Whispering-gallery-mode (WGM) microresonators with high quality (Q) factors and small mode volumes (V) underpin a breath of important applications in optical sensing, nonlinear optics, optomechanics, and quantum optics. It has been the focus and frontier of recent researches in the field of optics physics. Conventionally, WGMs are dominantly confined inside high-refractive-index dielectric microcavities via total internal reflection (TIR), with a small portion of mode energy extended into the ambient environment (i.e., evanescent field) for both light coupling and optical sensing. Thus, the light-matter interaction is limited inside the cavities. In contrast, exterior WGMs have been long sought after for optical sensing applications due to their large mode overlap with the surroundings and thus more sensitive to perturbation. Typically, they can be excited by utilizing plasmonic resonant cavities and slot-waveguide structures. The plasmonic microcavity can strongly confine the electromagnetic field by taking advantage of surface plasmon resonance (SPR). However, the large intrinsic metallic loss greatly degrades their Q factors. Hybrid schemes have therefore been proposed, e.g., by attaching metallic nanoparticles (NPs) to dielectric microcavities. But the long-standing problem of metallic loss in SPR is, in principle, unavoidable. Another way to achieve exterior WGMs is based on slot-waveguide structures, in which the excited modes are confined in between a nanogap (slot) of two closely placed dielectric media, e.g., planar slot-waveguide ring resonators. Light confinement and enhancement is caused by large discontinuity of the electric field at high-index-contrast interfaces, rather than TIR or SPR. However, the subwavelength vertical slot structure requires high-precision fabrication techniques. Slot-waveguide modes are known to be vulnerable to roughness induced extrinsic scattering, which imposes their main loss. The fabrication of a vertical nano-slot with high aspect ratios and ultrasmooth sidewalls is still a challenge and at high cost. To overcome these drawbacks, horizontal slot-waveguide schemes have been utilized for better gap control. But, the mature nanofabrication technique for both geometries is limited to CMOS compatible materials, most of which lack second-order nonlinearity.

Figure | Left: Schematic of the double-layer LNOI microdisk and its SEM image. Right: Transverse intensity profiles of interior and exterior WGMS.

We, together with our collaborator Prof. Ya Cheng from East China Normal University, demonstrate that a double-layer crystalline lithium niobate thin film (LNTN) microdisk supports exterior slot-waveguide WGMs with high-Q factors. The geometry consists of two vertically stacked LNTF microdisks separated by a nanoscale gap (~138 nm), and forms a horizontal slot-waveguide microdisk resonator. The microcavity features high-Q slot-waveguide WGMs at the telecommunication band, in excess of 100,000. The surface roughness of the LNTF by chemomechanical polishing is better than 1 nm, as confirmed by atomic force microscopy reconstruction. The characteristics of these slot-waveguide modes over their counterparts are is that they experience dual enhancement from cavity and slot structures. Our configuration on the double-layered lithium niobate on insulator (LNOI) platform would also be beneficial for novel effects with the material’s exceptional properties, e.g., optical nonlinearity, electro-optics, and piezoelectricity.

Crystalline LN lacking of inversion symmetry has a wide transparent window, strong second-order nonlinearity, electro-optic effects and piezoelectricity, as well as a higher Young’s module, which would be further exploited for other photonic applications like EOM, frequency conversion and electrical field sensing. Beyond their promising application in optical sensing, cavity optomechanics can also benefit from our scheme. For instance, double-layer microdisks, whose WGMs exhibit strong optical gradient force, have already been exploited for optomechanics. The newly developed double-layer crystalline microcavity would still open the door for a wealth of applications in optical sensing and cavity optomechanics. This also holds potential for on-chip nonlinear optics in LNOI photonics by using stacked microcavities. Besides, with the building block of optical microcavity for photonic chips, the proposed LNOI microresonators also pave the way for cavity quantum electrodynamics, quantum optics and quantum information technologies. This research will definitely attract intensive interest in the near future.

Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.253902