Abstract
Keywords
Introduction
In recent years, the field of nonlinear nanophotonics has grown enormously and attracted tremendous attention due to a vast range of related applications, including ultrasensitive sensing
Bound states in the continuum (BICs) recently emerge as a novel approach to manipulating the light-matter interactions
Infrared (IR) imaging and spectroscopy based on nonlinear metasurfaces have been recognised as a potential substitute for commercial IR imaging detectors due to its superiority of being ultra-compact, containing fewer components for signal conversion
Here, we propose the employment of silicon membrane metasurfaces for augmenting THG process. Our proposed membrane metasurfaces consist of dimer holes (see
Figure 1.
Results and discussion
Design and analysis
We consider an amorphous silicon (a-Si) membrane metasurfaces consisting of a silicon slab with dimer airy holes arranged periodically on a glass substrate, as schematically shown in
We first characterise the mode properties of such membrane metasurfaces by calculating the band structure based on the Massachusetts Institute of Technology Photonic-Bands (MPB) open sources
Figure 2.(
By controlling the offset x0, one can transform such an ideal BIC into a quasi-BIC. This phenomenon has been explained from the view of symmetry. This explanation is intuitive and concise to summarise the formation of the BIC, but hardly shows the process of the energy exchange from the bound modes to the external modes. Taking Mode 1 as an example, we then investigate and characterise the modes via spherical and Cartesian multipolar analysis. This enables us to observe it from the view of multipolar transformations, manifesting itself as a Fano feature in the optical response spectrum.
We perform the spherical and Cartesian multipolar analysis
By controlling the offset x0, we can control the excitation and the coupling between the bright mode py and the dark mode TD, and thus control the energy exchange of the BIC with the external radiation channels, and the radiation damping rate at the resonance. When the offset increases from x0=120 nm to x0=135 nm, the radiation damping rate and the width of the resonance decrease significantly. The radiation has decreased accordingly as the excitation of the bright mode py decreases, as illustrated in
Figure 3.Calculated linear transmission spectra (
Fabrication and characterisation
We fabricated our sample using the standard electron beam lithography (EBL). First 235 nm amorphous silicon was deposited on a glass substrate via plasma-enhanced chemical vapor deposition (PECVD). It was followed by EBL on a positive resist, called ZEP, which was used as a mask for inductively coupled plasma etching (ICP). Finally, the residual ZEP resist was removed from the surface.
Figure 4.(
We then measured the transmission spectra of our fabricated metasurfaces with different offsets, as shown in
Figure 5.Measured linear spectra of metasurface samples with different offsets (
We further investigate the angular dependence of the transmission spectrum by changing the incident angle θ, of the y-polarised pump light along xz plane, as shown in θ, the redshift of the two resonances has been observed. This matches well with the calculated band structure.
TH emission was measured under the normal incidence of a pump laser with different central wavelengths. A femtosecond laser beam with 150 fs pulse width and 80 MHz repetition rate was focused to a spot size of 21
Figure 6.(
We further demonstrate the ability of THG imaging with the designed metasurfaces.
Figure 7.(
We successfully achieve the IR imaging with a clear multi-stripe pattern based on THG process. Such an approach suggests a novel platform for realising ultra-thin all-optical infrared imaging devices without the need of additional pump light. We believe that increasing the conversion efficiency of nonlinear generations based on the high-Q resonances and a large light-confined volume, such as employing light trapping assisted by slow light, increasing the spatial resolution based on small imaging units, carefully designing the optical setup to reduce the aberrations and fabricating the metasurfaces with larger areas can further improve the quality of IR imaging. In addition, compared to utilising the pump light to amplify the signal light in the SFG-based imaging, the THG-based imaging with only one input wavelength is more easily to be constructed in experiments. This simplicity may expand its application in sensing and infrared detecting.
Conclusions
In summary, we have demonstrated the potential of membrane metasurfaces for upconversion of an IR image to the visible wavelengths by the THG process based on Si material. Via tuning the offset x0, we successfully convert the symmetry-protected BICs into quasi-BIC type resonances with infinite Q-factor. Furthermore, we reveal the formation mechanisms of those BICs from the views of symmetry and multipolar analysis. Meanwhile, after the test and characteristics of fabricated sample, we demonstrate infrared imaging as one of potential applications of enhanced THG based on Si membrane metasurfaces. The employment of membrane metasurfaces can further enhance the nonlinear generation in the aspect of offering a smaller unit size and a larger mode volume within the nonlinear material, which expands a range of applications, particularly all-optical infrared imaging at room temperature, and nonlinear spectroscopy.
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