high-Q Microcavity Research Articles

Tailoring the Energy Levels and Cavity Structures toward Organic Cocrystal Microlasers.

Organic cocrystals with unique energy-level structures are potentially a new class of materials for the development of versatile solid-state lasers. However, till now, the stimulated emission in cocrystal materials remains a big challenge possibly because of the nonradiative charge-transfer (CT) transitions. Here, for the first time, we report organic cocrystal microlasers constructed by simultaneously tailoring the energy levels and cavity structures based on the intermolecular halogen-bonding interactions. The intermolecular interactions triggered different self-assembly processes, resulting in distinct types of high-quality resonant microcavities. More importantly, the halogen-bonding interactions alleviated intermolecular CT and thus brought about a favorable four-level energy structure for the population inversion and tunable lasing in the cocrystals.

Implementation of a Two-Qubit C-NOT Quantum Gate in a System of Two Double Quantum Dots, a Microcavity, and a Laser

We study the possibility of implementing a two-qubit controlled-NOT gate operation in a structure consisting of two semiconductor double quantum dots placed in a high-Q optical microcavity and governed by a resonant laser field. The effect of relaxation processes on the dynamics of the two-electron system is discussed. The dissipation rates allowing quantum error correction algorithms to be used are determined. The existing additional excitation channel (laser pulse) is shown to weaken the effect of the nonideality of the cavity on the evolution of the states. Optimal electron coupling coefficients in the quantum dots with control fields are fitted, in which the controlled qubit is switched with the highest probability and the gate implementation takes several hundreds of picoseconds.

Single-shot condensation of exciton polaritons and the hole burning effect

A bosonic condensate of exciton polaritons in a semiconductor microcavity is a macroscopic quantum state subject to pumping and decay. The fundamental nature of this driven-dissipative condensate is still under debate. Here, we gain an insight into spontaneous condensation by imaging long-lifetime exciton polaritons in a high-quality inorganic microcavity in a single-shot optical excitation regime, without averaging over multiple condensate realisations. We demonstrate that condensation is strongly influenced by an incoherent reservoir and that the reservoir depletion, the so-called spatial hole burning, is critical for the transition to the ground state. Condensates of photon-like polaritons exhibit strong shot-to-shot fluctuations and density filamentation due to the effective self-focusing associated with the reservoir depletion. In contrast, condensates of exciton-like polaritons display smoother spatial density distributions and are second-order coherent. Our observations show that the single-shot measurements offer a unique opportunity to study fundamental properties of non-equilibrium condensation in the presence of a reservoir.

Dynamics of a Spinor Exciton–Polariton System in Laterally Strained GaAs Microcavities under Resonant Photoexcitation

The evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.

Saturable absorption by carbon nanotubes on silica microtoroids

Saturable absorption is a key technology for shaping the waveform of light such as in passive mode-locking. The combination of high-Q optical microcavities with a saturable absorber allows stable lasing and soliton formation. This work describes saturable absorption by carbon nanotubes (CNTs) on silica microtoroids. CNTs, which are saturable absorbers capable of a fast response time and broadband absorption, were grown on silica microtoroids by chemical vapor deposition (CVD). Raman spectroscopy revealed that the CNTs are in good quality (G/D ratio ∼ 7) and about 1.0 nm in diameter, thus confirming that a sample for use in the telecommunication band can be prepared by CVD. A counter-propagating pump-probe experiment enabled us to investigate the characteristics of CNTs as saturable absorbers while suppressing thermo-optic bistability in a microcavity system. The results revealed a saturable absorption coefficient of 0.042 cm−1, a saturable intensity of 25.9 MW/cm2, and a modulation depth of 28%. This is the first step toward the demonstration of the robust mode-locking in a silica microtoroid consisting of CNTs.

Fast light in the generation configuration of stimulated Brillouin scattering based on high-Q micro-cavities.

In this article, we reported self-pumped stimulated Brillouin scattering (SBS)-induced fast light in a micro-resonator. The optically induced thermal effect in the micro-resonator will lead to a shift of the dispersion spectrum and make the SBS gain occurred in the anomalous dispersion regime. The group delay could be experimentally optimized from -91.0 microsecond to 2.6 microsecond by changing the modulation frequency in a microsphere with its diameter of 175 μm. The experimental results will benefit applications utilizing anomalous dispersion such as gyroscope.

Effect of phonon-bath dimensionality on the spectral tuning of single-photon emitters in the Purcell regime

We develop a theoretical frame to investigate the spectral dependence of the brightness of a single-photon source made of a solid-state nanoemitter embedded in a high-quality factor microcavity. This study encompasses the cases of localized excitons embedded in a one-, two-, or three-dimensional matrix. The population evolution is calculated based on a spin-boson model, using the noninteracting blip approximation. We find that the spectral dependence of the single-photon source brightness (hereafter called spectral efficiency) can be expressed analytically through the free-space emission and absorption spectra of the emitter, the vacuum Rabi splitting, and the loss rates of the system. In other words, the free-space spectrum of the emitter encodes all the relevant information on the interaction between the exciton and the phonon bath to obtain the dynamics of the cavity-coupled system. We compute numerically the spectral efficiency for several types of localized emitters differing by the phonon bath dimensionality. In particular, in low-dimensional systems where this interaction is enhanced, a pronounced asymmetric energy exchange between the emitter and the cavity on the phonon sidebands yields a considerable extension of the tuning range of the source through phonon-assisted cavity feeding, possibly surpassing that of a purely resonant system.

Research progress on organic micro/nanoscale lasers

Micro/nanoscale lasers that can deliver intense coherent light signals at (sub)wavelength scale have attracted great interest because of their promising applications ranging from on-chip information processing to high-throughput sensing. Compared with traditional inorganic materials, organic materials are ideal platforms to construct high performance microlasers, mainly because of their superiority in flexibly assembled structures for high-quality microcavities and abundant excited-state processes with large active cross sections for high gain emissions. This review begins with an overview of the research evolution of organic microlasers in terms of microcavity resonators and energy-level gain. Then a series of strategies to tailor the microcavity structures and excited-state dynamics of organic materials for the modulation of lasing performances are highlighted. Subsequently, we present the design and construction of organic-microlaser-based hybrid structures with advanced photonic functionalities. In the following part, we introduce their applications in chemical sensing, biosensing and integrated nanophotonics. Finally, we provide our outlook on the current challenges as well as the future direction of organic microlasers. It is anticipated that this review will provide inspiration for the development of miniaturized lasers with desired performances by tailoring of excited-state processes and microcavity structures toward practical applications.

Динамика спинорной экситон-поляритонной системы в латерально сжатых GaAs микрорезонаторах при резонансном фотовозбуждении

AbstractThe evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.

Investigation of a Polariton Condensate in Micropillars in a High Magnetic Field

The photoluminescence of a nonequilibrium polariton condensate in cylindrical and rectangular micropillars etched on the surface of a high-Q GaAs microcavity is investigated in magnetic fields of up to 12 T. The measurements are carried out under different levels of nonresonant optical pumping with nanosecond laser pulses for a wide range of cavity detuning. As far as nonresonant excitation produces a high density of excitons in a reservoir, it should be expected that the exciton–polariton interaction, which depends on the pump level, has a considerable effect on the Zeeman splitting and polarization of the condensate. However, measurements of the Zeeman splitting and polarization in high magnetic fields demonstrate that only minor changes take place up to the highest available pump levels. This means that, in the case under study, the effect of exciton–polariton interaction on the polariton system is insignificant. At the same time, the data obtained provide an estimate for the exciton density in the reservoir. In contrast to cylindrical micropillars, the photoluminescence of the condensate in rectangular micropillars consists of two perpendicularly linearly polarized lines which retain a high degree of linear polarization even in a field as high as 12 T. The Zeeman splitting in this case is nearly independent of the pump power. The degrees of both circular and linear polarization change with pump power, but these changes are noticeably smaller than the ones predicted theoretically. This indicates that the system of polaritons in micropillars deviates considerably from thermodynamic equilibrium.

Detailed Balance Limit of Efficiency of Broadband-Pumped Lasers

Broadband light sources are a wide class of pumping schemes for lasers including LEDs, sunlight and flash lamps. Recently, efficient coupling of broadband light to high-quality micro-cavities has been demonstrated for on-chip applications and low-threshold solar-pumped lasers via cascade energy transfer. However, the conversion of incoherent to coherent light comes with an inherent price of reduced efficiency, which has yet to be assessed. In this paper, we derive the detailed balance limit of efficiency of broadband-pumped lasers and discuss how it is affected by the need to maintain a threshold population inversion and thermodynamically dictated minimal Stokes’ shift. We show that lasers’ slope efficiency is analogous to the nominal efficiency of solar cells, limited by thermalisation losses and additional unavoidable Stokes’ shift. The lasers’ power efficiency is analogous to the detailed balance limit of efficiency of solar cells, affected by the cavity mirrors and impedance matching factor, respectively. As an example we analyze the specific case of solar-pumped sensitized Nd3+:YAG-like lasers and define the conditions to reach their thermodynamic limit of efficiency. Our work establishes an upper theoretical limit for the efficiency of broadband-pumped lasers. Our general, yet flexible model also provides a way to incorporate other optical and thermodynamic losses and, hence, to estimate the efficiency of non-ideal broadband-pumped lasers.

Electric-field sensor based on a double quantum dot in a microcavity

A scheme for an optical quantum external-electric-field sensor based on a double quantum dot placed in a high-Q semiconductor microcavity is proposed. A model of the dynamic processes occurring in this system is developed, its spectral characteristics are investigated, and the noise stability of the sensor is examined. It is demonstrated that, owing to design features, the device has a number of advantages, including high sensitivity, the presence of different excitation and measurement channels, and the possibility of accurate determination of the spatial field distribution.

Tunable Third-Harmonic Generation from Polaritons in the Ultrastrong Coupling Regime

Strong interparticle interactions between polaritons have traditionally stemmed from their exciton component. In this work, we impart a strong third-order nonlinearity to a polaritonic mode by embedding a nonlinear polymethine dye within a high-Q all-metal microcavity. We demonstrate nonlinear microcavities operating in the ultrastrong coupling regime with a normalized coupling ratio of 62%, the highest reported to date. When pumping the lower polariton branch, we observe tunable third-harmonic generation spanning the entire visible spectrum, with internal conversion enhancements more than 3 orders of magnitude larger than in bare films. Transfer matrix calculations indicate that the observed enhancements are consistent with the enhanced pump electric field.

Controlling Tamm Plasmons for Organic Narrowband Near-Infrared Photodetectors

Organic spectrometers are attractive for biomedicine and industrial process monitoring but are currently limited in terms of spectral selectivity and the accessible wavelength range. Here, we achieve narrowband enhancement of the below-gap near-infrared response of charge-transfer (CT) excitations in organic photodiodes by introducing them into a high-quality microcavity. The device architecture includes a nonconductive distributed Bragg reflector and thin metal electrodes, leading to the formation of sharp Tamm plasmon-polariton resonances. We demonstrate how to tailor the arising multimode spectra for spectroscopic photodetectors and present efficient single-resonance devices with remarkable line widths below 22 nm, which are partially transparent for visible wavelengths. Taking advantage of the spectrally broad CT band, we vary the resonator thickness to provide a proof of concept that benefits from the spectral selectivity of our high-quality microcavities. Finally, utilizing transfer-matrix calculati...

Exceptional points enhance sensing in an optical microcavity.

Sensors play an important part in many aspects of daily life such as infrared sensors in home security systems, particle sensors for environmental monitoring and motion sensors in mobile phones. High-quality optical microcavities are prime candidates for sensing applications because of their ability to enhance light-matter interactions in a very confined volume. Examples of such devices include mechanical transducers, magnetometers, single-particle absorption spectrometers, and microcavity sensors for sizing single particles and detecting nanometre-scale objects such as single nanoparticles and atomic ions. Traditionally, a very small perturbation near an optical microcavity introduces either a change in the linewidth or a frequency shift or splitting of a resonance that is proportional to the strength of the perturbation. Here we demonstrate an alternative sensing scheme, by which the sensitivity of microcavities can be enhanced when operated at non-Hermitian spectral degeneracies known as exceptional points. In our experiments, we use two nanoscale scatterers to tune a whispering-gallery-mode micro-toroid cavity, in which light propagates along a concave surface by continuous total internal reflection, in a precise and controlled manner to exceptional points. A target nanoscale object that subsequently enters the evanescent field of the cavity perturbs the system from its exceptional point, leading to frequency splitting. Owing to the complex-square-root topology near an exceptional point, this frequency splitting scales as the square root of the perturbation strength and is therefore larger (for sufficiently small perturbations) than the splitting observed in traditional non-exceptional-point sensing schemes. Our demonstration of exceptional-point-enhanced sensitivity paves the way for sensors with unprecedented sensitivity.

Quasi bound states in the continuum with few unit cells of photonic crystal slab

Bound states in the continuum (BICs) in photonic crystal slabs represent the resonances with an infinite quality (Q)-factor, occurring above the light line for an infinitely periodic structure. We show that a set of BICs can turn into quasi-BICs with a very high Q-factor even for two or three unit cell structures. They are explained by a viewpoint of BICs originating from the tight-binding of individual resonances of each unit cell as in semiconductors. Combined with a reciprocal-space matching technique, the microcavities based on quasi-BICs can achieve a Q-factor as high as defect-based PhC microcavities. These results may enable the experimental studies of BICs in a compact platform as well as realizing high-Q mirrorless microcavities.

Direct measurement of polariton–polariton interaction strength

Exciton-polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions, typically using excitons in semiconductor quantum wells (QWs) placed at the antinodes of the same cavity. Quantum phenomena such as Bose-Einstein condensation (BEC), quantized vortices, and macroscopic quantum states have been reported at temperatures from tens of Kelvin up to room temperatures, and polaritonic devices such as spin switches \cite{Amo2010} and optical transistors have also been reported. Many of these effects of exciton-polaritons depend crucially on the polariton-polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty of determining the absolute densities of polaritons and bare excitons. Here we report the direct measurement of the polariton-polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 40 $\mu$m to the center of a laser-generated annular trap, we are able to separate the polariton-polariton interactions from polariton-exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons squarely into the strongly-interacting regime. When there is a condensate, we see a sharp transition to a different dependence of the renormalization on the density, which is evidence of many-body effects.

Investigation of a nonequilibrium polariton condensate in cylindrical micropillars in a strong magnetic field

We analyze the photoluminescence of a nonequilibrium polariton condensate in cylindrical micropillars etched on the surface of a high-Q GaAs microcavity in a wide range of detunings in a magnetic field up to 12 T for various levels of nonresonant laser pumping by nanosecond pulses. With such a method of excitation, a considerable effect of the interaction of the reservoir of photoexcited excitons with the condensate on the Zeeman splitting of the polariton condensate levels can be expected, which can lead to a decrease in its value and even to sign reversal. However, the measurements of photoluminescence in a wide range of optical excitation densities show that Zeeman splitting weakly depends on the optical pumping (its variation does not exceed 15% of the splitting in a field of 12 T). The estimation of the exciton density in the reservoir based on these data gives a value lower than 108 cm–2. In addition, a noticeable decrease (by a factor of about 1.8) in the polariton condensation threshold in a magnetic field is detected.

Wafer-scale arrays of high-Q silica optical microcavities.

On-chip high-Q microcavities possess significant potential in terms of integration of optical microresonators into functional optoelectronic devices that could be used in various applications, including biosensors, photonic-integrated circuits, or quantum optics experiments. Yet, despite the convenience of fabricating wafer-scale integrated microresonators with moderate Q values using standard microfabrication techniques, surface-tension-induced microcavities (STIMs), which have atomic-level surface roughness enabling the observation of Q values larger than 106, could only be produced using individual thermal treatment of every single microresonator within the devised area. Here, we demonstrate a facile method for large-scale fabrication of silica STIMs of various morphologies. Q values exceeding 106 are readily obtained using this technique. This study represents a significant advancement toward fabrication of wafer-scale optoelectronic circuitries.

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

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)