Detection of Low-Concentration Biological Samples Based on a QBIC Terahertz Metamaterial Sensor.

Quasi-bound state in the continuum (QBIC) can effectively enhance the interaction of terahertz (THz) wave with matter due to the tunable high-Q property, which has a strong potential application in the detection of low-concentration biological samples in the THz band. In this paper, a novel THz metamaterial sensor with a double-chain-separated resonant cavity structure based on QBIC is designed and fabricated. The process of excitation of the QBIC mode is verified and the structural parameters are optimized after considering the ohmic loss by simulations. The simulated refractive index sensitivity of the sensor is up to 544 GHz/RIU, much higher than those of recently reported THz metamaterial sensors. The sensitivity of the proposed metamaterial sensor is confirmed in an experiment by detecting low-concentration lithium citrate (LC) and bovine serum albumin (BSA) solutions. The limits of detection (LoDs) are obtained to be 0.0025 mg/mL (12 µM) for LC and 0.03125 mg/mL (0.47 µM) for BSA, respectively, both of which excel over most of the reported results in previous studies. These results indicate that the proposed THz metamaterial sensor has excellent sensing performances and can well be applied to the detection of low-concentration biological samples.

Terahertz spectroscopy detection of zinc citrate in flour and milk powder mixtures.

Zinc citrate (ZC) has been widely used in food as an important nutritional supplement. Accurate detection of ZC in food is important for health and safety. In this study, THz time-domain spectroscopy (THz-TDS) is used to quantitatively detect ZC in flour and milk powder mixtures. In our research, 15 different contents of ZC in flour and milk powder mixtures were prepared and measured by THz-TDS. A partial least squares (PLS) model was established based on the quantitative analysis of the absorption coefficient data of these two mixtures at 0.5-3.0 THz. The R2 and rms error (RMSE) given by the PLS model prediction were, respectively, 0.999 and 0.14% ZC in flour and 0.999 and 0.20% ZC in milk mixtures, indicating the predictions of the PLS model are in excellent agreement with the experimental measurements. The results show that combining THz-TDS with the PLS model can be used for accurate, quantitative analyses of ZC in food mixtures.

A first-principles study on the electronic and optical properties of a type-II C2N/g-ZnO van der Waals heterostructure.

The structural, electronic and optical properties of a new van der Waals heterostructure, C2N/g-ZnO, composed of C2N and g-ZnO monolayers with an intrinsic type-II band alignment and a direct bandgap of 0.89 eV at the Γ point, are extensively studied using first-principles density functional theory calculations. The results indicate that the special optoelectronic properties of the constructed heterostructure mainly originate from the interlayer coupling and electron transfer between the C2N and g-ZnO monolayers, and the photogenerated electrons and holes are located on the C2N and g-ZnO layers, respectively, which reduces the recombination probability of the electron-hole pairs. According to Bader charge analysis, there are 0.029 electrons transferred from g-ZnO to C2N to form a built-in electric field of ∼9.5 eV at the interface. Furthermore, the tunability of the electronic properties of the C2N/g-ZnO heterostructure under vertical strain and electric field is explored. Under different strains, the type-II band alignment properties of the heterostructure are retained and the vertical compressive strain has a greater influence on the bandgap modulation than the vertical stretching strain. The implemented electric field also does not change the type-II band alignment but changes the bandgap of the heterostructure from 1.30 to 0.58 eV when the electric field strength varies from -0.6 to 0.6 V Å-1. In addition, the absorption spectrum of the C2N/g-ZnO heterostructure under solar light is also studied. The absorption range of the heterostructure varies from the ultraviolet to near-infrared region with the absorption intensity in the order of 105 cm-1. All of these studies indicate that the C2N/g-ZnO heterostructure has excellent electronic and optical properties and promising applications in nanoelectronics and optoelectronics.

Au-nanorod-clusters patterned optical fiber SERS probes fabricated by laser-induced evaporation self-assembly method.

Optical fiber surface-enhanced Raman scattering (SERS) probes provide a novel platform for liquid-phase in situ and remote SERS detections. However, it is still a challenge to fabricate noble metal nanostructures with large SERS enhancement factor (EF) onto optical fiber surfaces. In this article, we successfully prepare Au-nanorod cluster structures on optical fiber facets by a laboratory-developed laser-induced evaporation self-assembly method. It is demonstrated that the optimized optical fiber SERS probes show high detection sensitivity (10-10 M for rhodamine 6G solution, and 10-8 M for malachite green or crystal violet solution) and excellent reproducibility (relative standard deviation less than 6%). As the laser-induced evaporation self-assembly method is a simple and low-cost method capable of achieving automatic and reproducible preparations of cluster patterned optical fiber SERS probes, this work may find important application prospects in various liquid-phase SERS detection areas.

An Optically Tunable THz Modulator Based on Nanostructures of Silicon Substrates.

Nanostructures can induce light multireflection, enabling strong light absorption and efficient photocarrier generation. In this work, silicon nanostructures, including nanocylinders, nanotips, and nanoholes, were proposed as all-optical broadband THz modulators. The modulation properties of these modulators were simulated and compared with finite element method calculations. It is interesting to note that the light reflectance values from all nanostructure were greatly suppressed, showing values of 26.22%, 21.04%, and 0.63% for nanocylinder, nanohole, and nanotip structures, respectively, at 2 THz. The calculated results show that under 808 nm illumination light, the best modulation performance is achieved in the nanotip modulator, which displays a modulation depth of 91.63% with a pumping power of 60 mW/mm2 at 2 THz. However, under shorter illumination wavelengths, such as 532 nm, the modulation performance for all modulators deteriorates and the best performance is found with the nanohole-based modulator rather than the nanotip-based one. To further clarify the effects of the nanostructure and wavelength on the THz modulation, a graded index layer model was established and the simulation results were explained. This work may provide a further theoretical guide for the design of optically tunable broadband THz modulators.

Energy Transfer from Ce3+ to Tb3+ /Dy3+ /Mn2+ in Ca9 Ga(PO4 )7 Phosphors: Synthesis, Structure and Tunable Multicolor Luminescent Properties.

A series of Ca9 Ga(PO4 )7 :Ce3+ /Tb3+ /Dy3+ /Mn2+ phosphors with tunable color, in which Ce3+ acts as the sensitizer, was synthesized. Energy transfer (ET) from Ce3+ to Tb3+ /Dy3+ /Mn2+ was investigated in detail. Tb3+ /Dy3+ /Mn2+ single-doped Ca9 Ga(PO4 )7 can exhibit green, yellow, and red emission, respectively. Incorporating Ce3+ into a Tb3+ /Dy3+ /Mn2+ single-doped Ca9 Ga(PO4 )7 phosphor can remarkably promote the luminous efficiency of the Tb3+ /Dy3+ /Mn2+ ions. This enhancement originates from an efficient ET from Ce3+ to Tb3+ /Dy3+ /Mn2+ . The ET was validated by luminescence spectra, decay dynamics, and schematic energy levels. Moreover, the intensity ratio of red emission of Mn2+ to violet emission of Ce3+ was analyzed based on energy-transfer and lifetime measurements. In Ce3+ -Tb3+ , Ce3+ -Dy3+ , and Ce3+ -Mn2+ doped Ca9 Ga(PO4 )7 , the emitting color changed from violet to green, yellow, and red, respectively, which indicates the potential use of this new tunable phosphor in UV light-emitting diodes.

Terahertz spectroscopy detection of lithium citrate tetrahydrate and its dehydration kinetics.

Lithium citrate (LC) as a common food additive and also a psychiatric drug, usually in the form of tetrahydrate can gradually lose its crystalline water and convert into LC anhydrate at temperatures higher than the room temperature. In order to quickly distinguish the tetrahydrate from the anhydrate and to study the dehydration kinetics of the LC hydrates under the influence of the temperature, terahertz time-domain spectroscopy (THz-TDS) is utilized in this work. Experimental results show that the LC tetrahydrate at room temperature has an obvious absorption peak around 1.66 THz, while the LC anhydrate has no absorption peak at 0.5-3.0 THz. The absorption peak intensity of the LC tetrahydrate decreases continuously upon heating from 25 to 100 °C. Based on the normalized absorption peak area of the LC tetrahydrate around 1.66 THz, variation of its dehydration rate with the heating temperature is investigated and their relationship is fitted by the Arrhenius equation. The reaction activation energy of the LC tetrahydrate is derived to be 495.1 ±â€¯17.8 J/g with a deviation of about 3.7% from the traditional difference scanning calorimetry (DSC) measurement. These results indicate that THz-TDS can provide an efficient method to detect crystalline hydrates and can be applied to study the dehydration kinetics of crystalline hydrates with advantages of being fast, label-free and accurate.

Spectral filtering method for improvement of detection accuracy of Mg, Cu, Mn and Cr elements in aluminum alloys using femtosecond LIBS.

In this work, magnesium (Mg), copper (Cu), manganese (Mn) and chromium (Cr) in aluminum alloy samples were quantified by femtosecond laser-induced breakdown spectroscopy (fs-LIBS). The different parameters affecting the experimental results, including the laser pulse energy, moving speed of the 2D platform and spectral average number were optimized. The background signal preprocessing methods of median filtering (MF corrected) and Savitzky-Golay filtering (SG corrected) algorithms were used and the effect of the LIBS spectral analysis in the experiment investigated. The calibration curves of Mg, Cu, Mn and Cr elements were established separately and their corresponding detection limits (LODs) were calculated. After background correction, the LODs of Mg, Cu, Mn and Cr elements in MF corrected were 54.52, 11.69, 7.33 and 27.72 ppm, and in SG corrected were 59.15, 17.48, 14.75 and 31.97 ppm. The LODs of these elements in MF corrected and SG corrected have 1.4-5.2 and 1.2-2.5 improvement factors compared to those obtained using the fs-LIBS technique. This work demonstrates that background signal preprocessing methods are very helpful for improving analytical sensitivity and accuracy in quantitative analyses of aluminum alloys.

Terahertz Metamaterial Sensor for Sensitive Detection of Citrate Salt Solutions.

Citrate salts (CSs), as one type of organic salts, have been widely used in the food and pharmaceutical industries. Accurate and quantitative detection of CSs in food and medicine is very important for health and safety. In this study, an asymmetric double-opening ring metamaterial sensor is designed, fabricated, and used to detect citrate salts combined with THz spectroscopy. Factors that influence the sensitivity of the metamaterial sensor including the opening positions and the arrangement of the metal opening ring unit, the refraction index and the thickness of the analyte deposited on the metamaterial sensor were analyzed and discussed from electromagnetic simulations and THz spectroscopy measurements. Based on the high sensitivity of the metamaterial sensor to the refractive index of the analyte, six different citrate salt solutions with low concentrations were well identified. Therefore, THz spectroscopy combined with a metamaterials sensor can provide a new, rapid, and accurate detection of citrate salts.

Calculated Terahertz Spectra of Glycine Oligopeptide Solutions Confined in Carbon Nanotubes.

To reduce the intense terahertz (THz) wave absorption of water and increase the signal-to-noise ratio, the THz spectroscopy detection of biomolecules usually operates using the nanofluidic channel technologies in practice. The effects of confinement due to the existence of nanofluidic channels on the conformation and dynamics of biomolecules are well known. However, studies of confinement effects on the THz spectra of biomolecules are still not clear. In this work, extensive all-atom molecular dynamics simulations are performed to investigate the THz spectra of the glycine oligopeptide solutions in free and confined environments. THz spectra of the oligopeptide solutions confined in carbon nanotubes (CNTs) with different radii are calculated and compared. Results indicate that with the increase of the degree of confinement (the reverse of the radius of CNT), the THz absorption coefficient decreases monotonically. By analyzing the diffusion coefficient and dielectric relaxation dynamics, the hydrogen bond life, and the vibration density of the state of the water molecules in free solution and in CNTs, we conclude that the confinement effects on the THz spectra of biomolecule solutions are mainly to slow down the dynamics of water molecules and hence to reduce the THz absorption of the whole solution in confined environments.

Multicolor-tunable up-conversion emissions of Yb3+,Er3+/Ho3+ co-doped Ba3Lu2Zn5O11: crystal structure, luminescence and energy transfer properties.

Yb3+-Er3+ and Yb3+-Ho3+ co-doped Ba3Lu2Zn5O11 phosphors were successfully obtained. The structure of the as-synthesized phosphor was determined by Rietveld refinement. The up-conversion (UC) spectra of Ba3Lu2Zn5O11:Yb3+,Er3+ exhibits two prominent emission bands centered at 560 and 663 nm, which originate from the 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. Ba3Lu2Zn5O11:Yb3+,Ho3+ revealed a bright green emission at around 553 nm and negligible red emission peaking at 660 nm, which were assigned to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+, respectively. The up-conversion luminescence of the samples was studied as a function of the concentration of the dopants. The origin of the UC mechanism is discussed in detail by analyzing energy-level diagrams, the dependence of the UC emission intensity on pump power, and the lifetimes. The results indicate that the Ba3Lu2Zn5O11:Yb3+,Er3+/Ho3+ phosphors are excellent up-conversion emitters and have potential applications in display and illumination technologies.

Tricolor- and White Light-Emitting Ce3+/Tb3+/Mn2+-Coactivated Li2Ca4Si4O13 Phosphor via Energy Transfer.

Single-component tunable Li2Ca4Si4O13:Ce3+,Tb3+,Mn2+ phosphors were successfully synthesized at 950 °C. Li2Ca4Si4O13:Ce3+,Tb3+ exhibits two luminescence peaking at 430 and 550 nm, which originated from the allowed 5d → 4f transition of the Ce3+ ion and the 5D4 → 7F J (J = 6, 5, 4, 3) transition of the Tb3+ ion, respectively. Moreover, by codoping Ce3+ ions in the Li2Ca4Si4O13:Mn2+ system, yellow-red emission from the forbidden transition of Mn2+ could be enhanced. Under UV excitation, dual energy transfers (ETs), namely, Ce3+ → Mn2+ and Ce3+ → Tb3+, are present in the Li2Ca4Si4O13:Ce3+,Tb3+,Mn2+ system. The ET process was confirmed by the overlap of the excitation spectra, variations in the emission spectra, ET efficiency, and decay times of phosphors. In addition, quantum yields and CIE chromatic coordinates are presented. The emission color of these phosphors can be tuned precisely from blue to green via ET of Ce3+ → Tb3+ and from blue to yellow via ET of Ce3+ → Mn2+. White light can also be achieved upon excitation of UV light by properly tuning the relative composition of Tb3+/Mn2+. This result indicates that the developed phosphor may be regarded as a good tunable emitting phosphor for UV light-emitting diodes.

Stable optical trapping and sensitive characterization of nanostructures using standing-wave Raman tweezers.

Optical manipulation and label-free characterization of nanoscale structures open up new possibilities for assembly and control of nanodevices and biomolecules. Optical tweezers integrated with Raman spectroscopy allows analyzing a single trapped particle, but is generally less effective for individual nanoparticles. The main challenge is the weak gradient force on nanoparticles that is insufficient to overcome the destabilizing effect of scattering force and Brownian motion. Here, we present standing-wave Raman tweezers for stable trapping and sensitive characterization of single isolated nanostructures with a low laser power by combining a standing-wave optical trap with confocal Raman spectroscopy. This scheme has stronger intensity gradients and balanced scattering forces, and thus can be used to analyze many nanoparticles that cannot be measured with single-beam Raman tweezers, including individual single-walled carbon nanotubes (SWCNT), graphene flakes, biological particles, SERS-active metal nanoparticles, and high-refractive semiconductor nanoparticles. This would enable sorting and characterization of specific SWCNTs and other nanoparticles based on their increased Raman fingerprints.

Eigenfields and output beams of an unstable Bessel-Gauss resonator.

We evaluate the eigenfields of an unstable Bessel-Gauss resonator (UBGR) by use of the transfer-matrix method in which the transverse profiles and their corresponding losses of the UBGR are considered as the eigenvectors and eigenvalues of a transfer matrix so that the dominant mode fields and their losses of the UBGR can be readily extracted in terms of the matrix eigenvalue algorithm. Moreover, based on the eigenfields across two mirrors that resulted from the transfer-matrix method, we simulate the field distributions in the cavity and the propagation of output beams by means of the angular spectrum method. The computation results show that the UBGR easily produces a fundamental Bessel-Gauss mode of good quality, and the output beams retain the original Bessel-Gauss distribution during propagation.

Gaussian-reflectivity mirror resonator for a high-power transverse-flow CO2 laser.

A Gaussian-reflectivity mirror resonator is proposed to achieve high-quality laser beams. To analyze the laser fields in a Gaussian-reflectivity mirror resonator, the diffraction integral equations of a Gaussian-reflectivity mirror resonator are converted to the finite-sum matrix equations. Consequently, according to the Fox-Li laser self-reproducing principle, we describe the mode fields and their losses in the proposed resonator as eigenvectors and eigenvalues of a transfer matrix. The conclusion can be drawn from the numerical results that, if a Gaussian-reflectivity mirror is adopted for a plano-concave resonator, a fundamental mode can easily be obtained from a transverse-flow CO2 laser and high-quality laser beams can be expected.

Analysis of eigenfields in the axicon-based Bessel-Gauss resonator by the transfer-matrix method.

The axicon-based-Bessel-Gauss resonator (ABGR) has been proposed for the production of Bessel-Gauss beams. To analyze eigenfields of the ABGR with a plane or spherical output coupler, we present and demonstrate the transfer-matrix method. Since the method is slow to converge to eigenmodes of the ABGR by use of the Fox and Li iterative algorithm, in this paper the Huygens-Fresnel diffraction integral equations associated with ray matrices are converted into finite-sum matrix equations, and mode-fields and corresponding losses are described as eigenvectors and eigenvalues of a transfer matrix according to the self-reproducing principle of the laser field. By solving the transfer matrix for eigenvectors and eigenvalues, we obtain field distributions and losses of the dominant eigenmodes. Moreover, eigenfields across arbitrary interfaces between the axicon and the output coupler, and the propagation of output beams, are simulated by using the fast-Fourier transform (FFT). The calculation results reveal that because of the ABGR's poor transverse mode discrimination the ABGR should be improved to produce good-quality Bessel-Gauss beams.