Inscribing diffraction gratings in bulk silicon with nanosecond laser pulses
Résumé
Diffraction gratings are transversally inscribed in the bulk of monolithic crystalline silicon with infrared nanosecond laser pulses. Nanoscale material analyses of the modifications composing the gratings show that they rely on laser-induced stress associated with a positive refractive index change as confirmed with phase-shift interferometry. Characterizations of the optical properties of the gratings, including the diffraction angles and the efficiency of the different orders, are carried out. The refractive index change obtained from these measurements is in good agreement with the phase-shift measurements. Finally, we show that the grating diffraction efficiency depends strongly on the laser writing speed. Ultrafast laser pulses (i.e., in the femtosecond regime) offer the possibility to tailor the properties of glasses for writing wave-guides [1], storing data eternally [2], and inducing second-harmonic generation [3], for instance. By inscribing permanent diffraction gratings in the bulk of fused silica, Sudrie et al. have demonstrated that the control of the refractive index is essential for the functionalization of the material [4]. Transposing this technique to crystalline silicon (c-Si) would be of broad interest for applications in silicon photonics, terahertz physics, and Raman silicon lasers [5]. Moreover, the laser inscription of dif-fraction gratings in this semiconductor material would open the path to the writing of Bragg gratings, as it has existed in glasses for decades [6]. However, the main issue associated with the interaction between infrared (IR) femtosecond laser pulses and the bulk c-Si through a plane surface sample is the delo-calization of the energy mainly due to plasma defocusing [7]. To date, three strategies have been adopted for overcoming these limitations and functionalizing c-Si. The first one relies on femtosecond irradiation through a spherical interface for completely suppressing the refraction at the surface [8]. The second strategy consists of a femtosecond irradiation at a high repetition rate (250 kHz) provoking cumulative effects [9]. Finally, the much less costly third approach employs pulses with a duration on the order of or higher than 1 ps for drastically reducing the nonlinear and plasma effects protecting the bulk of c-Si [10-12]. 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 [13-16]. Therefore, this emerging long pulse duration regime could be suitable for functionalizing c-Si in the volume. Recently, Tokel et al. have suggested the possibility to inscribe gratings based on a negative refractive index change inside silicon for evaluating the efficiency of holograms written by back-reflected nanosecond laser pulses [11]. However, no characterizations of such components were provided. In this Letter, we demonstrate the inscription of diffraction gratings in the bulk of c-Si with nanosecond pulses at 0.01 mm/s writing speed and analyze their optical properties. Material characterizations of the laser-written lines composing the gratings show that the modifications consist of stress, as confirmed by quantitative phase-shift interferometry which, moreover, gives access to a refractive index change of 3.6 × 10 −3. The injection of IR continuous-wave (CW) light in the gratings enables us to evaluate their optical properties in terms of angles and diffraction efficiency of the different orders. Moreover, the measurements of the grating geometry, as well as the diffraction efficiency, allow us to retrieve a theoretical value for the refractive index change which is consistent with the one found by phase-shift measurements. Finally, gratings inscribed at 0.5 mm/s exhibit a mediocre diffraction efficiency, which is explained by the morphology of the written lines and confirms that the adjustment of the writing speed is paramount for optimizing the function given to the material. The experimental setup employed for writing diffraction gratings inside c-Si is schematically depicted in Fig. 1. It relies on 5 ns duration (full width at half-maximum) Gaussian laser pulses (in red) at a 1.55 μm wavelength and a 1 kHz repetition rate emitted by an Er-doped fiber laser (MWTech, PFL-1550). The beam is focused inside a 1 mm thick (100)-oriented mono-lithic c-Si sample by means of an objective lens of numerical aperture NA 0.7. At the focus positioned at the center of the sample, the beam is Gaussian with a waist w 0 1.4 μm Letter Vol. 43, No. 24 / 15 December 2018 / Optics Letters 6069 0146-9592/18/246069-04 Journal
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