Probing Ferroic States in Oxide Thin Films Using Optical Second Harmonic Generation

Amorphous, PMMA: DR1: surfactant and PMMA:DR1:TEOS thin films were prepared by dip-coating. All films were calcined at 70 °C for 3 hours. For nanostructured thin films two ionic surfactants were used, Sodium Dodecyl Sulfate (SDS) and Cetyltrimethyl ammonium bromide (CTAB) to obtain two different nanostructures of the PMMA matrix: lamellar, and hexagonal, respectively. X-ray diffraction studies were performed to determine the long-order structure tailored in the films. The measurements of the optical absorption and the second harmonic generation (SHG) intensity were carried out at different orientation arrangements of the chromophores embedded in the films. The chromophore orientation distributions were obtained by means of the corona technique. These distributions depend on the corona poling time. We physically modeled the optical absorption and the second harmonic generation experimental results as function of the corona poling time, employing only one fitting parameter related to the matrix-chromophore interactions. The physical model and the experimental results were in an excellent agreement. The experimental results fitted by the model are shown in plots of order parameter against corona poling time and SHG intensity against corona poling time. The amorphous films provide a larger SHG intensity values than those obtained from the nanostructured films. Thin films with lamellar structure have a SHG intensity bigger than those from hexagonal and PMMA:TEOS thin films.

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Strategic improvement of second and third harmonic generation in multifunctional Cu-Sn-S3 ternary semiconducting thin films

Thin solid film and interfaces analysis with Optical Second Harmonic Generation: application to nanotribology and to the study of the LaAlO3/SrTiO3 interface

Nonlinear optical phenomena such as second harmonic generation (SHG) have been studied in various ferroelectric liquid crystals. SHG efficiency depends on the molecular structure of ferroelectric liquid crystal and can be controlled by applied voltage. Phase matching of SHG has been well realized in these ferroelectric liquid crystals. SHG in polymeric ferroelectric liquid crystals, ferroelectric liquid crystal-conducting polymer composites, thin ferroelectric liquid crystal films and liquid crystals which exhibit flexoelectric effect have also been discussed.

Wide bandgap semiconductors such as gallium nitride (GaN) are essential constituents of future optical circuits, as their optical response can accommodate a broad wavelength range while suppressing two-photon absorption and free-carrier absorption effects that are encountered with silicon (Si) structures. Their direct wide bandgap is also favourable for the incorporation of active elements. Furthermore, nonlinear optical processes can be harnessed by exploiting the higher-order susceptibility tensors of the crystal structure to achieve advanced light control modalities, enabling all-optical processing and the generation of entangled photon states. We will report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Genetic optimization is applied to sweep parameter space for the highest cavity quality factors, and simultaneously accounting for power in-coupling. While there is a clear trade-off theoretically between coupling efficiency and Q-factor for a given cavity design, the upper limit on the Q-factor that is imposed by loss channels, given the disorder figure of current fabrication technology, makes room for introducing improved far-field coupling to enhance nonlinear processes without sacrificing the experimentally achievable light confinement. Far-field coupling is addressed through various PhC cavity designs, which enable the excitation of the fundamental mode with a Gaussian beam. Optimized two-dimensional PhC cavities with increased far-field coupling have been characterized with quality factors as high as 44′000, approaching the computed theoretical values. The strong enhancement in light confinement has enabled second harmonic generation (SHG) under continuous-wave excitation, with up-conversion from both 1300 nm and 1550 nm wavelength bands, confirmed by spectral and power dependence measurements. At 1550 nm, normalized SHG conversion efficiency as large as 2.4×10−3 W−1 are measured as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation

In attempts to fabricate thermally stable second-order nonlinear polymer thin films, we have investigated the second harmonic generation (SHG) from both nonlinear polymer and guest-host thin films. We have also investigated the role of capping on the SHG, temporal stability and relaxation of dipole alignment. Corona poling techniques were employed to orient the dopants into the noncentrosymmetric structure required to obtain the SHG. The effect of capping with a polymeric encapsulant below the glass transition temperature of the polymers on the unpoled and corona poled thin films was studied. Capping of the nonlinear polymer and guest host thin films have resulted in high SHG with good temporal stability. SHG signal falls drastically during the first 8 days after poling while no further significant decay in SHG signal was observed after about 33 days. Our investigations have identified the characteristics required for a good encapsulant on a non-con-ductive surface.

UV irradiation of ultrathin ${\mathrm{SiO}}_{2}$ on Si can lead to injection of both holes and electrons into the oxide. 4.9-eV photons lead to electron injection while 6.7-eV photons open up the hole injection channel. The holes and electrons can trap, generating electric fields of opposite signs. Second harmonic generation (SHG) is sensitive to both the sign and magnitude of the electric field at the buried interface, revealed by opposing electric field induced second harmonic (EFISH) generation contributions. The SHG response of UV-irradiated ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ slowly evolves to the SHG pattern characteristic for unirradiated ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}.$ We attribute the phenomenon to combined dynamics of electrons and holes at the ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interface.

The surface freezing and wetting behaviour at the liquid/vapour interface of Ga-rich alloys of Ga and Pb has been studied as a function of temperature at selected compositions both in the homogeneous liquid and the fluid two phase demixing regime. The interfacial phase transitions have been probed by second harmonic generation (SHG) in combination with plasma generation (PG) measurements at different cooling and heating cycles. In the composition range between the eutectic and monotectic points the occurrence of surface freezing is clearly indicated by distinct changes of the SHG and PG intensities, respectively. It is not observed below the eutectic composition for solution very dilute in Pb. Qualitatively, these results agree with the recent observations made in Ga–Bi alloys. A new wetting characteristic is found crossing the miscibility gap at constant composition with varying temperature. A Pb-rich macroscopic wetting film forms which must be concluded from the SHG intensities. This interfacial phase transition causes a clearly detectable drop of the sample temperature which is explained by the corresponding discontinuous change of the optical emissivity at the interface. These interfacial phenomena related with the miscibility gap have not been reported before.

Integrated nonlinear metasurfaces leading to high-efficiency optical second harmonic generation (SHG) are highly desirable for optical sensing, imaging, and quantum photonic systems. Compared to traditional metal-only metasurfaces, their hybrid counterparts, where a noncentrosymmetric nonlinear photonic material is incorporated in the near-field of a metasurface, can significantly boost SHG efficiency. However, it is difficult to integrate such devices on-chip due to material incompatibilities, thickness scaling challenges, and the narrow band gaps of nonlinear optical materials. Here, we demonstrate significantly enhanced SHG in on-chip integrated metasurfaces by using nanometer thin films of ferroelectric Y:HfO2. This material has the merit of CMOS compatibility, ultraviolet transparency up to 250 nm, and significant scalability down to sub-10 nm when deposited on silicon. We observe a 20-fold magnitude enhancement of the SHG intensity from the hybrid metasurface compared to a bare ferroelectric HfO2 thin film. Moreover, a 3-fold SHG enhancement is observed from the hybrid metasurface compared to a control structure using nonferroelectric HfO2, demonstrating a major contribution to the SHG signal from ferroelectric Y:HfO2. The effective second-order nonlinear optical coefficient χ(2) of Y:HfO2 is determined to be 6.0 ± 0.5 pm/V, which is comparable to other complex nonlinear photonic oxide materials. Our work provides a general pathway to build an efficient on-chip nanophotonic nonlinear light source for SHG using ferroelectric HfO2 thin films.

The guest-host effect of second harmonic generation (SHG) in ferroelectric liquid crystal (FLC) has been studied. It has been found that a marked enhancement of SHG is obtained by means of slight doping with dye such as p-nitroaniline (p-NA) and this effect depends on the molecular structures of both host FLC and guest dye. The magnitude and direction of the second order hyperpolarizability in p-NA doped FLC have been estimated by using the components of nonlinear optical coefficients which are obtained by the theoretical fitting of phase-matched curves of SHG.

Using in vivo second harmonic generation (SHG) and third harmonic generation (THG) microscopies, we tracked the course of collagen remodeling over time in the same melanoma microenvironment within an individual mouse. The corresponding structural and morphological changes were quantitatively analyzed without labeling using an orientation index (OI), the gray level co-occurrence matrix (GLCM) method, and the intensity ratio of THG to SHG (RTHG/SHG). In the early stage of melanoma development, we found that collagen fibers adjacent to a melanoma have increased OI values and SHG intensities. In the late stages, these collagen networks have more directionality and less homogeneity. The corresponding GLCM traces showed oscillation features and the sum of squared fluctuation VarGLCM increased with the tumor sizes. In addition, the THG intensities of the extracellular matrices increased, indicating an enhanced optical inhomogeneity. Multiplying OI, VarGLCM, and RTHG/SHG together, the combinational collagen remodeling (CR) index at 4 weeks post melanoma implantation showed a 400-times higher value than normal ones. These results validate that our quantitative indices of SHG and THG microscopies are sensitive enough to diagnose the collagen remodeling in vivo. We believe these indices have the potential to help the diagnosis of skin cancers in clinical practice.

We have detected second-order nonlinear optical response from an amphoteric mega-saccharides named sacran under a needle/ring electrode stimulus by using an optical second harmonic generation (SHG) microscope. SHG was observed from sacran in the neighborhood of the negatively biased needle electrode below -4.5 V with respect to the outer ring electrode. From the incident light polarization dependence of SHG, this sacran was suggested to be oriented toward the electrode needle. SHG of the sacran polymer was judged to be induced by sacran aggregates with net positive charge.

ZnO single crystal and epitaxial thin film studied by second harmonic generation and photoluminescence

We have found that the resonance energy of optical second-harmonic (SH) generation from Au films on NaCl(100) depends strongly on the film thickness. The resonance peak of the SH intensity was found at $2\ensuremath{\Elzxh}\ensuremath{\omega}=3.2\mathrm{eV}$ for the island film of mean thickness 0.4 nm, while it appeared at $2\ensuremath{\Elzxh}\ensuremath{\omega}=2.4\mathrm{eV}$ for the continuous film of thickness 40 nm. We suggest that the SH intensity peak observed at $2\ensuremath{\Elzxh}\ensuremath{\omega}=3.2\mathrm{eV}$ originates from a resonant transition from occupied to unoccupied electronic levels of the (100) surface on top of the Au islands. The SH intensity peak observed at $2\ensuremath{\Elzxh}\ensuremath{\omega}=2.4\mathrm{eV}$ for the film thickness of 40 nm originates from the roughness-induced linear dielectric property of the film.