Inspired by the structural color from the multilayer nanophotonic structures in Morpho butterfly wing scales, 3D lamellae layers in dielectric polymers (polymethyl methacrylate, PMMA) with n ∼ 1.5 were designed and fabricated by standard top-down electron beam lithography with one-step exposure followed by an alternating development/dissolution process of PMMA/LOR (lift-off resist) multilayers. This work offers direct proof of the structural blue/green color via lithographically-replicated PMMA/air multilayers, analogous to those in real Morpho butterfly wings. The success of nanolithography in this work for the 3D lamellae structures in dielectric polymers not only enables us to gain deeper insight into the mysterious blue color of the Morpho butterfly wings, but also breaks through the bottleneck in technical development toward broad applications in gas/liquid sensors, 3D meta-materials, coloring media, and infrared imaging devices, etc.
Layered 2D semiconductors are characterized by unique photoelectric properties and, therefore, constitute a new class of basic building block for next‐generation optoelectronics. However, their wide bandgaps limit the spectral responsivity to a narrow range. Here, a facile approach is demonstrated by integrating β‐NaYF4:Yb3+, Er3+ up‐conversion microcrystals (UCMCs) with monolayer‐MoS2 phototransistors to break this bandgap‐imposed barrier and to drastically extend the responsivity range. In essence, the UCMCs up‐convert a near‐infrared excitation at 980 nm to visible light of photons with energy matching the large bandgap (i.e., 1.8 eV) of monolayer‐MoS2, thereby activating the phototransistor with remarkable photocurrent and minimum interference. This approach leads to preservation of the excellent electrical merits of monolayer‐MoS2 and simultaneous retention of its low dark current and high photoresponsivity to the above‐bandgap lights. Significantly, an enhancement by over 1000 times is achieved for both responsivity and specific detectivity at 980 nm excitation. Moreover, the rate of response is kept identical to that when the MoS2 phototransistor is excited by a visible light. Therefore, integrating with UCMCs can enable the emerging 2D semiconductors of wide bandgap to respond to infrared excitations with high efficacy and without sacrificing their performance in the visible region.
There are currently growing needs for polarimetric imaging in infrared wavelengths for broad applications in bioscience, communications and agriculture, etc. Subwavelength metallic gratings are capable of separating transverse magnetic (TM) mode from transverse electric (TE) mode to form polarized light, offering a reliable approach for the detection in polarization way. This work aims to design and fabricate subwavelength gold gratings as polarizers for InP-based InGaAs sensors in 1.0-1.6 μm. The polarization capability of gold gratings on InP substrate with pitches in the range of 200-1200 nm (fixed duty cycle of 0.5) has been systematically studied by both theoretical modeling with a finite-difference time-domain (FDTD) simulator and spectral measurements. Gratings with 200 nm lines/space in 100-nm-thick gold have been fabricated by electron beam lithography (EBL). It was found that subwavelength gold gratings directly integrated on InP cannot be applied as good polarizers, because of the existence of SPP modes in the detection wavelengths. An effective solution has been found by sandwiching the Au/InP bilayer using a 200 nm SiO2 layer, leading to significant improvement in both TM transmission and extinction ratio. At 1.35 μm, the improvement factors are 8 and 10, respectively. Therefore, it is concluded that the Au/SiO2/InP trilayer should be a promising candidate of near-infrared polarizers for the InP-based InGaAs sensors.
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