Heyuan Guan

Tungsten disulfide (WS 2 ) based all-fiber-optic humidity sensor.

We demonstrate a novel all-fiber-optic humidity sensor comprised of a WS2 film overlay on a side polished fiber (SPF). This sensor can achieve optical power variation of up to 6 dB in a relative humidity (RH) range of 35%-85%. In particular, this novel humidity fiber sensor has a linear correlation coefficient of 99.39%, sensitivity of 0.1213 dB/%RH, and a humidity resolution of 0.475%RH. Furthermore, this sensor shows good repeatability and reversibility, and fast response to breath stimulus. This WS2 based all-fiber optic humidity sensor is easy to fabricate, is compatible with pre-established fiber optic systems, and holds great potential in photonics applications such as in all-fiber optic humidity sensing networks.

Fabrication of Side-Polished Single Mode-Multimode-Single Mode Fiber and Its Characteristics of Refractive Index Sensing

This paper presents a low-cost, flexible, and highly efficient wheel polishing techniques for the fabrication of sidepolished single mode-multimode-single mode fiber (SP-SMSF). The evolution of transmission spectrum of SP-SMSF is measured, simulated, and discussed. The good linear relationship between polished depth (PD) and polish-induced loss has relatively high linear correlation at 95%, allowing us to monitor and control the critical parameter PD in line and in real time. Several desirable SPSMSF with PD = 9.6 , 15, 20.6 μm were fabricated successfully. Their characteristics of refractive index (RI) sensing are investigated experimentally. The results show that SP-SMS sensitivity increases as RI increases, approaching its maximum when the latter gets close to its core. The maximum sensitivity of the SP-SMSF with PD = 20.6 μm is 1190 nm/RIU, comparable to that of chemically etched SMSF. The dependence of the sensitivity on the PD of SP-SMSF is also measured and discussed, showing that an increase in PD can improve the sensitivity of SP-SMSF. In addition, such novel structure of SP-SMSF will provide a flat platform to implement various fiber devices.

Reduced graphene oxide for fiber-optic toluene gas sensing

A fiber-optic toluene gas sensor based on reduced graphene oxide (rGO) is demonstrated and its sensing property is investigated experimentally and theoretically. The rGO film is deposited on a side polished fiber (SPF), allowing the strong interaction between rGO film and propagating field and making the SPF sensitive to toluene gas. It is found that the sensor has good linearity and reversibility and can work at room temperature with the response and the recovery time of 256 s and the detection limit of 79 ppm. Moreover, a theoretical model for the sensor is established to analyze the sensing mechanism. Theoretical analysis indicates this type of sensor could work in a wide range of toluene gas concentration and shows that a significant rise in its sensitivity can be expected by adjusting the doping level or chemical potential of graphene.

Large spatial and angular spin splitting in a thin anisotropic ε-near-zero metamaterial

We show theoretically that after transmitted through a thin anisotropic ε-near-zero metamaterial, a linearly polarized Gaussian beam can undergo both transverse spatial and angular spin splitting. The upper limits of spatial and angular spin splitting are found to be the beam waist and divergence angle of incident Gaussian beam, respectively. The spin splitting of transmitted beam after propagating a distance z depends on both the spatial and angular spin splitting. By combining the spatial and angular spin splitting properly, we can maximize the spin splitting of propagated beam, which is nearly equal to the spot size of Gaussian beam w(z).

Tunable spin splitting of Laguerre–Gaussian beams in graphene metamaterials

Optical spin splitting has attracted significant attention owing to its potential applications in quantum information and precision metrology. However, it is typically small and cannot be controlled efficiently. Here, we enhance the spin splitting by transmitting higher-order Laguerre–Gaussian (LG) beams through graphene metamaterial slabs. The interaction between LG beams and metamaterial results in an orbital-angular-momentum- (OAM) dependent spin splitting. The upper bound of the OAM-dependent spin splitting is found, which varies with the incident OAM and beam waist. Moreover, the spin splitting can be flexibly tuned by modulating the Fermi energy of the graphene sheets. This tunable spin splitting has potential applications in the development of spin-based applications and the manipulation of mid-infrared waves.

Coreless side-polished fiber: a novel fiber structure for multimode interference and highly sensitive refractive index sensors

A novel fiber structure, coreless side-polished fiber (CSPF), is proposed and investigated to implement multimode interference (MMI) and high sensitive refractive index (RI) sensors. For such CSPF, the part of the cladding and the core of a single-mode fiber are side-polished off so as to make the remained cladding a D-shaped multimode waveguide. The excitation and evolution of MMI in the CSPF are simulated numerically. The simulation results show that the high-order modes excited within the D-shaped multimode waveguide are mainly TE0,1 (TM0,1)~TE0,6 (TM0,6) modes. Moreover, the RI sensing characteristics and the influences of residual thickness and dip wavelength on the sensitivity are investigated both numerically and experimentally. The experimental results show that the CSPF with a residual thickness of 43.1 μm can reach an ultra-high sensitivity of 28000 nm/RIU in the RI range of 1.442~1.444. It is also found that the sensitivity can be further increased by reducing the residual thickness and choosing the dip at a longer wavelength. Thanks to the ultra-high RI sensitivity and the ease of fabrication, the CSPF could provide a cost-effective platform to build high-performance fiber devices of various functions.

Tungsten disulfide wrapped on micro fiber for enhanced humidity sensing

Tungsten disulfide (WS2) sheet wrapped on the tapered region of micro fiber (MF) and its humidity sensing are proposed and demonstrated. WS2 coated MF (WS2CMF) is demonstrated to enhance the interaction and contact area between WS2 and the strong evanescent field of optical fiber. An enhancement in sensitivity (0.196 dB/%RH) of the WS2CMF is achieved in a RH range from 37%RH to 90%RH. Furthermore, the proposed WS2CMF shows a good repeatability from 40%RH to 75%RH and a rapid response to periodic breath stimulus. This WS2CMF holds great potential in all optical sensing networks owing to the advantages of high sensitivity, compact size and low cost.

The upper limit of the in-plane spin splitting of Gaussian beam reflected from a glass-air interface

Optical spin splitting has a promising prospect in quantum information and precision metrology. Since it is typically small, many efforts have been devoted to its enhancement. However, the upper limit of optical spin splitting remains uninvestigated. Here, we investigate systematically the in-plane spin splitting of a Gaussian beam reflected from a glass-air interface and find that the spin splitting can be enhanced in three different incident angular ranges: around the Brewster angle, slightly smaller than and larger than the critical angle for total reflection. Within the first angular range, the reflected beam can undergo giant spin splitting but suffers from low energy reflectivity. In the second range, however, a large spin splitting and high energy reflectivity can be achieved simultaneously. The spin splitting becomes asymmetrical within the last angular range, and the displacement of one spin component can be up to half of incident beam waist w0/2. Of all the incident angles, the spin splitting reaches its maximum at Brewster angle. This maximum splitting increases with the refractive index of the “glass” prism, eventually approaching an upper limit of w0. These findings provide a deeper insight into the optical spin splitting phenomena and thereby facilitate the development of spin-based applications.

Molybdenum disulfide nanosheets deposited on polished optical fiber for humidity sensing and human breath monitoring

One important use of molybdenum disulfide (MoS2) could be in making sensing and detection devices with optical chip or fiber. Here, MoS2 nanosheets coated on side-polished optical fiber (SPF) is proposed, which can enhance the localized interaction between evanescent light of fiber core and MoS2 nanosheets, this can motivate greatly sensing and detection performance. Moreover, the MoS2 nanosheet possesses exceedingly high surface/volume ratio. By combining the MoS2 nanosheets and the side-polished fiber, humidity sensing characteristics has been demonstrated. The optical transmitted power (OTP) of the MoS2-based SPF changes with a negative correlation to the variation of relative humidity (RH) in experiments. The OTP changes of the MoS2-based SPF as an exponential function can reach ~13.5dB (~54 fold enhancement) when the RH ranges from 40%RH to 85%RH. Furthermore, experiments on the monitoring of human breath have also been conducted to evaluate the response time (0.85 s) and the recovery time (0.85 s). The performance comparison between this proposed device and the other recent-developed fiber-optic humidity sensing devices in literature illustrates the superiority of the MoS2-based SPF in humidity sensing and monitoring of human breath, which paves a path for the MoS2 nanosheets to integrate in lab-on-fiber sensing and detection devices.

Halloysite Nanotube-Modified Plasmonic Interface for Highly Sensitive Refractive Index Sensing

We propose and demonstrate a novel strategy to modify the plasmonic interface by using a thin layer of halloysite nanotubes (HNTs). The modified surface plasmon resonance (SPR) sensor achieves a greatly improved sensitivity because the large surface area and high refractive index of the HNTs layer significantly increase the probing electric field intensity and hence the measurement sensitivity. More significantly, the thickness of the HNTs layer can be tailored by spraying different concentrations of HNTs ethanol suspension. The proposed sensors show significant superiority in terms of the highest sensitivity (10431 nm/RIU) and the enhancement fold (5.6-folds) over those reported previously. Additionally, the proposed approach is a chemical-free and environment-friendly modification method for the sensor interface, without additional chemical or biological amplification steps (no toxic solvents are used). These unique features make the proposed HNTs-SPR biosensor a simple, biocompatible, and low-cost platform for the trace-level detection of biochemical species in a rapid, sensitive, and nondestructive manner.