Enhanced optical absorption of monolayer WS2 using Ag nanograting and distributed Bragg reflector structures

The AlGaAs photocathode can be used in the field of underwater optical communication because of its fast response speed and adjustable spectral response range. In order to solve the problem that the low light absorption of the AlGaAs emission layer limits the improvement of its quantum efficiency, the distributed Bragg reflector (DBR) structure is used to reflect the light at a specific wavelength back to the emission layer to further increase the absorption rate, thus improving the response capability of the photocathode at 532 nm. The spectral response model of the AlGaAs photocathode with DBR structure is obtained by solving one-dimensional continuity equation. The optical model of the AlGaAs photocathode with enhanced response at 532 nm is established by the finite-difference time-domain method. The effects of the sublayer periodic pairs, the sublayer material and the thickness of emission layer and buffer layer on the absorption rate of emission layer are analyzed. The light absorption distributions of AlGaAs photocathode with and without DBR structure are compared, and the influence mechanism of DBR structure on the blue-green light absorption capacity of AlGaAs photocathode emission layer is clarified, which can provide a theoretical basis for designing its structural parameters. The results show that the DBR structure with a periodic pair of 20 and Al<sub>0.7</sub>Ga<sub>0.3</sub>As/AlAs has the best reflection effect on 532 nm light. Based on the DBR structure, when the thickness of the emission layer and buffer layer are 495 nm and 50 nm, respectively, the emission layer has the best absorption rate of 532 nm light. Furthermore, two kinds of AlGaAs photocathodes with and without DBR structure are prepared by the metal-organic chemical vapor deposition technology, and the reflectivity and profile structure of the grown samples are characterized. Then the Cs/O activation experiments are performed to compare the spectral response curves. It is found that the spectral response of the AlGaAs photocathode sample with DBR structure at 532 nm wavelength is about twice that of the sample without DBR structure.

Structural analysis of distributed Bragg reflector mirrors

GaN growth on Si with rare-earth oxide distributed Bragg reflector structures

We have developed a single-frequency tapered laser emitting at a wavelength of 1.55 pm. The ridge waveguide section enables only one lateral mode, while the DBR structure provides single ffequency operation. This tapered laser has 20 dl3 side-mode suppression at continuous wave (CW) power levels up to 0.6 W. 80 'YO of the power from this device remains in the central lobe of the far-field at a power level of 0.5 W. A Gaussian contact over the tapered section matches the current distribution to the lateral optical profile reducing self-focusing and beam instability effects. We are also developing both distributed feedback (DFB) and distributed Bragg reflector (DBR) angled-stripe semiconductor laser designs for high power. To obtain power levels approaching 10 W, we are using a lenslike waveguide that causes divergence along the lateral direction along with a second order grating to provide outcoupling along the surface of the device.

In this study, high performance nitride-based flip-chip (FC) light-emitting diodes (LEDs) using optimized distributed bragg reflector (DBR) were fabricated and compared with conventional FC-LED using silver (Ag) reflector. Most of FCLEDs are using the silver (Ag) as reflector due to its superior reflectance at visual spectrum region. However, A silver has detrimental problems such as electro-chemical migration and agglomerations, which resulting in reliability issues such as degradation of power drop, unstable operating voltage and leakage issues. Our DBR structure was designed to have 99% at whole visible spectrum range (400~750nm), which is higher reflectance than silver reflector (90~95%). Optical power is higher than higher than the Ag-LED up to 30% @ 500mA. As the current increases up to 1A, the gap slightly decreased. Reliability test results show stable optical power, operating voltage, and leakage maintenance.

The distributed Bragg reflectors (DBRs) are important optical components in a wide range of applications, such as Fabry-Perot optical resonators, hyperspectral sensors, micro-cavity structures, and waveguide lasers. In this thesis, we have developed, designed, and fabricated highly reflecting, and mechanically stable DBRs comprised of electron-beam deposited zinc selenide (ZnSe) and barium fluoride (BaF2) thin films and explored its use in optical filtering and hyperspectral sensing applications. Most published results are in the short-wavelength infrared (SWIR) and near-infrared (NIR) with limited results in the mid-wavelength infrared (MWIR). In this work, we have fabricated ZnSe/BaF2-based DBRs in the MWIR region. The fabricated DBRs showed a central wavelength of 4 µm and reflection exceeding 93%. A Fabry-Perot optical filter, that is comprised of two parallel DBR mirrors with half-wavelength optical cavity thickness was used to partially transmit and reflect the incident light at the target wavelength. The assembled fixed-cavity Fabry-Perot optical filters achieved a full-width half maximum (FWHM) of ~ 450 nm. We further showed that the FWHM of the optical filters can be reduced by ~ 50 % with the addition of nano-thick silicon monoxide (SiO) interface grading layer between every pair of the quarter-wave optical thick (QWOT) layers in the stack. The SiO interface grading layer acts as an anti-reflection coating layer to reduce reflection loss in the DBR multilayer structure. The SiO interface grading layer, ~ 80 nm thick, also has the potential to stop atomic inter-diffusion between the layers to minimize free carrier absorption loss. The principal thesis goal is to design and assemble fixed-cavity interface-graded Fabry-Perot optical filters (with SiO interfacial layers) that optimized optical transmission and spectral resolution. The thesis also proposes a low-cost micro-cavity structure that can be formed by embedding two-dimensional atomic crystals, such as black phosphorous into a DBR cavity structure. The proposed design can be used to increase absorption in 2D materials, which has the potential to increase the efficiency of next generation MWIR photodetectors and infrared sensors.

This paper reports on the design and simulation of a Multi Quantum Well (MQW) AlGaAs/GaAs single p-i-n junction solar cell with a Distributed Bragg Reflector (DBR) placed at back surface. The DBR structure reflects the part of the spectrum that benefits from absorption in the AlGaAs QW structure, while being transparent to the IR spectrum. The efficiencies and short-circuit currents for the AlGaAs MQW cell with and without DBR are simulated. The efficiencies and short-circuit currents of an optimized AlGaAs MQW cell with and without DBR are simulated. Results obtained show improvement in short circuit current, open circuit voltage and efficiency when both MQW and DBR were used. Simulation results predict an efficiency of about 28.4% for the MQW AlGaAs-DBR single p-i-n junction photovoltaic cell under the presence of both radiative and non-radiative recombination mechanisms.

Full-angle distributed Bragg reflectors (DBRs) consisting of numerous sub-DBRs with discrete central wavelengths have been developed to enhance performance of GaN-based flip-chip mini light-emitting diodes (FC mini-LEDs). However, relatively low reflectivity of full-angle DBRs at large angle incidence restricts further enhancement in performance of FC mini-LEDs. Here, we introduce a reflectivity optimization strategy for constructing high-reflectivity multiple-stack DBRs by rationally engineering the number of sub-DBRs and adjusting central wavelength distribution of sub-DBRs. Based on the reflectivity optimization strategy, we devise a Ti3O5/SiO2 quintuple-stack DBR which is composed of five sub-DBRs. Our quintuple-stack DBR maintains a high reflectivity (>97.5%) over a wide range of incident angles of light. Notably, compared with the full-angle DBR, our quintuple-stack DBR exhibits higher reflectivity at large angle incidence and thinner multilayer thickness. Furthermore, we demonstrate two types of GaN-based blue FC mini-LEDs with indium-tin oxide (ITO)/quintuple-stack DBR and ITO/full-angle DBR p-type ohmic contacts. Benefiting from superior reflection performance, blue FC mini-LED with ITO/quintuple-stack DBR achieves an enhancement of ∼5.8% in light output power at 10 mA, in comparison with blue FC mini-LED with ITO/full-angle DBR. Our work signifies an advancement towards high-reflectivity DBRs, which enables higher-performance FC mini-LEDs.

Down-conversion in Tm3 +/Yb3 + doped glasses for multicrystalline silicon photo-voltaic module efficiency enhancement

Optimized Tamm-plasmon structure by Differential Evolution algorithm for single and dual peaks hot-electron photodetection

We fabricated amorphous silicon (a-Si)-based distributed Bragg reflectors (DBRs) consisting of alternating dense/porous films (i.e., pair) for a center wavelength (λ(c)) of 0.96 μm by oblique angle deposition (OAD) technique using an electron-beam evaporation system. The dense (high refractive index, i.e., high-n) and porous (low-n) a-Si films were deposited at two incident vapor flux angles of 0° and 80° in the OAD, respectively. Their optical reflectance characteristics were investigated in the wavelength range of 0.6-1.5 μm, including theoretical comparison using a rigorous coupled-wave analysis method. Above three pairs, the reflectivity (R) of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. For the a-Si DBR with only three pairs, a broad normalized stop bandwidth (Δλ/λ(c)) of ∼22.5% was obtained at wavelengths of ∼0.87-1.085 μm, keeping high R values of >95%. To simply demonstrate the feasibility of device applications, the a-Si DBR with three pairs was coated as a high-reflection layer at the rear facet of GaAs/InGaAs quantum-well laser diodes (LDs) operating at λ=0.96 μm. For the LDs coated with three-pair a-Si DBR, external differential quantum efficiency (η(d)) was nearly doubled compared to the uncoated LDs, indicating the η(d) value of ∼50.6% (i.e., η(d)∼25.5% for the uncoated LDs).

We describe the design, fabrication, and characterization of mechanically stable, reproducible, and highly reflecting distributed Bragg reflectors (DBR) composed of thermally evaporated thin films of calcium fluoride (CaF2) and zinc sulfide (ZnS). CaF2 and ZnS were chosen as the low and high refractive index components of the multilayer DBR structures, with n = 1.43 and n = 2.38 respectively, because neither material requires substrate heating during the deposition process in order to produce optical quality thin films. DBRs consisting of seven pairs of CaF2 and ZnS layers, were fabricated with thicknesses of 96 and 58 nm, respectively, as characterized by high-resolution scanning electron microscopy (HR-SEM), and exhibited a center wavelength of λc = 550 nm and peak reflectance exceeding 99%. The layers showed good adhesion to each other and to the glass substrate, resulting in mechanically stable DBR coatings. Complete optical microcavities consisting of two such DBR coatings and a CaF2 spacer layer between them could be fabricated in a single deposition run. Optically, these structures exhibited a resonator quality factor of Q > 160. When a CaF2/ZnS DBR was grown, without heating the substrate during deposition, on top of a thin film containing the fluorescent dye Rhodamine 6G, the fluorescence intensity showed no degradation compared to an uncoated film, in contrast to a MgF2/ZnS DBR coating grown with substrate heating which showed a 92% reduction in signal. The ability to fabricate optical quality CaF2/ZnS DBRs without substrate heating, as introduced here, can therefore enable formation of low-loss high-reflectivity coatings on top of more delicate heat-sensitive materials such as organics and other nanostructured emitters, and hence facilitate the development of nanoemitter-based microcavity device applications.

A novel free-standing resonant-cavity-enhanced (RCE) Ge thin-film absorber is designed with a bottom distributed Bragg reflector (DBR) for infrared photodetection at 1550 nm wavelength based on a Si substrate. The free-standing structure offers a high degree of freedom in optimizing the number of periods, the layer arrangement, and the thickness of each layer of the DBR. A SiO2/Si DBR that is compatible with Si processing technology is used on the bottom of the Ge thin film to construct the RCE structure. Based on our theoretical study, the SiO2 low-index layer must be placed next to the Ge layer to utilize the large difference in the refractive indices between SiO2 and Ge to achieve high reflectance for the DBR. In this design, the structure can reach an absorbance of 85.90% with a DBR of three periods on the bottom and an antireflective oxide layer on the top surface of the Ge layer. Without the antireflective oxide layer, the absorbance is increased to 99.17%. The design paves the way for high-responsivity Ge infrared photodetectors in Si-based photonic systems.

The feasibility of a distributed Bragg reflector (DBR) based on wide band gap II-VI semiconducting alloys is reported. The reflectivity spectra of a Zn0.66Mg0.34Se/Zn0.74Cd0.26Se DBR are calculated within the stratified medium theory using matrix representation. The composition x for each alloy is taken in such a manner to obtain a convenient difference between the respective refractive indices of each constituent material. Both normal and oblique incidence are considered. For both cases the effect of the number of pairs has been studied. We found out that the reflectivity maximum is enhanced as the number of pairs increases. The DBR bandwidth is found to be dependent on the refractive indices difference. The reflectivity spectrum is shifted towards smaller wavelengths, and the Bragg wavelength is diminished as the angle of incidence is augmented.

We reported the fabrication and characteristics of distributed Bragg reflectors (DBRs), made of titanium dioxide (TiO2) dense/nanoporous film stacks, operating in the near-infrared wavelength region of 0.8-1.1 μm. To form the nanoporous TiO2 films with a low refractive index (low-n), the oblique angle deposition (OAD) technique was employed at a high incident vapor flux angle of 80 degrees. For the fabricated DBRs consisting of TiO2 dense/nanoporous (high-n/low-n) thin films, the high reflectance band was gradually increased and the stop bandwidth was narrowed with increasing the number of DBR pairs. Particularly, the TiO2 DBR with only 6 pairs exhibited a normalized stop bandwidth (Δλ/λ(c)) of -11.6% at a center wavelength (λ(c)) of 0.96 μm as well as high reflectance values of > 95% over a wide mid-infrared wavelength region of 0.9-1.01 μm (i.e., Δλ = 0.111 μm). Furthermore, the reflectance characteristics were investigated at incident light angles of 20-70 degrees for different polarized lights. For theoretical optical analyses, the reflectance calculations were also performed by a rigorous coupled-wave analysis method, showing a similar tendency to the experimentally measured data.