The detection of molecular vibrations in the mid-infrared(MIR) range (λ=3-30μm) at room temperature has applications in medical imaging, astronomical surveys, and quantum communication. Current technologies are limited by thermal noise and rely on energy-intensive cooled semiconductor detectors such as mercury cadmium telluride. In this talk, I will present our recent advancements to overcome this challenge using plasmonic nanogaps by upconverting low-energy MIR light into high-energy visible wavelengths that can be detected using silicon technologies capable of single photon detection^[1-3].

By strongly coupling surface plasmons between two metallic nanostructures spaced a few nanometers apart, light can be confined to extreme dimensions, allowing coherent coupling to electronic and vibrational states of the molecules assembled in the tiny gaps. The measured coupling strengths approach strong light-molecule coupling at room temperature^[4,5], enabling Purcell-enhanced light emission^[6] and optomechanics with bond vibrations^[7].

 

Recent advances in cavity medicated surface-enhanced Raman scattering (SERS) and light emission have opened new possibilities for single-bond vibrations in the MIR range. I will present three different methods for achieving this. The first method involves using nanoparticle-on-foil (NPoF) nanocavities that support both visible and MIR plasmonic hotspots, enabling the modulation of molecular SERS signals in the presence of MIR photons due to MIR absorption in the phonon resonance of the substrates^[2,8]. In the second approach, phonon absorption is suppressed, and a 140% amplification of the SERS anti-Stokes emission is observed when the MIR pump is tuned to a molecular vibrational frequency^[3,10]. The third approach involves assembling molecular emitters into a nanoscale cavity and continuously pumping them with optical energy, resulting in the upconversion of MIR light absorbed by the molecular vibrations into visible luminescence ^[1,9]. These demonstrations open unique possibilities not just for molecular spectroscopy and sensing but has wider implications in mode-selective chemistry and mid-infrared photonic devices. 

 

[1] Chikkaraddy, R, et al. Nature Photonics, 17(10), (2023) 865-871. 

[2] Chikkaraddy, R, et al. Light: Science & Applications 11.1 (2022): 19.
[3] Xomalis, A, et al. Science 374.6572 (2021): 1268-1271.
[4] Chikkaraddy, Rohit, et al. Nature 535.7610 (2016): 127-130.
[5] Ojambati, Oluwafemi S., et al. Nature communications 10.1 (2019): 1049.
[6] Chikkaraddy, Rohit, et al. Nano letters 18.1 (2018): 405-411.
[7] Benz, Felix, et al. Science 354.6313 (2016): 726-729.
[8] Chikkaraddy, Rohit, et al. ACS photonics 8.9 (2021): 2811-2817.
[9] Arul, Rakesh, et al. Light: Science & Applications 11.1 (2022): 281.
[10] Xomalis, Angelos, et al. Nano Letters 21.6 (2021): 2512-2518.