Interaction and Symmetry Breaking of Counterpropagating Light

Project Information

CounterLIGHT

Grant agreement ID: 756966

DOI

10.3030/756966

Project closed

EC signature date 16 November 2017

Start date 1 March 2018

End date 29 February 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 500 000,00

EU contribution

€ 1 500 000,00

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 4 - CounterLIGHT (Interaction and Symmetry Breaking of Counterpropagating Light)

Reporting period: 2023-01-01 to 2024-02-29

Summary of the context and overall objectives of the project

Optical whispering gallery microresonators are tiny rings made of a transparent material that can be used to store and manipulate light. Light can circulate in these resonator rings for more than a million round-trips without being absorbed or scattered out of the ring. The size of these resonators is typically between 100 micron and 1 mm, which makes them ideal building blocks for integrated photonic circuits. At low optical power levels, these resonators can be used as optical filters for wavelength division multiplexing and for sensing applications by monitoring changes in the resonance frequency. However, a key benefit of these resonators is the extremely high power enhancement of the light circulating within the rings. If a photon makes on average a million round-trips within the resonator before being scattered out, this means that the optical power circulating in the resonator is a million times higher than the power launched into the resonator. Together with the small size of the resonator this leads to extremely high power densities in which different lightwaves can start to interact with each other. Many nonlinear optical effects like Raman scattering, second and third harmonic generation, and frequency soliton frequency comb generation can be observed at low threshold powers in microresonators.

In this project we study the interaction of counterpropagating light within microresonators. In first experiments we have shown that the interaction of the counterpropagating light leads to a spontaneous splitting of resonance frequencies for the clockwise and counterclockwise circulating direction of light. This means that light needs a different colors in clockwise and counterclockwise direction in order to "fit" into the resonator.

Such a resonance splitting of the resonator modes has many interesting applications that we investigate:
I. The splitting of the resonance frequencies is very sensitive to rotation of the resonators and can be used for miniaturized optical gyroscopes. This is in particular interesting for the development of small optical gyroscopes with low power consumption e.g. for drones and self-driving cars.
II. The ring resonator can be used as optical diode by attaching it to two waveguides to send light in and out of the ring. When sending light of a given color through the resonator in clockwise direction, the light cannot return to its origin without changing its color because of the resonance frequency splitting. The miniaturized optical diodes that we can develop using this principle are important building blocks for integrated photonic circuits.
III. By controlling the splitting between clockwise and counterclockwise light states in a microresonator we can develop chip-based logic gates. By using several ring resonators on a photonic chip, this could be used for simple optical computations and routing of light for data transfer in computers or in telecom networks.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The main objective of the ERC Starting Grant "CounterLight" was the investigation of interaction of counterpropagating light in integrated photonic circuits and optical ring resonators. Our diverse work on this topic has led to more than 30 scientific publications. The following is a list of the most important outcomes related to each of the proposed work packages.

WP1: In work package 1 we investigated optical gyroscopes that are enhanced by the nonlinear interaction of counterpropagating light in microresonators. In this work, we build an optical gyroscope based on a whispering gallery microresonator, demonstrate the physical principle for the first time and test its performance. This work has led to the publication “Nonlinear Enhanced Microresonator Gyroscope”, Optica 8, 1219-1226.

WP2: In this work package we investigate the use of nonlinear interaction of counterpropagating light to build an optical diode, which is a device that supports light propagation in one direction, while blocking backwards propagating light. This work has led to the publication “Microresonator Isolators and Circulators Based on the Intrinsic Nonreciprocity of the Kerr Effect”, Optica 5, Issue 3, pp. 279-282.

WP3: Here, we investigated the use of clockwise and counterclockwise circulating light states in ring resonators to store and process data in photonic circuits. We investigate the switching times between light states and in separate work we build an all-optical logic gate for simple optical calculations. This technique can be used to build future optical computers, e.g. for AI applications and for neuromorphic computing. This has led to the publication “Logic Gates Based on Interaction of Counterpropagating Light in Microresonators”, J. Lightwave Technol. 38, 1414-1419 as well as the publication “Optical memories and switching dynamics of counterpropagating light states in microresonators”, Opt. Express 29, 2193-2203.

WP4: We investigated the use of counterpropagating light for optical sensors and near-field sensing. In our experiments we employed a Tungsten tip in order to control the optical fields in counterpropagating modes of a microresonator. In particular, this can be used to suppress back reflection of light in optical circuits, which is important in optical networks, gyroscopes and other precise optical sensors. This work has been published as “Coherent suppression of backscattering in optical microresonators”, Nature, Light Science and Applications 9, 204.

WP5: Here, we investigated different types of optical frequency combs in optical microresonators. Frequency combs are optical signals that consist of many colors, that are equidistantly spaced in optical frequencies. Similar to a ruler with equidistant marks to measure distances, an optical frequency comb can be used to measure optical frequencies with very high precision. This has led to the following publications: “Spectral Extension and Synchronisation of Microcombs in a Single Microresonator”, Nature Comms. 11, 6384, “Dark-Bright Soliton Bound States in a Microresonator”, Phys. Rev. Lett. 128, 033901, and “Low-Temperature Sputtered Ultralow-Loss Silicon Nitride for Hybrid Photonic Integration”, Laser & Photonics Reviews, 2300642.

In addition to the work mentioned above, we have published a large number of additional theoretical and experimental work based on the nonlinear interaction of light.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In our experiments we have shown the first demonstration of nonlinear optical diode based on a whispering gallery microresonator. We have shown a suppression of the backwards transmitted light through a microresonator by more than 24 dB, which means that 99.6% of the backward propagating light is blocked. This amount of isolation is comparable to bulky conventional optical isolators based on the Faraday effect, which are difficult to integrate on a microchip because of the required magnetic fields. In addition we have shown first results on the optical gyroscopes based on counterpropagating light in microresonators that are in line with the theoretical predictions. In another experiment we have shown a proof-of-principle demonstration of optical logic gates based on switching between clockwise and counterclockwise light states in a microresonator. In related experiments we have shown a Kerr effect-facilitated all-optical polarization controller in a linear fiber cavity, as well as coherent suppression of backscattering of light in whispering gallery ring resonators.

Optical microring resonator made of silicon nitride on a silicon chip.