Tip-Enhanced Molecular and Quantum Cavity Nano-Optics

With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, Professor Markus Raschke and his group at the University of Colorado are developing a technique called tip-enhanced strong coupling (TESC) as a new approach for measuring and controlling the properties of single quantum mechanical particles that are strongly coupled with light. The researchers form a very small cavity between a very sharp metallic tip in a scanning probe microscope and a mirror surface to trap light in a region of space containing the single particle.

Tuning the cavity size with atomic precision allows the team to control the interaction between light and matter in order to form light-matter hybrid states that are described by cavity quantum electrodynamics (cQED). Their approach overcomes traditional limitations in cavity size, number of particles being probed, and temperature requirements for the measurements.

The work provides a fundamental understanding of the properties of strong cavity-emitter interactions and enables room-temperature control of quantum coherent interactions, enabling new forms of single-molecule spectroscopy, quantum sensing, and control of photochemistry.

This project takes advantage of the newly developed approach of tip-enhanced strong coupling (TESC), which provides a pico-cavity mode volume controlled with atomic precision between a scanning plasmonic tip and the sample substrate. The resulting coupling strength exceeds decoherence even at room temperature to individually address, dynamically tune, actively control, and image single quantum emitters that are strongly coupled with light.

Through photoluminescence measurements, TESC is used to probe colloidal quantum dots and solid-state defects in 2D materials as model quantum emitters in order to establish the single emitter-single photon limit and to demonstrate entanglement, optical nonlinearities, and photon blockade in strong coupling. Extending into the femtosecond time domain, adiabatic femtosecond nano-focused TESC measures the competing relaxation pathways of these emitters using coherent four-wave mixing.

In combination with variable temperature TESC of solid-state emitters, and low-temperature tip-enhanced Raman spectroscopy (TERS) as a probe of intramolecular vibrational energy redistribution, the work leads to a quantum-state-resolved fundamental understanding of decoherence to coupled internal and external electronic and vibrational degrees of freedom. The work thus helps answer the question of the fundamental limit of engineering quantum coherence of electronic wavefunctions, phonons, and vibrations in chemical systems – information pertinent to any form of solid-state-based quantum technologies.

To achieve its goals, the project includes diverse graduate and undergraduate training, workforce development through cQED integration into the course curriculum, and industry collaborations to accelerate quantum emitter development for quantum information applications.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.