analysis and fabrication of hybrid metal-dielectric omnidirectional bragg reflectors

Analysis and fabrication of hybrid metal-dielectricomnidirectional Bragg reflectors

Nakeeran Ponnampalam and Ray G. DeCorby*Department of Electrical and Computer Engineering and TRLabs, University of Alberta, 7th Floor, 9107—116 Street N.W.,

Edmonton, Alberta, Canada, T6G 2V4

*Corresponding author: [email protected]

Received 19 July 2007; accepted 15 October 2007;posted 29 October 2007 (Doc. ID 85458); published 20 December 2007

We describe chalcogenide glass and polymer based Bragg reflectors with a metallic underlayer and use atransfer matrix model to analyze their performance. The angle-averaged reflectance of a hybrid mirrorapproaches unity for only a few periods and is much higher than that for a nonmetallized Bragg reflectoror for the metallic layer alone. For an angle-averaged reflectance greater than 0.99, the addition of a metallicunderlayer enables nearly a tripling of the omnidirectional bandwidth (from �110 to �305 nm) concurrentwith a significant reduction in the number of required periods (from 10.5 to 4.5). Hybrid mirrors of 4.5periods, with a 50 nm Au underlayer and overall thickness of �2 �m, were fabricated atop siliconsubstrates and characterized. They exhibit an omnidirectional stop band in the 1450–1750 nm wave-length range, in good agreement with theoretical predictions. © 2008 Optical Society of America

OCIS codes: 130.3120, 160.2750, 230.4170.

1. Introduction

For light incident from a lower index external me-dium, a one-dimensional (1D) Bragg mirror (of infi-nite extent and sufficiently high index contrast) canprovide complete and omnidirectional reflection [1,2].Mirrors of this type are called omnidirectional dielec-tric reflectors (ODRs). Hollow core fibers with ODRcladdings of up to 35 bilayers have been drawn froma rolled preform [3]. However, most ODRs reported inthe literature [1,2,4–11] contained only a small num-ber of bilayers (typically from three to nine). This is amore realistic scenario for ODR-enabled integratedoptics devices [12–16], which are usually fabricatedby serial deposition and patterning techniques.

Few-period ODRs do not necessarily provide atruly omnidirectional reflection band. Rather, theyoften exhibit deep dips in reflection within the nom-inal ODR band, for one or both polarization states,and at design-dependent incident angles (typicallyfar from normal incidence) [2,4,17]. These reflectiondips at near-grazing angles have important implica-tions for the design of integrated hollow waveguides

[12–15] and microcavities [16]. Baumeister [17]pointed out that the spectral, angular, and polariza-tion dependences of the reflection dips are deter-mined mainly by the index (i.e., high or low) andthickness of the last layer in the ODR (i.e., the layernearest to the incident medium). Gallas et al. [4] usedthis fact to design finite-period ODRs with maximumangle-averaged reflectance. Furthermore, other au-thors [14,15] have optimized the loss of air-core ODR-cladded waveguides by tuning the thickness of thelayer adjacent to the core. In this case, the effectiveincident angle is determined mainly by the core size,and minimization of propagation loss does not neces-sarily require that the omnidirectional properties ofthe cladding be considered [18]. Nevertheless, forpractical implementation of low-loss waveguides,compact waveguide bends [13], and high Q microcavi-ties [16], few-period ODRs with ultrahigh reflectance(R �� 0.99 [18,19]) for both polarization states, andat all angles of incidence, are highly desirable.

Hybrid metal-dielectric Bragg mirrors, in which adielectric multilayer [such as a quarter-wave-stack(QWS)] is terminated by a metal layer, are widelyused in vertical-cavity optoelectronic devices [20–22].They have been shown to combine the best featuresassociated with metallic mirrors (i.e., thinness) and

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30 APPLIED OPTICS � Vol. 47, No. 1 � 1 January 2008

dielectric mirrors (i.e., ultrahigh reflectance). Specif-ically, with the addition of a thin metal terminatinglayer, the reflectance and bandwidth of a few-perioddielectric stack can approach the values associatedwith an infinite stack. This concept has been appliedto vertical cavity lasers [20,21], hollow fibers [23],and LEDs [24]. However, to our knowledge, the om-nidirectional potential of such hybrid mirrors has notbeen analyzed in detail.

In the following, we show that for a given targetreflectance, metal-terminated ODRs enable a wideromnidirectional bandwidth compared to similarODRs without metal termination. We also analyzethe impact of dielectric absorption on the opticalproperties of the ODR. Finally, we provide experi-mental corroboration using a system of chalcogenideglass and polymer with a Au-termination layer.

2. Experimental Details

All of the results below are based on the Ge33As12Se55(IG2) glass and polyamide–imide (PAI) polymer mul-tilayer system that we have described elsewhere[9–11]. Briefly, the IG2 glass layers were depositedby thermal evaporation onto unheated substrates,and the PAI layers were deposited by spin-casting.Cleaned silicon substrates were first deposited with�5 nm of Cr (as an adhesion layer) followed by�40 nm of Au, chosen for its high reflectance in thenear infrared. On this metal-coated substrate, IG2and PAI layers were deposited in an alternating fash-ion (starting with a PAI layer, as shown schemati-cally in Fig. 1). Each PAI layer was soft-baked at90 °C for 5 min in air, and then hard-baked in anitrogen environment for 1 h at 200 °C. Nominallayer thicknesses of 150 and 290 nm were chosen forIG2 and PAI, respectively. As discussed elsewhere[10], this represents a filling factor � � 0.34 (for thehigh index material) that is slightly lower than thatfor a QWS of the same materials ��q � 0.39�. Fordirect comparison with experimental results, we usedthe aforementioned thickness values in all theoreti-cal analyses below. However, we have verified thatthe conclusions regarding bandwidth and angle-averaged reflectance are applicable to the QWS case.One of the experimental mirrors described has�135 nm thick IG2 layers, for a filling factor of 0.32.

We analyzed and fabricated mirrors ended by ei-ther a low index (polymer) or a high index (glass)layer. Note that ending a Bragg mirror with a highindex layer results in a higher normal-incidence re-flectance, and furthermore in higher reflectance forTE polarized light of any angle of incidence. However,it also results in low reflectance for TM polarizedlight over some range of (typically near-grazing) in-cident angles [14,18]. When considering both polar-ization states, ending an ODR with a low index layerenhances its omnidirectional properties [4]. Whilethe TE reflectance and the near normal incidence TMreflectance are reduced by low index termination, anincrease in the number of periods can compensate. Asdiscussed below, analogous conclusions can be drawnfor metal-terminated (hybrid) ODRs.

Experimental reflectance scans were obtained witha Perkin–Elmer Lambda-900 spectrophotometer, us-ing a Au mirror as reference. The absolute reflectanceof the test samples was estimated as follows: the rawdata (i.e., the relative reflectance data of the ODRobtained from the spectrophotometer) was multipliedby the reflectance of the Au reference mirror, on apoint-by-point basis. The reflectance of the gold mir-ror was estimated from analytical formulae in theliterature [22]. The accuracy of the extracted data isdiscussed in Section 5.

3. Simulation Details and Consideration of MaterialProperties

Theoretical curves were obtained using a standardtransfer matrix model [25]. The refractive indices ofIG2 glass and PAI polymer in the near infrared are�2.55 and �1.65, respectively. The dispersion curvesused for the real part of the refractive indices wereprovided elsewhere [9]. In some simulations, a non-zero extinction coefficient ��PAI� was included to as-sess the impact of absorption and volume scatteringin the PAI layers. As is the case for all C–H polymers[26,27], PAI exhibits characteristic absorption peaksin the near-infrared (NIR) corresponding to the vi-brational and rotational overtones of C–H, N–H,C–O, and O–H bonds. We estimated the NIR loss ofthe PAI films (see Fig. 2) by two methods. In the firstmethod, a 10 �m PAI film was spun-cast and curedon a low adhesion substrate. It was then peeled fromthe substrate and cut into pieces. The pieces werestacked between two glass wafers, and the extinctioncoefficient of PAI was extracted from spectrophotom-eter transmission scans. In the second method, theextinction coefficient was estimated from cutbackmeasurements (using a tunable laser source) onhighly confining, large-core PAI based waveguides.The absorption peaks at �1665 and �1380 nm areattributable to the vibrational�rotational overtonesof the aromatic C–H bond [28]. The small peak at�1490 nm likely derives from N–H bonds, which arepresent with relatively low density in PAI [29]. Fulldetails on these measurements will be reported else-where.

Both measurements likely overestimate the intrin-sic PAI loss. In the first case, the manual stacking of

Fig. 1. (Color online) Schematic cross section of a hybrid ODR isshown. The glass, polymer, and metal layers are described in detailin the text. Without the metal layer and assuming sufficient indexcontrast between the glass and polymer layers, the mirror is astandard (all-dielectric) ODR.

1 January 2008 � Vol. 47, No. 1 � APPLIED OPTICS 31

films probably leads to interfacial scattering by airbubbles and defects. In the second case, the propaga-tion loss includes a contribution from scattering dueto sidewall roughness (the strip waveguides wereformed by a dry etch process). Nevertheless, the dataare in good agreement with loss reported for otherpolymers [3,26,27]. PAI exhibits a relatively widetransparency window in the 1400–1600 nm wave-length range ��PAI � 5 � 10�5�, which is typical forhighly aromatic polymers [28]. However, the C–Hovertone centered near 1665 nm results in a higherloss in the 1600–1800 nm range ��PAI � 3 � 10�4�. Anaccurate analysis of the multilayers would considerthe wavelength dependence of both absorption andscattering [30]. Since our goal here is only to assessthe relative impact of dielectric loss, we ignored thesedetails and used an effective extinction coefficient atspecific wavelengths of interest.

Bulk IG2 glass exhibits negligible absorption(�IG2 � 10�7 [31]) for wavelengths longer than �1 �m.Although we have yet to accurately measure the lossof our IG2 thin films, pulsed laser deposited films ofIG2 glass were recently shown [32] to exhibit simi-larly low absorption in the NIR. Based on these facts,we assumed the IG2 layers to be lossless.

For the metallic underlayer, we used closed-formexpressions for the optical constants of Au providedby Rakic et al. [22]. We treated the metal bilayer as asingle Au layer in the simulations, since the Cr layeris very thin and is optically isolated (to first order) bythe Au layer.

While metallic loss is taken into account, most ofthe simulated results were obtained under the as-sumption of ideal (transparent and flat) dielectriclayers. For the metal-terminated mirror, peak re-flectance �0.999 is predicted for some incident an-gles. In practice, residual losses within the dielectriclayers will reduce the peak reflectance relative to theideal value [30]. These residual losses arise from ab-sorption in the dielectric layers, scattering by roughinterfaces, and scattering due to volume inhomoge-neities. It should be stressed that fabrication of mir-

rors with reflectance on the order of 0.999–0.9999 isnot trivial, and neither is the experimental verifica-tion of such high reflectance. Analytical approxima-tions for the absorption and scattering losses (fornormal incidence) in a Bragg mirror are available inthe literature [30,33]. For mirrors ended with a highor low index layer, the bulk absorption and scatteringlosses can be estimated using Eqs. (1a) or (1b), re-spectively:

VL � 2n0

�H �L

nH2 � nL

2, (1a)

VL �2

n0

�nL2�H nH

2�L��nH

2 � nL2�

. (1b)

Furthermore, the loss due to scattering by surface(interface) roughness can be estimated as

SSL � 82n0

nLTHL�nH

2 � nL2��

�0�2

, (2)

where VL is the reflectance reduction due to absorp-tion and volume scattering within the dielectric lay-ers, and SSL is the reflectance reduction due toscattering induced by interface roughness. nH and nL

are the refractive indices of the high and low indexlayers (�2.55 and �1.65 here), and n0 is the index ofthe incident medium. �H and �L are the effective ex-tinction coefficients of the high and low index layers,taking into account both volume absorption and vol-ume scattering effects. THL is the Fresnel transmit-tance for a single interface between a high and lowindex layer, � is the rms surface roughness (assumedequal for each interface), and �0 is the free-spacewavelength. Note that these equations were derivedfor the specific case of a QWS at its tuned wave-length (i.e., the wavelength corresponding to thequarter-wave layer thicknesses), and for a mirrorwith near-unity reflectance. In Section 4, we assessthe applicability of Eq. (1) to the hybrid mirrors, bycomparison with transfer matrix simulations.

We have previously reported that the rms rough-ness of IG2�PAI multilayers is as low as �0.2 nm [9].Using this estimate, the predicted SSL is very low(�10�5 for operation near 1600 nm). Furthermore,given the material extinction coefficients discussedabove (�IG2 � 0 and �PAI � 10�4), VL is predicted to below ��2 � 10�4�. Thus, based on Eqs. (1) and (2), itmight be possible to achieve R � 0.999 for IG2�PAImirrors operating at � � 1.6 �m (with a very tightlycontrolled fabrication process to ensure layer thick-ness uniformity and to minimize contaminants androughness). As mentioned, experimental verificationof such high reflectance is challenging. However, in-direct evidence that IG2�PAI mirrors can exhibitultrahigh reflectance was obtained from transmis-sion measurements on free standing ODRs [11] andfrom propagation loss measurements on hollowwaveguides [15].

Fig. 2. (Color online) Estimated extinction coefficient of PAI isplotted in the vicinity of the omnidirectional reflection band of themirrors described. The extinction coefficient was extracted from aspectrophotometer transmission scan on a relatively thick��100 �m� PAI layer (blue solid curve) and from cutback measure-ments on highly confining PAI-based waveguides (red dottedcurve).


4. Theoretical Results

To illustrate the enhanced performance of hybridmetal-dielectric mirrors, we first present results fromthe transfer matrix model. Unless otherwise indi-cated, the glass and polymer layers were assumedlossless, and to have thicknesses of 150 and 290 nm,respectively. Figure 3 shows the predicted reflectancefor three different mirrors on a Si substrate: a 4.5period dielectric mirror, a 50 nm Au mirror, and thehybrid mirror (i.e., the 50 nm Au mirror topped bythe 4.5 period dielectric mirror). Considering wave-lengths in the fundamental stop band of the dielectricmirror, the normal-incidence reflectance for the di-electric, Au, and hybrid mirrors is approximately 0.9,0.97, and 0.997, respectively. The spectral features(i.e., the reflection dips, etc.) of the dielectric andhybrid mirrors are nearly aligned in wavelength, butthe stop band characteristics of the hybrid mirror aremore fully developed (i.e., the peak reflectance is nearunity, and the stop band has a nearly square shape).Furthermore, intraband reflection dips (such as thatnear 1480 nm for grazing incidence of TE polarizedlight) are largely suppressed by the addition of themetallic underlayer.

The angular dependence of reflectance is moreclearly illustrated in Fig. 4, for a wavelength near thecenter of the omnidirectional stop bands of the dielec-tric and hybrid mirrors. In addition to their lowerreflectance mentioned above, both the dielectric andmetal mirrors exhibit a dip in reflectance �Rmin� 0.9� for TM polarized light at near-glancing inci-dence. For the hybrid mirror, the predicted reflec-tance does not drop below �0.996 over the entirerange of incident angles and for either polarization.Given that the hybrid mirror is not significantly

thicker or more complicated to fabricate than thedielectric mirror, the improvement in omnidirection-ality is quite remarkable.

In Fig. 5, the predicted reflectance of the hybridmirror versus incident angle is plotted for two cases:a 4 period mirror ending with a high index layer anda 4.5 period mirror ending with a low index layer. Asmentioned above, the addition of a low index layerresults in lower TE reflectance globally and in lowerTM reflectance for incident angles from normal up toapproximately 50°. However, it also eliminates thedeep dip in reflectance exhibited by the high indexterminated mirror for TM polarized light near 80°incidence [4].

Fig. 3. (Color online) Predicted reflectance for TM- (left column)and TE- (right column) polarized light, at the incident angles in-dicated, for a 4.5 period dielectric mirror (green dotted curve), a50 nm Au mirror (blue dashed curve), and a hybrid mirror of 4.5periods overtop 50 nm of Au (red solid curve).

Fig. 4. (Color online) Predicted reflectance at 1600 nm wave-length versus angle of incidence for TM- (upper plot) and TE-(lower plot) polarized light, for the 4.5 period dielectric mirror(green dotted line), the metal mirror (blue dashed curve), and thehybrid mirror (red solid curve).

Fig. 5. (Color online) Predicted reflectance of a hybrid mirror(with 50 nm Au underlayer) at 1600 nm wavelength is plottedversus angle of incidence. The red solid curve corresponds to a 4period, lossless mirror ended with a high index layer. The reddashed curve corresponds to the same 4 period mirror, but withfinite loss ��PAI � 10�4� for the low index layers. The blue dotted–dashed curve corresponds to a 4.5 period, lossless mirror endedwith a low index layer. The blue dotted curve corresponds to thesame 4.5 period mirror, but with �PAI � 10�4.


Figure 5 also shows the impact of finite loss ��PAI

� 10�4� in the low index PAI layers. For the mirrorending with a high index layer, VL � 2 � 10�4 pre-dicted by Eq. (1a) is in reasonable agreement with thetransfer-matrix result (VL � 3 � 10�4 at normalincidence). Furthermore, the impact of PAI loss in-creases with incident angle for TM polarized lightand decreases with incident angle for TE polarizedlight. This is due to the relative penetration of theevanescent field into the mirror (determined by theBloch wave vector), as explained in [1].

For the mirror ended with a low index layer, VL� 11 � 10�4 predicted by Eq. (1b) agrees very wellwith the transfer matrix results (VL � 12 � 10�4 atnormal incidence). The higher absorption loss in thiscase is not surprising, since a stronger interactionbetween the evanescent field and the low index me-dium is expected. Furthermore, the impact of the PAIloss on TM reflectance is highest for normal inci-dence, while the impact on TE reflectance variesnonmonotonically with incident angle. Clearly, inmirrors ended by a low index layer, there is a greaterneed to minimize the loss of the low index medium(arising from absorption and volume scattering).

One useful measure of omnidirectionality is theangle-averaged reflectance, simply defined as [34]

RAVG�� 2 �

0

�2

R� , ��d , (3)

where R is reflectance, � is free-space wavelength,and � is incident angle. In Fig. 6, this parameter (at1600 nm wavelength) is plotted versus the number ofbilayers in the dielectric and hybrid mirrors. For clar-ity, mirrors ended by a PAI layer were assumed. Forthe hybrid mirror, the predicted angle-averaged re-flectance of both polarization states exceeds 0.99 foronly 3.5 periods and exceeds 0.999 for 6.5 periods. By

comparison, 9.5 periods are required for the dielectricmirror to achieve an angle-averaged reflectance of0.99. Note that the angle-averaged reflectance is al-ways lower for TM-polarized light, as expected fromthe narrowing of the TM stop band with increasingangle of incidence. Not surprisingly, the addition ofthe metal underlayer is of greatest benefit for few-period mirrors [23]. For increasing number of bilay-ers, the metal layer is eventually isolated from theincident light and has little impact on reflectance.The convergence happens for a higher number of bi-layers in the TM case, due to the greater penetrationof TM-polarized light. The hybrid IG2�PAI mirror isof greatest advantage for period counts in the 3–10range, where it can enable angle-averaged reflec-tance �0.99–0.999�, much higher than that providedby the dielectric mirror.

Another significant advantage of the hybrid mirroris that it exhibits a much wider omnidirectionalbandwidth in practice. This arises from the align-ment of the spectral dips for the hybrid and dielectricmirrors of the same period count, as shown in Fig. 3.As is well known, the width of the stop band (i.e., thewavelength separation between the reflection min-ima at the stop band edges) decreases with increasingnumber of periods in a Bragg mirror. Since the hybridmirror produces a more fully developed stop band forsmall period counts, it effectively provides a wideromnidirectional stop band. Figure 7 compares thereflectance of a 4.5 period hybrid mirror with that ofa 10.5 period dielectric mirror, at various angles ofincidence. While these mirrors exhibit similar peakreflectance, the stop band bandwidth of the hybridmirror is clearly enhanced, especially at the longwavelength side of the band [20].

Fig. 6. (Color online) Plots of angle-averaged reflectance (RAVG)at � � 1600 nm versus number of bilayers are shown for thedielectric mirror (green dotted curve) and the hybrid mirror with50 nm Au underlayer (red solid curve). In each plot, the horizontaldashed curve indicates the angle-averaged reflectance of a 50 nmAu mirror.

Fig. 7. (Color online) Reflectance is plotted versus wavelength fora 10.5 period dielectric mirror (green dotted curve) and a 4.5 periodhybrid mirror with 50 nm Au underlayer (red solid curve), for bothpolarization states and various angles of incidence.


To illustrate further, suppose an omnidirec-tional reflector with peak angle-averaged reflectance�0.995 for both polarization states is desired, andthat the omnidirectional stop band is to be defined bythe criterion that the angle-averaged reflectance ex-ceeds 0.99. Per the discussion above, we need onlyconsider TM-polarized light in the analysis. Thislevel of reflectance requires a dielectric mirror of 10.5periods, but a hybrid mirror of only 4.5 periods, cor-responding to the cases shown in Fig. 7. Figures 8 and9 compare the angle-averaged reflectance of the twomirrors as a function of wavelength. Using the afore-mentioned criterion �RAVG � 0.99�, the omnidirec-tional bandwidth of the dielectric mirror is �110 nm,while that of the hybrid mirror is �305 nm. Katagiriet al. [23] have previously described analogouspractical advantages for nonomnidirectional metal-terminated Bragg mirrors.

5. Experimental Results and Discussion

The experimental reflectance of a 4.5 period hybridmirror is plotted versus wavelength in Fig. 10, alongwith the theoretically predicted data. There is closeoverall agreement between the experiment and the-ory. The discrepancy in the vicinity of narrow spectralfeatures (especially the reflection dips outside the

Fig. 11. (Color online) Experimental (red solid curves) and theo-retical (blue dashed curves) reflectance of a 5 period hybrid mirror(ended by an IG2 layer) are plotted versus wavelength for variousincident angles (15°, 30°, 45°, and 60°). For transfer matrix simu-lation, the IG2, PAI, and Au layer thicknesses were set to 135, 290,and 40 nm, respectively.

Fig. 8. (Color online) Angle-averaged reflectance is plotted versuswavelength for a 10.5 period dielectric mirror (green dotted curve)and a 4.5 period hybrid mirror with 50 nm Au underlayer (red solidcurve). In the TE case, the peak angle-averaged reflectance isslightly higher for the dielectric mirror ��0.999� than for the hy-brid mirror ��0.998�.

Fig. 9. (Color online) Angle-averaged reflectance versus wave-length for TM-polarized light incident on a 10.5 period dielectricmirror (green dotted curve) and a 4.5 period hybrid mirror with50 nm Au underlayer (red solid curve). Using a 99% angle-averaged reflectance criterion, the omnidirectional bandwidth ofthe dielectric mirror is �110 nm �1510–1620 nm�, while that of thehybrid mirror is �305 nm �1450–1755 nm�.

Fig. 10. (Color online) Experimental (red solid curves) and theo-retical (blue dashed curves) reflectance of a 4.5 period hybrid mir-ror (ended by a PAI layer) are plotted versus wavelength forvarious incident angles (15°, 30°, 45°, and 60°). For the transfermatrix simulation, the IG2, PAI, and Au layer thicknesses were setto 150, 290, and 45 nm, respectively.


main stop band) can in part be attributed to the beamdivergence and limited resolution of the spectropho-tometer employed. Furthermore, the measurementnoise increases for longer wavelengths due to lowersource power.

We also fabricated and characterized a 5 periodhybrid ODR (ended by an IG2 layer), and the resultsare shown in Fig. 11. In this case, the IG2 layers werechosen to be somewhat thinner ��135 nm�, and thebest fit was obtained for a thinner Au underlayer�40 nm�. Again, the global agreement between theexperimental and theoretical results is very good.

In Fig. 12, a close-up view of the 60° reflectancecurves from Figs. 10 and 11 are shown. It is apparentthat both the measurement noise and the error in theestimate of the absolute reflectance are on the orderof 0.01. In the latter case, the error derives from theuse of analytical formulae to estimate the reflectanceof the Au reference mirror. Confirmation of theoret-ically predicted reflectance on the order of 0.999 orgreater would require a more sensitive and accuratemeasurement technique, and is left for future work.

As discussed in Section 4, PAI absorption is ex-pected to have a greater impact on reflectance formirrors ended by a PAI layer. As shown in Fig. 5, thePAI terminated mirror is predicted to exhibit great-est evidence of PAI absorption for TE-polarized lightat �60° incidence. Consistent with this, the mainabsorption overtones of PAI (in the 1650–2000 nmwavelength range) are most clearly evident in theireffect on the experimental reflectance of TE light in-cident on the PAI-terminated mirror. Furthermore,by assessing the impact of the overtone centered at�1670 nm, a close inspection of the data in Figs. 10and 11 confirms that they corroborate the trendsshown in Fig. 5.

6. Summary and Conclusions

Using a transfer matrix model, we have shown thathybrid metal-dielectric mirrors (Bragg stacks with ametal underlayer) can have compelling advantagesas omnidirectional reflectors. Specifically, they canprovide a high angle-averaged reflectance for a low

number of layers. Furthermore, they can providesignificantly increased omnidirectional bandwidthcompared to all-dielectric (nonmetallized) mirrorscomprising the same materials.

We also analyzed the impact of loss in the low indexdielectric layers. Such loss has a much greater impacton the reflectance of mirrors ended by a low indexlayer, which must be weighed against the superioromnidirectionality of such mirrors. Finally, we pre-sented experimental results for hybrid mirrors ofchalcogenide glass and polymer with a Au under-layer. Within experimental accuracy, the omnidirec-tional properties of the fabricated mirrors are in goodagreement with theoretical predictions.

The work was supported by the Natural Sciencesand Engineering Research Council of Canada and byTRLabs. We thank Blair Harwood, Hue Nguyen, andYing Tsui for assistance with device fabrication.

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analysis and fabrication of hybrid metal-dielectric omnidirectional bragg reflectors

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