Band Edge Wavelength Research Articles

PCF-Cavity FBG Fabry-Perot Resonator for Simultaneous Measurement of Pressure and Temperature

In this paper, we propose a Photonic Crystal Fiber (PCF)-cavity fiber Bragg grating (FBG) Fabry-Perot resonator for simultaneous sensing of temperature and pressure. The proposed device is constructed by splicing a 1.5-mm long PCF segment between two identical saturated FBGs written in SMF-28 forming an FBG-PCF-FBG configuration. As a result, a resonance dip is created within the Bragg reflection curve. Taking advantage of the difference in pressure sensitivities between the two types of fiber, the discriminative measurement of pressure and temperature can be achieved from the shifts of the resonance wavelength and band-edge wavelength. In addition, the same temperature sensitivity of PCF and SMF-28 provides the convenience of eliminating the temperature variable and achieving temperature insensitive pressure measurement.

Design of a high figure of merit subwavelength grating based plasmonic sensor for detection of DNA hybridization

A SPR biosensor based on a metallic subwavelength grating structure is studied for detection of DNA hybridization. Performance and dispersion curve characteristics of the sensor are investigated numerically for variations of the grating geometry parameters by finite element method (FEM) as a numerical method. The results of this investigation are used to design a high figure of merit (FOM) SPR sensor in near-infrared (NIR) wavelength of 984nm. Also, performance of the sensor in visible wavelength of 633nm is studied and compared with the NIR one. This study shows that the performance changing with the grating geometry parameters can be elucidated well by evaluating the effects of those parameters especially fill factor (f.f.) on the dispersion curve. Numerical study results in obtaining a higher FOM for operating wavelengths in NIR range in contrary to visible wavelengths because a NIR wavelength can be adjusted as a band edge wavelength locating in the flattened part of the dispersion curve by selecting appropriate values of f.f. and grating thickness.

Synthesis and characterization of Cd1−xZnxS nanopowder and its nanostructured thin films

Cd1−x Zn x S (with x = 0, 0.01, 0.03, 0.05 and 0.1) nanopowder were synthesized by chemical precipitation method under N2 atmosphere at room temperature. Cd1−x Zn x S nanostructured thin films were deposited on the glass at 450 °C by spraying the colloidal suspension of the Cd1−x Zn x S nanoparticles. Optical and structural properties of both nanostructures were investigated. The prominent peaks of X-ray diffractions (XRD) of samples indicate that the Cd1−x Zn x S crystalline grain size in the nanostructured thin film is larger than the Cd1−x Zn x S nanoparticles in the nanopowder, due to growing of the grains, originated from high temperature of the substrate. UV-Vis absorption spectra show that both Cd1−x Zn x S nanostructures have smaller band edge wavelength in comparison with bulk samples. There is also an increase in blue shift value of the band gap of both Cd1−x Zn x S nanostructures, with increasing x, which can be attributed to Burstein-Moss effect. It was found that the variation of the band gap of Zn x Cd1−x S nanopowder and nanostructured thin films with Zn concentration are approximately linear with similar slopes. Photoluminescence (PL) spectra indicated that the emission peaks of both the samples are shifted toward the shorter wavelengths with increasing Zn+2 ions.

Enhancement of photoluminescence from defect states in ZnS random photonic crystal: An effect of electronic and photonic mode coupling

This paper reports on the enhanced defect state emission from ZnS in the form of a random photonic crystal (RPC) medium. ZnS photonic crystals with varied randomness are fabricated by colloidal self assembly of ZnS nanospheres (215 ± 10 nm). Reflection and transmission studies reveal mid band gap wavelength at ∼435 nm. The band structure calculated for BCC lattice with reduced packing fraction (53%) is in good agreement with experimental results. The reflection due to the photonic band gap diminishes with increased randomness in the nanosphere arrangement. The features of fluorescence from ZnS are modified in the RPC medium, resulting in suppression at wavelengths in the photonic band gap region and an enhancement at band edge wavelengths of 415 and 468 nm. This enhancement becomes less prominent with increasing randomness in the structure. Interestingly these two modes correspond to the electronic defect states of ZnS. Emission enhancement is shown to be due to the strong coupling of electronic defect states and photonic band edge states which is facilitated by randomly scattering slow Bloch modes in the ZnS RPC. Fabrication of RPCs by colloidal self-assembly with specifically designed degrees of randomness (leading to controllable features of emission) provides scope for the design of low threshold random lasing systems.

Phonon-assisted near-field activation of electron transfer

An optical near field should promote phonon-assisted multiple excitation in nanoscale structures. With the phonon-assisted process, greater catalytic activity is expected without heating. To confirm this effect, photo-induced current generation using platinum black electrodes in ferricyanide solution (an absorption band-edge wavelength of 470 nm) under visible light irradiation continuous wave [(CW), λ=532 nm ] was observed. Higher order dependence of the generated current density on the incident light power was observed, indicating two-step activation of electron transfer, which originated from the phonon-assisted near-field effect on the nanostructured surface of the electrode.

Enhancement of photocurrent in ultrathin active-layer photodetecting devices with photonic crystals

We demonstrate an enhancement of the photoelectric-conversion efficiency of an ultrathin (50 nm) silicon active-layer photodetecting device using a two-dimensional photonic crystal positioned nearby to boost the optical absorption. We show both experimentally and with simulations that the incident-light absorption within the active layer is enhanced by optical-resonance effects at the photonic band edge. We also find that a photonic crystal with deeper holes can lead to an even larger absorption enhancement due to better quality (Q)-factor matching between the photonic band-edge modes and the intrinsic material absorption. The experimentally observed photocurrent of the fabricated photonic-crystal sample is increased by a factor of ∼20 at the photonic band-edge wavelength relative to that of a control sample without the photonic crystal which is attributed to the improved Q matching.

Enhancement of spontaneous emission in three-dimensional low refractive-index photonic crystals with designed defects

In this work, we demonstrate that low refractive-index three-dimensional photonics crystals with a planar defect, fabricated with the direct laser writing method, can be used to effectively enhance the spontaneous emission (SE) of semiconductor quantum dots (QDs). To achieve this, we develop a controllable and reliable method to integrate quantum dots into the defect-embedded photonic crystals (PCs). Although the overlapping of different direction stop gaps in low refractive-index defect-free three-dimensional photonic crystals imposes a challenge to realize spontaneous emission enhancement at the band edge wavelength, we achieve a 15% enhancement of the spontaneous emission by introducing a tailored planar defect.

Theoretical and Experimental Investigation of the Incident-Power-Dependent Extinction Ratio of an Electroabsorption Modulator Integrated With a Distributed Feedback Laser

We have found that the extinction ratio (ER) of an electroabsorption modulator integrated with a distributed feedback laser strongly depends on the incident power to the modulator. Our theoretical consideration based on full device simulation reveals that the temperature change in the modulator dominates the phenomenon. To obtain the intrinsic extinction characteristics of the modulator, we use microscopic theory, which takes into account quantum mechanical many-body interactions in semiconductors and makes it possible to exclude unknown experimental fitting parameters. A heat-flux analysis is performed for the calculation of the temperature distribution in the entire device. As the incident power to the modulator increases, the photocurrent also increases, leading to an increase in the temperature of the modulator by some tens of degrees Celsius by Joule heating. The increase red-shifts the absorption spectrum of the modulator, and the detuning, which is the difference between the lasing wavelength and the band-edge wavelength of the modulator, becomes small, leading to a larger ER. The calculated results are in good agreement with the experiment, showing the validity of our discussion, and indicate that the ER of electroabsorption modulators is strongly affected by the internal heating of the device.

광자결정 광섬유기반 광신호 분배기 개발

A broadband optical coupler with a broad bandwidth of 1000 nm based on a photonic crystal fiber (PCF) is investigated. The PCF has 6 layers of air hole structures and a diameter of . The PCF-based coupler is fabricated by using a fused biconical tapering method based on heating and elongation processes. Changing temperature and an elongation length can control the bandwidth and the bandedge wavelength of the PCF-based broadband coupler. The diameter of the fused region in the PCF-based coupler was measured to be . The fabricated PCF-based coupler has a nearly-flat coupling ration of 3-dB in a broad bandwidth of 1000 nm, which is wider than that of the previously reported PCF-based coupler and that of the single-modefiber-based coupler. Since the resolution of optical coherence tomography system is proportional to the bandwidths of both an optical light sources and an interferometer, the fabricated PCF-based broadband optical coupler has a great potential for realization of a broadband interferogram.

Laser Emission from a Photopolymerized Cholesteric Blue Phase II

The authors report the preparation of a free-standing film of the cholesteric blue phase II by photopolymerization, and investigate its fluorescence characteristics upon optically pumping the sample. Before polymerization, laser emission was observed from the long band-edge wavelength of the selective reflection band of the blue phase. After polymerization, laser emission was not observed from the band-edge wavelength, but appeared to have red-shifted possibly due to a distortion of the helix.

Enhancement of nonlinear effects at the degenerate band edge of two-dimensional photonic crystals

The ability of two-dimensional (2D) photonic crystals (PC) for high enhancements of nonlinear processes is analyzed in the case of a degenerate band edge when two symmetrical diffracted beams are generated by Bragg diffraction in the 2D-PC. Calculations are performed using the very simple Bragg-coupled wave theory which only involves three coupled waves (the incident wave and the two diffracted waves) for the linear interaction. The validity of the approximation is proved for wavelengths lying at the neighborhood of the band edges of 2D-PC. Very large local-field intensities are predicted around the band-edge wavelengths, in particular for the upper band edge. Nonlinear propagation is studied through the analysis of degenerate four-wave mixing. For counterpropagating pump beams orthogonally sent onto the 2D-PC huge improvement of the phase conjugate reflectivity are predicted at least for small incidence angles of the signal beam. These results represent an improvement by a factor of 20 when compared to the case of a one-dimensional PC of the same thickness made of the same materials. As three intense phase conjugate beams are generated in the four-wave mixing interaction, the 2D-PC could be very interesting for the purpose of dense parallel optical signal processing. Moreover, the simple theoretical analysis developed in the paper can be used for any kind of 2D-PC.

Synthesis and properties of polyacetylenes having pendent phenylethynylcarbazolyl groups

Novel acetylene monomers substituted with phenylethynylcarbazolyl groups, 3-[(4-octylphenyl)ethynyl]-9-propargylcarbazole (1), 3,6-bis[(4-octylphenyl)ethynyl]-9-propargylcarbazole (2), 9-(4-ethynylphenyl)-3-[(4-octylphenyl)ethynyl]carbazole (3), and 9-(4-ethynylphenyl)-3,6-bis[(4-octylphenyl)ethynyl]carbazole (4) were synthesized, and polymerized with Rh+(nbd)[η6–C6H5B−(C6H5)3] and WCl6–n-Bu4Sn catalysts. The corresponding polyacetylenes with number-average molecular weights ranging from 9200 to 94 000 were obtained in 20–98% yields. The IR spectra of the polymers revealed that acetylene polymerization took place at the terminal ethynyl group, while the ethynylene group remained intact. The UV–vis absorption band edge wavelengths of W-based poly(3) and poly(4) were longer than those of the other polymers. W-Based poly(4) emitted fluorescence with the highest quantum yield (41%). Poly(1) exhibited excimer-based fluorescence in dilute solution.

Degradation of 4-chlorophenol in TiO 2, WO 3, SnO 2, TiO 2/WO 3 and TiO 2/SnO 2 systems

The present study was undertaken to evaluate the degradation performance of 4-chlorophenol (4-CP) using TiO 2/WO 3 and TiO 2/SnO 2 systems. A BET surface area analyzer, UV–vis spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron spectroscopy for chemical analysis (ESCA) were employed to characterize the photocatalyst. The band edge wavelength increased to 475 nm and gap energy decreased to 2.61 eV in the TiO 2/WO 3 system as compare to the single TiO 2. Although the specific surfaces area of TiO 2/WO 3 decreases due to its larger size as compared to either TiO 2 or WO 3, the 4-CP degradation efficiency significantly increased as compared to single TiO 2 or WO 3 system at 435 nm wavelength. The TiO 2/WO 3 degradation of 4-CP at 369 nm was in fact inhibited. For TiO 2/SnO 2, the degradation efficiency also suffered at 369 nm, and only slightly increased compared to otherwise hardly 4-CP degraded in single TiO 2 or SnO 2 system. Since there is a significant accumulation of byproducts, the buildup of these intermediates on the catalyst surface may be responsible for their poor performance.

The role of hydrostatic stress in determining the bandgap of InN epilayers

This letter establishes a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy, the authors find that the films are under primarily hydrostatic strain. The corresponding stress results in a shift in the band edge to higher energy. The effect is significant and may be responsible for some of the variations in InN bandgap reported in the literature.

Synthesis and properties of polyacetylenes carrying N-phenylcarbazole and triphenylamine moieties

Novel acetylene monomers containing N-phenyl-substituted carbazole (Cz) and triphenylamine (TPA) groups, namely, 3-ethynyl-9-phenylcarbazole ( 1) and p-( N,N-diphenylamino)phenylacetylene ( 2) were synthesized, and polymerized with several Rh-, W-, and Mo-based catalysts. Poly( 1) and poly( 2) with high number-average molecular weights (15 500–974 000) were obtained in good yields (77–97%), when [(nbd)RhCl] 2–Et 3N (nbd = norbornadiene) was used as a catalyst. The polymers exhibited UV–vis absorption peaks derived from the Cz and TPA moieties at 250–350 nm and polyacetylene backbone above 350 nm. The UV–vis absorption band edge wavelengths of the polymers were longer than those of the corresponding monomers. Poly( 2) exhibited a UV–vis absorption peak at a longer wavelength than poly( 1) did, which indicates that poly( 2) has main chain conjugation longer than that of poly( 1). The molecular weights and photoluminescence quantum yields of the polymers obtained by the polymerization using [(nbd)RhCl] 2–Et 3N were larger than those of the Rh +(nbd)[ η 6-C 6H 5B −(C 6H 5) 3]-based counterparts. The cyclic voltammograms of the polymers indicated that they had clear electrochemical properties; the onset oxidation voltage of poly( 1) was higher than those of N-alkyl-substituted Cz derivatives. The polymers showed electrochromism and changed the color from pale yellow to blue by application of voltage, presumably caused by the formation of charged polaron at the Cz and TPA moieties. The temperatures for 5% weight loss of the polymers were around 350–420 °C under air, indicating the high thermal stability.

Optical properties and electric field enhancement in cholesteric liquid crystal containing different periodicities

We study a defect mode in a one-dimensional photonic band gap of a cholesteric liquid crystal (CLC) consisting of two helicoidal periodicities. The optical properties of this CLC are analyzed using 4×4 transfer matrix and finite difference time domain (FDTD) methods. In calculated transmission spectra of this CLC, one of the defect modes always appears at the band edge wavelength of the inner CLC having a different helix to that of two sides of CLCs. Furthermore, the electric field analysis of this CLC has also been demonstrated by the FDTD method. At the defect mode wavelength, the electric field enhancement is found to be significant larger than a normal CLC.

Defect Mode in Cholesteric Liquid Crystal Consisting of Two Helicoidal Periodicities

We study a defect mode in a one-dimensional photonic band gap of a cholesteric liquid crystal (CLC) consisting of two helicoidal periodicities. The optical properties of this CLC are analyzed using 4 ×4 transfer matrix and finite difference time domain methods. One of the defect modes always appears at the band edge wavelength of the inside CLC. Furthermore, this CLC is fabricated using photopolymerized CLC films. The transmission spectrum of this CLC is observed experimentally. Some peaks appear in the one-dimensional photonic band gap of this CLC, and the experimental results agree well with the calculated ones.

측면 연마된 열확산 코어 광섬유와 외부 물질의 분산 특성 차이를 이용한 단파장 통과 필터

코어가 확산된 단일모드 광섬유와 외부 물질 사이의 분산 특성의 차이를 이용하여 차단 특성이 우수한 단파장 통과 필터(short-pass filter)를 구현하였다. 두 물질의 결합 방법으로 측면 연마 기법을 도입하였다. 실험 결과 소자의 대역경계 파장(band edge wavelength)은 코어의 확산 정도로 조절이 가능하였다. 일반 통신용 단일 모드 대신에 열확산 코어 확산 광섬유를 도입함으로써 더 예리한 파장응답을 얻을 수 있었다. 열광학 효과를 이용하여 경계 파장을 400nm 이상 가변 할 수 있음을 관측하였다. We have demonstrated a fiber short-wavelength filter with a good cut-off property using dissimilar dispersive properties between? a thermally expanded cored fiber and an external medium. Side-polishing is applied to coupling between the fiber and the external medium. The experimental results revealed that the bend edge wavelength can be adjusted by controlling the degree of core expansion. Futhermore, the sharpness of wavelength response? was significantly? improved by employing expanded core fiber instead of a conventional single mode fiber. Tuning range of the band edge wavelength exceeded 400 m based on thermo-optic effect of the external medium.

In-line tunable band-edge filter based on a single-mode tapered fiber coated with a dispersive material

We report the use of a single-mode tapered fiber coated with a dispersive material as a tunable band-edge filter. The band-edge wavelength can be adjusted, over a range of /spl sim/500 nm (1150-1650 nm), in a simple way with the taper waist diameter, the interaction length, or with the refractive index of the dispersive material that coats the taper. Moreover, the fabrication procedure is simple and of low cost.

Changes induced in electrical properties and deep level spectra of p‐AlGaN films by treatment in hydrogen plasma and by proton implantation

The influence of hydrogen plasma treatment (250 °C, 0.5 h) and of proton implantation (proton energy of 200 keV, doses between 1012 and 1015 cm–2) on electrical properties of p-AlGaN films were studied. Hydrogen plasma treatment leads to substantial (more than an order of magnitude) decrease of the hole concentration in the surface layer. The dominant deep traps observed in current deep level transient spectroscopy were the Mg-related hole traps with activation energy of 0.17 eV and 0.19 eV. The concentration of both traps was strongly decreased after the hydrogen plasma treatment. At the same time the MCL intensity in the bandedge wavelength region was greatly increased. The results are explained by partial hydrogen passivation of Mg acceptors and of deep centers responsible for non-radiative recombination. Proton implantation was found to lead to a strong decrease of hole concentration, to a decrease in the intensity of the bandedge MCL signal and to a strong suppression of the signal from the hole traps with activation energy of 0.17 eV–0.19 eV. However, no new prominent deep centers were detected in deep level spectra after the implantation. It is suggested that the main centers responsible for the increased compensation of the layers are located in the upper part of the bandgap. Possible interplay between the hydrogen passivation of deep defects and introduction of new defects due to radiation and possibly due to hydrogen interaction with primary radiation defects will be discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)