Complete Report - University of New South Wales

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