## Abstract

We present an algorithm for designing high efficiency (∼98%), small-footprint (1.5–4 square vacuum wavelengths) couplers between arbitrary nanophotonic waveguide modes in two dimensions. Our “objective-first” method is computationally fast (15 minutes on a single-core personal computer), requires no trial-and-error, and does not require guessing a good starting design. We demonstrate designs for various coupling problems which suggest that our method allows for the design of any single-mode, linear optical device.

© 2012 Optical Society of America

## 1. Motivation

Optical mode conversion, the efficient transfer of photons from one guided mode to another, is a fundamental requirement in nanophotonics. For instance, efficient conversion between waveguides modes is essential for:

- Coupling to and from optical fiber [1], to communicate with the outside world.
- Coupling between various nanophotonic waveguides, since different waveguides are best suited for different applications. For example, ridge waveguides seem ideal for low-loss transport [2], but other waveguides, such as photonic crystal waveguides or slot waveguides, may be better suited for slow-light [3] or nonlinear optical devices based on localized field intensities [4].

In this paper, we present a method to solve the problem of designing single-mode, linear nanophotonic devices. We then demonstrate this method by designing coupling structures between various nanophotonic waveguides. Furthermore, we show that our method does not require a good initial design, is computationally fast, and can generate high efficiency couplers within a very small footprint.

## 2. Objective-first approach

Physical structures are typically designed by solving the following problem:

where*x*is the field variable and

*p*is the structure variable. Here,

*f*(

*x*), the

*design objective*, calculates the performance of the device (e.g. amount of power lost); while

*g*(

*x*,

*p*) is the underlying physical equation for the system (e.g. the electromagnetic wave equation).

In contrast, the objective-first approach solves

where ||*g*(

*x*,

*p*)||

^{2}is the

*physics residual*. We term this formulation “objective-first” because the design objective is prioritized even above satisfying physics; specifically, we force our design to always exhibit the desired performance (

*f*(

*x*) = 0), even at the expense of not perfectly satisfying the underlying physics which governs its operation.

The motivation behind this formulation is two-fold. First, this approach allows one to arrive at a locally-optimal design rapidly by allowing *x* and *p* to vary independently, as opposed to Eq. 1 where the value of *x* is completely dependent on *p*. Second, it enables an increase in the likelihood of arriving at a high efficiency design by forcibly imposing *f*(*x*) = 0, and thereby circumvents any local optima consisting of low-performance devices.

## 3. Objective-first design of waveguide couplers

We now demonstrate the objective-first approach by designing waveguide couplers in two-dimensions. As such, we do not analyze effects which are only present in full three-dimensional systems, such as out-of-plane losses. Specifically, we work in the two-dimensional transverse electric mode, choosing *H _{z}* as the field variable

*x*, and

*ε*

^{−1}(the inverse of the permittivity) as the structure variable

*p*. This results in the following form of the physics residual,

*ω*is the angular frequency, and

*μ*

_{0}is the permeability of free-space.

In order to produce a method applicable to the design of any linear nanophotonic device, we note that its performance will be completely characterized by the fields along its boundary, and therefore, we choose a boundary-value formulation for the design objective as shown in Fig. 1. Namely, we first compute the boundary fields needed to obtain perfect performance, ${H}_{z}^{\text{perfect}}$, and then form the design objective as

*∂H*/

_{z}*∂n*denotes the spatial derivative in the direction normal to the boundary.

Such a design objective is both extremely simple and widely adaptable to the design of linear nanophotonic devices in general. On the other hand, such a formulation often prohibits the physics residual from disappearing completely; however, we find that very good device performance is still obtained as long as the physics residual is sufficiently small.

Lastly, we employ an alternating directions strategy, where both *x* and *p* are solved iteratively [7] and limit the allowable values of *ε* to be between the permittivity of vacuum and of silicon,

*ε*= {

*ε*

_{0},

*ε*

_{silicon}}, is needed for manufacturing purposes; this will be pursued in a future work along with an analysis of robustness to fabrication imperfections. That said, the final designs presented here all have significant portions which are already binary.

#### 3.1. Coupler designs

We now present five results which suggest our method is indeed able to design arbitrary single-mode, linear nanophotonic devices. These results comprise of couplers between the following nanophotonic waveguides:

- coupling between waveguides of different refractive index and width (Fig. 2);
- coupling between waveguide modes of different order and symmetry (Fig. 3);
- coupling between waveguides that confine light using different principles (index guided vs. distributed Bragg reflection guided), i.e., between a slab waveguide and a photonic crystal fiber (Fig. 4)
- coupling from a dielectric to a plasmonic metal-insulator-metal waveguide (Fig. 5); and
- coupling from a dielectric waveguide to a (plasmonic) metal wire (Fig. 6).

Note, however, that the small footprint of the designs presented does not imply small, unresolvable feature sizes. On the other hand, assuming a vacuum wavelength of 1550 nm, the smallest possible feature in any of the examples presented is still 37 nm × 37 nm (see Fig. 2, for example), as given by the size of a single grid point; placing virtually all designs within reach of current-generation lithographic systems.

In addition, good starting points were not required; all initial designs were simply *ε* = 9 everywhere (a somewhat arbitrary guess, other values work as well). Finally, only 15 minutes on a single-core personal computer were needed to obtain each design.

In the appendix, we further demonstrate the generality of our method by presenting several additional examples including

- coupling between all four modes of a wide dielectric waveguide;
- coupling to selected channels of a set of five plasmonic waveguides.

## 4. Conclusion

We develop a fundamentally new approach to designing physical structures, which we term “objective-first”, in that we choose to satisfy the design objective even above satisfying the physical equation which governs its operation. We then apply this approach to the design of various nanophotonic waveguide couplers, and show that our method produces high-efficiency designs (∼ 98% efficiency) in small footprints (1.5–4 square vacuum wavelengths) without needing a good starting point. Furthermore, we suggest that such a methodology may be applicable to the design of any single-mode, linear nanophotonic device.

## A. Appendix A

Additional mode coupler designs In this section, we present additional mode coupler designs. A wide, high-index waveguide is used at both the input and output ports, and all permutations of couplers are fabricated among the first four propagating modes [see Figs. 7–12].

## B. Appendix B

Additional designs with wide, low-index input waveguide We now reproduce Figs 3–6 but instead use a wide, low-index waveguide as the input. Once again, this is to demonstrate the generality of our method; namely, that it can be applied to the design of nearly any single-mode, linear nanophotonic device [see Figs. 13–16].

## C. Appendix C

Additional designs with multiple output plasmonic waveguides We also present “selector” designs where photons are coupled to only one of five possible plasmonic output waveguides. We demonstrate these selector designs for both metal-insulator-metal and metal-wire plasmonic waveguides [see Figs. 17–22].

## Acknowledgments

This work has been supported by the AFOSR MURI for Complex and Robust On-chip Nanophotonics (Dr. Gernot Pomrenke), grant number FA9550-09-1-0704. The Matlab code used is available online [10].

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