of nonlinear propagation simulations at wavelengths between 1.55 and 2.35 µm. [ 201,202 ] Similar to the nonlinear propagation models presented in Section 3, the calculations rely on the solution of the nonlinear Schrödinger equation (NLSE). For 1‐µJ pulses, the domain of high free‐carrier density at 1.95‐ and 2.15‐µm wavelength is larger than at 1.55‐ and 2.35‐µm wavelength.…”

“…The beneficial aspect of near‐2‐µm wavelengths for modifying silicon was experimentally verified in ref. [202] by using 5‐ps duration pulses at 2.09‐µm wavelength emitted by a Ho:fiber‐seeded Ho:YAG chirped‐pulse amplifier.…”

“…Moreover, at 2‐µm wavelength, silicon exhibits a nonlinear refractive index peak (see Section 3.2, Figure 12), [ 47,48,98 ] which is favorable for producing relatively high electron density [ 102 ] and potential modifications. [ 31,202 ] By combining these two advantages, Gehlich et al., [ 220 ] and Mingareev et al. [ 221 ] have employed nanosecond laser pulses at 2‐µm wavelength for writing continuous lines at the exit surface of silicon.…”

Nonlinearity and wavelength control in ultrashort-pulse subsurface material processing

“…Several successful studies have been carried out since then [10][11][12][13][14][15][16][17][18][19][20]. Those sub-surface defects are targeted for such important industrial processes as thin wafer exfoliation [21], sub-surface structuring [17,21], data storage [22], waveguides [4,[22][23][24] and grating fabrication [6]. The common way to introduce sub-surface modifications in all these works is using multiple pulses and pulse trains [25].…”

“…Besides nonlinear multi-photon absorption light propagating through the material experiences several different processes, such as self-focusing [16,21] and plasmainduced de-focusing and reflection [11,15], to name a few. Those processes strongly depend on the wavelength and intensity of the pulse propagating through the material [27,28].…”