Nanophotonic circuits for single photon emitters

Nanophotonic circuits for single photon emitters. The work demonstrated in this thesis is dedicated to the engineering, simulation, fabrica-tion and investigation of the essential element base to develop hybrid fully integrated nanopho-tonic circuit with coupled single photon emitter on chip. Combining several individually opti-mized stages of photonic devices, interconnected by nanoscale waveguides on chip with eva-nescently coupled single photon emitter, is a key step to the realization of such a scheme. The main requirements which should be satisfied for building such a hybrid system on-chip, and are thus the subject of this Thesis, are, namely: integration of single photon photostable source with high Quantum Yield (QY) on chip, efficient coupling of the emitted light to nanophotonic cir-cuits, and efficient filtering of the excitation light. Silicon nitride-on-insulator was used in all the projects described in this Thesis as the platform for the realization of photonic circuits. It provides low-loss broadband optical transparency covering the entire visible range up to the near infrared spectrum. Furthermore, sufficiently high refractive index contrast of Si3N4 on SiO2 enables tight confinement of the mode in the waveguide structure and the realization of photonic circuits with small footprint. A drastic increase of the coupling efficiency of the emitted light into the waveguide mode can be achieved by placing single-photon emitter on photonic crystal cavity because of its high Quality factor and small mode volume enabling a high Purcell enhancement. To this end, a novel cross-bar 1D freestanding photonic crystal (PhC) cavity was developed for evanescent integration of single photon emitter, in particular Nanodiamonds (NDs), onto the region of the cavity. The novelty of this photonic structure is that collection of emitted light is provided via waveguide, which consists of PhC, whereas direct optical excitation is obtained through a crossed waveguide in the orthogonal direction of the in-plane cavity. Optimization of the PhC cavity architecture was performed via rounds of simulations and ver-ified by experimental measurements of fabricated devices on chip, which were found in excel-lent agreement. The next round of simulations was performed to define an optimal position of the source in the cavity region to achieve maximum Purcell enhancement, which was realized via Local Density of States (LDOS) computation. Thus, placing a single photon emitter into a determined position on the cavity region of the developed cross-bar 1D freestanding PhC enables an increase in the transmission coupling efficiency into cavity up to =71% in comparison with computed 41% in the case of coupling into waveguide mode of cross-bar structure without PhC. To block the pump light and at the same time transmit the fluorescent emitted light, compact and low-loss cascaded Mach–Zehnder interferometers (MZIs) tunable filters in the visible region embedded within nanophotonic circuit, were realized. Tunability was provided via thermo-optic effect. The design of this device, namely geometry and shape of the microheater, was optimized via thermo-optic measurements, to achieve low electrical power consumption (switching power of 12.2 mW for the case of a spiral-shape microheater), high filtration depth and low optical insertion loss. The novel design with double microheaters on top of both arms of single and cascaded MZIs allows doubling the range of the shifting amplitude of the interference fringes. The demonstrated architecture of tunable filter is multifunctional, namely allowing transmission and filtering of the desired wavelengths in a wide wavelength range. In particular, filtration depth beyond 36.5 dB of light with 532 nm wavelength and simultaneous transmission of light with 738 nm wavelength, which correspond respectively to excitation and emission wavelength of the silicon-vacancy color center in diamond, was demonstrated. The results were published in Ovvyan, A. P.; Gruhler, N.; Ferrari, S.; Pernice, W. H. P. Cascaded Mach-Zehnder interferometer tunable filters. Journal of Optics 2016, 18, 064011 https://doi.org/10.1088/2040-8978/18/6/064011 Another filter with non-repetitive stopband with bandwidth of several nanometers was developed in this thesis. A non-uniform Bragg grating filter with novel double Gaussian apodization was proposed, whose fabrication required a single lithography step. This optimized Bragg filter provides a 21 dB filtration depth with a 3-dB bandwidth of 5.6 nm, insuring negligible insertion loss in the best case, while averaged insertion loss in reflected signal is 4.1dB (including loss in splitter). One of the first Hybrid organic molecule Dibenzoterrylene (DBT) coupled on chip to a nanophotonic circuit was demonstrated in this thesis. DBT is a photostable single photon source in the near infrared spectrum at room and at cryogenic temperature, with almost unitary quan-tum yield. In order to protect the molecule against oxidization DBT was embedded in a host matrix – thin Anthracene crystal (DBT:Ac), which increases photostability. Mirror enhanced grating couplers were employed as convenient output ports for ridge Si3N4 waveguide to detect single photons emitted from integrated Dibenzoterrylene (DBT) molecules at room temperature. The coupling ports were designed for waveguide structures on transparent silica substrates for light extraction from the chip backside. These grating ports were employed to read out optical signal from waveguides designed for single-mode operation at λ=785 nm. DBT molecule was coupled evanescently to the waveguide, and upon excitation of isolated single molecule, emitted single photon signal was carried inside the waveguide to the outcou-pling regions. Using a Hanbury Brown and Twiss setup pronounced antibunching dip was read out from a single molecule via the grating couplers, which confirms the quantum nature of the outcoupled fluorescent light. Simulated and measured transmission coupling efficiency of sin-gle photon emission into the waveguide mode equals =42%. The results were published in P. Lombardi*, A. P. Ovvyan*, S. Pazzagli, G. Mazzamuto, G. Kewes, O. Neitzke, N. Gruhler, O. Benson, W. H. P. Pernice, F. S. Cataliotti, and C. Toninelli. Photostable Molecules on Chip: Integrated Sources of Nonclassical Light. ACS Photonics 2018, 5, 126−132, DOI: 10.1021/acsphotonics.7b00521. * P. Lombardi and A. P. Ovvyan contributed equally to this work. Engineered nanophotonic elements integrated in optical circuits with coupled single photon emitter on chip allow simultaneously to enhance the emitted light by coupling it into resonant PhC cavity modes, to spatially separate the excitation light from the enhanced single photon emission and to filter out pump light. Enhancement of the emission rate leads to a sig-nificant increase of the coupling efficiency into cavity. Beforehand performed simulations were an essential step in order to design, build and optimize the architecture of the nanophotonic devices. Local Density of States enhancement computation was especially necessary to pre-cisely determine optimized position of the source on PhC cavity region to obtain maximum enhancement of the emission rate. To evaluate transmission coupling efficiency of emitted light into the cavity (β-factor), an extra round of simulations was performed. The integrated photonic elements investigated and optimized in this Thesis, will be further employed for the realization of hybrid photonic circuits with integrated single photon sources: silicon-vacancy, nitrogen-vacancy centers in diamond as well as single organic molecule and semiconducting single-walled carbon nanotubes.