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Physics of semiconductor microcavity lasers

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1995 Semicond. Sci. Technol. 10 739 (http://iopscience.iop.org/0268-1242/10/6/002) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 37.44.207.131 This content was downloaded on 17/01/2017 at 21:36 Please note that terms and conditions apply.

You may also be interested in: Microscopic theory of non-equilibrium microcavity laser dynamics H C Schneider, F Jahnke and S W Koch Ultrafast non-equilibrium carrier dynamics in semiconductor laser mode-locking J Hader, M Scheller, A Laurain et al. Recent progress in quantum cascade lasers andapplications Claire Gmachl, Federico Capasso, Deborah L Sivco et al. Optical processes in microcavities R E Slusher Calculation of the gain saturation in cw semiconductor lasers with Boltzmann kinetics for Coulomb and LO phonon scattering S Schuster and H Haug Fast phenomena in semiconductor lasers Peter P Vasil'ev, Ian H White and John Gowar Semiconductor lasers with optical injection and feedback G H M van Tartwijk and D Lenstra Recent advances in VECSELs Arash Rahimi-Iman

Semicond. Sci. Technol. 10 (1995) 739-751. Printed in the UK

1

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TOPICAL REVIEW

Physics of semiconductor microcavity iasers ^...

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t Department of Phvsics and Materials Sciences Centre. Philipus . . Universitv. .. 5-35032Marburg, Germany i Semiconductor Phvsics Department. Sandia National Laboratow, Albuaueraue. . . . N M 87185-0350, USA Received 29 December 1994, accepted for publication 11 January 1995 Abstract. This review summarizes recent developments and successes in the theoretical modelling of t h e characteristics of semiconductor microcavity lasers. After a discussion of the basic laser properties, results of a quasi-equilibrium many-body

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microcavity laser operation not too far above the laser threshold. Non-equilibrium phenomena, such as spectral and kinetic hole burning as well as plasma heating effects, are analysed using a quantum kinetic approach. Comparisons with experimental observations are discussed, before open problems and future challenges are outlined.

1. What are microcavity lasers? Semiconductor microcavity lasers are tiny semiconductor lasers with an overall volume in the pm3 regime. up to now, such miniature lasers have been realized in two fundamentally different configurations. These are the vertical cavity surface emitting lasers (VCSEL) [ I 4 1 and the so-caiied microdisc iasers i i j . Both structures are shown schematically in figure 1. A microdisc laser consists of a disc of semiconductor material with a typical diameter of 2-5 pm, which is thinner than the optical wavelength. Under laser conditions high-Q whispering-gaiiery modes propagate inside the disc ciose to the circumference. These modes are confined inside the disc due to the large effective reflection coefficient. The VCSEL has a Fabry-P&ot resonator with very high mirror reflectivity. This high reflectivity is achieved with distributed Bragg retiectors (DBRsj, consisting of epitaxiaiy grown layers of different refractive indices [SI. The simplest DBR has alternating layers that are each 1/4 thick so as to provide constructive interference of the reflected waves from each interface. For a typical arrangement the resuiting frequency-dependent reflectivity is shown in figure 2. The more refined DBRs have complicated layer structures, which are designed to give graded potential energy differences to improve carrier transport without degrading the optical performance. Present DBRs consisting of as many as 30 to 40 layers can achieve over 99.9% reflectivity. For a review of the design details see, for example, 161. 0268-1242/95/060739+13$19.508 1995 IOP Publishing Ltd

The entire VCSE structure is that of an effectively o n e dimensional resonator with a length of a few micrometres. The active region between the mirrors consists either of bulk semiconductor material or of one or a few quantum wells embedded between cladding layers. As a consequence of the short cavity length it is possible to design a mode structure in Vcsm where only one high-Q iiangiiudinaij cavity. made exists within he specirai gain region of the semiconductor medium. Such devices exhibit nearly ideal single-mode operation.

2. Spontaneous emission properties Besides a number of promising practical advantages, which we will discuss in later sections of this review, the high-Q cavities in semiconductor microlasers offer interesting possibilities for the study of fundamental aspects of the interaction between light and the semiconductor medium. Many of these studies are extensions of investigations dealing with the modified radiative properties of atoms between highly reflecting mirrors. For example, it could be demonstrated (see, for example, [9])that the,spontaneous decay rate of excited atoms in high-Q cavities is enhanced if the atomic transitions are in resonance with a cavity mode. On the other hand the spontaneousdecay is inhibited if the transitions of excited atoms occur at frequencies which are strongly suppressed because of the negative feedback inside the cavity. Besides their general importance, such studies of spontaneous emission in optical microcavities are also of practical relevance for the performance of laser devices.

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all of the spontaneous carrier recombinations were to lead to emission into the laser mode (,9 = 1) and if non-radiative recombination processes were negligible, the emitted intensity would linearly increase from zero with increasing pump rate. This ideal configuration has been called the thresholdless laser [ 121. In conventional semiconductor lasers with low-Q resonators in the length range of a few hundred p m one has a large number of modes spectrally inside the gain region. Correspondingly the spontaneous emission of the active material is distributed over the large number of nonlasing modes and the spontaneous emission coupling into the laser mode is small. The factor 3, is typically of the order of io-' or even smaiier. Such iow 6 vaiues iead io a pronounced laser threshold. According to Fermi's golden rule the spontaneous photon emission probability of the active medium is given by the electronic transition probability from the excited to the .,*-*--A

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WAVE LENGTH (nm) Figure 2. Typical VCSEL design example with top mirror (241/4 layers), active region (bulk material), bonom mirror (45 A!4 layers) and substrate. The upper part shows the calculated longitudinal intensity distribution and the lower pari is t h e reflectivity of an external light beam.

For example, the intrinsic origin of a clear laser threshold is the ioss caused by ihe sponianeous emission inio ihe non-lasing modes. This part of the spontaneous carrier recombination presents a net loss of inversion for the laser process. The pump process has to overcome these losses before laser action becomes possible. On the other hand, spontaneous emission into the laser mode is necessary in order to s m t the lasing. Hence, a characteristic number for lasers is the spontaneous emission coupling, ,9, which defines the ratio of the spontaneous emission into the laser mode to the total spontaneous emission. It can be s h o w on the bzsis of B equa!inn ana!ysis [IOi II! that the amount of intensity increase at the laser threshold is determined by the spontaneous emission coupling. If 740

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Therefore, in optical resonators the spontaneous emission properties can be changed via changes of the photon DOS. A strongly enhanced photon DOS of the resonator mode and reduced photon DOS of other modes increases the spontaneous emission coupling efficiency. In the case of an ideal VCSEL, where all but one longitudinal mode in the gain region is suppressed, this single laser mode has a very high cavity Q due to the high (larger than 99.9%) effective mirror reflectivities. Consequently, the spontaneous emission coupling efficiency is mostly determined by the number of transverse modes. Generally, in VCSELs the spontaneous emission coupling is considerably improved in comparison to conventional semiconductor lasers and drastic reductions of the threshold pump current have been reported [ 1 4 ] . However, until now, only for whispering-gallery modes in microdisc lasers could an effective three-dimensional mode confinement be achieved. With a cavity volume of the order of the cubic wavelength only a few cavity modes remain and spontaneous emission coupling larger than 10% has been reported [13]. Even though semiconductor microdisc lasers have been used for investigations of fundamental importance, most application interests are currently focused on VCSEL structures. These devices offer a number of interesting posslbi!i!ies re!a!ed to the ease of fabrication and the potential for improved performance in comparison to that of conventional semiconductor lasers. Therefore. we will

Physics of semiconductor microcavity lasers concentrate our discussion in the following sections mostly on VCSELS, even though many of the general considerations are also valid for microdisc lasers. 3. Problems of conventional semiconductor lasers

To show the reasons for the growing interest in

VCSELs

let us first mention a few problems associated with conventional semiconductor lasers. The commonly used laser structure is called an 'edge emitter', because the laser emission occurs from an edge of the semiconductor chip. Gain is achieved by current injection pumping of a heterostructure, consisting of a semiconductor active layer sandwiched between materials with wider bandgaps. A common heterostructure has a GaAs active iayer between AlGaAs barriers. One of the functions of a heterostructure is the confinement of the charge caniers which are pumped into the active region and which are needed to obtain gain. This carrier confinement is achieved by designing the iaser smcture to be very thin in the direction of curreni flow, which is also the epitaxial growth direction. The conventional laser configuration works very well, as is evident from its wideranging applications in many electrooptical systems. On tine oiher hand, ihe combinaiion of edge emission and active layer shape gives rise to several disadvantages. Edge emission makes the laser transverse and lateral modes depend on the cross section of the gain region, which is very thin in the transverse dimension for carrier -2-e ^_.I ...:.I..:.. .L^ ,.-"a a:...---:-CA~U,,I,,,C,,,C,,, a,,u WLVS 111 ULS l d l G l d l "II,IG,:II>I"II ,U, UUqJUL ^..I.....

power. The resulting near field of the laser emission is then also highly elongated, which does not match well, for example, to the circular cross section of an optical fibre into which one might wish to couple the . . !aser eF2r::Gn. r i x e (he !%?stverse benm dimension is very small, typically of the order of 1 pm or less, the transverse beam divergence is rather high (typically x 50" full angle) because of diffraction. Therefore, the far field of the laser emission is also highly elongated. The ka!!ztigmatism !oge!her with.th.e high divergence makes design and fabrication of coupling optics rather challenging. Due to the long (m 1 mm) optical cavity, an edge emitter typically operates multimode. Approaches to single-mode operation involve embedded gratings or buried heterostructures, which increase unit cost and limit output power 1141. 4. How VCSELS can help

The discussion in the previous section makes it piausibie that a possible solution to the problems associated with the beam profile of typical edge emitters lies in a decoupling of the transverse and lateral optical modes from the cross section of the gain region. Exactly this is achieved in a VCSEL. Since the opticai cavity axis is in the verticai (epitaxial growth)duection (see figure l), the laser emission occurs from the surface of the wafer. This surface-emitting configuration has several desirable features. By being

independent of the gain layer cross section, there is more flexibility in the shape and size of the transverse optical mode. It is possible to make the beam circular (note that this makes it no longer necessary to distinguish between transverse and lateral dimensions). With an approximately 6 p m diameter circular output aperture, lowest-order (Gaussian-like) transverse mode operation with less than 10" beam divergence is possible [151. Furthermore, as mentioned earlier, the very short cavity length makes VCSEL inherently single mode. Other advantages of a VCSEL are even more practical in nature. Unlike an edge emitter, the VCSU mirrors are fabricated during the epitaxial growth of the entire wafer, so that the mirrors of hundreds of lasers are made simultaneously. With edge emitters, we can only make one-dimensional laser arrays, whereas with surface emitters truly two-dimensional arrays are routinely possible. To test an edge emitter one needs to expose the edges, which means cleaving the wafer and making the necessary facet preparations. These are time-consuming and therefore costly steps that have to be repeated for each laser. Surface emitters can be fabricated, tested and operated at the wafer level. 5. Problems with VCSELS

The advantages of VCSELs do not come without a price. Most apparent is the drastic reduction in the gain length, from hundreds of micrometres in an edge emitter to a few tens of nanometres in a VCSEL. This gain length reduction has to be compensated by the high cavity Q,putting great demands on the quality of the DER mirrors. However, there still remain complications of a more fundamental nature. The short high-Q VCSEL optical cavity has sharp, widely spaced resonances. In many cases, there is only one resonance under the gain curve. As a result, the laser threshold current is very sensitive to the position of this resonance in relation to the peak of the material gain spectrum. To achieve minimum threshold operation the optical resonance should be well aligned with the gain peak. A related complication arises since the cavity-gain alignment can be maintained typically only within a small temperature range, because the gain spectrum and the cavity resonances have different temperature dependences. The gain spectrum shifts in frequency because of the temperature dependences of the bandgap energy and the carrier distributions. The cavity resonances shifi with temperature because of thermally induced changes in the refractive indices of the mirror material and the material within the optical cavity. The result is a temperature 1

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Figure 15. (a) Gain peak wavelength and (b)comesponding threshold current density versus AI concentration in 6 nm AlGalnP quantum wells. In (b)the long-dashed curve is the contribution to the inversion and the short-dashed curve is the leakage current. in edge emitters. gain guiding gives better output power and efficiency because it allows an optical mode to take whatever form is necessary to maximize modal gain. In early 1994, epitaxial growth techniques for the phosphide compounds improved sufficiently to provide the control necessary for making short (lh) microcavities. This results in further improvement by eliminating from the microcavity extraneous materials that would otherwise contribute to the optical losses. It is with a l k microcavity device that the present records of 3 mW output power and 10% wall-plug efficiency were made [42]. A present goal is to extend VCSEL emission to shorter wavelengths. Edge emitters with tensile strained InGaP quantum wells have operated with TM polarization to below 610 nm 1431. However, a microcavity can only support the TE mode, which has lower gain in tensile strained quantum wells. Also, heating is a rather severe problem in a vCSEL, giving rise to higher current leakage and temperatureinduced bandgap energy shrinkage. In the worst case, all these factors may combine to prevent a repeat of the edge emitter’s success. Another possibility for shorter wavelengths is to use AlGaInP in the quantum wells. Regardless of the approach, the ability to achieve shorter lasing wavelengths is eventually limited by the available bandgap energy. For example, the r-X crossing in (AI,Gal-x)o.51~.~P,which occurs at x = 0.58, places an upper limit of 2.31 eV on the usable bandgap energy. Choosing (Alo.sGao.5)o.sIno.sP for the barrier material, figure 15(n) shows the possible reduction in the gain peak wavelength with increasing AI concentration in a 6 nm (Al,Gal-x)O.~InO.~f,Pquantum well. However, the threshold current also increases because of current leakage (see figure 15(b)). The leakage current rises because of the smaller electron and hole confinement potentials resulting from the higher quantum well bandgap energy. Note that the current density that actually contributes to the creation of the inversion remains relatively constant with AI concentration, even in the presence of a discontinuity at x = 0.38 due to the transition from two conduction subbands to a single conduction subhand. Figure 15 is obtained using the microscopic quasiequilibrium theory discussed earlier in this review, and it neglects the detrimental effects of oxygen which may be introduced into the quantum well along with the Al. 750

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600

650 700 Wavelength (nm)

Figure 16. Inversion and leakage contributions to the threshold current density versus laser wavelength for different In concentration in 6 nm (AlxGal~x)l~ylnyP quantum wells. The AI concentration is varied to change the wavelength. It is informative to plot the inversion and leakage contributions to the threshold current density versus the gain peak wavelength. This is done in figure 16, where the different wavelengths are obtained by changing the quantum well AI concentration, and each set of inversion and leakage current density curves corresponds to a given In concentration in the quantum well. For an In concentration of 0.50 the quantum well is unstrained, and larger (smaller) values result in compressive (tensile) strain. While optimal gain configurations exist, for example compressive strain appears to give the lower threshold current density, every set of curves shows a lower-wavelength limit of FZ 620 nm, after which cument leakage accounts for over half of the total current density. The same basic conclusion is also reached if we had varied the quantum well width instead of the strain. The approach to shorter wavelength in microcavity lasers may lie with the U-VI compounds or the nitride based m-V compounds.

Acknowledgments We thank R E Slusher, H M Gibbs and J McInerney for many stimulating discussion on semiconductor microcavity lasers. Parts of this work were supported through the Deutsche Forschungsgemeinschaft, through the Optical Circuitry Cooperative, through the National Science Foundation, as well as the US Department of Energy through contract DEACO4-94DP85000. Furthermore, we thank the HRLZ Jiilich for grants for CPU time.

References [I] Iga K, Koyanama F and Khoshita S 1988 J. Qrcantum Electron. 242 1845 [21 Faist I, Morier-Genoud F, Martin D, Ganiere I A and ReinhaFt F K 1988 Electmn. Lett. 24 629

Physics of semiconductor microcavity lasers [3] Gourley P L, Brennan T M, Hammons B E, Corzine S W, Geels R S, Yam R H, Scott I W and Coldren L A 1989 Appl. Phys. Lett. 54 1397 [4] Jewell J L. McCall S L, Lee Y H. Scherer A, Gossard A C and English J H 1989 Appl. Phys. Left. 54 1400 [SI Geels R S and Coldren L A 1990 Appl. Phys. Left. 57

1605 [6] Jewell 1 L, Harbison J P, Scherer A, Lee Y H and Florez L T 1991 IEEE J. Quuntiun Electron. 27 1332 [7] McCall S L, Levi A F I, Slusher R E. Pearlon S J and Logan R A 1992 Appl. Phys. Lett. 60 289 [8] Lee Y H, Jewell I L, Scherer A, McCall S L, Walker S 1, Harbison I P and Flarenz L T 1989 Elecrmn. Lett 25 1377 [9] Goy P, Raimond I M, Gross M and Haroche S 1983 Phys. Rev. Len 50 1903 Hulet R G, Hilfer E S and Kleppner D 1985 Phys. Rev. Lett. 55 2137 [lo] Yokoyama H and Brorson D 1989 J. Appl. Phys. 66 4801 [ l l ] Yamamoto Y, Machida S and BjOrk G 1991 Phys. Rev. A 44 657 [I21 De Martini F and Iacobovitz G R 1988 Phys. Rev. Leu. 60 1711 [I31 Slusher R E. Levi A F I, Mohideen U,McCall S L and Logan R A 1993 Appl. Phys. Lett. 63 1310 and private communication [I41 Thompson G H B Physics of Semiconductor Laser Devices (Chichester: Wiley) ch 6, 8 [15] Chang-Hasnain C L, Harbison J P, Hasnain G H, von Lehmen A C, Florenz L T and Stoffel N G 1991 IEEE .

_

I

Semiconducfor Lasers (New York \;an No&d

Reinhold) ch 6 [17] Chow W W, Koch S W and Sargent M I11 1994 Semiconductor Laser Physics (Berlin: Springer) [IS] Haug H and Koch S W 1994 Quantum Theory ofthe n"+;*..l "..A n ,".-,"....;VY..*".U,.Y".C'.'Y,..L.

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(Singapore: World Scientific) Crawford M H and Schneider R P 11 private communication Schneider R P Jr, Choquette K D, Lott 1A. Lear K L, Figiel I I and Malloy K 1 1994 IEEE Phofon Technol. Lptt. 6 313 Luttinger I M and Kohn W 1955 Phys. Rev. 97 869 Stringfellow G B 1972 J. Appl. Phys. 43 3455

1231 Chow W W, Schneider R P 11, Lott J A and Choquette K D 1994 Appl. Phys. Left 65 135 U241 Young D B, Conine S W, Peters F H, Scott I W, Thibeault B J and Coldren L A 1992 C o d Proc. LEOS '92, (Boston, MA, 1992) p 546 l25l. Chow W W, Corzine S W. Youne D B and Coldren L A . 1995 Appl. Phys. Left. submittid [261 Chow W W, Choquette M D and Gourley P L 1995 Appl. Phys. Lett. submitted 1271 . . Hasnain G. Tai K, Yanrr L. Wane Y H. Fischer R I. Wynn J D, Weir B, but& N 2 and Cho A Y 1991 IEEE J. Quantum Electron. 27 1377 [281 Coldren L A, Geels R S and ScoU I W 1992 Opt, Quanfum Elecrmn 24 105 [29] Corzine S W 1993 PhD Dissertufion UC Santa Barbara [301 Hadley G R, Lear K L, Warren M E, Scott I W and Corzine S W 1994 14th IEEE /U. Semiconductor Laser Conf: (Muni, Hawaii, 1994) paper P32 [311 Thode L E, Csanak G, So L L and Kwan T I T 1994 Proc. SPIE 2146 174 [321 Korenman V 1966 Ann. Phys., NY 39 72 [331 DuBois D F 1967 Lecfuresin Theoretical Physics vol IX C, ed W E Brittin and A 0 Banit (New York Gordon and Breach) p 469 [341 Henneberger K, Jahnke F and Herzel F 1992 Phys. Stufus Solidi b 173 423 Henneberger K, Henel F, Koch S W. Binder R, Paul A E and Scott D 1992 Phys. Rev. A 45 1853 [351 Jahnke F, Henneberger K, Schafer W and Koch S W 1993 J. Opt. Soc. Am. B 10 2394 [361 Jahnke F and Koch S W 1993 Opf.Lett. 18 1438 i37j Koch S 'iv' and iahnke F i994 Advances in Soiid Srate Physics vol 34, ed R Helbig (Braunschweig: Vieweg) p 259 Jahnke F and Koch S W 1994 Proc. SPIE 2146 I381 Iahnke F and Koch S W 1995 Phys. Rev. A submitted [391 Slusher R E , Mohideen U, Jahnke F and Koch S W 1994 ~~

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[41j Lott J A, Scheider R P, Choquette K D, kilcoyne S P and Figiel J J 1993 Electron. Lett. 29 1693 [421 Crawford M H, Schneider R P Jr, Choquette K D. Lear K L, Kilcoyne S P and Figiel J I 1995 Electron. _-_

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[431 Bour D P, Treat D W, Beemink K I, Krusor B S,Geels R S and Welch D F 1994 IEEE Phofon. Technol. Lett. 6 128

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