Invited paper-“Micro-nano optics”
2019, 48(2): 203001.
doi: 10.3788/IRLA201948.0203001
Recently, terahertz(THz) technology has developed rapidly, showing promising potential in the fields of communication, anti-terrorism, monitoring and biomedicine, etc. In particular, terahertz biosensor has attracted extensive attentions in biotechnology, because many biological molecules and materials have their finger prints in the THz absorption spectra, and the damage by the low power terahertz wave is low. However, the THz wave-matter interaction is relatively weak because of the mismatch between the long wavelength of THz wave and the size of biomolecules, which limits the performance of terahertz sensors. Current research interest is manipulating the spatial and spectral distributions of the electromagnetic fields based on the microstructures to enhance the sensitivity of the sensors. In this review, we are going to introduce the working mechanisms of various microstructure THz sensors and the recent progress, and then discuss their advantages and disadvantages, finally we conclude the major issues to be resolved and predict the future developing trend and potential applications.
2019, 48(2): 203002.
doi: 10.3788/IRLA201948.0203002
Due to the high energy, narrow distribution and breaking the bandgap limitation, plasmon induced hot electrons has been widely applied to extend the photoresponse spectra of the semiconductor, such as realizing the response spectrum of wideband semiconductor and silicon to visible and near-infrared range, respectively. Besides, the response spectrum can be adjusted by changing the plasmonic nanostructures, which has an important advantage for realizing silicon-based near-infrared photodectection.Firstly, the concept and mechanism of surface plasmon and plasmon enhanced internal photoemission were introduced. Then, the recent progress on near infrared hot electron photodetector based on silicon was summarized. The influence of the shape, size, distribution of plasmonic nanostructure on the generation and transportation of hot electrons were also summarized. Finally the challenges and potential future directions of surface plasmon enhanced near-infrared photodetector based on metal/Si Schottky heterojunction were discussed.
2019, 48(2): 203003.
doi: 10.3788/IRLA201948.0203003
In order to couple a terahertz Gaussian beam into a hyperbolic metal waveguide(HMWG), a tapered elliptical-hyperbolic metal waveguide(TEHMWG) was proposed as a connector. The input of the TEHMWG was rectangular, and the output was elliptical-hyperbolic. In the tapered process, the TE01 mode of rectangular metal waveguide was gradually transformed into the elliptical polarized mode of hyperbolic metal waveguide. According to WKB approximation, the slowly tapered waveguide eliminated the reflection and scattering. The coupling efficiency between the TE01 mode and the elliptical polarized mode was 94%. Through this connector, the coupling efficiency between a Gaussian beam and HMWG was improved to 69.1%.
Multiband absorbers comprised of one dimensional periodic Ag metallic thin layer and MoO3/SiO2 dielectric layer on a reflective Ag thick layer were fabricated by thermal evaporation and magnetron sputtering methods. Experimental results show that with the number of unit cell (N) increasing, the number of the absorption peaks increases accordingly and precisely equals N. For our fabricated devices with 14 nm thick Ag layer, 2 nm thick MoO3 layer, and 135 nm thick SiO2 layer, the integrated absorption efficiency over the wavelength range from 400 nm to 900 nm increases from 29.4%when N=1 to 57.2% when N=6, the trend of which is consistent with the calculation results. Moreover, measurements show that the absorption peaks are insensitive to the incident angles and polarizations. The multilayer absorbers were also fabricated on flexible polyethylene terephthalate substrates, which maintained their original absorption performances after bending for 1 000 times. The fabricated absorbers may have potential applications in areas like photovoltaics and thermal emission tailoring.
2019, 48(2): 203005.
doi: 10.3788/IRLA201948.0203005
A terahertz(THz) wave optical modulation system was introduced. The modulation was based on the coupling of THz waves and THz surface plasmon by a simple slit between a razor blade and a semiconductor wafer. The modulation process was realized by changing the illumination intensity on the intrinsic semiconductor surface. With or without the optical illumination, the plasma frequency of the semiconductor was larger or smaller than the frequencies of the surface plasmon. Therefore, the THz surface plasmon propagating on the semiconductor and the THz wave coupled from the surface plasmon can be switched on and off. In comparison with conventional THz modulation approaches, this method has the advantages of wide modulation bandwidth, high speed, low cost and room temperature operation etc., which are favorable to THz wave communication applications. The simulation and experimental results prove the feasibility of the THz wave modulation system.
2019, 48(2): 203006.
doi: 10.3788/IRLA201948.0203006
The optical property of the flashing hind wings of the Papilio maackii were measured by an angle resolved spectrum system. A multilayer reflection structure model was established to explain the mechanism of the structural color of the butterfly wings. The circular dichroism in the microstructure of the hind wings of the butterflies was measured and the polarization properties were verified. Based on the structural color, the sensing experiments of the refractive index of solutions by using butterfly wings were designed and implemented. The results show that the spectra of butterfly wings redshift with the increase of the refractive index of solutions. The research provides a technological platform for polarization and sensing applications based on the microstructures in nature.
