Engineering the spontaneous emission of diamond color centers using photonic structures

Abstract

Recently, the quantum emitters such as color centers in nanodiamonds have attracted a huge research interest due to its excellent optical and spin properties. These color centers are generally point defects in diamond crystal and may be present naturally or created artificially using ion beam bombardment. The most widely studied defect center in the diamond is the nitrogen vacancy (NV) center which is formed when two adjacent sites in a diamond crystal are altered. One of them is replaced by the nitrogen atom and other remains vacant and this pair is known as the NV center. NV center exists in two charge states namely neutral (NV0) and negative (NV-). Among these charge states, NV- is the most favourable as it has interesting optical and spin properties. It has a unique emission spectrum with characteristic features like the zero phonon line (ZPL) which corresponds to pure electronic transitions and the phonon sidebands (PSB) which corresponds to non-resonant phonon mediated electronic transitions. The PSB emission being incoherent in nature needs to be suppressed for many applications involving NV centers. At the same time, the ZPL which is very weak at room temperature needs to be enhanced so that the NV center can be used efficiently in various applications. In this thesis, we study the optical properties of NV center in nanodiamonds and devise methods which can enhance their utility in various applications ranging from quantum sensing to bio-imaging. We have achieved ~50% suppression in PSB emission intensity at room temperature by using engineered photonic structures called photonic crystals. A periodic variation of refractive index in the photonic crystals results in the Bragg diffraction of a certain set of frequencies, which constitute the photonic stopgap. The density of photon states in the stopgap is reduced in comparison to the homogeneous medium which according to the Fermi-golden rule results in the low emission rate. The low emission rate is dictated through low emission intensity and hence results in PSB suppression. We have also utilized another interesting property of the photonic crystals to enhance the ZPL of NV centers. This is done by tuning the blue band-edge of the photonic stopgap which shows the enhanced density of states, with the ZPL of NV center. Thus, we have achieved a long-standing goal of PSB suppression along with simultaneous enhancement of ZPL at room temperature. We also show just by a slight modification in the measurement geometry to achieve the opposite effect where we need to enhance the PSB emission for NV center based bio-medical applications. The emission dynamics of NV centers modified by the electromagnetic environment induced by the photonic crystal are also studied. The emission lifetime studies complement our results on the modification of emission intensity. The lifetime measurements directly dictate the changes in local density of states (LDOS) for the NV center in nanodiamonds decorated on photonic crystals. The emission lifetimes are modified according to the Barnett Loudon sum rule which says that the suppression in emission decay rate over a certain frequency range is compensated by its enhancement at some other transition frequencies and vice-versa. Thus, the suppression in emission rate at the stopgap wavelength is balanced the enhancement of emission rate at other frequencies that is at the blue-side and red-side of the photonic stopgap. The emission lifetime distributions are analysed using Kolmogorov-Smirnov test, which reveals that the measured lifetime distributions are truly distinct and they do not arise from same underlying distributions. Our studies also shed light on the fundamental charge-state conversion processes which are probed using the excitation power-dependent decay rate measurements. The results also reveal the effect of spin polarization on the emission lifetimes. We identified two power regimes in which the pump-dependent lifetime behavior is completely different for the reference and the resonant photonic crystal sample. The goal of simultaneous suppression of PSB emission along with the enhancement of ZPL is achieved at the low power regime. At the high power regime, the charge state conversion becomes inevitable and the LDOS induced modifications in emission lifetimes are diminished. The present thesis thus create a knowledge base in the NV center emission properties and stimulate the research in the field of quantum nanophotonics. The work finds the number of applications in different areas such as quantum information processing, nano-scale sensing, and biophysics.