Submitted by Pooja Pandey on Tue, 03/10/2023 - 09:49
When light and matter intersect, a realm of quasiparticles known as polaritons emerges. These polaritons hold the power to reshape a material's molecular vibrations and electronic properties, aspects that are crucial in determining a material's behaviour and functionality. Yet, the intricate mechanisms governing these transformations have long remained elusive. Now, a discovery by researchers based at the University of Cambridge in collaboration with University of Exeter promises to illuminate this enigmatic interaction.
The collaborative effort between the University of Cambridge and University of Exeter sets the stage for future research. Our next steps are to use these optical gratings to control the optoelectronic properties of materials placed on top of them, such as their light emission and carrier transport properties. Jeremy Baumberg
The interaction between light and matter, and the ability to push this interaction to its limits within optical cavities, has been instrumental in shaping modern physics and technology, from lasers to quantum computers. While researchers have extensively probed optical cavities in the microwave to optical range, the mid-infrared spectrum, with wavelengths spanning 5 to 20 microns, offers a promising arena due to its unique interplay with molecular vibrations, which exert control over optoelectronic properties. When the strength of the coupling between light and matter surpasses losses within the optical cavity, a phenomenon known as "strong coupling" materialises. In this regime, light and matter intertwine, giving rise to new quasiparticles known as polaritons.
Polaritons may potentially modify the molecular vibrations and electronic attributes of materials that dictates how a material behaves and functions. This has real-world impact in the design of better LEDs and lasers. However, investigating the nuances of this nanoscale coupling has proven to be challenging.
To overcome this challenge, researchers at Cavendish Laboratory designed an open metallic grating—a well-established platform with historical roots dating back to Lord Rayleigh's time. This grating enables the probing of mid-infrared polaritons with sub-diffraction limited resolution, achieved by measuring the Raman signatures of polaritons in the visible-wavelength range. The findings published in Physical Review Letters, elucidates how leveraging the power of resonant optical cavities, the researchers achieved a level of entwinement between light and molecular vibrations such that they became virtually indistinguishable, entering the realm of ‘strong coupling.’
“Traditionally, the detection of strong coupling relied on far-field optical techniques, but these methods overlooked local spatial variations. Hence, we utilised Raman microscopy to measure the light-molecule coupling strength at the sub-micron scale, revealing concealed signatures that escalates to an entirely new level, the ‘ultra-strong coupling’ regime,” says Rakesh Arul, PhD student, Cavendish Laboratory and first author of this study. “This ‘ultra-strong coupling’, exceeding far-field measurements by 350%, aligns with theoretical upper limits predicted for this molecular material. It promises to unlock new phenomena in the mid-infrared realm, such as photon blockade and superradiance.”
This discovery not only deepens our comprehension of how polaritons induce transformative changes in materials but also extends our exploration of these quasiparticles, impacting fields like chemistry and quantum optics. “The collaborative effort between the University of Cambridge and University of Exeter sets the stage for future research. Our next steps are to use these optical gratings to control the optoelectronic properties of materials placed on top of them, such as their light emission and carrier transport properties,” said Prof. Jeremy Baumberg, Professor of Nanophotonics at the Cavendish Laboratory.
Image:
Grating in action, producing a diffraction pattern with a red laser Credit: Rakesh Arul
Reference:
Rakesh Arul, Kishan Menghrajani, Marie S. Rider, Rohit Chikkaraddy, William L. Barnes, and Jeremy J. Baumberg,'Raman Probing the Local Ultrastrong Coupling of Vibrational Plasmon Polaritons on Metallic Gratings', Phys. Rev. Lett. 131,126902. DOI: 10.1103/PhysRevLett.131.126902
Featured in Physics (APS): https://physics.aps.org/articles/v16/s130