Applications of Optical Cavities in Modern Atomic, Molecular, and Optical Physics

Jun Ye1 ye@jila.colorado.edu; Theresa W. Lynn2    1 JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309-0440, USA
2 Norman Bridge Laboratory of Physics, Room 12-33, California Institute of Technology, Pasadena, California 91125, USA

I Introduction

For many contemporary physics experiments, the use of an optical cavity has become a powerful tool for enhancement in detection sensitivities, nonlinear interactions, and quantum dynamics. Indeed, an optical cavity allows one to extend the interaction length between matter and field, to build up the optical power, to impose a well-defined mode structure on the electromagnetic field, to enable extreme nonlinear optics, and to study manifestly quantum mechanical behavior associated with the modified vacuum structure and/or the large field associated with a single photon confined to a small volume. Experimental activities that have benefited from the use of optical cavities appear in such diverse areas as ultra-sensitive detection for classical laser spectroscopy, nonlinear optical devices, optical frequency metrology and precision measurement, and cavity quantum electrodynamics (cavity QED). Of course the most important application of optical cavities is in laser physics itself. However, in this article we will concentrate on the various applications of external optical cavities (independent from lasers) that take advantage of the common physical properties associated with resonator physics.

One of the important themes in laser spectroscopy is to utilize an extended interaction length between matter and field inside a high finesse cavity for an increased detection sensitivity. Two key ingredients are needed to achieve the highest sensitivity possible in detection of atomic and molecular absorptions: enhancement of the absorption signal and elimination of technical noise. While the absorption signal is enhanced by an optical cavity, it is important also to take measures to avoid technical noise; the sharp resonances of the cavity can introduce additional noise through frequency-to-amplitude noise conversion. Cavity vibration and drifts can also contribute noise beyond the fundamental, quantum-noise limit. In this article (Sections IV and V), we will discuss the application of various modulation techniques combined with suitable experimental configurations that let one benefit from the signal enhancement aspect of a cavity, at the same time suppressing the technical noise in the detection process. Such achievement has enabled studies of molecular vibration dynamics of weak transitions.

Another important theme is the application of optical frequency metrology for precision measurements. Section VI addresses the role of optical cavities in this context. Certainly the use of an optical cavity for laser frequency stabilization is critical for the development of super-stable optical local oscillators. The extreme quality factor (1015) associated with some “forbidden” optical transitions in cold and trapped samples of atoms and ions demands a similar or even higher quality factor on the optical probe source to take full advantage of the system coherence. Although the advent of femtosecond-laser-based optical frequency combs has to a certain degree reduced the utility of cavity-based frequency reference systems, stable optical cavities continue to provide important services in laser laboratories; either the cavity modes themselves provide optical frequency markers or a cavity helps enhance optical to microwave frequency coupling via an intracavity modulator. And one of the most important applications of optical cavities is still for laser frequency stabilization, with the scope now extended to cover ultrafast lasers as well. In fact, cavity-based ultra-sensitive detection of atomic/molecular absorptions represents an important approach to produce accurate and precise frequency references throughout the visible and near-infrared wavelength regions. We also note that some of the most demanding work in precision measurement is now associated with cavity-enhanced Michelson interferemetry to search for gravitational waves or violation of relativity laws.

A third theme that will be covered in this article (Section VII) is the exploration of quantum dynamics associated with the enhanced interaction between atoms and cavity field; where the structure of the cavity enables a large field amplitude associated with single intracavity photons, the system dynamics can become manifestly quantum and nonlinear. A high cavity finesse suppresses the dissipation rate associated with photon decay while a well-defined spatial mode of the cavity output field (associated with cavity decay) permits recovery of information about the intracavity dynamics with high quantum efficiency. Although cavities in the optical domain have not significantly influenced the atomic radiative properties in a direct manner, the enhanced coherent interactions between them present an ideal platform to study open quantum systems.

Before addressing these topics in detail, we begin with some broad comments and historical context in the remainder of this introduction. Section II is devoted to description and characterization of the optical cavities themselves, while Section III gives some simple physical arguments for the cavity enhancement effect that is crucial for applications ranging from classical spectroscopy to cavity QED.

A Signal Enhancement and Optical Field Buildup Inside a Cavity

Improving sensitivity for spectroscopy on an atomic or molecular sample by placing it inside an optical resonator is a well-known technique and is most commonly explained in terms of the multipass effect. In fact, it was realized in the early days of laser development that a laser cavity was useful for absorption enhancement [1], owing to the multipass effect and the delicate balance between the laser gain and intracavity absorption [2,3]. However, in most recent implementations, it is often preferable to separate the absorber from the laser, in order to extend the experimental flexibility and to characterize better working parameters. Kastler first suggested that a Fabry-Perot cavity be employed for the sensitivity of its transmission to small variations in absorption within the cavity [4]. The external cavity approach has since been applied to record both linear and high-resolution nonlinear molecular spectra [5–7]. In particular, enhancement cavities in the form of cavity ring down spectroscopy [8,9] have been extensively applied in the field of physical chemistry to study molecular dynamics and reaction kinetics.

While we defer a detailed discussion of cavity enhancement effects to Section III, here we make a quick note of the advantages associated with an optical resonator. The well-known multipass effect leads to an enhancement of the effective absorption length by a factor of 2F/π, where F is the cavity finesse. Additionally, the intracavity optical power is built up relative to the input power via constructive interference, which allows for the study of nonlinear interactions even with low-power laser sources. An often taken-for-granted benefit in practice is that although the intracavity interaction is powered by a high field amplitude, the cavity reduces the output power to a reasonable level acceptable for subsequent photo-detectors. Alternatively, high intracavity power can be extracted using high-speed optical switching devices; this forms the basis of a cavity-based optical amplifier to be discussed in Section VI.D. Additionally, the geometrical self-cleaning and mode matching of the optical waves inside a cavity is important both for eliminating pointing-direction related noise and for...