Includes bibliographical references (page 2618).We describe studies of an index-guided ~4×3 µm2 vertical cavity surface emitting laser with a cw, room temperature 133 µA threshold current and 53% slope efficiency fabricated using selective wet thermal oxidation to provide optical and electrical confinement. While the device operates strictly single mode, a large number of transverse modes are evident in the subthreshold luminescence observed in both vertical and lateral directions. Despite the lumped nature of the index region, the transverse mode wavelengths agree very well with those predicted for a conventional distributed waveguide structure. Waveguide dimensions and an effective index step of 2% between the active and cladding region are determined by fitting the modal data.This work was supported by the United States Department of Energy under Contract No. DE-AC04-94AL85000

Choquette, Kent D.

Kilcoyne, Sean Patrick

Lear, Kevin L.

Schneider, Richard P.

We describe studies of an index-guided ~4×3 µm2 vertical cavity surface emitting laser with a cw, room temperature 133 µA threshold current and 53% slope efficiency fabricated using selective wet thermal oxidation to provide optical and electrical confinement. While the device operates strictly single mode, a large number of transverse modes are evident in the subthreshold luminescence observed in both vertical and lateral directions. Despite the lumped nature of the index region, the transverse mode wavelengths agree very well with those predicted for a conventional distributed waveguide structure. Waveguide dimensions and an effective index step of 2% between the active and cladding region are determined by fitting the modal data.This work was supported by the United States Department of Energy under Contract No. DE-AC04-94AL85000

Schneider, Richard P., author

Choquette, Kent D., author

Lear, Kevin L., author

Kilcoyne, Sean Patrick, author

American Institute of Physics, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

tlynceheedtiveModal analysis of a small surface emitting laser with a selectivelyoxidized waveguideK. L. Lear,a) K. D. Choquette, R. P. Schneider, Jr., and S. P. KilcoyneSandia National Laboratories, Photonics Research Department 1312, MS 0603, P. O. Box 5800,Albuquerque, New Mexico 87185-0603~Received 6 February 1995; accepted for publication 16 March 1995!We describe studies of an index-guided;433 mm2 vertical cavity surface emitting laser with a cw,room temperature 133mA threshold current and 53% slope efficiency fabricated using selective wethermal oxidation to provide optical and electrical confinement. While the device operates strictsingle mode, a large number of transverse modes are evident in the subthreshold luminesceobserved in both vertical and lateral directions. Despite the lumped nature of the index region, ttransverse mode wavelengths agree very well with those predicted for a conventional distributwaveguide structure. Waveguide dimensions and an effective index step of 2% between the acand cladding region are determined by fitting the modal data. ©1995 American Institute ofPhysics.e-r--Recently reported vertical cavity surface emitting lase~VCSELs! incorporating aluminum oxide layers have fetured threshold current reductions to 91mA1 and power con-version efficiency increases to 50%.2 The low index of theoxide provides optical confinement, evident in the spectrathe latter device, which is critical to realizing smaller dvices. In this letter, we investigate waveguiding in a laserthe same structure and fabrication as the high efficiency obut with smaller area for reduced threshold and single-moperation. A substantial modal structure, however, is evidin subthreshold luminescence, and it is used to analyzenature of waveguiding in an aluminum oxide confined sface emitting laser for the first time. The modal dispersidata, which is enhanced in lateral emission spectra, is usedetermine the effective dimensions of the waveguide andsize of the effective index step from the active to claddiregion.The laser has been realized by producing a lateral instep and current aperture due to conversion of AlGaAs tooxide by selective lateral oxidation.3 The initial epitaxialstructure including Al0.96Ga0.04As/GaAs distributed Braggreflectors and a triple InGaAs quantum well in a gradAlGaAs cavity is similar to that described previously,4 ex-cept that the aluminum mole fraction of the AlGaAs mirrlayer closest to the cavity has been increased fromx50.96 tox50.98. Devices are formed in mesas created by etchaway;4 mm of the surrounding material so as to exposeedge of the cavity layers. A portion of the AlGaAs layestarting from the exposed edge are then converted to an etrically insulating aluminum oxide with a refractive index o;1.55 by wet thermal oxidation5 at 425 °C for a few hours.The x50.98 layer oxidizes most rapidly3 and the resultantoxide protrudes furthest into the mesa to provide electriand optical confinement about the active region wherelayer has not oxidized. A backside substrate contact andnular contact on the mesa complete the top-emitting ladiode. Figure 1 shows a top view and cutaway orthogoa!Electronic mail: kllear@sandia.gov2616 Appl. Phys. Lett. 66 (20), 15 May 1995 0003-6951Downloaded¬29¬Oct¬2004¬to¬129.82.229.173.¬Redistribution¬subjersa-ofe-ofnesodeenttheur-ond tothengdexanedoringtherslec-fcalthisan-sernalview of the laser. The square shape of the oxide apertur~active region! resembles the outline of the mesa from whichthe oxidation proceeded. Microscopic inspection of the lumi-nescent area indicates an approximately 333 mm2 activearea, roughly consistent with the waveguide dimensions extracted below.The room-temperature, cw light-current characteristicfor this laser is presented in Fig. 2. This characteristic wasstable with no noticeable changes during experiments ovethe course of several weeks. It is quite linear in the rangeshown, reaching a maximum power of 1.5 mW at 2.5 mAbefore rolling over due to heating effects. The threshold current of 133mA is more than an order of magnitude smallerthan the minimum threshold observed in lasers made fromthe same epitaxial material using proton implantation, sincesuch gain guided devices suffer from diffraction losses.6 Thisthreshold current has been achieved with relatively high outFIG. 1. ~a! Structure of the VCSEL diode shown in top view and cutawayorthogonal view. High aluminum content layers~gray! are converted to ox-ide ~black! including most of the layer directly above the quantum wellregion ~dashed line! except for a small rectangular active region~shown inwhite in the top view!. An annular metal contact~cross hatched! allows fortop emission.~b! Arrangement for collecting luminescence from the edge ofthe laser mesa. The fiber was positioned along thex andy axes in line withthe mesa to collect the spectra shown in Figs. 3~a! and 3~b!, respectively./95/66(20)/2616/3/$6.00 © 1995 American Institute of Physicsct¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jsp, atalltedne-dedehethere-edvityastalthssisalr or-n-hetialeity.beallhichn aesi-arethee,tlypec-earre,ngly. Intepostedrenge--s in-fit-wsput coupler transmission so that the differential quantumficiency just above threshold is 53%. Higher mirror refletivity would result in even lower threshold currents. Fexample, a recently reported, lateral injection, oxide confinVCSEL using a dielectric deposited top mirror exhibited amA threshold and a slope efficiency of 22%.1 For reference,the lowest reported threshold for an edge-emitting lase165mA, and this device had a slope efficiency of 0.4%.7 TheVCSEL reported here lases strictly in the fundamental mup to and beyond rollover. The moderate effective index sof 2% in this oxidized device, as compared to the very lasemiconductor to air index step in etched devices, promthe single-mode behavior. In comparison to etched devithis structure is also more suitable for short wavelength temitting lasers and offers lower resistance for active regdiameters less than about 4mm.While only the fundamental mode reaches lasing threold, the full modal structure is evident in subthreshold aplified spontaneous emission spectra as shown in Fig. 3.three spectra corresponding to the emission in the threethogonal directions were taken at a drive current of 100mA.The vertical emission from the surface of the laser was clected using a 0.31 nA microscope objective, while the hozontal emission was collected using a 100mm core opticalfiber as indicated in Fig. 1~b!. The emission was collected foFIG. 2. The light vs current characteristic for a small, index guided,emitting VCSEL fabricated by selective oxidation.FIG. 3. The subthreshold~100mA! emission spectra for a small VCSEL ithe orthogonal horizontal~a! x direction and~b! y direction and in the~c!vertical direction. The computed positions of transverse modes, in exceagreement with the data, are shown as lines at the top of the figure.peaks are labeled with the transverse mode order indices determinedthe analysis.Appl. Phys. Lett., Vol. 66, No. 20, 15 May 1995Downloaded¬29¬Oct¬2004¬to¬129.82.229.173.¬Redistribution¬subjeef-c-ored91r isodeteprgeotesces,op-ionsh-m-Theor-ol-ri-rtwo orthogonal horizontal directions~along thex andy axes!aligned with the sides of the mesa. The fundamental modethe longest wavelength, is observed in the spectra forthree directions. The additional spectral lines are associawith higher order transverse modes, recalling that the owavelength cavity supports only a single longitudinal moin the region shown with the next nearest longitudinal mooccurring around 640 nm. The broad feature in tz-direction luminescence at 945 nm corresponds toFabry–Perot resonance in the cavity under the oxidizedgion. The resonance is shifted from that in the unoxidizregion, since the change in the index alters the optical calength.8Transverse mode filtering in the horizontal directionwell as the small cavity dimensions contribute to horizonspectra having well-defined transverse mode wavelengand readily assigned mode indices. The following analywill verify that the strong, narrow peaks in the horizontspectra correspond to waveguided modes that are higheder in the direction of emission but are strictly the fundametal mode in the orthogonal lateral direction. We attribute tsuppression of higher order orthogonal modes to the spafiltering of the far field by the limited size of the fiber corwhich corresponds to selection of wavevectors in the cavThe detection of higher order orthogonal modes couldenhanced by laterally displacing the fiber position. The smwaveguide size leads to modes that are well separated, wimproves the relative accuracy of mode wavelengths givefixed spectrometer resolution. The modal lines in thex direc-tion are more closely spaced than those observed in thydirection indicating a slightly asymmetric~rectangular!waveguide with thex dimension larger than they dimension.Based on the following analysis, computed modal potions for the peaks are indicated in Fig. 3. The modeslabeled with the notationl, m, where the first index,l, is thex-component mode order, and the second index,m, is they-component mode order. Because of the difference inphase shift for TM and TE reflections from the waveguidthe two polarizations for a given transverse mode are slighnondegenerate. The splitting was not resolved in these stra and is calculated to be small, 1–6 GHz, due to the nunity index ratio. In an ideal distributed waveguide structuthe evanescent tails of the confined modes should be stroattenuated before reaching the outer edges of the mesacontrast, the present structure relies on a large index sover a relatively small fraction of the cavity length with nindex step for most of the cavity. The low index layer thuacts as a lumped optical element rather than a distribuguide. The strength of the lateral radiation from this structumay indicate that the lumped nature of the waveguidimore effectively scatters or diffracts light out of the wavguide.A zig-zag ray model9 is now used to analyze the observed modal structure and extract waveguide parametercluding the lateral dimensions and the index step. Curveting of the lateral modal dispersion presented in Fig. 4 alloa determination of the Fabry–Perot wavelength,kz , the ratioof the cladding to cavity index,c5nclad/ncav, and the lat-eral dimensions,dx anddy . The zig-zag ray formulation istopnllentThefrom2617Lear et al.ct¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jsp--.-used to establish a transcendental equation for the latwavevector eigenvalues9Akl ,m2 2~ky,m2 1kz2!ncavdx22 arctanS 1c2dTMA~ky,m2 1kz2!2kl ,m2 c2kl ,m2 2~ky,m2 1kz2! D 5 lp,where dTM51 for the TM mode and is zero for the TEmode and the cavity index has been factored out so thatwave-vector values are those external to the cavity whn51. Using the set of data forkl ,0 from thex-direction spec-tra, this equation was fit to obtain the three unknown quatities: the effective waveguide width,ncavdx513.0960.07 mm, the index ratio,c50.980460.0007, and themagnitude of the orthogonal component of the fundamenwave vector, Aky,02 1kz256.495 1460.000 09. Theanalysis of the k0,m data for the y direction yieldedncavdy510.8660.0 mm, the index ratio, c50.981960.0006, and the magnitude of the orthogonal componenthe fundamental wave vector,Akx,02 1kz256.495 4160.000 19. The fit to thedata is excellent as shown in Fig4 and quantified with a regression figure of better thR50.9999. The errors in the above parameters are thfrom the fit. The two sets of data agree well on the indratio of c50.981. The excellent agreement with the theofor a distributed waveguide is particularly noteworthy sincthe decreased index layer occupies only;3% of the effec-tive cavity length.The expectation value for the index has been calculain both the cavity and cladding regions. This effective indevalue is calculated as the spatial average index weighby the optical intensity in the cavity, ^n&5^Ecav(z)unuEcav(z)&/^Ecav(z)uEcav(z)&, where Ecav(z) isthe electric field. The calculated cavity index is^n&cav53.26, sothat the physical waveguide cross section is 433.3mm2. This index also results in an effective index steof Dn5(12c)^n&cav50.062. The effective claddingindex, computed using the index profile in the cladding bthe electric field distribution of the unoxidized cavity, i^n&clad53.16, giving a calculated index step of 0.10, inrough agreement with the above value. The uncertainty invertical extent of the low index oxide may contribute to thFIG. 4. Modal dispersion for the two horizontal emission spectra. The linshow theoretical fits used to obtain the adjustable parameters of index rand waveguide dimension.2618 Appl. Phys. Lett., Vol. 66, No. 20, 15 May 1995Downloaded¬29¬Oct¬2004¬to¬129.82.229.173.¬Redistribution¬subjeceraltheeren-talt of.anoseexryetedxted.0putstheedifference in the index step obtained by the two methods. Ineither case, the index step is much larger than those of;1023 typically employed for lateral confinement in edge-emitting lasers,10 but smaller than the huge index step of.2occurring in etched post VCSELs.The small lateral dimensions of the unoxidized cavityresult in a calculated in-plane mode spacing of 30–40 nm atwavelengths around 980 nm. The 2% effective index stepprovides only a small reflectivity for these modes, so thatlateral cavityQ factor is low. No distinct spectral features areassociated with amplified spontaneous emission from thesemodes, although a broad baseline in the horizontal signals isattributed to broadband lateral spontaneous emission. A fac-tor of 2 to 4 decrease in cavity size would separate in-planemodes sufficiently to place them outside the gain region.This in combination with higher lateral index steps couldproduce a partial suppression of in-plane spontaneous emission and thus lower the threshold current.We have demonstrated a small vertical cavity surfaceemitting laser with optical and electrical confinement pro-vided by a selectively oxidized layer. The laser has a roomtemperature 133mA threshold current and 53% slope effi-ciency. Many transverse modes are clearly seen in the subthreshold luminescence, but the laser operates single modeThe transverse mode wavelengths, taken from three-dimensional spectra, have been used to calculate waveguiding parameters including a moderate index ratio of 0.981 forthis structure. Yet smaller index steps are desirable for singlemode operation in larger diameter devices and can be ob-tained by positioning the oxidized layer further from the cav-ity and thus decreasing the field overlap with the oxidizedlayer. Despite the lumped nature of the index guiding, thetransverse modal dispersion agrees extremely well with thatof a conventional distributed waveguide.The authors thank R. Hadley for useful discussions andJ. Banas, J. Figiel, K. Geib, and J. Nevers for technical as-sistance. This work was supported by the United States De-partment of Energy under Contract No. DE-AC04-94AL85000.1D. L. Huffacker, J. Shin, and D. G. Deppe, Electron. Lett.30, 1946~1994!.2K. L. Lear, K. D. Choquette, R. P. Schneider, Jr., S. P. Kilcoyne, and K. M.Geib, Electron. Lett.31, 208 ~1995!.3K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, and K. M. Geib, Elec-tron. Lett.30, 2043~1994!.4K. L. Lear, R. P. Schneider, Jr., K. D. Choquette, S. P. Kilcoyne, J. J.Figiel, and J. C. Zolper, Photon Technol. Lett.6, 1053~1994!.5F. A. Kish, S. J. Caracci, N. Holonyak, Jr., J. M. Dallasasse, K. C. Hsieh,S. C. Smith, and R. D. Burnham, Appl. Phys. Lett.59, 1755~1991!.6K. L. Lear, S. P. Kilcoyne, and S. A. Chalmers, Photon. Technol. Lett.6,778 ~1994!.7T. R. Chen, B. Zhao, L. E. Eng, Y.H. Zhuang, and A. Yariv, in14th IEEEInternational Semiconductor Laser Conference~IEEE, Maui, HI, 1994!, p.26.8K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., and K. M. Geib~unpub-lished!.9H. C. Casey, Jr. and M. B. Panish,Heterostructure Lasers~Academic, SanDiego, 1978!.10G. R. Hadley, Opt. Lett.14, 308 ~1989!.esatioLear et al.t¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jsp

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Modal analysis of a small surface emitting laser with a selectively oxidized waveguide

Modal analysis of a small surface emitting laser with a selectively oxidized waveguide

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