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Physics Seminar

Plasmonics around the clock: new chiral optical effects and applications in quantum optics

Speaker: Prof Ventsislav K Valev (University of Bath)
Date: Wednesday 16 October 2019
Time: 15:00
Venue: N/3.28

This seminar will focus on three absorbing lines of research into nanophotonics, representing three consecutive time scales of the physics involved. It will begin with the first experimental observation of a chiroptical effect that was predicted 40 years ago [1,2]. Next, it will consider the smallest backjets (‘nanojets’) ever created and will discuss how they can serve to assemble novel metamaterials [3,4]. Finally, drawing inspiration from steampunk science fiction, it will illustrate how a vapour stabilization technique can greatly enhance quantum sensors [5]. When light shines on metal nanoparticles (NPs), it is initially (fs timescale) absorbed by the electrons. These electrons give rise to “instantaneous” nonlinear optical processes, such as second harmonic (SH) generation, whereby two photons at the fundamental frequency are converted into a single photon at twice that frequency \Omega. This SH generation is promising for applications, based on frequency conversion (for laser manufacturing), on nonlinear optical characterization[6,7] (e.g. microscopy) and on metasurfaces (for ultrathin telecom components). Our team recently demonstrated that in chiral metal NPs (those that lack mirror symmetry) the intensity of light, scattered at the SH frequency, is proportional to the chirality, see Fig. 1 [in the poster]. This effect was predicted 40 years ago, it is >10,000 more sensitive than corresponding linear optical effects and it could enable safer pharmaceuticals.

At the ps timescale following illumination, the energy of electrons transfers to the NP lattice. The transfer gives rise to lattice vibrations (phonons) that are promising for applications in photoacoustic imaging and in the three branches of nano metalworking: forming, cutting and joining. We will consider examples of all three, focusing on the formation of nanojets in the electromagnetic hotspots of gold NPs and on joining (welding) gold NPs together in continuous strings. Nano metalworking is an exciting, emerging field that is largely unexplored.

At the ns timescale following illumination, the energy transfers from the NP’s lattice to its environment, in the form of heat. Such heating processes find applications in nanorobotics, in cancer treatment, in heat-assisted magnetic recording, in steam/vapor generation and now in quantum sensors. Indeed, numerous quantum technologies are enabled by alkali metal vapors: atomic clocks, ultra-fine frequency lasers, atomic traps, optically pumped magnetometers, etc. The metal atoms are kept in a vacuum, within containers made of metal or glass. Unfortunately, upon colliding with the container walls, the atoms lose their quantum states or condense. Current methods to address these problems are slow, costly and impractical to scale up. We developed a gold NP-based coating, whereby the NPs serve as tiny heating elements on the interior of the container walls, which prevents the atoms from sticking to them. Our technology offers 1,000 times faser control of the atomic vapor pressure and could enable wearable helmets for human magnetoencephalography.

1. Phys Rev. X 9, 011024 (2019). 2. J. Chem. Phys. 70, 1027 (1979). 3. Adv. Mater. 24, OP29-OP35 (2012). 4. Nat. Commun. 5, 4568 (2014). 5. Nat. Commun. 10, 2328 (2019). 6. ACS Nano 13, 3896–3909 (2019). 7. Nano Lett. 19, 165-172 (2019).