The local inscription of strong birefringence by ultrashort laser pulses facilitates the fabrication of manifold photonic devices, such as data storage devices. One intriguing feature of these nanograting-based data units is to delete and rewrite new nanograting voxels by changing the laser polarization orientation during inscription. However, up to now no comprehensive picture of this complex physical process exists. Thus we performed optical retardance measurements as well as microscopic analyses, such as small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) to address this issue.<p> </p>Our results reveal that only few laser pulses already lead to an erasure of nanometric pores which is mapped by the total (X-ray) scattering volume as well as by the strong reduction of the initial form birefringence. Simultaneously, new nanostructures form which arrange in individual grating planes with ongoing irradiation. However, since the rewrite process is no ideal mechanism some of the old sheets remain, which perturb the quality of the new nanograting. When rewriting multiple times the glass becomes even more porous due to repetitive annealing and quenching. This promotes the formation of new inhomogeneities and in turn leads to an increase in optical retardance.
The versatility of ultrashort laser pulses as a tool for laser materials processing has augmented particular interest in the
past decade. Especially birefringent modifications, so-called nanogratings, have found to exhibit tremendous potential
for manifold photonic functionalities. These self-assembling structures, orienting always perpendicular to the laser
polarization, have been up to now extensively studied in bulk fused silica. Commonly it is assumed that the formation of
nanogratings is actually limited to anomalous glasses like silica or slightly doped silica. However, we recently found that
even in glasses like borosilicate or BK7 nanogratings can be observed within certain parameter regimes.
Here we present an extensive study of the fundamental constituents of nanogratings in bulk borosilicate glass using small
angle X-ray scattering (SAXS) in combination with focused ion beam milling (FIB) and scanning electron microscopy
(SEM). The irradiation produces void-like sheets (10-20 nm wide) as well as elongated cracks of up to 400 nm. In
contrast to nanogratings in fused silica, borosilicate shows a significant smaller optical retardance. The cumulative action
of several hundreds of laser pulses lead to the formation of individual grating planes with a period of about 60 nm (at an
inscribing laser wavelength of 800 nm) while the well-known λ/2n (n-refractive index) period is prevented. This has
never been observed for ultrashort pulse induced nanogratings so far.
Within recent years the phenomena of so-called nanogratings induced by tightly focussed femtosecond laser pulses has gained significant interest. These self-organized structures appearing after several laser pulses show strong formbirefringence which allows, when combining with the three-dimensional freedom of the direct laser writing technique, to fabricate versatile functionalities. However, the underlying structure has been the subject of intensive debate since the discovery of the nanogratings ten years ago. In order to uncover the primary constituents of nanogratings typical visualisation techniques (e.g. SEM) rely on cleaving and subsequent etching of laser treated samples. Fine details are effectively erased by such invasive preparation methods. Recent investigations based on exclusively cleaved samples report on hollow cracks embedded within the bulk material. However, these time-consuming imaging methods only provide two-dimensional cross sections and can hardly address the evolution of cracks (size, shape) depending on various laser parameters. To overcome these limitations we performed a comprehensive study of nanopores and cracks using small-angle x-ray scattering (SAXS) in combination with focussed ion beam milling (FIB) and scanning electron microscopy (SEM). By probing nanogratings inscribed in the bulk of fused silica we found nanopores with dimensions of (30x25x75)nm<sup>3</sup> and (280x25x380)nm<sup>3</sup>. While the dimensions remain constant with ongoing laser exposure and different pulse energies the nanopore shape changes from cuboidal cracks to ellipsoidal.
Sub-wavelength structures are a crucial ingredient for modern optics. A class of ultrashort laser pulse induced, selforganized modifications in bulk transparent materials have attracted particular interest in recent years. Despite the multitude of potential applications of these so-called “nanogratings”, their underlying structure on the nanometer scale has been the subject of intensive debate throughout the decade since their discovery: Are they merely continuous modulation patterns of the material density, or do they consist of a substructure of hollow cavities? As nanogratings are embedded within the bulk material the conventional visualization technique relies on polishing and subsequent etching to excavate the modifications. However, such invasive sample preparation effectively erases sub-100 nm features. Moreover, they only provide access to two-dimensional cross sections. To overcome these limitations, we employed small angle X-ray scattering (SAXS), focused ion beam (FIB) milling and scanning electron microscopy (SEM) to reveal the underlying three-dimensional structure of nanogratings. Our results show that small cavities are the primary constituents of the nanogratings. These cavities grow predominantly during the first 100 laser pulses and reach a final size of about 30x200x300 nm<sup>3</sup>. Prolonged exposure to laser pulses increases the absolute number of cavities. Their threedimensional arrangement forms characteristic periodic planes of nanogratings.
By the use of stroboscopic laser pump - x-ray probe techniques and x-ray scanning techniques the structural relaxations of gold nanoparticles have been resolved on the 50 ps time scale. The structural dynamics are addressed by several methods including power scattering and small angle scattering (SAXS) to resolve microscopic and mesoscopic length scales of the composite system. The laser power is a direct measure of the dissipated heat. Thus the caloric reaction and melting transition can be monitored as function of temperature, particle size and time. Nonlinear effects are observed with femtosecond excitation, attributed to ablation. While the phenomenology for nanoparticle suspensions and surface supported monolayers display similar energetics, structure formation processes are strongly altered on the surface due to interparticle interactions.
Zone folded coherent acoustic phonons were generated in a multilayered GaSb/InAs epitaxial heterostructure via rapid heating by femtosecond laser pulses. These phonons were probed by means of ultrafast x-ray diffraction. Phonons both from the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the absorption index due to photogenerated free carriers.