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Laser & Photonics Review

Realization of controllable photonic molecule based on three ultrahigh‐Q microtoroid cavities (Laser Photonics Rev. 11(2)/2017)

Chao Yang, Xiaoshun Jiang, Qian Hua, Shiyue Hua, Yuan Chen, Jiyang Ma, Min Xiao

Published Mar 1, 2017• 4 citations
Publisher

Abstract

Photonic molecules have been shown to be a very useful platform for studying fundamental physics and practical applications. In recent years, double-cavity photonic molecules have been used in many important applications. Here, an approach to realize the fully-controlled triple-cavity photonic molecules, based on three ultrahigh-Q microtoroids with two different configurations, is experimentally demonstrated. By controlling the coupling strengths and resonant frequencies of the high-Q microcavities, interesting phenomena, such as super-mode evolution, dark state, double anti-crossing, as well as transition from double-EIT to double-EIA, are observed. This fully-controllable triple-cavity photonic molecule sets a unique stage for future investigations of photonic devices and cavity-QED effects. (Picture: Chao Yang et al., article number 1600178, in this issue)

DOI
10.1002/lpor.201770021

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Photonic molecules have been shown to be a very useful platform for studying fundamental physics and practical applications. In recent years, double-cavity photonic molecules have been used in many important applications. Here, an approach to realize the fully-controlled triple-cavity photonic molecules, based on three ultrahigh-Q microtoroids with two different configurations, is experimentally demonstrated. By controlling the coupling strengths and resonant frequencies of the high-Q microcavities, interesting phenomena, such as super-mode evolution, dark state, double anti-crossing, as well as transition from double-EIT to double-EIA, are observed. This fully-controllable triple-cavity photonic molecule sets a unique stage for future investigations of photonic devices and cavity-QED effects. (Picture: Chao Yang et al., article number 1600178, in this issue)
All-optical analogs of electromagnetically induced absorption (EIA) and electromagnetically induced transparency (EIT) are important for light storage, sensing, and optical communication. Here, we report results of a computational study focusing on the EIA and EIT characteristics of triple-microcavity hosting one-dimensional photonic crystals (PHCs). In contrast to previous experimental investigation of dual PHC cavities, realization EIA and EIT in the triple-cavity PHC is not restricted to reflection and transmission, respectively. In addition, the dispersive response of the triple-cavity is also distinct since EIA in the triple-cavity may lead to the generation of both sub- and superluminal light pulses. Furthermore, the dispersion of the system at either output may be reversed by altering the distribution of the coupled light among the three cavities. Finally, we discuss the experimental implementation of these features based on a single monolithic PHC and show tuning of the EIA and EIT phenomenon over a broad spectral range by using strain-tunable polymer-based PHC microcavities. Among others, these findings are advantageous for realizing ultra-narrow bandwidth optical filters, all-optical switching, temporal control of cavity quantum electrodynamics effects, and on-chip dispersion control.
In this paper, we analyze a chain-linked triple-cavity photonic molecule (TCPM) with controllable coupling strengths between the cavities on their spectral properties and field (energy) distributions by solving eigenvalues and eigenvectors of the Hamiltonian matrix based on coupled-mode theory. Phase transition is extended from double-cavity photonic molecules (DCPMs) to TCPMs, and evolutions of the supermode frequencies and linewidths are analyzed, which have synchronous relations with the degree of coherence between adjacent optical microcavities and energy distributions in the three cavities, respectively. We develop a superposition picture for the three supermodes of the TCPM, as interferences between supermodes of sub-DCPMs. In particular, we demonstrate the abnormal properties of the central supermode in TCPMs, such as the dark state in the middle cavity and phase shift when energy is flowing between the side cavities; these are promising for information processing and remote control of energy. General properties of TCPMs are summarized and limitations on linewidths are given. Finally, we make an interesting analog to intracavity electromagnetically induced transparency in multilevel atomic systems using the flexible TCPM platform under appropriate conditions.
A single two-level emitter coupled to a single mode of an electromagnetic resonator constitutes the most fundamental instance of light--matter interaction. Realizations of this type of system are investigated in the field of cavity quantum electrodynamics (cavity QED). Particularly important are schemes in which the coupling is strong enough to exceed the spontaneous emission rate as well as cavity damping. In this case, the coherent exchange of excitation between source and photon is the dominating process, providing the unique possibility to deterministically control the quantum evolution of the system. Great progress has recently been accomplished, technologically as well as conceptually, and the focus section in this issue of Journal of Optics B: Quantum and Semiclassical Optics features five articles highlighting important recent developments in the field. Strong coupling between single atoms and the electromagnetic cavity field have been achieved in two distinct regimes, the optical and the microwave region. The latter is represented by the micromaser, a high-Q microwave cavity pumped by a beam of highly excited Rydberg atoms. It has proved to be an almost ideal system for the generation of non-classical field states. A constraint in these experiments is the fact that the field inside the cavity can only be examined indirectly, by analysing the occupation of atoms leaving the cavity. While only two atomic states were previously identified for this purpose, V A Reshetov shows in his article (pages 119--126) that more information on the field may be extracted by taking advantage of the Zeeman substructure of the atomic levels. In the optical domain of cavity QED, by contrast, the state of the resonator field may be probed directly by measuring the light leaking from the cavity using photodetectors. In order to minimize damping, high-quality cavities must be employed. In most optical experiments these are Fabry--Perot resonators with ultra-high reflectivity mirrors, with losses as low as a few parts per million being state of the art. At the same time, the coupling of atoms and photons is enhanced by reducing the length of the cavity, so that strong coupling conditions are reached. A very attractive application of this type of cavity QED setup is the possibility of exerting control over the quantum dynamics of the system. An impressive example is the implementation of quantum feedback, reported in the paper by W P Smith and L A Orozco (pages 127--134). Using information gained from detecting a single photon escaping from the cavity, active feedback is applied to halt the dynamical evolution of the system. This is the first time active feedback has been used to modify the quantum evolution of a system at the single-photon level. The authors discuss how detunings between the subsystems affect the quantum feedback, an issue particularly relevant for ensembles of atoms with a spread in detuning. Perfect quantum feedback in this case requires the use of additional parameters to control the system. The majority of optical cavity QED experiments performed so far have used as a medium atoms traversing the cavity on random trajectories, such as, for example, thermal atomic beams. In this case, the observation of quantum effects is degraded in two ways. The Doppler effect and the finite transit time lead to line broadening, while the spatial structure of the cavity mode results in atoms in different locations experiencing different coupling with the photonic field. J E Reiner et al (pages 135--142) investigate the impact of these effects on quantum fluctuations in their cavity QED system. In order to overcome line broadening, they have developed a continuous cold atomic beam, extracted from a magneto-optical trap. An even better control of broadening effects and fluctuating coupling strength is provided by trapping the atoms inside the cavity and cooling them to temperatures at which their position spread is smaller than the optical wavelength. Much recent activity has been devoted to developing such an ultimate cavity QED system. A Vukics et al (pages 143--153) present a theoretical proposal for dynamically trapping and cooling atoms inside a cavity. By driving the atoms with a standing wave injected from the side of the cavity and by weakly exciting the cavity field itself, they predict three-dimensional trapping of the atom for periods around 1 second, an extremely long time on the typical scale of cavity QED dynamics. The essential component of their scheme is the interference between the two driving fields, determining the temperature of the particle and the stable trapping positions. In the quest for better confinement of radiation, lately, an appealing alternative to the Fabry--Perot cavity has been investigated -- the optical microsphere resonator. A sphere with a diameter of a few tens of microns, it supports so-called whispering gallery modes near its circumference, for which quality factors up to 10 10 have been measured. In order to exploit the correspondingly low field damping rates for cavity QED, a method must be found to couple atomic or solid state emitters to these modes. S Göotzinger et al (pages 154--158) report the coupling of single nanocrystals on the surface of a microsphere through the evanescent field of the mode. Selective excitation of a single nano-emitter was accomplished with a scanning confocal microscope. The system is a promising candidate for cavity QED with single-quantum emitters. The ability to fully control the quantum dynamics of atoms and light is beginning to spawn early applications. The most notable example is quantum information processing. This requires the coherent manipulation of single-quantum systems, which is ideally provided by cavity QED interaction in the strong-coupling limit. The development of practical devices will be among the principal future tasks for research in the field. This will include physical systems, which have not been covered in this issue, such as semiconductors, quantum dots as emitters, or photonic crystals as resonator structures. Cavity QED is therefore guaranteed to remain a vigorous field in the years to come.
We implement permanent spectral tuning to bring lifetime-limited emitters into collective resonance within an integrated photonic cavity. This addresses a fundamental challenge in solid-state cavity QED: combining multiple coherent quantum emitters with scalable nanophotonics. Our hybrid approach decouples emitter synthesis from nanophotonic fabrication using straightforward techniques that make cavity QED broadly accessible. High doping densities allow us to couple several coherent emitters to a single cavity mode, while optically-induced frequency shifting provides long-lived spectral control. By tuning two molecules into resonance, we demonstrate controlled formation of collective quantum states, establishing a scalable platform for many-body cavity QED. This opens pathways toward chemically-designed quantum systems where optical properties are engineered through synthetic chemistry.
The article gives an introduction into some of the basic cavity QED concepts, highlights some recent achievements and addresses some of the perspectives opened up by the ability to precisely control individual atomic and photonic quantum objects. Optical cavity QED with its ability to detect individual atoms and single photons plays an increasingly important role for various applications. The importance of optical cavity QED for the advance of quantum electronics can be highlighted by the rapidly growing number of research groups, the exponentially increasing number of publications and citations, and the increasing number of cavity designs ranging from the standard standing-wave Fabry-Perot cavity with dielectric mirrors to nanostructured slabs with a photonic band gap and high-quality running-wave microtoroids with optical fibre coupling.