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On-chip noninterference angular momentum multiplexing of broadband light

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Science  13 May 2016:
Vol. 352, Issue 6287, pp. 805-809
DOI: 10.1126/science.aaf1112

A twist on optical multiplexing

Information can be encoded using various properties of light. Optical multiplexing frequency, brightness, and polarization have played crucial roles in information technologies, high-capacity data storage, high-speed communications, and biological sensing. Angular momentum is another degree of freedom that could increase capacity further. Typically, however, the bulk optical elements used to determine the angular momentum of light limit possible on-chip processing. Ren et al. take a nanophotonics approach to measure and sort light co-propagating with different states of angular momentum (see the Perspective by Molina-Terriza). The approach is promising for on-chip multiplex processing of optical signals.

Science, this issue p. 805; see also p. 774

Abstract

Angular momentum division has emerged as a physically orthogonal multiplexing method in high-capacity optical information technologies. However, the typical bulky elements used for information retrieval from the overall diffracted field, based on the interference method, impose a fundamental limit toward realizing on-chip multiplexing. We demonstrate noninterference angular momentum multiplexing by using a mode-sorting nanoring aperture with a chip-scale footprint as small as 4.2 micrometers by 4.2 micrometers, where nanoring slits exhibit a distinctive outcoupling efficiency on tightly confined plasmonic modes. The nonresonant mode-sorting sensitivity and scalability of our approach enable on-chip parallel multiplexing over a bandwidth of 150 nanometers in the visible wavelength range. The results offer the possibility of ultrahigh-capacity and miniaturized nanophotonic devices harnessing angular momentum division.

In the age of information technology, optical multiplexing using physical dimensions of light, including space (1), frequency (2), brightness (3), color (1, 4), polarization (1, 5, 6), mode (7), and lifetime (8), has played a crucial role in high-definition displaying (35), high-capacity data storage (1, 6), high-speed communications (7), and highly sensitive biological sensing (8). As one of the most fundamental physical properties in both classical and quantum optics, angular momentum (AM) of light—including spin angular momentum (SAM) possessed by circularly polarized light and orbital angular momentum (OAM) manifested by the helical wavefront of light—has emerged as a physically orthogonal multiplexing approach to high-capacity optical communications ranging from free-space (9) to compact optical fibers (10). However, macroscale interference-based detection methods through hologram-coding (9, 10) or phase-shifting (11, 12) of AM-carrying beams have imposed a fundamental physical limit for realizing such a principle at a chip-scale footprint.

The advance of strong light-confinement nanophotonic approaches has been a major propellant of miniaturized optical circuits to harness AM of light. The chip-scale generation and transmission of AM-carrying beams on silicon-integrated circuits have been realized through whispering gallery mode resonators (13) and resonant microring fibers (10). However, these approaches are resonant in nature, leading to a narrow bandwidth down to several nanometers. Surface plasmon polaritons (SPPs) capable of strong light confinements have long been pursued to overcome the size limitation of nanophotonic devices and, hence, potentially facilitate the chip-scale multiplexing of SAM through the SAM-distinguishing nanostructures (1418). Even though the OAM generators mediated by SPPs have been demonstrated either through digitalized metasurfaces with a helical phase (19) or geometric metasurfaces based on spin-orbit interaction (20), the extrinsic nature of OAM (21) with helical wavefronts restricts its detection to a phase-sensitive interference-based method through a holographic metasurface (22), which inevitably degrades the perceptive devices for on-chip applications.

The concept of our on-chip noninterference AM multiplexing of broadband light is illustrated in Fig. 1. Without losing the generality, coaxially superposed AM-carrying beams with four selected AM modes [l0 = –4, s = –1 (AM1); l0 = –2, s = –1 (AM2); l0 = +2, s = +1 (AM3); and l0 = +4, s = +1 (AM4); where l0 and s are the modal indices for OAM and SAM, respectively (Fig. 1A)] propagate through a nanoring aperture (NRA) multiplexing unit that consists of shallow nanogrooves and the spatially shifted mode-sorting nanoring slits of different sizes (Fig. 1B and fig. S1A). The nanogroove structures act as the metal-dielectric interfaces to convert the AM modes carried by photons into SPPs and to spatially route the excited plasmonic AM modes to the locations of the nanoring slits. A set of AM-carrying beams of l0 = ±1, ±2, ±3, ±4 and s = ±1 (fig. S2) can be adopted to excite a range of plasmonic AM modes (determined by total AM L = l0 + s + ls, where ls is the geometrical topological charge arising from the nanogrooves), with a distinguished spatial separability from the structure depicted in fig. S1A. The formation of the spatial separability by nanogrooves provides a physical ground for AM mode sorting. As a result of the distinctive AM mode-sorting sensitivity by nanoring slits, the plasmonic AM modes can be selectively coupled out through the slits that have different sizes and spatial shifts (Fig. 1C). Furthermore, the nonresonant AM mode-sorting sensitivity by the nanoring slits enables the AM multiplexing over a broad bandwidth. As such, a large-scale NRA-structured AM multiplexing chip (NAMMC) (Fig. 1D) consisting of an array of individually addressable NRAs, wherein NRA units are separated by a spacing larger than the diffraction-limit distance, allows for on-chip processing of an AM-multiplexed image in parallel through a multibeam approach (Fig. 1E).

Fig. 1 The principle of on-chip noninterference AM multiplexing of broadband light.

(A) Four selected AM beams [l0 = –4, s = –1 (AM1); l0 = –2, s = –1 (AM2); l0 = +2, s = +1 (AM3); and l0 = +4, s = +1 (AM4)] are coaxially overlapped as the AM-superposed beams. (B) Schematic of a NRA multiplexing unit consisting of nanogroove structures and the mode-sorting nanoring slits. (C) Mechanism for AM mode-sorting by nanoring slits that have different sizes and lateral shifts. (D) NAMMC integrated by an array of 8 NRA units by 8 NRA units. (E) Concept of on-chip processing of AM-multiplexed images over a broadband by the NAMMC. λ, wavelength.

In terms of the operation mechanism, we considered a nanoring slit enclosed by a concentric nanogroove (ls = 0) in a gold film. Throughout this paper, the width of the nanoring slit is fixed as 50 nm. We carried out a full vectorial approach for the analysis of the AM mode in the nanoring slit (23). With respect to the cut-off AM mode (fig. S3) of the nanoring slit, the calculated effective indices of the eigen-AM modes of L = ±1 and ±3 indicate that the lower AM mode of L = ±1 can be supported by both slits, with inner radii of 75 nm (Rin1) and 200 nm (Rin2), but the higher AM mode of L = ±3 can only be maintained by the slit with Rin2 (Fig. 2A). Moreover, the effective index differences almost remain flat in visible wavelengths, which indicates the nonresonant nature of AM modes supported by nanoring slits and lays the foundation for multiplexing broadband light.

Fig. 2 Distinctive AM mode-sorting selectivity by a nanoring slit of varying size.

(A) Theoretically calculated effective index differences (Δneff) (red curves) and MFs (black curves) for the plasmonic modes with total AM of L = ±1 (solid lines) and L = ±3 (dashed lines) for a nanoring slit with Rin1 = 75 nm (top) and Rin2 = 200 nm (bottom), respectively. (B and C) Scanning electron microscopy (SEM) images of the fabricated NRAs consisting of concentric nanogrooves and nanoring slits with inner radii of Rin1 (see fig. S1B for 45° view) and Rin2 (see fig. S1C for 45° view), respectively. The insets show enlarged views of the nanoring slits (scale bars, 100 nm). (D) Numerically calculated (curves) and experimentally confirmed (triangles) AM mode-sorting selectivity spectra of the AM beams of l0 = –2, s = +1 (L = –1) and l0 = +2, s = +1 (L = +3) for nanoring slits with inner radii of Rin1 (top) and Rin2 (bottom), respectively. The red color indicates the bandwidths (defined as the selectivity ≥ 0.1) of AM mode-sorting selectivity by nanoring slits.

The outcoupling (transmittance) efficiency of nanoring slits can be determined by the mode matching between the eigen-AM mode supported by nanoring slits (fig. S4, A to C) and the plasmonic AM mode excited from nanogrooves (fig. S4, D to F). A mode-matching factor (MF) can be defined (23) so that we intuitively understand the distinctive AM mode-sorting selectivity by the nanoring slits. The MF can be selectively maximized from its dependence on the illumination wavelength and the slit radius (fig. S4, G to I). As an example, the black curves in Fig. 2A reveal that plasmonic modes with total AM of L = ±1 and ±3 can be distinctively coupled out through nanoring slits with Rin1 and Rin2, respectively. In addition, theoretical analysis of the fundamental symmetries in nanophotonics (24, 25) provides physical insight into the NRA exhibiting the distinctive sensitivity on the total AM of SPPs, which yields additional flexibility in the subsequent chip design operating by different SAM and OAM combinations with the given total AM.

The distinctive AM mode-sorting selectivity, as defined in (23), can be experimentally verified for AM modes of L = ±1 and ±3 over a broad bandwidth of 150 nm in visible wavelengths (Fig. 2D). As an illustration, fig. S5 shows transmissive patterns of the AM beams at a wavelength of 640 nm. The physical principle of the distinctive AM mode-sorting selectivity can be extended to other wavelengths, such as telecommunication bands ranging from 1.45 to 1.65 μm (fig. S6).

The principle of the AM mode-sorting selectivity by nanoring slits of different sizes can be adopted for chip-scale multiplexing of AM-superposed beams if two nanoring slits with Rin1 and Rin2 are used concentrically. As shown in Fig. 3, A to D, two sections of the circular nanogrooves were spatially shifted in opposite directions, yielding ls = +2. Additionally, AM beams of AM1 and AM2 can excite plasmonic AM modes corresponding to L = –3 and –1, respectively, leading to the distinctive transmittance from the concentrically aligned nanoring slits. The capacity of the AM mode-sorting multiplexing can be increased by laterally shifting one of the circular nanogroove sections and the enclosed nanoring slit in opposite directions (Fig. 3, E to H). Using this nanogroove-shifting principle, AM beams with OAM modes ranging from l0 = –4 to +4 and SAM modes of s = –1 and +1 can be coupled out by the two spatially shifted nanoring slits that have different locations and sizes, with the smallest footprint of 4.2 μm by 4.2 μm (fig. S7).

Fig. 3 Experimental characterization of chip-scale AM multiplexing based on double concentric and spatially shifted nanoring slits enclosed by sections of spatially shifted nanogrooves.

(A) SEM image of the double nanoring slits (inset), with Rin1 and Rin2 enclosed by the two sections of shifted grooves with ls = +2. (B) Simulated total intensity distributions of the AM beams of AM1 (top) and AM2 (bottom) in the longitudinal planes of the nanoring slits. (C) Experimental far-field intensity distributions of the AM beams of AM1 and AM2 in the transverse planes. (D) Experimental cross-section plots of the far-field intensity distributions in (C), as labeled by the dashed white lines. A.U., arbitrary units. (E to H) Counterparts of (A) to (D), based on the spatially shifted nanoring slits with Rin1 and nanogrooves with ls = +2 (left) and ls = –2 (right). In (F) to (H), top images correspond to AM2 (top); bottom images correspond to AM3.

Based on the AM mode-sorting principle, we can achieve on-chip multiplexing of multiple AM modes (Fig. 4). We used two concentric double nanoring slits (Fig. 4A and fig. S1D) to selectively couple out the AM beams of AM1, AM2, AM3, and AM4, which can easily be evidenced by their distinctive transmissive patterns in the far-field region (fig. S8). As such, chip-scale AM multiplexing by dynamically switching on individual AM beams can be directly observed at different wavelengths (Fig. 4B and fig. S9), with a modal cross-talk as low as –17 dB (Fig. 4C).

Fig. 4 Four-state AM multiplexing through a NRA unit and parallel AM- and wavelength-division multiplexing through the large-scale NAMMC.

(A) SEM image of the single NRA (see fig. S1D for 45° view) in the NAMMC and the two concentric double nanoring slits (inset). (B) Experimental characterization of the four-state AM multiplexing by dynamically switching on the AM-superposed beams. The images are presented in pseudo-colors. (C) Measured modal cross-talk of the four AM modes at different wavelengths. (D) SEM image of the NAMMC. (E) Experimentally reconstructed AM- and wavelength-coded images retrieved from the four AM modes (AM1, AM2, AM3, and AM4) (Fig. 1A) at the three different wavelengths.

The broadband feature of the noninterference AM multiplexing by the chip-scale NRA can enable a multiplexing chip constructed by an array of NRAs (i.e., the NAMMC) to carry out both AM- and wavelength-division multiplexing in parallel. The NAMMC, which consists of an array of 8 NRA units by 8 NRA units, was fabricated (Fig. 4D) and illuminated by an array of 8 by 8 multibeams carrying well-defined SAM and OAM (23) (figs. S10 and S11). Consequently, Fig. 4E shows the experimentally reconstructed AM- and wavelength-coded images (100 pixels by 100 pixels), which were constructed one piece at a time through the dynamic area-by-area coding method (23). In addition, we show that the NAMMC is also capable of displaying the AM-coded image by simultaneously addressing the four AM information channels (fig. S12).

In bulky optics, OAM multiplexing is outperformed by conventional multiplexing techniques, in terms of multiplexing capacity. However, OAM multiplexing outperforms other techniques in nanoscale systems with a small space-bandwidth product (26). In general, the AM mode-sorting sensitivity of the NRAs can be extended to spiral nanogroove systems with different ls (figs. S13 and S14) and to multiple concentric nanoring slits. This generalization can be advantageous for further reduction of the NRA footprint while multiplexing optical beams with a greater number of AM modes. The noninterference operating principle in NRAs removes the requirement of bulky interference-based optics, and the associated nonresonance nature can largely increase the multiplexing capacity in conjunction with the wavelength-division multiplexing in a broad band. The large-scale NAMMC can be further integrated with chip-scale AM generators and could thereby offer compact on-chip AM applications in optical communications, information display, data storage, and data encryption.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/352/6287/805/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S14

References (2730)

REFERENCES AND NOTES

  1. Materials and methods and supplementary text are available as supplementary materials on Science Online.
Acknowledgments: We thank X. Li for help with the ion beam lithography, G. Gervinskas and F. Eftekhari from the Melbourne Centre for Nanofabrication for their fabrication efforts, H. Lu for technical assistance with the waveguide calculation, and J. Storteboom for assistance with using the Spectra-Physics Inspire laser system. This work was supported under the Australian Research Council Laureate Fellowship program (grant FL100100099). M.G. acknowledges support from the Australian Research Council Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS) (project CE110001018). X.L. acknowledges support from the Australian Research Council (grant DE150101665). All data related to the experiments described in this manuscript are archived on a lab computer at Swinburne University of Technology.
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