1 Introduction
Designing metamaterials or composite materials, in which computational tools select composable components to recreate desired properties that are not present in the constituent materials, is a crucial task for a variety of areas of engineering (acoustic, mechanics, thermal/electronic transport, electromagnetism, and optics) [20]. For example in metalenses, the components are subwavelength scatterers on a surface, but the device diameter is often wavelengths [21]. Applications of such optical structures include ultracompact sensors, imaging, and spectroscopy devices used in cell phone cameras and in medical applications [21]. As the metamaterials become larger in scale and as the manufacturing capabilities improve, there is a pressing need for scalable computational design tools.
In this work, surrogate models were used to rapidly evaluate the effect of each metamaterial components during device design [38], and machine learning is an attractive technique for such models [2, 18, 3, 19]. However, in order to exploit improvements in nanomanufacturing capabilities, components have an increasing number of design parameters and training the surrogate models (using bruteforce numerical simulations) becomes increasingly expensive. The question then becomes: How can we obtain an accurate model from minimal training data? We present a new activelearning (AL) approach—in which training points are selected based on an error measure (Fig. 3)—that can reduce the number of training points by more than an order of magnitude for a neuralnetwork (NN) surrogate model of partialdifferential equations (PDEs). Further, we show how such a surrogate can be exploited to speed up largescale engineering optimization by . In particular, we apply our approach to the design of optical metasurfaces: large (– wavelengths ) aperiodic nanopattered ( structures that perform functions such as compact lensing [51].
Metasurface design can be performed by breaking the surface into unit cells with a few parameters each (Fig. 1) via domaindecomposition approximations [38, 25], learning a “surrogate” model that predicts the transmitted optical field through each unit as a function of an individual cell’s parameters, and optimizing the total field (e.g. the focal intensity) as a function of the parameters of every unit cell [38] (Sec. 2). This makes metasurfaces an attractive application for machine learning (Sec. 4) because the surrogate unitcell model is reused millions of times during the design process, amortizing the cost of training the model based on expensive “exact” Maxwell solves sampling many unitcell parameters. For modeling the effect of a –
unitcell parameters, Chebyshev polynomial interpolation can be very effective
[38], but encounters an exponential “curse of dimensionality” with more parameters
[7, 48]. In this paper, we find that a NN can be trained with orders of magnitude fewer Maxwell solves for the same accuracy with parameters, even for the most challenging case of multilayer unit cells many wavelengths () thick (Sec. 5). In contrast, we show that subwavelengthdiameter design regions (considered by several other authors [2, 17, 18, 32, 3, 19]) require orders of magnitude fewer training points for the same number of parameters (Sec. 3), corresponding to the physical intuition that wave propagation through subwavelength regions is effectively determined by a few “homogenized” parameters [15], making the problems effectively lowdimensional. In contrast to typical machinelearning applications, constructing surrogate models for physical model such as Maxwell’s equations corresponds to interpolating smooth functions with no noise, and this requires new approaches to training and active learning as described in Sec. 4. We believe that these methods greatly extend the reach of surrogate model for metamaterial optimization and other applications requiring moderateaccuracy highdimensional smooth interpolation.Recent work has demonstrated a wide variety of opticalmetasurface design problems and algorithms. Different applications [33] such as holograms [45], polarization [4, 36], wavelength [49], depthoffield[6], or incident angledependent functionality [29] are useful for imaging or spectroscopy [1, 52]. Ref. Pestourie et al. [38] introduced an optimization approach to metasurface design using Chebyshevpolynomial surrogate model, which was subsequently extended to topology optimization ( parameters per cell) with “online” Maxwell solvers [26]. Metasurface modeling can also be composed with signal/imageprocessing stages for optimized “endtoend design” [44, 27]. Previous work demonstrated NN surrogate models in optics for a few parameters [28, 34, 41], or with more parameters in deeply subwavelength design regions [2, 17]. As we will show in Sec. 3, deeply subwavelength regions pose a vastly easier problem for NN training than parameters spread over larger diameters. Another approach involves generative design, again typically for subwavelength [3, 19] or wavelengthscale unit cells [30], in some cases in conjunction with largerscale models [18, 32, 17]. A generative model is essentially the inverse of a surrogate function: instead of going from geometric parameters to performance, it takes the desired performance as an input and produces the geometric structure, but the mathematical challenge appears to be closely related to that of surrogates.
Active learning (AL) is connected with the field of uncertainty quantification (UQ), because AL consists of adding the “most uncertain” points to training set in an iterative way (Sec. 4) and hence it requires a measure of uncertainty. Our approach to UQ (Sec. 4) is based on the NNensemble idea of Ref. Lakshminarayanan et al. [24] due to its scalability and reliability. There are many other approaches for UQ [16, 12, 43, 47, 10], but Ref. Lakshminarayanan et al. [24] demonstrated performance and scalability advantages of the NNensemble approach. In contrast, Bayesian optimization relies on Gaussian processes that scale poorly ( where is the number of training samples) [31, 5]. To our knowledge, the work presented here is the first to achieve training time efficiency (we show an order of magnitude reduction sample complexity), design time efficiency (the actively learned surrogate model is at least two orders of magnitude faster than solving Maxwell’s equations), and realistic largescale designs (due to our optimization framework [38]), all in one package.
2 Metasurfaces and surrogate models
In this section, we present the neuralnetwork surrogate model used in this paper, for which we adopt the metasurface design formulation from Ref. Pestourie et al. [38]. The first step of this approach is to divide the metasurface into unit cells with a few geometric parameters each. For example, Fig. 1(left) shows several possible unit cells: (a) a rectangular pillar (“fin”) etched into a 3d dielectric slab [22] (two parameters); (b) an Hshaped hole (four parameters) in a dielectric slab [2]; or a (c) multilayered 2d unit cell with ten holes of varyings widths considered in this paper. As depicted in Fig. 1(right), a metasurface consists of an array of these unit cells. The second step is to solve for the transmitted field (from an incident planewave) independently for each unit cell using approximate boundary conditions [38, 26, 22, 50], in our case a locally periodic approximation (LPA) based on the observation that optimal structures often have parameters that mostly vary slowly from one unit cell to the next [38]. (Other approximate boundary conditions are also possible [25].) For a subwavelength period, the LPA transmitted far field is entirely described by a single number—the complex transmission coefficient . One can then compute the field anywhere above the metasurface by convolving these approximate transmitted fields with a known Green’s function, a neartofarfield transformation [14]. Finally, any desired function of the transmitted field, such as the focalpoint intensity, can be optimized as a function of the geometric parameters of each unit cell [38].
In this way, optimizing an optical metasurface is built on top of evaluating the function (transmission through a single unit cell as a function of its geometric parameters) thousands or even millions of times—once for every unit cell, for every step of the optimization process. Although it is possible to solve Maxwell’s equations “online” during the optimization process, allowing one to use thousands of parameters per unit cell requires substantial parallel computing clusters [26]. Alternatively, one can solve Maxwell’s equations “offline” (before metasurface optimization) in order to fit to a surrogate model
(1) 
which can subsequently be evaluated rapidly during metasurface optimization (perhaps for many different devices). For similar reasons, surrogate (or “reducedorder”) models are attractive for any design problem involving a composite of many components that can be modeled separately [3, 19, 35]. The key challenge of the surrogate approach is to increase the number of design parameters, especially in nonsubwavelength regions as discussed in Sec. 3.
In this paper, the surrogate model for each of the real and imaginary parts of the complex transmission is an ensemble of independent neural networks (NNs) with the same training data but different random “batches” [13] on each training step. Each of NN is trained to output a prediction
and an error estimate
for every set of parameters . To obtain these and from training data (from bruteforce “offline” Maxwell solves) we minimize [24]:(2) 
over the parameters of NN . Equation (2) is motivated by problems in which
was sampled from a Gaussian distribution for each
, in which case andcould be interpreted as mean and heteroskedastic variance, respectively
[24]. Although our exact function is smooth and noisefree, we find that Eq. (2) still works well to estimate the fitting error, as demonstrated in Sec. 4. Each NN is composed of an input layer with 13 nodes (10 nodes for the geometry parameterization and 3 nodes for the onehot encoding
[13]of three frequencies of interest), three fullyconnected hidden layers with 256 rectified linear units (ReLU
[13]), and one last layer containing one unit with a scaled hyperbolictangent activation function
[13] (for ) and one unit with a softplus activation function [13] (for ). Given this ensemble of NNs, the final prediction (for the real or imaginary part of ) and its associated error estimate are amalgamated as [24]:(3)  
(4) 
3 Subwavelength is easier: Effect of diameter
Before performing active learning, we first identify the regime where active learning can be most useful: unitcell design volumes that are not small compared to the wavelength . Previous work on surrogate models [2, 17, 18, 32, 3, 19] demonstrated NN surrogates (trained with random samples) for unit cells with
parameters. However, these NN models were limited to a regime where the unitcell degrees of freedom lay within a subwavelengthdiameter volume of the unit cell. To illustrate the effect of shrinking design volume on NN training, we trained our surrogate model for three unit cells (Fig.
2(right)): the main unit cell of this study is deep, the small unit cell is a vertically scaleddown version of the normal unit cell only deep, and the smallest unit cell is a version of the small unit cell further scaled down (both vertically and horizontally) by . Fig. 2(left) shows that, for the same number of training points, the fractional error (defined in Methods) on the test set of the small unit cell and the smallest unit cell are, respectively, one and two orders of magnitude better than the error of the main unit cell when using training points or more. (The surrogate output is the complex transmission from Sec. 2.) That is, Fig. 2(left) shows that in the subwavelengthdesign regime, training the surrogate model is far easier than for larger design regions (.Physically, for extremely subwavelength volumes the waves only “see” an averaged effective medium [15]
, so there are effectively only a few independent design parameters regardless of the number of geometric degrees of freedom. Quantitatively, we find that the Hessian of the trained surrogate model (secondderivative matrix) in the smallest unitcell case is dominated by only two singular values—consistent with a function that effectively has only two free parameters—with the other singular values being more than
smaller in magnitude; for the other two cases, many more training points would be required to accurately resolve the smallest Hessian singular values. A unit cell with large designvolume diameter ( is much harder to train, because the dimensionality of the design parameters is effectively much larger.4 Activelearning algorithm
Here, we present an algorithm to choose training points that is significantly better at reducing the error than choosing points at random. As described below, we select the training points where the estimated model error is largest, given the estimated error from Sec. 2.
The algorithm used to train each of the real and imaginary parts is outlined in Fig. 3 and Algorithm 1. Initially we choose uniformly distributed random points to train a first iteration
over 50 epochs
[13]. Then, given the model at iteration , we evaluate (which is orders of magnitude faster than the Maxwell solver) at points sampled uniformly at random and choose the points that correspond to the largest . We perform the expensive Maxwell solves only for these points, and add the newly labeled data to the training set. We train with the newly augmented training set. We repeat this process times.Essentially, the method works because the error estimate is updated every time the model is retrained with an augmented dataset. In this way, model tells us where it does poorly by setting a large for parameters where the estimation would be bad in order to minimize Eq. (2).
5 Activelearning results
5.1 Orderofmagnitude reduction in training data
We compared the fractional errors of a NN surrogate model trained using uniform random samples with an identical NN trained using an activelearning approach, in both cases modeling the complex transmission of a multilayer unit cell with ten independent parameters (Fig. 4(right)). With the notation of Sec. 4, the baseline corresponds to , and equal to the total number of training points. This corresponds to no active learning at all, because the points are chosen at random. In the case of active learning, , , and we computed for and . Although three orders of magnitude on the loglog plot is too small to determine if the apparent linearity indicates a power law, Fig. 4(left) shows that the lower the desired fractional error, the greater the reduction in training cost compared to the baseline algorithm; the slope of the activelearning fractional error () is about 30% steeper that that of baseline (). The activelearning algorithm achieves a reasonable fractional error of in twelve times less points than the baseline, which corresponds to more than one order of magnitude saving in training data (much less expensive Maxwell solves). This advantage would presumably increase for a lower error tolerance, though computational costs prohibited us from collecting orders of magnitude more training data to explore this in detail. For comparison and completeness, Fig. 4(left) shows fractional errors using Chebyshev interpolation (for the blue frequency only). Chebyshev interpolation has a much worse fractional error for a similar number of training points. Chebyshev interpolation suffers from the “curse of dimensionality”—the number of training points is exponential with the number of variables. The two fractional errors shown are for three and four interpolation points in each of the dimensions, respectively. In contrast, NNs are known to mitigate the “curse of dimensionality” [11].
5.2 Application to metalens design
We used both surrogates models to design a multiplexer—an optical device that focuses different wavelength at different points in space. The actively learned surrogate model results in a design that much more closely matches a numerical validation than the baseline surrogate (Fig. 5). As explained in Sec. 2
, we replace a Maxwell’s equations solver with a surrogate model to rapidly compute the optical transmission through each unit cell; a similar surrogate approached could be used for optimizing many other complex physical systems. In the case of our twodimensional unit cell, the surrogate model is two orders of magnitude faster than solving Maxwell’s equations with a finite difference frequency domain (FDFD) solver
[8]. The speed advantage of a surrogate model becomes drastically greater in three dimensions, where PDE solvers are much more costly while a surrogate model remains the same.The surrogate model is evaluated millions of times during a metastructure optimization. We used the actively learned surrogate model and the baseline surrogate model (random training samples), in both cases with training points, and we optimized a tenlayer metastructure with unit cells of period nm for a multiplexer application—where three wavelengths (blue: nm, green: nm, and red: nm) are focused on three different focal spots (m, m), (, m), and (m, m), respectively. The diameter is m and the focal length is m, which corresponds to a numerical aperture of . Our optimization scheme tends to yield results robust to manufacturing errors [38] for two reasons: first, we optimize for the worst case of the three focal spot intensities, using an epigraph formulation [38]; second, we compute the average intensity from an ensemble of surrogate models that can be thought of as a Gaussian distribution with , and and are defined in Eq. (3) and Eq. (4), respectively,
(5) 
where is a Green’s function that generates the farfield from the sources of the metastructure [38]. The resulting optimized structure for the activelearning surrogate is shown in Fig. 5(bottom).
In order to compare the surrogate models, we validate the designs by computing the optimal unit cell fields directly using a Maxwell solver instead of using the surrogate model. This is computationally easy because it only needs to be done once for each of the unit cells instead of millions of times during the optimization. The focal lines—the field intensity along a line parallel to the twodimensional metastructure and passing through the focal spots—resulting from the validation are exact solutions to Maxwell’s equations assuming the locally periodic approximation (Sec. 2). Fig. 5(top) shows the resulting focal lines for the activelearning and baseline surrogate models. A multiplexer application requires similar peak intensity for each of the focal spots, which is achieved using worst case optimization [38]. Fig. 5(top) shows that the actively learned surrogate has smaller error in the focal intensity compared to the baseline surrogate model. This result shows that not only is the activelearning surrogate more accurate than the baseline surrogate for training points, but also the results are more robust using the activelearning surrogate—the optimization does not drive the parameters towards regions of high inaccuracy of the surrogate model. Note that we limited the design to a small overall diameter ( unit cells) mainly to ease visualization (Fig. 5(bottom)), and we find that this design can already yield good focusing performance despite the small diameter. In earlier work, we have already demonstrated that our optimization framework is scalable to designs that are orders of magnitudes larger [39].
Previous work, such as Ref. Chen and Gu [9]—in a different approach to activelearning that does not quantify uncertainty—suggested iteratively adding the optimum design points to the training set (reoptimizing before each new set of training points is added). However, we did not find this approach to be beneficial in our case. In particular, we tried adding the data generated from LPA validations of the optimal design parameters, in addition to the points selected by our active learning algorithm, at each training iteration, but we found that this actually destabilized the learning and resulted in designs qualitatively worse than the baseline. By exploiting validation points, it seems that the active learning of the surrogate tends to explore less of the landscape of the complex transmission function, and hence leads to poorer designs. Such exploitation–exploration tradeoffs are known in the activelearning literature [16].
6 Concluding remarks
In this paper, we present an activelearning algorithm for composite materials which reduces the training time of the surrogate model for a physical response, by at least one order of magnitude. The simulation time is reduced by at least two orders of magnitude using the surrogate model compared to solving the partial differential equations numerically. While the domaindecomposition method used here is the locally periodic approximation and the partial differential equations are the Maxwell equations, the proposed approach is directly applicable to other domaindecomposition methods (e.g. overlapping domain approximation [25]
) and other partial differential equations or ordinary differential equations
[42].We used an ensemble of NNs for interpolation in a regime that is seldom considered in the machinelearning literature—when the data is obtained from a smooth function rather than noisy measurements. In this regime, it would be instructive to have a deeper understanding of the relationship between NNs and traditional approximation theory (e.g. with polynomials and rational functions [7, 48]). For example, the likelihood maximization of our method forces to go to zero when . Although this allows us to simultaneously obtain a prediction and an error estimate , there is a drawback. In the interpolation regime (when the surrogate is fully determined), would become identically zero even if the surrogate does not match the exact model away from the training points. In contrast, interpolation methods such as Chebyshev polynomials yield a meaningful measure of the interpolation error even for exact interpolation of the training data [7, 48]. In the future, we plan to separate the estimation model and the model for the error measure using a metalearner architecture [10], with expectation that the metalearner will produce a more accurate error measure and further improve training time. We believe that the method presented in this paper will greatly extend the reach of surrogatemodel based optimization of composite materials and other applications requiring moderateaccuracy highdimensional interpolation.
Methods
6.1 Trainingdata computation
The complex transmission coefficients were computed in parallel using an opensource finite difference frequencydomain solver for Helmholtz equation
[40] on a GHz Core Intel Xeon E processor. The material properties of the multilayered unit cells are silica (refractive index of 1.45) in the substrate, and air (refractive index of 1) in the hole and in the background. In the normal unit cell, the period of the cell is 400 nm, the height of the ten holes is fixed to 304 nm and their widths varies between 60 nm and 340 nm, each hole is separated by 140 nm of substrate. In the small unit cell, the period of the cell is 400 nm, the height of the ten holes is 61 nm, and their widths varies between 60 nm and 340 nm, there is no separation between the holes. The smallest unit cell is the same as the small unit cell shrunk ten times (period of 40 nm, ten holes of heigth 6.1 nm and width varying between 6 nm and 34 nm).6.2 Metalens design problem
The complex transmission data is used to compute the scattered field off a multilayered metastructure with unit cells as in Ref. Pestourie et al. [38]. The metastructure was designed to focus three wavelengths (blue: nm, green: nm, and red: nm) on three different focal spots (m, m), (, m), and (m, m), respectively. The epigraph formulation of the worst case optimization and the derivation of the adjoint method to get the gradient are detailed in Ref. Pestourie et al. [38]. Any gradient basedoptimization algorithm would work, but we used an algorithm based on conservative convex separable approximations [46]. The average intensity is derived from the distribution of the surrogate model with and the computation of the intensity based on the local field as in Ref. Pestourie et al. [38],
where the notation denotes the complex conjugate, the notations and are simplified to , and , and the notation is dropped for clarity. From the linearity of expectation,
(6)  
(7) 
where we used that and .
6.3 Activelearning architecture and training
The ensemble of NN was implemented using PyTorch
[37] on a GHz Core Intel Xeon E processor. We trained an ensemble of NN for each surrogate models. Each NN is composed of an input layer with 13 nodes (10 nodes for the geometry parameterization and 3 nodes for the onehot encoding [13] of three frequencies of interest), three fullyconnected hidden layers with 256 rectified linear units (ReLU [13]), and one last layer containing one unit with a scaled hyperbolictangent activation function [13] (for ) and one unit with a softplus activation function [13] (for ). The cost function is a Gaussian loglikelihood as in Eq. (2). The mean and the variance of the ensemble are the pooled mean and variance from Eq. (3) and Eq. (4). The optimizer is Adam [23]. The starting learning rate is . After the tenth epoch, the learning rate is decayed by a factor of . Each iteration of the active learning algorithm as well as the baseline were trained for epochs. The choice of training points is detailed in Sec. 4. The quantitative evaluations were computed using the fractional error on a test set containing points chosen at random. The fractional errorbetween two vectors of complex values
and is(8) 
where is the Lnorm for complex vectors.
Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
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Acknowledgements
This work was supported in part by IBM Research, the MITIBM Watson AI Laboratory, the U.S. Army Research Office through the Institute for Soldier Nanotechnologies (under award W911NF13D0001), and by the PAPPA program of DARPA MTO (under award HR00112090016).
Author contributions
RP, YM, PD, and SGJ designed the study, contributed to the machinelearning approach, and analyzed results; RP led the code development, software implementation, and numerical experiments; RP and SGJ were responsible for the physical ideas and interpretation; TVN assisted in designing and implementing the training. All authors contributed to the algorithmic ideas and writing.
Competing interests
The authors declare no competing financial or nonfinancial interests.
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