Fabrication and tailoring of SESAMs

A SESAM is a semiconductor heterostructure and is therefore fabricated by means of molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) [Kel99]. To reduce the strain within the layer structure the lattice constants of the dif-ferent semiconductor materials should be similar. In Fig. 3.5 the band gap energy of different semiconductor materials is plotted versus their lattice constant. Since the lattice constant of GaAs (5.64 A) and AlAs (5.66 A) are just slightly different [Yu03], they are well suitable for layer structures. Furthermore, by varying the material com-position of compound semiconductors the lattice constant and the band gap energy can be adjusted. Hence, the band structure and the opto-electronic properties of the ma-terial can be tuned continuously for best fulfillment of requirements regarding growth conditions and optical properties.

Thus, the material composition of the SESAM as well as the layer thicknesses and layer structure determine the optical properties of the device, respectively its characterizing parameters.

3.4 Fabrication and tailoring of SESAMs

In general, for an optimum reflectivity change it is necessary that the quantum well material absorbs the laser wavelength. Consequently, for a laser wavelength of 1030 nm In_xGa_1-xAs as absorber material is suitable. The absorption increases with the amount of quantum well material. Hence, increasing the number of quantum wells increases the absorption and leads to a higher modulation depth [Col12]. Increasing the thickness of the quantum wells can also result in a higher modulation depth, but increases the insertion losses, too, leading to higher nonsaturable losses [Kel99].

Apart from the material composition and layer structure defect states and dielectric coatings do also influence the saturable absorber.

Since defect states typically act as recombination centers, the slow time constant of the SESAM decreases/increases when the amount of defects increases/decreases. There are different methods to influence the amount of defects.

Typical growth temperatures of the active layer of SESAMs are 200-400^◦C [Kel96].

Hence, by low temperature growth the amount of defect states increases and the electron-hole recombination time decreases [Gup92, Sie96, Hai99, Jas09]. Another method to increase the amount of defects is proton [Gop01] or ion bombardment [Led97, Del98, Hak08], and beryllium doping [Hai99, Pra98, Che98]. Since nitrogen in-corporation causes defects that act as nonradiative recombination centers [Pat04], one can control the electronhole recombination time also by adding a certain amount of ni-trogen [Gui07, Hsu08, Sch12a]. In Fig. 3.6 some reflectivity transients of SESAMs with In_0.3Ga_0.7As (left column) and In_0.3Ga_0.7As_0.985N_0.015 quantum well absorbers (right column) are shown. It is obvious that due to nitrogen incorporation the recombina-tion time is strongly reduced. For example the slow time constant of the un-annealed SESAM with In_0.3Ga_0.7As_0.985N_0.015quantum wells is ten times faster than the slow time constant of the un-annealed sample with In_0.3Ga_0.7As absorbers [Sch12a]. However, the trade-off of these techniques is that an increase of defect states also increases the non-saturable losses [Bro95, Kel99, Sch12a].

Another method to influence the SESAM parameters is post-growth rapid thermal annealing (RTA). Photoluminescence (PL) measurements of semiconductor materials have shown that post-growth annealing causes (i) an improvement of the PL-efficiency [Wan99, Xin00, Li01, Sun04, Liv06, Pak08], which implies a reduction of nonradiative point defects and stress, (ii) a reduction of the spectral linewidth [Wan99, Xin00], (iii) a blue shift of the PL-peak [Sun04, Liv06] mainly due to an increase of the band gap, and (vi) an interdiffusion of the atoms [Alb02].

Hence, annealing manipulates the optical properties of a SESAM and its carrier dy-namics [Sun04, Hak08], in particular the absorber recombination time can be slowed down [Led97]. However, the effect of RTA depends on the absorber material [Sch12a].

The transients in Fig. 3.6 (right column) show that RTA strongly influences the re-combination time of SESAMs with InGaAsN quantum well absorbers. The slow time constant could be increased by a factor of 6 from 20 ps of the un-annealed sample up to 130 ps of the sample annealed at 700^◦C. In contrast, the slow time constant of

0 2 4 6 8 10 12

Figure 3.6: Reflectivity transients of SESAMs with In_0.3Ga_0.7As (left column) and In_0.3Ga_0.7As_0.985N_0.015 (right column) quantum well absorbers. The samples are an-nealed at different temperatures. The arrow marks the direction of increasing pump fluence. The slow time constant for low fluence transients is noted.

3.4 Fabrication and tailoring of SESAMs

SESAMs with InGaAs quantum wells (see Fig. 3.6 (left column)) did not change due to annealing. The recombination time of the un-annealed sample was already more than 200 ps indicating that the structure was grown nearly defect-free.

Another advantageous consequence of RTA is that annealing removes short-lived de-fects and reduces nonsaturable losses [Hak08, Jas09]. All these results show that by defect incorporation and subsequent annealing the carrier dynamics of a saturable ab-sorber can be tailored according to the requirements needed to mode-lock lasers.

Another design related parameter to influence the SESAM are dielectric coatings. As already mentioned the simplest form of the top mirror of a SESAM is the GaAs/air interface. According to Fresnel equation the reflectivity at an interface between two materials with different refractive indexes n₁ and n₂ is R = ((n₁−n₂)/(n₁+n₂))² [Dem04], resulting in a reflectivity of 30 % for GaAs/air. By dielectric coatings the re-flectivity, respectively the field intensity inside the SESAM, can be varied. Antiresonant dielectric coatings reduce the field intensity inside the SESAM compared to the incident intensity, whereas resonant coatings increase it. Since the saturation fluence depends on the field intensity within the absorber structure,F_sat increases/decreases, if the field intensity is reduced/enhanced [Bro95].

Changing the saturation fluence implies changing the modulation depth, too. In [Sar12b] it is shown that the product of saturation fluence and modulation depth is an intrinsic parameter of the absorber material and does not change due to coating processes: F_sat∆R_mod ≈ const. Therefore, if the coating decreases the field intensity by a factor α, the saturation fluence increases by α, whereas the modulation depth decreases by α⁻¹. In the low fluence regime, where TPA and FCA can be neglected, the carrier dynamics are not affected, so that the absorber recovery time remains con-stant independent of the coating of the SESAM [Kra12]. Typical dielectric material compositions are SiO₂/Si₃N₄ [Sar12b], SiO₂/TiO₂ [Bro95], and SiO₂/Ta₂O₅ [Mar08].

The influence of different coatings on the nonlinear reflectivity of SESAMs and their damage thresholds is given in [Sar12b].

Another crucial parameter for SESAM characterization and the SESAM performance within a laser resonator is the roll-over parameterF₂. For a sech²-pulse and a roll-over just due to TPAF₂ can be calculated as

F₂ = τ_P

0.585R

structureβ_TPA(z)n²(z)(|E_n(z)|²)²dz, (3.3) whereβ_TPA(z) is the TPA coefficient of the material, n(z) is the refractive index, E_n(z) is the normalized electric field within the SESAM, and τ_P is the pulse length of the laser pulse [Gra05]. F₂ is direct proportional to τ_P, leading to a stronger roll-over for shorter pulses. However, a smaller F₂ leads to smaller effective modulation depth. For high-power lasers the effective modulation depth should be as high as possible, so that as much of the accessible modulation depth as possible can be used for mode-locking.

Thus, a high F₂-parameter is required.

According to Eq. (3.3) the roll-over parameter is determined by the integration over the complete SESAM structure. This reflects the fact that TPA does not only occur in

the absorber region but also in the spacer layers and even in the Bragg mirror [Gra05].

Since GaAs exhibits a high TPA-coefficient, TPA could be decreased by the reduction of GaAs as a basic material of the SESAM structure. For example GaAs based spacer layers can be exchanged by AlAs spacer layers which exhibit a smaller TPA-coefficient [Gra05]. Coating the SESAM also affects the roll-over and increases F₂, because the TPA coefficient of typical dielectric coatings is smaller than that of GaAs [Sar12b].

For mode-locking high-power lasers a high damage threshold is an indispensable con-dition. As it is also shown in [Sar12b] a highF₂-parameter can lead to a high damage threshold. Therefore, apart from a higher effective modulation depth, a weak roll-over also results in higher damage thresholds. Consequently, coating SESAMs with dielec-tric layers does not only allow for tailoring the saturation fluence and the modulation depth, but can also lead to higher damage thresholds.

In summary, by means of growth condition, post-growth processes, and design issues the parameters of saturable absorber mirrors can be tailored according to the require-ments for mode-locking lasers.

3.5 SESAM requirements for high-power thin-disk