This paper presents a chemical sensor using a metamaterial absorber, which consisted of an Au Bottom layer, a FR4 substrate and a double-split-square-resonator (DSSR).The resonance generated by DSSR, which is extraordinarily sensitive to changes of the effective dielectric constant around the capacitive gap. The proposed sensor shows a great sensitivity and a high value of Q by a creative periodic DSSR structure and the Au Bottom layer. In addition, the relationship between the absorption frequency and chemical concentration is demonstrated by simulation.

Subwavelength interference in ultra-thin bilayer metamaterials is investigated, where both near- and far-field interactions play important roles. A hybrid coupling model is established, which clearly shows the different contributions of near- and far-field couplings to electromagnetic responses: the former is only responsible for the energy level splitting, while the latter mainly reshapes the profile of the resonance spectra. Accordingly, the reflection spectrum exhibits a very sharp subradiant resonance within the envelope of a superradiant resonance and interference between them leads to a sharp Fano resonance. Under two-antisymmetric-beam illumination, the subradiant mode can be selectively excited, while the superradiant mode is highly suppressed, attributing to their different mode symmetry. A sharp coherent perfect absorption (CPA) peak with well-defined Lorentzian lineshape is achieved under critical conditions, relating to the completely excitation of subradiant mode. The peak frequency can be tuned by tailoring the near-field coupling, which alters the energy-level splitting. Ultrathin bilayer terahertz metamaterials with flexible polyimide substrate and interlayer are fabricated and tested by THz time domain spectroscopy, showing very good agreement with the theoretical and numerical results. This work provides a method of how to extract narrow subradiant resonance from an asymmetric Fano lineshape, which may enlighten the way for selective mode excitation via coherent illumination.

We have been developing a compact and low-cost flow cytometry unit (Figure 1) suitable for a variety of water monitoring needs. We have been developing an integrated flow cytometry chip, which comprises a pre-treatment part and a sheath flow-forming part with a twisted microchannel structure, biological cells were successfully detected at the proposed chip [1][2]. We constructed a compact and low-cost monitoring unit with a light emitting diode (LED) based optical setup. In this presentation, we introduce the compact and low-cost flow cytometry unit and demonstrate the ability to measure the particles.

Figure 2 shows a schematic of flow cytometry chip and an image of the twisted flow cytometry chip. Figure 3 shows a photograph of the fabricated chip. The flow cytometry chip made of Polydimethylsiloxane (PDMS) and the sheath flow-forming part is twisted by 180 degrees (°). The length of the sheath flow channel was designed to be twisted by 90° at second junction and the sheath flow was formed. Figure 4 shows a schematic of the optical setup. A laser and a PMT of the conventional optical setup were changed to the LED and the optical sensor in the new setup. We assessed the detecting ability of the compact and low-cost flow cytometry unit using standard fluorescent particles.

Figure 5 shows a photograph of the compact and low-cost flow cytometry unit. The unit is a 60 mm cubic. Water including the fluorescent particles was used as the sample fluid. The flow rates of the sheath fluid and the sample fluid were set to 0.10 µl/sec and 0.017 µl/sec, respectively. The mean flow velocity at observation point is 1.3 mm/sec. Figure 6 shows the signals from particles. The signals of particles were successfully detected using the LED based optical setup.

In summary, we constructed the compact and low-cost flow cytometry unit for monitoring particles in water and the signal of particles were successfully detected.

This paper reports a preliminary study of electric field induced (EFI) optical rectification (OR) in (001), (110) and (111) surface layers of germanium (Ge). It is well known that the second-order nonlinear optical effects are theoretically absent in Ge due to the inverse symmetry. In fact, space charge regions (SCRs) exist in surface layers of Ge, and electric fields in SCRs can break the inverse symmetry and induce the so-called “EFI OR”. OR is an important mechanism to be used in terahertz (THz) generation. A. Urbanowicz et al. indicated that EFI OR played an important role in the observed THz emission from Ge surfaces [1]. Besides, EFI OR can be used to characterize surface properties of crystals with symmetry centers such as Si [2]. We have demonstrated EFI OR in Si(001) surface layers in previous work [3]. However, there are few relative researches on OR in Ge.

The samples used in experiments are all near-intrinsic Ge single crystals, sandwiched between two metal electrodes. The structures, sizes and orientations of the sample are shown in Fig. 1, as well as the measurement system. The thickness of Ge(001) and Ge(110) crystals is 3 mm and the resistivity is about 60 Ω×cm. The thickness of Ge(111) crystals is 1 mm and the resistivity is more than 35Ω×cm. The light source is a fiber laser with the wavelength of 1964 nm. A chopper is used to input a reference signal into the lock-in amplifier. The polarization of the polarizer is along the x axis. If the azimuth of linearly polarized light with respect to the x axis is θ, when the light propagated along the y axis, the dc polarization along the z axis can be expressed as,

                                                                                                                                    (1)

where  is the permittivity of Ge,  is the optical electric field of the probing beam, and  and  are the components of the effective second-order susceptibility tensor. By rotating the half-wave plate, we measured the dependence of OR signals on the azimuth θ. The OR signals in Ge(001), Ge(110) and Ge(111) surface layers are shown in Fig. 2 to Fig. 4. According to fitted curves, OR signals all show cosine dependences on the azimuth θ. In Fig. 2, the fitted curve can be written as,

                                                                                                                                     (2)

If any contribution to OR signals from absorption is neglected, according to Eqs. (1) and (2), the ratio of  in Ge(001) surface layer is calculated to be about 0.91, which means that the two susceptibilities are close. Therefore, the sum of the two susceptibilities is much larger than the difference and there is a large background in the measured OR signals. Using the same method, the ratio of  in Ge(110) and Ge(111) surface layers are calculated to be 0.91 and 1.07.

We also measured the distribution of OR signals in Ge(001) surface layers along the surface normal direction. The result is shown in Fig. 5, as well as the simulation curve, which agrees well with the experimental data. Distance between the two peaks of OR signals is 2920 μm, which is consistent with the thickness of crystal after being polished. According to the simulation, the ratio of the maximum electric field intensities and the ratio of the width of SCRs in No. 1 and No. 2 Ge(001) surface layers are both deduced to be 1:0.966. It is proved that EFI OR is a method to analysis the surface properties of crystals with symmetry centers, such as Ge and Si.

Organic-inorganic halide perovskites are presently in the limelight because of their outstanding optoelectronic properties for applications ranging from photovoltaics, light emission, lasing and even radiation detection. Presently, the power conversion efficiencies of perovskite solar cells have exceeded 20% while the external quantum efficiencies perovskite light emitting diodes have breached the 10% mark. In this talk, I will review the photophysical mechanisms of the workhorse methylammonium lead halide (CH₃NH₃PbI₃) system. In addition, I will also present our latest photophysics results on (a) slow hot carrier cooling in perovskite nanocrystals [1]; (b) overcoming the slow bimolecular recombination in 3D perovskites [2]; and (c) giant five photon absorption in core-shell perovskite nanoparticles.

References

[1] M. J. Li, S. Bhaumik, T. W. Goh, M. S. Kumar, N. Yantara, M. Graetzel, S. G. Mhaisalkar, N. Mathews*, and T. C. Sum*, “Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals”, Nature Communications 8:14350 (DOI: 10.1038/ncomms14350) (2017)

[2] G. Xing, B. Wu, X. Wu, M. J. Li, B. Du, Q. Wei, J. Guo, E. K. L. Yeow, T. C. Sum* and W. Huang*, “Transcending the slow bimolecular recombination in lead-halide perovskites for electroluminescence”, Nature Communications 8:14558 (DOI: 10.1038/ncomms14558) (2017)