High Reflection Range Research Articles

Optoelectronic Properties of In0.87Ga0.13As0.25P0.75(001)β2(2×4) Surface: A First-Principles Study.

InGaAsP photocathode surface affects the absorption, transport and escape of photons, and has a great influence on quantum efficiency. In order to study InGaAsP photocathode surface, the electronic structure, work function, formation energy, Mulliken population and optical properties of In0.87Ga0.13As0.25P0.75(001)β2(2×4) reconstruction surface were calculated from first principles. Results show that stabilized the In0.87Ga0.13As0.25P0.75(001)β2(2×4) surface is conducive to the escape of low-energy photoelectrons. The narrow bandgap and emerging energy levels of the reconstruction surface make the electron transition easier. Under the action of the dipole moment, the electrons transfer from inner layers to the surface during the surface formation process. By contrast to the bulk, the surface absorption coefficient and reflectivity considerably decrease, and the high-reflection range becomes narrower as the falling edge redshifts. On the contrary, the surface transmissivity increases, which is conducive for the photons passing through the surface into the bulk to excite more photoelectrons. Meanwhile, the higher absorption coefficient of surface in low-energy side is favorable for long-wave absorption. The dielectric function peaks of the surface move toward the low-energy side and peak values decrease.

Photonic crystal films with high reflectance based on mesoporous silica in the extreme ultraviolet range

In the present work, mesoporous silica films with hexagonally packed perpendicular nanochannels treated as a 2D photonic crystal in extreme ultraviolet (EUV) range are explored by combining theories and experiments. The simulation results show that this film structure exhibits high reflectivities in a wide band range from 5 nm to 11 nm and at broad incidences angle range. The better reflective properties can be ascribed to its excellent photonic crystal structure corresponding to short spectral region. Experimentally, highly ordered mesoporous silica films with vertical mesochannels were fabricated by electrochemically assisted self-assembly method. Moreover, the reflectances of the fabricated film at 5–11 nm region were also tested using synchrotron radiation, which can reach up to 80.4% at 2°from grazing incidence direction. Meanwhile, a relative high reflectance can be maintained at 5–11 nm at incidence angles of 2–7°. This research results offer a new strategy to developing a promising extreme ultraviolet optical film with high reflectance and broad band range.

Theoretical and experimental study of Fermi Bragg reflector as application for thin silicon solar cells

The conception of efficient Bragg reflectors (BRs) based on dielectric layers is needed to improve the light trapping in thin film epitaxial silicon solar cells. Porous silicon multilayer structures (PSMS) used as BR show its potential to enhance optical confinement of low energy photons. In the present work, a novel design of PSMS will be studied in order to extend the reflectivity to long wavelength by adjusting BR thicknesses according to the Fermi profile law. Such new design named Fermi Bragg reflector (FBR) includes different configurations. Several FBR parameters are used in the simulation study, which is carried out by transfer matrix method. The optimal structures were realized using electrochemical etching dissolution of P+ type silicon substrate in hydrofluoric acid solution (HF) at two temperature conditions: room temperature (20 °C) and −20 °C. The measured optical response of FBR shows an enlargement in the width of high reflection range in comparing to the conventional BR. More than 80% of the photons were reflected in the spectral range 636–1155 nm for the FBR performed at ambient temperature and in the range 750–1353 nm to that carried out at −20 °C. This result proves that the FBR formed on crystalline silicon could enhance the light trapping in thin film silicon solar cells.

The enlargement of high reflectance range in ultra-narrow bandpass filter with disordered one-dimensional photonic crystal

The conventional ultra-narrow bandpass filter structure has only a very limited width of high-reflectance range. This study, by introducing disorder into one-dimensional (1D) photonic crystal, attempts to enlarge the width of high-reflectance range while keeping the ultra-narrow bandpass. Enlargement by 46.8% was obtained after theoretical design. Since this structure contains some degree of disorder already, it has a strong tolerance of the variation of layer thicknesses. Unlike studies using conventional periodic structures, theoretical statistical results in this study demonstrate that high quality remains even after allowing for ±5% variation of layer thicknesses. This indicates that only a very low thickness control precision is required in the future and the production difficulty is immensely lowered. To put the construction to test, a structure has been developed and demonstrated by a magnetron reactive sputtering coating system, which agrees with the theoretical result very well. By introducing disorder into the periodic 1D photonic crystal structure, the high-reflectance range is significantly extended by 37%, with an ultra-narrow pass band of 0.8 nm and intensity of 82%.

Semitransparent Polymer Solar Cells with 5% Power Conversion Efficiency Using Photonic Crystal Reflector

Efficient semitransparent polymer solar cells (ST-PSCs) have been fabricated with one-dimensional photonic crystals (1DPCs) as a high reflector. The 1DPCs are composed of several pairs of WO3 (65.8 nm)/LiF (95.5 nm). By optimizing the pairs of WO3/LiF, 1DPCs can reflect the light back into the ST-PSCs due to the photonic band gap, when the high reflectance range of 1DPCs is matched with absorption spectrum of the active layer. ST-PSCs with 8 pairs of 1DPC exhibit an attractive performance. The short-circuit current density (Jsc) and power conversion efficiency (PCE), respectively, reach to 9.76 mA/cm(2) and 5.16% compared to 8.12 mA/cm(2) and 4.24% of the reference ST-PSCs without 1DPCs. A maximum enhancement of 20.2% in Jsc is obtained and the PCE increases by ~21.7%. This approach provides a simple, fascinating and promising method to realize the highly efficient ST-PSCs toward applications.

Highly efficient and high transmittance semitransparent polymer solar cells with one-dimensional photonic crystals as distributed Bragg reflectors

We demonstrate that one-dimensional photonic crystals as distributed Bragg reflectors can effectively improve the performance of semitransparent polymer solar cells (PSCs) based on the blend of P3HT:ICBA. The one dimensional distributed Bragg reflectors (1D DBRs) are composed of N pairs of WO3/LiF which are thermally evaporated on Ag anode. Due to its photonic bandgap, 1D DBRs can reflect the light totally back into the PSCs when the high reflectance range of 1D DBRs is well matched with absorption spectrum of the active layer. A maximum power conversion efficiency (PCE) of 4.12%, a highest transmittance of 80.4% at 660nm and an average transmittance of 55.6% in the wavelength range of 600–800nm are obtained in the case of N=8, corresponding enhancement of 24.1% in PCE when compared with the device without the 1D DBRs.

Light scattering losses of high reflection dielectric multilayer optical devices

Light scattering losses from dielectric multilayer are becoming increasingly important for designing high precision performance optical devices. In this paper, we applied the bi-directional reflectance distribution function of optical multilayer and analyzed the total reflectance scattering losses based on both the completely correlated and non-correlated interface models to compare with a high reflection 17-layer optical multilayer deposited on roughness of 2.8 nm substrates. The experimental result supports the completely correlated interface model as firstly the wavelength dependence is in good agreement with the phase change of the calculated result and secondly the calculated scattering intensity of the completely correlated interface scattering model is approximately the same as that of the measured scattering spectrum, while the intensity using the non-correlated interface scattering model is significantly higher than the measured result in the high transmission ranges and lower in the main band of the high reflection range.

DISORDERED ONE-DIMENSIONAL METALLIC-DIELECTRIC PHOTONIC CRYSTAL REFLECTOR

The reflected properties of one-dimensional frequency-dependent metallic-dielectric photonic crystals are investigated when disorders are introduced for the first time. It is demonstrated that disordered metallic-dielectric photonic crystal provides remarkably high reflection range compared with the corresponding period metallic-dielectric one when the degree of disorder is moderately chosen, and a wider stop band will be obtained with the increasing of periods. At last, the reflected properties influenced by incident angle for different polarizations are also calculated and discussed.

Enhancement of omnidirectional high-reflection wavelength range in one-dimensional ternary periodic structures: a comparative study

A comparative study of enhancement of omnidirectional high-reflection wavelength range in one-dimensional ternary structured photonic crystals (PCs) was carried out. The PCs investigated are of the dielectric-birefringent type and the dielectric-metallic type. These were compared with a 1D all-dielectric ternary PC investigated earlier by us. The dielectric-birefringent PC gives a larger range enhancement as compared to the all-dielectric one, together with a reduction in the number of periods of the structure. Maximum range enhancement is obtained in the metallodielectric ternary PC, with a further reduction in size, as only six layers need to be deposited in this case. This implies easier fabrication methods and lower costs.

Estimation method of high-reflecting region of periodic layered media

A concise method of estimating high-reflection range for layered media composed of finite periodic unit is presented. Based on the Floquet theorem, the photonic bandgap properties of periodic layered media are analyzed, and the wavelength range of high-reflection region for periodic layered media is discussed. The relation between high-reflection region of layered media and forbidden band of the periodic unit is discussed. Numerical results show that the center wavelength of high-reflection region coincides with the one of the forbiddan band of periodic unit. Furthermore, as the periodic unit number of layered media is increased, the depth and width of high-reflection region become close to those of forbidden band of the periodic unit. Finally, the variations of photonic bandgap properties of periodic layered media with respect to the incidence angle and polarization have also been discussed.

Expanding high reflection range in a dielectric multilayer reflectorby disorder and inhomogeneity

A systematical study is presented on the impact of disorder andinhomogeneity on the high reflection range (HRR) in a one-dimensionalbinary dielectric multilayer reflector for various incident angles. It isfound that the introduction of disorder can expand the HRR for bothtransverse electric (TE) and transverse magnetic (TM) waves, irrespectiveof the incident angle. The HRR is found to become wider as the degree ofdisorder increases, until the effect becomes saturated. On the other hand,for the TE wave only, the introduction of inhomogeneity may broaden thewidth of the HRR regardless of the incident angle. While for the TM wave,the effect of inhomogeneity depends on the angle of incidence, and it canexpand the HRR only for the case of a small incident angle. The phenomenonis expected to find significant applications in the optimal design ofbroadband binary dielectric omnidirectional reflectors.

The reflective properties of broadband optical multilayers

Binary multilayers with a certain kind of disorder may exhibit high reflection range over a very wide bandwidth. The dependencies of this bandwidth on the center wavelength, random layers sequence, refractive contrast of the coating materials, layer thickness errors and the angle of incidence have been studied numerically. Our simulations have shown that broadband reflectors can be designed for various applications. The reflective curves for staggered multilayers are also computed. The results show that the broadening effect of irregular multilayers is dominated by its degree of disorder. A broadband TiO2-SiO2 multilayer is fabricated. The measured width of the transmittance less than 1% for this multilayer covers 400 nm, from 700 to 1100 nm, which is just the tuning range of Ti: sapphire lasers.

Broadband optical reflector—an application of light localization in one dimension

A broad-band optical reflector based on the localization of light for one-dimensional disordered systems is proposed and demonstrated both theoretically and experimentally. The binary disordered multilayer reflector consists of alternating random layers of two coating materials. Its high reflection range is much wider than that of standard λ/4 stacks and can be obtained by a conventional coating method and ordinary materials. This kind of reflector will have significant applications in laser technology and optical equipment.