Light emission is a power-hungry process. Reducing the brightness of a smartphone's screen means longer battery life, as does reducing any unwanted energy from the screen. In order to reduce power consumption, photons have to be emitted in a more controlled manner. This is particularly important for the stable, efficient management of single-photon sources for quantum information, miniaturized light sources, and numerous other applications. Researchers at the University of Twente (UT) demonstrated a way to control the emission of photons with record-setting precision, by using nanophotonic tools — specifically, tiny chemical chains of polymer brushes — to hold photon sources in place. Through their demonstration, the team showed that excited light sources can be reduced by nearly 50×. The researchers studied the spontaneous emission of lead sulfide (PbS) nanocrystal quantum dots in 3D photonic band gap crystals made from silicon (Si). The nanocrystals were covalently bound to polymer brush layers that were grafted from the Si-air interfaces inside the nanostructure. The crystals had an inverse woodpile structure that consisted of two perpendicular sets of crossing pores. “The polymer brushes are grafted in solution from pore-surfaces inside a so-called photonic crystal made from silicon,” researcher Andreas Schulz said. “Quite a tricky experiment! So, we were very excited when we saw in separate X-ray imaging studies that the photon sources were sitting at the right positions on top of the brushes.” A graphical depiction of a finite 3D photonic crystal in free space. Because the crystal has a complete 3D photonic band gap, ubiquitous vacuum fluctuations incident on the crystal’s surface (shown as red wavelets) are forbidden from entering and are thus reflected from the crystal’s surfaces. Therefore, an excited two-level quantum system (atom, ion, molecule, or quantum dot) embedded inside the crystal is shielded from the fluctuations and cannot decay by spontaneously emitting a photon. As a result, the excited state becomes more stable. Courtesy of J Phys Chem. The researchers characterized the presence and position of the quantum dots by using synchrotron X-ray fluorescence tomography. This was done at a high 17 kilo-electron-volt photon energy to obtain large penetration depths and efficient excitation of the elements of interest. Using the technique, the researchers observed that the quantum dots resided along the axes of the pores. The team observed both continuous wave emission spectra and time-resolved, time-correlated single photon counting. In time-resolved measurements, the researchers found that the total emission rate increased significantly when the quantum dots were transferred from suspension to the Si nanostructures. The likely cause of this increase, the team hypothesized, was quenching caused by copper catalysts. In this regime, continuous wave emission spectra are known to be proportional to the radiative rate and thus to the local density of states. In spectra normalized to those taken on flat silicon outside the crystals, the researchers observed a broad and deep stop band, which they attributed to a 3D photonic band gap with a relative bandwidth of up to 26%. The researchers observed a reduction in photon emission that was 4 to 30x stronger than the previously reported record for inhibitions. They found that the shapes of the relative emission spectra matched well with the theoretical density of states spectra, which was calculated with plane-wave expansion. The team observed that broadband strongly reduced the emission inside the 3D band gap, in agreement with theory, and enhanced emission above the band gap. “The theory predicts zero light since it pertains to a fictitious, infinitely extended crystal. In our real finite crystal, the emitted light is non-zero, but so small it’s a new world record,” researcher Marek Kozon said. The polymer brushes used for the experiment are from a set of tools developed by the team, called the MINT-toolbox, comprising tools from the scientific disciplines of mathematics, informatics, natural sciences, and technology. “Our multi-toolbox offers opportunities for completely new applications that profit from strongly stabilized, excited states,” professor Willem Vos said. “These are central to photochemistry and could become sensitive chemical nanosensors.” Putting the MINT-toolbox to work to explore the properties of light, via experiments like the one undertaken to study the spontaneous emission of PbS quantum dots, could lead to new ways to increase the efficiency of miniature lasers and other small-scale light sources. It could enable reductions in perturbations due to vacuum fluctuations in qubits for photonic circuits and improve solar energy harvesting. The research was published in the Journal of Physical Chemistry C (www.doi.org/10.1021/acs.jpcc.4c01541).
