2.1 Ultrafast solid-state lasers - ETH - the Keller Group

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86 2.1.4 Loss modulation [Ref. p. 134 Beam Epitaxy (MBE) or with Metal-Organic chemical Vapor deposition (MOVPE). MBE gives us the additional flexibility to grow semiconductors at lower temperatures, down to ≈ 200 ◦ C, while MOVPE usually requires growth temperatures of ≈ 600 ◦ C to break up the incident molecules on the wafer surface during the growth. Lower growth temperatures lead to microscopic structural defects which act as traps for excited carriers and thus reduce the recovery time, which is beneficial for the use in ultrafast lasers. Optimized materials combine an ultrafast recovery time with low saturation fluence, high modulation and small nonsaturable losses. This material optimization issue has been addressed for ion-implanted [99Led] and LT-grown [96Sie, 99Hai1, 99Hai2] semiconductor saturable absorbers. 2.1.4.3.3 Semiconductor saturable absorber materials Semiconductor materials offer a wide flexibility in choosing the emission wavelength of the lasers. It ranges from ≈ 400 nm in the UV using GaN-based materials to ≈ 2.5 μm in the mid-infrared using GaInAsSb-based materials. More standard high-performance semiconductor material systems which can be grown today cover the infrared wavelength range from 800 nm up to 1.5 μm. Semiconductor compounds used for these wavelengths are AlGaAs (800 nm to 870 nm), InGaAs (870 nm to about 1150 nm), GaInNAs (1.1 μm to1.5μm), or InGaAsP (1.5-μm range). A larger wavelength range for a given material composition may only be obtained at the expense of increased defect concentrations because of an increased lattice mismatch to a given substrate material. Generally, bulk quantum-well and quantum-dot semiconductor saturable absorbers have been used. Especially, the quantum-dot saturable absorbers turned out to be advantageous for the integration into the VECSEL structure [04Lor, 07Maa]. 2.1.4.3.3.1 InGaAs/GaAs/AlGaAs semiconductor material system This material is best-suited for the 800 nm–1.1 μm wavelength range because of the near-perfect lattice match between GaAs and AlGaAs. InGaAs saturable absorbers have been grown on AlAs/GaAs Bragg mirrors and have been the material of choice for SESAMs at an operation wavelength of ≈ 1 μm. However, thicker InGaAs saturable absorbers above the critical thickness had surface striations that introduced too much scattering losses to be used inside a laser [94Kel]. Low-temperature MBE growth (see more details in Sect. 2.1.4.3.2) resulted in strain-relaxed structures with surfaces that were optically flat, but with strongly increased defect densities. For SESAM applications this is actually advantageous, and has been exploited to optimize the dynamic response of the SESAM. InGaAs saturable absorbers on AlAs/GaAs Bragg mirrors have even been used at an operation wavelength of 1.3 μm [96Flu2, 97Flu] and 1.55 μm [03Spu, 04Zel]. However, these highly strained layers with high indium content exceed the critical thickness, and show significant nonsaturable losses due to strain and defect formation. Optimized low-temperature MBE growth, however, (Sect. 2.1.4.3.2) allowed improved InGaAs SESAMs to support stable mode-locking and Q-switching in diode-pumped solid-state lasers. 2.1.4.3.3.2 GaInAsP/InP semiconductor material system This material system can be lattice-matched on the InP substrate but suffers from low refractiveindex contrast and poor temperature characteristics. Due to the low refractive-index contrast, a high number of InP/GaInAsP mirror pairs are required to form Distributed Bragg Reflectors (DBRs). This demands very precise control of the growth to achieve DBRs with uniform and accurate layer thickness. Landolt-Börnstein New Series VIII/1B1
Ref. p. 134] 2.1 Ultrafast solid-state lasers 87 2.1.4.3.3.3 GaInNAs semiconductor material Recently, dilute nitrides (i.e. GaInNAs) have attracted strong attention for laser devices in the telecommunication wavelength range between 1.3 μm and 1.55 μm that can use high-contrast GaAs/AlGaAs DBR mirrors [02Har, 02Rie]. Adding a few percent of nitrogen to InGaAs has two advantages: a redshift of the absorption wavelength and a reduction of the lattice mismatch to GaAs. The drawback is that the nitrogen incorporation decreases the crystalline quality, which is a big challenge for the fabrication of active devices. However, SESAMs are passive devices relying on fast defect-induced nonradiative carrier recombination to allow for short-pulse generation. GaInNAs saturable absorber on GaAs/AlsAs Bragg mirrors operating at 1.3 μm have been demonstrated for solid-state laser mode-locking. The first GaInNAs SESAM was reported to mode-lock a quasi-cw pumped Nd:YLF and Nd:YALO laser at 1.3 μm [02Sun]. Self-starting stable passive cw modelocking of a solid-state laser with a GaInNAs SESAM was demonstrated more recently [04Liv]. A detailed study of the absorber properties and the mode-locking behavior revealed that GaInNAs SESAMs provide low saturation fluences and possess extremely low losses [04Liv, 04Sch2, 05Gra3]. These SESAMs supported mode-locking at repetition rates of 5 GHz and 10 GHz [05Spu2]. In 2003, GaInNAs SESAMs at 1.5 μm were shown to mode-lock Er-doped fiber lasers but had too much loss for solid-state lasers [03Okh]. Just recently for the first time successful mode-locking of a solid-state laser at 1.54 μm using a GaInNAs SESAM has been demonstrated [05Rut]. 2.1.4.3.3.4 AlGaAsSb semiconductor material Another interesting long-wavelength semiconductor saturable absorber material is based on antimonide. The quaternary alloy AlGaAsSb has a wide band-gap tunability (1.55 μm to0.54μm) and intrinsically low modulation depth [03Saa, 04Ost]. Similar to InGaAsP, AlGaAsSb is latticematched to InP, but its absorption edge is not as steep as the one of InGaAsP [87Ada]. Therefore, operating the absorber in the bandtail results in a sufficiently small modulation depth (i.e. usually below 0.5 %) suitable for high-repetition-rate lasers. An Sb-based SESAM can be grown by MOVPE with AlGaAsSb/InP DBRs [06Ost]. Compared to InGaAsP, AlGaAsSb forms a high refractive-index contrast with InP (0.4) allowing for a lower number of Bragg periods. The first antimonide SESAM self-started and mode-locked a 61-MHz Er:Yb:glass laser [04Gra]. More recently, this was extended to an Er:Yb:glass laser at 10 GHz, 1535 nm and with 4.7 ps pulse duration [06Gra]. 2.1.4.3.3.5 GaAs wafer for ≈ 1 μm Simple GaAs wafers have been used as saturable absorbers to mode-lock [04Kon] and Q-switch [00Li, 01Che1] solid-state lasers at a wavelength of ≈ 1 μm. Photo electrons in the conduction band are generated from mid-gap defect states (i.e. EL2) present in GaAs wafers. These EL2-defects have similar properties as the arsenic antiside point defects in LT-grown materials (Sect. 2.1.4.3.2). This transition, however, has a very high saturation fluence in the range of 1 mJ/cm 2 [06Li] which is typically about 100 times higher than the standard valence-to-conduction band transition generally used for SESAMs. This strongly increases the tendency for Q-switching instabilities (Sect. 2.1.6.8). 2.1.4.3.3.6 Semiconductor-doped dielectric films Saturable absorbers based on semiconductor-doped dielectric films have been demonstrated [99Bil]. In this case, InAs-doped thin-film rf-sputtering technology was used which offers similar advantages as SESAMs, i.e. the integration of the absorber into a mirror structure. At this point, however, Landolt-Börnstein New Series VIII/1B1
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2.1 Ultrafast solid-state lasers - ETH - the Keller Group

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