Narrow Photonic Band Gap Research Articles

Aperiodic Bragg Reflectors for Tunable High-Purity Structural Color Based on Phase Change Material.

Tunable thin-film coating-based reflective color displays have versatile applications including image sensors, camouflage devices, spatial light modulators, and intelligent windows. However, generating high-purity colors using such coatings have posed a challenge. Here, we reveal high-purity color generation using an ultralow-loss phase change material (Sb2S3)-based tunable aperiodic distributed Bragg reflector (A-DBR). By strategically adjusting the periodicity of the adjacent layers of A-DBRs, we realize a narrow photonic bandgap with high reflectivity to generate high-purity orange and yellow colors. In particular, we demonstrate an A-DBR with a large photonic bandgap tunability by changing the structural phase of Sb2S3 layers from amorphous to crystalline. Moreover, we experimentally tailor multistate tunable colors through external optical stimuli. Unlike conventional nano thin-film coatings, our proposed approach offers an irradiance-free, narrowband, and highly reflective color band, achieving exceptional color purity by effectively suppressing reflections in off-color bands.

Designing the Structure of a One-Dimensional Photonic Crystal with a Given Spectrum of the Reflection Coefficient

A method for solving the inverse problem of designing the structure of a one-dimensional photonic crystal is proposed and experimentally implemented. It is known that a one-dimensional photonic crystal with a spatial sinusoidal modulation of the refractive index, has a narrow photonic bandgap at a frequency related to the spatial frequency of this sinusoid. A reverse engineering method is proposed for one-dimensional photonic crystals with an arbitrary given reflection spectrum by expanding this spectrum into elementary photonic band gaps and then summing them. The application of this method to fabricate examples of photonic crystals with simple shapes of spectral reflection curves is demonstrated.

Designing the Structure of a One-Dimensional Photonic Crystal with a Given Spectrum of the Reflection Coefficient

A method for solving the inverse problem of designing the structure of a one-dimensional photonic crystal is proposed and experimentally implemented. It is known that a one-dimensional photonic crystal with a spatial sinusoidal modulation of the refractive index, has a narrow photonic bandgap at a frequency related to the spatial frequency of this sinusoid. A reverse engineering method is proposed for one-dimensional photonic crystals with an arbitrary given reflection spectrum by expanding this spectrum into elementary photonic band gaps and then summing them. The application of this method to fabricate examples of photonic crystals with simple shapes of spectral reflection curves is demonstrated.

Mode Dispersion in Photonic Crystal Organic Light-Emitting Diodes

Similar to an electronic lattice determining the motion of electrons in solids, photonic crystals (PhCs) are periodic photonic nanostructures that determine the propagation of photons. By incorporating PhCs into organic light-emitting diodes (OLEDs), the device efficiency and emission spectra can be modified, which can be explained and predicted by the mode dispersion. In this work, we experimentally measure the mode dispersion of 1-D and 2-D PhC OLEDs at different azimuthal angles with angle-resolved electroluminescence spectra. The results are explained using an intuitive geometry approach, which shifts and slices the cone-shaped optical modes to obtain the mode dispersion of PhC OLEDs. We note that the weak cavity mode and a narrow photonic band gap are visible only after eliminating the intrinsic emitter spectrum in the air mode dispersion. In the end, we discuss the implication of mode dispersion on the OLED light extraction.

Photonic Shape Memory Polymer Based on Liquid Crystalline Blue Phase Films.

Photonic shape memory (SM) polymers based on liquid crystalline blue phase (BP) films have been fabricated by self-assembly and subsequent photopolymerization of liquid-crystal mixtures. These freestanding BP films exhibit narrow photonic band gaps and high reflectivity in the visible wavelength range. Multiple blue-shift colors are achieved by SM programming process at different mechanical pressures. The blue-shift colors can be attributed to a decrease of effective BP pitch along the viewing direction caused by the compressed deformation of the BP films, which are confirmed by a three-dimensional interometric profile. The deformed BP films can recover to their original shapes and reflecting colors by heating the polymer films to temperatures above the glass-transition temperature. Quantitative relationships between the shape change and optical response are established for understanding this SM effect. What is more, the temporary photonic patterns can be reversibly written and erased for dozens of cycles without apparent degradation, making these freestanding BP films appealing as rewritable photonic papers and optical sensors.

Anodization fabrication of 3D TiO2 photonic crystals and their application for chemical sensors

Three-dimensional (3D) TiO2 photonic crystals (PCs) with multilayers films were prepared using a periodic pulse anodic oxidation method. Unlike the typical morphologies of bamboo-like tubes with the reinforced compact parts at the interface regions and also tube walls under constant anodization processes. The periodically concave structure was found outside of the TiO2 nanotubes under multipulsed anodization voltage condition, so that the periodic layered structure with different porosity was obtained along the longitudinal direction of tubes. The thickness of the tube and the size of pit could be tunable and controllable by modifying the duration of the high voltage and low voltage. The PCs have uniform narrow photonic band gap (PBG), steep band edge, which were applied successfully in chemical sensors. The formation mechanism of periodic structure was also discussed.

Broadening of effective photonic band gaps in biological chiral structures: From intrinsic narrow band gaps to broad band reflection spectra

Under normal illumination with non-polarized light, reflection spectra of the cuticle of golden-like and red Chrysina aurigans scarabs show a structured broad band of left-handed circularly polarized light. The polarization of the reflected light is attributed to a Bouligand-type left-handed chiral structure found through the scarab's cuticle. By considering these twisted structures as one-dimensional photonic crystals, a novel approach is developed from the dispersion relation of circularly polarized electromagnetic waves traveling through chiral media, to show how the broad band characterizing these spectra arises from an intrinsic narrow photonic band gap whose spectral position moves through visible and near-infrared wavelengths.

Preparation of narrow photonic bandgaps located in the near infrared region and their applications in ethanol gas sensing

In this work, we demonstrated the use of a promising sensor based on alumina Photonic Crystals (PC) which can be applied in the field of gas detection owing to the well structured PC with an ultra-narrow Photonic Bandgap (PBG), of which the full width at half maximum is only 30 nm. This type of PC was prepared successfully by using a compensation voltage mode method, which can effectively compensate for the chemical corrosion and the slowing down of ion exchange, resulting in a uniform diameter of the pores in the anodic alumina oxidation (AAO) from beginning to end. The as-prepared sample was applied to detect the concentration of organic vapor, such as ethanol, in this experiment. A large PBG shift is achieved when the concentration of ethanol changes, and the mechanism was also discussed theoretically. This sensor had a good repeatability due to the chemical and physical stability of the matrix material (alumina). These results indicate the potential applications of the as-prepared AAO based PC in the sensing field.

Controllable preparation of the ordered pore arrays anodic alumina with high-quality photonic band gaps

High-quality photonic crystals (PCs) based on the ordered pore arrays anodic alumina are fabricated by the anodic oxidation method using the newly-designed periodic oxidation voltage wave. The obtained PCs own uniform narrow photonic band gaps (PBGs), steep photonic band edges, zero transmittances inside of the PBGs and high transmittances outside of the PBGs. Importantly, the high-quality PBGs of the anodic alumina PCs are tunable and controllable in the range from 350 nm to 1330 nm by modifying the anodizing voltage waveform. The benefit of newly-designed periodic oxidation voltage wave is to improve the optical properties of anodic alumina PCs.

Infrared dielectric mirrors based on thin film multilayers of polystyrene and polyvinylpyrrolidone

Thin film polymer multilayers were prepared by spin coating alternating layers of polystyrene and polyvinylpyrroli- done. Samples with 10, 20, 30, 40, and 50 layers were prepared with individual layer thickness values in the range 223-508 nm. These samples were measured using a Fourier transform infrared spectrometer and were found to display narrow photonic band gaps (∼0.04 to 0.06m wide) in their spectral response over the wavelength range 1.6 − 2.6m. The position of the photonic band gaps was controlled by varying the thickness of the individual lay- ers within the multilayer structures. This was achieved by varying the spin speed used during the deposition of the polymer layers. The peak reflectance of the multilayers was controlled by vary- ing the number of layers within the multilayer samples giving values in the range 20-80% (corresponding to transmittances of 80-20%). Calculated transmittance spectra were also obtained using an optical transfer matrix method. These calculated spec- tra were shown to be in good agreement with the experimental data obtained. These experiments demonstrate a facile approach to the production of low cost dielectric mirrors that have tai- lored photonic properties over a range of wavelengths that are currently important for applications in fibre optic based telecom- munications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 732-739, 2011

Decay lifetime and evolution of semiconductor quantum dots in photonic crystals

Spontaneous emission from GaAs/AlGaAs quantum dots (QDs) embedded in photonic crystals with a narrow photonic band gap is studied theoretically. The results show that the decay lifetime is very sensitive to the sizes of QDs, and both inhibited and accelerated emission can occur, which had been indicated in a previous experiment. The Weisskopf–Wigner approximation, good for atoms and molecules, may be incorrect for QDs. A damped Rabi oscillation of the excited state with the transition frequency outside the photonic band gap may appear, which is impossible for atoms and molecules.

Dependence of second-harmonic generation in ferroelectric liquid crystals with a defect on the optical gap

Second-harmonic generation (SHG) in ferroelectric liquid crystal with a twist defect is studied by de Vries' solution. It is shown that the width of the photonic band gap for the SH wave is an important factor to determine the saturation length. SHG is effectively enhanced by using narrow photonic band gaps.