Intrinsic Detection Limit Research Articles

Embedded racetrack microring resonator sensor based on GeSbSe glasses.

In this article, a compact racetrack double microring resonator (MRR) sensor based on Ge28Sb12Se60 (GeSbSe) is investigated. The sensor device consists of a racetrack microring, an embedded small microring, and a strip waveguide. Electron beam lithography (EBL) and dry etching are used to fabricate the device. The compact racetrack double MRR device are obtained with Q-factor equal to 7.17 × 104 and FSR of 24 nm by measuring the transmission spectrum. By measuring different concentrations of glucose solutions, a sensitivity of 297 nm/RIU by linear fitting and an intrinsic limit of detection (iLOD) of 7.40 × 10-5 are obtained. It paves the way for the application of chalcogenide glasses in the field of biosensing.

Two-Way Detection of COVID-19 Spike Protein and Antibody Using All-Dielectric Metasurface Fluorescence Sensors.

COVID-19 (or SARS-CoV-2) has deeply affected human beings worldwide for over two years, and its flexible mutations indicate the unlikeliness of its termination in a short time. Therefore, it is important to develop a quantitative platform for direct COVID-19 detection and human status monitoring. Such a platform should be simpler than nucleic acid amplification techniques such as polymerase chain reaction, and more reliable than the disposable test kits that are based on immunochromatography. To fulfill these requirements, we conducted proof-of-concept experiments for the quantitative detection of spike glycoprotein peptides and antibodies in one platform, i.e., all-dielectric metasurface fluorescence (FL) sensors. The high capability to enhance FL intensity enabled us to quantitatively measure the glycoproteins and antibodies more efficiently compared with the previous methods reported to date. Furthermore, the intrinsic limit of detection in the metasurface FL sensors was examined via confocal microscopy and found to be less than 0.64 pg/mL for glycoprotein peptides. Moreover, the sensors had a dynamic range more than five orders that of the target concentrations, indicating extremely high sensitivity. These two-way functions of the metasurface FL sensors can be helpful in reducing daily loads in clinics and in providing quantitative test values for proper diagnosis and cures.

High Q-factor, ultrasensitivity slot microring resonator sensor based on chalcogenide glasses.

In this article, the chalcogenide slot waveguide is theoretically studied, and the highest power confinement factors of the slot region and the cladding region are obtained to be 36.3% and 56.7%, respectively. A high-sensitivity chalcogenide slot microring resonator sensor is designed and fabricated by electron-beam lithography and dry etching. The structure increases the sensitivity of the sensor compared with the conventional evanescent field waveguide sensor. The cavity has achieved a quality factor of 1 × 104 by fitting the resonant peaks with the Lorentzian profile, one of the highest quality factors reported for chalcogenide slot microring resonators. The sensor sensitivity is measured to be 471 nm/RIU, which leads to an intrinsic limit of detection of 3.3 × 10--4 RIU.

Highly Sensitive Refractive Index-Based Sensor for DNA Hybridization Using Subwavelength Grating Waveguide

A biosensor for detecting DNA hybridization via refractive index sensing has been demonstrated using a subwavelength grating (SWG) waveguide. Waveguide dimensions are selected by parametric optimizations to increase the mode-analyte overlap with considerations of typical silicon-on-insulator fabrications technology. DNA layer is added with a linker layer on silicon pillars of the SWG waveguide to optimize and detect the DNA hybridization. Some essential characteristics such as mode overlap factor, change in effective refractive index, waveguide sensitivity, and shift in resonance wavelength are computed for the different dimensional parameters of the SWG waveguide in DNA hybridization along with mode field intensity and normalized power using the finite element method. The DNA hybridization is shown by variation in normalized power of interacting light in cladding region of the waveguide for 0% to 100% fractional change of dsDNA and ssDNA. Waveguide sensitivity, shift in resonance wavelength, device sensitivity, and intrinsic limit of detection are obtained to ∼ 0.157, 7.01, 605 nm/RIU, and 1.30 × 10−4 RIU, respectively, for the optimized structure of SWG waveguide in sensing of DNA hybridization, which could be suitable aspects for detecting DNA hybridization.

Performance improvement of a silicon nitride ring resonator biosensor operated in the TM mode at 1310 nm.

Silicon-based ring resonators have been demonstrated to be a key element to build lab-on-chip devices due to their ability to perform as label-free photonic sensors. In this work, we demonstrate photonic biosensing using silicon nitride ring resonators operated in the TM mode around 1310 nm wavelengths. Our results show that operating the devices using the TM mode results in an increased sensitivity in comparison with the typically used TE mode, while working at 1310 nm wavelengths compared to 1550 nm contributes to an increased quality factor. As a result, a reduction in the intrinsic limit of detection is achieved, indicating the suitability of TM modes in the 1310 nm regime for biosensing using integrated photonics.

Sensitivity and intrinsic limit of detection improvement in a photonic refractive-index sensor

In this work, we study the bulk refractive index (RI) sensitivity (S) and intrinsic limit of detection (ILOD) of a micro racetrack resonator (MRR) based RI-sensor. Sensing measurements are performed with solutions of sodium chloride (NaCl)-water. The sensing device includes an array of multiple silicon-oxy-nitride (SiON) MRRs, fibre-coupled to a vertical-cavity surface-emitting laser (VCSEL) operating at 850 nm. We particularly choose 850 nm for sensing measurement, since the water absorption coefficient at this wavelength is three orders of magnitude lower than at 1550 nm (another conventional wavelength in biosensing). Different concentrations of NaCl–water solutions (spanning from 0.0% to 1.8% weight/weight), are flowed on top of the MRR surface in the cladding layer. Measuring of whispering gallery mode (WGM) resonance wavelength shifts in NaCl–water solutions, confirming a S and ILOD of 31.66 nm/RIU and 5.98×10−4 RIU, respectively. Furthermore, with exact engineering of the physical parameters of the system, the best S and ILOD of 325.85 nm/RIU and 6.14×10−5 RIU are found, respectively. The ILOD parameter delineates the minimum amount of RI or correspondingly minimum concentration of a biological solution, which can be detected by a RI-sensor. In this regard, this device can find applications in the exact measurement of NaCl in real biological sample solutions.

Label-Free Detection of Morpholino-DNA Hybridization Using a Silicon Photonics Suspended Slab Micro-Ring Resonator

In this study, deoxyribonucleic acid (DNA) hybridization was demonstrated using a suspended silicon photonics micro-ring resonator with a 90 nm-thick slab and morpholino as the capture probe. Complementary DNA of various concentrations were tested achieving a surface sensitivity of 2.12 nm/nM, a detection limit of 250 pM, and an intrinsic detection limit of 36.9 pM. A bulk sensitivity of 98 nm/RIU and an intrinsic detection limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.03\times 10^{-3}$</tex-math></inline-formula> RIU was also measured upon exposure to isopropanol/deionized water solutions. With these characteristics, the suspended 90 nm slab ring sensor proved as a promising candidate for lab-on-a-chip bio-sensing applications.

High-sensitivity refractive index sensor based on Ge–Sb–Se chalcogenide microring resonator

In this article, we report a liquid sensing device based on a Ge–Sb–Se microring resonator (MRR) operating at a wavelength of 1550 nm. The transmission loss of the waveguide is 4.3 dB/cm, and the intrinsic Q factor of the MRR is 7.74 × 104, with an extinction ratio of approximately 40 dB. By detecting the different concentrations of NaCl solutions, we observe that the sensitivity of our proposed device is approximately 123 nm/RIU, and the intrinsic limit of detection is approximately 3.24 × 10−4 RIU. Compared with the other materials, such as silicon, Si3N4, or polymer, the moderate refractive index (RI) of Ge28Sb12Se60 glass allows it to obtain a high sensitivity and a high Q factor, while maintaining a strong confinement on the light field to avoid suffering from bending loss under a small bending radius, with a compact structure.

On Noise Performance of Dual-Gated Silicon FET Biosensors With Schottky Contacts

We analyzed the low-frequency noise (LFN) of dual-gated field-effect transistor (DG-FET) biosensors with Schottky contacts. We found the flicker noise at the sensing insulator-semiconductor interface to be the major noise source while employing Schottky contacts to have minimal noise contribution with a sufficiently large back-gate bias voltage. The measured noise dependence on transconductance further indicated the presence of a nonuniform energy distribution of interface trap density at the said sensing interface. Based on these findings, we argued that the DG structure is advantageous over its single-gated (SG) counterpart; although they possess the same intrinsic lower limit of detection (LLOD), the former could offer a larger signal gain at the optimum LLOD thanks to sufficient channel carrier supply through back-gating instead of biasing the sensing interface toward band edge with higher trap density.

Narrowband Bragg filters based on subwavelength grating waveguides for silicon photonic sensing.

Subwavelength grating (SWG) waveguides have been shown to provide enhanced light-matter interaction resulting in superior sensitivity in integrated photonics sensors. Narrowband integrated optical filters can be made by combining SWG waveguides with evanescently coupled Bragg gratings. In this paper, we assess the sensing capabilities of this novel filtering component with rigorous electromagnetic simulations. Our design is optimized for an operating wavelength of 1310 nm to benefit from lower water absorption and achieve narrower bandwidths than at the conventional wavelength of 1550 nm. Results show that the sensor achieves a sensitivity of 507 nm/RIU and a quality factor of 4.9 × 104, over a large dynamic range circumventing the free spectral range limit of conventional devices. Furthermore, the intrinsic limit of detection, 5.1 × 10-5 RIU constitutes a 10-fold enhancement compared to state-of-the-art resonant waveguide sensors.

Al2O3:Yb3+ integrated microdisk laser label-free biosensor

Whispering gallery mode resonator lasers hold the promise of an ultralow intrinsic limit of detection. However, the widespread use of these devices for biosensing applications has been hindered by the complexity and lack of robustness of the proposed configurations. In this work, we demonstrate biosensing with an integrated microdisk laser. Al2O3doped with Yb3+ was utilized because of its low optical losses as well as its emission in the range 1020-1050 nm, outside the absorption band of water. Single-mode laser emission was obtained at a wavelength of 1024 nm with a linewidth of 250 kHz while the microdisk cavity was submerged in water. A limit of detection of 300 pM (3.6 ng/ml) of the protein rhS100A4 in urine was experimentally demonstrated, showing the potential of the proposed devices for biosensing.

Parallel-Coupled Dual SiOxNy Racetrack Resonators as Biosensors with High Improved Intrinsic Limit of Detection

This study describes a chip-integrated optical biosensor based on dual coupled racetrack resonators. A physical analysis of the device is carried out using conformal transformation and coupled-mode theory. The intrinsic limit of detection is improved by a factor of 100, compared to a conventional single-resonator system, with better dynamic range as well. The proposed sensor could be a promising candidate for analyzing different components of blood samples in medical diagnostics, which is especially important for the early detection of critical cancers, such as prostate cancer.

Subwavelength Grating Metamaterial Racetrack Resonator for Sensing and Modulation

Subwavelength grating metamaterial racetrack ring resonator (SGMRTR) increases the coupling strength between the bus waveguide and the ring resonator and thus better extinction ratio can be achieved. In this paper, we summarized our recent work on SGMRTRs. We show that with the improved extinction ratio, the intrinsic detection limit of refractive index sensors can be pushed closer to the theoretical limit. We have also demonstrated the first high-speed optical modulator based on electro-optic polymer infiltrated subwavelength grating metamaterial waveguide ring resonator with measured 3-dB bandwidth of 41.4 GHz. The power consumption for digital communication is estimated to be 2.55 f J/bit.

Optimized Bent Part Coupling SiON Racetrack Resonators for Biological Sensing

In this paper, we investigate the optimized parameters of bent part coupling silicon oxynitride micro-racetrack optical resonators coupled to a straight waveguide, for an efficient design of label-free biosensor devices. A systematic engineering of waveguide-resonator characteristics for optimum geometry and field-overlap with analytes is proposed. Different parameters of system, such as coupling, intrinsic, and total quality factors of $Q_{\kappa }$ , $Q_{i}$ , and $Q_{t}$ , sensitivity ( $S$ ) and figure of merit, or intrinsic limit of detection (ILOD), are examined with interest of taking into account the dispersion effect in calculations. To the best of our knowledge, considering dispersion effect in calculations has been proposed for the first time in bent part coupling racetrack resonator-based biosensors. We have shown the effective role of dispersion on the best optimized parameters of the biosensors. These investigations result in high amounts of $S$ (435 nm/RIU) and $Q_{t}$ (≥45000), simultaneously for the proposed biosensors. The devices have been optimized for operation at a wavelength of 850 nm. The biosensing performance of our biosensor is compared with lately reported theoretical and experimental investigations. Based on the ILOD calculations, the performance of our sensor structure is improved by a factor of 0.10 compared with a resonator-based biosensor, reported lately.

Characteristics optimization in single and dual coupled silicon-on-insulator ring (disk) photonic biosensors

Silicon-on-insulator (SOI) evanescent field whispering gallery mode (WGM) ring (disk) sensors are promising components for biological sensing applications. We study the optimum design parameters that result in the highest sensitivity (S) and figure of merit (FOM) simultaneously, for single and dual coupled SOI WGM ring (disk) based sensors. These devices are respectively consisting of one and two silicon ring (disk) resonator coupled to a straight waveguide. Periodic behavior of coupling parameters in biological solutions, are modelled by analytical conformal transformation method and coupled mode theory. These methods are verified by experimental results. We demonstrate optimum design parameters at twelve different configurations of single SOI ring (disk) based sensors. Based on the results, the calculated quality factor of second TE WGM resonance at these resonators in water solutions is Qt ≥ 25000. This gives S and FOM parameters to be 193 (134) nm/RIU and 3112 (2161) /RIU, for single ring (disk) resonator sensor. We also obtain an ultra-improved intrinsic limit of detection (ILOD=7.32 × 10−8 RIU) biosensor based on two coupled ring resonators. The ILOD value, shows that our structure performs two and three orders of magnitude better than respectively two and three SOI cascade ring resonators based sensors, reported lately.

Fast analytical modelling of an SOI micro-ring resonator for bio-sensing application

In this work, we present a closed-form model of silicon-on-insulator (SOI) micro-ring resonators, based on the conformal transformation method. We compute the propagation characteristics of the whispering gallery modes at resonance wavelengths. The results of the proposed model are compared to numerical simulations based on the finite element method (FEM). The proposed method allows comparable accuracy to that of FEM simulations, with a significant reduction in computer resource requirements (time, speed, memory). To further validate the proposed modelling, we also compare the modelling results to experimental data. Finally, we model a ring resonator for biosensing applications. We compute the bulk refractive index sensitivity (S), the figure of merit (FOM) and the intrinsic limit of detection (ILOD). In the geometry under study, our proposed model predicts an ideal ILOD of refractive index units, which is 0.24 smaller than what has been reported in SOI optimized structure. These results validate the proposed modelling technique for the fast design of optical biosensors where both the figure of merit and the geometries can be optimized.

Probing the fundamental detection limit of photonic crystal cavities

The accurate characterization of single nanoparticles in colloidal solutions in terms of their size and morphology is important for medical diagnostics and aerosol investigations. However, it is challenging to achieve a high throughput for very small (sub-100 nm) particles. In particular, it is not well established what the fundamental limits are on the trade-off between speed and the smallest detectable particle. Here we study these limits for the case of refractive index sensing based on resonant photonic crystal cavities. Importantly, we have reached a regime where the fundamental thermal fluctuations set the intrinsic detection limit for acquisition sampling times tacq larger than 3 μs. Such an intrinsic fundamental limit corresponds to 1/2000th the linewidth of the optical spectrum of photonic crystal cavities of effective mode volume as small as 0.06 μm3. The results of this work indicate that it is possible to monitor up to 33 million particles per second with a particle size down to 34 nm, making it a promising technique for fast real-time biosensing.

Experimental Study of the Detection Limit in Dual-Gate Biosensors Using Ultrathin Silicon Transistors.

Dual-gate field-effect biosensors (bioFETs) with asymmetric gate capacitances were shown to surpass the Nernst limit of 59 mV/pH. However, previous studies have conflicting findings on the effect of the capacitive amplification scheme on the sensor detection limit, which is inversely proportional to the signal-to-noise ratio (SNR). Here, we present a systematic experimental investigation of the SNR using ultrathin silicon transistors. Our sensors operate at low voltage and feature asymmetric front and back oxide capacitances with asymmetry factors of 1.4 and 2.3. We demonstrate that in the dual-gate configuration, the response of our bioFETs to the pH change increases proportional to the asymmetry factor and indeed exceeds the Nernst limit. Further, our results reveal that the noise amplitude also increases in proportion to the asymmetry factor. We establish that the commensurate increase of the noise amplitude originates from the intrinsic low-frequency characteristic of the sensor noise, dominated by number fluctuation. These findings suggest that this capacitive signal amplification scheme does not improve the intrinsic detection limit of the dual-gate biosensors.

Silicon based On-chip Sub-Wavelength Grating Ring and Racetrack Resonator BioSensors

ABSTRACTIn this paper, we experimentally study the unique surface sensing property and enhanced sensitivity in subwavelength grating (SWG) based microring resonator biosensors versus conventional ring resonator biosensors. In contrast to a conventional ring, the 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 propagation path of the optical mode. It leads to an unique property of thickness-independent surface sensitivity versus common evanescent wave sensors; in other words, the surface sensitivity remains constantly high with progressive attachment of biomolecules to the sensor surface. To increase the robustness of performance of ring shaped circular SWG biosensors, we experimentally demonstrate silicon SWG racetrack resonators. A quality factor of 9800 and bulk sensitivity (S) is ∼429.7 nm/RIU (refractive index per unit) results in an intrinsic detection limit (iDL) 3.71×10-4 RIU in racetrack SWG biosensors while still retaining the accumulated surface thickness properties of circular rings.

Comparison analysis of energy loss between micro clamped–clamped and clamped-free beam in vertical motion flux modulation magnetic sensor

The 1/f noise is one of the main noise sources of giant magnetoresistive sensors, which will cause intrinsic detection limit at low frequency. To suppress this noise, a vertical motion flux modulation (VMFM) integrated with simple microelectromechanical-systems (MEMS) structure is proposed. However, the energy loss of VMFM structure is not considered yet. In this paper, energy loss between MEMS clamped---clamped beam (C---C beam) and clamped-free beam (C-F beam) used as VMFM structure are compared. In this comparison, quality factor (Q) is proposed to characterize the energy loss of the VMFM structure. Theory analysis and experimental results show that quality factor of a C---C beam is higher than that of a C-F beam, indicating a lower energy in VMFM. Finally, the sensitivity of a VMFM magnetic sensor modulated by a C---C beam is tested, and the sensitivity gets 3.85 mV/V/Gs.