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