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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTSimulated reflectivity spectra ofPSi DBRs with varying numbersof bilayers; layer thicknesseswere adjusted to fix the shortwavelengthstop band edgeat 1550nm (marked with reddashed vertical line).Figure 4.5.68Band structure in the neighborhoodof the fundamental band gap ofan infinite 1D photonic crystal(left) and group velocity in thematerial (right).Figure 4.5.67We have previously reported room-temperature UCluminescence from Er-doped nanoporous Si whenexcited at very high power densities with a 1550nmlaser. In the past year we have focused on increasingthe efficiency of this UC luminescence by modifyingthe PSi structure as described above.The dependence of the efficiency of the UC processon irradiance is non-linear in the range of “sunlike”incident power. That is, the concentrationof the incident radiation into a small Er-dopedregion results in greater efficiency than for nonconcentratedradiation over a proportionally largeareadevice. Typically, concentration is achievedusing lenses or non-imaging optics, but we haveexamined an analogous electromagnetic fieldenhancement in the vicinity of optically-active Erions by the excitation of slow-light modes in thesePSi:Er DBRs.Field enhancement in 1D PSi:Erphotonic crystalsThough their use preceded the advent of thephotonic crystal concept, DBRs can be describedas one-dimensional photonic crystals. The regularvariation of the optical thickness of multiplealternating layers results in wavelength-dependentinterference behaviour manifesting as discretebands of allowed and disallowed electromagneticmodes, as shown in the left panel of Fig. 4.5.67. The“edge” of a photonic band is a spectral region inwhich the effect of coherent scattering processeschanges abruptly between transmission andreflection of incident photons as determined bythe superposition of Bloch waves. Insofar as thegroup velocity v gof an incident wave can be saidto transition between “positive” and “negative”values - implying energy propagation into and outof the structure, respectively, in bands of strongtransmission and strong reflection - there exists aninversion point at which v gis zero, corresponding toa standing wave established by the superposition ofscattered Bloch components. As can be seen in theright panel of Fig. 4.5.67, group velocity is drasticallysuppressed across a broad region near the zeropoint.For steady-state equilibrium conditions,this slowing of energy propagation requires aproportional increase in energy density, that is,an enhancement of the electromagnetic fieldstrength within the structure. It can be shown thatthe efficiency increase for a two-step UC process isproportional to (c/nv g– 1)I 0, where c is the speed oflight in vacuum, n is the average refractive index ofthe multilayer material, and I 0is the incident fieldintensity [4.5.78]. In this way the slow-light modeacts to augment the interaction that convertsenergy from the field into atomic potential energy,resulting in a boost in the efficiency of the processuntil saturation is reached.Simulation of field enhancement in 1DPC structuresUsing a 1D transfer matrix calculation, we simulatedthe reflectivity characteristics of a series of PSi DBRswith 10, 20, 30 and 40 high-/low-porosity bilayers.As shown in Fig. 4.5.68, the reflectivity characteristicof a DBR contains a wide photonic stop-band.The structural parameters were tuned slightly ineach case as the number of bilayers increased toproduce a short-wavelength band edge minimumat 1550nm, the excitation wavelength of interest(marked with a dashed red line).Maximum transmission of ~1550nm light into thestructure is clearly required for efficient couplinginto the Er ions. From Fig. 4.5.68 it is evident thatan increased number of layers “compresses” thereflectivity characteristic, resulting in a steeper94
Intensity-valued mode profile in DBRswith varying numbers of high/lowporositybilayers for a leading band edgefixed at ~1550nm. All intensities arenormalized to the incident field intensityand each plot is independently colorscaledto show maximum detail. The airand substrate interfaces with the DBR areindicated by the dashed black (upper)and red (lower) lines, respectively.ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTFigure 4.5.69band edge and a narrower transmission windowbetween the stop band and the neighboringinterference fringe.Figure 4.5.69 shows the calculated field intensityprofile for these DBRs in the spectral regionbetween 1500 and 1600nm. The results havebeen normalized to the incident field intensityand each plot is independently colour-scaled toshow maximum detail. They clearly demonstratethat considerable field enhancement is possiblethroughout the depth of such a multilayer structure,with peak relative intensities of more than 11 forthe 30-bilayer structure and more than 18 for the40-bilayer structure. However, it is also clear that theregions of enhancement are increasingly narrowlyconcentratedas the number of bilayers increases.This is due to the additional interference fringesin the reflectivity characteristic of the DBR thatarise with additional layers, as is apparent from Fig.4.5.69. Calculations for more than 40 layers showthat enhancements of up to 80 are possible for alarge number of bilayers though the peak narrowsto less than 1nm wide for 100 bilayers.ConclusionThese simulations show that drastic field intensityenhancement is in principle achievable in Er-dopedPSi DBRs. Our current work is focused on thefabrication of suitable samples by anodic chemicaletching, electroplating, and high-temperatureannealing. Angular-dependent room temperaturephotoluminescence studies are underway toprobe the effect of the photonic band edge on theefficiency of UC in the optically-active Er ions withinthe structures. This Si-based approach to efficientup-conversion is conceptually promising and byoptimising our fabrication techniques to improvethe quality of our optical structures we expect it willprove to be valuable work.4.5.5 Plasmonics for 3 rdGeneration structuresResearcher:Supriya PillaiPlasmonics is the study of the interaction of lighton a thin metal dielectric interface resulting inthe collective oscillation of the free electrons inthe metal. This gives rise to exciting propertiesthat are very different from the bulk metal. It isan emerging area of technology for photovoltaicapplications with potential for light managementat the nanoscale. Ever since our proof-ofconcept results were published in 2007 [4.5.79],there has been a huge increase in interest inincorporating nanoparticles on solar cells dueto their demonstrated light trapping properties.The large optical polarisation associated with thenanoparticles results in a strong enhancement ofthe electric field around the vicinity of the particlesalong with strong scattering. This optical property isbeing utilised for improving light trapping in thinfilmsolar cells.The scattering or near field enhancement isstrongest at the surface plasmon resonance (SPR)frequency and this can be tuned by changing thesize, shape, dielectric medium around the particles.SPR tunability offers control in manipulating lightin the regime where the absorption of the solarcells are weak. Our recent work (see section 4.4Thin Film) has shown the benefits of having theparticles on the rear as opposed to previous workwith front located particles [4.5.80]. By having theparticles on the back of the cell any potential lossdue to absorption in the metal or losses due tosub-resonance photocurrent suppression resultingfrom destructive interference of the scattered andtransmitted light into the semiconductor layercan be avoided [4.5.81]. An additional advantageis that because the scattering plasmonics layeris electrically decoupled from the cell, theantireflection layer on the front and the plasmons95
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