Tunneling Electron Induced Single-molecule Single-photon Emission

Single-photon sources are one of the key issues in quantum information technology.Single-emitter quantum systems,such as semiconductor quantum dots,atomic particles,color centers and single molecules,have been investigated for their potentials as single-photon sources.Among them,molecular single-photon sources are attractive because of the stable and identical nature of molecules as well as the tunable photon energies over a wide range.Due to either the optical diffraction limit or the difficulty to make nanoscale electrode-molecule contact,previous single-photon studies have to rely on particular methods to disperse emitters in a very dilute manner so that only one emitter is excited and gives out photons that are free of multi-photon events.On the other hand,electrically driven nano emitters and single-photon sources are important for future optoelectronic integtation at the nanoscale.However,it remains to be experimentally verified whether an isolated single molecule located in-between a pair of nano-electrodes can emit fluorescent photons upon electrical excitation and further act as a single-photon source?not to mention the characterization and control of such electrically driven single-photon sources at the nanoscale.In this thesis,thanks to the highly localized excitation by tunneling electrons and sub-nanometer resolution in a scanning tunneling microscope(STM),we can selectively excite an isolated single molecule to realize electrically driven single-molecule fluorescence.Second-order photon correlation measurements indicate a photon antibunching behavior for the emitted photons and thus confirm the nature of single-photon emission.Furthermore,the property of such single-photon emission can be modified and optimized either by changing the geometry of the nanocavity or by manipulating the architecture of emitters.The thesis is composed of the following four chapters,as detailed below.In chapter one,we present a brief introduction of the research background.The introduction begins with the concept of surface plasmon with special emphasis on the localized plasmonic enhancement effects,which is followed by a brief description of STM and STM induced luminescence(STML).Two essential issues in achieving STM induced molecular fluorescence are stated:quenching suppression and resonant nanocavity plasmonic enhancement.Then,a brief introduction to the single-photon source is given,including physics behind single-photon emission,detection methods and related research status.At the end of this chapter,the feasibility and advantage of realizing single-photon emission with STML are discussed,followed by a description of the STM and optical setup in our lab used in the experiments of this thesis.In chapter two,we study the STML of pentacene and meso-tetrakis(3,5-ditertiarybutylphenyl)-porphyrin(H2TBPP)on the decoupling layer of copper oxides(CuO).The single pentacene molecule is found to selectively adsorb on the c(6x2)reconstruction area with all the molecules aligning along the same direction.Although the intrinsic molecular fluorescence is quenched due to insufficient decoupling,the pentacene molecule is found to modulate nanocavity plasmonic emission in a bias-polarity dependent way.At positive bias,pentacene molecules will remarkably enhance the emission intensity while at negative bias the plasmonic emission is suppressed.Such bias-dependent modulation phenomenon can be interpreted by the change of localized density of states(LDOS)in different bias polarities.Unstable molecular fluorescence can be detected on the top of some post-evaporated H2TBPP clusters with unclear molecular orientation.These results suggest that the decoupling effect of the CuO monolayer(ML)is insufficient to generate molecule-specific emission from the the first layer of molecules,but it does help to achieve molecular fluorescence from the relatively higher molecular clusters such as H2TBPP.In chapter three,we investigate the STML of zinc tetraphenylporphyrin(ZnTPP)on the sodium chloride(NaCl)decoupling layer since NaCl has a higher dielectric constant and can improve decoupling efficiency.Well-defined single ZnTPP molecules are found to adsorb on clean and flat NaCl islands through in situ evaporation.In this sample,STM induced fluorenscence from isolated single ZnTPP molecules can be detected at negative bias.Polarization detection has been carried out on emitted fluorescent photons with or without molecules and the results show that photons have been highly polarized along the tip axial direction,suggesting the active involvement of the nanocavity plasmonic dipole dominated by the STM tip in the light emission process.Second-order correlation functions measured by the Hanbury Brown and Twiss(HBT)setup have poor signal and noise ratios since the fluorescent emission is not strong enough to accumulate coincidence events.In this chapter,we successfully achieve single-molecule electroluminescence,which indicates that the NaCl thin film could be a good decoupling layer for subsequent single-molecule single-photon emission studies.In chapter four,we demonstrate the realization of electrically driven single-photon emission by using both thicker NaCl islands to achieve better decoupling performance and phthalocyanines(H2Pc,ZnPc)to obtain higher quantum yield.Single-molecule electroluminescence can be detected for almost all Pc molecules on the 4th-ML NaCl island.The second-order correlation functions of emitted photons measured by the HBT setup show evident photon antibunching dips,comfirming unambiguously the single-photon emission behavior.The single-photon purity for single Pcs on 4ML NaCl is found down to g2(0)=0.09.The dip does not fall ideally to zero probably because of the temporal resolution limit of the HBT setup and the minor contribution from the coherent nanocavity plasmon emission.This single-photon emission shows nearly identical features in terms of both spectra and second-order correlation functions.Additional HBT experiments indicate that the property of the single-photon emission can be modified by changing the geometry of the nanocavity around a single molecule,since a smaller distance between the emitter and nano-electrodes will lead to a higher controbution from the nanocavity plasmonic emission,resulting in a poorer single-photon purity.Furthermore,we can use STM manipulation to construct coupled molecular dimers and demonstrate not only the photon antibunching effect for these structures as well,but also the improvement on the quality of the single-photon emission with higher intensity and narrower linewidth.These findings verify experimentally that the dipole-dipole coupled molecular architecture indeed behaves as an entangled system,and offer a new method to manipulate the single-photon source on demand.