- Email: [email protected]
Semiconductor components for femtosecond semiconductor disk lasers grown by MOVPE
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 5187–5190
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Semiconductor components for femtosecond semiconductor disk lasers grown by MOVPE M. Zorn a,, P. Klopp b, F. Saas b, A. Ginolas a, O. Kru¨ger a, U. Griebner b, M. Weyers a a b
¨ r Ho ¨chstfrequenztechnik (FBH), Gustav-Kirchhoff-Str. 4, D-12489 Berlin, Germany Ferdinand-Braun-Institut fu ¨ r Nichtlineare Optik und Kurzzeitspektroskopie (MBI), Max-Born-Str. 2A, D-12489 Berlin, Germany Max-Born-Institut fu
a r t i c l e in fo
abstract
Available online 11 July 2008
Diode-pumped semiconductor disk lasers (SCDLs), also known as optically-pumped semiconductor vertical-external-cavity surface-emitting lasers (OPS-VECSELs), are promising light sources for achieving high output power in combination with nearly diffraction-limited beam quality as well as for generating short pulses at very high repetition rates. Combining a SCDL gain section with a semiconductor saturable absorber mirror (SAM) and a pump laser diode allows for simple mode-locked all-semiconductor laser designs. The design of these SAM and SCDL gain structures grown by metalorganic vapor phase epitaxy (MOVPE) is presented discussing the different approaches to obtain short pulses. For the SAM structures the common design using an As-implanted and annealed quantum well (QW) was replaced by a structure using a surface-near QW, which caused a significant reduction of the relaxation time. SCDL gain structures with 4–13 QWs and different barrier designs were tested. The shortest pulses were achieved with an asymmetric 4-QW-graded-index barrier design. Pumping this optimized SCDL gain element with an 840 nm laser diode, pulses as short as 290 fs at a repetition rate of 3 GHz and a wavelength of 1036 nm have been obtained. & 2008 Elsevier B.V. All rights reserved.
PACS: 42.55.Px 42.60.By 42.60.Fc 81.05.Ea 81.15.Gh 81.70.Fy Keywords: A3. Metal-organic vapor phase epitaxy B2. Semiconducting III–V materials B3. Semiconductor disk laser B3. Vertical-external-cavity surfaceemitting laser
1. Introduction
2. Experimental procedure
Since the first presentation of an optically-pumped semiconductor vertical-external-cavity surface-emitting laser (OPS-VECSEL) in the late 1990s [1,2], which is herein afterwards referred to as semiconductor disk laser (SCDL) due to the similar concept compared to the solid-state disk laser [3], a lot of work has been done to improve the output power of cw lasers and to reduce the pulse length of short-pulse laser systems based on this idea. A maximum cw output power of 30 W [4] was reported and pulses slightly shorter than 500 fs were obtained [5–7]. For an all-semiconductor setup, the main components of the short-pulse laser are a semiconductor saturable absorbing mirror (SAM) and a SCDL gain element pumped by a high-power laser diode. Compared to laser systems based on ion-doped dielectric media, the all-semiconductor approach has the advantage that emission and absorption wavelengths of the respective elements can be tailored over a wide range. The SAMs and the SCDL gain elements studied here both consist of a distributed Bragg reflector (DBR) mirror with a center wavelength around 1030 nm and a quantum well (QW) section also designed for operation near 1030 nm.
All epitaxial layers were grown in an Aixtron 200/4 metalorganic vapor phase epitaxy (MOVPE) reactor in 3 200 configuration. The sources used were trimethylgallium, trimethylaluminum, trimethylindium, arsine and phosphine. The GaAs (1 0 0) substrates used were off-oriented 21 towards ð1 1¯ 0Þ. For the SAM structure the DBR mirror was grown at 770 1C, while the QW was grown at 510 1C. The SCDL gain elements were grown in reverse order to enable substrate removal for better thermal performance. The active periodic gain structure was grown at 650 1C and followed by the DBR mirror at 700 1C. After epitaxy, the SAMs and SCDL gain elements were soldered on a CuW submount [8]. For the SCDL elements soldered upside down the substrate was removed by wet chemical etching. The elements were finally mounted on a copper heat sink. For pump-probe measurements a mode-locked Nd:glass laser was used, delivering 150-fs pulses at 1060 nm (High-Q, model IC127). Photoluminescence measurements were performed with a semiconductor laser diode emitting at 797 nm (Laser2000, model APM60). Reflectance was measured using a tungsten white-light source. Dispersion was measured using a white-light interferometer. Complete laser systems were build, operating at around 1030 nm emission wavelength, for pulse durations in the sub-picosecond range with GHz repetition rates (details are given in Ref. [9]).
Corresponding author. Tel.: +49 30 6392 2676; fax: +49 30 6392 2685.
E-mail address: [email protected] (M. Zorn). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.017
ARTICLE IN PRESS 5188
M. Zorn et al. / Journal of Crystal Growth 310 (2008) 5187–5190
3. SAM structures
4. SCDL gain structures
MOVPE-grown SAM structures for this wavelength range usually consist of a single InGaAs QW embedded in the center of a l/2 GaAs layer grown on an AlAs/(Al)GaAs DBR mirror as shown in Fig. 1a with the indium content in the QW defining the operation wavelength [10]. A SiNx (or comparable) antireflection (AR) coating is usually applied. To achieve very short relaxation times of the carriers in the QW, the structures are implanted with arsenic (As) ions [11] creating As-clusters and point defects. Afterwards, the samples are annealed for 20 min at 600 1C under arsenic overpressure to reduce lattice deformation. The recombination time is strongly reduced mainly by the formation of point defects [12]. A small blue-shift of the excitonic peak wavelength is also observed [11,12]. A different approach is to grow (In)GaAs layers at very low temperatures in molecular beam epitaxy (MBE), which leads to As-clusters also strongly reducing the carrier lifetime [13]. An alternative approach is shown in Fig. 1b, where the SAM structure employs a surface-near QW without implantation to achieve short relaxation times via surface states. The thickness of the GaAs cap was varied, and compared to absorbers with a 5 nm GaAs cap, those with a 2 nm GaAs cap showed a faster dominant relaxation time, i.e. only about 1 ps against 5 ps (Table 1), and were therefore selected for the laser experiments. The structures are additionally capped with a SiNx coating. Two different SiNx thicknesses were tested, resulting in a non-resonant and a resonant design. Best results with respect to short-pulse length have been obtained using the non-resonant design. The two different structures (As-implanted and surface-near QW) were investigated using a pump-probe setup. Fig. 2 shows the delay of the As-implanted structure (blue open squares) and that of the surface-near QW (green full circles). After a first similar decay, the As-implanted SAM showed a slower relaxation. These measurements were fitted using the following double exponential decay:
Similar to the SAM structures the SCDL gain chips also consist of an AlAs/GaAs DBR mirror followed by an active region. The active structure consists of InGaAs QWs and (Al)GaAs barriers. Free carriers are generated by the pump light absorbed in the barriers and drift into the QWs, where they recombine. To increase the gain, the QWs are placed in the antinodes of the electric field pattern of the infrared laser light. SCDL gain structures have been developed using different barriers and QW numbers (4–13 QWs). We have investigated two different barrier configurations: the simpler one uses GaAs barriers and is denoted in the following as step-index structure (STIN, Ref. [17]). The second barrier design uses graded AlxGa1xAs layers having an aluminum content of x ¼ 0 at the QW side and of x ¼ 0.2 at the AlGaAsP strain compensation layer (noted here as GRIN structure,
y ¼ y0 þ A1 ex=t1 þ A2 ex=t2
Table 1 Results of the pump-probe SAM characterization SAM structure
t1 (ps) t2 (ps) Modulation depth (%)
Saturation fluence (mJ/cm2)
As-implanted, annealed QW Surface-near QW (2 nm cap) Surface-near QW (5 nm cap)
0.3 0.3 0.3
20 10 –
3.4 1.0 5.4
0.5 0.5 –
The modulation depth and the saturation fluence were determined for the first two samples only.
(1)
Table 1 compares the fit results together with investigations of the nonlinear reflectivity [14]. The As-implanted SAM showed a relaxation time t2 of 3.4 ps, while the near-surface SAM had a significantly reduced relaxation time t2 of 1 ps. Additionally, the near-surface SAM showed a reduced saturation fluence. With this SAM structure, no additional process steps such as implantation and annealing were needed. In conclusion, the SAM structure applying a surface-near SAM is a promising approach for an easily producible device. Both SAM types have been successfully applied also in Yb-based femtosecond lasers [15,16].
Fig. 2. Pump-probe measurements of an As-implanted and annealed QW (blue open squares) and a surface-near-QW (green full circles) SAM structure.
Fig. 1. Different concepts for SAM structures: (a) As-implanted and annealed QW and (b) surface-near QW both placed on a DBR mirror.
ARTICLE IN PRESS M. Zorn et al. / Journal of Crystal Growth 310 (2008) 5187–5190
Ref. [17]). Strain compensating layers were introduced, since they were found to be crucial due to the high strain incorporated by the InGaAs QWs [18]. The mounting of the SCDL gain element is sophisticated, because an efficient heat removal is needed for efficient operation. Since the main heat resistance is given by the thick GaAs substrate, the substrate is removed as described above. Finally, the mounted chips were AR coated. Fig. 3 shows the reflectance measurement of a SCDL gain chip before AR coating (left axis). A reflectance stop band centered around 1030 nm is observed, since the element was designed to serve as a mirror of the laser cavity. Due to the missing AR coating, a resonance is formed within the gain structure resulting in the reflectance decrease at 1026 nm. The corresponding photoluminescence spectrum is also shown in Fig. 3 (right axis). Comparing the spectra measured with and without AR coating, the influence by the cavity resonance effect is clearly visible. Without AR coating, the peak is shifted towards the resonance wavelength and a second small peak appears at 970 nm near the stop band edge. Only after AR coating the undisturbed PL emission can be measured, since the resonance has practically disappeared due to the minimized reflectance at the SCDL/air interface.
Fig. 3. Measured reflectance (left axis) and photoluminescence (right axis) of a SCDL gain structure with 6 QWs and GaAs barriers (STIN structure). The photoluminescence is additionally measured after AR coating (dashed line).
5189
Fig. 5. Design of the 4 QW-GRIN structure with irregular QW distribution.
The AR coating has also a strong influence on the group delay dispersion (GDD) of the gain element as shown in Fig. 4, where an uncoated and a coated structure are compared. We obtained short-pulse operation in the below-ps range only with AR-coated SCDL gain chips. In the uncoated case, the element shows the spectral-phase characteristics of a Gires–Tournois interferometer (GTI) (green solid line in Fig. 4). For femtosecond SCDLs, the structure thickness and hence the spectral positions of the GTI resonances are chosen such that the GTI is operated in the vicinity of zero dispersion. However, these positions depend on the operation temperature, as can also be seen from Fig. 4. Furthermore, the dispersion varies considerably over the spectral width of a short pulse, and there are also dispersion dynamics due to the interplay of GTI resonance and the gain or absorption spectrum changed by the pulse. To a smaller degree the above mentioned findings are also valid for the SAM structures used here, since the GDD is proportional to the square of the thickness of the structure [19]. With AR coating, the measured dispersion is low for all wavelengths, i.e. always below the resolution limit of our white-light interferometer (blue solid line in Fig. 4). Nevertheless, we can expect a thin structure with 4 QWs to have lower residual dispersion compared to a thicker one with, e.g. 6 or 13 QWs, which motivated the development of the 4-QW gain medium for shortest pulses. Additionally, in the optimized structure the 4 QWs were not placed regularly (i.e. one QW per antinode of the electric field), but we used a double QW in the first antinode and omitted the third period as displayed in Fig. 5. Thereby, when using 840 nm pump light, the excitation of the 4 QWs is inhomogeneous, resulting in a broadening of the gain spectrum, which supports shorter pulses.
5. Complete laser system
Fig. 4. Group delay dispersion (GDD) of a 4 QW GRIN-barrier gain structure. An AR-coated sample (blue solid line) is compared with an uncoated sample made from the same wafer (green solid line). The latter shows a significant temperature dependence as indicated by the other curves.
First investigations started with 13 QWs in a GRIN gain section. With these structures frequency-doubled picosecond pulses at 489 nm have been achieved with a repetition rate of 1.88 GHz [20]. A systematic comparison of the two barrier designs, GRIN and STIN, was done with 6 QWs in a linear continuous-wave oscillator [17]. The GRIN design exhibited a better performance with respect to output power, i.e. no roll-off at higher pump powers. We refer this to a reduction of the free-carrier concentration in the barriers by an enhanced carrier drift towards the QWs due to the graded bandgaps. The all-semiconductor setup for generating ultra-short pulses consisting of the SAM, the SCDL gain element and a pump diode is described in detail in Ref. [9]. The SAM had a surface-near QW as described above. Pulses with a duration of 590 fs were obtained using the 6-QW-STIN structure at an emission wavelength of 1040 nm [10]. Switching to the 4-QW-GRIN design with two single QWs and a double QW pulses as short as 290 fs at a repetition rate of 3 GHz were achieved. The SCDL emission wavelength was 1036 nm, with optical pumping at 840 nm. To the best of our
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M. Zorn et al. / Journal of Crystal Growth 310 (2008) 5187–5190
knowledge, this pulse duration is the shortest observed from a semiconductor laser oscillator.
6. Summary The design and fabrication process of the two main semiconductor components for short-pulse SCDLs, SAM and SCDL gain structure, have been optimized. The usually As-implanted and annealed QW in the SAM structure was replaced by a surface-near QW resulting in a reduced dominant relaxation time of 1 ps. Different designs of the active SCDL gain section were tested using different barrier designs and QW numbers. An AR coating was found to be important to decrease the pulse length by reducing the dispersion of the gain element. With an asymmetric 4-QW-graded-barrier gain element an all-semiconductor laser system emitting pulses as short as 290 fs at a repetition rate of 3 GHz and a wavelength of 1036 nm was realized.
Acknowledgments H. Lawrenz is acknowledged for performing the etch process and the reflectance and photoluminescence measurements. R. Olschewsky is acknowledged for mounting the chips. The AR coating of the SCDL gain chips was done by Berliner Glas KGaA. Furthermore, the authors like to thank T. Roos and his team for excellent mechanical support. This work was funded by the German Bundesministerium fu¨r Bildung und Forschung (BMBF) under Grant no. 13 N 8569 and the EU within the Berlin ProFIT program under Grant no. 10135476.
References [1] J.V. Sadunsky, S.R.J. Brueck, IEEE Photonics Technol. Lett. 8 (1996) 313. [2] M. Kuznetsov, F. Hakimi, R. Sprague, A. Mooradian, IEEE J. Sel. Top. Quantum Electron. 5 (1999) 561. [3] A. Giesen, H. Hu¨gel, A. Voss, K. Wittig, U. Brauch, H. Opower, Appl. Phys. B 58 (1994) 365. [4] J. Chilla, S. Butterworth, A. Zeitschel, J. Charles, A. Capara, M. Reed, L. Spinelli, Proc. SPIE 5332 (2004) 143. [5] A. Garnache, S. Hoogland, A.C. Tropper, I. Sagnes, G. Saint-Girons, J.S. Roberts, Appl. Phys. Lett. 80 (2002) 3892. [6] S. Hoogland, A. Garnache, I. Sagnes, J.S. Roberts, A.C. Tropper, IEEE Photonics Technol. Lett. 17 (2005) 267. [7] U. Keller, A.C. Tropper, Phys. Rep. 429 (2006) 67. [8] U. Zeimer, J. Grenzer, D. Korn, S. Do¨ring, M. Zorn, W. Pittroff, U. Pietsch, F. Saas, M. Weyers, Phys. Status. Solidi. (a) 204 (2007) 2753. [9] P. Klopp, F. Saas, M. Zorn, M. Weyers, U. Griebner, Opt. Express 16 (2008) 5770. [10] F. Saas, G. Steinmeyer, U. Griebner, M. Zorn, M. Weyers, Appl. Phys. Lett. 89 (2006) 141107. [11] M.J. Lederer, V. Kolev, B. Luther-Davies, H.H. Tan, C. Jagadish, J. Phys. D: Appl. Phys. 34 (2001) 2455. [12] J.W. Tomm, V. Strelchuk, A. Gerhardt, U. Zeimer, M. Zorn, H. Kissel, M. Weyers, J. Jimenez, J. Appl. Phys. 95 (2004) 1122. [13] E.S. Harmon, M.R. Melloch, J.M. Woodall, D.D. Nolte, N. Otsuka, C.L. Chang, Appl. Phys. Lett. 63 (1993) 2248. [14] F. Saas, PhD thesis, Humboldt-Universita¨t Berlin, 2008. [15] U. Griebner, S. Rivier, V. Petrov, M. Zorn, G. Erbert, M. Weyers, X. Mateos, M. Aguilo´, J. Massons, F. Dı´az, Opt. Express 13 (2005) 3465. [16] S. Rivier, X. Mateos, J. Liu, V. Petrov, U. Griebner, M. Zorn, M. Weyers, H. Zhang, J. Wang, M. Jiang, Opt. Express 14 (2006) 11668. [17] F. Saas, V. Talalaev, U. Griebner, J.W. Tomm, M. Zorn, A. Knigge, M. Weyers, Appl. Phys. Lett. 89 (2006) 151120. [18] M. Zorn, T.K. Tien, J.W. Tomm, H. Kissel, U. Zeimer, F. Saas, U. Griebner, M. Weyers, Proc. 11th European Workshop on Metalorganic Vapour Phase Epitaxy, June 2005, Lausanne, Switzerland, F09, p. 309. [19] M. Moenster, U. Griebner, W. Richter, G. Steinmeyer, IEEE J. Quantum Electron. 43 (2007) 174. [20] O. Casel, D. Woll, M.A. Tremont, H. Fuchs, R. Wallenstein, E. Gerster, P. Unger, M. Zorn, M. Weyers, Appl. Phys. B 81 (2005) 443.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Semiconductor components for femtosecond semiconductor disk lasers grown by MOVPE M. Zorn a,, P. Klopp b, F. Saas b, A. Ginolas a, O. Kru¨ger a, U. Griebner b, M. Weyers a a b
¨ r Ho ¨chstfrequenztechnik (FBH), Gustav-Kirchhoff-Str. 4, D-12489 Berlin, Germany Ferdinand-Braun-Institut fu ¨ r Nichtlineare Optik und Kurzzeitspektroskopie (MBI), Max-Born-Str. 2A, D-12489 Berlin, Germany Max-Born-Institut fu
a r t i c l e in fo
abstract
Available online 11 July 2008
Diode-pumped semiconductor disk lasers (SCDLs), also known as optically-pumped semiconductor vertical-external-cavity surface-emitting lasers (OPS-VECSELs), are promising light sources for achieving high output power in combination with nearly diffraction-limited beam quality as well as for generating short pulses at very high repetition rates. Combining a SCDL gain section with a semiconductor saturable absorber mirror (SAM) and a pump laser diode allows for simple mode-locked all-semiconductor laser designs. The design of these SAM and SCDL gain structures grown by metalorganic vapor phase epitaxy (MOVPE) is presented discussing the different approaches to obtain short pulses. For the SAM structures the common design using an As-implanted and annealed quantum well (QW) was replaced by a structure using a surface-near QW, which caused a significant reduction of the relaxation time. SCDL gain structures with 4–13 QWs and different barrier designs were tested. The shortest pulses were achieved with an asymmetric 4-QW-graded-index barrier design. Pumping this optimized SCDL gain element with an 840 nm laser diode, pulses as short as 290 fs at a repetition rate of 3 GHz and a wavelength of 1036 nm have been obtained. & 2008 Elsevier B.V. All rights reserved.
PACS: 42.55.Px 42.60.By 42.60.Fc 81.05.Ea 81.15.Gh 81.70.Fy Keywords: A3. Metal-organic vapor phase epitaxy B2. Semiconducting III–V materials B3. Semiconductor disk laser B3. Vertical-external-cavity surfaceemitting laser
1. Introduction
2. Experimental procedure
Since the first presentation of an optically-pumped semiconductor vertical-external-cavity surface-emitting laser (OPS-VECSEL) in the late 1990s [1,2], which is herein afterwards referred to as semiconductor disk laser (SCDL) due to the similar concept compared to the solid-state disk laser [3], a lot of work has been done to improve the output power of cw lasers and to reduce the pulse length of short-pulse laser systems based on this idea. A maximum cw output power of 30 W [4] was reported and pulses slightly shorter than 500 fs were obtained [5–7]. For an all-semiconductor setup, the main components of the short-pulse laser are a semiconductor saturable absorbing mirror (SAM) and a SCDL gain element pumped by a high-power laser diode. Compared to laser systems based on ion-doped dielectric media, the all-semiconductor approach has the advantage that emission and absorption wavelengths of the respective elements can be tailored over a wide range. The SAMs and the SCDL gain elements studied here both consist of a distributed Bragg reflector (DBR) mirror with a center wavelength around 1030 nm and a quantum well (QW) section also designed for operation near 1030 nm.
All epitaxial layers were grown in an Aixtron 200/4 metalorganic vapor phase epitaxy (MOVPE) reactor in 3 200 configuration. The sources used were trimethylgallium, trimethylaluminum, trimethylindium, arsine and phosphine. The GaAs (1 0 0) substrates used were off-oriented 21 towards ð1 1¯ 0Þ. For the SAM structure the DBR mirror was grown at 770 1C, while the QW was grown at 510 1C. The SCDL gain elements were grown in reverse order to enable substrate removal for better thermal performance. The active periodic gain structure was grown at 650 1C and followed by the DBR mirror at 700 1C. After epitaxy, the SAMs and SCDL gain elements were soldered on a CuW submount [8]. For the SCDL elements soldered upside down the substrate was removed by wet chemical etching. The elements were finally mounted on a copper heat sink. For pump-probe measurements a mode-locked Nd:glass laser was used, delivering 150-fs pulses at 1060 nm (High-Q, model IC127). Photoluminescence measurements were performed with a semiconductor laser diode emitting at 797 nm (Laser2000, model APM60). Reflectance was measured using a tungsten white-light source. Dispersion was measured using a white-light interferometer. Complete laser systems were build, operating at around 1030 nm emission wavelength, for pulse durations in the sub-picosecond range with GHz repetition rates (details are given in Ref. [9]).
Corresponding author. Tel.: +49 30 6392 2676; fax: +49 30 6392 2685.
E-mail address: [email protected] (M. Zorn). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.017
ARTICLE IN PRESS 5188
M. Zorn et al. / Journal of Crystal Growth 310 (2008) 5187–5190
3. SAM structures
4. SCDL gain structures
MOVPE-grown SAM structures for this wavelength range usually consist of a single InGaAs QW embedded in the center of a l/2 GaAs layer grown on an AlAs/(Al)GaAs DBR mirror as shown in Fig. 1a with the indium content in the QW defining the operation wavelength [10]. A SiNx (or comparable) antireflection (AR) coating is usually applied. To achieve very short relaxation times of the carriers in the QW, the structures are implanted with arsenic (As) ions [11] creating As-clusters and point defects. Afterwards, the samples are annealed for 20 min at 600 1C under arsenic overpressure to reduce lattice deformation. The recombination time is strongly reduced mainly by the formation of point defects [12]. A small blue-shift of the excitonic peak wavelength is also observed [11,12]. A different approach is to grow (In)GaAs layers at very low temperatures in molecular beam epitaxy (MBE), which leads to As-clusters also strongly reducing the carrier lifetime [13]. An alternative approach is shown in Fig. 1b, where the SAM structure employs a surface-near QW without implantation to achieve short relaxation times via surface states. The thickness of the GaAs cap was varied, and compared to absorbers with a 5 nm GaAs cap, those with a 2 nm GaAs cap showed a faster dominant relaxation time, i.e. only about 1 ps against 5 ps (Table 1), and were therefore selected for the laser experiments. The structures are additionally capped with a SiNx coating. Two different SiNx thicknesses were tested, resulting in a non-resonant and a resonant design. Best results with respect to short-pulse length have been obtained using the non-resonant design. The two different structures (As-implanted and surface-near QW) were investigated using a pump-probe setup. Fig. 2 shows the delay of the As-implanted structure (blue open squares) and that of the surface-near QW (green full circles). After a first similar decay, the As-implanted SAM showed a slower relaxation. These measurements were fitted using the following double exponential decay:
Similar to the SAM structures the SCDL gain chips also consist of an AlAs/GaAs DBR mirror followed by an active region. The active structure consists of InGaAs QWs and (Al)GaAs barriers. Free carriers are generated by the pump light absorbed in the barriers and drift into the QWs, where they recombine. To increase the gain, the QWs are placed in the antinodes of the electric field pattern of the infrared laser light. SCDL gain structures have been developed using different barriers and QW numbers (4–13 QWs). We have investigated two different barrier configurations: the simpler one uses GaAs barriers and is denoted in the following as step-index structure (STIN, Ref. [17]). The second barrier design uses graded AlxGa1xAs layers having an aluminum content of x ¼ 0 at the QW side and of x ¼ 0.2 at the AlGaAsP strain compensation layer (noted here as GRIN structure,
y ¼ y0 þ A1 ex=t1 þ A2 ex=t2
Table 1 Results of the pump-probe SAM characterization SAM structure
t1 (ps) t2 (ps) Modulation depth (%)
Saturation fluence (mJ/cm2)
As-implanted, annealed QW Surface-near QW (2 nm cap) Surface-near QW (5 nm cap)
0.3 0.3 0.3
20 10 –
3.4 1.0 5.4
0.5 0.5 –
The modulation depth and the saturation fluence were determined for the first two samples only.
(1)
Table 1 compares the fit results together with investigations of the nonlinear reflectivity [14]. The As-implanted SAM showed a relaxation time t2 of 3.4 ps, while the near-surface SAM had a significantly reduced relaxation time t2 of 1 ps. Additionally, the near-surface SAM showed a reduced saturation fluence. With this SAM structure, no additional process steps such as implantation and annealing were needed. In conclusion, the SAM structure applying a surface-near SAM is a promising approach for an easily producible device. Both SAM types have been successfully applied also in Yb-based femtosecond lasers [15,16].
Fig. 2. Pump-probe measurements of an As-implanted and annealed QW (blue open squares) and a surface-near-QW (green full circles) SAM structure.
Fig. 1. Different concepts for SAM structures: (a) As-implanted and annealed QW and (b) surface-near QW both placed on a DBR mirror.
ARTICLE IN PRESS M. Zorn et al. / Journal of Crystal Growth 310 (2008) 5187–5190
Ref. [17]). Strain compensating layers were introduced, since they were found to be crucial due to the high strain incorporated by the InGaAs QWs [18]. The mounting of the SCDL gain element is sophisticated, because an efficient heat removal is needed for efficient operation. Since the main heat resistance is given by the thick GaAs substrate, the substrate is removed as described above. Finally, the mounted chips were AR coated. Fig. 3 shows the reflectance measurement of a SCDL gain chip before AR coating (left axis). A reflectance stop band centered around 1030 nm is observed, since the element was designed to serve as a mirror of the laser cavity. Due to the missing AR coating, a resonance is formed within the gain structure resulting in the reflectance decrease at 1026 nm. The corresponding photoluminescence spectrum is also shown in Fig. 3 (right axis). Comparing the spectra measured with and without AR coating, the influence by the cavity resonance effect is clearly visible. Without AR coating, the peak is shifted towards the resonance wavelength and a second small peak appears at 970 nm near the stop band edge. Only after AR coating the undisturbed PL emission can be measured, since the resonance has practically disappeared due to the minimized reflectance at the SCDL/air interface.
Fig. 3. Measured reflectance (left axis) and photoluminescence (right axis) of a SCDL gain structure with 6 QWs and GaAs barriers (STIN structure). The photoluminescence is additionally measured after AR coating (dashed line).
5189
Fig. 5. Design of the 4 QW-GRIN structure with irregular QW distribution.
The AR coating has also a strong influence on the group delay dispersion (GDD) of the gain element as shown in Fig. 4, where an uncoated and a coated structure are compared. We obtained short-pulse operation in the below-ps range only with AR-coated SCDL gain chips. In the uncoated case, the element shows the spectral-phase characteristics of a Gires–Tournois interferometer (GTI) (green solid line in Fig. 4). For femtosecond SCDLs, the structure thickness and hence the spectral positions of the GTI resonances are chosen such that the GTI is operated in the vicinity of zero dispersion. However, these positions depend on the operation temperature, as can also be seen from Fig. 4. Furthermore, the dispersion varies considerably over the spectral width of a short pulse, and there are also dispersion dynamics due to the interplay of GTI resonance and the gain or absorption spectrum changed by the pulse. To a smaller degree the above mentioned findings are also valid for the SAM structures used here, since the GDD is proportional to the square of the thickness of the structure [19]. With AR coating, the measured dispersion is low for all wavelengths, i.e. always below the resolution limit of our white-light interferometer (blue solid line in Fig. 4). Nevertheless, we can expect a thin structure with 4 QWs to have lower residual dispersion compared to a thicker one with, e.g. 6 or 13 QWs, which motivated the development of the 4-QW gain medium for shortest pulses. Additionally, in the optimized structure the 4 QWs were not placed regularly (i.e. one QW per antinode of the electric field), but we used a double QW in the first antinode and omitted the third period as displayed in Fig. 5. Thereby, when using 840 nm pump light, the excitation of the 4 QWs is inhomogeneous, resulting in a broadening of the gain spectrum, which supports shorter pulses.
5. Complete laser system
Fig. 4. Group delay dispersion (GDD) of a 4 QW GRIN-barrier gain structure. An AR-coated sample (blue solid line) is compared with an uncoated sample made from the same wafer (green solid line). The latter shows a significant temperature dependence as indicated by the other curves.
First investigations started with 13 QWs in a GRIN gain section. With these structures frequency-doubled picosecond pulses at 489 nm have been achieved with a repetition rate of 1.88 GHz [20]. A systematic comparison of the two barrier designs, GRIN and STIN, was done with 6 QWs in a linear continuous-wave oscillator [17]. The GRIN design exhibited a better performance with respect to output power, i.e. no roll-off at higher pump powers. We refer this to a reduction of the free-carrier concentration in the barriers by an enhanced carrier drift towards the QWs due to the graded bandgaps. The all-semiconductor setup for generating ultra-short pulses consisting of the SAM, the SCDL gain element and a pump diode is described in detail in Ref. [9]. The SAM had a surface-near QW as described above. Pulses with a duration of 590 fs were obtained using the 6-QW-STIN structure at an emission wavelength of 1040 nm [10]. Switching to the 4-QW-GRIN design with two single QWs and a double QW pulses as short as 290 fs at a repetition rate of 3 GHz were achieved. The SCDL emission wavelength was 1036 nm, with optical pumping at 840 nm. To the best of our
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knowledge, this pulse duration is the shortest observed from a semiconductor laser oscillator.
6. Summary The design and fabrication process of the two main semiconductor components for short-pulse SCDLs, SAM and SCDL gain structure, have been optimized. The usually As-implanted and annealed QW in the SAM structure was replaced by a surface-near QW resulting in a reduced dominant relaxation time of 1 ps. Different designs of the active SCDL gain section were tested using different barrier designs and QW numbers. An AR coating was found to be important to decrease the pulse length by reducing the dispersion of the gain element. With an asymmetric 4-QW-graded-barrier gain element an all-semiconductor laser system emitting pulses as short as 290 fs at a repetition rate of 3 GHz and a wavelength of 1036 nm was realized.
Acknowledgments H. Lawrenz is acknowledged for performing the etch process and the reflectance and photoluminescence measurements. R. Olschewsky is acknowledged for mounting the chips. The AR coating of the SCDL gain chips was done by Berliner Glas KGaA. Furthermore, the authors like to thank T. Roos and his team for excellent mechanical support. This work was funded by the German Bundesministerium fu¨r Bildung und Forschung (BMBF) under Grant no. 13 N 8569 and the EU within the Berlin ProFIT program under Grant no. 10135476.
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