Tunable Modal Birefringence in a Low‐Loss Van Der Waals Waveguide

Abstract

path difference between orthogonally polarized guided modes via the modal birefringence of the waveguides. [8] Unlike the bulk birefringence which is predefined by the material, the modal birefringence of waveguides are tunable with constituent materials and waveguide geometry. [9] However in practice the manufacture of integrated phase retarders with designed modal birefringence is particularly challenging because of their stringent fabrication tolerance. [10] Therefore, new materials allowing layer-by-layer control and fine mechanical processing are highly demanded.Due to the difference between their interlayer and intralayer bonding strengths, van der Waals (vdW) crystals are both mechanically and optically anisotropic. [11,12] In light of this intrinsic outof-plane bi-anisotropy, vdW crystals are promising materials for the phase retardation applications in integrated optical circuits: the mechanical anisotropy permits the precise layer-by-layer manufacturing of the waveguide structure while the optical anisotropy provides one more degree of freedom to control its modal birefringence. vdW crystals with hyperbolic optical responses such as hexagonal boron nitride (h-BN) [13,14] and α-phase molybdenum trioxide (α-MoO 3 ) [15,16] have been demonstrated to support highly confined phonon polaritons in the mid-infrared (MIR) frequency range. However, these polaritonic modes are inherently inadequate for the integrated phase retardation applications (polariton-assisted polarization control of far-field light with metasurfaces made of vdW materials is possible [17] ), as a result of their inevitable transmission loss (imposed by the Kramers-Kronig relationships between dispersion and dissipation [18,19] ) and the absence of transverseelectric (TE) polarized modes. [20,21] An alternative way is to resort to vdW crystals with elliptic anisotropy at frequencies far away from any resonant absorption bands. In such case, the imaginary parts of the permittivity tensor are usually negligible and result in positive real parts. [22] The near-zero imaginary parts of permittivity guarantee the low-loss transmission of the supported waveguide modes, while the positive real parts allow the co-occurrence of TE and transverse-magnetic (TM) polarized modes. Transition metal dichalcogenides (TMDs) are expected to exhibit elliptic light dispersion in a broad frequency range (note that h-BN is also elliptically anisotropic out of its Reststrahlen bands, the reason TMDs are better for our proposed applications will be discussed at a later stage); [23] van der Waals (vdW) crystals are promising candidates for integrated phase retardation applications due to their large optical birefringence. Among the two major types of vdW materials, the hyperbolic vdW crystals are inherently inadequate for optical retardation applications since the supported polaritonic modes are exclusively transverse-magnetic (TM) polarized and relatively lossy. Elliptic vdW crystals, on the other hand, represent a superior choice. For example, molybdenum disulfide (...

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

“…A diagonal permittivity tensor can describe it with two optical constants corresponding to the crystallographic ab-plane and the c-axis. 16 Interestingly, these anisotropic properties of TMDCs were qualitatively demonstrated back in 1967 by Liang et al, 17 but only currently attracted significant importance in experiments dealing with novel regimes of light-matter interactions 18,19 comprising excitonpolariton transport, 20 Zenneck surface waves, 21 tunable modal birefringence, 22 and anapoleexciton polaritons. 23 Although a recent pioneering work by Hu and co-workers 16 reported a birefringence value of Δn = 1.4 for MoS2 at λ = 1530 nm, the values of asymmetric dielectric responses of MoS2 in a wide wavelength interval have so far remained unknown.…”

Femtosecond exciton dynamics in WSe2 optical waveguides

“…n the steady-state, it is customary to characterize the behavior of optical waveguide modes with the in-plane component of the complex wavevector, q r = q 1,r + iq 2,r [1][2][3][4][5] ; the subscript r stands for "reference". The real component of the wavevector, q 1,r , describes the phase velocity of the waveguide mode; the imaginary component of the wavevector, q 2,r , characterizes attenuation.…”

Self-assembly of liquid crystals in nanoporous solids for adaptive photonic metamaterials

“…Effective optical properties, not achievable by base materials, are possible [2-8], specifically adjustable birefringence. This is of particular relevance for important polarisation management components, such as waveplates and compensators, that play an essential part in modern optical communication systems [9].A versatile route to fine-tune optics of porous materials is filling the pore space with a liquid crystal (LC) [10,11]. In the bulk case LCs play a dominant role for the manipulation of optical anisotropy [12].…”