SAM – saturable absorber mirror

The saturable absorber layer consists of a semiconductor material with a direct band gap slightly lower than thephoton energy. During the absorption electron-hole pairs are created in the film. The relaxation time τ of thecarriers has to be a little bit longer than the pulse duration. In this case the back side of the pulse is still free ofabsorption, but during the whole period between two consecutive pulses the absorber is non saturated andprevents Q-switching.Because the typical relaxation time due to the spontaneous photon emission in a direct semiconductor is about 1ns, some precautions has to be done to shorten it drastically.Two technologies are used to introduce lattice defects in the absorber layer for fast non-radiative relaxation ofthe carriers:●●low-temperature molecular beam epitaxy (LT-MBE)ion implantation.The parameters to adjust the relaxation time in both technologies are the growth temperature in case of LT-MBEand the ion dose and annealing parameters in case of ion implantation.Typical values of the relaxation time of SOCs are between τ = 1 .. 10 ps.> Reflectance bandwidthThe reflectance bandwidth of the SOC has to be larger than the pulse bandwidth. The reflectance bandwidth isdetermined by the ratio of the refractive indices n H /n L of the layers in the Bragg mirror thin film stack. Moreabout Bragg-mirrors ...The relative spectral width w = ∆λ/λ of the high reflectance zone of a common semiconductor AlAs/GaAs thinfilm stack is about 0.1. Therefore the width of the high reflectance zone of an AlAs/GaAs Bragg-mirror with acentre wavelength of 1000 nm is about 100 nm. This results in a minimum pulse duration of about 20 fs.> AbsorbanceAn ideal SOC has a constantsaturable absorption for allwavelengths of the pulsespectrum. The absorption ofa direct semiconductorincreases heavily withincreasing photon energy,starting at the gap energyof the semiconductormaterial. In case of aquantum well structure theabsorption increases as astep-like dependency on thephoton energy due to theone-dimensionalquantisation of free carriers.In any case the result is anincreasing saturableabsorption of a SOC withdecreasing wavelength(increasing photon energy).Consequently, thereflectance versuswavelength curve of a SOCreveals under non-saturatedconditions a decreasingreflectance with decreasingwavelength.SeongKyeong Photonics 15 info@skphotonics.com