Fabrication of Uniform Phase Structures in the Bulk of a N-BK7 Glass Using Ultrashort Laser Pulses
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ication of Uniform Phase Structures in the Bulk of a N-BK7 Glass Using Ultrashort Laser Pulses A. S. Yeremyana, *, M. L. Sargsyana, and P. K. Gasparyanb aCANDLE b
Synchrotron Research Institute, Yerevan, Armenia Yerevan State University, Yerevan, Armenia *e-mail: [email protected]
Received February 11, 2020; revised March 26, 2020; accepted April 8, 2020
Abstract—We study the possibility of application of direct laser writing technique for the fabrication of large-size uniform phase structures in the bulk of the borosilicate glass N-BK7. The morphology and the type of modifications induced by the sub-picosecond (500 fs) pulses are studied depending on the energy dose of irradiation. We show that with moderate focusing and at pulse energies below the threshold of self-focusing effects, it is possible to choose writing regimes (the scanning speed, the repetition rate of pulses) which allow the fabrication of extended structures with flat modification of the glass refractive index in the layer-by-layer scanning mode. At higher peak intensities of the laser, the Kerr nonlinearities affect the pulse propagation and the dynamics of the relaxation processes, and the resulting structures are essentially inhomogeneous with the alternating sign of refractive index change. To validate the approach, a buried phase structure with large dimensions was fabricated showing a rather uniform refractive index change over the entire processed range. Keywords: Direct laser writing, refractive index change, femtosecond laser, glass processing DOI: 10.3103/S106833722003007X
1. INTRODUCTION The microprocessing of transparent materials using ultrashort-pulse lasers is a subject of intensive research and is increasingly used as a method for the fabrication of devices and components of optoelectronics and integrated optics. One of the most studied and widely used properties of this method is the possibility of direct precision writing of 3D-structures in glasses, crystals, and polymers [1–6]. The advantage of using the ultrashort pulses (of the order of or less than a picosecond) for this type of recording is the ability to achieve extremely high intensities and initiate nonlinear (multiphoton) absorption processes in precisely defined areas inside the material. As a result, laser irradiation modes can be implemented in micro- and nanoscale regions, leading to various structural or phase changes and, consequently, to modification of the local optical properties of the material—from a smooth change of the refractive index to the formation of micro-voids or birefringent zones [7–9]. Among the above types of modifications, the refractive index change (RIC) is perhaps the most interesting because of the great potential of practical applications. In particular, the possibility of inducing a positive RIC is considered as a direct and inexpensive method for producing active and passive waveguides in crystals and glasses [2, 6, 10–14], as well as filters and many other passive elements of integrated optics [1–5]. As a transparent medium for t
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