In the last several years, metasurfaces, artificially-designed arrays of subwavelength optical scatterers, have been employed to achieve versatile and comprehensive control of the key constitutive properties of light at the nanoscale [4], [212]–[214]. To date, metasurfaces have been used to demonstrate a number of low-profile optical components with important capabilities including wavefront engineering [1], [215], focusing [7], [8], [72], [140], [153], [216], polarization control and detection [166], [217]–[220], holographic imaging [73], [221], and quantum light control [164], [222], [223].
Recently, actively-tunable metasurfaces have emerged as a transformational concept in the field of photonics owing to their ability to provide post-fabrication modulation, and thereby, the capability to control the wavefront of the scattered light in real-time. This leads to the demonstration of a wide range of photonic functions [61], [161], [224], and hence, the realization of compact and fast low-profile nanophotonic devices capable of beam steering, active polarization switching, and formation of reconfigurable metalenses. To date, different approaches have been employed in order to realize reconfigurable metasurfaces. In these approaches, the reconfigurable metasurfaces are commonly obtained by incorporating an active material into the otherwise passive
metasurface structures. Then, by applying an external stimulus, the dielectric permittivity of the active material can be dynamically controlled.
Amongst the active meta-devices presented so far, the metasurfaces hybridized with tunable CMOS compatible materials, such as highly-doped semiconductors like ITO [46], [80], [84], [90], [93], [119], [225]–[229] have enabled an approach to achieve strong light-matter interaction with ultrafast control over the optical properties of individual unit cell elements, which in aggregate enable the realization of tunable spatially-varying phase/amplitude gradients.
Thanks to its large refractive index change, possessing an ENZ condition, and short response time at NIR wavelengths including particularly the 1.55 μm telecommunication wavelength [34], [35], [67], ITO has attracted a great deal of interest to be used as an electro-optically tunable active medium. Despite remarkable progress toward ITO-based active metasurfaces in the reflection mode [34], [35], [67], [230]–[232], there has been a lack of demonstration of reconfigurable devices in the transmission mode via an ultrathin flat metasurface based on ITO permittivity modulation.
A theoretical study of a transmissive structure based on ITO-integrated multi-material nanowires was proposed [233]. However, the non-planar design of the metasurface would make the fabrication of the device not to be very favorable. In another study, metallic slits filled with ITO [234] were utilized to provide tunable metasurfaces working in transmission mode. The design, however, was several wavelengths thick, and as a result, not appropriate to be employed as a low-profile optical component that can be used in photonic integration. It is noteworthy that ITO-based tunable metasurfaces were proposed that could provide modulation of the reflection/transmission amplitude via coupling the incident beam to guided mode (GM) resonances [235], [236] or employing Huygens modes in dielectric resonators [62].
However, no phase modulation was achievable using these devices. Moreover, tunable metasurfaces presented so far have been only able to perform phase modulation in either reflection or transmission mode at a predesigned operating wavelength.
In this chapter, we will present an ITO-based all-dielectric metasurface that could provide phase modulation in both reflection and transmission modes. The proposed metasurface is mainly composed of Si which is of particular interest for on-chip
photonic integrated devices and can separately control the transformations of both reflected and transmitted beams in real-time at two different wavelengths (dual-mode operation). To achieve such a device, one needs to have a careful choice of the constituent materials and the geometrical shape of nanoresonators. The structural parameters of the nanostructure should then be optimized in order for the device to provide dual-mode tunability.
As mentioned in the previous chapters, when applying an external bias across the ITO layer, one could observe a modulation of the dielectric permittivity in a very small region. As a result, these hybrid metasurfaces operate by spectrally overlapping the geometrical antenna resonance and the ENZ permittivity regime, and also spatially overlapping the metasurface element mode profile with the active ITO layer. This places a stringent requirement of having resonant excitation of highly confined fields in the active layer to achieve ITO-based tunable metasurfaces with widely-tunable optical responses. On the other hand, the strong field confinement in the accumulation region of ITO will lead to a considerable field absorption in this region. This will result in a limited efficiency of the ITO-based tunable metasurfaces, necessitating the search for alternative approaches to achieve reconfigurable metasurfaces.
Along the same line, an all-dielectric GaAs tunable metasurface was previously proposed that could achieve refractive index modulation via free carrier generation by using an optical pump [237]. Even though this approach could enable a picosecond response time, the requirement of an ultrafast pump laser source is not desirable for many low-power compact nanophotonic applications. Moreover, when using optical pumping as the modulation mechanism, the area at which the refractive index is tuned will be determined by the size of the focused laser spot. As a result of a large laser spot size, individual control of subwavelength metasurface elements would not be possible, limiting the applications of optically-tunable platforms. Consequently, to achieve independent control of individual metasurface elements, electrical modulation of the optical response of the metasurface is preferable.
Prior research has shown that metasurfaces incorporating patterned graphene layers [58], [238], and ITO-integrated metasurfaces [95] with individually-controllable elements could actively modulate the properties of the scattered light via application of a bias voltage. However, the efficiency of the mentioned plasmonic metasurfaces is low. Thus, developing a dielectric active metasurface platform that can dynamically
tailor the wavefront of scattered light by modulation of individual antenna elements remains an outstanding research challenge.
Electrical tuning of the coupling between metasurface resonances and intersubband transitions in multiple-quantum-wells has been explored for applications such as tunable filters and optical modulators at mid-infrared wavelengths [239]–[241].
In this chapter, we will describe an MQW-based electrically tunable metasurface platform that could provide amplitude and phase modulation in the NIR wavelength range. This all-dielectric metasurface utilizes III-V compound semiconducting MQW structures as resonant elements. When applying an electric field across the MQW metasurface elements, the complex refractive index of the MQW will be electro- optically tuned via the QCSE [242] especially at wavelengths near the MQW bandgap.
This leads to a continuous modulation of the optical response of the metasurface. The QCSE is widely used in high-performance electro-optical components such as high- speed modulators [243].
In this approach, we combine the well-established MQW technology with subwavelength antennas to create an active metasurface platform for diverse nanophotonic applications. Each MQW resonator of our metasurface design supports a hybrid resonant mode with a relatively high quality factor, enabling optical modulation under applied bias. This active device concept is then employed to experimentally demonstrate beam steering by electrically controlling the optical response of individual metasurface elements.