Emergence of Structural Anisotropy in Optical Glasses Treated to Support Second Harmonic Generation

Abstract

Structural alterations in v-SiO2 induced by "thermal poling", a treatment which makes the glass able to double the frequency of an impinging infrared light, are revealed by neutron diffraction as a breakdown of the macroscopic isotropy. This leads to concomitant changes in the vibrational density of states measured by inelastic neutron scattering. The observations are found to be consistent with the emergence of partial ordering within the glassy matrix along the direction of an electrostatic field applied during the poling treatment. 42.70.Ce, 42.65.Ky Few discoveries have puzzled the optics community more than the emergence of visible (green) light from optical fibers after strong irradiation by an infrared laser [1]. This frequency-doubling phenomenon known as Second Harmonic Generation (SHG) is not expected to take place in a centrosymmetric material such as the amorphous silica fiber-core, which shows no measurable second-order optical susceptibility χ (2) [2]. The process to be efficient also requires well defined phase-matching between the interacting waves to allow for constructive interference, and this seems even more difficult to fulfill within the glassy medium. Several plausible explanations about the origin of the phenomenon have been put forward. One of the most widely accepted [3] does not involve structural modifications to accomplish the breakdown of the glass radial symmetry. Rather, it postulates the emergence of a spatially modulated local dc field, E 0 , which, via a third-order nonlinearity (χ (3) ) (finite in isotropic materials), induces a spatially modulated second order nonlinearity (χ (2) ∝ χ (3) E 0 ) able to double the pump frequency. The achievement of a permanent χ (2) in optical glasses has focused a large research effort, which lead to the discovery of alternative poling techniques. In actual fact, the phenomenon can be produced by application of a high voltage (∼ 5 kV, just below dielectric breakdown) to glass plates at moderate temperatures (∼ 540 K − 580 K, compared with ≈ 1475 K where the glass melts). This method, known as "thermal" poling [4], provides permanent second-order nonlinear responses comparable to those shown by inorganic crystals. Whether the mechanism(s) leading to the emergence of a second order nonlinearity in "thermally-poled" glasses differ from those of photoinduced SHG or not needs to be clarified. At any rate, the relevant point stems from the possibility this method has opened up for developing inexpensive integrated optical frequency converters and electro-optic modulators.Up to now studies on the microscopic structure and/or dynamical alterations associated with poling are scarce. Significant changes between poled and native fibers at rather well defined frequencies have been reported from Raman studies [5]. Direct interpretation of such Raman data is however hampered by the need of knowledge of the possible structural alterations in order to assess the concomitant variations in the Raman matrix elements governing the signal intensities. Here we ...

Second harmonic generation induced by poling in borophosphate bulk and thin film glasses

“…At the same time Alley et al [11] interpreted the proton conduction observed during poling, by a decrease of hydroxyl Si-OH bonds associated with an increase of non-bridging oxygen (Si-O À ). Simultaneously, an increase of less tensed Si-O-Si groups is observed perpendicularly to the poled surfaces which is in agreement with the results given by Cabrillo et al, illustrating the structural anisotropic modifications produced by the poling [30].…”

Thermally poled silica samples are structurally heterogeneous: Electron diffraction evidence of partial crystallization

“…Although they appear randomly oriented in our measurements, the absolute orientation is lost during the abrasion process. The anisotropy previously observed by neutron diffraction 4 lends support to the possibility of a preferential orientation. However, the fact that the nonlinearity is mostly located a few microns below the anode surface makes it unlikely that the intrinsic (2) of the nanocrystals is the sole cause of the effect.…”

“…The estimate of the density of ionizable defects required to develop such a spacecharge is a mere few parts per million. Recently, neutron diffraction and inelastic scattering 4 have shown that the glass structure of ''poled'' silica plates as described by the g(r) radial distribution functions, becomes anisotropic and significantly differs from that of native samples. From data on hand at the time, crystallization of part of the glassy matrix into a cristobalite polymorph of silica appeared as a reasonable possibility.…”

X-ray absorption and infrared reflectance of poled silica glass for second harmonic generation

“…2 In most of the reported experiments on SHG in fused silica, the signal was regarded as essentially coming from a thin layer of a few microns depth near the anode surface. [1][2][3][4][5] However, according to Le Calvez, Freysz, and Ducasse, SH signal was also induced in the cathode surface area. 3 At the same time, the authors did not observe modulations in the Maker fringe pattern due to the interference of the SH signals generated from both surfaces of the poled glass sample.…”

“…4 They concluded to large microscopic alterations in the anodic poled region ͑about 5-m-deep below the anode surface͒ along the direction of the poling field, involving at least next nearest Si neighbors. They effectively evidenced an elongation of the shortest Si-Si distance from 3.06 to 3.23 Å in this direction and concluded to the opening of Si-O-Si bond angles in the same direction.…”