Fabrication and optical characterization of the active metasurfaces

Chapter V: Nano-electromechanical spatial light modulator enabled by asym-

5.4 Fabrication and optical characterization of the active metasurfaces

99 respectively. In contrast, the calculated reflected power coefficients of the +1 st (0th) order diffraction are 2.53% (5.69%) and 4.66%(7.47%) in Figs.5.3fand5.3g, respectively. Similarly, the blazed gratings with positive phase gradients are also investigated. In Fig. 5.3h, the arrangement of the gap sizes are simply reversed compared to the arrangement of the negative phase gradient blazed gratings shown in Fig5.3e. Then, the reversed nanomehcanical displacements realize the positive phase gradient of ²_𝑝^𝜋

𝑔 , expecting to result in the dominant +1st order diffraction at the angle of 𝜃_𝑔. The reflected power coefficient spectra of the positive phase gradient blazed gratings are plotted in Figs. 5.3i and 5.3j. The dominant +1st order diffraction and the suppressed 0th and −1st order diffractions are observed in Figs 5.3i and 5.3j. At the design wavelength of 1529 nm, the reflected power coefficients of the+1st order diffraction are 10.1% and 18.0% in Figs.5.3iand5.3j, respectively. At the same wavelength, the calculated reflected power coefficients of the−1st (0th) diffraction order are 4.26% (7.46%) and 7.11%(8.49%) in Figs. 5.3i and5.3j, respectively. For both periodicities of 4 and 6, the negative phase-gradient gratings used in Figs.5.3fand5.3gperform more efficiently than the positive phase- gradient gratings used in Figs.5.3iand5.3j. The same trend can be also found in the case of periodicity of 1 (see Fig.5.A.4for details). We expect that these differences inherently result from the asymmetry of the structure with respect to𝑥-axis. Besides, at the design wavelength of 1529 nm, reflected power coefficients of all available diffraction orders are plotted in Fig.5.A.5, showing that all high-order diffraction components are suppressed compared to the desired diffraction order.

5.4 Fabrication and optical characterization of the active metasurfaces

1480 1500 1520 1540 1560 0.4

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0V 4V 5V 6V 7V 8V Wavelength (nm) Rdevice/R

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Figure 5.4: Optical characterization of dynamic properties of the asymmetric nano-electromechanical metasurface. aOptical image of the fabricated metasur- face. Electrodes in the device are wire-bonded to a custom printed circuit board.

b-d Scanning electron microscopy images of the metasurface. Every pair of the nanostructure is connected to the electrodes. Scale bars in b, c, and d denote 500, 10, and 1 𝜇m, respectively. eSchematic of electrical configuration. Four different electrical biases,𝑉₁, 𝑉₂, 𝑉₃, and𝑉₄, are periodically applied to every four pairs of the nanostructures. Inf-h,𝑉₂and𝑉₄are equally changed with𝑉₁and𝑉₃grounded.

fMeasured reflection spectra for TE-polarized normally incident light. The spectra are measured under six different biases and plotted in different colors. The applied bias for each color is shown in legend. The reflection spectra are normalized by the reflection from a gold electrode. gMeasured intensity modulation under different biases at the wavelength of 1524 nm. The applied bias varies from 0V to 8V. h Measured phase shift of the metasurface at the wavelength of 1524 nm as a function of the applied biases from 0V to 8V. Error bars represent standard deviations of the estimated phase shifts.

the design shown in Fig.5.1. Figure5.4eillustrates the electrical configuration of the device, showing that every four pairs of nanostructures are connected to four different electrodes. In Fig.5.4e,𝑉₁,𝑉₂,𝑉₃, and𝑉₄denote the four different applied biases. The voltage differences between the neighboring nanostructures locally determine the gap sizes. The electrical configuration shown in Fig.5.4eenables the nano-electromechanical modulation with periodicity of 4.

We characterize the tunable optical properties, implementing the scheme shown in Fig.5.3a. While𝑉₁ and𝑉₃ are grounded,𝑉₂ and𝑉₄ are connected to the external biases. When the TE-polarized light is normally incident, the reflection spectra are measured under different external biases and plotted in Fig.5.4f (see Methods and Fig. 5.A.6for details). Without any bias, the resonance dip was observed around

101 1526 nm in Fig.5.4f, showing good agreement with the simulated resonance dip at 1529 nm shown in Fig. 5.3b. We believe that the small deviation results from slight errors in fabrication. When the bias changes from 4V to 7V, blue-shift of the resonances and decrease of the minimum reflection are observed in Fig. 5.4f, showing great agreement with the simulated results in Fig.5.3b. As an objective lens in the setup cannot capture the diffraction at∼44 degree, the decrease of minimum reflection in Fig.5.4fcan be explained by the increase of the±1st-order diffractions (see Fig.5.A.4for details). However, the spectrum measured with the bias of 8V in Fig.5.4fresults in increase of the minimum reflection, abrupt broadening of the resonance, and a large spectral shift of−8nm, deviating from the simulation results in Fig. 5.3b. We believe that the deviation results from non-negligible bending of the nanostructures at the large applied bias.

We experimentally investigate electrical modulations of reflection and reflected phase. Like the measurements in Fig.5.4f, we implement the electrical configuration shown in Fig.5.3a. To measure the intensity as a function of the applied bias, the reflection is measured at 1524 nm by increasing the applied bias from 0 to 8V. The measured reflection is plotted as a function of the applied bias in Fig.5.4g, showing that the measured reflection is higher than 50% (see Supplementary Fig. 5.A.7 for measured amplitude modulations at different wavelengths). Figure. 5.4g can be qualitatively explained by the blue shifts observed in Fig. 5.3b and Fig. 5.4f.

In Fig. 5.4f, the resonance dip is placed at 1526 nm without any bias. As the applied bias increases, the resonance is continuously blue-shifted so the resonance dip moves towards the measured wavelength of 1524 nm. From the position of the dip in Fig. 5.4g, the resonance dip is placed at 1524 nm when the applied bias is around 6.91V. For the further increase of the bias, the resonance dip is blue-shifted below 1524 nm and the shift results in increase of the reflection. In addition, we measure the phase modulation at 1524 nm by varying the applied bias. For the measurement of the phase shifts, we employ a Michelson-type interferometer setup [159]. As the field of view of the objective lens is larger than the device size, the input light illuminates the metasurface and unpatterend regions at the same time and forms fringes with a reference beam at the image plane. The phase shift is mainly evaluated by the shift of the fringes on the metasurface, while we ensure that the fringes on the unpatterned regions are unchanged. Figure5.4hshows measured phase shifts as a function of the applied bias at the wavelength of 1524 nm. First of all, the device achieves large phase modulation over 312^◦within the applied bias of 8V. Although the measured phase modulation is 61^◦larger than the simulated phase

5.5 Experimental demonstration of electrically controllable diffraction