Measuring Light-Matter Interactions in Chip-Based Optical Cavities

Measuring interactions between light and matter has both fundamental and practical importance. While devices such as lasers usually involve the collective interaction of thousands of photons and atoms, this project focuses on systems in which the interaction between a single photon and a single atom is important. Such systems are a proving ground for ideas in quantum mechanics and serve a testbeds for quantum-information-processing applications. Our goal is to develop measurement tools to probe these interactions in solid-state materials. We use a chip-scale system that features a semiconductor quantum dot embedded in a nanofabricated optical resonator, which we interrogate using an optical-fiber-based waveguide probe that links microscopic on-chip structures to macroscopic laboratory equipment.

Various technologies have been developed to isolate individual atoms, molecules, ions, and other single emitters, and to study their optical properties. For example, light in an optical cavity can be localized to a small volume and persist for thousands of cycles before decaying, providing the dual benefit of a strong electromagnetic field and a long interaction time. This localization forms the basis of cavity-enhanced optical spectroscopy, one important aspect of this project.

If a cavity is small enough and the rate of light loss is low, the interaction between a single photon and an emitter can be stronger than any decay process in the system. In this “strong coupling” regime of cavity quantum electrodynamics (cavity QED), the emitter and the light wave form a coupled quantum system, similar to two atoms forming a molecule.  Prospective applications of cavity QED include low-power optical switching, light-emitting devices that produce single photon pulses for cryptography, and quantum information processing networks.
 
This project involves a solid-state realization of cavity QED operating at technologically important telecommunications wavelengths. The heart of this computer-chip-sized system is an indium arsenide quantum dot.  This nanoscopic structure is sometimes called an “artificial atom.”  It exhibits many of the properties of a single atom even though it consists of thousands of atoms. The quantum dot is embedded in a gallium arsenide microdisk cavity with a diameter of 2.5 micrometers. Both the quantum dot and the cavity are created using nanofabrication technology similar to that used to make integrated circuits. To minimize unwanted decay processes in the quantum dot, the chip is cooled to below 15 Kelvin.

Performing measurements on such a small system is one of the principal challenges of this work. We accomplish this with a tapered optical fiber waveguide, fabricated out of standard telecommunications optical fiber. The waveguide diameter starts at 125 micrometers, standard for an optical fiber, and gradually decreases to about 1 micrometer, before gradually increasing back to 125 micrometers. The tapered fiber is positioned so that its narrowest section is adjacent to the microdisk. (See Fig. 1.) At this juncture, a fraction of the light sent through the waveguide then tunnels into the microdisk, where it circulates around its periphery and interacts with the quantum dot.  Some of the circulating light tunnels back into the waveguide. We measure the properties of the exiting light to identify signatures of the quantum dot interaction.
     
This project is a collaborative effort with colleagues at the California Institute of Technology (Caltech) and the University of Rochester. Spectroscopic studies performed to date have confirmed that a strongly-coupled system has been formed, and we have begun to explore its linear and nonlinear properties. The system has also proved to be a sensitive tool for elucidating important quantum-dot properties that can be challenging to measure with other techniques.

At CNST, we are developing new methods and instrumentation to complement the work of our collaborators.  We aim to increase the efficiency, sensitivity, and stability of our measurements through improvements in the cavity/fiber waveguide system and the cryogenic probe station in which measurements are conducted.  We have done detailed electromagnetic simulations to determine the efficiency with which we can perform direct single emitter spectroscopy using optical fiber taper waveguides and optimized fiber-coupled semiconductor waveguides. These methods will enable measurements of bare quantum dot properties with improved spectral and temporal resolution, enhancing our ability to quantitatively characterize the effects of the cavity on the quantum dot.  Finally, these measurement tools can support technology development involving other solid-state systems, including those involving colloidal quantum dots, single molecules, and impurity color centers.  

Left: Chip-scale microdisk-quantum dot system
Right:  Scanning electron microscope image of a GaAs microdisk optical cavity resonator

Relevant Publications/Reports:

• Linear and nonlinear optical spectroscopy of a strongly-coupled microdisk-quantum dot system, K. Srinivasan and O. Painter, Nature 450(7171), 862-865 (2007).

• Mode coupling and cavity-quantum-dot interactions in a fiber-coupled microdisk cavity, K. Srinivasan and O. Painter, Phys. Rev. A. 75 023814 (2007).

• Single quantum dot spectroscopy using a fiber taper waveguide near-field optic, K. Srinivasan, A. Stintz, S. Krishna, and O. Painter, Appl. Phys. Lett 91(9) 091102 (2007).

 

Nanophotonics