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Photoluminescence measurements of quantum-dot-containing semiconductor microdisk resonatorsusing optical fiber taper waveguides

Kartik Srinivasan,1, ∗ Andreas Stintz,2 Sanjay Krishna,2 and Oskar Painter1

1Thomas J. Watson, Sr., Laboratory of Applied Physics,California Institute of Technology, Pasadena, California91125

2Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico(Dated: February 2, 2008)

Fiber taper waveguides are used to improve the efficiency of room temperature photoluminescence measure-ments of AlGaAs microdisk resonant cavities with embedded self-assembled InAs quantum dots. As a near-fieldcollection optic, the fiber taper improves the collection efficiency from microdisk lasers by a factor of∼ 15-100times in comparison to conventional normal incidence free-space collection techniques. In addition, the fibertaper can serve as a efficient means for pumping these devices, and initial measurements employing fiber pump-ing and collection are presented. Implications of this worktowards chip-based cavity quantum electrodynamicsexperiments are discussed.

PACS numbers: 42.60.Da, 42.50.Pq, 42.70.Qs

I. INTRODUCTION

The ability to efficiently couple light into and out of semi-conductor microcavities is an important aspect of many mi-crophotonic technologies1, and plays a vital role in chip-basedimplementations of cavity quantum electrodynamics (cQED)for quantum networking and cryptography2,3,4. While somegeometries, such as micropillar cavities, exhibit highly direc-tional emission that can be effectively collected5,6, couplingto wavelength-scale semiconductor microcavities is in generalnon-trivial4,7,8,9, due to a number of factors. These includethe size disparity between the modes of the microcavity andthose of standard free-space and fiber optics, as well as therefractive index difference between semiconductors and glassor air, as well as the potentially complicated cavity mode pro-files sustained by these devices. One technique that we haverecently employed to couple efficiently to semiconductor mi-crocavities is evanescent coupling through an optical fiberta-per waveguide10.

The fiber taper11,12 is simply a standard single mode fiberthat has been heated and stretched down to a minimum di-ameter on the order of a wavelength. Such fiber tapershave been used as near-ideal coupling channels for glass-to-glass coupling with silica-based microcavities such asmicrospheres13,14,15,16and microtoroids17. Our recent exper-iments have indicated that they can also serve as efficientcouplers to high-refractive index semiconductor-based de-vices, such as photonic crystal waveguides18, photonic crystalcavities10,19 and microdisks20,21. Here, we consider the useof fiber tapers within active semiconductor devices consistingof AlGaAs microdisk cavities with embedded InAs quantumdots21.

In Section II, we qualitatively describe the issues addressedin this paper, as well as the methods used in device fabrica-tion and the experimental setup we use. In Section III, webriefly review passive measurements in the 1200 nm wave-length band to determine the intrinsic optical losses of theop-tical resonant cavities under study. In Section IV, we presentexperimental results demonstrating the improvements thatre-

sult when free-space collection is replaced by fiber-based col-lection in photoluminescence measurements, while in SectionV, we present initial results on microdisk lasers that employboth fiber pumping and fiber collection. Finally, in SectionVI, we consider some of the applications of this work to fu-ture experiments.

II. PRELIMINARY DISCUSSION AND EXPERIMENTALMETHODS

The specific devices we consider in this work are AlGaAsmicrodisk cavities with embedded quantum dots (QDs). Theepitaxy used is shown in Table I, and consists of a singlelayer of InAs quantum dots embedded in an InGaAs quantumwell22, which is in turn sandwiched between layers of AlGaAsand GaAs to create a 255 nm thick waveguide. This DWELL(short for dot-in-a-well) material has a room temperatureground state emission peak at around 1190 nm (Fig. 3(a)), andis grown on top of a 1.5µm Al0.70Ga0.30As layer that eventu-ally serves as a support pedestal for the microdisk. Fabrica-tion of the microdisks (Fig. 1) is accomplished through thefollowing series of steps: (i) deposition of a 200 nm SixNymask layer, (ii) electron-beam lithography and a subsequentreflow of the resist, (iii) inductively-coupled plasma reactiveion etch (ICP-RIE) of the SixNy layer, (iv) ICP-RIE of the QD-containing waveguide layer, (v) photolithography and isola-tion of the microdisk onto a mesa stripe that is several micronsabove the rest of the chip (Fig. 1(b)), and (vi) HF acid etch ofthe Al0.70Ga0.30As layer to form the pedestal which supportsthe disk. The fabricated microdisks in this work areD ∼4.5µm in diameter.

The free-space collection from a whispering gallery mode(WGM) of a microdisk is a function of a number of factors,including the position and numerical aperture (NA) of the col-lection lens, the radiation pattern, and the quality factor(Q)of the resonant mode. Optical losses from the microdisk in-clude not only the (ideal) radiation due to radial tunnelingoflight from the disk periphery, but also scattering losses due tosurface roughness imperfections at the disk edge and material

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TABLE I: Epitaxy for 1-DWELL microcavity lasers.

Layer Materials Thickness

Surface cap layer GaAs 100◦

A

Top waveguidelayer

Al0.30Ga0.70As 400◦

A

Top waveguidelayer

GaAs 740◦

A

Quantum welllayer

In0.15Ga0.85As 60◦

A

Quantum dot layer InAs 2.4 monolayer

Barrier layer In0.15Ga0.85As 10◦

A

Bottom waveguidelayer

GaAs 740◦

A

Bottom waveguidelayer

Al0.30Ga0.70As 500◦

A

Sacrificial bufferlayer

Al0.70Ga0.30As 15000◦

A

Substrate GaAs N/A

absorption. For high-refractive index (n∼ 3.5) III-V semicon-ductor microdisks, surface roughness scattering is typicallythe dominate form of radiation from the microcavity. The in-trinsic radiation loss of semiconductor microdisks is almostnegligible in all cases, save the smallest of microdisks; theradiationQ of the lowest radial order WGM of theD ∼ 4.5µm microdisks studied here is greater than 1016 at the QDemission wavelength of 1200 nm, and is greater than 106 forD ∼ 1.5 µm. As such, any light that is collected through free-space methods is the result of scattering of the WGM off im-perfections in the microdisk8, a relatively inefficient and non-directional process. Bulk material absorption and absorptiondue to surface states also play a role, and for even high-purity(nominally undoped) AlGaAs microdisks we have found ab-sorption losses on par with scattering losses in the 1.2-1.5 µmwavelength range forD ∼ 5 µm microdisks fabricated usingthe above described procedure (note that absorption lossesrapidly increased below 1µm wavelength, an effect we arecurrently studying further). This results in a situation wherethe more perfect the microdisk is made (through reduction insurface roughness), and the further theQ factor is improved,the more difficult it becomes to collect light from the resonantmodes. Although there may be some potential in modifyingthe disk geometry4,8 to improve this situation (for example, byetching a shallow second-order grating in the microdisk sur-face), the ability to do this while maintaining highQ factorscould be of potential difficulty. The most successful methodtodate for increasing collection efficiency from semiconductormicrodisk resonators seems to be placement of the collectionoptics in the plane of the disk23,24, resulting in more effectivecapture of the predominantly low-angle scattered light.

FIG. 1: (a)-(b) Scanning electron microscope (SEM) images of fab-ricated microdisk structures. Image (b) shows the isolation mesa thatis incorporated in order to aid in the taper testing. An optical fibertaper aligned to the side of a microdisk is also visible in this image.

The fiber taper offers an attractive alternative because it pro-vides a means to directly couple light out of the WGMs, with-out relying upon the weak intrinsic radiation of the microdiskor the non-directionality of surface roughness scattering. Thisevanescent near-field coupling, which is a function of the in-tegrated modal overlap of the microdisk and taper modes overthe interaction region25,26, has been demonstrated to be appre-ciable in previous works with small diameter semiconductormicrodisks20,21where phase-matching between the glass fibertaper waveguide and the semiconductor microdisk is not aslimiting. While the fiber taper does load the cavity mode, andthus degrade itsQ, the key point is that the added loss is pri-marilygood lossin the sense that it can be efficiently collectedinto the taper mode of interest16,19. This allows for the loadedQ to be maintained at a high value while simultaneously ob-taining high collection efficiency20,21. The situation is analo-gous to the case of a Fabry-Perot cavity where one mirror isintentionally made to have a slightly lower reflectivity forout-put coupling, which limits theQ of the cavity, but not beyondsome acceptable level. While in that case, the cavityQ is fixedby the mirror reflectivities, here we have some flexibility overtheQ and the amount of loading by adjusting the cavity-taperseparation.

To compare free-space and fiber-taper-based collection, weuse the experimental setup depicted in Fig. 2, which con-sists of a fiber taper probing station that has been incorporatedinto a standard photoluminescence (PL) measurement setup.The taper probing station consists of a motorized X-Y stage(50 nm encoded resolution) on which the microcavity chip isplaced, while the fiber taper waveguide is held in a “u”-shaped

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FIG. 2: Note: Figure is to extend over both columns. (a)Experimental setup for studying the QD-microdisk devices. The sample is mountedon an X-Y stage with 50 nm encoded resolution for positioning, and a pump beam atλp = 830 nm is directed through an ultra-long workingdistance objective lens (NA = 0.40) at normal incidence to the sample surface. The free-space pump laser power is monitored by using a830 nm wavelength 50/50 non-polarizing beamsplitter (NPBS) with a calibrated photodetector (PD) on one of the ports. The QD free-spacephotoluminescence in the 1200 nm band is collected at normalincidence from the sample surface using the same objective lens for pumpfocusing, is transmitted through the 830 nm NPBS and a long-pass pump rejection filter (LPF), and is finally collected intoa multi-mode fiber(MMF) using an objective lens with NA = 0.14. The pump laser and photoluminescence beams are shown as light gray and black arrows,respectively. To allow for fiber taper measurements, the fiber taper is strung across the sample and positioned in the near-field of the microdiskfrom above, thus allowing simultaneous (normal incidence)free-space and fiber taper optical pumping and photoluminescence collection. Theoutput of the fiber taper can either be connected to an InGaAs photodetector (PD) for wavelength scans using the swept tunable laser sourcein the 1200 nm band, or to an optical spectrum analyzer (OSA) for analysis of the photoluminescence from the microdisk. The OSA is alsoused to analyze the free-space photoluminescence using a MMF input. Alignment of the pump beam and the fiber taper to the microdiskis performed by imaging through the pump and collection objective lens, as shown in the inset (a mirror flips in-and-out ofthe free-spacephotoluminescence beam path to direct the image to a 12X zoomimaging system). (b) Schematic of the fiber taper probing geometry in whichthe fiber taper is mounted in a “u”-shape configuration on a Z-axis stage with 50 nm encoded resolution for near-field positioning of the taper.(c) Schematic of the taper-to-microdisk interaction region, showing the resonant fiber taper coupling to WGMs of the microdisk.

configuration on an acrylic mount as shown in Fig. 2(b). Theacrylic fiber mount is attached to a separate motorized Z-axisstage (50 nm encoded resolution) so that the fiber taper canbe precisely aligned to the microdisk (the taper moves verti-cally and the sample moves in-plane). The entire taper prob-ing setup (motorized stages, microcavity chip, and fiber taperwaveguide) is mounted onto a larger manually actuated X-Y-Z

stage that is positioned underneath a ultra-long working dis-tance objective lens (NA = 0.4). This microscope objectiveis part of a PL setup that provides normal incidence pumpingand free-space collection from the samples. The pump laserin the majority of the measurements is a 830 nm laser diodethat is operated in quasi-continuous-wave operation (280 nspulse width, 300 ns period). The pump beam is shaped into

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a Gaussian-like profile by sending the laser beam through asection of single mode optical fiber, and is then focused ontothe sample with a spot size that is slightly larger than the sizeof the microdisk (area∼ 18µm2). Luminescence from the mi-crodisks is wavelength resolved by a Hewlett Packard 70452Boptical spectrum analyzer. All of the measurements presentedhere were performed with the sample maintained in a roomtemperature environment, with no active cooling or tempera-ture control.

This integrated setup allows for a number of different mea-surements to be made. Passive measurements of the microdiskresonant modes are performed by connecting the input of thefiber taper to a 1200 nm band scanning tunable laser. Thepolarization of the tunable laser output at the fiber taper inter-action region with the microdisk (coupling to the WGMs ofthe microdisk is polarization sensitive) is controlled using apaddle wheel polarization controller (PC). The light transmit-ted past the microdisk resonator is fed from the optical fibertoa photodetector in order to monitor the wavelength-dependenttransmission. Photoluminescence measurements can be donein any of four potential configurations (i) free space pumping,free space collection: here, the fiber taper plays no role, andthe vertically emitted power from the disks is collected into amultimode optical fiber that is then fed into the OSA; (ii) freespace pumping, fiber taper collection: here, the output of thefiber taper is connected to the OSA; (iii) fiber taper pumping,free space collection: here, the input of the fiber taper is con-nected to a fiber-coupled pump laser; (iv) fiber taper pumping,fiber taper collection: here, the free-space optics used in thestandard PL measurements play no role.

III. MEASUREMENT OF CAVITY Q IN THE 1200NMWAVELENGTH BAND

The devices studied in this work have been previously char-acterized in the 1400 nm band, andQs as high as 3.6×105

have been measured21. Those measurements were done atwavelengths significantly red-detuned from the QD emis-sion band, where QD absorption and material absorption inthe GaAs/AlGaAs waveguide layers are expected to be quitesmall. To confirm that the cavityQs are still high near theground-state QD emission wavelength (peaked near 1190 nmas shown in Fig. 3(a)), we perform passive fiber-taper-basedmeasurements10,20,21in the 1200 nm band49. The input of thetaper is connected to a fiber-coupled scanning tunable laserwith a wavelength range of 1215-1265 nm and a spectral res-olution of better than 0.01 pm. The transmission past the mi-crodisk is monitored by connecting the taper output to a fiber-pigtailed photodetector. The taper-WGM coupling is sensi-tive to polarization, and here coupling was optimized for TE-like WGMs with electric field polarized predominantly in theplane of the microdisk. The fiber taper is brought into thenear-field of the microdisk by first vertically aligning the fibertaper within the plane of the microdisk and then bringing it infrom the side towards the disk edge (Fig. 2(c)). Resonancesbegin to appear within the taper’s transmission spectrum whenit is several hundred nanometers to the side of the microdisk.

The high-Q resonances within the transmission spectrum, un-der closer inspection, are seen to consist of a pair resonancesdips (inset to Fig. 3(b)). The two resonance dips (doublets)correspond to standing wave modes that are formed whensurface scattering couples and splits the initially degenerateclockwise and counterclockwise traveling wave WGMs of themicrodisk. Such doublet resonances have been observed by anumber of authors for different whispering-gallery geometriesand materials20,27,28.

FIG. 3: (a) Photoluminescence from an unprocessed region ofthe1DWELL material. (b) Passive taper-based measurements of ami-crodisk in the 1200 nm band. The insets show high resolution scansfor two sets of doublet modes for this device. These high resolutionscans are taken when the taper-microdisk separation is a fewhundrednm and the depth of coupling is∼5-10 %.

Finite-element frequency mode solutions29 of the WGMresonances of the microdisks studied in this work (D = 4.5µm) show that the free spectral range (FSR) is∼ 40 nm in the1200 nm wavelength band for TE-polarized modes of low ra-dial mode number (q = 1,2,3,4). Higher radial order WGMs(q≥ 5) are expected to show up only very weakly in the fibertaper transmission owing to their small radiation limited Qfactors (. 104) and significantly larger overlap with the sup-port pedestal. From the broad spectral wavelength scan shownin Fig. 3(b), a pair of deeply coupled resonant modes sepa-rated by a full FSR are observed (λ ∼ 1222 and 1263 nm),as well as several other deeply coupled resonant modes. Dueto the extended nature of the higher order radial modes and

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their better phase-matching to a low-index glass waveguidesuch as the fiber taper, the coupling to the lowest orderq = 1WGM is typically lower than that of theq = 2 mode for simi-lar sized microdisks20,26. We believe that these doublet modesat λ ∼ 1222 nm andλ ∼ 1263 nm are first order (q = 1) ra-dial modes, while the mode atλ ∼ 1242 nm is probably aq = 2 radial mode. The broader and more weakly coupled in-termediate modes are most likely higher order radial modes,q = 3,4 (higher order slab modes in the vertical direction ofmicrodisk are also a possibility, though less likely due to theirreduced radiationQ). Examining the linewidth of the doubletresonances when the taper is relatively far away from the mi-crodisk gives an estimate for the cold-cavity, unloadedQ ofthe modes.Qs as high as 2.2×105 at∼1260 nm and as highas 1.9×105 at ∼1220 nm are measured in these microdisks(insets of Fig. 3(b)), the latter of which is only 30 nm red-detuned from the peak of the QD emission spectrum (Fig.3(a)). TheseQ factors are still quite high for a wavelength-scale AlGaAs microcavity23,24,30,31,32, and correspond to acavity decay rate ofκ/2π ∼ 0.6 GHz for resonant modes withan effective mode volume of onlyVeff ∼ 6(λ/n)3. Neverthe-less, some degradation in the quality factors from those previ-ously measured in the 1400 nm band are observed. These arebelieved to be at least in part due to absorption in the QD lay-ers, as evidenced by the emission in the PL spectrum at thesewavelengths (Fig. 3(a)).

IV. IMPROVED COLLECTION EFFICIENCY WITHFIBER TAPERS

We now turn to the heart of the current work, which is astudy of the gains in efficiency that can be achieved by us-ing optical20,21 fiber tapers as a collection tool in PL mea-surements. This is initially done by comparing the amount ofpower obtained in free-space and fiber taper collection con-figurations, while maintaining identical free-space pumpingconditions (in terms of pump-beam intensity and pump beamposition). The free-space collection for a microdisk that hasbeen pumped at normal incidence with∼ 580 W/cm2 at 830nm is shown in Fig. 4(a). This pump intensity incident is nearthe laser threshold for this device (see below), and we see thatthe peak height atλ ∼ 1193.5 nm is∼ 30 pW. For comparisonto the fiber taper collection described below, an estimate ofthe optical losses in the free-space collection setup were made(after removal of the pump rejection filter). By measuring thecollected pump laser power reflected off of the mirror-qualitysurface of the AlGaAs epitaxy, and assuming a 30% reflec-tion coefficient from the AlGaAs surface, 43% of the reflectedpump beam was collected into the OSA. Additional limita-tions in the normal incidence free-space collection stem fromthe finite numerical aperture of the collecting lens (NA=0.4)which covers only 4% of the full 4π steradians.

Next, we consider the use of the optical fiber taper as a col-lection optic in the PL measurements. To obtain an estimate ofthe amount of coupling between the taper and the microdisk,the free-space pump beam is blocked, and passive measure-ments at wavelengths that are slightly red-detuned from the

FIG. 4: (a) Emission spectrum and (b) Light-in-vs-Light-out (L-L)curve for normal incidence free-space collection from a (free-space)optically pumped microdisk. Fiber taper collected (c) emission spec-trum and (d) L-L curve from the same microdisk resonator, underidentical pumping conditions. The emission spectrums for both thefree-space and fiber taper collection configurations were taken nearlaser threshold at∼ 580 W/cm2 of incident pump beam intensity.The fiber taper collected power included that from the forward prop-agating transmission channel only. Unless otherwise specified, in allof the measurements described in this work, a detector sensitivity of∼ 1 pW is used, and the resolution bandwidth of the OSA is set to0.1 nm.

QD emission are performed as described above in Section III.Since the FSR for the low radial number WGMs of the mi-crodisks studied here is∼ 41 nm in the 1200 nm wavelengthband, the modes coupled to and studied passively are typicallya single FSR red-detuned from the lasing mode. For most ex-periments, the taper is placed in direct contact with the topedge of the microdisk, which increases the amount of couplingfrom that shown in Fig. 3(b) to transmission depths between30−60%. For this initial measurement, a resonance depth of∼38% is obtained for a cavity mode atλ ∼ 1238.1nm, whichgives us a qualitative estimate for the coupling to WGMs over-lapping the peak of the gain spectrum50. This coupling depthcorresponds to a taper collection efficiencyη0 ∼ 11%, whereη0 is defined19 as the fraction of the optical power from thecavity resonant mode that is coupled into the fundamentalfiber taper mode in the forward propagating transmission di-rection. Other loss channels from the microdisk include in-trinsic loss of the cavity in absence of the taper, parasiticcou-pling into higher-order, non-collected modes of the fiber taper,and for the standing wave modes studied here, coupling intothe backwards propagating fundamental taper mode. For themoderate coupling depths measured here, the taper couplingefficiency into the forward and backward propagation direc-tions is approximately equal, thus yielding an overall taper

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coupling efficiency ofη′0 ∼ 22% for this WGM.

Once this level of coupling has been achieved, the tun-able laser output is blocked, the free-space pump beam is un-blocked, and the output of the fiber taper is disconnected fromthe photodetector and connected to the OSA to measure theemitted power from the microdisk. Fig. 4(c) shows the re-sulting spectrum collected by the fiber taper in the forwardpropagating transmission direction. We see that at the wave-lengthλ ∼ 1193.5 nm, the peak height is∼ 725 pW, which isnearly a factor of 25 times improvement over the peak heightobserved in normal incidence free-space collection. In addi-tion, a significant amount of power is present within modesthat were not detectable in the free-space case (the noise floorof the OSA was approximately 1 pW), due to the poor effi-ciency of collection in this configuration.

This straightforward comparison of the collected powersfor a single pump power is not necessarily the most appropri-ate comparison, however. The reason for this is that the fibertaper loads the cavity, thus decreasing theQ of the resonantmodes and increasing the threshold pump power, so that fora given pump power the laser is not equally above thresholdin the two measurements. Another, more appropriate compar-ison is the differential collection efficiency above threshold,which we labelξ. This is determined by measuring a light-in-light-out (L-L) curve for the microdisk and taking the slopeof this curve above threshold. In these curves, the light-outis taken to be the total power within the laser line, while thelight-in is taken to be the estimated absorbed pump power.The absorbed pump power is determined by multiplying thepump beam intensity by the area of the microdisk to get anincident pump power (the beam overlaps the entirety of thedisk), and then multiplying this value by the absorption ofthe microdisk at 830 nm. We estimate this absorption to be∼ 10%, assuming an absorption coefficient of 104 cm−1 inthe GaAs, quantum well, and QD layers33, and a reflectioncoefficient of 30% at the GaAs-air interfaces at the top andbottom of the disk. The resulting L-L curves are shown inFig. 4(b),(d) for both free-space and fiber-taper collection.We see that the threshold pump power has indeed increased inthe case of fiber-taper collection, but that the differential ef-ficiency has also significantly improved and is more than 70times that of the free-space value.

To study the tradeoffs betweenξ and threshold moreclosely, in Fig. 5(a) several L-L curves are plotted, each for adifferent taper position with respect to the microdisk (note thatthe microdisk studied here is not the same as the one studiedabove, but the qualitative behavior is identical). The differenttaper positions correspond to a varying level of coupling be-tween the microdisk and taper, which we again qualitativelyestimate through measurements of the coupling to a microdiskWGM that is red-detuned from the QD emission in the 1200nm band. From Fig. 5(a), we see that in general, both thethreshold power andξ increase with increasingη0. As mightbe expected, in the course of these measurements it was pos-sible in some cases to load the microdisk strongly enough todegrade the initial laser mode’sQ to the point that it no longerlases, and a different mode (with a higher loadedQ) begins tolase.

FIG. 5: (a) L-L curves for free-space pumping and fiber taper collec-tion at different taper positions. For each curve, we note the thresh-old pump power (Pth), the fiber taper collection efficiency (η0) for amode that is red-detuned from the peak QD emission, and the abovethreshold differential efficiency (ξ). (b) Scatter plot of the differen-tial efficiency for fiber taper (filled circles) and free-space collection(open circles) for a number of different microdisk lasers. For thesemeasurements the fiber taper collected power included that from theforward propagating transmission channel only.

A number of different microdisk devices have been studied,and the results described above are fairly consistent from de-vice to device, withξ routinely 1-2 orders of magnitude largerwhen fiber taper collection is employed. A scatter plot forsome of this data is shown in Fig. 5(b). Despite the signifi-cant improvement obtained using the fiber taper, we see thatthe largestξ measured is roughly 10 nW/µW, which meansthat only 1% of the pump photons are converted to a col-lected signal photon, and we should thus consider whyξ isfar below unity. We first note that when considering collec-tion into both directions of the fiber taper,ξ is actually 2%for the standing wave WGMs of the microdisks studied here.A measure of the fiber taper collection efficiency of the mi-crodisk WGM laser light,η0, from the passive wavelengthscans described above indicate that the external fiber tapercol-lection efficiency should be as high as∼ 22% (correspondingto 11% for the forward transmission direction only). The total

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loss through the fiber taper and all of the fusion-splices andconnections in the fiber path was measured to be∼ 1.6dB inthese experiments. If symmetric loss in the taper about the mi-crodisk coupling region is assumed, this corresponds to a lossof ∼ 17% of the WGM laser photons collected by the taperbefore they reach the OSA. Taken together, these two factorsput an estimate of the upper bound on the fiber-coupled exter-nal laser efficiency of 18% for collection into both directionsof the fiber.

The roughly order of magnitude difference between themeasured (2%) and expected (18%) differential laser effi-ciency may be a result of several factors involving the complexdynamics within the DWELL active region. Previous mea-surements of DWELL injection lasers in stripe geometries22

indicate that the internal quantum efficiency of the quantumdots isηQD

i ∼ 0.5 (this is roughly the percentage of carrierscaptured by the QDs in the DWELL structure that contributeto stimulated emission above threshold). This factor can cer-tainly change from growth to growth, and given that the laserthreshold values are roughly 2-2.5 times higher than that mea-sured in previous work on identically fabricated devices froma different wafer growth21, we might qualitatively expectξ tobe reduced by a factor in the range of 2-5 due toηQD

i .Both the spectral and spatial distribution of carriers within

the microdisk may also lead to reductions in the laser differ-ential efficiency through incomplete clamping of the sponta-neous emission into the non-lasing modes of the microdiskabove threshold. To examine such effects in our structures,we measure L-L curves (Fig. 6(a)) for a number of the mostprominent non-lasing WGMs of the microdisk studied in Fig.4. We see that the emission into these modes is largelyclamped above the threshold for the lasing mode (estimatedto be 10.7µW of absorbed pump power). The aggregate effectis clearly seen in Fig. 6(b), where the power into the non-lasing WGMs has been summed and plotted along with theL-L curve for the lasing mode. Such clamping has been re-ported by other authors for similarly sized microdisks34, whilesmaller microcavity devices with a larger laser mode sponta-neous emission rate, have exhibited a gradual rollover and/orincomplete clamping of spontaneous emission7,34,35,36. Mea-surement of the background spontaneous emission into non-WGM, radiation modes of the microdisk was performed us-ing free-space collection (the fiber taper is much more sen-sitive to WGM emission than to emission from the center ofthe microdisk into radiation modes), and did show incompleteclamping of the spontaneous emission. This sort of spatialhole burning has been predicted in numerical modeling of mi-crodisk cavities37. If this is the case, the effective pump area islimited to a region about the WGM. Assuming that the WGMradial width is approximately(λ/neff), whereneff is the effec-tive refractive index in the plane of the microdisk51, this cor-responds to a 7µm2 area in the devices under test here. Sincethe total disk area is∼ 16µm2, then only 7/16 of the pumpphotons would be effectively pumping the WGM. Includingthis factor brings the expected value ofξ within the range ofexperimentally measured values.

Aside from reducing taper loss (loss< 0.5dB can be eas-ily achieved in our lab),η0 is the main parameter that can be

FIG. 6: (a) L-L curves for non-lasing modes of the disk studied inFig. 4. (b) Total power in the non-lasing modes (shown by opencircles), showing saturation for pump values close to the thresholdvalue for the lasing mode (shown by asterisks). Note that they-axisscale for the lasing mode is 15 times larger than that for the non-lasing modes.

improved upon to increaseξ. This can be done by adjustingthe geometry of the disk (using thinner disks, for example)to bring the effective index of the WGMs of the semicon-ductor microdisks closer to that of the silica fiber taper, sothat more efficient coupling can be obtained. A study of suchmodifications in Si microdisk structures has been undertaken,and the regimes of critical coupling and overcoupling havebeen achieved26. In addition, if spatial hole burning is signif-icant, another factor that could potentially be improved isthemethod of pumping. In particular, the pumping beam could beshaped to preferentially pump the perimeter of the microdisk(i.e., an annular-shaped beam could be used). Alternately,asdiscussed below, a fiber taper could be used to pump the mi-crodisk.

V. FIBER-PUMPED MICRODISK LASERS

In addition to improving the collection efficiency, opticalfiber tapers have the potential for improving the pump effi-

8

ciency of these QD-containing microdisks; such an effect hasin fact been demonstrated in previous work on doped glassmicrocavities38,39,40. In particular, if the pump laser is reso-nant with a WGM of the microdisk, light can be absorbed withhigh efficiency, and in the case of critical coupling, completepower transfer can be achieved. This should be contrastedwith the case of free-space pumping, where only a small per-cent (10% for the devices we’ve considered here) of the in-cident pump light is absorbed by the device, and some of thisabsorption is in a region (the center of the microdisk) that doesnot contribute to useful gain for the resonant WGMs37. On theother hand, fiber taper pumping delivers pump light directlytothe perimeter of the disk. As we shall discuss in the followingsection, the ability to efficiently optically access microcavi-ties is also of particular importance to future quantum opticsexperiments.

FIG. 7: (a) L-L curve for a QD-microdisk device where the fibertaper is used for both pumping (atλ = 967.6 nm) and collection.The absorbed power was estimated to be 66% of the input power inthe fiber taper. (b) Sub-threshold spectrum for this device taken at anestimated absorbed power of 37µW (highlighted point in (a)).

For an initial demonstration, we use a tunable laser oper-ating in the 980 nm band as a pump source. The 830 nmpump laser is not used in this experiment because the absorp-tion within the microdisk at this wavelength is too large toallow uniform pumping of the microdisk perimeter (the pump

light is absorbed before a single round trip around the cav-ity can be made). At 980 nm, the material absorption is stillrelatively high (in particular, the quantum well layer willbehighly absorbing), so that theQ980 of WGM modes near thepump wavelength are not expected to exceed a few hundred.The 980 nm tunable laser is connected to the fiber taper input,and the fiber taper output is connected to the OSA. The taperis contacted to the side of the microdisk, and the pump wave-length and polarization of the tunable laser are manually ad-justed until the collected power in the OSA is maximized (thisis necessary in order to resonantly couple to a mode withinthe pump wavelength band). A typical L-L curve and sub-threshold spectrum are shown in Fig. 7. We see that a sig-nificant amount of power is collected into the fiber taper, andthat in particular, the sub-threshold spectrum shows a numberof well-resolved modes with a good signal-to-noise ratio. Theestimated absorbed pump power in the microdisk displayed inFig. 7(a) corresponds to 66% of the input power in the fibertaper, and is found by taking the difference in the 980 nmband taper transmission between when the taper is displacedtens of microns above the microdisk (no coupling) and whenit is in contact with the microdisk (strongly coupled). We notethat the pump threshold value in this pumping geometry isonly about a factor of two less than theincidentpump powerin the 830 nm free-space pumping, and is significantly largerthan what might be expected (ideally, the pump power hereshould be less than theabsorbedpump power in the 830 nmpumping). This is most likely a result of the relatively crudemethod we have employed to estimate the power absorbed inthe microdisk; a much more accurate method for determin-ing the coupled pump power uses the wavelength-dependenttransmission of the fiber taper to map out the resonance linedue to the WGM at the pump wavelength. Here, the strong ab-sorption of the microdisk in the 980 nm band makes it difficultto separate resonantly coupled power from scattering lossesat the taper-microdisk junction. In order to more carefullystudy the efficiency of this fiber-pumping and fiber-collectingconfiguration, experiments in which an excited state of thequantum dots is resonantly pumped through the fiber taper arecurrently underway.

VI. DISCUSSION AND FUTURE APPLICATIONS

As mentioned in the introduction of this paper, efficient op-tical access to wavelength-scale microcavities is of greatim-portance to quantum optics and information processing appli-cations currently being investigated within cavity QED sys-tems. In almost any application involving the coherent trans-fer or manipulation of quantum information, loss is a signifi-cant detriment. As described in Ref. 41, current implementa-tions of linear optics quantum computing require a near-unitycollection efficiency of emitted photons from a single photonsource. The same is true for applications involving quantumrepeaters in a quantum network42. A solution that is oftenproposed is to embed the single photon emitter within a mi-crocavity with a high spontaneous emission coupling factorβ,so that the majority of emitted photons are coupled into the

9

microcavity mode. However, it is important to note that evenfor a β = 1 microcavity, it is still necessary to have a methodto effectively collect all of the photons that are radiated bythat one cavity mode43. Also, from a very practical perspec-tive, efficient collection of emitted light from a microcavity isof premium importance for optical telecommunication wave-lengths> 1µm, where the dark count rates from single photoncounters are often 2-3 orders of magnitude larger than the Sisingle photon counters used at shorter wavelengths44.

An efficient coupling channel can also enable a number ofdifferent types of experiments. Having access to this cou-pling channel makes the cavity transmission (and reflection)an experimentally accessible parameter whose behavior canbe monitored to detect signatures of specific types of systembehavior. In recent experimental measurements of couplingbetween a single quantum dot and a resonant mode of a semi-conductor microcavity5,6,30,32,45, spontaneous emission fromthe coupled system is the only parameter measured. Alterna-tively, vacuum-Rabi splitting in a system consisting of a singleQD coupled to a cavity mode can be detected by simply mea-suring the transmission through the cavity as a function of theinput wavelength to the cavity; such an experiment is directlyanalogous to the experiments done with cooled alkali atomscoupled to a Fabry-Perot cavity46,47. Non-linear effects, suchas optical bistability and photon blockade, and coherent con-trol of the quantum system can also be more easily performedthrough the optical transmission or reflection channel of a mi-crocavity. Perhaps most importantly, by knowing the preciselevel of coupling between the fiber taper and the microcavity,the number of photons injected into the cavity can be preciselycalibrated. This is obviously of paramount importance in ex-periments that involve few or single cavity photons.

Finally, it is important to note that while some of the advan-tages described above are also available to in-plane waveg-uides that are microfabricated next to the microcavities, thefiber taper provides a level of flexibility that the microfabri-cated waveguides do not, as described in detail in Ref. 48.In particular, it can accommodate a number of different cav-ity geometries, which is very important for resonators suchas microdisk cavities, where developing a suitable in-planewaveguide is not necessarily trivial (in this case, due to theundercut nature of the microdisks). Furthermore, fiber tapers

can be routinely fabricated with low losses (∼ 0.1-0.5 dB inour lab), while in-plane waveguides still require a device toprovide low-loss coupling into and out of them which poses asimilar, albeit slightly less difficult, problem as coupling di-rectly into the microcavity. In addition, employing opticalfibers to couple to the microcavities makes these devices easyto integrate into larger, fiber-optic based networks. As opticalfibers are the low-loss transmission medium of choice, sucha fiber-based device architecture would be suitable for imple-mentation into quantum networks such as those proposed inRef. 42.

In conclusion, we have demonstrated the utility of fibertapers in the photoluminescence measurements of QD-containing microdisk cavities. Improvements in collectionefficiency of 1-2 orders of magnitude in comparison to nor-mal incidence free-space measurements are exhibited, and ini-tial measurements on microdisk lasers employing fiber taperpumping and collection have been presented. The largest dif-ferential laser efficiency was measured to be as high as 2%when accounting for collected light into both directions ofthe fiber taper. From direct passive measurements of the mi-crodisks at wavelengths 30 nm red detuned from the ground-state QD emission peak, the fiber taper collection efficiencyof the laser light (including all optical losses of the opti-cal fiber and fiber taper) is estimated to be 18% (again in-cluding both directions of emission within the fiber taper).With further modifications of the microdisk structure to en-able better phase-matching of the fiber taper waveguide andthe whispering-gallery modes of the high-refractive indexmicrodisk26, significant increases in coupling efficiency, ap-proaching unity, are believed possible with this technique.

VII. ACKNOWLEDGEMENTS

The authors thank Matt Borselli, Paul Barclay and TomJohnson for valuable discussions and assistance in settingupsome of the measurement equipment, and Matt Borselli fornumerical simulations of the microdisk modes. K.S. thanksthe Hertz Foundation for its graduate fellowship support.

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49 The WGMs in the 1400 nm wavelength band are expected to havevery similar radiation and scattering losses as those in the1200nm band for the microdisk geometries studied here. Differencesin Q at these two wavelengths are thus expected to be indicativeof wavelength-dependent material absorption losses.

50 The coupling between the taper and microdisk can be differentfor different cavity modes, so this technique is used primarily asa qualitative guide.

51 Finite-element simulations have shown this to be an accurate es-timate.