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

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
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