Design and Evaluation of Fundamental-Mode and Polarization-Stabilized VCSELs With a Subwavelength Surface Grating

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 3, MARCH 2006 231

Design and Evaluation of Fundamental-Modeand Polarization-Stabilized VCSELs With a

Subwavelength Surface GratingÅsa Haglund, Johan S. Gustavsson, Jörgen Bengtsson, Piotr Jedrasik, and Anders Larsson, Member, IEEE

Abstract—We demonstrate 850-nm oxide-confined ver-tical-cavity surface-emitting lasers (VCSELs) with a locallyetched subwavelength surface grating that are single-modeand polarization stable from threshold up to thermal roll-over,reaching 4 mW of output power. The side-mode suppressionratio (SMSR) is 30 dB and the orthogonal polarization sup-pression ratio (OPSR) is 20 dB. Moreover, no distortion of thefar-field beam profile is observed as a result of the surface grating.Our numerical calculations show that a carefully designed VCSELcan have a high simultaneous mode and polarization selectivitywithout a significant increase in loss for the favored fundamentalmode with polarization state perpendicular to the grating lines.This indicates characteristics such as threshold current and res-onance frequency will not be notably degraded. The calculationsalso show a low sensitivity to variations in grating etch depthand duty cycle, which relaxes fabrication tolerances. In our ex-perimental parametric study, where the oxide aperture diameter,surface grating diameter, and grating duty cycle were varied, thecombined mode and polarization selection was investigated. Foran optimum combination of oxide aperture and surface gratingdiameters of 4.5 and 2.5 m, respectively, the device is found to besingle-mode and polarization stable for a broad range of gratingduty cycles, from 55% to 75%, with only a small variation in otherlaser performances, which is in line with theory.

Index Terms—Polarization, single-mode, subwavelengthgrating, surface relief, vertical-cavity surface-emitting lasers(VCSELs).

I. INTRODUCTION

THE VERTICAL-CAVITY surface-emitting laser(VCSEL) is a well established light source in short

distance fiber-optic links and interconnects. There is a definitescope for further and more demanding applications, such asin spectroscopy, laser printing, optical storage and longerdistance communication. Improvement of laser properties forspecific applications would probably yield a larger commercialimpact of the VCSEL. For example, in the above-mentionedapplications a single mode output power of several milliwatts isoften needed, and frequently with a stable linear polarization asan additional requirement. Due to the relative large transverseextent in combination with a symmetric geometry and isotropic

Manuscript received September 2, 2005; revised November 23, 2005. Thiswork was supported in part by the Swedish Foundation for Strategic Research(SSF) and the European FAST ACCESS Project (IST-004772). The epitaxialmaterial was provided by IQE (Europe) Ltd.

The authors are with the Department of Microtechnology and Nanoscience,Chalmers University of Technology, SE-412 96 Göteborg, Sweden (e-mail:[email protected]).

Digital Object Identifier 10.1109/JQE.2005.863703

material properties, the VCSEL tends to lase in several trans-verse modes with an unpredictable state of polarization. Thelinear polarization states of the individual modes lie in the planeof the epitaxial layers, and due to the electro-optic effect theyare normally polarized in the [011] or the [011 crystallographicdirection [1], [2]. However, the polarization often randomlyswitches between these two directions because of temperature,injection current, and optical feed-back effects.

A number of techniques have been developed to increase thesingle fundamental mode output power from VCSELs, of whicha few are mentioned here. These are based on affecting the trans-verse guiding and/or introducing mode selective loss or gain. Byusing an extended cavity [3] the higher order modes experiencea higher diffraction loss and a smaller overlap with the gain re-gion compared to the fundamental mode. The epitaxial structuremust be carefully designed to avoid switching between differentlongitudinal modes. Another technique is to localize the gain re-gion near the optical axis of the device using proton implanta-tion such that it favors the fundamental mode [4]. This techniquerequires a high precision in the alignment of the oxide aper-ture and proton implantation, and the small gain aperture hasso far resulted in a high differential resistance and a highly non-linear light–current characteristic. The concept of antiguidinghas also been explored [5], where lateral reflectors were usedto decrease the high radiation loss for the fundamental mode.The complex fabrication still remains an issue for this tech-nique. Finally, a spatially modulated mirror loss, achieved byetching a shallow surface relief, has proven to be successful inselecting the fundamental mode in oxide-confined VCSELs [6].This technique allows for relative large oxide apertures, whichdecrease the differential resistance and thereby self-heating, al-lowing for higher output powers [7]. The alignment between theoxide aperture and the surface relief is important, however, aself-aligned method [8] can be used to circumvent this problem.

The polarization state in VCSELs can be controlled by intro-ducing anisotropy in the waveguide such that only one polariza-tion state is supported, or by providing a polarization dependentgain/loss. For example, anisotropic gain can be achieved by epi-taxial growth on non-(100) substrates [9], [10]. Unfortunately,the OPSR was found to be significantly reduced during modula-tion [11]. This was later improved by combining the non-(100)growth with strained quantum wells (QWs) [12]. The growthand post-processing can be cumbersome. For instance, the oxi-dation rates are very different in the and orthogonalcrystallographic directions, making it difficult to produce a sym-metric oxide aperture [11]. Asymmetric current injection [13] is

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232 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 3, MARCH 2006

another technique that can produce anisotropic gain, where theoptical field polarized perpendicular to the direction of the trans-verse injection is favored. The asymmetric injection also affectsthe electro-absorption, which can in fact improve the polariza-tion selection. However, the technique has until now been in-sufficient in pinning the polarization in the direction [14],indicating that it does not dominate over other effects. Intro-ducing anisotropic strain by etching deep trenches close to theVCSEL aperture along the preferential polarization direction[15] has also shown to improve the OPSR. A related techniqueis to use strained T-bars [16], which has reduced polarizationflipping. Successful polarization control can be achieved by ex-ternal optical feedback that is polarization dependent, for ex-ample using an amorphous silicon subwavelength transmissiongrating [17], but this is a nonmonolithic solution. An attractivemonolithic technique is to use a surface grating so that a po-larization sensitive top mirror is obtained. Both semiconductor[18] and metal/semiconductor [19]–[21] surface gratings havebeen demonstrated, where the grating period is larger than theoptical wavelength in the material. The laser performance is thenvery sensitive to the grating geometry and an optimized designrequires rigorous electromagnetic modeling.

Only a few techniques have been proposed that canachieve mode and polarization control simultaneously. Theseinclude using small asymmetric cavity geometries, e.g.,rhombus-shaped, dumbbell-shaped [22], or rectangular-shaped[23]. Special care has to be taken when designing these cavitiesin order to minimize nonradiative recombination and diffrac-tion losses. Small asymmetric oxide apertures have also beeninvestigated [24]. The small aperture results in a high differ-ential resistance and a low output power, and the asymmetryyields a noncircular far field [25]. Another technique is to usephotonic crystal patterns with defects [26] that are etched deepinto the top mirror. These devices often show a high thresholdcurrent, which is caused by increased scattering loss, diffractionloss, and nonradiative surface recombination. Also, the deeplyetched photonic crystal normally obstructs the current injection,which often results in a high differential resistance. Finally,a locally etched surface grating can be used for combinedmode and polarization control in oxide-confined VCSELs. Thistechnique is much related to the above-mentioned surface relieftechnique for mode control, sharing many of its advantages.A grating with a period larger than the optical wavelength inthe material has been presented [27] and recently we havedemonstrated both experimentally [28] and theoretically [29]the use of a subwavelength grating. The advantage of usinga subwavelength grating compared to a larger grating periodis that the diffraction related losses and beam degradation areminimized. The fabrication is, however, challenging since itrequires high-resolution definition. It should be mentioned thatinstead of having a surface grating, an elliptical surface reliefcan be used [30], however, the polarization control has shownto be rather weak [31].

In this work, we have experimentally studied the achievablesingle-mode and polarization stable output power of 850-nmoxide-confined VCSELs with a locally etched subwavelengthsurface grating. We have also investigated how sensitive the per-formance is to the grating geometry by a detailed parametric

Fig. 1. Schematic top and cross-sectional views of two subwavelength surfacegrating VCSEL designs for simultaneous fundamental mode and polarizationselection. In (a), a conventional epitaxial structure is used and in (b) an extra�=4-thick layer is added to the epitaxial structure so that the grating structure is“inverted” as compared to the conventional case.

study. This provides information on the required precision inthe fabrication of an optimized design. Numerical calculationsof the polarization dependent modal loss have also been per-formed, which give a first indication of the range of gratingparameters that can yield a high combined mode and polariza-tion selectivity. The paper is organized as follows. In Section IIthe concept, design, and fabrication of subwavelength surfacegrating VCSELs is discussed and some representative experi-mental results are shown. The dependence of the mode and po-larization selection on grating etch depth, grating duty cycle,grating region diameter, and oxide aperture diameter are pre-sented in Section III, and a conclusion is given in Section IV.

II. SUBWAVELENGTH SURFACE-GRATING VCSELS

A. Technique for Combined Mode and Polarization Control

The purpose of having a locally etched subwavelength sur-face grating is to introduce a mode and polarization dependenttop mirror loss. Fig. 1 illustrates two ways of incorporating thegrating, which favors the fundamental mode with a polarizationstate parallel to the grating lines in case (a) and perpendicularto the grating lines in case (b). In both cases an approximatelyquarter-wavelength deep surface depression is etched in acircular area concentric with the oxide aperture. In the conven-tional case (a), a standard epitaxial VCSEL structure is used, andthe depression consists of an inner grating region and an outerfully etched region. In the “inverted” case (b), an extra -thicktopmost layer is added to the epitaxial structure, and the depres-sion consists only of a grating region. For both cases the topmirror loss is high in the area just outside the grating regionbecause the reflections at the semiconductor–air interface arein antiphase with the rest of the reflections further down in themirror stack. This is utilized to provide the fundamental modeselection. In an oxide-confined VCSEL the transverse modes

HAGLUND et al.: DESIGN AND EVALUATION OF FUNDAMENTAL-MODE AND POLARIZATION-STABILIZED VCSELs 233

Fig. 2. Schematic cross-sectional view of the studied oxide-confined VCSELdesign with an inverted subwavelength surface grating.

are guided by the oxide aperture. By having a grating regiondiameter that is smaller than the oxide aperture diameter the in-tensity distribution of the higher order modes will have a largeroverlap with this high-loss region compared to the fundamentalmode, and, therefore, experience a higher modal loss. The po-larization selection is provided by the grating region. Since theoptical field has a wavelength larger than the grating period it ex-periences the subwavelength grating as an anisotropic homoge-nous medium, having different effective indices for the paralleland perpendicular polarizations that are both higher than theindex of air but lower than the index of the grating material. Theanisotropic effective index can be tailored by the duty cycle suchthat for a polarization direction perpendicular to the grating linesit is close to the index of air and in the parallel direction it is closeto the index of the grating material. Thus, relatively strong an-tiphase reflections can effectively be produced by the grating forthe polarization state perpendicular to the grating lines in case(a) and parallel to the grating lines in (b), while in-phase reflec-tions are effectively produced for the corresponding orthogonalpolarization states, which, therefore, are favored.

B. Device Design and Fabrication

The investigated 850-nm ( - cavity) oxide-confined VCSELdesign is schematically illustrated in Fig. 2. The epitaxial struc-ture consists of an active region with three GaAs quantumwells and a top and bottom DBR with 22 and 34 layer pairs ofAl Ga As–Al Ga As, respectively. Graded interfacesand modulation doping is used in the mirror layers to reducethe differential resistance while maintaining a low free-car-rier absorption loss. A 30-nm-thick Al Ga As layer ispositioned just above the active region close to a node of thelongitudinal standing-wave intensity distribution. This layer isselectively oxidized to form the oxide aperture, which providesboth current and optical confinement. The resulting opticalwave-guiding is designed to have an effective index differencebetween the core (defined by the oxide aperture diameter) andcladding of . For selecting the fundamentalmode with a fixed polarization state the inverted subwavelengthsurface grating technique is used. This is because it has thefabricational advantage of significantly relaxing the tolerancein etch depth in a similar way as for an inverted surface re-lief technique reported earlier [7] which only provided modecontrol. Numerical calculations also indicate that the invertedtechnique offers a lower sensitivity to variations in the grating

Fig. 3. Microscope image (left) and SEM images (middle and right) of aVCSEL with an inverted subwavelength surface grating.

duty cycle. Thus, a 60-nm-thick topmost GaAs layer is addedto the epitaxial structure and a 60-nm-deep surface grating,having a period of 120 nm, is etched in a circular region intothe top mirror. This is indeed a subwavelength grating since thewavelength in GaAs is 240 nm. For a subwavelength grating,the anisotropy in effective index increases somewhat with areduced grating period [29]. A period of 120 nm is chosen sinceit enables fabrication of a broad range of duty cycles.

The epitaxial structure was grown by IQE (Europe) Ltd.using metal-organic chemical vapor deposition (MOCVD). Thegrating was first defined using a JEOL JBX-9300FS electronbeam lithograph, a 47 nm-thick ZEP520 resist, and o-xylenefor the development. There was no compensation for proximityeffects. The grating was then dry etched using an Oxford Ionfab300 chemically-assisted reactive ion beam etcher (CARIBE).An 1:1 ratio of Argon and Chlorine gas was applied to achievea low surface roughness and near-vertical 80 sidewallswith an etch rate of 17 nm/min. The next step was to deposita Ti–Au top ring contact and then to define the mesa by e-beamlithography to obtain a good alignment between the oxideaperture and the grating region. The mesa was dry etched usingthe same CARIBE equipment as for the grating, but in thiscase with an 1:2 ratio of Borontrichloride and Chlorine gas,resulting in an etch rate of 0.15 m/min. The mesa wasthereafter oxidized to form the oxide aperture, and the structurewas planarized using benzocyclobutene (BCB). Finally, the topp-bondpad and bottom n-contact were deposited. Fig. 3 showsmicroscope and scanning electron microscope (SEM) imagesof a fabricated subwavelength surface grating VCSEL.

C. Experimental Results

Fig. 4 shows measured polarization resolved light–current( – ) characteristics, optical spectra, and far-fields for two fab-ricated subwavelength surface grating VCSELs, which experi-mentally demonstrate the efficiency of the mode and polariza-tion selection. For best possible comparison the two VCSELsutilize a common epitaxial structure, and are located next toeach other with a separation distance of 250 m. Both deviceshave a 4.5- m oxide aperture diameter (resulting in a 90differential resistance), a 2.5- m grating region diameter, and a60% grating duty cycle. The duty cycle is defined as the ridge toperiod ratio. The grating grooves are oriented in the crys-tallographic direction in VCSEL (a) and in the perpendicular[011] direction in VCSEL (b). For the polarization resolved –characteristic measurements a focusing lens, a Glan–Thompsonpolarizer (having a 50-dB polarization extinction ratio), and abroad-area detector were used. The lens and polarizer had an-tireflective coatings and the detector was slightly tilted to mini-mize optical feedback effects. The – characteristics show that


Fig. 4. Output power (polarization resolved) and OPSR versus current, optical spectra, and far-field, for a subwavelength surface grating VCSEL with the gratinglines oriented along (a) the [011] direction, and (b) along the [011] direction. The oxide aperture diameter is 4.5 �m, the grating region diameter is 2.5 �m, thegrating duty cycle is 60%, and the grating etch depth is 60 nm.

both lasers are polarization stable, having a 20 dB power ratiobetween the two orthogonal polarization states (OPSR) fromthreshold up to the thermal roll-over where they reach 4 mWof output power. The polarization state is in both cases perpen-dicular to the grating grooves, i.e., it rotates with the orienta-tion of the grating. For the optical spectrum measurements agraded-index multimode fiber and an optical spectrum analyzer(having a 0.08-nm resolution) were used. To assure that all ex-isting transverse modes are coupled with equal efficiency, thefiber had a large core diameter (62.5 m) and was butt-coupledto the VCSEL. The optical spectrum measurements show thatboth lasers are also single-mode, having an SMSR of 30 dBover the entire operational range. The far-field was scanned be-tween 0 and 90 from the optical axis by a broad-area detectorwith a pin-hole in front. The detector was positioned 20 cmfrom the VCSEL and the pinhole diameter was 1.75 mm, re-sulting in a 0.5 angle resolution. The dynamic range of thesetup is 25 dB. The far-field measurements show that thereis no indication of beam distortion, i.e., no sidelobes, as a re-sult of the grating. To verify that the combined mode and polar-ization selection indeed stems from the subwavelength gratingthe performance was compared with two other VCSELs on thesame chip. They had the same geometry as VCSELs (a) and (b)apart from one having no subwavelength surface grating and noextra -thick topmost layer, i.e., it was a conventional “mul-timode” VCSEL, referred to as VCSEL (c) and the other havinga 0% duty cycle, i.e., it had a disk-shaped surface depressionthat only provides mode selection, referred to as VCSEL (d).VCSEL (c) was found to be multimode with 2–4 transversemodes depending on the current, and has an OPSR that variesunpredictably between 0 and 10 dB from threshold up to thermalroll-over. VCSEL (d) was single-mode, however, the polariza-tion state switches at a drive current twice the threshold currentand once again at the thermal roll-over. The threshold current,initial slope efficiency, and maximum output power are 0.23

mA, 0.65 W/A, 4.1 mW for VCSEL (c) and 0.26 mA, 0.73 W/A,3.8 mW for VCSEL (d), which is very similar to the values forVCSELs: (a) 0.30 mA, 0.80 W/A, 4.1 mW and (b) 0.32 mA,0.85 W/A, 3.9 mW, indicating that the subwavelength gratingdoes not significantly affect other important laser properties. Formore details on these experimental results, see [28].

III. DEPENDENCE OF MODE AND POLARIZATION SELECTION

ON GRATING GEOMETRY

To investigate the dependence of the combined mode and po-larization control on the grating geometry an experimental para-metric study was performed. The maximum polarization stablesingle mode output power was measured for different gratingduty cycles, grating region diameters, and oxide aperture diam-eters. The threshold current was also measured to further inves-tigate the grating induced loss for the favored fundamental modeand polarization state. A low loss increase for this mode due tothe grating is desirable to maintain a low threshold current and ahigh resonance frequency. To get a first indication of favorablegrating parameters and for limiting the number of fabricatedVCSELs with different grating geometries, numerical calcula-tions of the polarization dependent modal loss were first per-formed. Since the optical fields experience the subwavelengthgrating as a homogeneous material, a scalar method developedby Hadley [32] was applied. This method has frequently beenused to numerically calculate the optical fields in oxide-confinedVCSELs, which are characterized by near-paraxial propagationwith polarization states in the plane of the epitaxial layers. InHadley’s method, the longitudinal and transverse dependenciesof the electric field are separated. The longitudinal variationof the field is computed from an eigenvalue equation, wherethe real and imaginary part of the eigenvalue are related to acavity effective index and cavity loss, respectively. This is donefor all separate transverse regions. The results are then input to


Fig. 5. Calculated polarization dependent modal cold-cavity loss as a functionof grating etch depth for a subwavelength surface grating VCSEL with a 60%and 0% grating duty cycle. The oxide aperture diameter is 4.5�m and the gratingregion diameter is 2.5 �m.

another eigenvalue equation for the transverse variation of thefield. The real and imaginary parts of this eigenvalue are relatedto the modal wavelength and modal loss, respectively. The Afro-mowitz model [33] was applied to compute the refractive indexfor the different AlGaAs compositions in the epitaxial structure.For the subwavelength grating the anisotropic effective indicesfor the electric field polarized parallel or perpendicular to thegrating lines were approximated by well known analytical ex-pressions [34]. To assure that this approximation is valid fora 120-nm grating period the effective index values were com-pared with those obtained from two-dimensional finite-differ-ence time-domain calculations, and good agreement was found.

A. Grating Etch Depth

Fig. 5 shows calculated polarization dependent modal lossas a function of grating etch depth for a VCSEL with a 0%and a 60% grating duty cycle. The oxide aperture diameter is4.5 m and the grating-region diameter is 2.5 m. The modalloss is calculated for a polarization state parallel and per-pendicular to the grating lines. For a 60% duty cycle anda 60-nm etch depth a 20 cm mode selectivity, i.e., lossdifference between the fundamental mode (LP01) and the firsthigher order mode (LP11) both with a favored , and simul-taneously a 15 cm polarization selectivity, i.e., loss differ-ence between and , are achieved. More-over, these high values are maintained for etch depths 20 nmfrom the targeted etch depth of 60 nm without significantly in-creasing the loss for the favored . Thus, only asmall performance variation in threshold current, output power,and mode and polarization selectivity can be anticipated for thisbroad range of etch depths. The modal loss for a VCSEL witha disk-shaped surface depression (0% duty cycle) that only pro-vides mode-selection is included in Fig. 5 for comparison. At60-nm etch depth the loss for the favored is negli-gibly increased when the grating duty cycle is changed from 0%to 60%. The low sensitivity in etch depth around 60 nm is also

Fig. 6. Calculated polarization dependent modal cold-cavity loss as a functionof grating duty cycle for a subwavelength surface grating VCSEL. The oxideaperture diameter is 4.5 �m, the grating region diameter is 2.5 �m; and thegrating etch depth is 60 nm.

maintained. Finally one can note that the favored polarizationstate for the LP01-mode can be switched by etching the gratingdeeper.

B. Grating Duty Cycle

Fig. 6 shows calculated modal loss as a function of gratingduty cycle for a VCSEL with a grating etch depth of 60 nm. Theoxide aperture diameter is 4.5 m and the grating region diam-eter is 2.5 m. For duty cycles up to 80% a high mode selec-tivity of 20 cm is achieved without significantly increasingthe loss for the favored compared to a 0% duty cycle.The polarization selectivity increases with duty cycle for dutycycles up to 80%. This is a result of the increased anisotropyin the effective refractive index of the subwavelength grating.For the index becomes closer to the index of the gratingmaterial for larger duty cycles, providing stronger antiphase re-flections. Meanwhile for duty cycles up to 80% the index for

is maintained relative close to the index of air, resulting inin-phase reflections. Note that for duty cycles 70% the polar-ization selectivity actually exceeds the mode selectivity. Thus,a high polarization selectivity of 15 cm is achieved for abroad range of a duty cycles, from 50% to 80%, for which alsoa high mode selectivity is obtained. For duty cycles 80% theloss for the favored increases rapidly and the modeselectivity is particularly lowered. This is expected consideringthat a very high duty cycle almost corresponds to having an un-etched grating region, which provides a high top mirror loss thatis mode and polarization insensitive.

Physical subwavelength surface-grating VCSELs, corre-sponding to those simulated in Fig. 6, with grating duty cycleswithin the promising 50% to 80% range as well as a 0%grating duty cycle have been fabricated. Fig. 7 shows measuredmaximum single-mode and polarization stable output power

and threshold current for a particular duty cycle.is defined as the maximum output power where an

dB and an 20 dB are maintained


Fig. 7. Measured P and I as functions of grating duty cycle for asubwavelength surface grating VCSEL. The oxide aperture diameter is 4.5 �m,the grating region diameter is 2.5 �m, and the grating etch depth is 60 nm.

from threshold up to . For duty cycles between 55% and75% a high of 4 mW, limited by the thermal roll-over,is obtained while having a low of 0.4 mA, which is verysimilar to that of a 0% duty cycle. Note that the VCSEL witha 0% grating duty cycle is single-mode but not polarizationstable. For the highest duty cycle of 80%, is significantlyreduced and is increased, which is in line with the cal-culations for very large duty cycles. From measured opticalspectra and polarization-resolved light–current characteristicsit is observed that the onset of the limits . Thelower mode selectivity from an 80% duty cycle is thus unableto prevent the onset of higher order modes before the thermalroll-over. The onset of higher order modes is mainly caused bythe effects of spatial hole burning (spatial depletion of carriersin the active region from intense stimulated recombination),which are normally elevated at higher currents [35]. Further,in conventional “multimode” VCSELs the polarization state ofthe lasing LP11-mode is usually orthogonal to the polarizationstate of the lasing LP01-mode [1]. This is not the case for thesubwavelength surface grating VCSELs with an 80% gratingduty cycle, implying that the grating also significantly influ-ences the loss for the as the calculations indicate.To summarize, the small variations in performance amongthe subwavelength surface grating VCSELs with duty cyclesbetween 55% and 75% point to a low sensitivity to duty cyclevariations around a targeted duty cycle of 65%.

C. Grating Region Diameter

Fig. 8 shows calculated modal loss as a function of gratingregion diameter for a VCSEL with an oxide aperture diam-eter of 4.5 m. The grating duty cycle is 60% and the gratingetch depth is 60 nm. The polarization selectivity increases withgrating region diameter due to the increased overlap ofthe LP01-mode intensity distribution with the grating region.The mode selectivity on the other hand has an optimalthat is approximately half the oxide aperture diameter .

Fig. 8. Calculated polarization dependent modal cold-cavity loss as a functionof grating region diameter for a subwavelength surface grating VCSEL. Theoxide aperture diameter is 4.5 �m, the grating duty cycle is 60%, and the gratingetch depth is 60 nm.

For the mode selectivity decreases because theoverlap of the LP01-mode intensity distribution with the areaoutside the grating region increases significantly, and the loss forthe LP01-mode thereby approaches the high loss for the higherorder modes. For the mode selectivity also de-creases due to a reduced overlap of the LP11-mode intensitydistribution with the high-loss area outside the grating region,and the loss for the LP11-mode thereby approaches the low lossfor the LP01-mode.

VCSELs corresponding to those in Fig. 8, with grating regiondiameters between 2 and 3.5 m, i.e., close to the calculatedoptimum, as well as an 8- m grating region diameter, have beenfabricated. Note that the VCSEL with an 8 m grating regiondiameter has a duty cycle of 0% and thus in effect represents aconventional multimode VCSEL. Fig. 9 shows measuredand for the different grating region diameters. In this casewith m the most favorable is 2.5 m, givinga of 4 mW. A deviation in of 0.5 m from theoptimal 2.5 m reduces with 30% to 40%. Further, avery small decrease in is observed when is changed from2 to 3.5 m. Compared with the VCSEL with an 8- m gratingregion diameter and a 0% grating duty cycle, the conventionalmultimode VCSEL, the is only slightly higher. Thus, for theinvestigated VCSELs the incorporation of a surface grating doesnot notably affect the threshold current. Finally, for and2.5 m is limited by the thermal roll-over but forand 3.5 m is found limited by the onset of the

, which again is in line with the calculations. According toFig. 8 the mode selectivity is 20 cm and the polarizationselectivity is 10 cm for and 2.5 m, while themode selectivity is 20 cm and the polarization selectivityis 10 cm for and 3.5 m. Thus, if these numbershold also for the fabricated devices, the experiments show that aquite high mode selectivity of 20 cm is required to assurefundamental mode operation over the entire operational range,


Fig. 9. Measured P and I as functions of grating region diameter for asubwavelength surface grating VCSEL. The oxide aperture diameter is 4.5 �m,the grating duty cycle is 60%, and the grating etch depth is 60 nm.

Fig. 10. Calculated polarization dependent modal cold-cavity loss as afunction of oxide aperture diameter for a subwavelength surface gratingVGSEL with an experimentally optimized grating region diameter. The gratingduty cycle is 60% and the grating etch depth is 60 nm.

while a more moderate polarization selection of 10 cm issufficient for polarization stabilization.

D. Oxide Aperture Diameter

Fig. 10 shows calculated modal loss as a function of oxideaperture diameter for VCSELs with experimentally optimizedgrating region diameters with respect to .The duty cycle is 60% and the grating etch depth is 60 nm. Theoverall trend for the investigated range of oxide aperture diam-eters, of 3-5.5 m is that the calculated loss for the differentmodes and polarization states as well as their mutual loss dif-ferences do not change significantly with oxide aperture diam-eter. The correlated variations of the curves is due to that the

Fig. 11. Measured P and I as functions of oxide aperture diameter fora subwavelength surface grating VCSEL with an experimentally optimizedgrating region diameter. The grating duty cycle is 60% and the grating etchdepth is 60 nm.

fraction is somewhat different for the different de-vices. In general, for larger oxide apertures the optical field be-comes less confined in the transverse direction. If the fraction

is maintained as increases, the loss for the favoredincreases and the mode and polarization selectivity

decreases by the reduced overlap between the LP01-mode inten-sity distribution and the grating region. To compensate for thiseffect, i.e., maintain an optimized mode and polarization selec-tivity, one should increase such that the fractionincreases proportionally with . In fact, a higher mode se-lectivity can be achieved in a subwavelength surface gratingVCSEL having a larger oxide aperture since the overlap be-tween the LP01- and the LP11-mode intensity distribution is re-duced, but on the other hand, the loss for the favoredsomewhat increases as a result of the reduced optical confine-ment. For the studied VCSELs, however, the effect of reducedoptical confinement with oxide aperture is very small since theinvestigated range of oxide apertures is rather limited and theoptical wave-guiding from the oxide aperture in this design isquite strong .

Fig. 11 shows measured and for the same VCSELsas in Fig. 10. A maximum of 4 mW, limited by thethermal roll-over, is achieved with m. For smalleroxide apertures is also limited by the thermal roll-overbut decreases simply due to the fact that the available outputpower is reduced. The latter is a consequence of the strongerself-heating from the increased differential resistance.

For larger oxide apertures the available output power in-creases, however, decreases by the onset of thebefore the thermal roll-over. The higher order mode onset iscaused by the current injection into the active region becomingincreasingly nonuniform with increased oxide aperture, lowerin the central region and higher in the region around the edgeof the oxide aperture. This current injection profile favors thehigher order modes by primarily aggravating the effects ofspatial hole burning, which makes it more difficult to sustain


fundamental mode operation at higher currents despite the highmode selectivity of 20 cm that the surface grating pro-vides. If the nonuniformity in the current injection in some wayis reduced it should be possible to achieve a higher maximum

with a larger oxide aperture diameter. The uniformity canin principle be improved by increasing the current confinementwhile maintaining the optical confinement in the VCSEL. Thiscan be accomplished by, for example, using a tapered oxideaperture or by using proton implantation where the protonaperture is made smaller than the oxide aperture. The increasein will, however, be limited since there is a trade-offwith the increased self-heating from having a smaller currentaperture, which reduces the available output power. Further, an

mA is obtained for the limited range of investigatedoxide aperture diameters. The small increase in for thelargest oxide apertures is mainly caused by the increased activevolume, where a higher current is simply required to obtain agiven injected current density. The more nonuniform currentinjection into the active region also contributes to a higherthreshold current. This occurs since the higher injected currentdensity in the region around the edge of the oxide apertureresults in a larger amount of unused carriers that are lost byspontaneous recombination.

IV. CONCLUSION

We have experimentally investigated the combined modeand polarization control in 850-nm oxide-confined VCSELshaving a locally etched subwavelength surface grating (120-nmperiod) with different geometries. The surface grating intro-duces a mode and polarization dependent top mirror loss, andis incorporated via an inverted scheme: an extra -thicktopmost layer is added to the epitaxial structure, and thena -deep subwavelength surface grating is etched in acircular region, concentric with the oxide aperture. Basically,the spatial extent of the grating region determines the modeselectivity while the grating lines provide the polarization se-lectivity. Numerical calculations indicate that the fundamentalmode with a polarization state perpendicular to the grating linesis strongly favored for a grating region diameter that is abouthalf the oxide aperture diameter and for a grating duty cycleof 60%. This monolithic technique for combined mode andpolarization control in VCSELs allows for the use of relativelarge oxide apertures, which reduces device heating and enablesa higher output power. The use of a subwavelength grating min-imizes diffraction losses and beam degradation, but it requiresa high-resolution definition. In the fabrication we have usedelectron beam lithography for defining the grating and also themesa. Ultimately, electron beam lithography should be avoidedin large-volume production since it is associated with a highcost for large-area writing. Concerning our two exposures, thewriting time can be kept small for the grating exposure sincethe grating area is small. The mesa exposure can be replacedby a self-alignment technique [8], which accommodates forthe rather critical alignment between the grating and mesa.Looking for alternatives to electron beam lithography to furtherreduce the fabrication complexity, the nano-imprint technology

[36] seems promising, where features as small as 5 nm havebeen demonstrated [37]. Another potential alternative is to useholographic lithography.

We experimentally demonstrate devices, having an optimized4.5- m oxide aperture diameter, 2.5- m grating region-diam-eter, and 60% grating duty cycle, that are single-mode and po-larization stable (with polarization state perpendicular to thegrating lines) from threshold up to the thermal rollover, pro-ducing 4 mW of output power, and having a low thresholdcurreqt of 0.5 mA. Comparing to conventional “multimode”devices that do not have a surface grating and an extra -thicktopmost layer, the threshold current is negligibly higher as a re-sult of the grating. This indicates that the loss for the favoredfundamental mode is not significantly higher in a properly de-signed subwavelength surface grating VCSEL, and other impor-tant laser characteristics such as the resonance frequency shouldthus not be significantly degraded. It should be noted that a pos-sible loss increase due to the grating will in any case mainlybe a “useful” loss to the laser output, resulting in a somewhathigher slope efficiency. Another important observation from theexperimental parametric study is that high power VCSELs withpolarization stable, single-mode operation and a low thresholdcurrent can be achieved for a broad range of grating duty cycles(from 55 to 75%), which is also consistent with the results fromthe numerical calculations. No degradation of the far-field as aresult of the grating was observed for the ensemble of investi-gated devices.

The low performance sensitivity to variations in grating dutycycle, together with a similar low sensitivity to variations ingrating etch depth (40 to 80 nm), which the inverted gratingscheme benefits from, significantly relaxes the fabrication tol-erances of the grating. The effective index character of a sub-wavelength grating (making it fairly insensitive to the detailedshape of the grating grooves) also contributes to facilitate fabri-cation. Thus, the technique provides a promising route to highyield, moderate-cost, large-volume production.

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Åsa Haglund received the M.Sc. degree in physics from Göteborg University,Göteborg, Sweden, in 2000.

She then joined the Photonics Laboratory, Department of Microtechnologyand Nanoscience, Chalmers University of Technology, Göteborg, Sweden, andis currently working toward the Ph.D degree. Her research is focused on design,fabrication, and characterization of high-speed VCSELs, and mode and polar-ization control in VCSELs using surface structures.

Johan S. Gustavsson received the M.Sc. degree in electrical engineering andthe Ph.D. in photonics from Chalmers University of Technology, Göteborg,Sweden, in 1998 and 2003, respectively. His Ph.D. thesis dealt with mode dy-namics and noise in vertical-cavity surface-emitting lasers (VCSELs).

Presently, he is an Assistant Professor at the Photonics Laboratory, De-partment of Microtechhology and Nanoscience, Chalmers University ofTechnology. His current research includes design, modeling, and characteri-zation of long wavelength semiconductor lasers, and mode and polarizationcontrol in VCSELs using surface structures.


Jörgen Bengtsson received the M.Sc. degree in engineering physics and thePh.D. degree in photonics in 1997 from Chalmers University of Technology inGöteborg, Sweden, in 1993 and 1997, respectively.

Presently, he is an Associate Professor at the Photonics Laboratory , ChalmersUniversity of Technology. A major research topic has been the development ofnovel passive optical components using the inherent multifunctional capabilitiesof diffractive optics. More recently, this research was extended to the modelingand design of devices with diffractive structures integrated with, or being partof, active components such as DBR lasers and VCSELs, to create laser sourceswith new desired characteristics. Currently, his research is also oriented towardthe manipulation of partially coherent radiation from UV lasers as well as themodeling of disc lasers for high-power emission in the visible spectrum.

Piotr Jedrasik received the M.Sc. degree in electronics from Technical Uni-versity of Lodz, Lodz, Poland in 1982. In 1998, he received the D.Sc. degreefrom UIA University of Antwerp, Antwerp, Belgium for his work on proximityeffects correction in electron beam lithography.

He was with FONICA, Lodz, Poland, from 1982 to 1985 where he was de-veloping microprocessing industrial control systems. From 1986 to 1994 he waswith Solid State Physics Department, University of Lodz, where he worked withsolid-state thin films fabrication and characterization. From 1989 he became theElectron Beam Lithography Laboratory (EBLL) Manager with main activitybeing microfabrication of variety of microelectronic devices on piezoelectric,semiconducting, and polymeric materials. He joined VISIELAB of RUCA Uni-versity of Antwerp in 1994 as an assistant working with artificial intelligencemethods for multidimensional signal processing. From 1998 to 2000, he helda postdoctoral position at Applied Solid State Physics, Chalmers University ofTechnology, Göteborg, Sweden. He joined the staff at the MC2 Process Labora-tory, Chalmers University of Technology in 2000. At MC2, he is a Researcherin the nanoprocessing group with main activity being research around electronbeam nanolithography.

Anders Larsson (M’01) received the M.Sc. and Ph.D. degrees in electrical en-gineering from Chalmers University of Technology, Göteborg, Sweden, in 1982and 1987, respectively.

He was with the Department of Applied Physics, California Institute of Tech-nology, Pasadena, from 1984 to 1985, and from 1988 to 1991, he was with theJet Propulsion Laboratory, Pasadena. In 1991, he joined the faculty at ChalmersUniversity of Technology, Göteborg, Sweden. His background is in the areaof quantum-well materials and devices for optical communication, optical in-formation processing, and infrared detection. Currently, his research is focusedon surface-emitting lasers (vertical-cavity surface-emitting lasers and opticallypumped disk lasers), and new gain materials (GaInNAs and metamorphic ma-terials) for semiconductor lasers.

Design and Evaluation of Fundamental-Mode and Polarization-Stabilized VCSELs With a Subwavelength Surface Grating

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