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Bistable localized emission states in a broad-area vertical-cavity surface-emitting lasers with frequency-selective feedback. Y. Tanguy1, T. Ackemann1, R. Jäger2 1SUPA,Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, Scotland, UK. 2ULM Photonics, Lise-Meitner-Str. 13, 89081 Ulm,Germany. email: [email protected] Summary Small-area bistable lasing spots, indicators of cavity solitons, can be created at different positions within the aperture of a broad-area VCSEL with frequencyselective feedback. These spots can be switched on and off with an incoherent injected field. Introduction Recent years showed remarkable progress in achieving, controlling and understanding of bistable soliton-like emission states, which are usually referred to as cavity solitons (CS), in semiconductor microcavities. They are discussed as `bits' for future all-optically and potentially massively parallel information processing schemes. Currently realized schemes rely on a passive or active semiconductor microcavity driven by a broad-area holding beam (HB) of high spatial and temporal coherence [1]. This HB and its frequency detuning with the vertical-cavity surface-emitting lasers (VCSEL) cavity resonance induce bistability, a necessary condition for the appearance of CS. We are following a different approach for a self-sustained cavity soliton laser pumped only by incoherent means and investigate a VCSEL with frequencyselective feedback, from a grating in a Littrow configuration, therefore removing the need of a HB and simplifying the scheme. Previous theoretical analysis showed that this system exhibits bistability between the non-lasing off-state and lasing states [2], a potentially suitable situation for CS. Discussion The broad-area bottom-emitting VCSEL emits at 980nm, and is electrically pumped through a 200μm circular oxide aperture. The experimental apparatus is shown in Figure 1. The external cavity includes two lenses and a holographic grating in a Littrow configuration and is adjusted to be self-imaging in order to keep the high Fresnel number of the VCSEL. The grating frequency was chosen to coincide approximately with the longitudinal resonance of the VCSEL cavity below the solitary laser threshold. With a careful alignment of the feedback from the grating, several bistable spots appeared spontaneously in the near-field. Their switch-on and switch-off are abrupt, and their bistability range on the order of several mA. The spot size is typically 10μm, although this may vary upon the near-field location. Frequency spectra of one spot demonstrated that it can operate on a single longitudinal mode, with a 10MHz linewidth and 10dB side lobe attenuation. These spots can exist at many though not all positions within the aperture and seem to be pinned by local defects in the near-fields, such as defects lines or other local inhomogeneities in the active layer. Thereafter we examined the possibility of switch-on and switch-off of these localised emissions with an injected field, or writing beam (WB). A tunable source beam was shaped to obtain a 20 μm beam waist onto the VCSEL, and its wavelength adjusted to be near the Figure 1: Experimental set-up. The external grating wavelength. Two of the cavity is self-imaging, and beam samplers are spontaneously appearing spots were positioned to couple out a fraction of the beam chosen, at locations away from the nearto the detection part, and also to allow the field boundaries and defect lines. The injected field to enter the cavity. VCSEL was biased to be within the bistability range of these two spots. Figure 2 displays near-field intensity distributions during the experiment. The spots switch-on could be obtained by applying the WB directly onto the spot locations, while for the switch-off the WB is slightly misaligned to be located 10-15μm away from the spots. We note that both switch-on and switch-off are incoherent processes, as the WB and spot frequencies do not need to be identical. a) b) c) d) e) f) g) h) i) Picture 1: Near-fields showing the successive switch-on of two spots with an injected incoherent field (brightest spot), and their switch-off. The later is accomplished by injecting the WB beside the spot locations. a) Both spots are off, b) injection of WB, c) one spot was switched-on and stays, d) injection of WB at another location, e) second spot switched-on, f) WB injected beside the first spot, g) first spot was switched-off and does not reappear, h) injection of writing beam to switch-off the second spot, i) second spot remains off. Conclusions We demonstrated the existence of small-area bistable lasing spots in a frequency selective feedback scheme. These could be switched on/off with an incoherent injected field. These spots seem to possess all properties expected from cavity solitons, though we caution that the possible role of material defects in localization needs to be addressed by future studies. References [1] S. Barland, J. Tredicce, M. Brambilla, L. Lugiato & al., Nature, 419, 699-702, 2002. [2] N. Loiko, M. Sondermann, K. Jentsch, T. Ackemann, Optics Communications, 259, 823-833, 2006.