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Intensity interferometry experiments in a scanning transmission electron microscope : physics and applications

Sophie Meuret

Published Nov 16, 2015• 3 citations
Publisher

Abstract

Quantum optics performed at the nanometer scale is an important challenge, especially for quantum emitters characterization. They can be point defects in material (few ang- ströms) or confined structures of a few nanometers. A way to reach this scale is by using cathodoluminescence (CL) performed in a scanning transmission electron microscope (CL- STEM), which has only recently been done [1]. However, when aiming at studying the statistical properties of the light coming out of a CL experiment, which is necessary to e. g. study the quantum nature of Single Photon Emitters (SPE) emission, dedicated expe- riments on top of regular CL ones have to be designed. Few months before my arrival in the STEM-group of the LPS, an intensity interferometry experiment (HBT) that measures the autocorrelation function g(2) of the CL signal intensity was built [2]. It is well known that the clear signature of SPE as measured in photoluminescence (PL) is antibunching in the g(2)(τ), namely that the autocorrelation function is always less than one. It was re- cently demonstrated on a famous SPE, the Nitrogen vacancy (NV) defect in diamond, that CL-STEM is similar to PL when only one SPE is involved. In this thesis we will see how CL-STEM allowed to characterize a new point defect in h-BN, showing the relevance of HBT experiments in a CL-STEM for discovering and characterizing new SPE. However, by studying the excitation of multiple SPE in CL, we discovered a new emission phenomenon, characterized by a huge bunching effect of the g(2)(τ) function (g(2)(0) > 35), in complete contradiction to PL measurements and expectations (g(2)(0)<1). In my thesis manuscript, this surprising effect will be experimentally investigated, theoretically explained and applied to lifetime measurement at the nanometer scale. Because quantum optics is often linked to quantum plasmonics, I will present, to conclude, a theoretical proposal, in collaboration with J. Garcia de Abajo, about quantum plasmonics measurement in a STEM.

DOI
10.70675/604709b7z087bz4f89z8d24z18e46972265a

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Quantum optics performed at the nanometer scale is an important challenge, especially for quantum emitters characterization. They can be point defects in material (few ang- ströms) or confined structures of a few nanometers. A way to reach this scale is by using cathodoluminescence (CL) performed in a scanning transmission electron microscope (CL- STEM), which has only recently been done [1]. However, when aiming at studying the statistical properties of the light coming out of a CL experiment, which is necessary to e. g. study the quantum nature of Single Photon Emitters (SPE) emission, dedicated expe- riments on top of regular CL ones have to be designed. Few months before my arrival in the STEM-group of the LPS, an intensity interferometry experiment (HBT) that measures the autocorrelation function g(2) of the CL signal intensity was built [2]. It is well known that the clear signature of SPE as measured in photoluminescence (PL) is antibunching in the g(2)(τ), namely that the autocorrelation function is always less than one. It was re- cently demonstrated on a famous SPE, the Nitrogen vacancy (NV) defect in diamond, that CL-STEM is similar to PL when only one SPE is involved. In this thesis we will see how CL-STEM allowed to characterize a new point defect in h-BN, showing the relevance of HBT experiments in a CL-STEM for discovering and characterizing new SPE. However, by studying the excitation of multiple SPE in CL, we discovered a new emission phenomenon, characterized by a huge bunching effect of the g(2)(τ) function (g(2)(0) > 35), in complete contradiction to PL measurements and expectations (g(2)(0)<1). In my thesis manuscript, this surprising effect will be experimentally investigated, theoretically explained and applied to lifetime measurement at the nanometer scale. Because quantum optics is often linked to quantum plasmonics, I will present, to conclude, a theoretical proposal, in collaboration with J. Garcia de Abajo, about quantum plasmonics measurement in a STEM.
Intensity interferometry experiments in a scanning transmission electron microscope : physics and applications L'optique quantique réalisée à l'échelle du nanometer est un défit crucial, surtout pour la caractérisation d'émetteur de photon unique. Ces émetteurs sont des défauts ponctuels dans des matériaux (quelques angströms) ou des structures confinées de quelques nanomètres. Une façon d'atteindre cette échelle est d'utiliser la cathodoluminescence (CL) dans un microscope électronique à transmission à balayage (CL-STEM) [1]. Cependant, lorsque l'on cherche à étudier les propriétés statistique d'émission de la lumière sortant d'une expérience de CL, ce qui est nécessaire pour étudier par exemple la nature quantique d'émetteur de photon unique (SPE), une expérience dédiée s'ajoutant à l'expérience de CL-STEM doit être réalisée. Quelques mois avant mon arrivé dans le groupe STEM du LPS, une expérience d'interférence des intensités (HBT) qui mesure la fonction d'autocorrélation g(2)(τ) du signal de CL a été construit [2]. Il est bien connu que la signature univoque d'un SPE en photoluminescence (PL) est l'antibunching, c'est à dire que le g(2)(τ) est toujours inférieur à un. Il a été récemment démontré que lorsque seulement un SPE est excité la CL-STEM est similaire à la PL sur un célèbre SPE, le centre NV dans le diamant. Dans cette thèse nous montrerons comment la CL-STEM a permis de caractériser un nouveau défaut ponctuel dans le h-BN, montrant la pertinence de l'expérience HBT dans un CL-STEM pour découvrir et caractériser de nouveaux SPE. Cependant, en étudiant l'excitation de multiple SPE en CL, on a découvert un nouveau phénomène d'émission, caractérisé par un grand effet de regroupement (bunching) dans la fonction g(2) (g(2)(0) > 35), en complète contradiction avec les mesures de PL et ce que l'on pourrait attendre (g(2)(0)< 1). Dans mon manuscrit de thèse, cet effet surprenant a été expérimentalement étudié, expliqué théoriquement et appliqué à la mesure de temps de vie à l'échelle du nanomètre. Parce que l'optique quantique est souvent liée à la plasmonique quantique, je présenterai pour conclure une proposition théorique en collaboration avec Javier Garcia de Abajo pour étudier la plasmonique quantique dans un microscope électronique à transmission à balayage.
We report on the experimental demonstration of single-photon state generation and characterization in an electron microscope. In this aim we have used low intensity relativistic (energy between 60 and 100 keV) electrons beams focused in a ca. 1 nm probe to excite diamond nanoparticles. This triggered individual neutral nitrogen-vacancy centers to emit photons which could be gathered and sent to a Hanbury Brown-Twiss intensity interferometer. The detection of a dip in the correlation function at small time delays clearly demonstrates antibunching and thus the creation of nonclassical light states. Specifically, we have also demonstrated single-photon states detection. We unveil the mechanism behind quantum states generation in an electron microscope, and show that it clearly makes cathodoluminescence the nanometer scale analog of photoluminescence. By using an extremely small electron probe size and the ability to monitor its position with subnanometer resolution, we also show the possibility of measuring the quantum character of the emitted beam with deep subwavelength resolution.
The rapid growth of quantum technologies has driven the development of quantum defects for both single photon sources as well as for spin qubit and sensor applications. These artificial atoms in wide bandgap solid-state materials such as diamond and hBN offer the possibility of a truly quantum deterministic source. These single photon emitting quantum defects exhibit antibunching that can be quantified by performing the Hanbury-Brown Twiss (HBT) experiment. The HBT experiment allows one to evaluate the second-order autocorrelation (g(2)(τ )) function, which is a central measure in determining the quality of single photon emission. To enhance the various emission parameters such as intensity, wavelength, and directionality from these defects the development of photonic structures become essential. Here, we study single photon emission from a diamond nanopillar array containing Nitrogen Vacancy (NV) centers. We spatially map the g(2)(τ ) function and radiative lifetimes in these samples and correlate the two quantities. Crucially, we observe that the measured antibunching varies as a function of position of the objective relative to the nanopillar, even across the same pillar. This likely shows the dependence of the apparent antibunching to the angle of excitation and detection in such photonic structures. Our results highlight the importance of structural and geometric factors in g(2)(τ ) measurements of emitters, especially when embedded in photonic structures. These should be carefully understood in evaluating the quality of the single photon sources.
The nitrogen-vacancy (NV) center is a defect in the structure of diamond, roughly the size of a single atom, that possesses properties that make it applicable to quantum information processing, high-resolution magnetic resonance imaging, and probing biological systems. When an NV center is excited by green light of wavelength 523 nm, it emits red light of in the range from 600-800 nm tens of nanoseconds later. A confocal microscope can be used to excite and collect emitted photons from NV centers. By detecting the intensity of the emitted red photons, we can determine the quantum state of a single NV center, which is needed to build various quantum computing, sensing and imaging devices. The autocorrelation method is used to detect antibunching in the emission of red photons and ascertain that the observed photons are being emitted from a single NV center. We have built an autocorrelation device that measures, with sub-nanosecond accuracy, the statistical distribution of the arrival times of the red photons emitted from a diamond sample at a photon counter. The device can measure fixed time values with a time resolution of 120 ps FWHM. For uncorrelated sources measured for 120 seconds at the TAC full range of 100 ns, the average counts in each histogram bin were about 0.01% of the predicted value. The autocorrelation device that we have built costs about 10 times less than commercial units used by other NV researchers.
Abstract Nitrogen-vacancy (NV) centers in diamond are reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers, NV0 and NV–, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, an understanding of the dynamics between the different states still remains challenging. Here, conversion from NV– to NV0 due to electron-induced carrier generation is shown. Ultrafast pump–probe cathodoluminescence spectroscopy is presented for the first time, with electron pulses as pump and laser pulses as probe, to prepare and read out the NV states. The experimental data are explained with a model considering carrier dynamics (0.8 ns), NV0 spontaneous emission (20 ns), and NV0 → NV– back transfer (500 ms). The results provide new insights into the NV– → NV0 conversion dynamics and into the use of pump–probe cathodoluminescence as a nanoscale NV characterization tool.