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Polarimetry enabled by nanophotonics

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Science  16 Nov 2018:
Vol. 362, Issue 6416, pp. 750-751
DOI: 10.1126/science.aau7494

Light beams consist of oscillatory electric (and magnetic) fields having a certain amplitude, phase, and frequency. In transverse waves, the state of polarization (SoP) characterizes how the electric field oscillates in the plane perpendicular to the propagation direction. Light-matter interactions strongly depend on the SoP, so its complete measurement is of paramount importance in a wide array of disciplines including chemistry, imaging, optical communications, and astronomy. However, measuring the SoP of a light beam, the main goal of polarimetry, is much trickier than knowing its intensity or frequency, because it involves the simultaneous measurement of the four Stokes parameters, which even account for the case of unpolarized light.

For decades, polarimeters have consisted of a combination of linear retarders, polarizers, and quarter-wave plates, which were able to obtain the four required measurements by spatial or temporal splitting of the incoming light beam (1). Such macroscopic polarimeters, widely used in many applications, are complex, bulky, and expensive; there have been few attempts to miniaturize them, with the notable exception of the fiber-grating polarimeter, highly useful in fiber optics (2). Recent advances in nanoscience and nanotechnology have unveiled new ways to shrink polarimeters, with all of the subsequent advantages that miniaturization and on-chip integration may bring (3).

Because the optical response of nanostructures depends on the polarization of the incident light beam, it should be possible to retrieve the SoP from outgoing signals. Possible polarization states are linearly polarized light along the horizontal (I0) or vertical (I90), and right-handed (σ+) and left-handed (σ) circularly polarized light. Then, a metasurface (an array of nanostructures) can be designed to scatter such polarization states into different directions (see the figure, left), which enables the retrieval of the Stokes parameters at a certain wavelength (48). The scattering paths could even be parallel to the metasurface and guided on the chip substrate to facilitate on-chip processing and detection (5).

However, the light-matter interaction enabling the polarization-dependent response in these implementations is distributed among all of the elements of the array, so the device footprint is much larger than one square wavelength. Local nanoscale measurement of the SoP requires polarization-dependent photonic responses in individual metallic or dielectric nanostructures. For instance, plasmonic nanostructures could be engineered in certain in-plane shapes that produce optical hot spots depending on the SoP of the illumination (see the figure, middle). An absorbing semiconductor placed in the hot-spot regions can generate output photocurrents proportional to the SoP (9). Although the device footprint is much smaller than in the case of metasurface polarimeters, this approach still requires at least four nanostructures to fully retrieve the SoP. Thus, this polarimeter would efficiently work for transverse light beams but would fail when measuring the SoP of complex, structured beams that have variations of the local polarization in the transverse plane (10).

The SoP at a single spatial point and in a single-incident light pulse can be determined by mixing both previous approaches—that is, by measuring the polarization-dependent scattering from individual nanostructures. As a result of spin-orbit interaction (11), the direction of the scattered light paths depends on the incident polarization. Moreover, the scattering paths can be defined with lithographically etched waveguides that support different guided modes that will carry all the information needed to retrieve the SoP (see the figure, right) (12). In principle, there is no limit for downscaling the size of the scatterer as long as it partly scatters the incoming beam. Remarkably, spin-orbit interactions would also enable a full-vector description of the incident light, beyond the transverse picture (13).

Nanoscale polarimeters

Transverse light, with an electric field E, wave vector k, and wavelength λ, illuminates a set of nanostructures. In all cases, measuring at least four outputs enables the retrieval of the state of polarization. Dielectric and metallic nanostructures are depicted in blue and yellow, respectively.

GRAPHIC: V. ALTOUNIAN/SCIENCE

By a suitable design (materials, shape, and size) of the underlying nanostructures, the previous approaches can be used to build ultracompact polarimeters across the entire electromagnetic spectrum, even into the terahertz regime. Such polarimeters would be extremely broadband; by calibrating the polarization scattering paths or absorption at each working wavelength, they could be used for spectropolarimetry (6, 12). Notably, they may also be implemented with resonant dielectric nanostructures (14), which would avoid the ohmic losses inherent in metals and also facilitate mass manufacturing in silicon chips. Although the absorptive approach is highly appropriate to build a device for SoP measurement that blocks beam propagation, the approaches based on scattering may operate in a nondestructive way with low insertion losses and are highly suitable for an in-line configuration.

On-chip polarimeters should easily displace their bulk free-space counterparts because of the inherent advantages of integration, such as low cost, reliability, repeatability, and integration with electronics (8). Several applications of on-chip polarimeters could immediately open up. Low-cost in-line polarimeters with low insertion losses operating at telecommunication wavelengths could monitor SoP in real time in present and future optical communication networks that use polarization multiplexing schemes. Spin-orbit polarimeters with near-atomic dimensions may locally test the polarization of single photons in quantum systems and networks. Now that the fundamentals for on-chip nanoscale polarimeters have been settled, it is time to make them a practical reality.

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