Low saturation fluence in a semiconductor saturable electroabsorber mirror operated in a self-biased regime

Abstract

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...

Use of the Franz–Keldysh effect in self-biased p-i-n-heterostructures for saturable absorber mirrors

“…The alternative mechanism of absorption saturation is the electroabsorptive mechanism which acts via a combination of electroabsorption effect and field screening by photogenerated carriers. In a recent paper [7], we have shown that the electroabsorption effect in extremely shallow QWs (ESQWs) [8,9] is well suited for implementation of very low saturation fluence self-biased surface-normal electroabsorptive SESAMs.…”

“…Here, we generalize the treatment of Franz-Keldysh effect in self-biased heterostructures conducted in [7] to analyze both bulk and ESQW structures and also to include resonator effects and to show how the device performance can be optimized using these routes. We also analyze the thermal properties of the SESAM and compare them to those of existing constructions.…”

Self-biased saturable absorber mirror demonstrating very low saturation fluence

“…2b) are the calculated transmittance T ¼ P t /P in (P t being the transmitted power) and absorptance A ¼ 1-R-T of the SESAM against incident pulse energy. Note that, in the present study, the reflectance R saturates at a value of R ¼ 0.9, rather than R ¼ 1 as in [6,7] This is because with R DBR ¼ 0.97, approximately 0.1 of the incident power is transmitted through the DBR into the substrate, as illustrated by the (theoretical) curves in Fig. 2b.…”

“…As the peak laser power incident on the SBSESAM was only 1 mW, the laser was emitting optical pulses t p ' 200 ns long in order to accumulate the energy necessary to saturate the SESAM. Owing to the deliberately large load resistance (R L 200 kV), the discharge of the P -i-N structure capacitance through R L during the pulse was negligible despite the long t p , and the SESAM was operating in the t p R L C regime analysed in [6,7]. In this regime, the reflectance R ¼ P r /P in (P r and P in being the reflected and incident power) is determined at the end of the optical pulse.…”

“…In this latter case, however, the SESAM construction ceases to be selfcontained, presenting integration problems, and energy expended and released can become significant. In [6,7], a SESAM utilising the Franz-Keldysh effect due to built-in voltage in a self-biased P -i-N heterostructure, with a bulk or extremely shallow quantum well absorbing i-layer, has been proposed and analysed theoretically. The saturation flux of the proposed device when operated with incident pulse durations down to a few picoseconds, has been shown to compare favourably with existing all-optical constructions.…”