Emerging
Quantum light and light–matter coupling
Non-classical light in integrated photonic circuits, and molecules coupled strongly enough to a confined field that the light becomes part of the chemistry.
Background
Before the ultrafast work, my research was in quantum optics: the theoretical modeling of non-classical states of light in integrated photonic circuits.
Quantizing the electromagnetic field gives two non-commuting quadrature operators, \(X\) and \(Y\), whose noise obeys \(\Delta X\,\Delta Y = 1/4\). Coherent and vacuum states divide that noise evenly. A squeezed state does not: the noise in one quadrature falls below the vacuum level, which is what makes squeezed light useful for measurement below the standard quantum limit — as in the interferometers of LIGO — and as a source of entangled photons for quantum sensing, computing, and communication.
At the time, most demonstrations used bulk optics, which does not scale. We proposed generating squeezed states on-chip instead, using enhanced spontaneous parametric down-conversion in a single-waveguide side-coupled AlGaAs microring resonator. A pulsed second-harmonic field couples into the ring, generates fundamental-wave photons inside it, and couples back out. Solving the system in the interaction picture shows that for a narrow-pulse coherent input and small cross-coupling, the photons leaving the waveguide are single-mode squeezed light.
Where this is going
Two directions, both drawing on the same light–matter machinery as the nanoparticle and molecular projects.
Non-classical light in photonic circuits. Extending the generation schemes to other nonlinearities, such as the Kerr effect; modeling how non-classical light interacts with atoms and molecules, including quantum coherence, resonance energy transfer, and enhanced coupling under photonic confinement; and identifying where non-classical states give a real advantage in precision measurement.
Polariton chemistry. Coupling molecules to a quantized field — inside an optical cavity, or near a plasmonic nanoparticle — creates hybrid light–matter states called polaritons. What makes this different from conventional photochemistry is that it does not rely on large photon numbers: in the strong coupling regime, reaction pathways can be modified even with no photons initially present in the cavity. The relevant tools are cavity quantum electrodynamics on one side and molecular dynamics on the other, which is the combination the group already works with.
The plasmonic connection is what makes this a natural extension rather than a change of subject. The induced near fields characterized in the imaging project are exactly the confined fields that put a nearby molecule into strong coupling.