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

Abstract

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.

Self-biased saturable absorber mirror demonstrating very low saturation fluence

“…1) was grown in a P -i -N photodiode low-finesse resonator configuration by molecular beam epitaxy, with subsequent processing to create a device mesa structure D '70 mm in diameter. The structure parameters were very close to those used in the theoretical paper [7], with the total thickness of the P -i-N layers satisfying the resonance condition at l ¼ 0.89 mm (Fig. 1) The specific capacitance for a broad-area planar structure identical to that of the SESAM was measured as 40 nF cm 22 , which is close to a simple estimate using a parallel-plate capacitor formula.…”

“…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.…”

“…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.…”

“…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.…”

“…The solid curve in Fig. 2a is the theoretical dependence R(E in ), obtained for l ¼ 0.89 mm using the formulas and material parameters from [7] and the reflectivities of the low-finesse resonator (R DBR ¼ 0.97, R tf ¼ 0.3). To illustrate the good agreement of the measured and theoretical curve shapes, we also show, as a dashed curve, the same dependence, with the small-signal reflectance artificially adjusted to fit exactly the measured value of R ¼ 0.84.…”