THEIR sophisticated structure make vertical-cavity surface-emitting

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 12, DECEMBER 2007 1227 Spatiotemporal Turn-On Dynamics of Grating Relief VCSELs Christian Fuchs, Tobias Gensty, Pierluigi Debernardi, Gian Paolo Bava, Johannes Michael Ostermann, Rainer Michalzik, Asa Haglund, Anders Larsson, and Wolfgang Elsäßer, Senior Member, IEEE Abstract Within a joint collaboration between modeling, technology and experiments, we investigate the polarization-resolved spatial emission and turn-on dynamics of oxide-confined vertical-cavity surface-emitting lasers with an integrated surface-relief grating. By applying a time-resolved imaging technique we demonstrate that the introduced high dichroism also maintains its influence dynamically. This leads to highly polarization-stable spatially fundamental Gaussian mode emission on 100-ps timescale. Finally, the achieved progress, but also the limits of this promising stabilization scheme are discussed. Index Terms Semiconductor laser, spatiotemporal dynamics, spectra and polarization, vertical-cavity surface-emitting lasers (VCSELs). I. INTRODUCTION THEIR sophisticated structure make vertical-cavity surface-emitting lasers (VCSELs) at the same time a very interesting subject of actual basic [1] [4] and application-oriented research [5], [6]. The manifold interactions between optical field and semiconductor material result in spatiotemporal phenomena [7] [11], a complex polarization behavior due to their circular symmetric structure [4], [12], [13], including a particular interesting behavior under optical feedback and quite unique and sophisticated noise properties. Among all those phenomena, the concurrence of both polarization [14] and mode stabilization [15] are of particular interest in many applications, such as, e.g., optical data communication [6], [16], [17] and sensing [18]. Here, by our fast imaging investigations we address the question, if polarization suppression and modal selection can be obtained simultaneously also on Manuscript received June 14, 2007; revised August 17, 2007. Part of this work originates from a collaboration within COST 288 ( Ultrafast and nanoscale photonics ) supported in part by the European Community and the European Science Foundation. The work from Darmstadt-Torino was supported in part by a German-Italian VIGONI project (DAAD-CRUI). The work of A. Haglund was supported in part by a VISTA EU project, on a short term mission to Ulm University, Ulm, Germany, under J. Mähnß. C. Fuchs and T. Gensty were with Darmstadt University of Technology, 64289 Darmstadt, Germany. W. Elsäßer is with the Institute of Applied Physics, Darmstadt University of Technology, 64289 Darmstadt, Germany (e-mail: elsaesser@physik.tu-darmstadt.de). P. Debernardi and G. P. Bava are with Istituto Elettronica e Ingegneria Informazione Telecomunicazioni (IEIIT-CNR), Politecnico di Torino, 10129 Torino, Italy. J. M. Ostermann and R. Michalzik are with the Institute of Optoelectronics, Ulm University, 89069 Ulm, Germany. A. Haglund and A. Larsson are with the Photonics Laboratory, Micro-technology and Nanoscience, Chalmers University of Technology, 41296 Göteborg, Sweden. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2007.908118 Fig. 1. Schematic depiction of the grating relief. short timescales. Therefore, we investigate the spatiotemporal dynamics with particular emphasis on polarization control in the nano- and subnanosecond turn-on regime of VCSELs with a surface-integrated grating relief [19] [23]. We study the systematic influence of the grating on the emission behavior of VCSELs with different grating parameters and oxide aperture diameters. We apply here for the first time repetitive and single-shot detection techniques with high spatial and temporal resolution (up to 50 ps) to a grating relief VCSEL, different from [24] where the dynamical behavior of grating VCSELs was studied by means of a time trace analysis. By polarization-resolved near field images we demonstrate the achievement of a polarization stable turn-on dynamics of the fundamental mode on a 50-ps timescale and give further evidence on the successful applicability of the grating relief stabilization concept for even larger aperture VCSELs. The contribution is structured as follows: After this short introduction we describe briefly the investigated VCSEL structure, followed by the experimental setup. Next, we investigate the influence of the various grating parameters on the polarization and spectral mode suppression under continuous wave (CW) conditions. Then, we depict the repetitive and nonrepetitive contributions in the spatial and polarization-resolved turn-on dynamics for selected lasers. We close with a summary and an outlook. II. LASER STRUCTURES AND EXPERIMENTAL SETUP In the past, there has been a large variety of proposals to select and to stabilize the polarization of the VCSEL s emitted light, as anisotropic gain [25], noncylindrical resonators [26], [27], integrated polarization-selective mirrors [22], [28], and external polarization-selective feedback [29]. Here, we pursue an approach based on a grating etched into a quarter-wavelength thicker cap-layer on top of the upper laser Bragg mirror (the so called inverted design [30]). Limiting the geometrical extension of the grating onto a size smaller than the active area, usually defined via the current-confining diameter of the oxide aperture, results in an inverted grating relief concept [18]. Here, we shall demonstrate the successful transversal and polarization 0018-9197/$25.00 2007 IEEE

1228 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 12, DECEMBER 2007 Fig. 2. Typical P-I characteristics (left scale) and OPSR (right scale) for a standard (left) and a grating relief VCSEL (right); 4-m oxide aperture, 2.5-m diameter grating relief grating, with 0.7-m period and 55nm etch depth. mode control via the grating relief concept implemented simultaneously in a one-step technology process [20], [21]. For this purpose, we investigate standard AlGaAs-based VCSEL structures, comprising three quantum wells embedded into a -cavity which is sandwiched between a 34-pair n-doped bottom Bragg mirror and a 22-pair p-doped output mirror. The current and mode confinement is achieved by an oxide aperture with diameters ranging from 3 to 7 m and we investigate inverted gratings with periods from 0.6 to 1.1 m in steps of 0.1 m and etch depth ranging from 23 to 85 nm. Fig. 1 illustrates schematically the grating relief parameters. All investigated VCSELs with their different parameters have been realized on one chip which has been mounted temperature-stabilized with individual laser access via a contact needle. The improved modal performance is analyzed by comparison with standard VCSELs of the same oxide diameters and with a homogeneous etching of the whole outcoupling area to the same depth as the adjacent grating relief device. Therefore, prior to the time-resolved investigations, a series of VCSELs with constant oxide aperture diameter of 4 m has been characterized with respect to their static polarization mode and spectral mode suppression to find the optimum set of parameters of the grating relief. A comparison to the standard reference VCSELs shows the significantly improved static emission properties. An example of polarization resolved curves for a reference and a grating-relief device are shown in Fig. 2, where the beneficial effects of the grating relief on both polarization and transverse mode operation is obvious. In the graphs also the orthogonal polarization suppression ratio (OPSR) is depicted which is defined as, where and are the intensities of the polarizations parallel (0 ) and orthogonal (90 ) to the grating. The desired high time resolution and the full 2-D spatial resolution have been achieved by the TRIDA method (temporally resolved imaging by differential analysis) which allows to investigate the repetitive dynamics of the near field in the 50-ps time domain [31], [32]. The nonrepetitive parts in the dynamics have been investigated by a fast gated single-shot intensified charge coupled device (ICCD) camera. These single shot pictures complement the TRIDA technique. The combination of both experimental methods allows a comprehensive picture of the repetitive and nonrepetitive turn-on emission dynamics, in particular with respect to the polarization. III. EMISSION PROPERTIES UNDER CW-OPERATION Here, we investigate the influence of the grating relief on the polarization behavior under continuous-wave (CW) operation and compare them with modeling results. The narrow 4.0- m oxide aperture enables to limit the number of transverse modes. First, we investigate a grating with a relief diameter of 4.5 m, which therefore covers a little bit more than the whole active area, i.e., it provides a polarization-dependent reflectivity for the fundamental mode as well as for the higher order modes. Then, a smaller relief diameter of 2.5 m is investigated, which by covering only a smaller central area counteracts the occurrence of higher transverse modes due to the reduced mirror reflectivity outside of the grating relief. For both grating reliefs, we measured polarization-resolved light power current ( - ) characteristics and determined the orthogonal polarization suppression ratio (OSPR) for a large variety of grating parameters. Fig. 3 shows a survey of the OPSR for VCSELs with 4.0- m oxide aperture diameter and 4.5- m relief diameter for different etch depths and grating periods. The OPSR given here is computed as an average of current dependent OPSR (see Fig. 2) between 10% and 100% of maximum optical power. The different trends and regimes can be summarized as follows: VCSELs with a small etch depth of only 23 nm exhibit only a weak polarization control. The major polarization is randomly oriented either perpendicular (blue) or parallel to the grating (red). The rows with 39 and 55 nm exhibit high values for the OPSR for small grating periods (0.6 to 0.9 m). The 70-nm VCSELs have a weaker OPSR for intermediate grating periods, whereas the 85-nm row shows again an improvement. The whole regime of the smaller grating periods exhibits an emission which is polarized perpendicular to the grating. Grating periods of 1.0 and 1.1 m have a tendency to emit polarized parallel to the grating, with the 1.1- mrow having a high OPSR. The 23-nm row VCSELs exhibit in general a smaller OPSR due to the lower dichroism. One VCSEL in the lower right corner (85 nm grating depth and 1.1- m grating period) is special due to a technology lack resulting in a too high oxide aperture diameter. In summary, the surface-grating relief concept enables a superior OPSR over a large range of pump currents and a large regime of grating parameters. The observed OPSR value can now be compared with theoretical modeling calculation of the dichroism. The relative dichroism [14], where and are the

FUCHS et al.: SPATIOTEMPORAL TURN-ON DYNAMICS OF GRATING RELIEF VCSELs 1229 Fig. 3. OPSR for a 4.0-m oxide aperture diameter VCSEL with a 4.5-m diameter relief grating and different grating parameters. Each square represents one laser on the chip. Blue colors mean polarization perpendicular to the grating lines, red parallel. The color saturation intensity encodes the polarization suppression. The varying colors denote a decreasing OPSR from, e.g., +20 db to +15 db with increasing current. Fig. 5. OPSR for a 4.0 m oxide aperture diameter VCSEL with a 2.5 m diameter relief grating and different grating parameters. Blue colors mean polarization perpendicular to the grating lines, red parallel. The color saturation intensity encodes the polarization suppression. The varying colors denote a decreasing OPSR from, e.g., larger than 5 db to smaller than 5 db with increasing current. Fig. 4. Calculated dichroism as a function of etching depth for the VCSELs with 4.0-m oxide aperture and 4.5-m relief diameter for grating periods from 0.6 to 1.1 m. threshold gains of the polarizations parallel (0 ) and orthogonal (90 ) to the grating, has been calculated within a full vectorial laser model [28], [33], which allows to study comprehensively the parameter dependencies of the relief grating [20], [28]. Here, we concentrate on the most prominent dependencies in the connection with the time-resolved nearfield dynamics of the following Sections IV and V. The calculated dichroism is depicted in Fig. 4 for the realized parameter regime of the grating parameters. A negative value for means that the favored polarization is orthogonal to the grating. The comparison of the simulations with the measured values shows a good agreement in a large parameter range. Large negative values occur for small grating periods and intermediate etch depths where the measurements also show a high negative OPSR. The transition from orthogonal to parallel polarization for a grating period larger than 1 m is also confirmed. The alternating polarization control for VCSELs with 23-nm etch depth and small grating period corresponds to a small calculated dichroism. The highest value of is observed for a period of 0.8 m and 55-nm etch depth. The experimentally observed grating parameter dependence of the OPSR is in good agreement with the calculations of. It has to be noted that this value is superior by one order of magnitude in comparison to those obtained for comparable elliptical pure relief structure VCSELs [27]. Reducing the relief diameter from 4.5 to 2.5 m, the grating only overlaps with the fundamental modes and no longer with higher transverse modes which then consequently experience higher losses with an in principle improved single-mode performance. The results for the OPSR of these VCSELs are depicted in the survey of Fig. 5. A comparison with the 4.5 m results shows a decrease in the measured OPSR. This is due to a lack of transverse mode control which makes higher order modes to come into play, confirmed by the fact that this occurs mainly for grating relief depths apart from the (59 nm) value which provides the highest reflectivity contrast. Since the size of the grating relief is limited, these modes experience no polarization control. Therefore, since the OPSR is measured as an average between threshold to thermal rollover, it contains also higher transverse mode contributions. Consequently, the overall OPSR of these VCSELs is reduced compared to VCSELs with a larger grating relief. However, the tendency for emission parallel to the grating for periods of 1.1 m and perpendicular for intermediate grating depths and small periods can still be seen in Fig. 5. Such a reduced OPSR is thus not a signature of a poor polarization control of the fundamental mode, which has polarization properties similar to those depicted in Fig. 3. This is proven by the fact, that for those cases where the relief concept results in a complete transverse mode control, the good polarization control with a high OPSR is achieved again. VCSELs with 55-nm etch depth and 0.7- and 0.9- m grating period exhibit an extraordinary high OPSR of better than 20 db, in contrast to that of adjacent lasers. This is accompanied with a unique spectral mode suppression ratio (SMSR) of more than 30 db. These two VCSELs with their high OPSR and SMSR show convincingly that the grating can fix the polarization whereas the relief can simultaneously suppress the higher order modes as supported by high-resolution spectral characterization measurements.

1230 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 12, DECEMBER 2007 In summary, it is found that for grating relief diameters larger than the oxide aperture diameter: all modes experience a polarization control; there is no spatial mode control. For grating relief diamter smaller than the oxide aperture diameter: polarization control of only the fundamental mode (instead of all modes); higher order modes are suppressed. In both cases, the best results are obtained for grating periods comparable to the emission wavelength, and grating depth equal to. For that latter case, the investigations of the influence of a grating relief on the polarization mode suppression, accompanied by theoretical calculations of the dichroism showed that OPSR larger than 20 db can be achieved in a large regime of the characteristics. This achievement of a polarization-stable emission in a single spatial mode by exploring the relief grating technique will consequently dramatically extend the field of applications of VCSELs. In the next section we study the dynamical aspects in the turn-on behavior on a 50-ps time scale. The investigations are performed with the already discussed oxide diameter VCSELs, supplemented by even larger ones. Therefore, the key question, if the positive aspects of the grating relief concept can also be kept in the fast turn-on dynamics of the lasers, is directly addressed. IV. POLARIZATION-RESOLVED TURN-ON NEAR-FIELD DYNAMICS We begin now the polarization-resolved dynamics investigations with the TRIDA technique in order to study the repetitive part of the dynamics. We shall investigate VCSEL structures which have already shown up a very interesting static polarization behavior. At the end of these dynamics investigations single-shot images will then complement and support the results. A. 4- m Oxide Diameter and 2.5- m Relief Grating VCSELs and High-Power Emission The positive influence of the relief gratings on the emission characteristics is now investigated with respect to their picosecond timescale turn-on dynamics by using the TRIDA technique [31], [32]. The VCSELs are driven with rectangular electrical current pulses of widths from 400 to 3000 ps length in increasing steps of 50 ps. The polarization separated nearfields are imaged onto a CCD camera. 1 The dynamic evolution of the near field images within 50 ps is obtained via a differential analysis of the captured 2-D images for consecutively increasing pulse length. For this study, we consider the device with 4- m oxide aperture, 2.5- m grating relief with 0.7- m period and 55-nm depth, which exhibits a CW-OPSR of 20 db and a CW-SMSR of 30 db. We compare then these results with those of the corresponding standard VCSEL (adjacent on chip and with same oxide diameter). In the first step, both lasers are compared at high outputpower, where the standard VCSEL emits in both polarizations. In the next step, the low power regime 1 In this publication we refer to nearfield image as the laser facet being imaged via an objective lens on the CCD camera. Fig. 6. 50 ps time-resolved near field emission of standard VCSEL (left) and 2.5-m grating relief VCSEL (right) both with 4-m oxide aperture at high driving. The drive currents are set to 4.7 and 6.5 ma for the standard and grating VCSEL respectively, which correspond to the same CW-power of 2.1 mw. The grating is oriented horizontally with respect to the picture. is investigated where also the standard VCSEL emits in only one polarization direction (see Fig. 2). The near fields have been investigated for the same CW-output power of 2 mw, which occurs at different operation currents as indicated by the vertical lines in Fig. 2. The time-resolved near field turn-on emission is depicted for both lasers in Fig. 6, where the intensity is linearly color-coded. The standard VCSEL starts at 400 ps 2 with an emission of the fundamental mode in both polarization directions. The emission in the 90 polarization is more pronounced. The emission in 0 switches after 450 ps to the first-order mode which then becomes continuously more pronounced until at 1150 ps both intensities are comparable. The grating relief VCSEL shows an impressive complete suppression in the turn-on of the weak 0 polarization and an emission in the fundamental mode persistent for all times, in contrast to that of the standard VCSEL. In that case, spatial hole burning [32], [34], [35] leads already to a transition to the higher order mode in the 0 polarization 50 ps after turn-on, i.e., at 450 ps. The competition of both polarizations for gain leads to a spatially complementary behavior in both polarization directions. The transverse modes contributing at longer times to the emission are determined by a smaller spatial overlap. This behavior is characteristic for VCSELs with an oxide aperture larger than 3 m. Here, we demonstrate for the first time the complete suppression of transverse and polarization mode dynamics of a VCSEL with a surface relief grating. The grating 2 The indicated times are to be understood as a combination of turn-on delay and electrical signal propagation time to the VCSEL device.

FUCHS et al.: SPATIOTEMPORAL TURN-ON DYNAMICS OF GRATING RELIEF VCSELs 1231 Fig. 8. 50-ps time-resolved near field emission of 5-m oxide aperture diameter standard VCSEL (left) and two 5-m oxide aperture diameter grating relief VCSELs (middle 3.0-m relief grating; right 4.5 m relief grating) for low pump current. The drive currents are set to 3.0 and 3.2 ma for the standard and grating VCSELs respectively, which correspond to the same CW-power of 1.5 mw. The grating is oriented vertically with respect to the picture. Fig. 7. 50-ps time-resolved near field emission of standard VCSEL (left) and 2.5-m grating relief VCSEL (right) both with 4-m oxide aperture for lower driving currents of 1.3 and 3.6 ma for the standard and grating VCSEL respectively, which correspond to the same CW-power of 0.55 mw. The grating is oriented horizontally with respect to the picture. related high dichroism guarantees that the orthogonal polarization does not reach threshold. Therefore, all the gain in the active zone is available for the dominant polarization so that a higher optical output in the fundamental mode is achieved. B. 4- m Oxide Diameter and 2.5- m Relief Grating VCSELs and Low-Power Emission In the low power operation regime (0.55 mw), both the standard and the relief grating VCSEL show a very high OPSR under CW operation. The time-resolved near fields are depicted in Fig. 7. The grating VCSELs shows, as already demonstrated in the previous paragraph, a polarization stable emission perpendicular to the grating. The standard VCSEL starts with emission in both polarizations on the fundamental mode. At the onset of the emission at 400 ps they have the same intensity; the intensity of the 0 polarization decreases and is very weak at 1150 ps. The dominant 90 polarization persistently emits on the fundamental spatial mode without a subsequent transition to higher modes. These results show that the mechanism of polarization selectivity for the standard VCSEL which works well under CW-conditions is not sufficient to enable single polarization emission at the start of the emission. The investigations of the turn-on dynamics of the 4- m VC- SELs at low power show that the mechanism of the relief grating is also functional in the turn-on regime. The high dichroism, as also confirmed by the modeling, guarantees the emission governed by the grating orientation, in contrast to the standard VC- SELs and this behavior persists up to high output power, as shown previously. C. 5- m Oxide Aperture Diameter and 3.0- and 4.5- m Relief Diameter VCSELs Larger oxide diameters enable higher output powers, however accompanied with the tendency of emission on multiple transverse modes. After the successful demonstration of the fundamental mode stabilization aspects in the previous paragraphs, now the influence of the relief diameter on the transverse modes is investigated. The dynamics for 5- m oxide diameter VCSELs, in standard configuration and with 3.0- and 4.5- m relief diameters (grating period of 0.7 m and etch depth 55 nm) are depicted in Figs. 8 and 9 for low and high pumping, respectively. The OPSR under CW-operation is very small for the standard VCSEL and reaches up to 17 db for the two grating VCSELs. For low pumping, the standard VCSEL starts on both polarizations with the same intensity in the fundamental mode. The 90 polarization develops at 450 ps the first-order mode and at 500 ps emits simultaneously with the same intensity in both modes. At 600 ps, the fundamental mode intensity decreases and then the first-order mode dominates. The 0 polarization exhibits the same trend and at 2050 ps, a first-order mode appears with horizontally oriented spots. This reflects the typical complementary character in the two polarization directions [32]. The relief grating VCSEL starts in the 90 polarization. The 4.5- m relief VCSEL starts to emit only at 450 ps; the top image shows the spontaneous emission. Both VCSELs are dominated by fundamental mode emission. The 4.5- m VCSEL shows a transition to a higher order mode only at times larger than 2000 ps. The turn-on of the 3.0- m grating relief VCSEL occurs in

1232 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 12, DECEMBER 2007 Fig. 9. 50-ps time-resolved near field emission of 5-m oxide aperture diameter standard VCSEL (left) and two 5-m oxide aperture diameter grating relief VCSELs (middle 3.0-m relief grating; right 4.5-m relief grating) for higher pumping currents of 7.6 and 7.3 ma for the standard and grating VCSELs, respectively, which correspond to CW-powers of 3.4 and 4.0 mw. the fundamental mode in 90 polarization. However, also some elongation effects of the near field intensity in the grating line direction are visible and a tighter near field of the grating VCSELs in comparison to the standard VCSEL. At even higher pump currents, as illustrated in Fig. 9, all lasers show the occurrence of the higher order modes already immediately after turn-on. However, the two grating VCSELs keep their high dominance of the 90 polarization direction with the effective suppression of the 0 polarization. Even though the polarization selection via the grating works still very efficiently, the transverse mode control is more difficult to be achieved at high pumping currents. D. 7- m Oxide Aperture Diameter and 4.5- m Relief Diameter Finally, the investigations of the near field dynamics of VCSELs with 7- m oxide aperture and 4.5- m relief diameter (grating period of 0.7 m and 55 nm etch depth) have been performed. The stabilizing grating influence is clearly visible in the CW-characteristics where OPSR between 10 and 12 db were achieved in comparison to a very weak, nearly vanishing value for the standard VCSEL. The time-resolved TRIDA near fields are depicted in Fig. 10. The standard VCSELs exhibits a rich modal and polarization behavior. For the grating VCSEL, a relative good control of the polarization in the 90 direction is visible and we see the typical complementary behavior of the two polarizations. These last two investigations for larger oxide aperture diameters of 5 and 7 m clearly show that the modal selectivity for higher aperture diameters seems to be a critical issue, whereas the polarization stability seems to be maintained much better. Therefore, these results enable us to deduce the limits of the combined polarization and modal stabilization by the grating Fig. 10. Time-resolved near field emission of 7-m oxide aperture standard VCSEL (left) and grating relief VCSELs with 4.5-m relief grating (period of 0.7 m and 55-nm etch depth). The CW-equivalent output power amounts to 3.6 mw achieved with a current of 5.8 ma for the standard VCSEL and 3.3 mw for 5.8 ma for the grating VCSEL. The grating is oriented horizontally. relief technique. The design rules for the successful modal control are analogous to those of the standard relief technique [36]. Therefore, the etching depth has to be close to, providing the highest reflectivity contrast, and the oxide aperture should not be too large, in order to prevent supplementary transverse effects, such as thermal and current crowding, which then play a determining role. V. NONREPETITIVE DYNAMICS Single shot images [32] with 200 ps time resolution give even more insight and complement the TRIDA investigations of the repetitive dynamics. Single shot images with an exposure time of 1 ns at various time windows obtained for the 4- m oxide aperture and 2.5- m relief VCSEL in comparison to standard VCSELs are depicted in Fig. 11. They confirm that the repetitive two polarization turn-on behavior of the standard VCSELs effectively consists out of a stochastic turn-on, either in the 0 polarization (40% of the cases) or in the 90 polarization (60% of the cases). After approximately 1100 ps a transition from the 0 polarization to the 90 polarization takes place, consistent with the repetitive dynamics observations of Fig. 7. For the standard VCSEL both polarizations have the same threshold, but the polarization selectivity is governed statistically [37]. The grating

FUCHS et al.: SPATIOTEMPORAL TURN-ON DYNAMICS OF GRATING RELIEF VCSELs 1233 Fig. 11. Time series of single-shot near field images of a standard VCSEL (left) and the 2.5-m grating relief VCSEL (right), both with 4-m oxide aperture diameter taking at a time after turn-on as indicted in ps. The pictures stand for a representative selection of many single-shot images taking with 1 ns of exposure time. The percentage number at the bottom indicates the relative probability of the occurence of this polarization. VCSEL turns on in 100% of the images with a polarization perpendicular to the grating, remaining stable in that polarization also for longer time scales. VI. CONCLUSION We have studied the influence of surface relief gratings on the repetitive and nonrepetitive turn-on dynamics of VCSELs with different oxide aperture diameters and compared them with standard VCSEL. In all cases, the standard VCSELs showed a rich and complex polarization and mode dynamics, which originate from spatial hole burning and competition on the available gain. VCSELs with relief grating can completely suppress these effects for an oxide aperture diameter of 4 m. The high dichroism of the grating guarantees the stable turn-on on a single polarization. The high polarization selectivity, as known from CW-operation conditions remains stable even on a 50-ps time scale for the full pump current range. The restriction on the fundamental mode emission is enabled via the relief profile, which introduces a reduced reflectivity for the periphery of the VCSEL. This fundamental mode emission also remains stable in the turn-on process within the same short time scale. Theoretical simulations support the experimental findings that the grating parameters need to be carefully chosen. Best results are achieved with a grating period corresponding to the emission wavelength and the grating depth equals to. The investigated mechanism for polarization and modal control is in-principle even functional at higher oxide apertures, even though inhomogeneities, such as thermal and current crowding issues, can limit the full benefit. REFERENCES [1] X. Hachair, F. Pedaci, E. Caboche, S. Barland, M. Giudici, J. K. Tredicce, F. Prati, G. Tissoni, R. Kheradmand, L. A. Lugiato, I. Protsenko, and M. Brambilla, Cavity solitons in a driven VCSEL above threshold, IEEE J. Sel. Topics Quantum Electron, vol. 12, no. 3, pp. 339 351, May/Jun. 2006. [2] M. P. van Exter, M. B. Willemsen, and J. P. Woerdman, Polarization fluctuations in vertical-cavity semiconductor lasers, Phys. Rev. A, vol. 58, pp. 191 4205, 1998. [3] M. P. van Exter, R. F. M. Hendriks, and J. P. Woerdman, Physical insight into the polarization dynamics of semiconductor vertical-cavity lasers, Phys. Rev. A, vol. 57, p. 2080, 1998. [4] M. SanMiguel, Q. Feng, and J. V. Moloney, Light-polarization dynamics in surface-emitting semiconductor-lasers, Phys. Rev. A, vol. 52, pp. 1728 1739, 1995. [5] P. Schnitzer, M. Grabherr, R. Jager, F. Mederer, R. Michalzik, D. Wiedenmann, and K. J. Ebeling, GaAs VCSELs at = 780 and 835 nm for short-distance 2.5-Gb/s plastic optical fiber data links, IEEE Photon. Technol. Lett., vol. 11, no. 7, pp. 767 769, Jul. 1999. [6] J. S. Gustavsson, A. Haglund, J. Bengtsson, and A. Larsson, High-speed digital modulation characteristics of oxide-confined vertical-cavity surface-emitting lasers: Numerical simulations consistent with experimental results, IEEE J. Quantum Electron., ser. 8, vol. 38, pp. 1089 1096, Aug. 2002. [7] C. Degen, I. Fischer, W. Elsäßer, L. Fratta, P. Debernardi, G. P. Bava, M. Brunner, R. Hovel, M. Moser, and K. Gulden, Transverse modes in thermally detuned oxide-confined vertical-cavity surface-emitting lasers, Phys. Rev A, vol. 63, pp. 23 817, 2001. [8] H. Li, T. L. Lucay, J. G. McInerney, and R. A. Morgan, Transverse modes and patterns of electrically pumped vertical-cavity surface-emitting semiconductor-lasers, Chaos Solitons Fractals, vol. 4, p. 1619, 1994. [9] S. P. Hegarty, G. Huyet, P. Porta, J. G. McInerney, K. D. Choquette, K. M. Geib, and H. Q. Hou, Transverse-mode structure and pattern formation in oxide-confined vertical-cavity semiconductor lasers, J. Opt. Soc. Amer. B, vol. 16, no. 11, pp. 2060 2071, Nov. 1999. [10] C. J. Chang-Hasnain, J. P. Harbison, G. Hasnain, A. C. Von Lehmen, L. T. Florez, and N. G. Stoffel, Dynamics, polarization, and transverse mode characteristics of vertical cavity surface emitting lasers, IEEE J. Quantum Electron, vol. 27, no. 6, pp. 1402 1409, Jun. 1991. [11] L. Fratta, P. Debernardi, G. P. Bava, C. Degen, J. Kaiser, I. Fischer, and W. Elsässer, Spatially inhomogeneously polarized transverse modes in vertical-cavity surface-emitting lasers, Phys. Rev. A, vol. 64, pp. 31 803, 2001. [12] C. Degen, B. Krauskopf, G. Jennemann, I. Fischer, and W. Elsässer, Polarization selective symmetry breaking in the near-fields of vertical cavity surface emitting lasers, J. Opt. B: Quantum Semiclass. Opt, vol. 2, pp. 517 525, 2000. [13] J. Martin Regalado, F. Prati, M. SanMiguel, and N. B. Abraham, Polarization properties of vertical-cavity surface-emitting lasers, IEEE J. Quantum Electron, vol. 33, no. 5, pp. 765 783, May 1997. [14] J. M. Ostermann, P. Debernardi, and R. Michalzik, Optimized integrated surface grating design for polarization-stable VCSELs, IEEE J. Quantum Electron, vol. 42, no. 7, pp. 690 698, Jul. 2006. [15] H. Martinsson, J. A. Vukusic, M. Grabherr, R. Michalzik, R. Jager, K. J. Ebeling, and A. Larsson, Transverse mode selection in large-area oxide-confined vertical-cavity surface-emitting lasers using a shallow surface relief, IEEE Photon. Technol. Lett., vol. 11, no. 8, pp. 1536 1538, Aug. 1999. [16] A. Larsson, C. Carlsson, J. Gustavsson, A. Haglund, P. Modh, and J. Bengtsson, Direct high frequency modulation of VCSELs and applications in fibre optic RF and microwave links, New J. Phys., vol. 6, p. 176, 2004. [17] A. Haglund, J. S. Gustavsson, P. Modh, and A. Larsson, Dynamic mode stability analysis of surface relief VCSELs under strong RF modulation, IEEE Photon. Technol. Lett., vol. 17, no. 8, pp. 1602 1604, Aug. 2005. [18] J. M. Ostermann, F. Rinaldi, P. Debernardi, and R. Michalzik, VC- SELs with enhanced single-mode power and stabilized polarization for oxygen sensing, IEEE Photon. Technol. Lett., vol. 17, no. 11, pp. 2256 2258, Nov. 2005. [19] P. Debernardi and G. P. Bava, Coupled mode theory: A powerful tool for analyzing complex VCSELs and designing advanced device features, IEEE J. Sel. Topics Quantum Electron., vol. 9, no. 3, pp. 905 917, May/Jun. 2003. [20] J. M. Ostermann, P. Debernardi, C. Jalics, A. Kroner, M. C. Riedl, and R. Michalzik, Surface gratings for polarization control of single and multimode oxide-confined vertical-cavity surface-emitting lasers, Opt. Commun, vol. 246, pp. 511 519, 2005. [21] J. M. Ostermann, P. Debernardi, C. Jalics, and R. Michalzik, Polarization-stable oxide-confined VCSELs with enhanced single-mode output power via monolithically integrated inverted grating reliefs, IEEE J. Sel. Topics Quantum Electron, vol. 11, no. 5, pp. 982 989, Sep./Oct. 2005.

1234 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 12, DECEMBER 2007 [22] J. S. Gustavsson, A. Haglund, J. A. Vukusic, J. Bengtsson, P. Jedrasik, and A. Larsson, Efficient and individually controllable mechanisms for mode and polarization selection in VCSELs, based on a common, localized, subwavelength surface grating, Opt. Exp., vol. 13, pp. 6626 6634, 2005. [23] A. Haglund, J. S. Gustavsson, J. Bengtsson, P. Jedrasik, and A. Larsson, Design and evaluation of fundamental-mode and polarization-stabilized VCSELs with a subwavelength surface grating, IEEE J. Quantum Electron, vol. 42, no. 3, pp. 231 240, Mar. 2006. [24] J. M. Ostermann, P. Debernardi, and R. Michalzik, Surface-grating VCSELs with dynamically stable light output polarization, IEEE Photon. Technol. Lett., vol. 17, no. 12, pp. 2505 2507, Dec. 2005. [25] N. Nishiyama, A. Mizutani, N. Hatori, M. Arai, F. Koyama, and K. Iga, Lasing characteristics of InGaAs-GaAs polarization controlled vertical-cavity surface-emitting laser grown on GaAs (311) B substrate, IEEE J. Sel. Topics Quantum Electron., vol. 5, no. 3, pp. 530 536, May/Jun. 1999. [26] K. D. Choquette and R. E. Leibenguth, Control of vertical-cavity laser polarization with anisotropic transverse cavity geometries, IEEE Photon. Technol. Lett., vol. 6, no. 1, pp. 40 42, Jan. 1994. [27] P. Debernardi, H. J. Unold, J. Maehnss, R. Michalzik, G. P. Bava, and K. J. Ebeling, Single-mode, single-polarization VCSELs via elliptical surface etching: Experiments and theory, IEEE J. Sel. Topics Quantum Electron., vol. 9, no. 5, pp. 1394 1405, Sep./Oct. 2003. [28] P. Debernardi, J. M. Ostermann, M. Feneberg, C. Jalics, and R. Michalzik, Reliable polarization control of VCSELs through monolithically integrated surface gratings: A comparative theoretical and experimental study, IEEE J. Sel. Topics Quantum Electron, vol. 11, no. 1, pp. 107 116, Jan./Feb. 2005. [29] C. I. Wilkinson, J. Woodhead, J. E. F. Frost, J. S. Roberts, R. Wilson, and M. F. Lewis, Electrical polarization control of vertical-cavity surface-emitting lasers using polarized feedback and a liquid crystal, IEEE Photon. Technol. Lett., vol. 11, no. 2, pp. 155 157, Feb. 1999. [30] H. J. Unold, S. W. Z. Mahmoud, R. Jäger, M. Golling, M. Kicherer, F. Mederer, M. C. Riedl, T. Knödl, M. Miller, R. Michalzik, and K. J. Ebeling, Single-mode VCSELs, in Proc. SPIE Vertical-Cavity Surface-Emitting Lasers VI, 2002, pp. 218 229. [31] A. Barchanski, T. Gensty, C. Degen, I. Fischer, and W. Elsäßer, Picosecond emission dynamics of vertical-cavity surface-emitting lasers: Spatial, spectral, and polarization-resolved characterization, IEEE J. Quantum Electron, vol. 39, no. 7, pp. 850 858, Jul. 2003. [32] K. Becker, I. Fischer, and W. Elsäßer, D. Lenstra, G. Morthier, T. Erneux, and M. Pessa, Eds., Spatiotemporal emission dynamics of VCSELs: Modal competition in the turn-on behavior, Proc. SPIE, vol. 5452, p. 452, 2004. [33] G. P. Bava, P. Debernardi, and L. Fratta, Three-dimensional model for vectorial fields in vertical-cavity surface-emitting lasers, Phys. Rev. A, vol. 63, pp. 023 816, 2001. [34] C. Degen, I. Fischer, and W. Elsäßer, Transverse modes in oxide confined VCSELs: Influence of pump profile, spatial hole burning, and thermal effects, Opt. Exp., vol. 5, pp. 38 47, 1999. [35] A. Valle, J. Sarma, and K. A. Shore, Spatial holeburning effect on the dynamics of vertical-cavity surface-emitting laser-diodes, IEEE J. Quantum Electron, vol. 31, no. 8, pp. 1423 1431, Aug. 1995. [36] A. Haglund, J. S. Gustavsson, J. Vukusic, P. Modh, and A. Larsson, Single fundamental-mode output power exceeding 6 mw from VC- SELs with a shallow surface relief, IEEE Photon. Technol. Lett., vol. 16, no. 2, pp. 368 370, Feb. 2004. [37] M. B. Willemsen, M. U. F. Khalid, M. P. van Exter, and J. P. Woerdman, Polarization switching of a vertical-cavity semiconductor laser as a Kramers hopping problem, Phys. Rev. Lett., vol. 82, pp. 4815 4818, 1999. Tobias Gensty received thee diploma and the Ph.D. degree, from Darmstadt University of Technology, Darmstadt, Germany, in 2002 and 2005, respectively, both in physics. He is currently holding a government position. Pierluigi Debernardi received the degree in electronics engineering from Politecnico di Torino, Torino, Italy, in 1987. Since 1989, he has been with the Istituto Elettronica e Ingegneria Informazione Telecomunicazioni, an institute of the Italian National Council of Research at the Politecnico di Torino, Torino, Italy. Gian Paolo Bava received the degree in electrical engineering from Politecnico di Torino, Torino, Italy, in 1961. Since then he has been working at Dipartimento di Elettronica, Politecnico di Torino, where he has been a full Professor of microwave techniques since 1976. Johannes Michael Ostermann received a Master of Science degree from the University of Massachusetts. In 2002 he graduated in Physics, and in 2007 earned a Ph.D. (Dr.- Ing.), both with distinction from Ulm University. He also stayed for one year at the IEIIT-CNR c/o Politecnico di Torino, Italy. Rainer Michalzik received the Dipl.-Ing. degree in electrical engineering from the Technical University of Braunschweig, Braunschweig, Germany, in 989 and the Dr.-Ing. degree from the University of Ulm, Ulm, Germany, in 1996. He is leading the VCSELs and Optical Interconnects Research Group, Institute of Optoelectronics, Ulm University, Ulm, Germany. Aåsa Haglund received the M.Sc. degree in physics from Göteborg University, Göteborg, Sweden, in 2000 and the Ph.D. degree in electrical engineering from Chalmers University of Technology, Göteborg, Sweden, in 2005. She is currently a Research Associate at the Photonics Laboratory, Chalmers University of Technology. Anders Larsson received the M.Sc. and Ph.D. degrees in electrical engineering from Chalmers University of Technology, Göteborg, Sweden, in 1982 and 1987, respectively. From 1984 to 1985, he was with the Department of Applied Physics, California Institute of Technology, Pasadena, and from 1988 to 1991 with the Jet Propulsion Laboratory, Pasadena. In 1991, he joined the faculty at Chalmers University of Technology, where he became Professor in 1994. Wolfgang Elsäßer received the Dipl. from University of Karlsruhe, Karlsruhe, Germany, in 1980 and the Ph.D. degree from University of Suttgart, Suttgart, Germany, in 1984, both in physics. He is a Professor in the Institute for Applied Physics, Darmstadt University of Technology, Darmstadt, Germany, heading the Semiconductor Optics Group. Christian Fuchs received the diploma degree from the Darmstadt University of Technology, Darmstadt, Germany, in 2006.