Chapter 8Semiconductor SaturableAbsorbersSofar we only considered artificial saturable absorbers, but there is of coursethepossibilitytouserealabsorbersformodelocking.Aprominentcandidatefor a saturable absorber is semiconductor material, which was pioneered byIslam, Knox and Keller [1][2][3] The great advantage of using semiconductormaterials is that the wavelengthrange over which these absorbers operatecan be chosen by material composition and bandstructure engineering, ifsemiconductor heterostructures are used (see Figure 8.1).Even though, thebasic physics of carrier dynamics in these structures is to a large extent wellunderstood [4], the actual development of semiconductor saturable absorbersfor mode locking is still very much ongoing.289
Chapter 8 Semiconductor Saturable Absorbers Sofar we only considered artificial saturable absorbers, but there is of course the possibility to use real absorbers for modelocking. A prominent candidate for a saturable absorber is semiconductor material, which was pioneered by Islam, Knox and Keller [1][2][3] The great advantage of using semiconductor materials is that the wavelength range over which these absorbers operate can be chosen by material composition and bandstructure engineering, if semiconductor heterostructures are used (see Figure 8.1). Even though, the basic physics of carrier dynamics in these structures is to a large extent well understood [4], the actual development of semiconductor saturable absorbers for mode locking is still very much ongoing. 289
290CHAPTER8.SEMICONDUCTORSATURABLEABSORBERSImageremovedduetocopyrightrestrictions.Pleasesee:Keller,U.,UltrafastLaserPhysics,InstituteofQuantumElectronics,SwissFederal InstituteofTechnologyETHHonggerberg—HPT,CH-8093Zurich,Switzerland.Used with permission.Figure8.l:EnergyGap,correspondingwavelengthandlatticeconstantforvarious compound semiconductors. The dashed lines indicate indirect tran-sitions.30-40Pairs3.5a3.032.5"n'e22.011.51.06.06.57.07.5z (μum)GaAsAlAsQW orBulk LayerFigure 8.2: Typical semiconductor saturable absorber structure. A semicon-ductor heterostruture (here AlAs/GaAs) is grown on a GaAs-Wafer (20-40pairs).The layer thicknesses are chosen to be quarter wave at the centerwavelength at whichthelaser operates.This structures acts as quarter-waveBraggmirror. On top of the Bragg mirror a half-wave thick layer of the lowindex material (here AlAs) is grown, which has a field-maximum in its center.At the field maximum either a bulk layer of GaAlAs or a single-or multipleQuantum Well (MQW) structure is embedded, which acts as saturable ab-sorber for the operating wavelength of the laser.Figure by MIT OCW
290 CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS Figure 8.1: Energy Gap, corresponding wavelength and lattice constant for various compound semiconductors. The dashed lines indicate indirect transitions. Figure 8.2: Typical semiconductor saturable absorber structure. A semiconductor heterostruture (here AlAs/GaAs) is grown on a GaAs-Wafer (20-40 pairs). The layer thicknesses are chosen to be quarter wave at the center wavelength at which the laser operates. This structures acts as quarter-wave Braggmirror. On top of the Bragg mirror a half-wave thick layer of the low index material (here AlAs) is grown, which has a field-maximum in its center. At the field maximum either a bulk layer of GaAlAs or a single-or multiple Quantum Well (MQW) structure is embedded, which acts as saturable absorber for the operating wavelength of the laser. Keller, U., Ultrafast Laser Physics, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Hönggerberg—HPT, CH-8093 Zurich, Switzerland. Used with permission. 1.0 6.0 6.5 GaAs z (µm) Refractive Index Electric field strength, a.u. 7.0 7.5 0 1 2 3 4 1.5 2.0 2.5 3.0 3.5 30-40 Pairs AlAs QW or Bulk Layer Figure by MIT OCW. Image removed due to copyright restrictions. Please see:
2918.1.CARRIERDYNAMICSANDSATURATIONPROPERTIESA typical semiconductor saturable absorber structure is shown in Figure8.2. A semiconductor heterostruture (here AlAs/GaAs) is grown on a GaAs-Wafer (20-40 pairs). The layer thicknesses are chosen to be quarter waveat the center wavelength at which the laser operates. These structures actas quarter-wave Bragg mirror. On top of the Bragg mirror, a half-wavethick layer of the low index material (here AlAs) is grown, which has afield-maximum in its center. At the field maximum, either a bulk layer ofa compound semiconductor or a single-or multiple Quantum Well (MQW)structure is embedded, which acts as a saturable absorber for the operatingwavelengthof thelaser.Theabsorbermirrorservesasoneoftheendmirrorsin the laser (see Figure 8.3).Ti:S, 2.3 mm,0.25%,DopingArgonPumpM1M2AocDAUFSJPrismsM3M1-3:R=10 cmSem.Sat.Abs.Figure 8.3: The semiconductor saturable absorber, mounted on a heat sink.is used as one of the cavity end mirrors.A curved mirror determines thespot-size of the laser beam on the saturable absorber and, therefore, scalesthe energy fluence on the absorber at a given intracavity energy.8.1Carrier Dynamics and Saturation Prop-ertiesThere is a rich ultrafast carrier dynamics in these materials, which can befavorably exploited for saturable absorber design. The carrier dynamics inbulk semiconductors occurs on three major time scales (see Figure 8.4 [5])When electron-hole pairs are generated, this excitation can be considered
8.1. CARRIER DYNAMICS AND SATURATION PROPERTIES 291 A typical semiconductor saturable absorber structure is shown in Figure 8.2. A semiconductor heterostruture (here AlAs/GaAs) is grown on a GaAsWafer (20-40 pairs). The layer thicknesses are chosen to be quarter wave at the center wavelength at which the laser operates. These structures act as quarter-wave Bragg mirror. On top of the Bragg mirror, a half-wave thick layer of the low index material (here AlAs) is grown, which has a field-maximum in its center. At the field maximum, either a bulk layer of a compound semiconductor or a single-or multiple Quantum Well (MQW) structure is embedded, which acts as a saturable absorber for the operating wavelength of the laser. The absorber mirror serves as one of the endmirrors in the laser (see Figure 8.3). Figure 8.3: The semiconductor saturable absorber, mounted on a heat sink, is used as one of the cavity end mirrors. A curved mirror determines the spot-size of the laser beam on the saturable absorber and, therefore, scales the energy fluence on the absorber at a given intracavity energy. 8.1 Carrier Dynamics and Saturation Properties There is a rich ultrafast carrier dynamics in these materials, which can be favorably exploited for saturable absorber design. The carrier dynamics in bulk semiconductors occurs on three major time scales (see Figure 8.4 [5]). When electron-hole pairs are generated, this excitation can be considered
292CHAPTER 8.SEMICONDUCTOR SATURABLEABSORBERSas an equivalent two-level system if the interaction between the carriers isneglected, which is a very rough assumption.ELOhhh-/kiFigure 8.4: Carrier dynamics in a bulk semiconducotr material. Three timescales can be distinguished. I.Coherent carrier dynamics,which at room tem-perature may last between 10-50 fs depending on excitation density. II. Ther-malization between the carriers due to carrier-carrier scattering and coolingto the lattice temperature by LO-Phonon emission. III. Carrier-trapping orrecombination [5]FigurebyMITOcW.There is a coherent regime (1) with a duration of 10-50 fs depending onconditions and material. Then in phase (1I), carrier-carrier scattering setsin and leads to destruction of coherence and thermalization of the electronand hole gas at a high temperature due to the excitation of the carriers highin the conduction or valence band. This usually happens on a 60 -100 fstime scale. On a 300fs - lps time scale, the hot carrier gas interacts withthe lattice mainly by emitting LO-phonons (37 meV in GaAs). The carriergas cools down to lattice temperature. After the thermalization and coolingprocesses, the carriers are at the bottom of the conduction and valence band
292 CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS as an equivalent two-level system if the interaction between the carriers is neglected, which is a very rough assumption. Figure 8.4: Carrier dynamics in a bulk semiconducotr material. Three time scales can be distinguished. I. Coherent carrier dynamics, which at room temperature may last between 10-50 fs depending on excitation density. II. Thermalization between the carriers due to carrier-carrier scattering and cooling to the lattice temperature by LO-Phonon emission. III. Carrier-trapping or recombination [5]. There is a coherent regime (I) with a duration of 10-50 fs depending on conditions and material. Then in phase (II), carrier-carrier scattering sets in and leads to destruction of coherence and thermalization of the electron and hole gas at a high temperature due to the excitation of the carriers high in the conduction or valence band. This usually happens on a 60 - 100 fs time scale. On a 300fs - 1ps time scale, the hot carrier gas interacts with the lattice mainly by emitting LO-phonons (37 meV in GaAs). The carrier gas cools down to lattice temperature. After the thermalization and cooling processes, the carriers are at the bottom of the conduction and valence band, Eg E e - LO e - e lh hh | k | I II III Figure by MIT OCW
8.1.CARRIER DYNAMICS AND SATURATIONPROPERTIES293respectively. The carriers vanish (11l) either by getting trapped in impuritystates, which can happen on a 100 fs - 100 ps time scale, or recombine overrecombination centers or byradiation on a nanosecond time-scale. Carrier-lifetimes in III-VI semiconductors can reach several tens of nanoseconds andin indirect semiconductors like silicon or germanium lifetimes can beinthemillisecond range. The carrier lifetime can be engineered over a large rangeof values from 100 fs - 30ns, depending on the growth conditions and purityof the material.Special low-temperature growth that leads to the formationor trapping and recombination centers as well as ion-bombardment can resultin very short lifetimes [9].Figure 8.5 shows a typical pump proberesponseof a semiconductor saturable absorber when excited with a 100 fs long pulseThe typical bi-temporal behavior stems from the fast thermalization (spectralhole-burning)[7] and carrier cooling and the slow trapping and recombinationprocesses0.50.4Intraband thermalization0.30.20.1Carrierrecombination0.00.01.02.03.0Time delay (ps)Figure 8.5: Pump probe response of a semiconductor saturable absorbermirror with a multiple-quantum well InGaAs saturable absorber grown atlow temperature [3]Figure byMIT OCW.With the formula for the saturation intensity of a two-level system Eq(2.145), we can estimate a typical value for the saturation fluence F. (satu-ration energy density) of a semiconductor absorber for interband transitions.The saturation fluence FA, also related to the absorption cross-section A, is
8.1. CARRIER DYNAMICS AND SATURATION PROPERTIES 293 respectively. The carriers vanish (III) either by getting trapped in impurity states, which can happen on a 100 fs - 100 ps time scale, or recombine over recombination centers or by radiation on a nanosecond time-scale. Carrierlifetimes in III-VI semiconductors can reach several tens of nanoseconds and in indirect semiconductors like silicon or germanium lifetimes can be in the millisecond range. The carrier lifetime can be engineered over a large range of values from 100 fs - 30ns, depending on the growth conditions and purity of the material. Special low-temperature growth that leads to the formation or trapping and recombination centers as well as ion-bombardment can result in very short lifetimes [9]. Figure 8.5 shows a typical pump probe response of a semiconductor saturable absorber when excited with a 100 fs long pulse. The typical bi-temporal behavior stems from the fast thermalization (spectral hole-burning)[7] and carrier cooling and the slow trapping and recombination processes. Figure 8.5: Pump probe response of a semiconductor saturable absorber mirror with a multiple-quantum well InGaAs saturable absorber grown at low temperature [3]. With the formula for the saturation intensity of a two-level system Eq. (2.145), we can estimate a typical value for the saturation fluence Fs (saturation energy density) of a semiconductor absorber for interband transitions. The saturation fluence FA, also related to the absorption cross-section σA, is 0.0 0.0 1.0 2.0 3.0 0.1 0.2 0.3 0.4 0.5 Reflectivity Time delay (ps) Carrier recombination Intraband thermalization Figure by MIT OCW
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:
296CHAPTER8.SEMICONDUCTORSATURABLEABSORBERScommunication. The structure is a GaAs/AiAs-Bragg-mirror with an InPhalf-wavelayer andan embedded InGaAsPquantumwell absorberwith aband edge at 1.530 μm. The mirror is matched to air with an Al0 single-layer Ar-coating. At low fluence (5.6 μJ) the bleaching dynamics of theQWsaredominant.Athigherfluences.two-photonabsorption(TPA)andfree carrier absorption (FCA) in the InP half-wave layer develop and enven-tually dominate [13].1.0x10入=1.5 μm eer o eue5.0x10°0.0pump fluence (μ/cm)-1125.62ay28.-222.. 56-5.0x103-2046-28Delay (ps)Figure 8.8: Differential reflectivity measurements of a semiconductor sat-urable absorber mirror (GaAs/AlAs-Bragg-mirror and InP half-wave layerwith embedded InGaAsP quantum well absorber for the 1.55 μm range. ThemirrorismatchedtoairwithanAlOsingle-layerar-coating).Atlowfluencethe bleaching dynamics of the QWs are dominant.At higher fluences,TPAand FCA develop and enventually dominate [13]Langlois,P.etal."Highfluenceultrafastdynamicsof semiconductor saturableabsorbermirrors."AppliedPhysicsLetters75(1999):3841-3483.Usedwithpermission.The assumption that TPA and FCA are responsiblefor this behaviour hasbeen verified experimentally. Figure 8.9 shows differential reflectivity mea-surementsunder highfluenceexcitation at 1.56 μm for a saturable absorbermirrorstructureinwhichabsorptionbleachingisnegligible(solidcurve).Thequantum well was placed close to a null of the field. A strong TPA peak isfollowed by induced FCA with a single ~5ps decay forFCA.Both of thesedynamics do not significantly depend on the wavelength of the excitationas long as the excitation remains below the band gap. The ~ 5ps decay is
296 CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS communication. The structure is a GaAs/AlAs-Bragg-mirror with an InP half-wave layer and an embedded InGaAsP quantum well absorber with a band edge at 1.530 µm. The mirror is matched to air with an Al203 singlelayer Ar-coating. At low fluence (5.6 µJ) the bleaching dynamics of the QWs are dominant. At higher fluences, two-photon absorption (TPA) and free carrier absorption (FCA) in the InP half-wave layer develop and enventually dominate [13]. Figure 8.8: Differential reflectivity measurements of a semiconductor saturable absorber mirror (GaAs/AlAs-Bragg-mirror and InP half-wave layer with embedded InGaAsP quantum well absorber for the 1.55 µm range. The mirror is matched to air with an Al203single-layer ar-coating). At low fluence the bleaching dynamics of the QWs are dominant. At higher fluences, TPA and FCA develop and enventually dominate [13]. The assumption that TPA and FCA are responsible for this behaviour has been verified experimentally. Figure 8.9 shows differential reflectivity measurements under high fluence excitation at 1.56 µm for a saturable absorber mirror structure in which absorption bleaching is negligible (solid curve). The quantum well was placed close to a null of the field. A strong TPA peak is followed by induced FCA with a single ∼ 5ps decay for FCA. Both of these dynamics do not significantly depend on the wavelength of the excitation, as long as the excitation remains below the band gap. The ∼ 5ps decay is Langlois, P. et al. "High fluence ultrafast dynamics of semiconductor saturable absorber mirrors." Applied Physics Letters 75 (1999): 3841-3483. Used with permission
2978.2.HIGHFLUENCEEFFECTSattributedtocarrierdiffusionacrosstheInPhalf-wavelayer[13]Thedashedcurveshowsthedifferentialabsorptionofa~350μm thickInPsubstrateinwhich a standing-wave pattern is not formed and the ~ 5ps decay is absent.The inset in Figure 8.9 shows the power dependence of TPA and FCA. Asexpected, TPA and FCA vary linearly and quadratically, respectively, withpumppower.The pump-induced absorption of theprobe(TPA)is linearlydependent on the pump power. Since FCA is produced by carriers that aregenerated by the pump alone via TPA,FCA scales with the square of thepumppower.0.02=1.56 μm-0.2-0.4酒pezieuon0.004-0.6oTPA-0.8FCA0.000--1.0PPumpfluence(μu/cmm)224608Delay (ps)Figure 8.9: Differential reflectivity measurements under high fluence excita-tion at 1.56 μmfora saturableabsorbermirror structureinwhichabsorptionbleaching is negligible (solid cuve). The ~ 5 ps decay for FCA is attributedto carrier diffusion across the InP half-wave layer.The dahed curve showsthe differential absorption of a ~ 350 μm thick InP substrate in which astanding-wave pattern is not formed. (Inset) Linear and quadratic fluencedependence of the TPA and FCA components,respectively.Langlois.P.etal."Highfluenceultrafastdynamicsof semiconductor saturableabsorbermirrors.AppliedPhysicsLetters75(1999):3841-3483.Usedwithpermission.These high fluence effects lead to strong modifications of the saturationcharacteristics of a saturable absorber.The importance of the high fluenceeffects was first recognized in resonant absorbers (see Figure 8.10). The fieldinsidetheabsorberisenhancedbyaddingatopreflectorandaproperspacerlayer. This leads to an effective lower saturation fuence when viewed with
8.2. HIGH FLUENCE EFFECTS 297 attributed to carrier diffusion across the InP half-wave layer [13] The dashed curve shows the differential absorption of a ∼ 350 µm thick InP substrate in which a standing-wave pattern is not formed and the ∼ 5ps decay is absent. The inset in Figure 8.9 shows the power dependence of TPA and FCA. As expected, TPA and FCA vary linearly and quadratically, respectively, with pump power.The pump-induced absorption of the probe (TPA) is linearly dependent on the pump power. Since FCA is produced by carriers that are generated by the pump alone via TPA, FCA scales with the square of the pump power. Figure 8.9: Differential reflectivity measurements under high fluence excitation at 1.56 µm for a saturable absorber mirror structure in which absorption bleaching is negligible (solid cuve). The ∼ 5 ps decay for FCA is attributed to carrier diffusion across the InP half-wave layer. The dahed curve shows the differential absorption of a ∼ 350 µm thick InP substrate in which a standing-wave pattern is not formed. (Inset) Linear and quadratic fluence dependence of the TPA and FCA components, respectively. These high fluence effects lead to strong modifications of the saturation characteristics of a saturable absorber. The importance of the high fluence effects was first recognized in resonant absorbers (see Figure 8.10). The field inside the absorber is enhanced by adding a top reflector and a proper spacer layer. This leads to an effective lower saturation fluence when viewed with Langlois, P. et al. "High fluence ultrafast dynamics of semiconductor saturable absorber mirrors." Applied Physics Letters 75 (1999): 3841-3483. Used with permission
298CHAPTER8.SEMICONDUCTORSATURABLEABSORBERSrespect to the intracavity fluence or intensity.Therefore, high fluenece effectsare already reached at low intracavity intensities (see Figure 8.9)422pairsInGaAs12Sropuoinaeuonye()D07.07.55.05.56.06.5z (μm)Figure 8.10: A top reflector is added to the semiconductor saturable absorbersuch that the field in the quantum well is resonantely enhanced by about afactorof 1oincomparisontothenonresonantcase.Theon,E.R.,etal."Two-photon absorption insemiconductorsaturableabsorbermirrors."AppliedPhysicsLetters74(1999):3927-3929.Usedwithpermission
298 CHAPTER 8. SEMICONDUCTOR SATURABLE ABSORBERS respect to the intracavity fluence or intensity. Therefore, high fluenece effects are already reached at low intracavity intensities (see Figure 8.9). Figure 8.10: A top reflector is added to the semiconductor saturable absorber such that the field in the quantum well is resonantely enhanced by about a factor of 10 in comparison to the non resonant case. Theon, E. R., et al. "Two-photon absorption in semiconductor saturable absorber mirrors." Applied Physics Letters 74 (1999): 3927-3929. Used with permission