Photonic Crystal Microcavities for Cavity QED

Photonic crystal optical microcavities show promise for applications to experimental cavity quantum electrodynamics (QED). The goal of this project is to demonstrate experimentally the feasibility of these cavities for our group's cavity QED experiments.

Cavity QED

The cavity QED system consists of a small number of atoms interacting strongly with photons confined in a high-quality optical cavity. It is one of few experimentally realizable systems to date in which the intrinsic quantum mechanical coupling dominates losses due to dissipation, providing a setting in which one can quantitatively study the dynamics of an open quantum system under continuous observation. Furthermore, the presence of a single atom or photon is sufficient to affect the properties of the system in this strong coupling regime. Quantum state mapping between atomic and optical states becomes possible, which has profound implications towards realization of quantum information processing. (See [1,2], and the Kimble group web page.)

All of the cavity QED experiments to date have used Fabry-Perot cavities, which require special efforts to stabilize. A pair of high reflectivity supermirrors form the cavity, with their spacing (typically 10-100 microns) stabilized to ~1fm by active servo control to maintain resonance with the atomic transition of interest. Obtaining an error signal involves using an auxiliary laser and another transfer cavity in order to stabilize the frequency of the auxiliary laser [3]. The desire to avoid this extensive labor overhead, and the requirement of scalability in applications to quantum information processing, motivate the search for a new experimental paradigm for cavity QED.

Photonic crystal defect optical cavities

A photonic crystal is a fabricated material with a spatially periodic dielectric constant, for example a dielectric slab with a lattice of holes etched in it. Some lattice types can exhibit a photonic bandgap, a range of wavelengths of light for which propagation through the material in certain directions is not allowed. A defect in the lattice, for example a missing hole, can give rise to localized modes with wavelengths within the photonic bandgap, thus acting as an optical cavity. For references on photonic crystals, see [4,5].

A simulated localized electromagnetic mode in a photonic crystal defect cavity.

Photonic crystal cavities have several potential advantages over Fabry-Perot cavities for cavity QED experiments. The properties of a photonic crystal optical cavity are determined by its geometry, which is controlled through the fabrication process. This avoids the need for active stabilization, greatly simplifying the experiments. The photonic crystal microcavity can also be integrated with single atom trapping schemes using lithographically patterned magnetic microtraps (see Benjamin Lev's work). Finally, the photonic crystal system may prove to be more scalable than the current cavity QED paradigm.

Our research

The goal of our project is to demonstrate experimentally the feasibility of photonic crystal defect microcavities for our group's cavity QED experiments. Numerical simulations [6-8] have yielded cavity designs that would be suitable for cavity QED, but these models have yet to be fully tested. Our work therefore involves the design, fabrication, and characterization of cavities. We do all three of these in-house, in collaboration with the Caltech nanofabrication group.

From a theoretical perspective, we have been developing techniques based on optimal control theory for designing photonic crystals suitable for strong-coupling cavity QED experiments [6]. Our design goals utilize algorithmic approaches to cavity design rather than trial and error. We have shown that it is possible to derive analytic solutions to the 2-dimensional photonic crystal design problem, and have extended the same optimization principles to planar structures using numerical methods. By posing the pbg design problem as a mathematical inverse problem, we were able to balance the seemingly competing featuers of small mode volume, high quality (Q) factors and large intracavities fields. Using inversion methods, we have proposed highly localized defect crystals with Q's larger than one hundred thousand.

We make our prototype cavities in the nanofabrication laboratories of Prof. Axel Scherer [9]. The main techniques we use are electron beam lithography and chemically-assisted ion beam etching (CAIBE).

The samples are characterized using two techniques. First, we examine the geometry of the completed structures with a scanning electron microscope (SEM). We obtain images that are analyzed in MATLAB and Mathematica to determine the quality of the structures. Second, we measure the optical reflectance of the structures. High-Q modes appear as sharp features in the reflected spectrum.

Layout of our optical characterization apparatus.

(last updated 24 February 2003)

References