High Q-factor microwave Fabry-Perot resonator with distributed Bragg reflectors

A Fabry-Perot resonator operating at 39 GHz, with two pairs of quarter-wavelength single-crystal quartz Bragg reflectors has been realized. For the length of 98.26 mm, its Q-factor is about 560,000, which is 4.3 times better than for the same resonator without Bragg reflectors. Rigorous finite-difference frequency-domain analysis has been applied to the problem and is compared with simplified semi-analytical solutions. Good agreement between theoretical and experimental resonant frequency and Q-factors has been obtained. Thermal compensation of the resonant frequency of the Fabry-Perot has been proposed employing rods and cylinders made of metals with different thermal expansion coefficients.     

Si-based photonic quantum dots with the self-similar distributed Bragg reflectors

In this letter, optical microcavities are deposited directly on mesa-patterned substrates, and three dimensional Bragg cavities are formed due to the conformal deposition process. The room temperature photoluminescence spectra of three-dimensional Bragg microcavity exhibit a series of discrete resonant peaks. Optical field profiles of corresponding resonant peaks were given, by finite difference time domain simulation. These discrete peaks obviously show cavity resonant characteristics and optical field is mostly localized in the cavity layer. Also size dependence of cavity modes was concluded from our experimental results.     

Infrared ellipsometry characterization of porous silicon bragg reflectors

We investigate porous silicon Bragg reflectors in a nondestructive manner using variable angle-of-incidence infrared spectroscopic ellipsometry. In addition to the thickness, volume porosity, inhomogeneity, and optical anisotropy, properties of the solid content of the porous material are investigated in terms of dielectric function and surface chemistry. The material was found to have positive birefringence. The high sensitivity of the technique is employed to detect and identify infrared resonant absorptions related to different Si-H as well as Si-O-Si vibrational modes. The average electrical resistivity of the solid content of the porous material is determined to be 0.03 Omega cm, which is larger than the corresponding bulk value of 0.019 Omega cm. Furthermore the average carrier concentration in the porous material shows a decrease from 6.2 x 10(18) cm(-3) to 4 x 10(18) cm(-3).     

Sputtered hydrogenated amorphous silicon thin films for distributed Bragg reflectors and long wavelength vertical cavity surface emitting lasers applications

In this work, we report a study of hydrogenated amorphous silicon (a-SiH) films deposited by radio frequency magnetron sputtering for application in Vertical Cavity Surface Emitting Lasers (VCSEL) elaboration. The influence of the hydrogen dilution in the plasma during the deposition on the optical and surface properties is investigated. After selection of the deposition parameters, a-SiH films have been combined with amorphous silicon nitride (a-SiN_x) films to provide high reflectivity Bragg reflectors. Distributed Bragg reflector (DBR) based on these quarter wavelength thick dielectric layers have been realized and characterized by optical measurements and compared with theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.2% at 1.6 μm and a large spectral bandwidth of 700 nm have been reached with only four and a half periods of a-SiH/a-SiN_x deposited on a glass substrate. Residual absorption at 1.55 μm has been measured to be as low as 60 cm⁻¹ with a-SiH layers, compared with 400 cm⁻¹ loss with amorphous silicon without hydrogenation step. Finally, DBR comprising six a-SiH/a-SiN_x periods have been included in an InP-based VCSEL. Laser emission is demonstrated at room temperature in continuous wave operation with a photopumping experiment.     

Investigation of thermal and optical properties of distributed Bragg reflectors by photothermal deflection spectroscopy

We present the study of thermal and optical properties of 15.5 pair of GaAlAsSb/AlAsSb layers deposited upon a GaSb substrate by photothermal deflection spectroscopy. This stacking of layers constitutes a distributed Bragg mirror.     

Light emission from porous silicon

Room-temperature visible luminescence observed in porous silicon is one of the most significant discoveries of this decade as it opens up the possibility of silicon-based optoelectronics afresh. The exact mechanism of this different luminescence behaviour of porous silicon, compared to crystalline silicon, is not well established. In this paper results of a combination of infrared absorption, and photoluminescence emission and excitation spectroscopies will be described to show that the nanocrystallite nature of porous silicon and chemical environment at the surface are the important aspects of this novel luminescence behaviour.     

Fabrication of human IgG sensors based on porous silicon interferometer containing Bragg structures

A simply modified biosensor based on protein A-modified distributed Bragg reflectors (DBR) porous silicon (PSi) chip for the detection of human immunoglobin G (IgG) are developed. The fabrication, optical characterization, and surface derivatization of DBR PSi are investigated. The sensor system studied consist of multi-layer of porous silicon modified with protein-A. The sensor is operated by the measurement of the reflection peak in the white light reflection spectrum. Molecular binding is detected as a shift in wavelength of reflection peaks.     

Infrared light emission from porous silicon

Very intense broad sub-bandgap infrared (IR) light emission around 1,550 nm was observed on porous silicon by photoluminescence (PL) measurements. The integrated intensity of the IR signal is two orders of magnitude higher than that of the band–band emission in Cz silicon. PL measurements with the sample immersed in different media, e.g., in HF and H₂O₂, confirmed that the broad IR band originates from the Si/SiO _x interface. Electroluminescence spectroscopy was carried out on a porous silicon p–n junction sample contacted with indium-tin oxide. The IR band was detected at room temperature at both forward and reverse bias. The results indicate that radiative recombination through interface states is very efficient at room temperature.     

多孔硅与聚甲基丙烯酸甲酯复合光致发光特性研究

多孔硅与有机材料复合可以改善多孔硅的光致发光特性。用化学腐蚀的方法制备了多孔硅，通过不同方法实现了多孔硅与聚甲基丙烯酸甲酯(PMMA)的复合。实验结果表明，用旋涂法实现的PMMA固化后再与多孔硅复合而制得的样品的结果最好，它与原始的多孔硅样品相比，发光峰发生了蓝移而且发光强度下降很小。PMMA层有限的厚度和PMMA对多孔硅表面的保护使复合后发光强度下降很小。制备的多孔／PMMA复合体系的发光强度几乎不随时间而下降，这可能是由于PMMA有效地隔绝多孔硅与空气的接触，保护了多孔硅的表面，不会产生更多的悬挂键。     

Optimized light emission from layered porous silicon structures

We discuss in detail the physics of light emission from porous silicon microcavities formed by periodically modulating the porosity to produce multilayered structures. Changing the porosity alters not only the refractive index and absorption but also the luminescence, resulting in a complex interplay of effects that has not yet been addressed in the literature as far as we know. A transfer matrix model is developed that accounts for the dispersion of the refractive index, absorption, and photoluminescence. A multilayer porous silicon mirror is found to emit light almost as well as a conventional distributed feedback microcavity system with a mid-stop-band resonant state.     

Blue-light emission from porous silicon induced by laser irradiation

Porous silicon coated with silica gel emitted light with higher energy after it had been irradiated by a Nd:YAG laser. SEM and XRD measurements demonstrate no crystal-to-amorphous transformation under the irradiation, which is presumably because of good protection of the material by silica coat. FTIR results show that the blue-light emission obtained is due to the formation of Si–C and Si–O–C surface bonds under the laser treatment.     

Enhanced photo luminescence from porous silicon on texturized surface

Porous silicon (PS) was formed on both polished and texturized single crystal silicon (100) by anodic etching. Photoluminescences (PL) from both of these silicon surfaces were measured and compared. A two-fold enhancement of PL from textured silicon surface was obtained. This enhancement could be ascribed to the geometry of the textured surface.     

Localized-surface-plasmon enhanced emission from porous silicon by gold nanoparticles

The porous silicon (PS) samples, decorated by Au nanoparticles (NPs) possessing localized-surface-plasmon (LSP) resonance, are prepared by the conventional anodization method. Photoluminescence (PL) is studied systematically, in particular, its dependence on the excitation power. It is found that undecorated PS samples exhibit a saturation behavior in PL intensity with increasing the pumping laser power, while the luminescence of Au-decorated PS hybrid samples have a purely linear dependence on the excitation power. In the linear response region of PS samples, addition of metal NPs layer moderately suppresses the emission while, in the saturation region, the net emission is enhanced by approximately up to 4-fold. Several possible mechanisms are discussed. We believe that the observed PL enhancement in saturation region is dominantly due to the resonant coupling between the LSP of Au NPs and the electronic excitation of PS, which inhibits the nonradiative Auger recombination process at high excitation power. These results indicate that the plasmon effect could be useful for designing even more efficient optoelectronic devices such as super bright light emitting devices and solar cells with high efficiencies. Despite many challenges, Au NPs can potentially be applied to introduce LSP resonance for the future silicon-based optoelectronics or photonics.     

Preparation of aligned silicon carbide whiskers from porous carbon foam by silicon thermal evaporation

Aligned silicon carbide whiskers were prepared from porous carbon foams by thermal evaporation of silicon. High-density silicon carbide whiskers were vertically deposited on the surface of siliconizing carbon foam. The whiskers were straight and hexagon-shaped with diameter of 1-2 μm and length of about 40 μm. They consisted of a single-crystalline zinc blende structure crystal in the [111] growth direction. The pore structure of carbon foam played an important role in determining distribution of the whiskers on the surface of siliconizing carbon foam. When carbon foam with higher porosity and larger pore size was employed, distributions of the whiskers were more ordered and more intensive. The whiskers were grown by the vapor-solid (VS) mechanism.     

Optical devices from porous silicon having continuously varying refractive index

We shall discuss two types of optical devices made using porous silicon, in which the refractive index distribution is continuous. In the rugate filter, the refractive index profiles is sinusoidal along the growth axis, while in the graded refractive index lenses that we fabricated from PSi, the refractive index is quadratic in the plane of the porous layer. We shall talk about the design and fabrication methods used to manufacture both of these device types, and show examples for each.     

On the origin of the visible luminescence from porous silicon

A correlation between the photoluminescence, chemical bonds and structure of c-Si subjected to electrochemical and laser induced chemical etching has been obtained by X-ray photoelectron spectroscopy, IR spectroscopy and Raman scattering. Photoluminescence is very strongly related to the chemical bonding and depends on the Si, O and C content and the presence of different oxidized states of Si which are different for porous silicon prepared on Si substrates of different electrical resistivity. Porous silicon having Si oxidized states typical for Si suboxides exhibits very intensive photoluminescence. The influence of the doping level of c-Si on the process of electrochemical etching as well as the influence of the laser power on the process of stain etching are discussed.     

Distributed Bragg reflector enhancement of electroluminescence from a silicon nanocrystal light emitting device

We demonstrate distributed Bragg reflector (DBR) enhanced electroluminescence from a silicon nanocrystal-based light emitting device. An a-Si/SiO₂ superlattice containing silicon nanocrystals serves as the intrinsic layer in an n-i-n device that is embedded in a DBR cavity consisting of alternating layers of silicon and silicon dioxide. The entire structure, including DBR, superlattice and contact layers, is deposited by plasma-enhanced chemical vapor deposition. The photoluminescence, electroluminescence (EL) and optical output power are measured and compared to a reference device. The DBR is found to enhance the peak EL intensity by a factor of 25 and the external quantum and power conversion efficiencies by a factor of 2.     

Processing of closed-cell silicon oxycarbide foams from a preceramic polymer

A novel processing route for fabricating closed-cell ceramic foams has been developed. The strategy for making the ceramic foams involves: (i) forming some shapes using a mixture of preceramic polymer and expandable microspheres by a conventional ceramic forming method, (ii) foaming the compact by heating, (iii) cross-linking the foamed body, and (iv) transforming the foamed body into ceramic foams by pyrolysis. By controlling the microsphere content and the pyrolysis temperature, it was possible to adjust the porosity ranging from 56 to 85%.     

Electrohydrodynamic forming of porous ceramic capsules from a preceramic polymer

Porous polymeric near-spherical capsules, ~ 3.6 mm in diameter, were prepared using an electrodydrodynamic process. These capsules were pyrolysed to porous ceramics, ~ 3 mm in diameter. The ceramic capsules had interconnected pores of ~ 1.3 μm in size, and large cells with a mean size of 28 μm. The larger pores resemble the cells in a typical ceramic foam and were evenly distributed throughout the structure. A large proportion of the ceramic capsules contained 65–70% porosity, and their compressive strength was 0.2–0.4 MPa.