Broadband focusing and collimation of water waves by zero refractive index

It is always a challenge to realize extreme and unusual values of refractive index for a broad range of frequencies. We show that when water is covered by a thick, rigid and unmovable plate, it behaves like a medium with zero refractive index for water waves at any frequency. Hence, by covering water with a plate of a concave or rectangular shape, water waves can be focused or collimated in a broad range of frequencies. Experiments were conducted to demonstrate these effects and results are in excellent agreement with numerical simulations.

Unlike electromagnetic and acoustic/elastic waves, water waves are mechanical waves that propagate along the water surface and with the restoring force provided by gravity 26,27 . For a constant water depth h, the dispersion of linear water waves is given by where v is the angular frequency, k is the wavenumber and g is the gravitational acceleration [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] . In this paper, we theoretically and experimentally show that by covering water with a rigid and unmovable plate, a zero wavenumber and zero refractive index can be created for water waves at any frequency. As a result, interesting phenomena such as focusing and collimation of broadband water waves can be further realized by using different shapes of plates.

Results
System description, water wave equation and zero refractive index. We consider linear, inviscid and irrotational water waves in infinite extent of water as shown in Fig. 1. Set the x-y plane horizontal, the z-axis positive upward and the bottom of water in the z 5 0 plane. The water depth is h 1 in both region I with x , 0 and region III with x . L. In region II with 0ƒxƒL, the water is covered by a thick, unmovable and rigid plate and has a depth of h 2 . For harmonic water waves, the velocity of water v is related to a potential W by v(x, y, z, t) 5 =W(x, y, z)e 2ivt 26,27 . W satisfies the following equations where the subscript l in h l is 1 and 2 in regions I/III and II, respectively. At the upper surfaces (z 5 h l ) of regions I/ III and II, the boundary conditions are LW Lz~v Here, g l 5 g 1 5 g in region I and g l 5 g 2 R ' in region II. Hence, the solution of Eqs.
Coupled mode theory. The field W~Qe ikyy and Q in regions I, II, III can be written respectively as where k y 5 k 11 sin h, h is the incident angle, x9 5 x 2 L and (A, C, E, G) and (B, D, F, H) are the amplitudes of leftgoing and rightgoing waves, respectively. At the interface (x 5 0) between regions I and II, the potential and velocity should be continuous [Q I 5 Q II and LQ I Lx~L By multiplying Eq. (5) with where Transfer matrix formalism. Eqs. (7) and (8) can be rewritten as From Eq. (9), we have where where P mj~dmj e ik2xjL . At the interface between regions II and III, we can also obtain From Eqs. (10) -(12), we have For incident waves from the left (A j 5 d 1j and H j 5 0), the transmission t 5 jG 1 /A 1 j 2 and reflection r 5 jB 1 /A 1 j 2 can then be calculated by Eq. (13). We note that Eqs. (10) -(13) represent the transfer matrix   A finite number of plane waves (jƒN) are adopted in numerical calculations. When pL/h 2 . 1, the evanescent waves (with j §2) can be neglected (i.e. N 5 1 is used) so that an analytic formula can be obtained for the transmission at normal incidence (h 5 0), where q~1 2 [see

Supplementary information for derivations].
Calculated transmission through water covered by a long rectangular plate. The above theory shows that water covered by a plate can be regarded as a zero-index medium (k 21 5 0) for water waves and exhibit a relatively high transmission for long wavelengths (L/l , 1). As an example, the transmission spectra are calculated for the case with h 1 5 h 2 5 6 mm and L 5 30 mm, as shown in Fig. 2a.
To obtain convergent results, 5 plane waves (N 5 5) have been adopted. It can be seen that for normal incidence (h 5 0), the accurate results can be well described by Eq. (14). A high transmission occurs for low frequencies (L/l , 1). For oblique incidence, all the waves become evanescent (Im(k 2xj ) 5 k 11 sin h . 0) in region II, so that the transmission decays almost exponentially with increasing frequency [Fig. 2a]. As a result, at a given frequency, a relatively large transmission occurs only when the incident angle is around zero, and this phenomenon becomes more apparent with increasing the frequency [ Fig. 2b]. Eqs. (2) -(4) are an accurate 3D theory for linearized water waves and can be approximately replaced by a 2D equation 31,37 , where y(x, y) is the potential W at the water surface (z 5 h l ) and u 5 tanh(kh)/k is the effective depth. In Fig. 2a, we also show the results at normal incidence by Eq. (15). It can be seen that Eq. (15) can present almost the same results as those by Eq. (14).
Experimental observations and numerical simulations of focusing effect. The zero-index media can be applied to refract and focus waves 3,4 . For instance, one can focus plane waves with a concave lens made of a zero-index medium. Here, we conduct a waterwave experiment to demonstrate such a focusing effect. Our experimental setup is sketched in Fig. 3a. Water is placed in a vessel and partially covered by a concave, rigid and unmovable plate, which is obtained by cutting a half circle with radius of 20 cm on a 40 3 20.5 cm 2 rectangular plate [ Fig. 3b]. The water depths are 6 mm and 5 mm in the regions without and with the plate, respectively. The plate is impinged by a Gaussian water-wave beam with width of 28 cm. Since the vessel has a transparent bottom, patterns of water waves can be projected onto a screen and then recorded by a digital camera 32,35 . On the other hand, we also apply a finite-element method to simulate the experiment. Figures 4(a) -4(h) show the simulated and observed wave patterns for different frequencies. Good agreement is found between the theory and experiment. Here, water waves are normally incident on the flat side of the focusing lens (i.e. water covered by the concave plate), so that they do not refract at the entrance side. But when water waves leave the lens, they do refract at the exit side with a concave shape [Figs. 4a-4d]. All the outgoing waves are perpendicular to the exit side and thus directed to the focus of lens (i.e. the center of the half circle) [Figs. 4e-4h]. Since our zero index is independent on frequency, the focusing effect can be observed in a broad band of frequencies.
Experimental observations and numerical simulations of collimation effect. In addition to focusing external water waves, the plate-covered water can also be applied to modify the emission   property of an inner source, as shown in Fig. 5. Here, water is partially covered by a 32 3 7 cm 2 rectangular plate that is drilled with a small hole. The hole is connected to an air tube with oscillating pressure, so that a point source of water waves can be generated in the plate. At the exit side of the plate, the emergence angle of water waves are always zero for arbitrary incident angle. As a result, a collimated beam can be observed in the water region without plate [Figs. 5a-5d]. Again, the experimental results are well described by the theory, showing that the water covered by the unmovable plate indeed possesses a zero index for water waves.

Discussion
In the above experiments, the focusing and collimation effects were demonstrated by using a plates with size larger than the water depth. We note that the effects can also be realized by using a plate that has a size smaller than the water depth but larger than the wavelength. When the water depth is much larger than the wavelength (h 1 ?l), the velocity of water is almost zero near the bottom (z , h 1 2 20l). Hence, in numerical simulations, h 1 and h 2 can be replaced by h 1 2 h d and h 2 2 h d , respectively, where h d 5 h 1 2min(h 1 , 20l). By using five wave modes (N 5 5), transmission spectra (similar to the curves in Fig. 2) can also be obtained.
Region II in our water-wave system, exhibiting an infinite effective gravitational acceleration (g 2 5 0) and normal depth (h 2 5 h 1 ), has its optical analogue that is a medium with zero magnetic permeability (m 5 0) and normal dielectric constant (e 5 1). Such a single zero (m 5 0) medium can be realized by using a photonic crystal with a square lattice of dielectric cylinders in air 43 . Near a particular frequency, the focusing and collimation effects have also been observed for electromagnetic waves. In contrast, since our infinite effective gravity is valid for water waves at any frequency, our effects can occur in a broad range of frequencies.
In summary, we have shown theoretically and experimentally that by covering water with a rigid and unmovable plate, a zero wavenumber and zero refractive index can be achieved for water waves in a broad range of frequency. By using plates with different shapes, interesting phenomena including focusing and collimation have been demonstrated in water waves. The results suggest a new mechanism for controlling the propagation of water waves and may find applications in wave energy focusing and extraction.