Bessel beams are invariant solutions to the Helmoltz equation that can also propagate, with finite pulse energy at high intensity, in a quasi-invariant regime in transparent dielectrics. Homogeneous energy is deposited along a line focus by infrared ultrashort pulses. If the cone angle is sufficiently high, the laser-deposited energy density is enough to open nanochannels in glasses or sapphire with a single laser pulse. This has found applications in the field of glass cutting via the technique of "stealth dicing".
Here we address two important challenges in this field. First, high quality Bessel beams are essential for controlled energy deposition. Second, the maximal angle used up to here for channel drilling was 26° for 800 nm laser central wavelength. This enabled the formation of channels with diameters down to typically 300 nm in glass and sapphire. It is questionable if higher cone angles could also produce channels with potentially smaller diameters.
Here, we generate high quality Bessel-Gauss beams with a setup based on reflective, off-axis axicons. The Bessel zone exceeds 100 µm for cone angles up to 35 degrees. This corresponds to central spot diameter down to 0.5 µm FWHM. We qualified these beams with a 100 fs laser source centered at 800 nm wavelength. We report nanochannel drilling down to typically 100 nm over at least 30 µm length in glass.
Our approach opens novel perspectives for high quality Bessel beam generation but also for the highly confined laser-matter interaction for high precision processing of transparent dielectrics.
Strong ion migration in shown to enable the production of high refractive index contrast waveguides by fs-laser writing
in a commercial (Er,Yb)-doped phosphate based glass. Waveguide writing was performed using a high repetition rate fslaser
fibre amplifier operated at 500 kHz and the slit shaping technique. Based on measurements of the NA of
waveguides, the positive refractive index change (Δn) of the guiding region has been estimated to be ∼1-2 x10-2. The
compositional maps of the waveguides cross-sections performed by X-ray microanalysis evidenced a large increase of
the La local concentration in the guiding region up to ~25% (relative to the non-irradiated material). This large
enrichment in La was accompanied by the cross migration of K to a neighbouring low refractive index zone. The
refractive index of the La-phosphate glass increases linearly with the La2O3 content (Δn per mole fraction increase of
La2O3 ≈ 5x10-3) mainly because of the relative mass of the La3+ ions. The density increase without substantial
modification of the glass network was confirmed by space-resolved micro-Raman spectroscopy measurements showing
minor variations in the (PO2)sym vibration Raman band. These results provide evidence for the feasibility of adapting the glass composition for enabling laser-writing of high refractive index contrast structures via spatially selective
modification of the glass composition.
Nonlinear laser processing of dielectrics with ultrafast lasers has been extensively studied over the last years and successfully applied to the production of photonics and micro-fluidic devices. Still, problems related to the presence of strong optical nonlinearities make it difficult to optimize the spatial intensity distribution in the focal region (SIDFR) in some cases. Methods providing a rapid estimate of the latter, even approximately, can be of great help for optimizing processing strategies and in other applications conditioned by nonlinear propagation like spatial soliton shaping. We have developed a numerical method for estimating the SIDFR inside a dielectric material, considering nonlinear absorption, nonlinear refraction and spherical aberration for laser beams with arbitrarily shaped wavefront. It is based on a generalized adaptive fast-Fourier evolver and has been successfully tested for flat wavefronts in subsurface processing. In this work we demonstrate its applicability to complex wavefronts, like those that can be generated with spatial light modulators (SLM). For this purpose the beam wavefront is described using Zernike polynomials before being propagated inside the material for different depths, pulse parameters. The results obtained show that under certain conditions, nonlinearities can be not only controlled and pre-compensated but also exploited for producing tailored SIDFRs.
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