Abstract
Reconfigurable metalenses are compact optical components composed by arrays of metaatoms that offer unique opportunities for advanced optical systems, from microscopy to augmented reality platforms. Although poorly explored in the context of reconfigurable metalenses, thermooptical effects in resonant silicon nanoresonators have recently emerged as a viable strategy to realize tunable metaatoms. In this work, we report the proofofconcept design of an ultrathin (300 nm thick) and thermooptically reconfigurable silicon metalens operating at a fixed, visible wavelength (632 nm). Importantly, we demonstrate continuous, linear modulation of the focallength up to 21% (from 165 μm at 20 °C to 135 μm at 260 °C). Operating under rightcircularly polarized light, our metalens exhibits an average conversion efficiency of 26%, close to mechanically modulated devices, and has a diffractionlimited performance. Overall, we envision that, combined with machinelearning algorithms for further optimization of the metaatoms, thermally reconfigurable metalenses with improved performance will be possible. Also, the generality of this approach could offer inspiration for the realization of active metasurfaces with other emerging materials within field of thermonanophotonics.
1 Introduction
Optical metasurfaces are twodimensional (2D) arrangements of metaatoms with subwavelength spacing that are engineered to precisely control and manipulate the properties of a light beam, such as its phase, amplitude, and polarization. Their evolution in the last two decades has enabled the miniaturization of numerous optical components, including holographic optical elements [1–3], beam deflectors [2, 4], and flat lenses [5–8]. More recently, reconfigurable metasurfaces have enabled active control of the optical properties revolutionizing the design, functionality and application domains of optical components and devices [9–12]. Phasechange materials [9, 12], [13], [14], stretchable substrates [15–17], strainfield engineering [18], thermooptical effects [19–22], and freecarrier modulation [10, 11, 23, 24] are some of the engineering strategies used to achieve reversible changes of the metasurface function.
Reconfigurable (varifocal) metalenses (RMLs) [14, 15, 20, 22, 24], in particular, have attracted increasing attention thanks to their enabling potential for tunable optics in microscopy systems [25], depth sensor devices [26, 27] as well as virtual and augmented reality platforms [28]. Moreover, these RMLs can uniquely combine focal length modulation with complex functionalities, such as multiwavelength [27] operation, spectroscopy and polarization routing [29], that are critical for compact and smart imaging devices/sensors.
The realization of continuously reconfigurable MLs, however, presents significant challenges. Indeed, to generate a converging wavefront with a desired focal length, the nanoscatters constituting a ML must introduce a prescribed (parabolic) phase delay at each position along the ML radius (Figure 1a–c). To dynamically change the focal length, a distinct phase shift must be achieved at each lattice site. Thus, RMLs require the nontrivial realization of both a parabolic spatial phaseprofile and a spatially varying phase shift (Figure 1d and e).
Figure 1:
To date, stretchable substrates [30] and strainfield engineering [18], which entail a mechanical modification of the ML structure, have demonstrated excellent reconfiguration capabilities with good efficiency (up to 30 and 60% respectively) and large focal length tuning (up to 20 and 100% respectively). Yet, there is a growing interest for nonmechanical modulation approaches such as phasechange materials, thermooptical effects [31] as well as tuning of freecarriers and excitonresonances [32]. Indeed, these designs can offer unique opportunities in terms of fast modulation speed, including ultrafast alloptical control [33], and reduced device thickness, i.e., ultrathin lenses [34]. Furthermore, achieving the desired modulation using a single uniform external control of the local optical properties (e.g., temperature or electrical potential), would greatly simplify device design, fabrication and integration, accelerating the deployment in realword components.
Thermooptic effects represent an attractive approach for the realization of dielectric RMLs [22]. Indeed, they entail a continuous and smooth change in the material optical properties (the refractiveindex as shown in Supplementary Figure 1) and are robust against thermalcycling. Yet, the lowmagnitude of typical thermooptical coefficients is generally assumed to limit the applicability of this strategy. Interestingly, silicon nanoresonators [5, 28] have been recently shown to exhibit pronounced shifts in their optical resonances (Figure 1b) both under external heating [19, 21] and for alloptical [35] modulation. Thus, by leveraging the amplification of thermooptical effects by opticalresonance modes, siliconbased thermally reconfigurable dielectric metalenses (TRMLs) could become a competitive and CMOScompatible solution [36]. However, a viable design for such an ultrathin, tunable ML with fixed operation wavelength in the visible regime has not been demonstrated yet.
Here we report a proofofconcept design of an ultrathin (300 nm thick) and thermooptically reconfigurable silicon ML operating at a fixed wavelength in the visible regime (632 nm). We demonstrate that, using thermooptic effects, it is indeed possible to achieve continuous modulation of the focallength beyond the depthoffocus of the lens. Specifically, operating under rightcircularly polarized light, our TRML exhibits a change of 21% in the focal length, with a linear shift from 165 μm at 20 °C to 135 μm at 260 °C. The average conversion efficiency of the lens is 26%, close to the performance of mechanically modulated devices, while its Strehl ratio is 0.99, confirming a diffractionlimited performance. Importantly, in our design, we rely on a spatially uniform temperature increase of the structure, overcoming the need for a spatially varying modulation input and potentially enabling an alloptical photothermal modulation approach. Concurrently, in this work, we report an automatized methodology to design a reconfigurable metalens, compute its layout and verify the expected performance. Overall, although further optimization of the metaatom design is needed to boost the performance of these components [37], our results demonstrate that TRMLs can be a viable solution for active tuning of optical systems.
2 Results
2.1 Thermally reconfigurable metalens phase profile and choice of design parameters
A TRML must be composed of an array of nanoscatters (metaatoms – Figure 1a–c) that, at all temperatures T, satisfies a quadratic phase profile [38] along its radius r:
where
where Δf is the desired focallength variation achievable at the maximum temperature variation ΔT. Using this assumption for
where
We observe that in Eqn. (3) the required temperaturedependent phase profile ϕ(r,T) has been decomposed into a temperatureindependent initial phase profile, ϕ
_{0}(r), and a spatially varying phase shift, Δϕ, which depends on the temperature. Tuning of the phase shift at each lattice site can be achieved either by a structured external control [34] T(r) that generates different modulation inputs along the radius, i.e.,
The analytical phase profile described in Eqn. (3) is uniquely defined once λ, ΔT, f _{0}, Δf, and the ML radius R are defined. We note that for each set of these constraints, a specific phase shift range Δ^{max} ϕ must be attained with the metaatoms (Figure 1c), where:
Seeking a favorable compromise between tunability and transmission efficiency of the ML in the visible regime, we set λ = 632 nm as the operating wavelength (see Supplementary Information S1, Figure 1b and Supplementary Figure 1 for further details). We also considered a maximum temperature variation ΔT = 240 °C, compatible with simple electric heaters (T
_{max} = 260 °C). Next, we quantified Δf/f
_{0} as a function of the lens numerical aperture (NA), its radius R as well as Δ^{max}
ϕ and we studied the maximum focal length variation versus the depth of focus (Figure 1f and Supplementary Figure 3). From this detailed analysis we observed that, in percentage, Δf/f
_{0} decreases for larger R and for smaller Δ^{max}
ϕ. Aiming for a ML with a numerical aperture (NA) of at least 0.08, comparable with commercial ultracompact objectives, and with a focal length variation at least equal to the depth of focus, we defined the target phase profile of the TRML using the following parameters:
2.2 Metalens design: silicon nanofin library and digitalization of the target phase profile
The response of individual metaatoms must be engineered such that their phase parameter space (ϕ and Δϕ) satisfies the requirements imposed by the target phase
Nonresonant silicon nanopillars, which only support waveguidemodes [5, 39, 40], are insufficient to realize an ultrathin, subμm thick ML (h = 300 nm) because, for ΔT = 240 °C, they can achieve a maximum phase shift
To introduce additional degrees of freedom for engineering the phase profile, we therefore adopted anisotropic metaatoms (nanofin) composed of two silicon nanopillars waveguides with different nanopillar length and width that act as coupled waveguides (Figure 1c). Indeed, these anisotropic nanofins offer the opportunity to leverage the PancharatnamBerry phase [42, 43] and obtain an additional π phase accumulation [44] (see Methods for further details).
As shown in Figures 1c and 2a, the engineered nanofins are characterized by a set of geometrical parameters
Figure 2:
We used numerical simulations (COMSOL Multiphysics^{®}, see Methods and Supplementary Note S3 for further details) to study the electromagnetic response of the nanofins. To obtain our nanofin library, we first performed a parameter sweep computing the transmission efficiency versus the maximum phase shift for every geometry # at a fixed angle θ = 0 and for ΔT = 240 °C. Interestingly, as shown in Supplementary Figure 14, there appears to be a tradeoff between the maximum phase shift and the transmission efficiency. From this first study, we selected those geometries (Figure 2a) with an average transmission efficiency above 15% (Supplementary Table 1) that could also cover the required Δ^{max}
ϕ ∼100 deg. The geometric parameters of these selected nanofins are shown in Supplementary Figure 6a and listed in Supplementary Table 1. Next, for every selected geometry #, we performed a sweep over the rotation angle θ and input temperature T, obtaining the complete dataset of phase
Overall, we observed that the thermooptical effect combined with the PancharatnamBerry (geometrical) phase effect in our engineered silicon nanofins provides a large range of phase and phase shift values enabling the design of a thermally reconfigurable metalens (TRML).
2.3 Thermally reconfigurable metalens
We simulated the propagation of a beam focused by our TRML obtaining the threedimensional intensity profile of the focused beam (see Methods). From a qualitative inspection of the intensity profiles along the propagation direction, we verified that the focal length decreases as the temperature increases (Figure 3a and b). The focal length shifts of the metalenses were determined by measuring their point spread functions at increasing temperatures (from T = 20 °C up to T = 260 °C, with ΔT = 30 °C) along the propagation direction (zaxis) with 1 μm resolution. Furthermore, we observed that both the pointspreadfunction (PSF) of the beam focus and the depth of focus become narrower at higher temperatures. Thus, the lens NA increases with temperature, as expected (Figure 3c and d, f). From a quantitative analysis, we obtained that the ML focal length changes from f(20 °C) ∼ 165 μm to f(260 °C) ∼ 135 μm.
Figure 3:
Due to intrinsic diffraction of low Fresnel number lens [45], our ML, exhibits an unavoidable deviation of its focal length from the ideal focal length defined with geometric optics. In particular, the peak irradiance position Z
_{
p
} derived from diffraction theory lies at an average distance δ = Z
_{
p
} − f ∼ 27 μm from the target focal length f over all temperatures. Nonetheless, the reported design satisfies all the tunability requirements. Indeed, the total focal length modulation, Δf ∼ 30 μm, is comparable to the final depth of focus, z
_{0} ∼ 35 μm. Furthermore, we observe that both the pointspreadfunction (PSF) of the beam focus and the depth of focus become narrower at higher temperatures as the NA increases from NA(20 °C) = 0.10 to
Importantly, from the deviation of the discretized phase profile from the analytical one (Supplementary Figure 5d and e), we obtain a ML Strehl ratio of S ∼ 0.99 > 0.8, indicative of a diffractionlimited behavior of our design (see Methods for further details). Indeed, as shown in Figure 3f, the value of the fullwidth at half maximum of the Airy disk at the focal plane (
We also characterized our TRML by measuring the focusing efficiency of the focal spot under RCP incident light. We defined the focusing efficiency as the focal spot power divided by transmitted power through an aperture with the same radius as the designed metalens. Our TRML exhibits an average focusing efficiency equal to 26%. Although, this value is limited by the optical losses and reduced transmission (intrinsically limited up to 50% by the polarization conversion and by the conversionefficiency of the nanofin metaatoms Supplementary Table 1), our result is comparable to that of mechanically actuated reconfigurable metalenses [30, 46, 47].
3 Discussion
Similar to other adaptive optic approaches [46, 48], the TRML proposed in this work is affected by chromatic aberrations. Our metalens is designed to operate at one wavelength of λ = 0.632 μm with an effective focal length f ∼ 150 μm and an effective NA = 0.1. At this operating wavelength, focal length and NA, and assuming that the fractional change in the focal length is equal to the fractional change in the wavelength (Δf/f = Δλ/λ), the theoretical operation bandwidth [46, 49] should be
Our design principle enables also higher NA thermally tunable metalens. As the proof of concept, we show in Supplementary Figure 16 the beam propagation resulting from the design of a thermally tunable metalens with NA = 0.4, radius R = 4.55 μm and a total focal length shift of 10% (from 10 μm up to 11 μm). This design is based on the same nanoresonators set identified for our initial design and has been obtained following the same methodological approach. In general, the maximum focal length shift is limited by the tradeoff between the metalens NA and radius as well as the tradeoff between the metalens NA and the maximum thermal phase shift provided by the nanoresonators (Supplementary Figure 1 and Supplementary Figure 2). Such tradeoff also impacts the design of MLs with larger diameters, as they require a large maximum phase shift. However, selecting longer operation wavelengths, the ML design present a less stringent tradeoff between maximum focal length shift and NA (see Supplementary Figure 16).
In this regard, although the thermally tunable metalens presented here has a small submillimeter aperture, our methods can be easily adapted to generate metalens designs with higher diameters while keeping the exact same set of nanofin resonators and parameter space. However, to reach high quality design while increasing the ML radius it is recommended either to decrease the total focal length variation requirement or to extend the parameter space by introducing other geometries to cover the range of the total phase thermal shift that will be required while increasing the ML radius. In particular, one possibility to increase the maximum phase shift is to combine both positive and negative phase shifts of the metaatoms. Moreover, it would be interesting to explore the use of a material with a higher thermooptic coefficient, such as germanium [51, 52], and to operate at infrared (telecom) wavelengths.
Concerning the TRML realization and operation, we envision the possibility of using a ring microheater to provide a uniform temperature bias to the lens. Indeed, this approach in combination with thin substrates can result in excellent temperature uniformity across the lens (Supplementary Figure S17). Implementation of a microheater underneath the lens would likely provide faster response [20]. Yet, perturbations to the input polarization and optical losses could hinder the ML operation. More interestingly, we envision the possibility of leveraging optical heating for fast alloptical modulation of the metalens [35]. In this case, collective heating effects are expected to result in a nonuniform temperature profile across the radius of the ML. While these should be carefully assessed in the design phase, the proposed methodology is already capable of accounting for an arbitrary temperature profile. From the longterm stability point of view, adverse effects of thermal stresses (i.e., delamination) are not expected for the low temperatures considered for this analysis. However, much higher operation temperatures as well as thermal fatigue over many thermal cycles should be carefully assessed experimentally towards fast and reliable modulation.
4 Conclusions
Overall, we have demonstrated a thermally reconfigurable metalens based on thermooptic effects in silicon nanofin resonators. Our design enables a continuous modulation of the focal length with closeto diffraction limited optical performance and competitive focusing efficiency. Our approach combines a geometrical phase approach with a temperature dependent phase shift to tune the lens properties using a spatially uniform temperature input. Increasing the complexity of the metaatom geometry and introducing more resonance modes [53] would offer a broader parameter space to search for TRML designs with advanced optical properties (high NA, high efficiency). Thus, we envision that coupling our approach with machinelearning algorithms [54, 55] capable of identifying nonintuitive metaatom shapes, which cover a large phase shift range with high conversion efficiencies, will significantly improve the performance of thermally reconfigurable silicon metalenses. More broadly, the proposed design approach, based on geometric phase combined with thermooptical effects, is expected to offer inspiration for the future realization of a broad class of active metasurfaces within the emerging field of thermonanophotonics [36].
5 Methods
5.1 Strehl ratio and focusing efficiency
Strehl ratio is defined as the ratio between the square of the electric field amplitude at the center of the designed intensity profile and the square of the electric field amplitude at the center of the ideal point spread function (PSF) [56–58]. It is given by the following equation:
where
The root mean square deviation of the designed ML phase profile from the analytical phase profile results to be σ[ϕ(T
_{0})] = 2.5 deg at 20 °C and
The efficiency is computed as the ratio between the total left circular polarized light intensity at the focal point upon conversion from right (RCP) to left (LCP) polarized light divided by the total light intensity (RCP plus LCP) which would be transmitted and focused assuming no losses due to absorption or reflection.
5.2 Simulation of the silicon nanoresonators
The geometry of all nanoresonators was simulated with a fixed gap between the two nanopillars (g = 60 nm) and height (h = 300 nm). The gap has been minimized to maximize the coupling and its value has been chosen based on fabrication considerations such as the resolution of ebeam lithography. The nanopillar high was fixed at about h ∼ λ/2 to ensure good mode coupling.
We simulated the interaction of circularly polarized light (electrical field E ) with our nanoscatterers to extract both the phase ϕ and the transmission T (coincident with the polarization conversion efficiency from RCP to LCP) of the electric field. We thus used the radio frequency (RF) module of COMSOL Multiphysics v5.5 to solve the Maxwell’s equation:
where
Each silicon nanofins unit was simulated setting a periodic boundary condition along the transverse direction with respect to the propagation of light and a perfectly matched layer and input/output ports boundary conditions along the longitudinal direction.
We used the scattering parameter S _{21}, measured from the eigenmode expansion of the electromagnetic field at the output port 2 (see Supplementary Figure 7), to extract both the transmission T and phase ϕ of the electric field converted from right to left circularly polarized light:
where
and where the input RCP light incident on the input port 1 (S _{1}, Sparameter of incident wave) under sapphire substrate, and the output LCP component (S _{2}) at the exit port 2 are:
Note that, while performing COMSOL simulation, we inverted both the sign of output phase and the input/output polarization definition compared to the ones used in the formalism described above for the phase profile design of our metalens. We are inverting the phase and polarization because in COMSOL the phase convention is based on:
Thus, while in our formalism an increase of the phase value indicates retardation (delay accumulation) in phase profile, in COMSOL, a phase increase introduces an electrical field anticipation.
5.3 Thermally tunable metalens design method
The design of our metalens (ML) is based on three main steps written in MATLAB.
In the first step, the software computes the maximum focal length variation and numerical aperture (NA) for a metalens characterized by the inputs set by the user. At this stage, the input data simulated in COMSOL are loaded and shown and the analytical phase profile based on the user input parameters is created and displayed. The metalens design optimization and the distribution of the different NF# geometries over the ML surface, are performed during the second step where the metalens layout is created and the Strehl ratio is computed. In the last step the Beam Propagation Method (BPM) is used to retrieve the actual ML focus profile and to extract the ML focus position, its FWHM and its depth of focus and its efficiency at each temperature. Lastly, the ML performance is compared with the behavior of an ideal lens. Further details can be found in the Supplementary note S1 and in the following paragraph.
5.4 Beam propagation method (BPM)
In this section, we present the derivation of the theory supporting the beam propagation method here implemented to study the behavior of a beam focused by our designed metalens [59, 60]. All numerical calculations of the intensity profiles were performed in MATLAB environment.
When considering a plane wave incident on a homogeneous, isotropic, and linear medium, the electromagnetic wave equations reduce to the Helmholtz equation for E :
In the paraxial limit, the twodimensional Fourier transform of the electric field E transverse to the propagation axis Z and at a certain fixed position z (i.e., the E field in the z = 0 plane) can be written as:
where k
_{
x
}, k
_{
y
} are the spatial frequencies coordinates of the Cartesian transverse coordinates x and y and ϕ the phase value in the
Then, the inverse Fourier transform becomes:
Knowing that:
and replacing the
E
field expression in the Helmholtz equation with the Fourier representation just derived above, we find that the evolution of the field in the propagation direction Z, in the Fourier space corresponds to a multiplication for a factor
In other words, the Fourier spectrum of
E
, in an arbitrary image plane at the z location, corresponds to the spectrum in the object plane (z = 0) multiply by the factor
where
is the Fourier transform of the electrical field at the ML plane (z = 0) and ϕ and E _{0} and are the phase and the electrical field amplitude of each nanoresonator at the position (x, y) on the z = 0 plane respectively. The intensity profile of the beam focused by the designed metalens can be finally retrieved from the squared of the electrical field propagation:
5.5 Geometrical phase
The phase delay induced by dielectric nanostructures on the incident light can be controlled by tuning either the refractive index, the relative rotation or the resonances of the nanostructures. Both on and offresonance based approaches are strictly related to the operating wavelength, the refractive index of the utilized materials and the geometry and location of the nanoantennas. On the contrary, the relative rotation of the nanopillars introduces a geometrical phase delay which depends only on the geometrical asymmetry of the nanostructures and on the polarization of the incident light.
According to the Pancharatnam–Berry geometric effect [42, 43], circularly polarized light incident on a periodic layer consisting of subwavelength and anisotropic structures, with different orientations θ respect to the reference, is transmitted as the sum of two components: one with the same phase delay and the same handedness of the incident light and the other with a phase delay ϕ proportional to the rotation angle θ of the structure and with opposite handedness. In further detail, if the electric field incident on the nanostructure is circularly polarized, that is,
where the entire operation is represented by the matrix:
where t
_{l} and t
_{s} are the complex transmittance coefficients corresponding to an incident light linearly polarized along the long and the short axis of the nanofin respectively, θ is the nanostructure rotation angle with respect to its long axis in the rotation,
The design of our ML relies on the electric field component converted from righthanded circularly polarized light
Funding source: Schweizerischer Nationalfonds zur Fӧrderung der Wissenschaftlichen Forschung
Award Identifier / Grant number: Eccellenza Grant #194181
Award Identifier / Grant number: Spark Grant #190809
Funding source: H2020 Marie SkłodowskaCurie Actions
Award Identifier / Grant number: 898315
Acknowledgments
A.A. acknowledges the support of the Marie SkłodowskaCurie Actions #898315. F.K. acknowledges the support of the Swiss National Science Foundation (Eccellenza Grant #194181). T.V. Tsoulos acknowledges the support of the Swiss National Science Foundation (Spark Grant # 190809).

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

Research funding: SNSF Eccellenza Grant #194181; SNSF Spark Grant #190809; H2020 Marie SklodowskaCurie Actions #898315.

Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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