Light does not stay confined to small regions in space or time, but wave packets of light have the natural tendency to broaden: In space this is due to diffraction, in time this is due to dispersion, at least in a medium. On the other hand, for optical communications, storage or future all-optical processing techniques, it is important to counteract this natural tendency and to confine light to the smallest possible dimensions. Though great partial success was achieved by linear optical element as optical fibres and other waveguides, it was always the dream of researchers to confine light by self-action. This is why the concept of solitary waves — or more loosely speaking of solitons — received a lot of attraction during the last decades. A soliton is a wavepacket in which the tendency to broaden is balanced by nonlinearities.
Due to the outcoupling losses through the mirrors, spontaneous emission and nonradiative decay, cavities are actually a dissipative environment. Hence cavity solitons are an example for dissipative solitons. This has important consequences:
They have fixed amplitude and width for constant parameters.
They have bistable, i.e. can be present or absent under the same conditions.
Within the bistable range, one can set and reset them by external optical control beams.
‘They possess mobility (in contrast to micro-machined pixels): Since they are self-localized, they can exist anywhere within a pumped area on the one hand and react to any – possibly dynamical gradients on the other hand.’
Hence, they have potential in future all-optical processing applications. We are using the concept of a cavity soliton to realize small microlasers within a broad-area vertical-cavity surface-emitting laser (VCSEL). These can be switched on and off by optical means.
We term this device a cavity soliton laser.
For a realization, we use a VCSEL with 200 micron emission aperture with feedback from a grating serving as a frequency-selective element. The competition between two resonances, the cavity resonance of the VCSEL and the one by the grating filter, makes the system bistable. Initially, they are detuned and the VCSEL is in a zero amplitude emission states with a high carrier density. The two resonance can became aligned because a high amplitude emission state with a correspondingly lower carrier density leads to a change of intracavity refractive index and hence resonance condition.
Please Click Here for real time movie of spatial emission structure of a broad-area VCSEL showing the spontaneous switch-on and switch-off of laser solitons if the current is ramped up and down slowly.
Click Here for a video showing the independent writing and erasure of all 23=8 bit combinations possible for three solitons demonstrating the possibility of a complete external control of emission state (see IEEE J. Quant. Electron. 2009 for details)
Current work is directed on understanding and controlling the choice of frequency, phase and polarization of these guys and how this influences their interaction. For applications, miniaturization is required.