Enhanced Anti-stokes Luminescence via Acoustic to Optical Phonon Conversion in Pyramid Arrays

Optical phonon assisted anti-Stokes photoluminescence (ASPL) events have been already well exploited in several systems. Here, we report the structural influence of nanopyramid arrays in enhancing ASPL, correlating its time-delay with acoustic phonon propagations. The pyramid arrays were fabricated on top of GaN-based LED structure incasing the active region of InGaN multiple quantum wells. Upon abrupt femtosecond excitation at 405 nm which generates carriers selectively within InGaN layers, we observed enhanced ASPL only around the pyramids whose spectra lies above GaN bandgap at room temperature. From the linear increment of ASPL with excitation power, the dominating mechanism was rather ascribed to the phonon-assisted absorption than the multi-photonic conversion. Most interestingly, to study acoustic (AC) phonons' contribution to ASPL, time-resolved PL was employed to display the time delay in ASPL matching the propagation time from active AC epicenter of InGaN MQWs to the point of sub-micronic diameter within the pyramid. This delay was separately confirmed from coherent phonon spectroscopy whose increasing temporal lineshape of AC dynamic oscillations manifested the ascending AC propagation toward the tip side of the pyramids, followed by plateau and decaying features. In this way, ASPL could be possibly influenced by the spatially accumulated AC phonons.