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Highly sensitive, simple and multiplex detection capabilities are key criteria of point-of-care (POC) diagnosis in clinical samples. Here, a simple and highly sensitive multi-analyte detection technique is proposed by using photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor that employs both internal and external sensing approaches. The proposed sensor can detect two different analytes simultaneously by the internal and external plasmonic micro-channels. The light propagation through the sensor is controlled by the scaled-down air-holes to excite the free electrons of the plasmonic metal layers. The light-guiding and sensing properties of the sensor is numerically analyzed by using the Finite Element Method (FEM). The proposed sensor shows the maximum wavelength sensitivities (WS) of 12,000 nm/refractive index unit (RIU), and 10,000 nm/RIU, for the internal and external sensing approaches, respectively, and corresponding resolution of 8.33×10-6 RIU and 1.0×10-5 RIU. Moreover, the hybrid sensor is applicable to detect unknown analyte refractive index (RI) in the range of 1.33 to 1.40 which covers extensively investigating analytes such as viruses, different cancer cells, glucose, proteins and DNA/RNA. Due to high sensing performance with multi-analyte detection capability, the proposed sensor can play a significant role to detect bio targets at the POC platform.

We conceptualized and numerically investigated a photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor for rapid detection and quantification of novel coronavirus. The plasmonic gold-based optical sensor permits three different ways to quantify the virus concentrations inside patient’s body based on different ligand-analyte conjugate pairs. This photonic biosensor demonstrates viable detections of SARS-CoV-2 spike receptor-binding-domain (RBD), mutated viral single-stranded ribonucleic acid (RNA) and human monoclonal antibody immunoglobulin G (IgG). A marquise-shaped core is introduced to facilitate efficient light-tailoring. Analytes are dissolved in sterile phosphate buffered saline (PBS) and surfaced on the plasmonic metal layer for realizing detection. The 1-pyrene butyric acid n-hydroxy-succinimide ester is numerically used to immobilize the analytes on the sensing interface. Using the finite element method (FEM), the proposed sensor is studied critically and optimized for the refractive index (RI) range from 1.3348-1.3576, since the target analytes RIs fluctuate within this range depending on the severity of the viral infection. The polarization-dependent sensor exhibits dominant sensing attributes for x-polarized mode, where it shows the average wavelength sensitivities of 2,009 nm/RIU, 2,745 nm/RIU and 1,984 nm/RIU for analytes: spike RBD, extracted coronavirus RNA and antibody IgG, respectively. The corresponding median amplitude sensitivities are 135 RIU-1, 196 RIU-1 and 140 RIU-1, respectively. The maximum sensor resolution and figure of merit are found 2.53 × 10−5 RIU and 101 RIU-1, respectively for viral RNA detection. Also, a significant limit of detection (LOD) of 6.42 × 10−9 RIU2/nm is obtained. Considering modern bioassays, the proposed compact photonic sensor will be well-suited for rapid point-of-care COVID testing.

A simple design of photonic crystal fiber (PCF)–based surface plasmon resonance (SPR) sensor is proposed and investigated numerically for the detection of large analyte refractive index (RI) range. An open U-shaped analyte channel is introduced at the top side and along the length of the fiber to detect unknown analytes. The U-shaped channel helps to generate large amount of surface plasmon resonance on the plasmonic metal layer (gold) by reducing the distance between core and sensing medium. The optical properties and sensitivity of the sensor are analyzed by using the finite element method. The simulation results suggest that the sensor can detect analyte refractive index ranging from 1.33 to 1.44 with maximum wavelength sensitivity of 66,000 nm/RIU and resolution of 1.52 × 10−06 RIU. It also exhibits the maximum amplitude sensitivity of 2,940 RIU−1 with sensor resolution of 3.40 × 10−05 RIU. Furthermore, a good figure of merit (FOM) of 1734 RIU−1 and maximum limit of detection (LOD) of 2.29 × 10−11 RIU2/nm are achieved by the proposed sensor. Additionally, the fabrication tolerance in terms of pitch, air-hole diameter and channel size is investigated. Finally, we can anticipate that the sensor will be a suitable candidate to detect unknown analytes such as biological or biochemical samples for its high sensitivity with wide sensing range.

Light control capability of photonic crystal fiber (PCF) is a unique feature which can be applied to improve biosensing and plasmonic performance. Here, we reported alphabetic-core microstructure fiber-based plasmonic biosensor. Three different alphabetic R-, M-, and S-shaped cores of PCF-based plasmonic microstructures show controllable light propagation to enhance biosensor sensitivity and resolution. The light-guiding properties and sensing performance are investigated numerically using the finite element method (FEM). The proposed R-shaped core (RSC), M-shaped core (MSC), and S-shaped core (SSC) PCF-based plasmonic sensors show the maximum wavelength and amplitude sensitivities of 12,000, 11,000, 10,000 nm/RIU and 478, 533, and 933 RIU−1, respectively, in the refractive index (RI) range of 1.33 to 1.40. The sensors also exhibit promising wavelength resolution of 8.33 × 10−6, 9.09 × 10−6, and 1.0 × 10−6 RIU, with figure of merit (FOM) of 108, 143, and 217 RIU−1 for RSC, MSC, and SSC PCFs, respectively. The tunable sensing performance is also observed in design structures due to controllable light traveling path and their interaction with analytes. The proposed alphabetic-core PCF SPR sensors would be a promising candidate for the application of light controlling, trapping in microscale environment, and biosensing.

In this article an attempt has been made to determine the uni-directinal (001-direction) pressure effect on the optical gain spectra and corresponding emission wavelength of a designed type-II nano-scale heterostructure based on GaAsSb-InAs material system. As a trial, the range of pressure applied was kept as 1-3 GPa. On introducing the uni-directional (001-direction) pressure (in the range of 1 GPa to 3 GPa) on the GaAsSb-InAs QW heterostructure, the optical gain is reduced significantly from 3050 /cm to 1150 /cm, while the transition wavelength shows the red shift.

A type-II (with broken bandgap) W-shaped nano-heterostructure having layers combination of AlSb, InAs and GaAsSb compound semiconductors has been proposed which can be utilized as high intensity lasing source in MIR (mid infrared region). For this heterostructure, a multiband band k.p Hamiltonian has been simplified to compute the required carrier’s wavefunctions, their subband structures and matrix dipole elements accountable for the probabilistic transitions which results into the high optical gain. For 2-D charge carrier density of 1.5 × 1012 cm−2, the computed results confirm that only the light hole (LH) subbands take part in optical transition in order to produce the high optical gain of the order of ~8850 /cm which corresponds to ~5.2 µm. Keeping in view its high optical gain at ~5.2 µm, the proposed type-II AlSb/InAs/GaAsSb heterostructure can be useful in the environmental monitoring, particularly important for sensing the CO2, CO and NO toxic gases available in the polluted environment. Moreover, this type-II heterostructure can also play an important role in traditional applications such as industrial, medical, MIR spectroscopy, and telecommunications applications which require ~5200 nm wavelength.

Light trapping capability is the key phenomena of the photonic crystal fiber (PCF) which paves the broad range of applications. In this work, we control the light-guiding direction of the proposed PCFs via scaled-down approach to enhance the coupling between core-guided mode and surface plasmon polariton (SPP) mode resulting in the increase of sensitivity. We introduce the scaled-down air-holes in the first and second rings of the proposed PCF which provides the light guiding capability through the desire scaled-down region. A chemically stable plasmonic material gold is used outside the PCF structure to facilitate the detection of unknown analyte externally. Finite element method is used for numerical investigations, and the sensor performance is investigated by following the wavelength and amplitude interrogation methods. The proposed sensor shows the maximum wavelength sensitivities of 30 000 and 22 000 nm/RIU, and the maximum amplitude sensitivities of 1212 RIU−1 and 1506 RIU−1 for $x$ - and $y$ -polarized modes, respectively in the analyte refractive index (RI) range of 1.33 to 1.39. It also shows high sensor wavelength resolution of $3.33\times 10^{-6}$ RIU and $4.54\times 10^{-6}$ RIU, and a figure of merit about 508 and 500 for $x$ - and $y$ -polarized modes, respectively. It should be noted that the proposed PCF SPR sensor shows the significant sensing response for both polarization modes, as a result it will provide more freedom in choosing light source during practical implementation. Furthermore, the finding results indicates the proposed sensor will be effective for the detection of organic chemicals, biomolecules, and medical diagnostics.

Refractive index (RI) is a unique attribute of bio-analytes which highly relies upon their concentrations or the activity/infectivity of the microbes. In this paper, a dual-polarized surface plasmon resonance (SPR) RI sensor based on a decentered propagation-controlled core photonic crystal fiber (PCF) has been studied. Fine-tuned light guidance is realized by scaling down the core air holes. The external surface of the fiber is metalized with a thin gold layer that maximizes the chemical stability by reducing the chances of oxidation. The deposition of the core from the central position reduces the core to analyte distance resulting in efficient coupling as well as greater sensitivity. Finite element method (FEM) based numerical investigation warrants promising sensing performances in wavelength and amplitude interrogation methods. The proposed sensor is viable for the RI range from 1.33 to 1.39, where it demonstrates the highest wavelength sensitivities of 13,700 and 15,400 nm/RIU and highest amplitude sensitivities of 852 RIU-1 and 654 RIU-1 in x-and y-polarized modes, respectively. It also exhibits high sensor resolutions of 7.30 × 10−6 RIU and 6.49 × 10−6 RIU, and high figure of merits of 216 and 256 RIU-1 for x and y-polarized modes, respectively. Along with the liberty of selecting light sources, the highly sensitive nature of the sensor will enable accurate detections of different organic compounds and bio-molecules.

A photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor is proposed for real-time multi-analyte detection. Semicircular U-shaped slots are excavated inside the PCF to facilitate the excitation of surface plasmon polaritons by reducing the distance between the core and external analyte channels. The evanescent field is controlled via engineered scaled-down air holes. A plasmonic metal layer (gold) is deposited over the U-curved microchannel to enhance the surface plasmons. This asymmetric PCF SPR structure enables superior birefringence controllability, which results in a dominant y -polarized response in comparison to the x -polarized mode. The light-guiding and sensor performance parameters are numerically analyzed using the full vector finite element method. The proposed sensor can detect a wide refractive index (RI) range from 1.33 to 1.43. It demonstrates the maximum wavelength sensitivities of 2000 nm/RIU and 12,800 nm/RIU with sensor resolutions of 5 × 1 0 − 5 R I U and 7.813 × 1 0 − 6 R I U for channels 1 and 2, respectively. The sensor also exhibits a maximum amplitude sensitivity of 1119 R I U − 1 with a high figure of merit of 474 R I U − 1 . The simultaneous dual-analyte detection capability and highly sensitive response with a broad detection range make the proposed sensor distinct and will pave the way for miniaturized medical diagnostics and sensing applications.

A strong light coupling between core-guided mode and metal layer leads to the enhancement of plasmonic sensor performance significantly. In this work, a plasmonic metal coated micro-channel based surface plasmon resonance sensor is proposed where a micro-channel is introduced to increase the mode coupling by reducing the distance between the core and analyte channel. Gold (Au) is considered to coat the micro-channel and generate surface plasmon on the fiber surface. Titanium dioxide (TiO2) is employed to stick the Au layer tightly with the silica which assists in shifting the sensing wavelength from visible to mid-IR region. The light-guiding and bio-sensing properties of the proposed D-shaped sensor performance is numerically investigated by employing the finite element method. The sensor obtained maximal wavelength and amplitude sensitivities of 1,21,000 nm/Refractive index unit (RIU) and 1,405 RIU−1, respectively in the x-polarized mode. Moreover, the sensor exhibits an extremely high resolution of 8.26 × 10−7 RIU and the limit of detection (LOD) is 6.83 × 10−12 RIU2/nm, respectively, in the wavelength interrogation technique. To the best of our knowledge, the proposed sensor shows the highest wavelength sensitivity, resolution and LOD when compared to the existing PCF based SPR sensors. Due to the extremely sensitive response, the proposed sensor will enhance the unknown analyte detection capability significantly. Furthermore, the broad sensing range of analyte refractive index (RI) variation from 1.33 to 1.44 makes the sensor suitable for biochemical as well as medical diagnostic applications.

A grapefruit fiber-based mode-multiplex plasmonic sensor is proposed to detect three different analytes simultaneously and improve the detection accuracy. The proposed sensor exhibits the maximum wavelength sensitivity of 18,000 nm/RIU for y-polarized mode.

We propose a grapefruit fiber based mode-multiplex plasmonic sensor to detect three different analytes simultaneously. The proposed sensor shows the maximum wavelength sensitivity of 2000, 3000 nm/RIU for x-polarized and 18000 nm/RIU for y-polarized modes, respectively.

Light controllability, design flexibility, and non-linearity features of photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor enable high sensitivity in the field of biosensing. Here, bio-inspired butterfly-core shaped microstructure fiber-based plasmonic sensor is proposed where circular air-holes are arranged to enhance the sensing performance. Butterfly shaped core is designed to confine the incident light into the core by preventing light scattering through the cladding and helps to excite surface electron of plasmonic metal layer. Chemically stable plasmonic material gold is used to produce the SPR phenomenon. The analyte detection layer and the plasmon layer are located externally on the PCF surface to make the detection process more feasible. The sensor performance is studied based on the finite element method (FEM), and the structural parameters are tuned to obtain maximum sensor performance. This modified core-based sensor exhibits the maximum wavelength sensitivity (WS) of 56,000 nm/RIU and the amplitude sensitivity (AS) of 1,584 RIU-1 for the x-polarized mode. It also shows an improved sensor resolution (SR) of 1.8 ×10−6 RIU, along with a decent figure of merit (FOM) of 691 RIU-1. Moreover, this sensor can detect analyte refractive indexes (RI) within a broad RI range of 1.33 to 1.42 in the visible to near-infrared wavelength range (450–2100 nm). Finally, the proposed sensor may have possible application to detect organic chemicals, food quality, and diseases with high accuracy due to outstanding sensitivity and linearity.

A modified solid-core photonic crystal fiber (PCF)-based plasmonic sensor is proposed where light propagation through the PCF is controlled by scaling down of air holes. The modified core facilitates the easy excitation of the plasmonic surface, resulting in improved sensor performance. The chemically stable gold is externally coated on the PCF surface, which helps to establish surface plasmon resonance phenomena. The response of the sensor is analyzed based on the numerical method, and the design parameters are optimized to enhance the sensing performance. The asymmetric fiber-core structure provides the polarization controllability and significantly suppresses the y -polarized response to achieve a dominant x -polarized response and additional functionalities. The sensor exhibits a maximum wavelength sensitivity of 11,000 nm/RIU (refractive index unit) and sensing resolution of 9.09 × 10 − 6 RIU in the x -polarized mode. Also, the sensor exhibits maximum amplitude sensitivity of 631 R I U − 1 , and a good figure of merit is 157 R I U − 1 . Furthermore, the sensor can detect the unknown analytes’ refractive index (RI) in the sensing analyte RI range of 1.33 to 1.40, which will lead to finding the potential applications in biomolecules, organic chemicals, and environment monitoring.

Highly sensitive and miniaturized sensors are highly desirable for real-time analyte/sample detection. In this Letter, we propose a highly sensitive plasmonic sensing scheme with the miniaturized photonic crystal fiber (PCF) attributes. A large cavity is introduced in the first ring of the PCFs for the efficient field excitation of the surface plasmon polariton mode and proficient infiltration of the sensing elements. Due to the irregular air-hole diameter in the first ring, the cavity exhibits the birefringence behavior which enhances the sensing performance. The novel plasmonic material gold has been used considering the chemical stability in an aqueous environment. The guiding properties and the effects of the sensing performance with different parameters have been investigated by the finite element method, and the proposed PCFs have been fabricated using the stack-and-draw fiber drawing method. The proposed sensor performance was investigated based on the wavelength and amplitude sensing techniques and shows the maximum sensitivities of 11,000 nm/RIU and 1,420 RIU−1, respectively. It also shows the maximum sensor resolutions of 9.1×10−6 and 7×10−6 RIU for the wavelength and amplitude sensing schemes, respectively, and the maximum figure of merits of 407. Furthermore, the proposed sensor is able to detect the analyte refractive indices in the range of 1.33–1.42; as a result, it will find the possible applications in the medical diagnostics, biomolecules, organic chemical, and chemical analyte detection.

Simple structure, quick response, and highly sensitive miniaturized sensors are highly desirable for the broad range of sensing applications. In this work, we numerically investigated a highly sensitive photonic-crystal-fiber (PCF)-based plasmonic sensor for refractive index sensing. We consider gold as the plasmonic material, which is used outside the fiber structure to exhibit the plasmonic phenomena and to help detect the surrounding medium refractive index. The proposed PCF is designed to enable the evanescent field to interact with an external sensing medium leading to a highly sensitive response. The sensor performance has been investigated by wavelength and amplitude interrogation methods. The proposed sensor exhibits the maximum amplitude sensitivity of 2,843 RIU−1 with the sensor resolution of 3.5×10−6 RIU, which is the highest among the reported PCF SPR sensors, to the best of our knowledge. It also shows wavelength sensitivity of 18,000 nm/RIU and sensor resolution of 5.6×10−6 RIU. The figure of merit of the proposed sensor is about 400. The sensor response also allows us to detect the refractive index variation in the range of 1.33 to 1.41. Such promising results and broad sensing range ensure that the proposed sensor will be a suitable candidate for biological analytes and biochemical and organic chemical detections.