Aabid Patel

Void-nanograting transition by ultrashort laser pulse irradiation in silica glass.

The structural evolution from void modification to self-assembled nanogratings in fused silica is observed for moderate (NA > 0.4) focusing conditions. Void formation, appears before the geometrical focus after the initial few pulses and after subsequent irradiation, nanogratings gradually occur at the top of the induced structures. Nonlinear Schrödinger equation based simulations are conducted to simulate the laser fluence, intensity and electron density in the regions of modification. Comparing the experiment with simulations, the voids form due to cavitation in the regions where electron density exceeds 1020 cm-3 but is below critical. In this scenario, the energy absorption is insufficient to reach the critical electron density that was once assumed to occur in the regime of void formation and nanogratings, shedding light on the potential formation mechanism of nanogratings.

Eternal 5D data storage by ultrafast laser writing in glass

Securely storing large amounts of information over relatively short timescales of 100 years, comparable to the span of the human memory, is a challenging problem. Conventional optical data storage technology used in CDs and DVDs has reached capacities of hundreds of gigabits per square inch, but its lifetime is limited to a decade. DNA based data storage can hold hundreds of terabytes per gram, but the durability is limited. The major challenge is the lack of appropriate combination of storage technology and medium possessing the advantages of both high capacity and long lifetime. The recording and retrieval of the digital data with a nearly unlimited lifetime was implemented by femtosecond laser nanostructuring of fused quartz. The storage allows unprecedented properties including hundreds of terabytes per disc data capacity, thermal stability up to 1000 °C, and virtually unlimited lifetime at room temperature opening a new era of eternal data archiving.

Direct Writing with Tilted-Front Femtosecond Pulses

Shaping light fields in both space and time provides new degrees of freedom to manipulate light-matter interaction on the ultrafast timescale. Through this exploitation of the light field, a greater appreciation of spatio-temporal couplings in focusing has been gained, shedding light on previously unexplored parameters of the femtosecond light pulse, including pulse front tilt and wavefront rotation. Here, we directly investigate the effect of major spatio-temporal couplings on light-matter interaction and reveal unambiguously that in transparent media, pulse front tilt gives rise to the directional asymmetry of the ultrafast laser writing. We demonstrate that the laser pulse with a tilted intensity front deposits energy more efficiently when writing along the tilt than when writing against, producing either an isotropic damage-like or a birefringent nanograting structure. The directional asymmetry in the ultrafast laser writing is qualitatively described in terms of the interaction of a void trapped within the focal volume by the gradient force from the tilted intensity front and the thermocapillary force caused by the gradient of temperature. The observed instantaneous transition from the damage-like to nanograting modification after a finite writing length in a transparent dielectric is phenomenologically described in terms of the first-order phase transition.

Harnessing polarization spatio-temporal coupling: A new degree of freedom in ultrafast laser material processing

Polarization spatio-temporal coupling reveals new degree of freedom in ultrafast laser material processing. Control of modification in fused silica is demonstrated with the use of prism compressors and polarization azimuth of ultrashort pulse laser beam.

Geometric phase circular gratings fabricated by ultrafast laser nanostructuring for symmetric simultaneous spatio-temporal focusing

The recent advances in flat optics have challenged the limitations of conventional optics by implementing planar elements that instead of rely on dynamic phase manipulate light waves via subwavelength-spaced phase shifters with spatially varying phase response [1]. One of the approaches for designing geometric (Pancharatnam-Berry phase [2]) phase optical elements (GPOE) is to exploit the transparent dielectrics which originate form birefringence. The desired phase pattern of the wave is directly encoded in the optical axis orientation, and is equal to twice the rotation angle of the local retarder. A decade ago, the formation of self-organized subwavelength periodicity structures, referred to as nanogratings, in the bulk of silica glass after irradiation with ultrashort light pulses was observed [3]. Such a periodic assembly behaves as a uniaxial birefringent material with optical axis oriented parallel to the polarization of incident laser beam, and serves as a perfect candidate for designing geometric phase profiles of nearly any optical components. The performance of fabricated GPOE varies depending on the processing conditions reaching the efficiency and transmittance higher than 90%, as well as the phase gradient higher than π rad/μm [4]. The silica glass based optics with a damage threshold of 22.8 J/cm2 demonstrate the potential of high-power applications.

Solving the puzzle of non-reciprocity in ultrafast laser writing

The nanostructuring of transparent media with ultrashort laser pulses has attracted interest due to its unique applications. However, little is understood with respect to the physical mechanisms responsible for the peculiarities of the dielectrics inscribing with high intensity laser beams. It has been shown that spatio-temporal couplings (STC) inherent to the ultrashort pulses make inscribing sensitive to the writing direction [1, 2] and that anisotropic photosensitivity [3] originates from the pulse front tilt (PFT). More recently, a greater appreciation of STC in focussing has been gained, shedding light on previously unknown parameters of the pulse such as a lighthouse-like wavefront rotation [4] emphasizing the need for a better understanding of the STC effects in light-matter interaction. Nevertheless, understanding of the microscopic processes responsible for the modifications of dielectrics with ultrashort laser pulses and control of the writing process via STC is still lacking. Here we show with control of STC through the use of grating compressors, allows one to control and understand ultrafast phenomena associated with material modification. We reveal unambiguously that PFT gives rise to the non-kreciprocity during femtosecond laser writing in transparent media and induces either an isotropic damage-like structure or a self-assembled nanostructure depending on the writing direction. This phenomenon is known as the “quill-writing effect” (Fig. 1a). A switching of the modification regime is observed when the translation of the beam is in the direction of the tilt, which can be qualitatively described in terms of a first-order phase transition.

Dataset for Non-Paraxial Polarization Spatio-Temporal Coupling in Ultrafast Laser Material Processing

Dataset supporting the article Non-Paraxial Polarization Spatio-Temporal Coupling in Ultrafast Laser Material Processing by Aabid Patel, Vladmir T. Tikhonchuk, Jingyu Zhang, and Peter G. Kazansky in Laser and Photonics Review

Geometric phase holograms imprinted by femtosecond laser nanostructuring

We demonstrate direct-write laser nanostructuring of semiconductor thin-films and transparent dielectrics resulting in space-variant anisotropic materials. The continuous phase profile of nearly any optical component can be achieved solely by the means of geometric phase.

Dataset for Eternal 5D data storage by ultrafast laser writing in glass

Securely storing large amounts of information over relatively short timescales of 100 years, comparable to the span of the human memory, is a challenging problem. Conventional optical data storage technology used in CDs and DVDs has reached capacities of hundreds of gigabits per square inch, but its lifetime is limited to a decade. DNA based data storage can hold hundreds of terabytes per gram, but the durability is limited. The major challenge is the lack of appropriate combination of storage technology and medium possessing the advantages of both high capacity and long lifetime. The recording and retrieval of the digital data with a nearly unlimited lifetime was implemented by femtosecond laser nanostructuring of fused quartz. The storage allows unprecedented properties including hundreds of terabytes per disc data capacity, thermal stability up to 1000 °C, and virtually unlimited lifetime at room temperature opening a new era of eternal data archiving.

Current trends in multi-dimensional optical data storage technology

Optical data storage, renowned for its low energy consumption features, is an ideal candidate for data archiving. The major challenge is the lack of appropriate combination of storage technology and medium possessing the advantages of both high capacity and long lifetime.