What is a cavity soliton?

Roughly speaking, cavity solitons (CS) are solitary waves in a nonlinear optical cavity.
(See Applications , References and Goals of FunFACS)

As such, they have the following intriguing properties:

Light distributions self-localized in space with exponentially decaying tails on top of some homogeneous (or uniformly patterned) background (see figure):

In the simplest case it is a bright spot of light on a zero or low-intensity background. In a semiconductor microcavity they typically have a size of a couple of microns up to 10-20 microns.

"Self-localized" means that CS are independent of boundary conditions:

i.e. a CS is not the TEM00 fundamental mode of the cavity. Instead you should think of it as a localized nonlinear excitation. In the simplest case, stabilization can be envisaged as being due to a nonlinear self-lensing effect beating the spreading due to diffraction. In that case, you can think of the cavity soliton as a "soliton in a box", i.e., a "conventional" soliton (e.g. of the nonlinear Schrödinger equation) bouncing back and forth between the cavity end mirrors.
However, note that cavity solitons are more general than conventional "propagational" solitons. They do not only occur in self-focusing but also in purely absorbing and self-defocusing media.

Localization takes place in the two transverse directions orthogonal to the propagation direction, i.e. the cavity axis. In propagation direction the cavity soliton is boundary localized.

This applies to the "usual" stationary, spatial cavity soliton. However, the pulses of mode-locked laser can - at least in some important operating regimes - also be understood as "cavity solitons" in time. In fact, one of the objectives of this project is to combine this two limiting cases to form light bullets localized in all directions.

A CS is bistable, i.e. it can be present or absent.

Hence, it can be used for all-optical information storage and processing. In a broad-area system several CS can coexist. Hence, processing can be massively parallel, in principle. Obviously this is particularly interesting for applications if it is realised in a semiconductor micro-cavity. This was achieved in a previous project PIANOS.
See movies of experimental and theoretical results.

CS have "plasticity."

In an ideal homogeneous system CS can exist everywhere due to translational symmetry. If this symmetry is perturbed due to modulations or gradients of the background or by another CS, movement results, either perpetual or until some equilibrium is reached. This can be utilized advantageously in processing schemes. In contrast to emission based on arrays of micro-machined pixels, the soliton landscape you can create is continuous and reconfigurable. See movie:

For fixed parameters, CS have a fixed amplitude and width.

In a cavity, the solitons are subject to losses and driving. This fixes their amplitude, whereas you have a continuous family of solitons in conservative wave equations.


Note that this does not rule out the possibility of high-order cavity solitons with a different internal structure than the fundamental one.





Applications

These intriguing properties make CS attractive applications for in massively parallel all-optical photonics. We mention the possibility of all-optical switching and routing in modern high-speed communication systems as well as memories and optically addressable displays. Their plasticity makes them attractive for processing applications including drift and buffer registers. The latter ones are regarded as key elements for future photonic systems and motivate much of the current research on "slow light". Drifting CS are a completely different approach to this problem than the modification of group velocity in the vicinity of resonances usually considered.

 

From the fundamental point of view, CS are also intriguing since they are a specific example for spontaneous self-organization in spatially extended systems driven far from thermal equilibrium. In this context, they are usually referred to as "dissipative solitons".

In nonlinear optics, one other popular scheme for their investigations are simple single-mirror feedback arrangements, i.e. a thin slice of a nonlinear medium with a single plane mirror at some distance and driven by a broad-area laser beam.
As mentioned, other examples include hydrodynamics, gas discharges, shaken granular media, chemistry and biology. Obviously, also self-organization in nature tends to favor localized objects and not extended, periodic patterns.
Goals of FunFACS

FunFACS researches in Fundamentals, Functionalities and Applications of Cavity Solitons, especially on the realization of a Cavity Soliton Laser, i..e. a device which draws energy solely by an incoherent source. In contrast to devices with a holding beam, solitons in a cavity soliton laser have the freedom to choose polarization, frequency and phase.

We are pursuing three main approaches to a cavity soliton laser:

1. VCSEL with frequency-selective feedback see nonlinear photonics site of Strathclydefor details.

2. VCSEL with saturable absorber

3. VCSEL with injected signal representing a mixed case with a holding beam but a VCSEL above threshold see Hachier, IEEE QE 2006 for details.

Further details and explanations on potential applications and the goals of the project : Finding the right soliton for future networks

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References

Research on cavity solitons and dissipative solitons in nonlinear optics is quite diverse and it is hard to give justice to all groups involved. Hence, we only mention the network of the European Science Foundation PHASE (Phase Domains and Spatial Solitons in Nonlinear Optics) and the ESPRIT LTR project 28235 PIANOS (Processing of Information by Arrays of Non-linear Optical Solitons). Especially the latter one gave important advances with respect to the applicability of cavity by demonstrating cavity solitons in broad-area vertical-cavity semiconductor amplifiers (Barland et al., Nature 419, 699, 2002)

  • Links to papers and conference contributions from FunFACS can be found on the "Papers" and "Conference" buttons


    • As reference for further information, we mention the following recent review articles, special issues and books:
    1. M. Segev, Solitons: A universal phenomena of self-trapped wave packets, Opt. Photonics News 13, No. 2, 27 (2002)
    2. N. N. Rosanov, Spatial hysteresis and optical patters, Springer series in Synergetics, Springer (2002)
    3. L. A. Lugiato, Introduction to the Feature Section on Cavity Solitons: An Overview; IEEE J. Quantum Electron. 39, No. 2, 1 (2003)
    4. P. D Drummond, M. Haelterman and R. Vilaseca, Optical Solitons, J. Opt. B. 6, S159 (2004)
    5. N. Akhmediev, A. Ankiewicz (eds.), Dissipative Solitons, Lecture Notes in Physics, Vol. 661, Springer (2005)
    6. P. Mandel and M. Tlidi, Transverse dynamics in cavity nonlinear optics (2000-2003), J. Opt. B: Quantum Semiclass. Opt. 6, R60 (2004)
    7.L.A. Lugiato, Cavity Solitons in Semiconductor Devices in Dissipative Solitons: From Optics to Biology and Medicine, Lect. Notes Phys 751 , 978 (2008)

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