Dark Resonances Research Articles

The role of tunable nonlinear dark resonances on vacuum Rabi splitting and optical bistability in an atom-cavity system

The phenomenon of “dark resonances” is a well-known concept in quantum optics and laser spectroscopy. As a general rule, interactions involving in such a “dark state” lead to multiple quantum superposition states that interact coherently and are undesirable. In this paper, two types nonlinear interaction in an atomic cavity, namely the nested and cascaded interactions, are theoretically analyzed how the dark resonances form the dark state peak to modulate the vacuum Rabi splitting (VRS) and optical bistability (OB) behavior. In both the zero- and high order modes, there are four VRS peaks generated in the nested interaction and three in the cascade interaction. Dark resonance can modulate not only the peak number of VRS, but also the OB thresholds. It is found that dark state can determine the asymmetric OB distribution of nested type and symmetric OB distribution of cascade type. Besides that, the distinctive OB thresholds in two kinds of interaction also be studied. The observations not only conceptually extend the conventional “dark resonances” phenomenon, but also opens the door for a variety of new applications in tunable all-optical switch and quantum communication.

Coherent population trapping in optically thin 133Cs atomic vapor in a finite-size cell

We study the peculiarities of the coherent population trapping (CPT) dark resonance in atomic vapor confined in a finite-size cell. Taking into account the scattering of atoms on anti-relaxation coating of the walls, we analyze the line shape of the CPT resonance in 133 C s depending on various parameters, such as the width of laser radiation spectrum, the type of pumping, and the magnitude of static magnetic field for both lin || lin and l i n ⊥ l i n polarization schemes. We show that for different parameters, the dependence of the line shape on the size of a cell is different, i.e., different factors affect the CPT resonance nonadditively. Thus, in the case when CPT resonance is excited by a narrowband laser, the shape of the resonance weakly depends on the size of a cell due to the laser-induced narrowing effect. We also show that the distortion of the line shape of the CPT resonance caused by nonresonant hyperfine sublevels of atomic excited states is modified with the size of a cell. Our calculation reveals the important role of accounting for the hyperfine and Zeeman structure of atomic levels. We have shown that the contrast of the CPT resonance differs about 5 times from the prediction of the simple three-level Λ model. The obtained results can be used for the optimization of the quantum frequency standards based on the CPT effect. In the final part of the paper, on the basis of the shot noise approximation we estimate the short-term stability of such standards.

Intermolecular dark resonance energy transfer (DRET): upgrading fluorogenic DNA sensing.

The sensitivity of FRET-based sensing is usually limited by the spectral overlaps of the FRET donor and acceptor, which generate a poor signal-to-noise ratio. To overcome this limitation, a quenched donor presenting a large Stokes shift can be combined with a bright acceptor to perform Dark Resonance Energy Transfer (DRET). The consequent fluorogenic response from the acceptor considerably improves the signal-to-noise ratio. To date, DRET has mainly relied on a donor that is covalently bound to the acceptor. In this context, our aim was to develop the first intermolecular DRET pair for specific sensing of nucleic acid sequences. To this end, we designed DFK, a push–pull probe based on a fluorenyl π-platform that is strongly quenched in water. DFK was incorporated into a series of oligonucleotides and used as a DRET donor with Cy5-labeled complementary sequences. In line with our expectations, excitation of the dark donor in the double-labeled duplex switched on the far-red Cy5 emission and remained free of cross-excitation. The DRET mechanism was supported by time-resolved fluorescence measurements. This concept was then applied with binary probes, which confirmed the distance dependence of DRET as well as its potency in detecting sequences of interest with low background noise.

Dynamically tunable slow light characteristics in graphene based terahertz metasurfaces

We propose a graphene based metasurface exhibiting dynamic slow light behavior via electromagnetically induced transparency (EIT) effect in the terahertz regime. The unit cell of the metasurface consists of bright and dark graphene rod resonators in a planar orthogonal configuration. The onset and tunability of the EIT effects are investigated through manipulating the near field electromagnetic coupling among the bright and dark resonators. An analytical method based on three level Lorentz oscillator model is employed to validate the simulated results. Further, we investigate the dynamic tunability of EIT effect by altering the Fermi energy of the graphene resonators. A blue shift of 0.59 THz in the EIT transparency peak is observed as the Fermi energy is increased from 0.3 eV to 1.0 eV. Similar dynamic tunability is investigated for the slow light characteristics (i.e., group delay, group index & group velocity) along with the delay bandwidth product. It is observed that an increase in the Fermi energy ultimately leads to an increase in group delay, group index & delay bandwidth product with their corresponding maximum values as 0.76 ps, 22.78 & 0.14 respectively for Fermi energy as 1.0 eV. For this case, the normalized group velocity reaches a minimum value of 0.04, thus, confirming the dynamic tunability of slow light behavior in the studied metasurfaces. Our findings can lead to the development of active slow light components like modulators, sensors, filters, buffers, ultrafast switches, and compact delay lines etc. in the terahertz domain.

Dark blood cardiovascular magnetic resonance of the heart, great vessels, and lungs using electrocardiographic-gated three-dimensional unbalanced steady-state free precession

BackgroundRecently, we reported a novel neuroimaging technique, unbalanced T1 Relaxation-Enhanced Steady-State (uT1RESS), which uses a tailored 3D unbalanced steady-state free precession (3D uSSFP) acquisition to suppress the blood pool signal while minimizing bulk motion sensitivity. In the present work, we hypothesized that 3D uSSFP might also be useful for dark blood imaging of the chest. To test the feasibility of this approach, we performed a pilot study in healthy subjects and patients undergoing cardiovascular magnetic resonance (CMR).Main bodyThe study was approved by the hospital institutional review board. Thirty-one adult subjects were imaged at 1.5 T, including 5 healthy adult subjects and 26 patients (44 to 86 years, 10 female) undergoing a clinically indicated CMR. Breath-holding was used in 29 subjects and navigator gating in 2 subjects. For breath-hold acquisitions, the 3D uSSFP pulse sequence used a high sampling bandwidth, asymmetric readout, and single-shot along the phase-encoding direction, while 3 shots were acquired for navigator-gated scans. To minimize signal dephasing from bulk motion, electrocardiographic (ECG) gating was used to synchronize the data acquisition to the diastolic phase of the cardiac cycle. To further reduce motion sensitivity, the moment of the dephasing gradient was set to one-fifth of the moment of the readout gradient. Image quality using 3D uSSFP was good-to-excellent in all subjects. The blood pool signal in the thoracic aorta was uniformly suppressed with sharp delineation of the aortic wall including two cases of ascending aortic aneurysm and two cases of aortic dissection. Compared with variable flip angle 3D turbo spin-echo, 3D uSSFP showed improved aortic wall sharpness. It was also more efficient, permitting the acquisition of 24 slices in each breath-hold versus 16 slices with 3D turbo spin-echo and a single slice with dual inversion 2D turbo spin-echo. In addition, lung and mediastinal lesions appeared highly conspicuous compared with the low blood pool signals within the heart and blood vessels. In two subjects, navigator-gated 3D uSSFP provided excellent delineation of cardiac morphology in double oblique multiplanar reformations.ConclusionIn this pilot study, we have demonstrated the feasibility of using ECG-gated 3D uSSFP for dark blood imaging of the heart, great vessels, and lungs. Further study will be required to fully optimize the technique and to assess clinical utility.

Enhanced magnetic Lorentz force second harmonic generation originating from a double-resonances plasmonic metasurface

Artificial second harmonic generation (SHG) based on magnetic Lorentz force has attracted abundant attention from researchers because of the initial breakthrough in physics. It is still a challenging task to boost this type of SHG emission due to the relative lower efficiency and the specific polarization of artificial SHG. Here, we demonstrate an effective way to enhance the magnetic Lorentz force-based SHG in a double-resonances plasmonic metasurface. The design of our method is twofold: firstly, a dark resonance at fundamental frequency and a bright resonance at second harmonic frequency (SHF); secondly, polarization consistency between the bright resonance and the SHF signal. The results demonstrate that the SHF conversion efficiency of this mode-matching plasmonic metasurface can reach 1.4 × 10−9, which is enhanced by a factor of 5.17 compared to the case without the mode-matching mechanism. This high efficiency and free design of a plasmonic metasurface offer a promising way for the applications of nonlinear optics.

Microwave-assisted all-optical switching in a four-level atomic system

We study the all-optical switching of the weak probe field via a four-level atom system with the help of a microwave field and electromagnetically induced transparency effect. Here, the microwave field is used for coupling two-folded lower levels to which double dark resonances (DDRs) can arise. The atomic medium is switched from transparent to complete absorption under the DDRs. It is shown that the absorption spectrum, pulse propagations and all-optical switching dynamics of the probe field are dependent on the intensity of the microwave field, applied fields and probe detuning. Furthermore, the switching mode between the probe field and the microwave field can be antisynchronous or synchronous, depending on the probe detuning being adjusted to be equal to zero or ± $$\Omega _{{c}}$$ , respectively. Our results possibly are helpful in actual tests to design all-optical switching and optical storage devices in optical communications and signal processing.

Terahertz Polarization Conversion in an Electromagnetically Induced Transparency (EIT)‐Like Metamaterial

AbstractIn this paper, a unique metamaterial (MM) design incorporating both electromagnetically induced transparency (EIT) effect and polarization conversion is proposed. It consists of a cut‐wire as a bright resonator and a side coupled split‐ring resonator (SRR) as a dark resonator, on the one hand, the dark resonator can be excited by the bright resonator via the magnetic near‐field coupling, thereby contributing to a strong cross‐polarization conversion. On the other hand, the destructive interference between the sub‐radiant (dark) SRR and the super‐radiant (bright) cut‐wire results in an EIT effect. Benefiting from the destructive interference between the radiative and dark elements in the EIT MM, radiation loss is greatly suppressed and nearly perfect cross‐polarization conversion with high transmission and low‐loss is realized. In addition, the importance of symmetry is demonstrated by the phenomenon of polarization conversion and EIT effects disappearing when an identical cut‐wire is placed on the other side of the SRR. The proposed MM design provides a new way to address the loss problem in MM polarization devices. More importantly, this design principle of taking advantage of one area to solve the challenges in others may inspire new high‐performance multifunctional MM designs.

Bright Plasmons with Cubic Nanometer Mode Volumes through Mode Hybridization

We propose a new interpretation for light confinement in picocavities formed by ultrasmall metallic protuberances inside the gap of metal−insulator−metal nanoresonators. We demonstrate that the protuberances support dark resonances with mode volumes comparable to their geometric volumes and that their brightness can be enhanced by several orders of magnitude when they are strongly coupled with the modes of nanoresonators with nanometric dielectric spacers. With a simple and accurate closed-form expression, we clarify the role of gap plasmons in this coupling. Based on this understanding, we propose a general strategy, exploiting strong coupling to design extremely localized modes with cubic nanometer volumes and so-far unreached brightness.

Implications of gravitational-wave production from dark photon resonance to pulsar-timing observations and effective number of relativistic species

The coherent oscillation of axionic fields naturally drives copious production of dark photon particles in the early universe, due to resonance and tachyonic enhancement. During the process, energy is abruptly transferred from the former to the latter, sourcing gravitational wave generation. The resulting gravitational waves are eventually to be observed as stochastic background today. We report analytical results of this production and connect them to the recent pulsar-timing results by the NANOGrav collaboration. We show an available parameter space, around the mass $m_\phi \sim 10^{-13} \, {\rm eV}$ and the decay constant $f_\phi \sim 10^{16} \, {\rm GeV}$ with a dimensionless coupling of ${\cal O}(1)$, for our mechanism to account for the signal. A mechanism to avoid the axion over-dominating the universe is a necessary ingredient of this model, and we discuss a possibility to recover a symmetry and render the axion massless after the production. We also comment on potential implications of the required effective number of relativistic species to the determination of the present Hubble constant.

Excitation of symmetry protected modes in a lithium niobate membrane photonic crystal for sensing applications

Our theoretical study reveals the opportunity to develop an electric field sensor based on the exploitation of the symmetry protected mode (SPM) that we excite within an electro-optical material, namely lithium niobate (LN). The SPM consists of a dark Fano-like resonance that results from the combination of a discrete Bloch mode of a photonic crystal (PhC) with a continuum mode of a membrane, both of them made in LN. The dark character is linked to the structure geometry having a high degree of symmetry. The SPM excitation is then made possible thanks to an illumination under small oblique incidence, which breaks the symmetry of the configuration. This results in several ultra-sensitive and tunable Fano-like resonances with high quality factors up to 105 in the telecoms spectral range. Some of these resonances provide modes with a highly confined electric field inside LN. This confinement allows the enhancement of the electro-optic Pockels effect by a factor up to 5 × 104, thus exacerbating the detection sensitivity of the designed sensor.

Competition of Concentration Narrowing and Power Broadening of a Dark Resonance in a Ladder System of Rubidium Atoms: Specificities of Its Occurrence in Thin Spectroscopic Cells

The effect of electromagnetically induced transparency (EIT) in a three-level ladder system 5S1/2‒5P3/2 –5D5/2 of Rb atoms is investigated. The effect of spectral narrowing of a dark resonance (DR) as a function of atomic vapor density and thickness L of a spectroscopic cell containing Rb vapor is demonstrated. Thickness L was varied in the range of 390 nm to 4 mm, while atomic density N was increased up to ~1016 cm–3; in the process, high-intensity coupling radiation and weak probe radiation were used. The strongest effect, namely, a 22-fold spectral narrowing of the DR, was achieved in a cell of thickness L = 4 mm. The effect of spectral narrowing was decreasing with decrease in thickness L: a 2.4-fold spectral narrowing of the DR was observed at L = 2 μm. Spectral narrowing was nearly absent at L = 0.8 μm, and spectral broadening of the DR was observed with further decrease in the cell thickness down to L = 0.4 μm. A close to 100% DR contrast was achieved in nearly all cases at moderate atomic densities and high intensities of the coupling radiation. An interpretation of the effect of spectral narrowing and broadening of the DR is proposed.

Modulation induced splitting of the magnetic resonance

The splitting of magnetic resonance induced by a frequency modulated linearly polarized light field is presented for Hanle experimental configuration. The experiment is carried out with thermal Rubidium atoms in an anti-relaxation coated glass cell. The observed splitting corresponds to steady-state behaviour of the system over an integrated time scale that is longer than the modulation frequency. The splitting is only observed for dark resonances, with left and right circularly polarized light simultaneously coupled to the atomic system. The split components are separated by half of the modulation frequency with the central component lying at zero-magnetic field. The absence of odd split components at single photon resonance is consistent with the theoretical calculation. The different components of the split profile have distinct response to the direction of the transverse magnetic field. The response is captured by the theoretical calculation and the underlying physical mechanism is discussed. These studies will be useful in metrology and advancement of vector atomic magnetometer.

Features of magnetically-induced atomic transitions of the Rb D1 line studied by a Doppler-free method based on the second derivative of the absorption spectra

In this paper we show that the second derivative (SD) technique of the absorption spectra of Rb atomic vapours, confined in a nanocell with a thickness ℓ = λ/2 = 398 nm, allows us to achieve close to Doppler-free spectroscopy. The narrow linewidth and linearity of the SD signal response with respect to transition probabilities allows us to study separately, in an external transverse magnetic field (0.6 to 4 kG), a large number of the atomic transitions of 85Rb and 87Rb atoms. Atomic transitions |Fg, 0⟩ → |Fe = Fg, 0′⟩, for which the dipole moment is null in a zero magnetic field (so-called magnetically-induced transitions), show a gigantic increase in probability with an increasing magnetic field. When a magnetic field is applied to the vapour, we show the possibility of forming a dark resonance on these transitions by adding a coupling laser. We are therefore able to demonstrate a five-fold increase in the transmission of the probe radiation when the coupling laser is on. Theoretical calculations are in very good agreement with the experimental results.

Broadband Filter and Adjustable Extinction Ratio Modulator Based on Metal-Graphene Hybrid Metamaterials.

A novel multifunctional device based on a hybrid metal–graphene Electromagnetically induced transparency (EIT) metamaterial at the terahertz band is proposed. It is composed of a parallel cut wire pair (PCWP) that serves as a dark mode resonator, a vertical cut wire pair (VCWP) that serves as a bright mode resonator and a graphene ribbon that serves as a modulator. An ultra-broadband transmission window with 1.23 THz bandwidth can be obtained. The spectral extinction ratio can be tuned from 26% to 98% by changing the Fermi level of the graphene. Compared with previous work, our work has superior performance in the adjustable bandwidth of the transmission window without changing the structure of the dark and bright mode resonators, and has a high extinction ratio and dynamic adjustability. Besides, we present the specific application of the device in filters and optical modules. Therefore, we believe that such a metamaterial structure provides a new way to actively control EIT-like, which has promising applications in broadband optical filters and photoelectric intensity modulators in terahertz communications.

Demonstration of Fano resonance-based miniaturized cylindrical cloaking scheme

An experimental realization of a magnetic Fano-like resonance-based cylindrical cloaking scheme in the microwave frequency regime is presented in this paper. Fano-like resonance is excited by the coupling between the background electric dipole mode and the dark magnetic resonant mode in the composite. The excitation of Fano interference patterns significantly reduces scattering from the composite at resonance in the microwave regime and hence the target becomes undetectable from backscattering measurements. Multipolar scattering theory has been used to clarify the excitation of this Fano resonance and the designed cloak is characterized using monostatic and bistatic scattering measurements inside an anechoic chamber.

A novel DRET and FRET combined fluorescent molecule and its applications in sensing and bioimaging

The work presented herein is focused on the construction of a donor-acceptor system with large Stokes shift, which could be used for the detection of Fe3+ in different polarity environments by combining dark resonance energy transfer (DRET) and fluorescence resonance energy transfer (FRET) strategies. A silole derivative and a rhodamine 6 G derivative, linked by a flexible diethylenetriamine spacer, are chosen as donor and acceptor, respectively. Attributed to its AIE features, silole fluorophore could act both as dark energy donor in solution state and emissive energy donor in aggregation state, and the molecule possesses polarity-sensitive DRET and FRET processes from silole to rhodamine after bonding with Fe3+ ion. Thus, intense fluorescence emission from the rhodamine acceptor is obtained no matter whether the test is carried out in solution with high organic solvent content or high water content. Imaging experiments suggest that this probe could be an effective Fe3+ imaging agent for use in live cells.

A novel dark resonance energy transfer-based fluorescent probe with large Stokes shift for the detection of pH and its imaging application

Herein, we design and synthesize a pH-dependent turn-on fluorescent probe based on the dark resonance energy transfer (DRET) strategy. A silacyclopentadiene (silole) derivative with aggregation-induced emission (AIE) properties is selected as dark energy donor and a rhodamine B derivative with spirocyclic structure as energy acceptor. Attributed to the non-emissive character of silole in solution, fluorescence leakage from the donor is eliminated. Binding with H+ turns on the DRET process and energy of donor is converted to the acceptor's fluorescence output before non-radiative decay. The probe responds to pH variation from 2.75 to 5 in buffer solutions with a Stokes shift up to 220 nm. In addition, imaging experiments suggest that the probe is capable of determining pH fluctuations in live cells with good cell membrane permeability and low cytotoxicity.

Complementary Dark and Bright Plasmonic Nanocavities with Controllable Energy Exchange for SERS Sensing

AbstractA 3D nanocavity array with each unit composed of complementarily arranged downward and upward silver nanoshells is reported. Due to the underneath planar gold film, the two differently oriented nanoshells form alternatively closed and semi‐closed nanocavities, respectively, which support dark and bright resonance modes of localized surface plasmons (LSPs). For the symmetric design of such nanocavities, mutually independent multipolar LSPs are directly excited at normal incidence, so that the near‐field coupling between them is inhibited. However, when the geometrical symmetry is broken by introducing a breach on a common sidewall of the two adjacent nanocavities, strong interaction and consequently energy exchange between them become allowed. As a result, Fano coupling between the dark and bright LSPs is clearly observed as a sharp spectroscopic resonance. The most important feature of such a coupled dark‐ and bright‐plasmon system is that the resonance modes of two different LSPs can be tuned separately, enhancing largely the flexibility of the photophysical performance and controllability. In particular, these features enable promising applications of such structures in single‐molecular detection through fluorescence enhancement, surface enhanced Raman scattering (SERS), and surface plasmon lasers. SERS measurements verify excellent performance of such nanocavity arrays in high‐sensitivity detection of low‐concentration molecules.

Optical frequency analysis on dark state of a single trapped ion.

We demonstrate an optical frequency analysis method using the Fourier transform of detection times of fluorescence photons emitted from a single trapped 40Ca+ ion. The response of the detected photon rate to the relative laser frequency deviations is recorded within the slope of a dark resonance formed in the lambda-type energy level scheme corresponding to two optical dipole transitions. This approach enhances the sensitivity to the small frequency deviations and does so with reciprocal dependence on the fluorescence rate. The employed lasers are phase locked to an optical frequency comb, which allows for precise calibration of optical frequency analysis by deterministic modulation of the analyzed laser beam with respect to the reference beam. The attainable high signal-to-noise ratios of up to a MHz range of modulation deviations and up to a hundred kHz modulation frequencies promise the applicability of the presented results in a broad range of optical spectroscopic applications.