Editorial Feature

Recent Advances in Resonator Technologies

Resonators are devices used to amplify wavelengths,they exist in many devices and come in many forms. Resonators commonly exist in science as mechanical, acoustic (sound), optical (light), microwave and cavity resonators. As ‘resonator’ is a general term for a device which amplifies a given wavelength, resonators exist in many sub-disciplines of science.

The applications for resonators are vast. Within the many sub-disciplines, resonators are involved in particle accelerators, photonic devices, transmission lines, cars, NMR instruments, musical instruments, LASER’s, electronics, as well as many other technologies. Whilst their applications are mainly technological and telecommunication based, resonator research is dominantly pursued by Physicists.

Theory of Resonators

Resonators work using the theory of resonance. Resonance is when a stimulus, such as vibrations or external force, causes another system to oscillate with a higher amplitude at a specific frequency. Resonance can only occur inside a system that is adapted to store and transfer energy between two different storage modes.

Dependent upon the type of wavelength being amplified, the amplification mechanism inside the resonator varies. Resonators amplify wavelengths through cavities, commonly composed of mirrors or hollow spaces. Resonators can also be used in conjunction with each other in a device to form waveguides.

Cavities that utilize mirrors are used in optical resonators. Whilst this type of cavity is only used in one type of resonator, optical resonators are one of the most widely used. Optical cavities are composed of a series of mirrors. The light entering the cavity is reflected off the mirrors multiple times, producing a standing wave at a certain frequency. The orientation of the mirrors can be flat, spherical, concentric, confocal, hemispherical, concave-convex or plane-parallel (Fabry–Pérot).

Cavities with hollow spaces are used in many types of resonator, including acoustic, mechanical, electromechanical and cavity resonators. The hollow space inside the cavity enables the wavelengths to bounce off the cavity walls and amplify. The distance between the walls are tuned for each specific device and the distance between the walls determines which specific wavelength is amplified.

Recent Developments in Resonators

The interdisciplinary nature of resonators has allowed for new publications involving resonator research to be published frequently. Within the last week, Nature Communications and Scientific Reports (both open access Nature journals) alone have published three papers on varying aspects of resonator research.

  • Generating Biphotons in A Microring Resonator

A team of Researchers from the USA and Italy have created a theoretical model for the entanglement of bi-photon quantum light sources using spontaneous four wave mixing (SFWM) in an optically transparent AlGaN microring resonator. The photon pairs, known as idler photons, were broadly spaced in the ultraviolet-visible (UV-Vis) spectrum and are beneficial for facilitating interactions with ion-trap qubits.

The Researchers used mathematical models to predict the coupling factors for the pump, signal and idler waves using a III-Nitride blue wavelength laser to create potential values for the biphoton flux and cross-correlation in the device. The calculations have predicted that such a device could be capable of cross correlation values of 2.37×104 at a biphoton flux of 8.748×104 (resonator radius of 200 µm).

The research also detailed that higher order modes in the resonator can overcome the normal photon dispersion phases, allowing for bi-photons to be experimentally generated. If fabricated, future research could help to yield scalable and efficient quantum information technology systems.

  • Magnon Coupling in Coplanar Waveguide Resonator (CPWR)

Researchers from the University of Oxford, UK, have used a combination of theoretical measurements and experiments to study the coupling of magnons in a Yttrium-iron garnet (YIG) sphere. The Researchers used a half-wavelength superconducting coplanar waveguide resonator (CPWR) to study the various waveguide modes and frequency characteristics of magnons.

The Researchers performed measurements at millikelvin temperatures. The non-uniform microwave field in the CPWR allowed many different magnon modes to be studied. By shifting the magnetic bias within the device, the Researchers were able to shift the magnon modes in a controlled manner and study them.

The approach enabled the Researchers to identify the individual magnon modes and their coupling strength to other magnon modes. Additionally, the resonator could reduce the number of photons to less than one; the so-called quantum limit.

The research showcases the potential for the realization of new hybrid quantum devices. The ability of integrating superconducting qubits into high magnetic fields could lead Researchers to create magnon-integrated quantum information devices.

  • Coupling of Mechanical Resonators

Cross-continent Researchers from the Netherlands and USA have experimentally investigated the real-time dynamics of two unusually-coupled resonators. Coupling can occur with resonators of a similar frequency, but this research investigates two resonators that are separated both spatially and in frequency.

The Researchers found that by employing two laser beams, the resonators can optomechanically couple and produce a coherent transfer state. The Researchers did find that transferring mechanical energy between resonators incurred losses, however, the process still allowed for the two resonators to be efficiently coupled.

The Researchers have stipulated that the work could be extended to both adiabatic transfer states and quantum regimes to investigate how entanglement will affect mechanical oscillators. It is also possible that the work could help to generate new quantum information systems with large frequency separations.

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“Broadband biphoton generation and statistics of quantum light in the UV-visible range in an AlGaN microring resonator”- Leonardis F. D. et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-11617-y

“Strong coupling of magnons in a YIG sphere to photons in a planar superconducting resonator in the quantum limit”- Morris R. G. E., Scientific Reports, 2017, DOI:10.1038/s41598-017-11835-4

“Coherent optomechanical state transfer between disparate mechanical resonators”- Weaver M. J., et al, Nature Communications, 2017, DOI: 10.1038/s41467-017-00968-9

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