NOVELTY - Structure to convert incident radiation from one wavelength to another comprises: first layer comprising rare earth compound; and second layer comprising semiconductor operable as photovoltaic cell, such that incident radiation comprising first wavelength incident on first layer is converted into different wavelength(s), where at least portion of different wavelength(s) is absorbed by second layer and converted into electrical energy.    USE - As structure to convert incident radiation from one wavelength to another wavelength (claimed), useful in fabricating solar cell.    ADVANTAGE - The structure provides for up and down conversion of radiation (preferably solar radiation), where high energy wavelengths are down converted to lower energy and low energy wavelengths are up converted to higher energy to improve adsorption by photovoltaic device. The structure includes rare-earth (REO, N, P) based materials that can covert long wavelength photons to shorter wavelength photons, which can be absorbed in photovoltaic device (up-conversion); and (REO, N, P) materials that can absorb short wavelength photon and re-emit (downshifting) longer wavelength(s) photons. The wide spectral range of sunlight overlaps with multitude of energy transitions in rare-earth materials, and offers multiple up-conversion pathways. The refractive index contrast of rare-earth materials with silicon enables a distributed Bragg reflector (DBR) with greater than 90% peak reflectivity and stop band greater than 150 nm. The device includes the rare earth compounds in thin film format that is materially compatible with underlying silicon semiconductor. One advantage of this is control is provided over process both in tuning material to particular wavelengths and in reproducing process in manufacturing environment.    DETAILED DESCRIPTION - A structure for converting incident radiation from one wavelength to another comprises: a first layer comprising a rare earth compound; and a second layer comprising a semiconductor operable as a photovoltaic cell, such that incident radiation comprising a first wavelength incident on first layer is converted into at least one wavelength different than first wavelength, where at least portion of at least one wavelength different than first wavelength is absorbed by second layer and converted into electrical energy. The first layer comprises initial cladding layer and secondary cladding layer comprising compound of composition (RE1)x(J)y, and core layer comprising compound of composition (RE1mRE2n)oJp (where RE1 and RE2 are selected such that there is no overlap between energy levels in cladding and core layers; m, n, o, p, x, and y are greater than 0; and J is oxygen, nitrogen, and/or phosphorus) between initial and secondary cladding layers, where first layer is positioned between incident radiation and second layer such that at least portion of incident radiation is down-converted from higher energy to lower energy before being absorbed by second layer. The structure further comprises third layer comprising initial cladding layer and secondary cladding layer comprising compound of composition (RE1')r(J)s, and core layer comprising compound of composition (RE1'uRE3'v)wJk (where RE1' and RE3' are selected such that there is no overlap between energy levels in cladding layers and core layers; and u, v, w, k, r, and s are greater than 0) between initial and secondary cladding layers, where third layer is positioned such that incident radiation has passed through second layer into initial cladding layer, and at least portion of incident radiation is up-converted from lower energy to higher energy by third layer; distributed Bragg reflector on third layer away from second layer, comprising several sets of first and second layers, where first layer is of composition (RE)x(J)y and second layer is of composition (Si)l(Ge)k (where l, x, and y are greater than 0, and k is greater than or equal to 0); reflector layer placed such that third layer is between reflector layer and second layer; and fourth layer comprising secondary cladding layer comprising compound of composition (RE1)e(J)f, and core layer comprising compound of composition (RE1aRE4b)cJd (where a, b, c, d, e, and f are greater than 0) between secondary cladding layer and secondary cladding layer of third layer, such that incident radiation has passed through third layer, and at least portion of incident radiation is up-converted from lower energy to higher energy by fourth layer; and reflector layer such that secondary cladding layer of fourth layer is between reflector layer and core layer of fourth layer. The structure comprises several layers comprising semiconductor layer operable as photovoltaic cell and rare earth compound layer, and substrate; where first of several layers comprises semiconductor layer of composition Si(1-m')Gem' and rare earth layer of composition (RE1)x'(J)y'; second layer comprises semiconductor layer of composition Si(1-n')Gen' and rare earth layer of composition (RE2)u'(J)v'; and several layers are separated from substrate by semiconductor layer of composition Si(1-o')Geo' (where u', v', x', and y' are greater than 0; and m', n', and o' are greater than or equal to 0). An INDEPENDENT CLAIM is included for device comprising: a silicon semiconductor based superlattice comprising several layers that form several repeating units, where at least one of layers is optically active layer with at least one species of rare earth ion, and repeating units have two layers, in which first layer comprises rare earth compound (RE1)x'(J)y' and second layer comprises compound Si(1-m')Gem', or a superlattice that includes several layers that form several repeating units, where at least one layer is optically active layer with at least one species of rare earth ion, and repeating units comprise two layers, in which first layer comprises rare earth compound (RE1)x(J)y and second layer comprises compound (RE2mRE3n)oJp.    DESCRIPTION OF DRAWING(S) - The figure shows schematic illustration of rare earth oxide on silicon.