Efficient ultrafast laser writing with elliptical polarization

Abstract

Photosensitivity in nature is commonly associated with stronger light absorption. It is also believed that artificial optical anisotropy to be the strongest when created by light with linear polarization. Contrary to intuition, ultrafast laser direct writing with elliptical polarization in silica glass, while nonlinear absorption is about 2.5 times weaker, results in form birefringence about twice that of linearly polarized light. Moreover, a larger concentration of anisotropic nanopores created by elliptically polarized light pulses is observed. The phenomenon is interpreted in terms of enhanced interaction of circularly polarized light with a network of randomly oriented bonds and hole polarons in silica glass, as well as efficient tunneling ionization produced by circular polarization. Applications to multiplexed optical data storage and birefringence patterning in silica glass are demonstrated.

Ultrafast Laser Writing in Different Types of Silica Glass

“…Recently the observation of increased nanopore formation in silica glass when writing with elliptical polarization has been interpreted in terms of defects with low excitation energy, which have smaller band gap energy, promoting the formation of nanopores in the X-type modification. [42] One example of such defects with absorption at 2 eV is non-bridging oxygen-hole centers (NBO-HCs, ≡Si─O•), which are formed during nonlinear photoionization of silica glass. [43,44] Other common defects with low excitation energy are self-trapped holes (STHs, hole-based polaron), which result in the radiation-induced absorption in a wide spectral region ranging from 0.6-2.6 eV in silica glass.…”

Nanoscale investigations of femtosecond laser induced nanogratings in optical glasses

“…In this process it seems that nanopores may not have the chance to grow from a spherical shape but rather ellipsoid like the nanoplasma distribution itself for a linear polarization whereas a circular polarization would induce substantially spherical nanopores and thus no/low birefringence. 74 Following this view, type X is in fact the early birth of nanograting formation mostly observed for a low number of pulses and low energy. These type X modifications refer to oblate (for linear or elliptical polarization) nanopores with low birefringence and ultralow optical losses.…”

Impact of Glass Free Volume on Femtosecond Laser-Written Nanograting Formation in Silica Glass

“…Modeling studies of bulk nanogratings are usually based on a low dielectric constant seed as an initial "nanovoid" [15,20]. Some studies have suggested point defects like E'; ODC; and, generally, color centers [21], as well as transient defects like self-trapped electrons (STEs) [18,45] and self-trapped holes (STHs) [22] as seeds. In discussing potential precursors, we must consider the necessity of a lower change in the refractive index to facilitate the plasma field enhancement perpendicularly to light polarization.…”

“…An alternative theory [21] emphasizes the role of point defects and color centers as the initial precursors for nanograting formation. These defects are hypothesized to be either intrinsic, pre-existing within the material, or extrinsically induced by the initial laser pulse (e.g., STHs [22]).…”