294CHAPTER8.SEMICONDUCTORSATURABLEABSORBERSthen given byh2hfFA=IATA(8.1)OA2T2ZmMn?no(8.2)2T2ZroMThe value for the dipole moment for interband transitions in III-V semicon-ductorsisaboutd=O.5nmwithlittlevariationforthedifferentmaterials.Together with the a dephasing time on the order of T2 = 20 fs and a linearrefractive index no = 3, we obtainh?no35(8.3)FAcm22T,ZpOMFigure 8.6 shows the saturation fluence measurement and pump probe tracewith10fsexcitationpulsesat800nmonabroadbandGaAssemiconductorsaturable absorber based on a metal mirror shown in Figure 8.7 [11]. Thepumpprobetrace shows a50fsthermalizationtime and longtimebleach-ing of the absorption recovering on a 50 ps time scale due to trapping andrecombinationImageremovedduetocopyright restrictions.Please see:Jung,I.D.,et al."Semiconductor saturable absorbermirrors supporting sub-10 fs pulses."AppliedPhysicsB65(1997):137-150Figure 8.6: Saturation fluence and pump probe measurements with 10 fspulses on a broadband metal mirror based GaAs saturable absorber.Thedots are measured valuesand the solid lineis thefit to a two-level saturationcharacteristic [11]
294 CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS then given by FA = hf σA = IAτ A = ~2 2T2ZF ¯ ¯ ¯ M ¯ ¯ ¯ 2 (8.1) = ~2n0 2T2ZF0 ¯ ¯ ¯ M ¯ ¯ ¯ 2 (8.2) The value for the dipole moment for interband transitions in III-V semiconductors is about d = 0.5 nm with little variation for the different materials. Together with the a dephasing time on the order of T2 = 20 fs and a linear refractive index n0 = 3, we obtain FA = ~2n0 2T2ZF0 ¯ ¯ ¯ M ¯ ¯ ¯ 2 = 35 µJ cm2 (8.3) Figure 8.6 shows the saturation fluence measurement and pump probe trace with 10 fs excitation pulses at 800 nm on a broadband GaAs semiconductor saturable absorber based on a metal mirror shown in Figure 8.7 [11]. The pump probe trace shows a 50 fs thermalization time and long time bleaching of the absorption recovering on a 50 ps time scale due to trapping and recombination. Figure 8.6: Saturation fluence and pump probe measurements with 10 fs pulses on a broadband metal mirror based GaAs saturable absorber. The dots are measured values and the solid line is the fit to a two-level saturation characteristic [11]. Jung, I. D., et al. "Semiconductor saturable absorber mirrors supporting sub-10 fs pulses." Applied Physics B 65 (1997): 137-150. Image removed due to copyright restrictions. Please see:
2958.2.HIGHFLUENCEEFFECTSA typical value for the fluence at wich damage is observed on an absorberis on the order of a few mJ/cm2. Saturating an absorber by a factor of 10without damaging it is still possible :The damage threshold is stronglydependent on the growth, design, fabrication and mounting (heat sinking) ofthe absorber.Image removedduetocopyright restrictionsPlease see:Fluck,R.,etal."Broadbandsaturableabsorberfor1ofspulsegeneration."OpticsLetters21 (1996):743-745Figure 8.7: GaAs saturable absorber grown an GaAs wafer and transferedonto a metal mirror by post growth processing [10]8.2HighFluence EffectsTo avoid Q-switched mode-locking caused by a semiconductor saturable ab-sorber,the absorber veryoftenisoperatedfar abovethesaturationfluenceor enters this regime during Q-switched operation. Therefore it is also im-portant to understand the nonlinear optical processes occuring at high exci-tation levels [13].Figure 8.8 shows differential pump probe measurements onasemiconductorsaturableabsorbermirrorsimilartoFigure8.2butadaptedto the 1.55 μm range for the developement of pulsed laser sources for optical
8.2. HIGH FLUENCE EFFECTS 295 A typical value for the fluence at wich damage is observed on an absorber is on the order of a few mJ/cm2. Saturating an absorber by a factor of 10 without damaging it is still possible . The damage threshold is strongly dependent on the growth, design, fabrication and mounting (heat sinking) of the absorber. Figure 8.7: GaAs saturable absorber grown an GaAs wafer and transfered onto a metal mirror by post growth processing [10]. 8.2 High Fluence Effects To avoid Q-switched mode-locking caused by a semiconductor saturable absorber, the absorber very often is operated far above the saturation fluence or enters this regime during Q-switched operation. Therefore it is also important to understand the nonlinear optical processes occuring at high excitation levels [13]. Figure 8.8 shows differential pump probe measurements on a semiconductor saturable absorber mirror similar to Figure 8.2 but adapted to the 1.55 µm range for the developement of pulsed laser sources for optical Fluck, R., et al. "Broadband saturable absorber for 10 fs pulse generation." Optics Letters 21 (1996): 743-745. Image removed due to copyright restrictions. Please see: