Non‐paraxial polarization spatio‐temporal coupling in ultrafast laser material processing

Abstract

Two hundred years after Malus' discovery of optical anisotropy, the study of polarization‐driven optical effects is as active as ever, generating interest in new phenomena and potential applications. However, in ultrafast optics, the influence of polarization is frequently overlooked being considered as either detrimental or negligible. Here we demonstrate that spatio‐temporal couplings, which are inherent for ultrafast laser systems with chirped‐pulse amplification, accumulate in multi‐pulse irradiation and lead to a strongly anisotropic light‐matter interaction. Our results identify angular dispersion in the focus as the origin for the polarization dependence in modification, yielding an increase in modification strength. With tight focusing (NA ≥ ∼0.4), this non‐paraxial effect leads to a manifestation of spatio‐temporal couplings in photo‐induced modification. We devise a practical way to control the polarization dependence and exploit it as a new degree of freedom in tailoring laser‐induced modification in transparent material. A near‐focus, non‐paraxial field structure analysis of an optical beam provides insight on the origin of the polarization dependent modification. However, single pulse non‐paraxial corrected calculations are not sufficient to explain the phenomena confirming the experimental observations and exemplifying the need for multi‐pulse analysis.

Thermal Stability of Type II Modifications Inscribed by Femtosecond Laser in a Fiber Drawn from a 3D Printed Preform

“…Such polarization dependence is explained by boundary conditions and varying spatial-temporal properties of the ultrashort pulse laser beam. This was quantified with pulse front tilting, leading to anisotropic photosensitivity [21,22]. These results indicate that these 3D printed fibers exhibit analogous photosensitivity compared with SMF28, consistent with similar quality germanosilicate glass.…”

Direct Writing with Tilted-Front Femtosecond Pulses

“…While conceptually straightforward, in practice it is difficult to work in a regime of zero PFT. Any laser that has a built-in chirped pulse amplification design can produce residual spatio-temporal couplings due to slight misalignments 27 , which can lead to strong asymmetries in writing. In order to choose the appropriate laser intensity where PFT is zero, a thorough understanding of the spatio-temporal properties is necessary.…”

“…To correct for them, complicated diffractive element-based designs of SLM methods are necessary. Alongside that, the concept of SSTF in laser writing is to produce stronger modification closer to the focus, which would result in PFT at the focus 27 , 57 . Therefore, it might be advantageous to use PFT as an extra degree of freedom in direct laser writing.…”

Photonic circuits written by femtosecond laser in glass: improved fabrication and recent progress in photonic devices

“…SSTF offers another solution to control the cross-section. 30,[46][47][48][49] This technique relies on separating the spectral components of the fs laser pulse in space before focusing using a grating pair, which induces spatial chirping in the incident pulse and generates an array of beamlets at various frequencies. As different frequency components overlap spatially only around the focus, temporal focusing happens, and this results in the shortest pulse duration with the highest peak intensity.…”