Photonic molecules with a tunable inter-cavity gap

Abstract

Optical micro-resonators have broad applications. They are used, for example, to enhance light–matter interactions in optical sensors or as model systems for investigating fundamental physical mechanisms in cavity quantum electrodynamics. Coupling two or more micro-cavities is particularly interesting as it enlarges the design freedom and the field of application. In this context, achieving tunability of the coupling strength and hence the inter-cavity gap is of utmost importance for adjusting the properties of the coupled micro-resonator system. In this paper, we report on a novel coupling approach that allows highly precise tuning of the coupling gap of polymeric micro-resonators that are fabricated side by side on a common substrate. We structure goblet-shaped whispering-gallery-mode resonators on an elastic silicone-based polymer substrate by direct laser writing. The silicone substrate is mechanically stretched in order to exploit the lateral shrinkage to reduce the coupling gap. Incorporating a laser dye into the micro-resonators transforms the cavities into micro-lasers that can be pumped optically. We have investigated the lasing emission by micro-photoluminescence spectroscopy, focusing on the spatial localization of the modes. Our results demonstrate the formation of photonic molecules consisting of two or even three resonators, for which the coupling strengths and hence the lasing performance can be precisely tuned. Flexibility and tunability are key elements in future photonics, making our approach interesting for various photonic applications. For instance, as our coupling approach can also be extended to larger cavity arrays, it might serve as a platform for tunable coupled-resonator optical waveguide devices.

Strong Coupling in a Photonic Molecule Formed by Trapping a Microsphere in a Microtube Cavity

“…[3] In our work, instead of moving a tapered fiber on a spherical surface, we enabled coupling strength tuning by changing pump position along a line on the top of microtube, which is compatible for both farfield and nearfield means. Moreover, benchmarking with other studies on tuning the intercavity coupling strength of photonic molecules, our spa tially selective pumping scheme avoids the requirement of external techniques (e.g., an elastic substrate for strain engi neering [21,22] or an interdigital transducer for generating surface acoustic waves [23] ), and thus offer a simple and flexible way for regulating the mode coupling behaviors.…”

“…As such, one needs a deliberate control on both the cavity geometries and the intercavity coupling gap to ensure a good spectral match and efficient evanescent coupling between the coupled cavities. [18] Moreover, dynamic tuning of the intercavity coupling strength has been investigated in recent years, which were carried out by advanced and sophisticated techniques such as strain tuning, [21,22] acoustooptic control, [23] and precise micromanipulation techniques. [3] To extend and promote the research in the field of photonic molecules, it is of high interest to design novel photonic molecules extending from adjacent solid microcavities to thinwalled hollow cavities which possess intense evanescent field facilitating intercavity coupling and provide novel strategy for tuning of the coupling strength.…”

Split-disk micro-lasers: Tunable whispering gallery mode cavities

“…The preparation of PDMS substrates, the necessary pre-treatment to enhance its adhesion properties, the DLW structuring, and development process are identical to the process described by Siegle et al in an earlier publication. 39 Polymers can be easily doped with organic dyes to from active resonators, e.g., ring resonator lasers 42 or disk and goblet lasers. 12 Following these approaches to form active micro-resonators that enable lasing emission through optical pumping (see below), both types of photo-resists were doped with the laser dye Pyrromethene 597 (Radiant Dyes Laser & Accessories GmbH, Wermelskirchen, Germany) at a concentration of 25 µmol prior to the structuring process.…”

“…For this purpose we use an approach based on mechanical deformation of the elastomer substrate that has been used earlier for the realization of photonic molecules with tunable inter-cavity coupling gaps. 39 Therefore, the PDMS substrate is fixed between clamps and mechanically elongated using a micrometer screw. If the gap of a split-disk is structured orienting along the stretching direction, the gap distance can be reduced by exploiting the lateral contraction as illustrated in Fig.…”

“…21,38 For such tuning in larger cavity arrays, we have established an alternative platform by structuring the cavity arrays onto elastomeric substrates. 39 Elaborating this approach, we present here a novel type of a WGM micro-resonator with a wide tuning range and with the prospect of easy integration into cavity arrays: Polymeric splitdisk resonators. They consist of opposing half-disks with an intermediate air gap, structured with either electron beam lithography or 3D lithography (direct laser writing, DLW).…”