Strong coupling and induced transparency at room temperature with single quantum dots and gap plasmons

Abstract

Coherent coupling between plasmons and transition dipole moments in emitters can lead to two distinct spectral effects: vacuum Rabi splitting at strong coupling strengths, and induced transparency (also known as Fano interference) at intermediate coupling strengths. Achieving either strong or intermediate coupling between a single emitter and a localized plasmon resonance has the potential to enable single-photon nonlinearities and other extreme light–matter interactions, at room temperature and on the nanometer scale. Both effects produce two peaks in the spectrum of scattering from the plasmon resonance, and can thus be confused if scattering measurements alone are performed. Here we report measurements of scattering and photoluminescence from individual coupled plasmon–emitter systems that consist of a single colloidal quantum dot in the gap between a gold nanoparticle and a silver film. The measurements unambiguously demonstrate weak coupling (the Purcell effect), intermediate coupling (Fano interference), and strong coupling (Rabi splitting) at room temperature.

Nano‐Cavity QED with Tunable Nano‐Tip Interaction

“…Optical antennas in the form of AFM tips have previously been demonstrated as a powerful platform for controlling the optical properties of materials on the nanoscale from dark exciton emission to optical tuning via induced strain, and their utility can be further improved by employing a tilted tip geometry, as shown in Figure 1a, which leads to plasmonic vector field control and enhancement of both in‐plane and out‐of‐plane dipole moments and maximizes optical confinement . With these innovations, and with the ability to configure the nano‐tip cavity for maximum coupling strength, we achieve single‐emitter coupling that is comparable to the strongest coupling seen in PL, even from large ensembles of emitters, with the added ability to control the cavity coupling and mode volume through tip placement and to couple a series of different single quantum emitters to the same cavity.…”

“…In these systems, low temperature operation is generally required to sufficiently decouple emitters from their environment and reduce their dephasing rate below that of the cavity–emitter coupling strength. However, a new regime of strong cavity–emitter interaction has recently been established using plasmonic nano‐cavities with deep sub‐diffraction‐limited mode volumes . While plasmonic nanocavities extend quantum state control even to room temperature, this approach has relied largely on nano‐fabrication techniques to generate static plasmonic cavities, which limit the ability to tune, control, and image emitters in the strong coupling regime.…”

“…Recently, plasmonic nano‐antenna cavities in the form of highly polarizable metallic nanostructures with dimensions down to just a few nanometers, which allow for deep sub‐diffraction limited mode volumes, have been broadly employed to generate hybrid quantum states of plasmons and emitters . The increased interaction rate facilitated by these nanoscopic cavity mode volumes have enabled strong coupling to both ensembles and single emitters at room temperature …”

“…Over the past two years, strongly coupled plexciton states were observed in the emission of single emitters coupled to plasmonic cavities in several experiments at room temperature. Most of these employed nanoparticle‐on‐a‐mirror (NPoM) geometry in which an emitter is located between a metal nanoparticle and a planar metal substrate . This is a significant landmark for the field, as this modality provides unambiguous evidence of strong coupling and paves the way for new applications in single emitter quantum information and photonic quantum devices.…”

“…However, while significant progress has been made in extending single emitter strong coupling to scalable, room temperature platforms based on plasmonic nano‐cavities with deep sub‐diffraction‐limited mode volumes, this approach has focused primarily on the fabrication and implementation of static plasmonic cavities, which do not allow for imaging, optimization, or control of the spatially dependent cavity–emitter interaction strength. Indeed, recent work on a quantum dot coupled to a scanning plasmonic slot structure suggests evidence for reaching the strong coupling regime in single emitter PL at room temperature, yet is plagued by convoluted spectra with large background and competing artefacts due to charged exciton emission.…”