Nonreciprocal Magnetless Devices using 2D Materials

Lorentz reciprocity ensures that the response of a linear device is unchanged when excitation and observation points are swapped. This is a fundamental property that has limited the way in which electromagnetic signals are processed and transmitted. Non-reciprocal systems, which are not bounded by this symmetry, have become of critical importance throughout the entire frequency spectrum to realize devices like circulators and isolators, enabling radar operation, full-duplex communications, and to protect sensitive laser sources from reflections. Non-reciprocity has traditionally been achieved through magneto-optical effects, requiring lossy and bulky magnetic materials under strong biasing fields that are incompatible with modern technology trends, in constant pursuit of miniaturized, integrated, and affordable devices. Motivated by these shortcomings, recent years have seen a rapidly growing interest on alternative ways to break reciprocity, mainly through nonlinear materials and spatiotemporal modulations able to impart linear or angular momentum to the waves propagating within the system. Unfortunately, the use of these approaches in practical applications beyond radio-frequencies remains very challenging due to the difficulty of rapidly modulating the properties of materials. In a related context, the field of terahertz (THz) and infrared (IR) plasmonics and nanophotonics has recently been revolutionized by the development of graphene and other two–dimensional (2D) materials such as gapped Dirac transition metal dichalcogenides (TMDs) and black phosphorus (BP) that exhibit exceptional and reconfigurable electrical, thermal, and mechanical properties. Although strong magneto–optical effects in such materials permit to break reciprocity with magnetic bias, the development of novel, practical, and inexpensive magnetic–free routes to break and manipulate reciprocity at the micro/nanoscale is crucial for the next generation of silicon–compatible THz and IR technology with unprecedented performance and functionalities.

Inspired by these relevant and longstanding needs, this research line explores innovative and efficient approaches to break and engineer time–reversal symmetry in 2D plasmonic materials and to subsequently apply them to develop magnetless, integrated, ultrathin, and low-loss nonreciprocal devices at THz and IR frequencies. 

For instance, we have recently unveiled the unusual non-reciprocal and diffraction-less properties of surface plasmon polaritons propagating in drift-biased graphene-based metasurfaces. The effect of this drift current can be qualitatively understood as follows: since surface plasmons are collective charge oscillations coupled to light, they are strongly affected by these drifting charges and are either dragged or opposed by it, which causes guided waves to effectively see different media when propagating with or against the drift. Even though the drift velocities required to achieve strong non-reciprocal responses are difficult to obtain in most semiconductors and metals, graphene has recently opened new possibilities in this context thanks to its ultra-high electron mobility. We have shown that applying a drift-current on a graphene sheet leads to extremely asymmetric in-plane modal dispersions from terahertz to infrared frequencies, associated with plasmons with low-loss (high-loss and ultra-high confinement) traveling along (against) the bias. Strikingly, truly unidirectional wave propagation is prevented by the intrinsic nonlocal response of a graphene, a mechanism that shapes the energy flow over the surface. We have also shown that highly-directive hyperbolic plasmons completely immune to backscattering propagate obliquely along the drift in nanostructured graphene, as illustrated in Fig. 1a. Such response can be merged with spin-orbit interactions to efficiently launch collimated plasmons along a single direction while maintaining giant non-reciprocal responses. We believe that this platform opens a new paradigm to excite, collimate, steer, and process surface plasmons over a broad frequency band.  

We also proposed to gated graphene structures with time-varying biasing voltages to implement spatio-temporal modulations at THz and IR taking advantage of the ultrafast modulation that graphene can support (see Fig. 1, central panel). This approach has led to compact isolators and leaky-wave waves able to exhibit different transmission and reception features. The main drawback of this techniques is that it requires high-quality graphene able to support plasmons with low damping. This is critical not only for acceptable levels of insertion loss, but also for sufficiently strong non-reciprocal responses, as the spectral linewidth of the resonant states affects how effectively they exchange energy when not perfectly phase-matched. To reduce the burden on graphene quality, we have recently proposed to break reciprocity by using spatiotemporally modulated graphene as a perturbation of high-Q photonic modes in dielectric structures (Fig. 1, bottom panel). The resulting hybrid graphene-dielectric photonic devices are low-loss, silicon-compatible, frequency-scalable from THz to infrared and telecom wavelengths, robust against graphene imperfections, and exhibit large non-reciprocal responses using realistic biasing schemes. We emphasize the compatibility of this platform with well-known integrated photonic systems, as graphene is used here only to engineer the required non-reciprocal coupling between photonic states, having negligible effect on their impedance, wavenumber and field profile. Moreover, the unprecedented performance at IR frequencies makes this platform suitable to break emission and absorption symmetries in thermal management applications, such as thermophotovoltaic cells with increased efficiency.