Huiping Tian

Nanoscale photonic crystal sensor arrays on monolithic substrates using side-coupled resonant cavity arrays.

We present nanoscale photonic crystal sensor arrays (NPhCSAs) on monolithic substrates. The NPhCSAs can be used as an opto-fluidic architecture for performing highly parallel, label-free detection of biochemical interactions in aqueous environments. The architecture consists of arrays of lattice-shifted resonant cavities side-coupled to a single PhC waveguide. Each resonant cavity has slightly different cavity spacing and is shown to independently shift its resonant peak (a single and narrow drop) in response to the changes in refractive index. The extinction ratio of well-defined single drop exceeds 20 dB. With three-dimensional finite-difference time-domain (3D-FDTD) technique, we demonstrate that the refractive index sensitivity of 115.60 nm/RIU (refractive index unit) is achieved and a refractive index detection limit is approximately of 8.65×10-5 for this device. In addition, the sensitivity can be adjusted from 84.39 nm/RIU to 161.25 nm/RIU by changing the number of functionalized holes.

High sensitivity and high Q-factor nanoslotted parallel quadrabeam photonic crystal cavity for real-time and label-free sensing

We experimentally demonstrate a label-free sensor based on nanoslotted parallel quadrabeam photonic crystal cavity (NPQC). The NPQC possesses both high sensitivity and high Q-factor. We achieved sensitivity (S) of 451 nm/refractive index unit and Q-factor >7000 in water at telecom wavelength range, featuring a sensor figure of merit >2000, an order of magnitude improvement over the previous photonic crystal sensors. In addition, we measured the streptavidin-biotin binding affinity and detected 10 ag/mL concentrated streptavidin in the phosphate buffered saline solution.

Design of simultaneous high-Q and high-sensitivity photonic crystal refractive index sensors

Sensitivities (S) and quality factors (Q) have been trade-offs in label-free optical resonator sensors, and optimal geometry that maximizes both factors is under active development. In this paper, we demonstrate that the nanoslotted parallel multibeam cavity possesses unexplored high S and high Q. We achieve S>800  nm/RIU (refractive index unit) and Q>107 in liquid at telecom wavelength range when absorption is neglected. To the best of our knowledge, this is the first geometry that features both high S and Q factors, and thus is potentially an ideal platform for refractive index-based biochemical sensing.

High-Q and high-sensitivity width-modulated photonic crystal single nanobeam air-mode cavity for refractive index sensing

We propose a novel optical sensor based on a one-dimensional (1D) photonic crystal (PhC) single nanobeam air-mode cavity (SNAC). The performance of the device is investigated theoretically. By introducing a quadratically modulated width tapering structure, a waveguide-coupled 1D-PhC SNAC with a calculated high quality factor of 5.16×10(6) and an effective mode volume of V(eff)∼2.18(λ/n(si))(3) can be achieved. For the air mode mentioned above, the light field can be strongly localized inside the air region (low index) and overlaps sufficiently with the analytes. Thus, the suggested PhC SNAC can be used for high-sensitivity refractive index sensing with an estimated high sensitivity of 537.8 nm/RIU. To the best of our knowledge, this is the first PhC single nanobeam geometry that features both high Q-factors and high sensitivity, and is potentially an ideal platform for realizing ultracompact lab-on-a-chip applications with dense arrays of functionalized spots for multiplexed sensing.

Refractive index sensing utilizing parallel tapered nano-slotted photonic crystal nano-beam cavities

We demonstrate refractive index sensing using parallel tapered nano-slotted photonic-crystal nano-beam cavities with three-dimensional (3D) finite-difference time-domain (3D-FDTD) simulation. The electric field of the cavity mode is strongly concentrated in the slot region leading to a large light–matter overlap, which is expected to add a significant contribution to sensitivity, and thus we present high refractive-index sensitivity of more than 600  nm/refractive index units. Additionally, the quality (Q)-factor in the proposed design is theoretically investigated, and through tapering the diameter of the pores outside the Bragg mirrors in nano-beam cavities and the width of the adjacent nano-slots, an optimal Q-factor of 11770 is obtained. A high figure of merit (FOM=4637) of the designed model has been obtained. We anticipate that this geometry is potentially an ideal platform for refractive-index based bio-sensing.

Design Low Crosstalk Ring-Slot Array Structure for Label-Free Multiplexed Sensing

We theoretically demonstrate a low crosstalk ring-slot array structure used for label-free multiplexed sensing. The proposed sensors array is based on an array of three ring-slot and input/output line defect coupling waveguides. Each ring-slot cavity has slightly different cavity spacing and different resonant frequency. Results obtained using two dimensional finite-difference time-domain (2D-FDTD) simulation indicate that the resonant frequencies of each sensor unit in response to the refractive index variations are independent. The refractive index sensitivity is 134 ∼ 145.5 nm/RIU (refractive index unit) and the Q factors more than 104 can be achieved. The calculated detect limit lower than 1.13 × 10−4 RIU is obtained. In addition, an extremely small crosstalk lower than −25.8 dB is achieved among the array of three ring-slot cavities. The results demonstrate that this multiplexed sensor array is a promising platform for integrated optical devices and enables highly parallel label-free detection.

Microdisplacement sensor based on high-Q nanocavity in slot photonic crystal

A microdisplacement sensor formed by a fixed and mobile hole-array based slot photonic crystal (slot-PhC) components is demonstrated. The sensing technique is based on a nanoscale cavity with a high-Q factor in photonic crystals (PhCs). The high-Q nanocavity (H0-cavity) is formed by only laterally shifting two adjacent holes outwards slightly in the opposite direction. The properties of the microdisplacement sensor are analyzed theoretically and simulated using the finite-difference time-domain method. The simulation results indicate that with a proper operating frequency, a quasilinear measurement of microdisplacement is achieved with a sensitivity of 1.0a −1 ( a is the lattice constant) in the sensing range between 0.00a and 0.20a. Although other researchers such as Xu et al. 1 who demonstrated a micro displacement sensor possessing an equivalent sensitivity, the Q factor is only 40. In this paper, combined with harmonic analysis, we show numerically that an intrinsic Q value of up to 6×10 3 is achieved. In addition, it is worth mentioning that when the parameters of the H0-cavity are determined, the resonant frequency of the H0-cavity remains approximately constant as the mobile PhC segment shifts along the common axis. It will be easier and more accurate for measurements in practical applications.

Ultrahigh-

We present a novel optical sensor based on the design of ultrahigh-Q and low-mode-volume 1-D single photonic crystal (PhC) slot nanobeam cavity (SNC) in which the air-hole radius is parabolically tapered. The performance of the device is investigated theoretically. In order to achieve high Q-factor and high sensitivity simultaneously, the slot geometry is exploited to make the optical field strongly localized inside the low index region and overlaps sufficiently with the analytes. With the three-dimensional finite-difference time-domain (3D-FDTD) method, we demonstrate that the proposed single 1-D PhC-SNC sensor device possess an ultrahigh sensitivity (S) up to ~900 nm/RIU (refractive index unit, RIU) and a high Q-factor in air up to > 107 at the telecom wavelength range. The optimized figure of merit is > 107. In addition, an ultrasmall mode volume of Vm ~0.01 (λ/ηair)3 has been achieved, which is more than three orders of magnitude smaller than our previous works [Appl. Phys. Lett. 105, 063118 (2014)] and, thus, is potentially an ideal platform for realizing ultracompact laboratory-on-a-chip applications with dense arrays of functionalized spots for multiplexed gas sensing.

Q

In this paper, unique surface sensing property and enhanced sensitivity in microring resonator biosensors based on subwavelength grating (SWG) waveguides are studied and demonstrated. The SWG structure consists of periodic silicon pillars in the propagation direction with a subwavelength period. Effective sensing region in the SWG microring resonator includes not only the top and side of the waveguide, but also the space between the silicon pillars on the light propagation path. It leads to greatly increased sensitivity and a unique surface sensing property in contrast to common evanescent wave sensors: the surface sensitivity remains constantly high as the surface layer thickness grows. Microring resonator biosensors based on both SWG waveguides and conventional strip waveguides were compared side by side in surface sensing experiment and the enhanced surface sensing capability in SWG based microring resonator biosensors was demonstrated.

and Low-Mode-Volume Parabolic Radius-Modulated Single Photonic Crystal Slot Nanobeam Cavity for High-Sensitivity Refractive Index Sensing

We report the design of one dimensional photonic crystal nanobeam cavities with elliptical holes that is fully encapsulated in the water environment and can confine the light in the low index region. The proposed structure, based on the principle of gentle confinement of the electromagnetic field, is designed by tapering the width of the host photonic crystal waveguide away from the center of the cavity while keeping other parameters constant. With elliptical-hole low-index mode nanobeam cavities and tapered waveguide widths, through three dimensional finite-difference time-domain simulations, large band-gap and high reflectivity are achieved to confine the optical mode. Also, the electric field is confined in the elliptical holes, which helps to enhance the sensitivity. The simulation results demonstrate that we achieve the highest quality factor of 1.35 × 105 when 15 taper segments and 15 additional mirror segments are placed on both sides of the host waveguide. A sensitivity of 390 nm/RIU (refractive index unit) and mode volume of 2.23 (λres/nsi)3 are achieved in the water environment, thus showing exceptionally good on-chip sensing properties with respect to high sensitivity, small footprints and masses.