This is a repository copy of Low saturation fluence in a semiconductor saturable electroabsorber mirror operated in a self-biased regime. ReuseUnless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. The self-biased SESAM, realised for example in the AlGaAs material system, is schematically shown in Fig.1. It comprises a simple P-i-N structure where a narrow-bandgap electroabsorptive i (n 0 or p 0 ) layer is placed between P-and N-doped wide-gap layers. This structure is grown on top of a high-reflectivity undoped AlGaAs DBR (AlAs/Al 0.1 Ga 0.9 As in our specific case). The reflection coefficient of such a DBR with low optical losses can be made very close to one(see e.g. [9]). No voltage is applied to the device. The incident light has a photon energy slightly below the bandgap E g of the i-layer.The principle of operation of the saturable absorber in such a SESAM is very simple[10]. Light absorption in the i-layer is governed by the electroabsorption effect due to the built-in voltage V b of the P + − i − N + structure. In steady state, the photocurrent generated due to this electro-absorption results in a voltage drop ϕ on the load resistance.Therefore, the voltage on the P + − i − N + junction decreases, which means a decrease in the electric field in the i-layer. This leads to a decrease in the electro-absorption. An increase of the radiation power increases the potential ϕ and thus reduces the voltage across the junction and, correspondingly, lowers the value of the electro-absorption coefficient α.Saturable Franz-Keldysh absorption of such a kind has been experimentally demonstrated, 2 under steady-state operation, in a surface normal configuration in [11] and in a waveguide configuration in [12].Next, we need to specify the nature of the electroabsorptive layer. Phenomenologically, any kind of electroabsorption effect in the spectral region where absorption coefficient increases with electric field allows saturable absorption to be realised. Quantum Confined Stark Effect (QCSE) in multiple quantum wells structures (MQWs) [13], which allows obtaining a large variation in absorption in this spectral region, is usually used for practical realisation of electroabsorption modulation. Achieving this large absorption variation alongside a short recovery ti...
The effects of the material properties and the resonator enhancement of absorption on the performance of a semiconductor saturable absorber mirror (SESAM) utilizing the Franz-Keldysh effect are analyzed theoretically. Bulk as well as extremely shallow quantum well materials are shown to be suitable for fabrication of devices of this type. The saturation flux and recovery time of the proposed device when operated with picosecond incident pulses, as well as the thermal properties, are shown to compare favorably with existing all-optical SESAM constructions.
Brussels (Belgium).The vertical-cavity surface-emitting laser (VCSEL) has attracted much attention due to its numerous advantages with respect to the conventional edge-emitting lasers. The VCSEL generally emits a linear polarization but its direction is not fixed a priori since polarization switching may occur as the operating conditions, i.e. the injection current or the temperature, are modified [1]. The optical injection from an external source of light may help to control or stabilize the VCSEL polarization and its study is interesting both from the point of view of dynamical instabilities and for applications in high density optical storage and optical signal processing [2].We report on an experimental and theoretical study of the polarization dynamics of a VCSEL submitted to optical injection. In our configuration, the linearly polarized injected light is orthogonal to the light emitted by the free running VCSEL [2]. The optical injection leads to a bistable polarization switching with or without injection locking, the switching power being strongly dependent on the frequency detuning (Fig. 1). For large positive detunings, we observe the locking of the first order transverse mode to the master laser and the suppression of the fundamental transverse mode. The polarization switching induced by optical injection may be furthermore accompanied by a mode hopping regime, i.e. a random hopping between the two linearly polarized modes. The statistical properties of the residence time are influenced by the optical injection strength and frequency detuning. On the other hand, the optical injection may stabilize an otherwise mode hopping VCSEL. Our experiment unveils a rich nonlinear polarization dynamics, which we map in detail in the plane of the injection parameters (injection strength, frequency detuning). Several different time-periodic dynamics are observed as a result of multiwave mixing or Hopf bifurcation. These timeperiodic instabilities may further bifurcate to a period doubling route to chaos, as shown in Fig. 2: an increase of the injected power leads first to a polarization switching with injection locking (b). The stationary state then undergoes a
We characterize the sensitivity of imaging properties of a layered silver-TiO2 flat lens to fabrication inaccuracies. The lens is designed for approximately diffraction-free imaging with subwavelength resolution at distances in the order of a wavelength. Its operation may be attributed to self-collimation with a secondary role of Fabry-Perot resonant transmission, even though the first order effective medium description of the structure is inaccurate. Super-resolution is maintained for a broad range of overall thicknesses and the total thickness of the multilayer is limited by absorption. The tolerance analysis indicates that the resolution and transmission efficiency are highly sensitive to small changes of layer thicknesses.
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