Highly efficient, tunable single photon source based on single molecules

Mathias Steiner, Achim Hartschuh, Rafał Korlacki, Alfred J. Meixner
2007 Applied Physics Letters  
The authors studied spatially isolated terrylene molecules immobilized in a quasiplanar optical / 2-microresonator using confocal microscopy and spectroscopy at variable temperatures. At T = 1.8 K, they observed individual molecules relaxing into microresonator-allowed vibronic levels of their electronic ground state by emission of single fluorescence photons. Coupling the purely electronic transition of embedded molecules to the longitudinal photonic mode of the microresonator resulted in an
more » ... or resulted in an ultimate spectral narrowing and an increased collection efficiency of the emitted single photon wave trains. Single photon sources ͑SPSs͒ emitting optical wave trains that contain only one photon have recently attracted considerable scientific interest and they have numerous applications in spectroscopy and quantum optics. 1 Individual dye molecules fulfill the necessary condition for true SPS: The intensity correlation measured from single molecule fluorescence exhibits photon antibunching, i.e., there is a diminishing probability for emission of two photons at the same time. 2 Just recently, it has been reported that the zerophonon line ͑ZPL͒ emission of single dye molecules embedded in a crystalline matrix at cryogenic temperatures is a proper source of indistinguishable photons having ultralong coherence times in the range of nanoseconds. 3, 4 The performance of a SPS based on a single molecule can still be improved essentially by utilizing a planar optical microresonator 5,6 and this technique has been used to enhance the radiative efficiency of single quantum dots, see, e.g., Refs. 7-9 and references therein. The modified photonic mode density in the presence of the microresonator is determined by the mirror spacing and can enhance 10 or inhibit 11 the spontaneous emission rate of embedded emitters. Under cryogenic conditions, the fluorescence spectra of dye molecules reveal their vibronic fine structure. Having the purely electronic transition of a single molecule on resonance with the longitudinal photonic ͑forward͒ mode of the microresonator, we expect to observe a suppression of redshifted transitions to excited vibronic levels of the molecular electronic ground state. This would result in a stream of highly directed single photons with an extremely narrow spectral distribution determined by the width of the ZPL, as it has been suggested already several years ago. 1, 12 In this letter, we report optical studies of single terrylene molecules embedded in a quasiplanar optical / 2 microresonator that is easy to build, inexpensive, long term stable, and suitable for temperatures ranging from T = 300 K down to T = 1.8 K. In Fig. 1͑a͒ , a schematic of the microresonator is shown. Two silver mirrors M 1,2 were separated by a layer of an UV-polymerizing optical adhesive ͑NOA 61, Norland͒ with slightly varying thickness L͑x , y͒ realized by applying a punctual force to one of the mirrors during the polymerization. 13 The adhesive was doped with homoge-neously distributed and randomly oriented terrylene molecules ͑c ter Ϸ 10 −8 mol/ l͒. Light passing the microresonator in the / 2 regime obeys the transmission condition 14,15 Here, ⌬ i denotes the phase change due to reflection at the respective silver mirror i =1,2 with thickness d i ͑d 1 = 30 nm, d 2 = 60 nm; reflectivities R 1 = 0.7, R 2 = 0.9, and ͚ i ⌬ i = 1.9 at = 532 nm assuming normal incidence͒. The refractive index of the intracavity medium n pol is 1.56. The incidence angle of a parallel light beam with respect to the z axis is given by . The transmission condition ͓Eq. ͑1͔͒ yields the local mirror spacing L͑x , y͒ from the measured transmitted wavelength ͑x , y͒. The mirror spacing variation ⌬L͑x , y͒ / ⌬x , y Ӎ 10 −3 ensures a planar resonator geometry within the focal diameter of our microscope objective ͑numerical aperture= 0.85͒. The microresonator yields a cavity Q around 50 in the / 2 regime and provides single molecule a͒ Electronic mail: alfred.meixner@uni-tuebingen. de FIG. 1. ͑Color online͒ ͑a͒ Schematic diagram of the microscope head that is located in a bath cryostat. The microresonator consists of two silver mirrors M 1,2 evaporated on glass coverslips. The polymer layer between M 1,2 is doped with terrylene molecules. ͑b͒ Energy level diagram and excitationemission cycle of a fluorescent molecule embedded in the microresonator. Intersystem crossing to a dark molecular triplet state is neglected. APPLIED PHYSICS LETTERS 90, 183122 ͑2007͒
doi:10.1063/1.2736294 fatcat:bksv3uwaxrfmzj3gsm2trdnjwa