6.3. III-V A LL -D IELECTRIC A CTIVE M ETASURFACES
6.3.3. Fabrication and Measurement of All-dielectric MQW Metasurface
After finding the optimal design, we fabricate the MQW-based metasurface to experimentally investigate its optical response. To fabricate the metasurface samples, first, a bottom Ohmic contact consisting of Ge/Ni/Au with the thicknesses of 43 nm/30 nm/87 nm is deposited on the n-doped GaAs substrate of the MQW wafer by using electron-beam evaporation. Next, a 950 PMMA A9 electron-beam resist with the thickness of 1.5 μm is spin-coated on the front side of the prepared MQW wafer at the speed of 4000 rpm for 60 s. The spin-coated sample is then baked on a hot plate at a temperature of 180 °C for 3 minutes. Subsequently, the top Ohmic contacts as well as some alignment markers are patterned on the EBR using an electron-beam direct-write lithography system [VISTEC electron beam pattern generator (EBPG) 5000+] at an acceleration voltage of 100 keV with a current of 5 nA. Following the exposure, the sample is developed, and the metals are deposited using electron-beam evaporation.
After the lift-off processes, ZEP 520A EBR is spin-coated at the speed of 4000 rpm for 60 s, and the sample is then baked on a hot plate for 3 minutes at 180 °C. Afterward, the double-slit structures are patterned by the mentioned EBPG system with a current of 0.3 nA.
After baking the sample for 2 minutes at 110 °C and then developing it at about 10°C for 90 s, the patterned ZEP 520A EBR is used as a mask for the dry etch process
employed for the fabrication of double slits. In order to etch the slits, a III-V compound semiconductor etcher (ICP-RIE, Oxford Instruments System) is utilized with gas flow rates of Cl2: Ar = 5 sccm: 30 sccm under 5 mTorr chamber pressure for 80 seconds.
After removing the ZEP 520A EBR using remover PG, the double slits are obtained.
Finally, a third EBPG process patterns the resonators, followed by developing the exposed EBR. Then, the same dry etching technique is used with a chamber pressure of 3 mTorr for 8 minutes and the MQW-based metasurface sample is obtained after removal of the ZEP 520A EBR using remover PG.
Once we fabricated our sample, we use the optical setup presented in Fig. 6.6 in order to optically characterize the reflectance of the MQW metasurface. In this setup, a coherent NIR laser beam (Toptica Photonics CTL 950) is utilized as a light source. The laser beam is focused on the sample using a long working distance objective with 10×
magnification and 0.28 numerical aperture. Moreover, an uncollimated white light source from a halogen lamp is used to visualize the sample surface. Then, the reflected beam from the metasurface is captured by a power meter as a detector (Thorlabs PM100D).
Figure 6.6: Optical setup used for the measurement of the reflection spectrum of the MQW metasurface. Schematic of the custom-built setup. M: mirror, ND: neutral density filter (Thorlabs NDC-50C-4M), I: iris, L: lens, P: linear polarizer (Thorlabs LPNIR100-MP), BS:
beam splitter (Thorlabs CCM1-BS014), O: objective (Mitutoyo 10× magnification with 0.28 numerical aperture), PM: power meter [117].
Figure 6.7a shows an SEM image of the fabricated MQW metasurface sample. The measured reflectance spectra of the metasurface under different applied biases with the incoming light being polarized perpendicular to the MQW stripes are shown in Fig.
6.7b. As can be seen, in the absence of the applied bias, the two resonant dips consistent with the simulation results (Fig. 6.5b) are observed. When changing the applied bias from 0 to –10 V, a significant red-shift accompanied by an intensity decrement of the
shorter-wavelength resonance, corresponding to the hybrid Mie-GM resonance, is achieved. The red-shift of the resonance is obtained as a result of an increase of the real part of the refractive index, enabling the voltage-induced phase modulation provided by the metasurface. The alteration of the reflectance intensities at the resonant wavelengths indicates the change in the imaginary part of the refractive index, enabling amplitude modulation provided by the tunable MQW-based metasurface.
Figure 6.7: Measured tunable optical response of the MQW metasurface. (a) SEM image of MQW-based hybrid Mie-GM resonators with double slits. (b) Measured reflectance spectra of the metasurface for different applied bias voltages. (c) The measured relative reflectance spectrum of the hybrid Mie-GM resonant metasurface as a function of applied bias. (d) Measured phase modulation at the wavelengths of 917 nm (red dots) and 924 nm (blue dots).
Each data point corresponds to an average phase shift measured at four different positions on the sample while each error bar indicates the standard deviation of the obtained four data points [117].
At the wavelength of 938 nm, a negligible shift of the resonance wavelength and a small decrease of the on-resonance reflectance are observed. Due to the small refractive index change and the absence of high-Q resonances at the wavelengths longer than 940 nm, no significant reflectance modulation is achieved at these wavelengths. As a result, we
will focus on the tunable resonance at shorter wavelengths which corresponded to the hybrid Mie-GM resonance.
To gain further insight, Fig. 6.7c presents the relative reflectance spectra at different applied biases obtained via
Δ𝑅
𝑅0 = 𝑅(𝑉)−𝑅(𝑉=0)
𝑅(𝑉=0) (6.2)
As can be seen in Fig. 6.7c, decreasing the applied bias from 0 to –10 V results in a strong relative reflectance modulation. In particular, a relative reflectance modulation as high as 270% can be obtained at the wavelength of 917 nm. Increasing the wavelength would lead to a decrease in the relative reflectance modulation (a relative reflectance modulation of −45% is observed at the wavelength of 925 nm). Thus, the proposed III-V MQW resonator-based metasurface seems to be a promising candidate for tunable amplitude modulation with the operating wavelength around 917 nm.
Once we confirmed that our tunable MQW metasurface could provide large amplitude modulation, we experimentally evaluate the phase shift of the reflected beam under applied bias at wavelengths of 917 nm and 924 nm using a Michelson interferometer system [34], [35], [70]. To measure the phase shift, an incident laser spot is illuminated on the edge of the resonator-based metasurface. As a result, the reflected beam is partly coming from the metasurface, and partly from the unpatterned MQW heterostructure.
The interference fringes captured by the camera are then processed and fitted. By considering the unpatterned MQW heterostructure as a built-in phase reference, the phase shift is determined by calculating the relative displacement of interference fringes between the hybrid Mie-GM resonator region and the unpatterned region. The measured phase shifts as functions of applied bias at wavelengths of 917 nm (red dots) and 924 nm (blue dots) are plotted in Fig. 6.7d.
At the wavelength of 917 nm, decreasing the applied bias from 0 V to–7 V results in a continuous increase in the phase shift by 70°. Further decreasing the applied bias from –7 V to –10 V leads to a decrease of the phase shift to 50°. Moving the operating wavelengths away from the hybrid Mie-GM resonance results in weaker phase modulations, with the obtained phase shift being only 12° at the wavelength of 924 nm.
It should be noted that the large (small) phase shift, i.e., large (small) change of the real part of the refractive index accompanied by the significant (negligible) reflectance modulation, i.e., large (small) modulation of the imaginary part of the refractive index
obtained at the wavelength of 917 nm (924 nm), is consistent with the Kramers–Kronig relation.
6.3.4. Demonstration of Electrical Beam Switching and Beam Steering with the