Coupling nitrogen-vacancy centers in diamond to fiber-based Fabry-Pérot microcavities

This thesis investigates the coupling of the fluorescence of nitrogen-vacancy (NV) centers in diamond to tunable optical microresonators at ambient conditions, in particular in the regime of Purcell enhancement. We use fiber-based, open-access Fabry-Pérot cavities optimized for high finesse and ultra-small mode volume. Different regimes of cavity enhancement are studied that are complementary to each other: A first experiment relies on a high-finesse cavity with dielectric mirrors. The scaling laws of Purcell enhancement are explicitly demonstrated by a large-range variation of both the cavity mode volume (V = 16 − 600 µm^3 ) and the quality factor (Q = 6 · 10^3 − 2 · 10^6). We detect an enhancement of the emission spectral density by up to a factor of 300. The full potential of this resonator can be exploited with emitters having a linewidth which is narrower than the resonance linewidth of the cavity. This concept holds promise for the implementation of wavelength-tunable, narrow-band single-photon sources as well as the generation of indistinguishable single-photons at ambient conditions. However, for broad-band emitters like the NV center at room temperature, the emission lifetime is not affected noticeably in this configuration. In order to directly observe lifetime changes and Purcell-enhanced single-photon emission, we manufacture fiber-based cavities with silver-coated mirrors having ultra-small mode volumes, as small as V = 1.0 λ^3 = 0.34 µm^3. We demonstrate cavity-enhanced fluorescence imaging, which allows to locate and analyze several single NV centers with one cavity. The Purcell effect is evidenced by an enhanced fluorescence collection of up to 1.6 · 10^6 photons per second from single-NV centers and a tunable variation of the emission lifetime corresponding to an effective Purcell factor of up to 2. We furthermore investigate a benefcial regime of optical confinement where the Fabry-Pérot cavity mode is combined with additional mode confinement by the diamond nanocrystal itself, enabling sub-λ^3 mode volumes. We perform simulations that predict effective Purcell factors of up to 11 for NV centers and of up to 63 for silicon-vacancy centers, revealing a great potential for bright single-photon sources and effcient spin readout at ambient conditions.Diese Arbeit erforscht die Kopplung der Fluoreszenz von Stickstoff-Fehlstellen-Zentren (NV-Zentren) in Diamant mit durchstimmbaren optischen Mikroresonatoren bei Umgebungsbedingungen, insbesondere im Regime der Purcell Verstärkung. Hierzu benutzen wir faserbasierte, offen zugängliche Fabry-Pérot Resonatoren, die für hohe Finesse und ultrakleine Modenvolumen optimiert sind. Verschiedene, komplementäre Bereiche der Resonatorverstärkung werden untersucht. Ein erstes Experiment basiert auf einem Resonator mit hoher Finesse und dielektrischen Spiegeln. Das Skalierungsverhalten der Purcell Verstärkung wird ausführlich ausgewertet, indem man sowohl das Modenvolumen des Resonators (V = 16 − 600 µm^3 ) als auch dessen Güte (Q = 6 · 10^3 − 2 · 10^6) über einen weiten Bereich verändert. Die spektrale Leistungsdichte der Emission kann durch den Resonator um einen Faktor von bis zu 300 überhöht werden. Das gesamte Leistugsvermögen dieses Resonators kann mit schmalbandigen Emittern ausgenutzt werden, deren Emissionslinienbreite kleiner als die Linienbreite des Resonators ist. Dies ist ein vielversprechender Ansatz für die Umsetzung von schmalbandigen Einzelphotonenquellen mit durchstimmbarer Wellenlänge und für die Erzeugung ununterscheidbarer Einzelphotonen bei Umgebungsbedingungen. Jedoch bleibt die Lebenszeit der Emission für breitbandige Emitter, wie dem NV-Zentrum bei Raumtemperatur, in dieser Anordnung nahezu unbeeinflusst. Um eine Veränderung der Lebenszeit und durch den Purcell-Effekt verstärkte Einzelphotonenemission direkt zu beobachten, stellen wir Faserresonatoren mit silberbeschichteten Spiegeln und ultrakleinen Modenvolumen, bis hinab zu V = 1.0 λ^3 = 0.34 µm^3, her. Wir demonstrieren resonatorverstärkte Fluoreszenzbildgebung, die das Auffinden und Untersuchen von verschiedenen einzelnen NV-Zentren mit einem Resonator erlaubt. Der Purcell-Effekt wird über eine gesteigerte Aufsammlung der Fluoreszenz nachgewiesen, mit einer Rate von bis zu 1.6 · 10^6 Photonen pro Sekunde von einzelnen NV-Zentren und außerdem durch die abstimmbare Veränderung der Emissionslebenszeit, entsprechend einem effektiven Purcell Faktor von bis zu 2. Des Weiteren untersuchen wir ein vorteilhaftes Regime, in dem der Diamant Nanokristall selbst eine zusätzliche Einschränkung der optischen Mode bewirkt, die sich mit der Mode des Fabry-Pérot Resonators verbindet und Modenvolumen unter 1 λ^3 ermöglicht. Simulationen ergeben effektive Purcell Faktoren von bis zu 11 für NV-Zentren und von bis zu 63 für Silizium-Fehlstellen-Zentren, wodurch das große Potenzial für helle Einzelphotonenquellen und für effzientes Spin-Auslesen bei Umgebungsbedingungen aufgezeigt wird

Fiber Fabry-Perot cavity with high finesse

We have realized a fiber-based Fabry-Perot cavity with CO2 laser-machined mirrors. It combines very small size, high finesse F>=130000, small waist and mode volume, and good mode matching between the fiber and cavity modes. This combination of features is a major advance for cavity quantum electrodynamics (CQED), as shown in recent CQED experiments with Bose-Einstein condensates enabled by this cavity [Y. Colombe et al., Nature 450, 272 (2007)]. It should also be suitable for a wide range of other applications, including coupling to solid-state emitters, gas detection at the single-particle level, fiber-coupled single-photon sources and high-resolution optical filters with large stopband.Comment: Submitted to New J. Phys

Microcavity Strain Sensor for High Temperature Applications

A microcavity extrinsic Fabry-Perot interferometric (EFPI) fiber-optic sensor is presented for measurement of strain. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber with a femtosecond (fs) laser and is then self-enclosed by fusion splicing another piece of single-mode fiber. The fs-laser-based fabrication makes the sensor thermally stable to sustain temperatures as high as 800°C. The sensor exhibits linear performance for a range up to 3700 µε and a low temperature sensitivity of only 0.59 pm/°C through 800°C

Sensing with ultra-short Fabry-Perot cavities written into optical micro-fibers

Abstract not availableStephen C. Warren-Smith, Ricardo M. André, Jan Dellith, Tina Eschrich, Martin Becker, Hartmut Bartel

Sensores de fibra ótica para meios desafiantes

With the present work, the development of fiber optic sensor solutions for the application in challenging media was intended. New sensor structures based on the post-processing of optical fibers were addressed, taking into account their sensitivity to variations in the external environment. In a first stage, fiber Bragg gratings were embedded in lithium batteries, to monitor temperature in situ and operando. Due to the harsh chemical environment of the battery, fiber optic sensors revealed to be the most advantageous alternative, when comparing to the electronic sensors. Fiber sensors exhibited good sensitivities and fast responses, besides being less invasive, thus they did not compromise the battery response. Furthermore, they were chemically stable. Still in the framework of this theme, and with the objective of monitoring possible strain and pressure variations inside the batteries, new sensors based on in-line Fabry-Perot cavities have been proposed. These sensors were characterized in lateral load, strain, and temperature. In a later stage, the study focused on the development of configurations that allowed to obtain high-resolution and/or sensitivity sensors. One of such configurations was obtained by creating a hollow microsphere at the fiber tip. The sensor was used to detected concentration variations and refractive index of glycerin and water mixtures. The influence of the diaphragm size in the sensor response was also studied, as well as the temperature response. New sensors based on multimode interference have also been characterized, using a coreless silica fiber tip. First, the influence of different parameters, such as length and diameters were analyzed. The sensors were tested in different solutions of glucose and water. It was observed that the sensor diameter is a decisive factor in obtaining devices that are more sensitive to refractive index and, consequently, to concentration. The determination of the thermo-optic coefficient of water/ethanol mixtures was also addressed using a multimode fiber interferometer sensor. Finally, a multimode interferometer sensor was functionalized by depositing agarose throughout the structure, allowing to optimize the response of the sensors to the external environment.Com o presente trabalho pretendeu-se explorar soluções de sensores em fibra ótica para a aplicação em meios desafiantes. Novas estruturas sensoras baseadas em pós-processamento de fibra ótica foram abordadas, tendo em consideração a sua sensibilidade a variações do meio externo. Numa primeira etapa, foram embebidas redes de Bragg no interior de baterias de lítio, para monitorizar variações de temperatura in situ e operando. Devido ao complexo meio químico da bateria, os sensores em fibra ótica revelaram ser uma alternativa mais vantajosa em relação aos sensores elétricos, não só pela sensibilidade e rápida resposta, mas também pelo fato de não afetarem o desempenho da bateria. Além disso, os sensores usados revelaram ser pouco invasivos e quimicamente estáveis. Ainda no âmbito deste tema, e com o objetivo de monitorizar possíveis deformações e variações de pressão no interior da bateria de lítio, foram desenvolvidos novos sensores baseados em cavidades de Fabry-Perot do tipo in-line. Esses sensores foram caraterizados em pressão lateral, deformação e temperatura. Numa fase posterior, o estudo centrou-se no desenvolvimento de configurações que permitissem a obtenção de sensores com elevada resolução e/ou sensibilidade. Uma das configurações consistiu na formação de uma microesfera oca na ponta de uma fibra ótica. Esse sensor foi utilizado para detetar variações de concentração e índice de refração de misturas de glicerina e água. A influência do tamanho do diafragma na resposta do sensor também foi estudada, assim como a resposta em temperatura. Em seguida, desenvolveram-se novos sensores baseados em interferência multimodo, utilizando para tal uma ponta de fibra de sílica sem núcleo. Numa primeira abordagem analisou-se a influência de diferentes parâmetros, como o comprimento e o diâmetro dos sensores. Os sensores foram expostos a diferentes soluções de glucose e água. Verificou-se que o diâmetro do sensor é um fator decisivo para a obtenção de dispositivos mais sensíveis ao índice de refração e, consequentemente, à concentração. Foi também desenvolvido um sensor baseado em interferência multimodo que permitiu determinar o coeficiente termo-ótico de misturas de etanol e água. Por fim, procedeu-se à funcionalização de um sensor baseado em interferência multimodo através da deposição de agarose ao longo da estrutura, permitindo assim otimizar a sua resposta a variações do meio externo.Programa Doutoral em Engenharia Físic

Interferometric Fiber Optic Sensors

Fiber optic interferometers to sense various physical parameters including temperature, strain, pressure, and refractive index have been widely investigated. They can be categorized into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac. In this paper, each type of interferometric sensor is reviewed in terms of operating principles, fabrication methods, and application fields. Some specific examples of recently reported interferometeric sensor technologies are presented in detail to show their large potential in practical applications. Some of the simple to fabricate but exceedingly effective Fabry-Perot interferometers, implemented in both extrinsic and intrinsic structures, are discussed. Also, a wide variety of Mach-Zehnder and Michelson interferometric sensors based on photonic crystal fibers are introduced along with their remarkable sensing performances. Finally, the simultaneous multi-parameter sensing capability of a pair of long period fiber grating (LPG) is presented in two types of structures; one is the Mach-Zehnder interferometer formed in a double cladding fiber and the other is the highly sensitive Sagnac interferometer cascaded with an LPG pair

Optical fiber sensors by direct laser processing: a review

The consolidation of laser micro/nano processing technologies has led to a continuous increase in the complexity of optical fiber sensors. This new avenue offers novel possibilities for advanced sensing in a wide set of application sectors and, especially in the industrial and medical fields. In this review, the most important transducing structures carried out by laser processing in optical fiber are shown. The work covers different types of fiber Bragg gratings with an emphasis in the direct-write technique and their most interesting inscription configurations. Along with gratings, cladding waveguide structures in optical fibers have reached notable importance in the development of new optical fiber transducers. That is why a detailed study is made of the different laser inscription configurations that can be adopted, as well as their current applications. Microcavities manufactured in optical fibers can be used as both optical transducer and hybrid structure to reach advanced soft-matter optical sensing approaches based on optofluidic concepts. These in-fiber cavities manufactured by femtosecond laser irradiation followed by chemical etching are promising tools for biophotonic devices. Finally, the enhanced Rayleigh backscattering fibers by femtosecond laser dots inscription are also discussed, as a consequence of the new sensing possibilities they enableThis research was funded by the Ministerio de Economía y Competitividad of Spain (TEC2016-76021-C2-2-R), the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (PID2019- 107270RB-C21), and the Ministerio de Educación, Cultura y Deporte of Spain (PhD grant FPU2018/02797)

Femtosecond laser micro-machined optical fiber based embeddable strain and temperature sensors for structural monitoring

Structural monitoring technology is becoming increasingly important for managing all types of structures. Embedding sensors while constructing new structures or repairing the old ones allows for continual monitoring of structural health thus giving an estimate of remaining utility. Along with being embeddable, miniaturized sensors that are easy to handle are highly sought after in the industry where in-situ monitoring is required in a harsh environment (corrosive atmosphere, high temperatures, high pressure etc.). This dissertation demonstrates the use of femtosecond laser-fabricated Fabry-Perot interferometer (FPI) based optical fiber sensors for embedded applications like structural health monitoring. Two types of Fabry-Perot interferometer sensors, extrinsic FPI and intrinsic FPI, have been designed, developed and demonstrated for strain and temperature monitoring applications. The absence of any movable parts make these sensors easy-to-handle and easy to embed inside a material. These sensors were fabricated using a laboratory integrated femtosecond (fs) laser micromachining system. For the extrinsic Fabry-Perot interferometer (EFPI) design, the fs-laser was used to ablate and remove the material off the fiber end face while for intrinsic Fabry-Perot interferometer (IFPI) design, the laser power was focused inside the fiber on the fiber core to create two microstructures. The scope of the work presented in this dissertation extends to device design, laser based sensor fabrication, sensor performance evaluation and demonstration. Feasibility of using these sensors for embeddable applications was investigated. A new type of material called Bismaleimide (BMI) was used for demonstrating the embeddability of the sensors. Experimental results of strain and temperature testing are presented and discussed. The EFPI sensor has low temperature sensitivity of 0.59 pm/⁰C and a high strain sensitivity of 1.5 pm/µε. The IFPI sensor has the same strain sensitivity as EFPI but is 25 times more sensitive to the temperature. These sensors were tested up to 850 ⁰C in non-embedded condition and they produced a linear response. A hybrid approach combining the EFPI and IFPI sensors was demonstrated for simultaneous measurement of strain and temperature --Abstract, page iii