Intrinsic Waveguide Research Articles

Theoretical study of microcavity-enhanced absorption spectroscopy for mid-infrared methane detection using a chalcogenide/silica-on-fluoride horizontal slot-waveguide racetrack resonator.

The reported chalcogenide (ChG) rectangular waveguide sensors with a small evanescent field need a large waveguide length to obtain an enhanced light-gas interaction effect. To make such sensors compact and improve the light-gas interaction effect, a microcavity-enhanced absorption spectroscopy technique for methane (CH4) detection was proposed using a mid-infrared chalcogenide/silica-on-fluoride horizontal slot-waveguide racetrack resonator. For the horizontal slot waveguide, an equivalent sensor model (ESM) and related formulations were proposed to simplify the analysis of the racetrack resonator sensor model (RRSM), and the ESM was verified through a comparison between the theoretical result of ESM and the simulation result of RRSM based on the finite element method (FEM). Due to the use of a chalcogenide/silica-on-fluoride horizontal slot-waveguide structure, the waveguide parameters were optimized to obtain a high power confinement factor of 44.63% at the wavelength of 3291 nm, which is at least 5 times higher than other ChG rectangular waveguides. The waveguide length is reduced at least 30 times due to the use of the optimized chalcogenide/silica-on-fluoride horizontal slot-waveguide and racetrack resonator. The limit of detection (LoD) is 3.87 ppm with an intrinsic waveguide loss of 3 dB/cm and an amplitude coupling ratio of 0.1 for the resonator. The response time is less than 5 µs due to the small light-gas interaction area. The influences of environmental pressure and waveguide intrinsic loss on the sensing characteristics were discussed. The compact racetrack resonator sensor structure and equivalent analytical model can also be adopted in the design of an on-chip waveguide sensor for the detection of other gas species.

Transparency in a chain of disparate quantum emitters strongly coupled to a waveguide

We demonstrate the emergence of transparent behavior in a chain of periodically spaced non-identical quantums emitters coupled to a waveguide, in the special case when the inter-atomic separation is a half-integral multiple of the resonant wavelength, i.e. $kL$ is an integral multiple of $\pi$, with $k$ being the spatial frequency and $L$ the spatial periodicity. When equal but opposite frequency detunings are assigned in pairs to a system of even number of atoms, perfect transmission ensues. When the chain size is odd, a similar assignment leads to the disappearance of collective effects as the odd atom determines the spectral behavior. We also manifest the robustness of these features against dissipative effects and show, how the spectral behavior hinges significantly on the relative detunings between the atoms as compared to the decay rate. A key distinction from the phenomenon of Electromagnetically Induced Transparency (EIT) is that in the waveguide case, the presence of an intrinsic waveguide mediated phase coupling between the atoms strongly affects the transport properties. Furthermore, while reciprocity in single-photon transport does not generally hold due to the phase coupling, we observe an interesting exception for $kL = n\pi$ at which the waveguide demonstrates reciprocal behavior with regard to both the transmission and reflection coefficients.

Broadband light-concentration with near-surface distribution by silver capped silicon nanowire for high-performance solar cells

Silicon nanowire (SiNW) shows striking light-concentration ability and thus holds promising application potentials in photonic devices. However, its narrow working waveband strongly affects its performance over the entire visible spectrum. Here, a silver capped SiNW structure (Ag-cap/SiNW) is presented to broaden the working waveband of the pure SiNW. Discrete dipole approximation simulations show that, by this structure, the light-concentration waveband can be significantly broadened from 440–620nm (pure SiNW) to 300–620nm. Thus, using the Ag-cap/SiNW in the solar cell, the ideal photocurrent density can be enhanced 16% compared with that using pure SiNW. Furthermore, the concentrated light shows the feature of the strong near-surface distribution around the Ag-cap/SiNW. This distribution feature gives a reasonable explanation for the huge superiority of the radial junction SiNW solar cells to axial junction SiNW solar cells from optical aspect. Additionally, due to the intrinsic waveguide property of SiNW, this Ag-cap/SiNW also has great potential applications in photonic devices such as nanoscale optical biosensors and light-integrated-chips.

Tunable optical properties in engineered one-dimensional coupled resonator optical waveguides

In this paper, the optical properties in finite size tilted and engineered one dimensional coupled resonator optical waveguide have been investigated. The large dependence of the optical transmittance, dispersion parameter and its higher order slope such as transmittance group delay, third order dispersion and intrinsic waveguide induced loss on the oblique incidence and fraction factor, as the ratio of the optical thicknesses of two adjacent layers, have been studied. Our results showed that as a consequence of changing the fraction factor, at normal incidence, photonic band gap zone, flat portion of third order dispersion curve and maximum magnitude of the transmission group delay can be tuned in long range of wavelength (red shift) slightly. Despite of slight tuning the optical properties in one dimensional coupled resonator optical waveguide by changing the fraction factor, incidence angle has a significant large magnitude of tunability in the overall region of operational wavelength. This fact yields us by changing the incidence from 30 to 60, the operational wavelength can be tuned between two main optical communication windows, while by changing the fraction factor, dispersion and its higher order can be fine tuned in each of optical communication windows which are very useful in wavelength division multiplexing systems and planar lightwave circuits.

Polarisation-independent bidirectional triplexer using grating-assisted multimode interference coupler in silica-on-silicon platform

A bidirectional triplexer based on a Bragg grating assisted multimode interference (MMI) coupler has been designed and fabricated. The MMI coupler can multiplex/demultiplex the 1310 nm wavelength (i.e. upload data) and the 1490 nm wavelength (i.e. download data). Moreover, the grating written in the waveguide of the MMI can reflect the 1550 nm wavelength (i.e. download video signals) to the other download port. Using a polarised UV ArF excimer laser irradiation, intrinsic waveguide birefringence can be compensated. Therefore, the fabricated device is polarasation insensitive. The working bandwidths of the final triplexer for 1310, 1490, and 1550 nm are about 110, 50, and 15 nm, respectively.

Molecular concentration sensor based on the diffraction resonance mode of gold nanowiregratings

We demonstrate a new diffraction resonance mode in waveguide gold nanowire gratingstructures, which shows much higher sensitivity than the intrinsic waveguide resonancemode to the change in the environmental refractive index. This enables realization of amolecular concentration sensor. Measurements on ethanol/water mixture andglucose solutions in water with different concentrations indicate a sensitivity of thissensor as high as 400 nm per unit refractive index. However, the narrow-bandoptical response implies higher resolution than the displayed spectrum shift. Thissensor device provides a simple and practical approach to perform on-site andcontamination-free characterization of the concentrations of chemical and biologicalsolutions.

Photochemistry in Photonic Crystal Fiber Nanoreactors

We report the use of a liquid-filled hollow-core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow-core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8 nL cm(-1) in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin B(12), cyanocobalamin (CNCbl) to hydroxocobalamin ([H(2)OCbl](+)) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real-time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore, the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly--in our experiments one thousand times faster than in a conventional cuvette.

Adjustable dispersion compensating in one dimensional coupled resonator planar optical waveguides

In this paper, the optical properties in finite size one dimensional coupled resonator optical waveguide are investigated. The large dependence of the group velocity, dispersion parameter and its higher order slope such as transmission group delay, third order dispersion and intrinsic waveguide induced loss have been studied. By engineering the optical contrast ratio of the medium by using some accessible and compatible materials, according to the equivalent layers theory, the above mentioned parameters can either be adjusted or enhanced dramatically, according to the configuration of the optical networks and users’ requirements. Also, by setting proper choice of the optical contrast ratio, super flattening of the transmittance group delay, third order dispersion spectrum and reduced intrinsic waveguide induced loss can be realized. Our results show the potential applications of the device for preventing the satellite pulse creating and transmitting pulse information distortion in multichannel lightwave systems.

CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band

Submicrometer silicon photodiode waveguides, fabricated on silicon-on-insulator substrates, have photoresponse from <1270 to 1740 nm (0.8 AW <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 1550 nm) and a 3-dB bandwidth of 10 to 20 GHz. The p-i-n photodiode waveguide consists of an intrinsic waveguide 500times250 nm where the optical mode is confined and two thin, 50-nm-thick, doped Si wings that extend 5 mum out from either side of the waveguide. The Si wings, which are doped one p-type and the other n-type, make electric contact to the waveguide with minimal effect on the optical mode. The edges of the wings are metalized to increase electrical conductivity. Ion implantation of Si <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> 1times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at 190 keV into the waveguide increases the optical absorption from 2-3 dBmiddotcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 200-100 dBmiddotcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and causes the generation of a photocurrent when the waveguide is illuminated with subbandgap radiation. The diodes are not damaged by annealing to 450 degC for 15 s or 300 degC for 15 min. The photoresponse and thermal stability is believed due to an oxygen stabilized divacancy complex formed during ion implantation

Theory of Raman-Mediated Pulsed Amplification in Silicon-Wire Waveguides

We present a comprehensive theoretical study of pulsed stimulated Raman scattering in silicon wires. The pulse dynamics is described by a system of coupled equations, which describes intrinsic waveguide optical losses, phase shift and losses due to free-carriers (FCs) generated through two-photon absorption (TPA), first- and second-order frequency dispersion, self-phase and cross-phase modulation, TPA losses, and the interpulse Raman interaction. Furthermore, the influence of the FCs on the pulse dynamics is incorporated through a rate equation. The corresponding system of equations has then been numerically integrated, and phenomena such as noise-seeded Raman amplification, pulsed Raman amplification, and Raman-mediated pulse interaction have been described.

Development of stripped-cladding optical fiber sensors for continuous monitoring: I. Theoretical study of a referencing method for measuring refractive indices of fluids

Intrinsic waveguide optical sensors, IWOS, formed by an exposed core (modified in some cases with a material sensitive to a target parameter), in contact with a measured fluid, have diverse applications. We have identified a novel referencing method for this type of sensor. The referencing method is a self-calibrating procedure that allows for development of portable sensors capable of continuous monitoring. This paper examines the theoretical basis of the proposed referencing method and constitutes a first step in the practical design of such optical sensors. Since the most commonly measured parameter with such sensors is the real term (n) of the complex refractive indices (m=n+ki), real index of refraction for different fluids and glass waveguide materials are used to investigate sensor responses. Case studies, in which ncore>nmeasuredfluid and ncore<nmeasuredfluid are considered, suggest that an easily configurable optical photo-refractometer can be designed with a variable dynamic range. This means that IWOS physicochemical detection characteristics (extent of reaction, species concentrations, etc.) can be usefully related to refractive index variations through self-calibrating strategies. It is also shown that portable spectrophotometers allow for measurement of spectral signals that are diagnostic of the measured material.

Photonic states deep into the waveguide cutoff frequency of metallic mesh photonic crystal filters

We examine the optical properties of three-dimensional metallic photonic crystals made from a periodic stacking of thin metallic mesh layers separated by homogeneous dielectric films by means of a combination of the plane-wave-based transfer-matrix method and analytical modal solution approach. Although each metallic mesh layer can serve as a frequency-selective surface and involves an intrinsic long-wavelength waveguide cutoff to electromagnetic waves, pass bands and new band gaps can exist far below the cutoff frequency due to the global coupling effect among different mesh layers. The results for the transmission spectra and photonic band structures are in good agreement with existing experimental measurements. It is found that the position of the pass bands and band gaps strongly depends on the thickness and composite of the separation layer between the adjacent metallic mesh layers.

High birefringence control and polarization insensitive Bragg grating fabricated in PECVD planar waveguide with UV polarized irradiation

A method for rapid compensation of high intrinsic birefringence in plasma-enhanced chemical vapor deposition (PECVD) based planar waveguides is presented. The intrinsic waveguide birefringence is compensated by a large UV induced opposite birefringence generated with polarized UV ArF excimer laser irradiation (oriented normal to the waveguide axis) and enhanced further through hydrogen loading. The fabrication of polarization insensitive Bragg gratings in PECVD planar waveguides with a large intrinsic birefringence of 6.3×10−4 is demonstrated by trimming it to zero birefringence with a low fluence blanket UV polarized exposure. After annealing for removal of the hydrogen, the polarization insensitive nature of the Bragg grating is unchanged.

Photonic crystal waveguide-mode orthogonality conditions and computation of intrinsic waveguide losses.

We simulate the propagation of light in a W1 planar photonic crystal waveguide with the three-dimensional finite-difference time-domain method and apply an inner product against previously calculated mode profiles to the simulated field cross sections. We show that this inner product satisfies mode orthogonality for both photonic crystal waveguides and segmented waveguides and use the obtained data to evaluate waveguide losses.

Thin film optical waveguide type UV sensor using a photochromic molecular device, spirooxazine

New waveguide type UV sensor using photochromic dye as a molecular light transducer has been studied. When exposed to UV radiation, thin film UV sensing membrane that is composed of spirooxazine derivative and polymer matrix (PMMA, poly(methyl methacrylate)) shows remarkable light absorption in the visible range ( λ max =587 nm ) reversibly from the original characteristics of spirooxazine dye. Since this polymeric thin film can also act as an intrinsic waveguide, it is useful to construct planar waveguide type sensor. The output intensity of inputted and guided visible light ( λ=633 nm , He–Ne laser) through planar thin film waveguide was decreased according to the amount of photochromic dye absorbance by the UV photon energy in the optical path. The sensitivity of waveguide type UV sensor was 64 times greater than that of a traditional transmittance type polymer membrane.

Optical Switching by the Use of Guided Wave Geometry Composed of a Polymeric Thin-Film Containing Photochromic Dye

A new optical switching device based on photochromic dye isomerization in a planar type active waveguide was proposed. The polymeric thin-film composed of a well designed spirooxazine derivative shows new light absorption band in the visible range (γmax = 587 nm) due to very fast photochromism under UV irradiation. Since this polymeric thin-film can also act as an intrinsic waveguide, the output intensity of input and guided visible light (γ = 633 nm, He-Ne Laser) through planar thin-film was decreased effectively during UV irradiation.

Polarization-insensitive wavelength filters by birefringence compensation of vertical couplers

We demonstrate a novel birefringence compensation technique to reduce the polarization dependence of AlGaAs directional coupler wavelength filters operating in the 770 to 970 nm wavelength range. By using a multiple quantum well structure to compensate intrinsic waveguide birefringence, we fabricated vertical coupler filters with polarization-dependent wavelength shifts from 0.1 to 2 nm, significantly less than those achieved for conventional devices. Experimental results show that both the filter wavelength and insertion loss can be rendered polarization insensitive by this technique.

Optical waveguide modelling and design for evanescent field chemical sensors

Analytical and numerical techniques are presented for determining the sensitivity of optical waveguides as evanescent wave absorption sensors. Analytical expressions based on a ray optics approach are derived for planar and multimode fibre sensors. The matrix method is used for multilayer structures to determine both the sensitivity and the effect of intrinsic waveguide losses. It is shown how more complex waveguide geometries such as fibres with a D-shaped cross-section may be assessed by the finite-difference method combined with perturbation analysis. Some examples are given demonstrating how the results enable the design of sensors with optimum sensitivity.

Remote gas sensing with mid-infra-red hollow waveguide

Remote detection of ethylene has been demonstrated using infra-red hollow glass waveguides and a CO2 laser. By a method of intrinsic waveguide sensing, gas detection at ppm levels can be achieved.