1. Introduction

Chalcogenide phase change material (PCM) is a cornerstone for rewriteable optical data storage and future non-volatile electronic memory. The reversible transition between amorphous and crystalline states is generally achieved by heating, e.g. photothermal or electrical Joule heating. Research endeavors have been made to reduce phase change transition time, increase lifetime and minimize energy required by the phase change [1, 2, 3]. This research paves the way to not only ultra-fast data storage but also optical switching and tunable photonic devices. Among several conceptualized tunable structures, grating-based optical reflector [4], tunable chiral metamaterials [5], and tunable lattice resonances [6] are of particular note. In this context, we envisage the provision of dynamic optical beam shaping and steering by using PCM based optical antennas and antenna arrays.

Optical antennas and antenna arrays offer possibilities in manipulating light at sub-wavelength scale [7]. Similar to their counterparts at microwave frequencies, optical antenna arrays offer the capability of beam shaping and steering through controlling the phase of each antenna element. The phase control of optical antenna arrays can be implemented by either delay lines [8] or antenna geometry and location [9, 10, 11, 12, 13]. Identical antennas with uniform lattice are usually utilized when delay lines are employed to adjust the phase of antennas [8]. Although sophisticated antenna design and arrangement can be avoided, delay lines are space consuming and difficult to integrate. Without using delay lines, antenna elements have to be carefully designed to obtain the required phase by taking into account the geometry, arrangement, coupling between elements and micro/nano fabrication tolerance. Limited by the current micro/nano fabrication techniques, antenna elements usually have the same height. Sometimes, two or more antenna geometries have to be employed to facilitate the 2π phase tuning [10, 12]. It is worthwhile to mention that the delay line approach [8] allows on-chip light to be emitted, while reflection or transmission arrays [9, 10, 11, 12] only steer incoming light.

In the currently realized designs, optical antenna arrays are based on conventional metal or low loss dielectric materials. Utilizing phase change material, dynamic beam shaping and steering can be achieved with array structures. Antenna arrays with dynamic high-speed beam shaping and steering will have a great impact on future micro/nano devices. As one of the most promising applications, free-space optical inter/intra chip interconnects, e.g. between processor cores, can overcome the drawbacks of conventional thin wires connections, such as interconnect delay, limited distance and most importantly space occupation. It offers tremendous advantages of large bandwidth, fast data transfer rates, low loss, little crosstalk and compact integration [14, 15, 16]. In the chip level intra-chip prototype in [16], the point-to-point free-space optical link is enabled by a glass prism, which is bulky and consumes much space. By replacing the glass prism with a planar reflectarray, the size of the whole structure can be significantly reduced. Another clear advantage of the antenna array over the mirror is the capability of providing wireless links between multiple points on the same chip layer or different layers. It will allow not only high-speed interconnects in future integrated circuits, but also possibility of totally new system architecture.

In this paper, a tunable reflectarray is proposed as a pathway to PCM based optical antenna arrays for free-space inter/intra chip interconnects. The proposed reflectarray can be classified as a metasurface since it follows the generalized law of reflection [17]. The tunable operation is realized by switching between two states of PCM. The difference in refractive index of the amorphous and crystalline state of PCM will change the behaviour of the reflectarray. Fig. 1 illustrates a scenario of a dynamic binary link between a source and two receivers enabled by the tuntable reflectarray. The light from the source reaches the reflectarray with an arbitrary incident angle of α. In the crystalline state, the reflectarray reflects light towards Receiver 1 with a specular reflection angle of α. After Ge₂Sb₂Te₅ is converted to the amorphous state, the light is deflected towards Receiver 2 with an angle of θ, with respect to the Link 1. The incident angle of α can be negative Fig. 1(a) or positive Fig. 1(b), which correspondingly results in the reflection angle of θ−α or θ+α, with respect to the normal to the surface. It should be noted that the source and receivers can be located in different layers as long as the angle relationship is satisfied. The design principle, detailed process and numerical demonstrations are presented in the following sections. Phase change materials have been under development for a number years and are used in rewritable DVD technology with optical laser based switching and will be used in next generation phase change random access memory technology [18] with high speed electronic switching. Thus our reflectarrays will be able to adapt these approaches to develop a practical, low cost, high speed optical switching technology.

 

Fig. 1 A scenario of the dynamic inter/intra chip free-space optical interconnect. The PCM based tunable reflectarray is able to switch optical link between two receivers, i.e. Source to Receiver 1 and Receiver 2 at crystalline and amorphous state of PCM, respectively. The incident angle can be either −α (a) or α (b), which correspondingly results in the location angle of Receiver 2 of θ−α or θ+α.

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2. Tunable optical reflectarray design

The reflectarray consists of an array of equally separated antenna elements. In one state of PCM, antenna elements are designed to provide a constant phase gradient ϕ along the interface. It modifies the wavefront of incident light by altering its phase in a desired manner, as shown in Fig. 2. The gradient of phase shift generates an effective wavevector along the interface, which is able to deflect light from specular reflection, described by the generalized Snell’s law [19]. The deflected angle θ is independent of the incident angle α. From the triangular relation between ϕ/k, d and θ in Fig. 2, the deflected angle θ can be calculated as

 

Fig. 2 Operation principle of the reflectarray. The wavefront of the reflected light is modified to an angle of θ by an array of equally separating space d antenna elements with a constant gradient of phase jump ϕ along the interface.

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Ge₂Sb₂Te₅ is selected for the tunable reflectarray due to its commercial availability, fast tuning time between two states, large number of switching cycles and stability [2, 6]. The reflectarray is based on a structure where 60 nm Ge₂Sb₂Te₅ is sandwiched between two 30 nm silver layers. The Ge₂Sb₂Te₅ layer provides a tunable dielectric environment for the reflectarray. In the optical communication band of 1.55 μm, the measured permittivity of amorphous and crystalline state of Ge₂Sb₂Te₅ is 20.13+0.07j and 45.02+0.59j, respectively [21]. The major challenge in the design is the high dielectric constant of the amorphous Ge₂Sb₂Te₅ layer, which prevents a full 2π phase control when using only single type of antenna patch antenna. Therefore, a second type of antenna, a dielectric loaded antenna, has to be used to provide the missing phase shift angles. Fig. 3 shows the simulated phase of these two types of antennas with lattice of d = 500 nm at the wavelength of 1.55 μm. The phase changes versus the antenna parameters are obtained by employing a periodic boundary condition to mimic the infinite uniform array with normal incident plane-wave excitation [9, 22]. In Fig. 3(a), the dielectric loaded antenna with the loaded silver patch size of 150 × 150 nm is able to manipulate the phase of reflected beam from 180° to −30° by varying the size A of the Ge₂Sb₂Te₅ square block from 200 to 500 nm. A phase range from 10° to −180° of the reflected beam is achieved by changing the size L of the silver patch antenna from 50 to 300 nm in Fig. 3(b). The combination of the dielectric loaded antenna and patch antenna allows for a full 2π phase change, as shown in Fig. 3(c). In the proposed design, a subarray is composed of six antenna elements with a progressive phase shift of ϕ = 60°. The size of two dielectric loaded antennas and four patch antennas are indicated by red dots and blue squares in Fig. 3(c). The subarray is periodically arranged in both x and y directions, as shown in Fig. 4. Given the deflected angle, the lattice d, progressive phase shift ϕ and other parameters are chosen such that the antenna elements can be reliably fabricated. Small lattice spacing leads to phase curve with less steep slope and hence allows for increased fabrication tolerances. In other words, with a flat phase curve, drift from the designed size does not cause large phase offsets. On the contrary, large lattice spacing usually results in wide gaps between antenna elements but also requires tight fabrication tolerances due to the steep slope of the phase curve.

 

Fig. 3 Simulated reflection phase response of two types of antennas with lattice of 500 nm at the wavelength of 1.55μm at the amorphous state of GST. (a) Dielectric loaded antenna with the silver patch size of 150×150 nm. The reflection phase covers the range from 180° to −30°. (b) Patch antenna. The reflection phase covers the range from 10° to −180°. (c) The combination of two types of antennas spans 360°. The two red dots and four blue squares respectively represent selected dielectric sizes A = 273 and 324 nm and silver patch size L = 86, 157, 184 and 250 nm to form 6 antenna elements reflectarray with the corresponding reflection phase of 130°, 70°, 10°, −50°, −110° and −170°, respectively.

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Fig. 4 The sketch of the tunable reflectarray based on Ge₂Sb₂Te₅. Each subarray consists of two dielectric loaded antennas and four patch antennas.

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3. Numerical study and discussion

The finite difference time domain method (FDTD Solutions 8.7.1, Lumerical Inc.) is employed to simulate the designed reflectarray operating at the wavelength of 1.55 μm. Fig. 5(a) shows the simulated farfield of the tunable reflectarray in the crystalline state with an incident angle α = −10°. For a better representation, the source and receivers layout in Fig. 1(a) is plotted at the bottom of the farfield pattern. Since the gradient phase shift along the surface is designed for the amorphous state, the gradient phase shift is disturbed due to the different material permittivity in crystalline state. The angle of the reflected beam is equal to the incident angle α = −10°, as shown in Fig. 5(a). The farfield pattern represents the Link 1 between Source and Receiver 1 in Fig. 1(a).

 

Fig. 5 The simulated farfield of tunable reflectarray with incident angle α = −10°. (a). The farfield pattern represents the Link 1 between Source and Receiver 1 in Fig. 1(a) when PCM at the crystalline state; (b). The farfield pattern represents the Link 2 between Source and Receiver 2 in Fig. 1(a) when PCM at the amorphous state.

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In the amorphous state of Ge₂Sb₂Te₅, the wavefront of the reflected light is manipulated by the designed gradient phase shift of antenna elements. Consequently, the reflectarray deflects the beam to about 31° with respect to Link 1, according to Eq. (1). In Fig. 5(b), the simulated far field indicates that the beam is reflected to an angle of approximately θ+α = 21°, which is equal to the Link 2 between Source and Receiver 2 in Fig. 1(a). The weak specular reflection and negative first order reflection, resulting from antenna cross-coupling effects [9], can be suppressed by optimizing the size of antennas.

Fig. 6 shows the simulated farfield of the tunable reflectarray with an incident angle α = 10, which represents the system setup in Fig. 1(b). In this case, the reflected beam patterns are located at the angles of 10° and 41°, in respect to the normal of the surface. Thus, the simulated far fields in Fig. 5 and 6 confirm the capability of dynamically tuning reflected beam angle by switching between the two states of Ge₂Sb₂Te₅.

 

Fig. 6 The simulated farfield of tunable reflectarray with incident angle α = 10°. (a). The farfield pattern represents the Link 1 between Source and Receiver 1 in Fig. 1(b) when PCM at the crystalline state; (b). The farfield pattern represents the Link between Source and Receiver 2 in Fig. 1(b) when PCM at the amorphous state.

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4. Conclusion

In this paper, the concept of beam shaping and steering of a phase change material based optical antenna array has been illustrated by a tunable optical reflectarray. The reflected beam angle of the reflectarray can be tuned by switching between the amorphous and crystalline state of PCM Ge₂Sb₂Te₅. The combination of two types of antenna: a dielectric loaded antenna and a patch antenna, has been proposed to achieve the required 2π phase tuning of the reflected light. The capability of beam tuning has been proved numerically with FDTD simulations. The study has exploited PCM based tunable antenna arrays in free-space optical inter/intra chip interconnects. It is expected that antennas and antenna arrays with tunable and reconfigurable optical properties will lead to a rich variety of new nanophotonics applications.