First-order Bragg Diffraction Research Articles

Holographic fabrication of azo-dye-functionalized photonic structures

We have developed an azo-dye-doped acrylate-based material formulation for holographic lithography. Volume phase gratings and two-dimensional photonic crystals were demonstrated by recording the interference patterns of two and three coherent beams in this material formulation. The dependence of the first-order Bragg diffraction efficiency of the gratings on the exposure intensity of the writing light was investigated. We have achieved a volume phase grating with a diffraction efficiency of 72%. These azo-dye-functionalized photonic structures showed an excellent all-optical switching behaviour, which was attributed to the imaginary refractive index changes caused by the trans–cisisomerization-induced absorption changes of the azo-dyes. Such photonic structures are potentially useful in the future development of all-optical devices.

Lattice-parameter effects on diffracted transmission of GaN two-dimensional square-lattice photonic crystals

The lattice-parameter effects on the diffracted transmission of GaN square-lattice photonic crystals (2PhC) at the wavelength of 460 nm were studied by using a rigorous coupled wave analysis (RCWA). The impacts of lattice parameters on the diffracted transmission are calculated in the ranges for lattice pitch from 100 nm to 2000 nm, fill factor from 0.1 to 0.9 and grating height from 100 nm to 1000 nm, respectively. Our simulation results confirm that the lattice pitch is the dominant factor of the diffraction. It determines how many orders of diffraction occur by the 2PhCs. The larger the lattice pitch, the higher the diffraction orders come into play. Moreover, besides the first-order Bragg diffraction, higher diffraction orders from large pitches of PhCs are also beneficial to the light extraction improvement. The higher enhancement factors of the integrated transmission were obtained from a wide range of pitches with micro-scale GaN 2PhCs. Three different diffraction mechanisms through wave vector analysis were used to discuss the simulation results.

Diffracted transmission effects of GaN and polymer two-dimensional square-lattice photonic crystals

The effects of the lattice pitch of GaN and polymer square-lattice photonic crystal (2PhC) on the diffracted transmission were studied by using rigorous coupled wave analysis (RCWA). Besides the first-order Bragg diffraction, higher-order diffractions from large pitches of PhC are also significant to the light extraction improvement. Three different diffraction mechanisms are illustrated through wave vector analysis. The enhancement factors of integrated transmission are obtained from a wide range of pitches with micro-scale for both GaN and polymer 2PhC. The experimental angular-resolved transmission on a transparent polymer sample of 2 microm pitch 2PhC is comparable to the simulation.

Engineering a light-emitting planar defect within three-dimensional photonic crystals

Sandwich structures, constructed from a planar defect of rhodamine-B (RhB)-doped titania (TiO2) and two photonic crystals, were synthesized via the self-assembly method combined with spin-coating. The modification of the spontaneous emission of RhB molecules in such structures was investigated experimentally. The spontaneous emission of RhB-doped TiO2 film with photonic crystals was reduced by a factor of 5.5 over a large bandwidth of 13% of the first-order Bragg diffraction frequency when compared with that of RhB-doped TiO2 film without photonic crystals. The angular dependence of the modification and the photoluminescence lifetime of RhB molecules demonstrate that the strong and wide suppression of the spontaneous emission of the RhB molecules is due to the presence of the photonic band gap.

Increasing diffraction efficiency by heating phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate)

The diffraction efficiency of phase gratings formed by femtosecond laser irradiation in poly(methyl methacrylate) was increased by more than an order of magnitude by subsequent heating below the glass transition temperature (Tg). The first-order Bragg diffraction efficiency of a 10 μm period grating was 1.9% without heating, whereas it increased to 72% when heated at 70 °C for 500 h. This is because the induced refractive index changes (Δn) were increased by heating. From the Lorentz–Lorenz equation, one of the reasons why Δn increases by heating could be a slight volume contraction in the irradiated area.

Diffraction gratings inside bulk azodye-doped hybrid inorganic–organic materials by a femtosecond laser

We have investigated diffraction gratings fabricated inside bulk azodye-doped hybrid inorganic–organic materials by a focused near-IR 800 nm femtosecond laser directly. The first-order diffraction of the grating was measured using a 632.8 nm He–Ne laser. By changing the laser parameters such as the laser power, the scanning speed, and the grating period, we found that the first-order Bragg diffraction efficiency was strongly dependent on the parameters of the femtosecond laser. The results showed that the first-order Bragg diffraction efficiency can be increased when decreasing the laser power or increasing the grating periods and the scanning speed of the laser. The mechanisms were also analyzed briefly.

Holographic volume gratings in bulk perylene-orange-doped hybrid inorganic-organic materials by the coherent field of a femtosecond laser

Holographic volume gratings with high first-order Bragg diffraction efficiency (greater than 35%) were fabricated in bulk laser-dye-doped hybrid inorganic-organic materials by the coherent fields of a femtosecond laser. Observations of optical microscopy show that refractive-index-modulated volume gratings were realized inside the sample through multiphoton absorption process. The isomerization and alignment of the laser dye molecules are responsible for the grating formation. We suggest that the materials codoped with laser dye and azodye and with photoinduced gratings inside are promising materials for making the distributed feedback tunable lasers.

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams.