Negative refraction in the visible spectrum in photonic crystals: Search for focusing by fluorescent quantum dots inside synthetic opals

The negative refraction inside two dimensional (2D) photonic crystals (PC) and homogeneous structures was studied numerically by inserting a point source inside the negative index part of these materials. The three-dimensional (3D) counterparts of these structures, the synthetic opal, were examined experimentally by infiltrating them with CdS quantum dots (QDs). Confocal microscopy measurements in which we tracked the infiltration of the QDs inside the opal indicate the focusing of light emitted by QDs, which can be due to negative refraction occurring at the opal/glass interface. The formation of a focus can be an indication of the negative refraction happening in these synthetic opals in the [111] direction in its higher photonic band, above the photonic band gap (PBG). This result is very promising because, until now, negative refraction has not been seen in 3D photonic crystals in the visible region of light. This result was made possible due to the use of infiltrated QDs as internal light sources inside the porous photonic crystal, which appears to be a very useful technique for the study of other negative-index materials (NIM) effects. Finally, the possibility of self-focusing of second harmonic by QD in nonlinear opal was developed.