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

Abstract

The metrology of laser-induced damage usually finds a single transition from 0% to 100% damage probability when progressively increasing the laser energy in experiments. We observe that picosecond pulses at 2-µm wavelength focused inside silicon provide a response that strongly deviates from this. Supported by nonlinear propagation simulations and energy flow analyses, we reveal an increased light delocalization for near critical power conditions. This leads to a nonmonotonic evolution of the peak delivered fluence as a function of the incoming pulse of the energy, a situation more complex than the clamping of the intensity until now observed in ultrafast regimes. Compared to femtosecond lasers, our measurements show that picosecond sources lead to reduced thresholds for three-dimensional (3D) writing inside silicon that is highly desirable. However, strong interplays between nonlinear effects persist and should not be ignored for the performance of future technological developments. We illustrate this aspect by carefully retrieving from the study the conditions for a demonstration of 3D data inscription inside a silicon wafer.

Pulse-duration dependence of laser-induced modifications inside silicon

“…The absence of any damage at short pulse duration is symbolized by the 'A' region covering even shorter pulse durations by combining this observation with previous reports [2,9,21]. With the range of pulse durations accessible with the designed stretcher, we expect modification comes into picture as observed in some other previous works [5,23]. Interestingly, our shortest accessible pulse duration of 4 ps (due to the minimum distance that can be set between gratings) leads to the same conclusion as for the femtosecond pulse irradiation.…”

“…While single shot modifications are hardly detectable, we found that 1000 shots are usually appropriate for the growth of the modifications and unambiguous detection. Applying the pulse sequences at 1 kHz repetition rate, we do not introduce any thermal accumulation effect to account in these experiments [5,23] and we only benefit from incubation in comparison from a single-shot study. For the investigations at different pulse durations, the maximum pulse energy impinging the Si target is 1.95 µJ (measured after the objective lens) corresponding to the maximum fluence of 86 J/cm 2 with theoretical beam diameter 1.22λ/NA 2.4 µm in air.…”

“…The emergence of ultrafast laser writing in narrow bandgap materials offers the possibility for directly creating microdevices with 3D architectures for application fields as important as micro-electromechanics and silicon photonics. The first recent successes in locally tailoring the properties of silicon (Si) with ultrashort pulses tightly focused below the surface [1][2][3][4][5] have revealed responses strongly deviating from those extensively studied in dielectrics during the last two decades [6,7]. Till now, the investigations for internal modification of silicon have been focused on the short-wave infrared region of the spectrum (SWIR) and very modest dependence to the wavelength parameter is found as long as multiphoton absorption initiates localized energy deposition near the focus in an originally transparent Si target [8,9].…”

“…At the opposite, the shortest ultrashort pulses (sub-100 fs) usually fail in inducing permanent changes inside Si using conventional machining configurations [2,8,21,22]. One can also note contradictions in the intermediate picosecond regime where some argues that modification regimes are hardly achievable with pulses of few picosecond duration [5,23] while other have recently presented demonstration of waveguide writing in Si with sub-ps pulses and conditions very similar to those used in dielectric studies [3,4]. As we will show in this paper, the controversy persists because the space-time localization of light inside Si-bulk rely on a stronger interplay between nonlinear processes across the focus than for dielectrics [8,21,24,25].…”

“…The main contributions that prevent modification with the shortest pulses are beam depletion by multiphoton absorption, Kerr-induced phase distortions, screening and defocusing of strong plasma created prior the focus [2,5,8,24]. All these phenomena are intensity-dependent nonlinear effects increasingly limiting the energy density that can be delivered as the pulse duration is decreased.…”