Ultracompact refractive index sensor based on microcavity in the sandwiched photonic crystal waveguide structure

A refractive index (RI) sensor based on the two-dimensional photonic crystal is presented. The sensor is formed by a point-defect resonant cavity in the sandwiched waveguide structure. The transmission spectrums of the sensor with different ambient refractive indices ranging from n = 1.0 to n = 1.6 are calculated. The calculation results show that a change in ambient RI of Δn = 0.001 is apparent, the sensitivity of the sensor (Δλ/Δn) is achieved with 330 nm/RIU (when lattice constant a = 440 nm), where RIU means the refractive index unit; and the transmission efficiency in the RI range of 1.0-1.6 can reach about 40% to 70%, that make the detection of spectrum easy and feasible. The properties of the sensor are analyzed and calculated using the plane-wave expansion (PWE) method and simulated using the finite-difference time-domain (FDTD) method.     

Influence of preparation conditions on the dispersion parameters of sprayed iron oxide thin films

Iron oxide thin films were prepared by spray pyrolysis technique (SPT) at various substrate temperatures (T_sub) and different deposition time. X-ray diffraction (XRD) analysis showed that, at T_sub ≥ 350 °C, a single phase of α-Fe₂O₃ film is formed which has the rhombohedral structure. Moreover, the crystallinity was improved by increasing T_sub. The effect of T_sub as well as deposition time on the optical dispersion of these films has been investigated. The optical transmittance and reflectance measurements were performed by using spectrophotometer in the wavelength range from 300 to 2500 nm. The refractive index was determined by using Murmann's exact equation. It was observed that, the refractive index increased with increasing in both the T_sub and film thickness. The optical dispersion parameters have been evaluated and analyzed by using Wemple-Didomenico equation. The obtained results showed that, the dielectric properties have weak dependencies of growth temperature and film thickness. At T_sub ≥ 350 °C, the average values of oscillator energy, E_o and dispersion energy, E_d were found to be 5.96 and 34.08 eV. While at different thickness, the average values of dispersion energies were found to be 3.93 and 17.08 eV. Also, the average values of oscillator strength S_o and single resonant frequency ω_o were estimated 10.78 × 10¹³ m⁻² and 5.99 × 10¹⁵ Hz, while at different thickness were evaluating 4.81 × 10¹³ m⁻² and 6.11 × 10¹⁵ Hz. Furthermore, the optical parameters such as wavelength of single oscillator λ_o, plasma frequency ω_p, and dielectric constant ? have been evaluated. The carrier concentration N_opt by using Drud's theory was obtained the range of 5.07 × 10²⁵ m⁻³ to 1.04 × 10²⁶ m⁻³.     

High-power continuous-wave doubly resonant all-intracavity sum-frequency mixing deep blue laser at 447 nm

In this paper, a high-power continuous-wave deep blue laser at 447 nm with intracavity tripling was achieved. The deep blue laser at 447 nm is obtained by using a doubly cavity, and type-II critical phase matching KTP crystal for intracavity sum-frequency mixing. Through designing of the cavity, the optimum matching of modes and gains for the two wavelengths was obtained. With incident pump power of 30 W for the Nd:YVO₄ crystal and 16 W for the other Nd:YVO₄ crystal, the deep blue laser output of 3.5 W at 447 nm with TEM₀₀ mode was obtained, the beam quality M² value was equal to 1.8 in both horizontal and vertical directions at the maximum output power, and the power stability is better than 3% at the maximum output power during half an hour. The experimental results show that the intracavity sum-frequency mixing by doubly resonant is an effective method for high-power blue laser.     

High-k Mg-doped ZST for microwave applications

The (Zr_0.8Sn_0.2)TiO₄ material (ZST), has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1260-1320 °C for 2 h. The effects of sintering parameters like sintering temperature (T_s) and MgO addition (0.2 wt.%) on structural and dielectric properties were investigated. Bulk density increases from 4900 to 5050 kg/m³ with the increase of sintering temperature. The effect of MgO addition is to lower the sintering temperature in order to obtain well sintered samples with high value of bulk density. The material exhibits a dielectric constant ?_r ∼ 37 and high values of the Q × f product, greater than 45,000, at microwave frequencies. The dielectric properties make the ZST material very attractive for microwave applications such as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc.     

All-optical NRZ-OOK-to-RZ-OOK format conversions with tunable duty cycles using nonlinear polarization rotation of a semiconductor optical amplifier

We experimentally demonstrate an all-optical 10 Gb/s format conversion from non-return-to-zero (NRZ) on-off-keying (OOK) to return-to-zero (RZ)-OOK with tunable duty cycle in the whole C-band using nonlinear polarization rotation (NPR) arising in an semiconductor optical amplifier (SOA). The experimental results show that, by tuning the polarizer at the SOA output, an RZ signal with tunable duty cycle from 33% to 66% could be obtained with an extinction ratio(ER) over 10 dB. In addition, we show that the NRZ-to-RZ conversion with duty cycle of 33-66% can be obtained with less than 1 dB power penalty at the bit error ratio (BER) of 10⁻⁹. The device can facilitate the cross-connection between optical transmission networks employing different modulation formats.     

T-shaped channel drop filter based on photonic crystal ring resonator

Photonic crystal ring resonators are promising candidates for realizing all optical filters with acceptable transmission efficiency and quality factor values. In this paper, by putting a12-fold quasi crystal at the middle of on 7 × 7 square cavity we created a ring resonator structure and designed a T-shaped channel drop filter. The drop wavelength of our filter is at 1551 nm, with transmission efficiency and quality factor equal to 90% and 387. Our structure is composed of dielectric rods immersed in air. Because in this kind of structures the dominant band gap is in TM mode, all of our simulations have been done in TM mode. The total footprint of our filter is 242.4 μm², which makes it suitable for all optical integrated circuits.     

Si-based erbium-doped light-emitting devices

We report on the fabrication and performance of Si-based light sources. The devices consist of MOS structures with erbium (Er)-doped silicon rich oxide (SRO) film as gate dielectric. The devices exhibit electroluminescence (EL) at 1.54 μm at room temperature with a 0.2% external quantum efficiency. These devices show a high stability due to the silicon excess in the film. The Er-doped SRO films have been introduced in a Si/SiO₂ Fabry-Perot Microcavity in order to increase the spontaneous emission rate, the extraction efficiency and the spectral purity at the resonant wavelength. The active medium in the cavity has been electrically pumped and the conduction mechanisms have been analyzed. The EL spectra have also been acquired and compared with photoluminescence (PL) ones for the same resonant cavity light-emitting device (RCLED). The EL and PL peak intensities of the on-axis emission at the resonant wavelength are over 20 times above that of the similar Er-doped SRO film without a cavity. The Si-based RCLEDs exhibit different quality factors, ranging from 60 to 170. The spectra shape and intensity have been correlated with the quality factor. A high directionality of the emitted light, due to the presence of the resonant cavity, has also been observed: the overall luminescence is confined within 10° cone from the sample normal.     

Narrow band filters in both transmission and reflection with metal/dielectric thin films

We present a novel narrow band filter operating in both transmission and reflection for the first time to our knowledge. This proposed structure consists of one unsymmetrical dielectric Fabry-Perot cavity and an ultrathin metal film with n ≈ k. Theoretical analysis shows that both the reflectance and transmittance at the central wavelength are maximums. Due to the high absorption induced by the metal, a good rejection level can be obtained for a wide spectral range. In addition, the changes of peak value ratio R_max/T_max is also investigated by adjusting the amount of dielectric stacks. We finally demonstrate the experimental results to verify these designs.     

Large enhancement of Faraday rotation by localized surface plasmon resonance in Au nanoparticles embedded in Bi:YIG film

A large enhancement of the Faraday rotation, which is associated with localized surface plasmon resonance (LSPR), was obtained in a sample with Au nanoparticles embedded in a Bi-substituted yttrium iron garnet (Bi:YIG) film. On a quartz substrate, Au nanoparticles were formed by heating an Au thin film, and a Bi:YIG film was then deposited on them. A sample containing the Au nanoparticles produced by 1000 °C heating showed a resonant attenuation with narrower bandwidth in the transmission spectrum than nanoparticles of other samples formed by low-temperature heating. The sharp resonant Faraday rotation angle was 4.4 times larger than the estimated intrinsic Bi:YIG film at the LSPR wavelength; the angular difference was 0.14°. A discrepancy in the bandwidth between the transmission attenuation and the resonant Faraday rotation is discussed.     

Magnetic and microwave absorbing properties of polyaniline/γ-Fe2O3 nanocomposite

The conducting protonated polyaniline (ES)/γ-Fe₂O₃ nanocomposite with the different γ-Fe₂O₃ content were synthesized by in-situ polymerization. Its morphology, microstructure, DC conductivity and magnetic properties of samples were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), four-wire-technique, and vibrating sample magnetometer (VSM), respectively. The microwave absorbing properties of the nanocomposite powders dispersing in wax coating with the coating thickness of 2 mm were investigated using a vector network analyzers in the frequency range of 7–18 GHz. The pure ES has shown the absorption band with a maximum absorption at approximately 16 GHz and a width (defined as frequency difference between points where the absorption is more than 8 dB) of 3.24 GHz, when 10% γ-Fe₂O₃ by weight is incorporated , the width is broadened to 4.13 GHz and some other absorption bands appear in the range of 7–13 GHz. The parameter dielectric loss tan δ_e (=ε″/ε′) in the 7–18 GHz is found to decrease with increasing γ-Fe₂O₃ contents with 10%, 20%, 30%, respectively, but magnetic loss tan δ_m (=μ″/μ′) increases with increasing γ-Fe₂O₃ contents. The results show that moderate content of γ-Fe₂O₃ nanoparticles embedded in protonated polyaniline matrix may create advanced microwave absorption properties due to simultaneous adjusting of dielectric loss and magnetic loss.     

A novel design of a photonic crystal sensor with improved sensitivity

Biosensors, based on photonic crystal (PC), are emergent subject. The use of PCs in this area brought solutions to both miniaturization and integration challenges that have been facing research groups for long time. We are only recently, by engineering such defects, able to propagate light in complex structures containing molecules of different sizes and shapes. We propose a novel structure containing defects with various sizes. The PC is formed by a dielectric cylinders with permittivity 8.9 (alumina Al₂O₃) and a radius r = 0.2a (a is the square lattice constant), arranged in a square lattice. We use the finite difference time domain to investigate the sensitivity of the proposed sensor to water. The defect based sensing element is introduced in two directions 〈0 1〉 and 〈1 0〉. These simulations show a better sensitivity to water than other analytes. It appears in the transmission curves where the peak shifts to high frequency when the refractive index is changed.     

1 × 3 Beam splitter based on self-collimation effect in two-dimensional photonic crystals

The analysis and simulation result of a 1 × 3 beam splitter in a two-dimensional square-lattice photonic crystal is presented in this paper, where the light is self-collimated as dictated by the self-collimation effect. The frequency and the direction of propagation of the self-collimated beam are obtained by the equal-frequency contours (EFCs) plot which is calculated by plane wave expansion method. Then a line defect is introduced by simultaneously varying the radii and the dielectric constant of the rods along the proper direction, the self-collimated beam propagation in such structure is simulated by the two-dimensional finite-difference time-domain (2D FDTD) method with perfectly matched layer absorbing boundary conditions. The simulation results show that the self-collimated beam can be split into three beams. With the same principle, a 1 × 7 beam splitter is realized by introducing different line defects along (X direction. Such devices can greatly enhance photonic crystals for usage in high-density optical integrated circuits.     

One step towards the fabrication of a nanoscale Si-nc based laser cavity

In this paper, we report on the design of two major components of a laser architecture using Si-nc embedded in SiO₂ as the optical gain medium and sub-wavelength periodic structures to form the resonant cavity. Dimensions of the structures have been matched to near-infrared wavelengths (∼850 nm) of the maximum photoluminescent emission where optical gain has been observed from Si-nc. Both the front (FM) and rear (RM) mirrors have been fabricated by the implantation of Si ions (50 keV, 2×10¹⁷ Si⁺/cm²) through a mask, in order to produce a Bragg reflector by optical index contrast between the implanted and the non-implanted zones. Two closely spaced Bragg reflectors are used in the FM structure to allow a narrow bandpass (partial transmission) centered at 850 nm. The implanted structures have been annealed to produce Si-nc and passivation. Scanning electron microscopy (SEM) images show that the design dimensions of the structure have been obtained. Characterization of the structures by laser excitation reveals an optical gap in both mirrors between 825 and 870 nm, as per the design parameters. A quality factor Q∼95 and a reflectivity R∼0.2 have been measured for the FM. These results support the concept that a complete Si-nc based laser cavity can be built to emit coherent light.     

Effect of sintering temperature on structure and nonlinear dielectric properties of Ba0.6Sr0.4TiO3 ceramics prepared by the citrate method

Ba_0.6Sr_0.4TiO₃ ceramics were prepared by a citrate precursor method. The structure and nonlinear dielectric properties of the resulting ceramics were investigated within the sintering temperature range 1200-1300 °C. Adopting fine Ba_0.6Sr_0.4TiO₃ powder derived from the citrate method was confirmed to be effective in reducing the sintering temperatures required for densification. The ceramic specimens sintered at 1230-1280 °C presented relative densities of around 95%. A significant influence of sintering temperature on the microstructure and nonlinear dielectric properties was detected. The discrepancy in nonlinear dielectric behavior among the specimens sintered at different temperatures was qualitatively interpreted in terms of the dielectric response of polar micro-regions under bias electric field. The specimens sintered at 1230 and 1250 °C attained superior nonlinear dielectric properties, showing relatively low dielectric losses (tan δ) of 0.24% and 0.22% at 10 kHz together with comparatively large figure of merits (FOM) of 121 and 142 at 10 kHz and 20 kV/cm, respectively.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.     

Mixed water/OH structures on Pd(1 1 1)

Chemisorbed O and water react on Pd(1 1 1) at low temperatures to form a mixed OH/H₂O layer with a (√3 × √3)R30° registry. Reaction requires at least two water molecules to each O before the (2 × 2)O islands are consumed, the most stable OH/water structure being a (OH + H₂O) layer containing 0.67 ML of oxygen, formed by the reaction 3H₂O + O → 2(H₂O + OH). This structure is stabilised compared to pure water structures, decomposing at 190 K as OH recombines and water desorbs. The (√3 × √3)R30° − (OH + H₂O) phase cannot be formed by O/H reaction and is distinct from the (√3 × √3)R30° structure formed by O/H coadsorption below 200 K. Mixed OH/water structures do not react with coadsorbed H below 190 K on Pd(1 1 1), preventing this phase catalyzing the low temperature H₂/O₂ reaction which only occurs at higher temperatures.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

Design and electro-optical characterization of Si-based resonant cavity light emitting devices at 850 nm

We report on the fabrication and characterization of Si/SiO₂ Fabry-Perot microcavities. These structures are used to enhance the external quantum efficiency along the cavity axis and the spectral purity of emission from silicon rich oxide films that are used as active media to fabricate a Si based RCLED (resonant cavity light emitting devices). A new structure to electrically pump the active media in the resonant cavity has been designed. These structures are fabricated by chemical vapour deposition on a silicon substrate. The microcavities are tuned at 850 nm and present a quality factor ranging from 17 to 150 depending on the number of pairs constituting the dielectric mirrors. An enhancement of the electro and photoluminescence (PL) signal of 20 times is achieved for the selected emission wavelength. These cavities are characterized by TEM analysis to evaluate film uniformity, thicknesses and the densification after annealing processes for temperature ranging from 800 to 1100 °C. The electrical properties of the active media are analyzed. The electroluminescence spectral features are compared with PL spectra correlated with the quality factor of the cavities. The photometric diagram shows also a high directionality of the emitted light within a 30° cone from the sample normal.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.