Thin film Bragg deflection gratings for integrated optics

Two types of single mode thin film Bragg deflectors were investigated : periodic index waveguides (with the guiding layers posessing parallel boundaries) by diffusion through a metallic grating mask; and periodic thickness (corrugated) waveguides by forming photoresist gratings on top of the waveguides (four-layered devices), or with the relief patterns ion-beam etched (using argon ions) directly onto the surfaces of the guides (three-layered devices). The periodic index waveguides were fabricated by silver-sodium ion exchange in glass substrates, and proton-exchange or titanium indiffusion in LiNbO3 substrates. The metallic grating masks (periodicity = 3mum) on the surfaces of the guides prior to diffusion were formed by conventional photolithographic technique, followed by vacuum deposition of the metal and liftoff. Sputtered homogeneous 7059 glass waveguides on standard Fisher microscope slides were used in the manufacture of corrugated waveguides. Submicron gratings (periodicity = 0.3mum) were fabricated by holographic exposure of the photoresist-coated samples. General waveguide theory of homogeneous and inhomogeneous dielectric slab waveguides is presented, using both geometrical (or ray) optics and electromagnetic theory. Properties of periodic waveguides are also described, and the Bragg deflectors fabricated are analysed using one-dimensional coupled mode formalism and two-dimensional coupled mode formalism. The theory predicts two unique properties of oblique incidence not found in normal incidence: mode conversion (TE/TM) and variation of power across the width of the diffracted and transmitted beam. The fabrication techniques for both types of deflectors, including the three diffusion processes and the interferometric technique for producing submicron gratings, are discussed in some details. The waveguide and grating parameters were measured using standard techniques, and the grating groove depths were deduced from diffraction efficiency measurements. The periodicity is determined from the measurement of the autocollimation angle of the first diffracted order. The Bragg devices were tested using a visible HeNe laser. For periodic index waveguides, the experimental results are in good agreement with theoretical calculations using onedimensional coupled mode formalism. However, mode conversion (TE-TM), a unique property in oblique incidence of guided wave onto a grating element, was not observed because of the long periodicity (small Bragg angle) of the devices. Furthermore, a two-step diffusion process, using the first diffusion to form the waveguide and a second diffusion to produce the grating structure, was also demonstrated. This gives further flexibility to the diffusion technique, and the parameters in the first and second diffusion steps can be chosen independently for optimised performance. For the corrugated waveguides, the experimental results agree better with calculations using the two-dimensional coupled mode formalism. The two unique properties of oblique incidence: mode conversion (TE-TM) and non-uniform intensity distribution across the width of both the diffracted and transmitted beams, were observed. It is confirmed experimentally that for right angle deflection, the TE-TE coupling is zero. When the diffraction efficiency is high, the diffracted beam 'breaks up'. The results from this investigation suggest that the periodic index waveguides fabricated have distinct advantages over the corrugated counterparts in terms of lower scattering loss, ease of fabrication and mechanical ruggedness. The two unique properties-of oblique incidence may pose a problem-in certain applications, such as beam expanders and multiplexer/demulplexer. 'Breaking up' of the beams and crosstalk introduced by mode conversion must be taken into account when designing these devices

Symmetry selective third harmonic generation from plasmonic metacrystals

Nonlinear processes are often governed by selection rules imposed by the symmetries of the molecular configurations. The most well-known examples include the role of mirror symmetry breaking for the generation of even harmonics, and the selection rule related to the rotation symmetry in harmonic generation for fundamental beams with circular polarizations. While the role of mirror symmetry breaking in second harmonic generation has been extensively studied in plasmonic systems, the investigation on selection rules pertaining to circular polarization states of harmonic generation has been limited to crystals, i.e. symmetries at the atomic level. Here we demonstrate the rotational symmetry dependent third harmonic generation from nonlinear plasmonic metacrystals. We show that the selection rule can be imposed by the rotational symmetry of meta-crystals embedded into an isotropic organic nonlinear thin film. The results presented here may open new avenues for designing symmetry-dependent nonlinear optical responses with tailored plasmonic nanostructures.Comment: 13 pages, 3 figure

Composition determination of off-congruent Li-deficient MgO (5 mol%)-doped LiNbO

Off-congruent Li-deficient MgO:LiNbO3 crystals were prepared by carrying out post-grown Li-poor vapor transport equilibration (VTE) treatments on a number of 0.47 mm thick MgO (5 mol% in growth melt or 6 mol% in crystal)-doped, initially congruent LiNbO3 plates at 1100 °C over different durations ranged in 40–395 h. At first, the VTE-induced Li composition reduction was measured as a function of the VTE duration using the gravimetric method. Then, optical absorption spectroscopy was applied to study the crystal composition effects on the fundamental optical absorption edge and OH absorption characteristic parameters including the peaking position, band width, peaking absorption and band area. These crystal composition effects enable one to establish the optical methods used for determination of the crystal composition from the spectroscopic measurements. These optical methods overcome the demerit that the gravimetric method is limited to a specific VTE temperature or crystal thickness, and can be applied to design and produce an MgO-doped crystal with desired Li composition

Heterojunction Photocatalyst Loaded on Electrospun Nanofibers for Synergistic Enhanced Photocatalysis and Real-Time Temperature Monitoring

Bi2WO6:Ho3+, Yb3+/g-C3N4 (BHY/CN) photocatalysts are successfully loaded on polyacrylonitrile (PAN) nanofibers by electrospinning technology, which combines an upconversion effect and heterojunctions to achieve dual-functional characteristics. Polymer-modified photocatalytic materials offer a large specific surface area of 24.1 m2/g and a pore volume of 0.1 cm3/g, promoting the utility of solar energy. The introduction of rare earth ions and g-C3N4 optimizes the structural band gap, which broadens the light absorption range and promotes electron transfer. Moreover, the heterojunction between Bi2WO6 and g-C3N4 has suppressed the complexation of photoinduced carriers, further improving catalytic performance. The optimized photocatalysts have higher photocatalytic activity with degrading 92.6% tetracycline-hydrochloride (120 min) under simulated sunlight irradiation. The optical thermometry has also been achieved based on the fluorescence intensity ratio technique, where the maximum absolute and relative sensitivity values of BHY/CN-1:6@PAN are 3.322% K–1 and 0.842% K–1, respectively. This dual-functional nanofibers with excellent mechanical properties provide noncontact temperature feedback and efficient catalytic performance for better wastewater treatment and ecological restoration in extreme harsh environments