Design of a highly sensitive photonic crystal waveguide platform for refractive index based biosensing

In this paper, a photonic crystal waveguide platform on silicon-on-insulator substrate is proposed in order to realize a highly sensitive refractive index based biosensor. Following the design, the analysis of the sensor structure are made by using the three dimensional Finite Difference Time Domain method. The principle of sensing is based on the change in refractive index, which in turn changes the output spectrum of the waveguide. Results show that the sensitivity of the sensor depends mainly on the geometrical properties of the defect region of the photonic crystal structure. The phenomenon is verified for various samples having refractive index ranging from 1 (air) to 1.57 (Bovine serum albumin). Further, the structure is compared with few other conventional photonic crystal waveguide designs to analyze the sensing performance. The estimated value of sensitivity of the sensor is found to be 260 nm/RIU with a detection limit of 0.001 RIU. This high sensitivity can enhance the performance of low-concentration analytes detection.     

Quantum dot bio-conjugate: as a western blot probe for highly sensitive detection of cellular proteins

In the present study, we report a quantum dot (QD)-tailored western blot analysis for a sensitive, rapid and flexible detection of the nuclear and cytoplasmic proteins. Highly luminescent CdTe and (CdTe)ZnS QDs are synthesized by aqueous method. High resolution transmission electron microscopy, Raman spectroscopy, fourier transform infrared spectroscopy, fluorescence spectroscopy and X-ray diffraction are used to characterize the properties of the quantum dots. The QDs are functionalized with antibodies of prostate apoptosis response-4 (Par-4), poly(ADP-ribose) polymerases and β actin to specifically bind with the proteins localized in the nucleus and cytoplasm of the cells, respectively. The QD-conjugated antibodies are used to overcome the limitations of conventional western blot technique. The sensitivity and rapidity of protein detection in QD-based approach is very high, with detection limits up to 10 pg of protein. In addition, these labels provide the capability of enhanced identification and localization of marker proteins in intact cells by confocal laser scanning microscopy.     

Porous solgel fiber as a transducer for highly sensitive chemical sensing

A novel solgel process for making porous silica fiber and doping the fiber core with sensing material is described. A CoCl(2) -doped solgel fiber was fabricated and was used to construct an active-core optical fiber moisture sensor. Test results show that the sensitivity of the active-core optical fiber sensor is much higher than that of an evanescent-wave-based optical fiber sensor.     

Coherent transients as a highly sensitive probe for femtosecond pulse shapers

We report on the application of shaped femtosecond pulses on low-field chirped excitation of an atomic two-level system. The induced transient phenomena can be considered as a phase diagram of the excitation pulse. Their high sensitivity to the phase-modulated pulse is analyzed, by comparing numerical solutions of the time-dependent Schrödinger equation with experimental results. These coherent transients allow for high precision calibration of pulse-shaping setups where usual methods are less efficient. As an illustration, a comparison between 128-pixel and 640-pixel spatial light modulator pulse shapers is given. PACS 32.80.Qk; 42.50.Md; 42.65.Re; 82.53.KpThis revised version was published with corrections to the e-mail address of the corresponding author, to Fig. 1, to a typo in page 4: sensitivity instead of sensivity, and to the references, which now have all authors listed.     

Spectral-domain optical coherence reflectometric sensor for highly sensitive molecular detection

We describe what we believe to be a novel use of spectral-domain optical coherence reflectometry (SD-OCR) for highly sensitive molecular detection in real time. The SD-OCR sensor allows identification of a sensor surface of interest in an OCR depth scan and monitoring the phase alteration due to molecular interaction at that surface with subnanometer optical thickness sensitivity. We present subfemtomole detection sensitivity for etching of SiO(2) molecules and demonstrate its application as a biosensor by measuring biotin-streptavidin binding in a microfluidic device.     

Nanoscale microcavity sensor for single particle detection

Recently we demonstrated a biosensor based on a two-dimensional photonic crystal microcavity for detection of proteins. We present a theoretical and experimental study of a modified structure for single particle detection. With an active sensing volume of approximately 0.15 microm(3), the device is capable of detecting approximately 1 fg of matter. Its performance is tested with latex spheres with sizes that fall in the size range of a variety of viruses.     

Photonic crystal fiber based evanescent-wave sensor for detection of biomolecules in aqueous solutions

We demonstrate highly efficient evanescent-wave detection of fluorophore-labeled biomolecules in aqueous solutions positioned in the air holes of the microstructured part of a photonic crystal fiber. The air-suspended silica structures located between three neighboring air holes in the cladding crystal guide light with a large fraction of the optical field penetrating into the sample even at wavelengths in the visible range. An effective interaction length of several centimeters is obtained when a sample volume of less than 1 microL is used.     

Label-free tungsten disulfide quantum dots as a fluorescent sensing platform for highly efficient detection of copper(Ⅱ) ions

A fluorescent probe for the sensitive and selective determination of copper ion(Cu~(2+)) is presented. It is based on the use of tungsten disulfide quantum dots(WS_2 QDs) which is independent of the p H of solution and emits strong blue fluorescence. Copper ions could cause aggregation of the WS_2 QDs and lead to fluorescence quenching of WS_2 QDs. The change of fluorescence intensity is proportional to the concentration of Cu~(2+), and the limit of detection is 0.4 μM. The fluorescent probe is highly selective for Cu~(2+) over some potentially interfering ions. These results indicate that WS_2 QDs,as a fluorescent sensing platform, can meet the selective requirements for biomedical and environmental application.     

Photonic crystal waveguides for coarse-selectivity devices

We discuss devices based on photonic crystal planar waveguides for use as wavelength-selective applications via the mini-stopband extraction mechanism. We present experimental data for the core of a compact demux system. We show that spatial and spectral behaviours are as intended from coupled-mode theory, as well as from the finite-difference time-domain approach. Finally, we propose architectures with large crosstalk and some inherent fabrication-related advantages achieved by duplicating the basic multimode waveguide and using it as a spectral filter.     

Design of a highly sensitive photonic crystal refractive index sensor incorporating ring-shaped GaAs cavity

Two highly sensitive optical sensor topologies are proposed and simulated in this paper. The proposed structures are optimized to provide better performance characteristics such as sensitivity, detection limit, and quality factor. They are based on two-dimensional photonic crystals consisting of rectangular arrays of GaAs rods in SiO₂ substrates. Such lattices have bandgaps for transverse magnetic modes. Two-dimensional finite difference time domain and plane wave expansion methods are used for the simulation and analysis of the refractive index sensors and particle swarm optimization method is used to optimize the structural parameters. The designed structures show a high sensitivity to refractive index variations. They are able to detect refractive indices from 1.33 to 1.5. An excellent figure of merit equal to 737 RIU^?1 is observed for the proposed structure and a significant improvement is observed compared to the structures reported in the literature.     

高灵敏度空间点目标探测系统设计

本文面向空间点目标探测,设计了基于高灵敏度CMOS传感器的空间点目标探测系统。首先对CMOS传感器图像进行降噪,提高传感器的探测灵敏度;其次,采用DSP+FPGA嵌入式架构,设计了基于星图匹配信息构建的点目标探测算法,并详细介绍了算法原理和步骤。最后,采用电子星图模拟器对该探测系统进行测试。结果表明:该嵌入式系统具备1 024×1 024@20p格式视频的实时处理能力,可以探测6等星。当信噪比大于6,视轴指向误差小于1°时,对于不同运动速度、不同尺寸点目标均能准确探测,识别正确率接近100%。综合而言,该空间点目标探测方法的计算精度高、适应性强、可靠性高,能够应用于空间点目标的有效探测。     

3.3-microm microcavity light emitter for gas detection

A room-temperature resonant-cavity light source emitting at 3.327 microm is presented. It combines a CdHgTe light-emitting layer, grown by molecular beam epitaxy, and two evaporated YF(3)-ZnS Bragg mirrors. The emitter is optically pumped by a commercial low-power GaAs laser diode. Compared with an unprocessed sample, this microcavity device shows a drastic (10-fold) linewidth reduction, a 3.3-fold intensity increase at 3.327 microm , and a 2.4-fold angular-spread decrease. The emitted optical power is 15 microW , and the device is used as a light source in a basic gas-detection setup. Measurements of a butane-propane mixture in the 1 to 5x 10(-3) bar range with a 5-cm-long single-path gas cell are demonstrated.     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

Controlling the resonance of a photonic crystal microcavity by a near-field probe

We demonstrate theoretically that the resonance frequencies of high-Q microcavities in two-dimensional photonic crystal membranes can be tuned over a wide range by introducing a subwavelength dielectric tip into the cavity mode. Three-dimensional finite-difference time-domain simulations show that by varying the lateral and vertical positions of the tip, it is possible to tune the resonator frequency without lowering the quality factor. Excellent agreement with a perturbative theory is obtained, showing that the tuning range is limited by the ratio of the cavity mode volume to the effective polarizability of the nanoperturber.     

Photonic crystal tapers for ultracompact mode conversion

We have studied the coupling of a classic ridge waveguide with a two-dimensional photonic crystal (PC) waveguide, using finite-difference time-domain calculations. The ridge waveguide exhibits only a weak refractive-index confinement of light, as it is commonly used in buried-heterostructure or ridge-waveguide lasers. The light is coupled to a PC waveguide that consists of one missing row along the ?K direction in a triangular lattice of air cylinders in AlGaAs. We compare various designs for PC tapers with that of a classic taper and for butt coupling. The calculation yields high coupling efficiency that exceeds 80% for a 2.5-microm-long PC taper. In addition, the dependence of the efficiency on the PC air-fill factor and on alignment tolerances is analyzed.     

Photonic crystal waveguide directional couplers as wavelength selective optical filters

Waveguide directional couplers, formed by two closely spaced linear defect waveguides in a two-dimensional photonic crystal of air holes in a semiconductor matrix, are numerically studied using plane wave expansion and finite difference time domain methods. The coupling lengths are on a wavelength scale and show a strong wavelength dependence, allowing for the design of compact wavelength selective optical filters. Applications as a channel interleaver for the 1.55 μm wavelength range and as a micrometer sized 1.31/1.55 μm wavelength demultiplexer are presented.     

Tunable and highly sensitive temperature sensor based on graphene photonic crystal fiber

Optical fiber temperature sensors have been widely employed in enormous areas ranging from electric power industry, medical treatment, ocean dynamics to aerospace. Recently, graphene optical fiber temperature sensors attract tremendous attention for their merits of simple structure and direct power detecting ability. However, these sensors based on transfer techniques still have limitations in the relatively low sensitivity or distortion of the transmission characteristics, due to the unsuitable Fermi level of graphene and the destruction of fiber structure, respectively. Here, we propose a tunable and highly sensitive temperature sensor based on graphene photonic crystal fiber (Gr-PCF) with the non-destructive integration of graphene into the holes of PCF. This hybrid structure promises the intact fiber structure and transmission mode, which efficiently enhances the temperature detection ability of graphene. From our simulation, we find that the temperature sensitivity can be electrically tuned over four orders of magnitude and achieve up to ～ 3.34×10^-3 dB/(cm·℃) when the graphene Fermi level is ～ 35 meV higher than half the incident photon energy. Additionally, this sensitivity can be further improved by ～ 10 times through optimizing the PCF structure (such as the fiber hole diameter) to enhance the light-matter interaction. Our results provide a new way for the design of the highly sensitive temperature sensors and broaden applications in all-fiber optoelectronic devices.     

Photonic crystal resonator integrated in a microfluidic system

We report on a novel optofluidic system consisting of a silica-based 1D photonic crystal, integrated planar waveguides, and electrically insulated fluidic channels. An array of pillars in a microfluidic channel designed for electrochromatography is used as a resonator for on-column label-free refractive index detection. The resonator was fabricated in a silicon oxynitride platform, to support electro-osmotic flow, and operated at lambda=1.55 microm. Different aqueous solutions of ethanol with refractive indices ranging from n=1.3330 to 1.3616 were pumped into the column/resonator, and the transmission spectra were recorded. Linear shifts of the resonant wavelengths yielded a maximum sensitivity of Deltalambda/Deltan=480 nm/RIU (refractive index unit), and a minimum difference of Deltan=0.007 RIU was measured.     

Photonic crystal waveguides utilizing a modulated lattice structure

We experimentally demonstrate a new class of optical waveguide consisting of a-Si/SiO(2) autocloned photonic crystals with modulated lattice structure. The waveguide utilizes the macroscopic form birefringence of photonic crystals and confines light by the difference in the effective refractive index. A monopole modal field with spot diameters of 6.9 micromx6.5 microm was observed at a wavelength of 1.55 microm. The propagation loss of the waveguide at the wavelength was found to be ~4.2 dB/mm at most.     

Dipole-induced grating in a waveguide-coupled photonic crystal microcavity embedding a driven three-level emitter

We study optical transmission characteristics of a waveguide-coupled photonic crystal microcavity embedding a driven three-level dipole emitter. By spatially modulating the control field, alternating regions of high transmission and absorption can be produced in two waveguide channels which can be used for an amplitude grating.