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Active and Passive Photonics with Coupled Optomechanical Resonators

Final Report Summary - APPCOPTOR (Active and Passive Photonics with Coupled Optomechanical Resonators)

Photonics, the science and technology of light, is one of the most rapidly growing research fields in the last decades. Nowadays, photonics is broadly established as a key technology in extensive areas from lighting, data transmission, energy generation and manufacturing processes to health and care science, among others. Recent studies show that photonic science could revolutionise not only the current devices, but also create high-impact novel applications such as quantum cryptography and quantum computation.

One of the key components of the light-wave circuits is a resonator, a cavity where the light is trapped for a certain period of time enhancing the interactions with the matter. The resonators can be made of active or passive materials depending on how the optical properties interact with the light: changing (active materials) or remaining intact (passive materials). Moreover, light interaction with mater could generate photonic forces. This phenomenon constitutes a very promising research field denominated optomechanics, which is rapidly developing nowadays. Controlling the mechanical properties with light has allowed the demonstration of unprecedented phenomena such as the possibility of ground state cooling employing light (back-action cooling).

The Marie Curie Intra European Fellowship (IEF) project APPCOPTOR aims in the investigation of the fundamental properties of silicon based optical resonators. It covers several areas of interest: from the investigation of the vertical coupling among a bus waveguide, single resonator and coupled resonators, to cavity optomechanics employing passive and active materials. This report summarises the activities performed.

Research activities - Passive materials

Oscillatory coupling between a vertically coupled bus waveguide and resonator
It is well known that exists a unique distance between a coplanar resonator and its bus waveguide (gap) where the power injected into the cavity is maximised and thus the light transmitted thought the bus waveguide is minimised (condition denominated critical coupling). This behaviour corresponds to a point-coupling situation, where the interaction between the waveguide / resonator takes place at the closest point between the waveguide and the resonator. However, we demonstrate theoretically and experimentally that in the case of a vertical coupling scheme, where the resonator is placed above the waveguide, more than one condition of relative maximum power injected into the cavity as a function as the gap could be reached. The reason for this effect is the fact that the coupling takes place in a relatively wide zone of the resonator / waveguide where the gap is almost flat (flat zone). These findings are of wide interest to optical circuits and their applications to fundamental science and technological applications. For instance, the use of vertical coupling geometry in second-harmonic generation experiments will allow to freely tune the Q factor of the second-harmonic mode at 2? while keeping the pump at the fundamental frequency ? in critical coupling. Also, a cavity optomechanics experiment will benefit from the possibility to maintain high values of the cavity Q factor in spite of the coupling gap variations due to the mechanical vibrations of the resonator. These results have been published in Physical Review Letters 110, 163901 (2013).

Monolithic integration of high-Q wedge resonators vertically coupled to buried waveguides
Ultra-high quality-factor (UHQ) microresonators lack the integration with bus waveguides and therefore their impressive applications are limited to research labs. In fact, typical UHQ resonators, such as microspheres and microtoroids lack the possibility of integration into lightwave circuits due to their planarity constrains. Another example of UHQ resonators is the planar wedge resonator which, however, lack the possibility to side couple to coplanar waveguides. We have proposed and demonstrated experimentally the complete integration of wedge resonators with vertically coupled dielectric bus waveguides. In this approach, the resonator and the waveguide lay in different planes which permits to realise the optical components in independent and isolated technological steps. As a result, the waveguide remains intact when the wedge geometry of the cavity is formed. The results of this work open the door to an industrial mass-fabrication of UHQ resonators. Owing to the fact that the vertical coupling gap is accurately controlled through a deposition procedure, the lithographic techniques employed are conventional and inexpensive. The vertical coupling scheme permits to select different materials for the resonator and the waveguide. These features are valuable for a number of applications such as cavity optomechanics, non-linear optics, label-free sensing and integrated photonics. These results have been published in Optics Express, 20, 22934 (2012).

Cavity optomechanics in monolithic integrated resonators
In the vertical coupling scheme, the gap material can be selectively removed in order to form freestanding cavities. We have created ring resonators suspended into the air by means of spokes attached to a pedestal. We observed oscillations in the transmittance of the bus waveguide that represent fingerprints of the mechanical oscillations. To be published.

Research activities - Active materials

Silicon nanocrystals as a gain material
We have studied the zero phonon (direct) optical transition as a possible source of optical amplification in Si-nc. To this scope, investigation of the dynamics of the optical cavities in the form of optically active free-standing microdisks on a nanosecond time-scale and under high excitation conditions have been employed. Here, Dr Nikola Prtljaga has made the major contribution / essential work.

Optical bistability
Optical bistability (OB) has attracted large interest in photonics community since it enables the full optical implementation of switches, logical gates and memories. Additionally, this phenomenon permits to characterize several optical properties such as the different absorption channels of a resonator. We demonstrated the feasibility of the vertical coupling scheme for non-linear process such as OB employing Si-nc. In fact, this coupling strategy is very valuable for the non-linear process because the non-linear effects are generated exclusively in the resonator, the unique component fabricated with the active material. We confirmed the low thermo-optical coefficient of the nanocrystalline material and the negligible contributions from two-photon and excited-carrier losses that allow to avoid spoiling of the mode Q-factor up to input powers as high as 100 mW. By using numerical calculations, we confirm that the heat dissipation dynamics of our nanocrystalline material is reflecting mostly the properties of the silica host. These results, combined with previous reports on large Kerr nonlinearities of Si-ncs, indicate that efficient non-linear optical devices might be realized by proper engineering of Si-nc-based ultra-high-Q resonator devices. To be published.

All of these work and additional results to come from this project represent a small but a valuable step forward towards a possible next technological jump were the photonic technology is postulating as a clear candidate to overcome the limits of the electronics.