Two-dimensional beam scanning by tunable photonic spin Hall effect.

To the best of our knowledge, a novel tunable photonic spin Hall effect is proposed based on a pair of liquid crystal Pancharatnam-Berry (PB) lenses. Owing to the spin-dependent geometric phases, a PB lens focus or defocus the incident light field according to its spin angular momentum. By cascading two PB lenses with a small gap, the focus and defocus effects can be suppressed, and the transmitted light fields with opposite spin will be deflected toward opposite directions when the two PB lenses have a relative lateral displacement. The deflection angles vary linearly with the displacements, thus double-lines two-dimensional continuous beam scanning is achieved with a scanning angle of 39o × 39° and a beam diverging angle of 0.028o × 0.028°. The scanning beam is used to write different patterns on a 200 nm thick gold film. We believe this beam scanning system can find wide applications ranging from laser processing, Lidar, particle manipulation, to free space optical communications.

Off-plane quartz-enhanced photoacoustic spectroscopy.

In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam went neither through the prong spacing of the quartz tuning fork (QTF) nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by the radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of the acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as the excitation source, the noise level was suppressed by ∼2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.

Quartz-Enhanced Photoacoustic Spectroscopy-Conductance Spectroscopy for Gas Mixture Analysis.

A novel spectroscopic method, named quartz-enhanced photoacoustic spectroscopy-conductance spectroscopy (QEPAS-CS), was first developed for gas mixture analysis. In QEPAS-CS, the advantage of photoacoustic detection and conductance analysis was realized by a quartz tuning fork (QTF). Two-component gas analysis was done by photoacoustic detection and conductance detection. For an explicit application, natural spider silk was used as a water vapor transducer to modify the QTF, making a conductance sensing channel. A 2004 nm laser diode was used as an excitation source for a photoacoustic sensing channel. Such a QEPAS-CS sensor was used for H2O/CO2 gas mixture analysis in a cell incubator. This provides a solution to calibrate an infrared photoacoustic spectroscopy gas sensor. This example effectively confirms the capacity of multigas analysis by the QEPAS-CS sensor.

Helmholtz-resonator quartz-enhanced photoacoustic spectroscopy.

In this work, Helmholtz-resonator quartz-enhanced photoacoustic spectroscopy (HR-QEPAS) was developed for trace gas sensing. A pair of Helmholtz resonators with high-order resonance frequency was designed and coupled with a quartz tuning fork (QTF). Detailed theoretical analysis and experimental research were carried out to optimize the HR-QEPAS performance. As a proof-of-concept experiment, the water vapor in the ambient air was detected using a 1.39 µm near-infrared laser diode. Benefiting from the acoustic filtering of the Helmholtz resonance, the noise level of QEPAS was reduced by >30%, making the QEPAS sensor immune to environmental noise. In addition, the photoacoustic signal amplitude was improved significantly by >1 order of magnitude. As a result, the detection signal-to-noise ratio was enhanced by >20 times, compared with a bare QTF.

Side-excitation light-induced thermoelastic spectroscopy.

In this Letter, a side-excitation light-induced thermoelastic spectroscopy (SE-LITES) technique was developed for trace gas detection. A novel, to the best of our knowledge, custom quartz tuning fork (QTF) was used as a transducer for photon detection by the thermoelastic effect. The mechanical stress distribution on the QTF surface was analyzed to identify the optimum thermoelastic excitation approach. The electrode film on the QTF surface also works as a partially reflective layer to obtain a long optical absorption path inside the QTF body. With the long optical absorption length and the inner face excitation of the QTF, the thermoelastic effect was greatly enhanced. With an optimized modulation depth, a signal-to-noise ratio (SNR) improvement of more than one order of magnitude was achieved, compared to traditional LITES.

High-sensitivity humidity sensing of a U-shaped microfiber coated with porous methacryloxyethyl trimethyl ammonium chloride film.

We demonstrate an intensity-modulated humidity sensor based on a U-shaped microfiber coated with porous methacryloxyethyl trimethyl ammonium chloride (DMC) film. The high surface-to-volume ratios of the porous structure improve the interaction between the DMC film and water molecules, resulting in significantly enhanced sensitivity of the humidity sensor. In the humidity range of 34.0%RH to 50.0%RH, the humidity sensitivity of this microfiber sensor is up to 3.090 dB/%RH, which is six times higher than that of other fiber humidity sensors. The humidity detection range can be adjusted with high humidity sensitivity (≥1.685d B/% R H) by controlling the microfiber diameter and bent diameter. Furthermore, this type of sensor has a fast recovery time of 0.023 s and a response time of only 0.692 s. This type of sensor has broad potential applications in chemical processing, medical diagnostics, instrument manufacturing, and so on.

Joint spatial weak measurement with higher-order Laguerre-Gaussian point states.

Here, joint spatial weak measurements with higher-order Laguerre-Gaussian (LG) point states are investigated experimentally. From the intensity patterns of the final LG point states, two dimensional position operators 〈X〉 and 〈Y〉 as well as high-order position operators 〈XY〉, 〈X2 - Y2〉, 〈X3〉, and 〈Y3〉 are extracted simultaneously, from which both the complex weak values and joint weak values of two non-commuting observables can be obtained. The enhancement of joint weak values by the postselection state are analyzed. The simple relationship between the expectation values of position operators and the azimuthal and radial indexes of LG modes allows us to identify the mode indexes directly. A simple and robust scheme based on an optical window is demonstrated to monitor the LG mode indexes. These findings deepen the understanding of the weak measurement and provide an alternate and effective method for LG mode index monitoring.

High-power near-infrared QEPAS sensor for ppb-level acetylene detection using a 28 kHz quartz tuning fork and 10 W EDFA.

A high-power near-infrared (NIR) quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for part per billion (ppb) level acetylene (C2H2) detection was reported. A 1536 nm distributed feedback (DFB) diode laser was used as the excitation light source. Cooperated with the laser, a C-band 10 W erbium-doped fiber amplifier (EDFA) was employed to boost the optical excitation power to improve QEPAS detection sensitivity. A pilot line manufactured quartz tuning fork (QTF) with a resonance frequency of 28 kHz was used as the photoacoustic transducer. In the case of high excitation power, gas flow effect and temperature effect were found and studied. Benefitting from the low QTF resonance frequency, high excitation power, and vibrational-translational (V-T) relaxation promoter, a detection limit of ∼7 ppb was achieved for C2H2 detection, corresponding to a normalized noise equivalent absorption coefficient of 4.4×10-8cm-1 · W · Hz-1/2.

Clamp-type quartz tuning fork enhanced photoacoustic spectroscopy.

In this Letter, clamp-type quartz tuning fork enhanced photoacoustic spectroscopy (Clamp-type QEPAS) is proposed and realized through the design, realization, and testing of clamp-type quartz tuning forks (QTFs) for photoacoustic gas sensing. The clamp-type QTF provides a wavefront-shaped aperture with a diameter up to 1 mm, while keeping Q factors > 104. This novel, to the best of our knowledge, design results in a more than ten times increase in the area available for laser beam focusing for the QEPAS technique with respect to a standard QTF. The wavefront-shaped clamp-type prongs effectively improve the acoustic wave coupling efficiency. The possibility to implement a micro-resonator system for clamp-type QTF is also investigated. A signal-to-noise enhancement of ∼30 times has been obtained with a single-tube acoustic micro resonator length of 8 mm, ∼20% shorter than the dual-tube micro-resonator employed in a conventional QEPAS system.

Steering Nonlinear Twisted Valley Photons of Monolayer WS2 by Vector Beams.

Monolayer transition metal dichalcogenides have intrinsic spin-valley degrees of freedom, drawing broad interests due to their potential applications in information storage and processing. Here, we demonstrate the possibility of using cylindrical vector pumped beams, which are nonseparable in their polarization and spatial modes, to manipulate nonlinear valley-locked twisted-vortex emissions in monolayer tungsten disulfide (WS2). The second-harmonic (SH) photons from K and K' valleys are encoded with opposite optical vortices, thus allowing the SH beams to emerge as cylindrical vector beams with doubled topological orders compared to the fundamental beams. The conically refracted pumped beams allow us to generate the first-order SH cylindrical vector and full Poincaré beams via tuning the valley-locked emitted light field profiles. With fanshaped WS2 films breaking the axial symmetry of SH beams, the SH valley photons are routed to opposite directions. Our results pave the way to develop atomically thin nonlinear photonic devices and valleytronic nanodevices.

Spider Silk-Improved Quartz-Enhanced Conductance Spectroscopy for Medical Mask Humidity Sensing.

Spider silk is one of the hottest biomaterials researched currently, due to its excellent mechanical properties. This work reports a novel humidity sensing platform based on a spider silk-modified quartz tuning fork (SSM-QTF). Since spider silk is a kind of natural moisture-sensitive material, it does not demand additional sensitization. Quartz-enhanced conductance spectroscopy (QECS) was combined with the SSM-QTF to access humidity sensing sensitively. The results indicate that the resonance frequency of the SSM-QTF decreased monotonously with the ambient humidity. The detection sensitivity of the proposed SSM-QTF sensor was 12.7 ppm at 1 min. The SSM-QTF sensor showed good linearity of ~0.99. Using this sensor, we successfully measured the humidity of disposable medical masks for different periods of wearing time. The results showed that even a 20 min wearing time can lead to a >70% humidity in the mask enclosed space. It is suggested that a disposable medical mask should be changed <2 h.

Broadband mode-selective couplers based on tapered side-polished fibers.

We propose the broadband mode-selective coupler (MSC) formed with a side-polished six mode fiber (6MF) and a tapered side-polished small core single-mode fiber (SC-SMF) or an SMF. The MSCs are designed to allow the LP01 mode in the SC-SMF and SMF to completely couple to the LP01, LP11, LP21, LP02, LP31, LP12 modes in the 6MF over a broadband wavelength range. The phase-matching conditions of the MSCs are satisfied by tapering the SC-SMF and SMF to specific diameters. The tapered fibers are side-polished to designed residual fiber thickness using the wheel polishing technique. The effective indices of the side-polished fibers are measured with the prism coupling method. The MSCs provide high coupling ratio and high mode purity. High coupling efficiencies in excess of 81% for all the higher-order modes are obtained in the wavelength range 1530-1600 nm. For the LP01, LP11, LP21, LP02, LP31, LP12 MSCs at 1550 nm, the coupling ratios are 96.2%, 99.8%, 89.5%, 85.0%, 90.9%, 96.1%, respectively, and the mode purity of the MSCs is higher than 88.0%. The loss of the MSCs is lower than 1.8 dB in the wavelength range 1530-1600 nm. This device can be applied in broadband mode-division multiplexing transmission systems.

Ultrasensitive temperature sensor and mode converter based on a modal interferometer in a two-mode fiber.

This paper presents an ultrasensitive temperature sensor and tunable mode converter based on an isopropanol-sealed modal interferometer in a two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers. The sensor provides high-sensitivity temperature sensing by taking advantages of TTMF, isopropanol and the Vernier-like effect. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as strong evanescent field on its surface. The temperature sensitivity of the sensor can be improved due to the high thermo-optic coefficient of isopropanol. The Vernier-like effect based on the overlap of two interference spectra is applied to magnify the sensing capabilities with a sensitivity magnification factor of 58.5. The temperature sensor is implemented by inserting the modal interferometer into an isopropanol-sealed capillary. The experimental and calculated results show the transmission spectrum exhibit blue shift with increasing ambient temperature. Experimental results show that the isopropanol-sealed modal interferometer provides a temperature sensitivity up to -140.5 nm/°C. The interference spectrum has multiple dips at which the input LP01 mode is converted to the LP11 mode. This modal interferometer acts as a tunable multi-channel mode converter. The mode converter that can be tuned by varying temperature and mode switch is realized.

Self-powered and high-performance all-fiber integrated photodetector based on graphene/palladium diselenide heterostructures.

An all-fiber integrated photodetector is proposed and demonstrated by assembling a graphene/palladium diselenide (PdSe2) Van der Waals heterostructure onto the endface of a standard optical fiber. A gold film is covered on the heterostructure working as an electrode and a mirror, which reflects back the unabsorbed residual light for further reusage. Owing to the low bandgap of PdSe2, the all-fiber photodetector shows a broadband photoresponse from 650 to 1550 nm with a high photoresponsivity of 6.68×104 AW-1, enabling a low light detection of 42.5 pW. And the fastest temporal response is about 660 µs. Taking advantage of heterostructures, the photodetector can work in self-powered mode with the on/off ratio about 82. These findings provide new strategies for integrating two-dimensional materials into optical fibers to realize integrated all-fiber devices with multi-function applications.

High-performance photonic spin Hall effect in anisotropic epsilon-near-zero metamaterials.

A high-performance photonic spin Hall effect is demonstrated in an anisotropic epsilon-near-zero (ENZ) metamaterial based on the wave-vector-varying Pancharatnam-Berry phase. The giant out-of-plane anisotropy of ENZ metamaterial induces strong spin-orbit coupling. With a small incident angle, photons with opposite spins move along opposite transverse directions gradually. After transmitting through a submicrometer thick ENZ metamaterial, the spin photons are fully separated with a spin separation of 2.7 times beam waist and transmittance of 70.1%, allowing a figure of merit F up to 1.9. A practical ENZ metamaterial consisting of an Ag nanorod array is proposed, whose figure of merit is still up to 0.006. This high-performance photonic spin Hall effect provides an integrated and practical way for the development of spin-photonic devices.

Radial-cavity quartz-enhanced photoacoustic spectroscopy.

Radial-cavity quartz-enhanced photoacoustic spectroscopy (RC-QEPAS) was proposed for trace gas analysis. A radial cavity with (0,0,1) resonance mode was coupled with the quartz tuning fork (QTF) to greatly enhance the QEPAS signal and facilitate the optical alignment. The coupled resonance enhancement effects of the radial cavity and QTF were analyzed theoretically and researched experimentally. With an optimized radial cavity, the detection sensitivity of QEPAS was enhanced by >1 order of magnitude. The RC-QEPAS makes the acoustic detection module more compact and optical alignment comparable with a bare QFT, benefiting the usage of light sources with poor beam quality.

Wave-Vector-Varying Pancharatnam-Berry Phase Photonic Spin Hall Effect.

The geometric Pancharatnam-Berry (PB) phase not only is of physical interest but also has wide applications ranging from condensed-matter physics to photonics. Space-varying PB phases based on inhomogeneously anisotropic media have previously been used effectively for spin photon manipulation. Here we demonstrate a novel wave-vector-varying PB phase that arises naturally in the transmission and reflection processes in homogeneous media for paraxial beams with small incident angles. The eigenpolarization states of the transmission and reflection processes are determined by the local wave vectors of the incident beam. The small incident angle breaks the rotational symmetry and induces a PB phase that varies linearly with the transverse wave vector, resulting in the photonic spin Hall effect (PSHE). This new PSHE can address the contradiction between spin separation and energy efficiency in the conventional PSHE associated with the Rytov-Vladimirskii-Berry phase, allowing spin photons to be separated completely with a spin separation up to 2.2 times beam waist and a highest energy efficiency of 86%. The spin separation dynamics is visualized by wave coupling equations in a uniaxial crystal, where the centroid positions of the spin photons can be doubled due to the conservation of the angular momentum. Our findings can greatly deepen the understanding in the geometric phase and spin-orbit coupling, paving the way for practical applications of the PSHE.

Black phosphorus terahertz sensing based on photonic spin Hall effect.

A novel terahertz (THz) sensing scheme is proposed based on the photonic spin Hall effect (PSHE). By illumining a paraxial Gaussian THz beam onto a black phosphorus (BP)-based Tamm structure, the reflected beam will undergo in-plane spin splitting, i.e., the centroids of two opposite spin components separate spatially. Due to Tamm plasmon resonance, one of the spin components is very sensitive to the refractive index changes of the analyte layer sandwiched by monolayer BP and distributed Bragg reflector. The sensitivity of the spin-dependent shift can be up to 2804 mm/RIU with a refractive index resolution of ∼10-8 RIU. The sensitivity and dynamic sensing region can be flexibly tuned by the BP rotation angle, thickness of analyte layer, or operation frequency. Therefore, the proposed PSHE-based THz sensing provides a new avenue for the development of high-performance THz sensors; thus, we may find applications in chemical sensing and biosensing.

Quartz-enhanced photoacoustic spectroscopy exploiting a fast and wideband electro-mechanical light modulator.

A quartz-enhanced photoacoustic spectroscopy (QEPAS) gas sensor exploiting a fast and wideband electro-mechanical light modulator was developed. The modulator was designed based on the electro-mechanical effect of a commercial quartz tuning fork (QTF). The laser beam was directed on the edge surface of the QTF prongs. The configuration of the laser beam and the QTF was optimized in detail in order to achieve a modulation efficiency of ∼100%. The L-band single wavelength laser diode and a C-band tunable continuous wave laser were used to verify the performance of the developed QTF modulator, respectively, realizing a QEPAS sensor based on amplitude modulation (AM). As proof of concept, the AM-based QEPAS sensor demonstrated a detection limit of 45 ppm for H2O and 50 ppm for CO2 with a 1 s integration time respectively.

Exploiting black phosphorus based-Tamm plasmons in the terahertz region.

Polarization-sensitive Tamm plasmons are investigated in a multi-layer photonic configuration where a monolayer black phosphorus (BP) is coated on a Bragg mirror separated by a dielectric. Owing to the in-plane anisotropy of BP, the Tamm plasmon can be excited selectively by tuning the BP carrier density. Cross-polarization conversion occurs when the armchair direction of BP makes an angle with the incident plan, i.e., ϕ≠0 or 90°. The BP-based Tamm device can be used as an intensity modulator with a modulation depth up to ∼100% and an insertion loss smaller than -0.55 dB. By analyzing the polarization evolution carefully, a multichannel polarization division multiplexing scheme is proposed and discussed. These findings open a new avenue for exploiting versatile tunable THz devices based on the monolayer of BP.