Coherent Perfect Absorption in a Parametric Cavity-Ensemble System

All-dielectric photonic crystals or metasurfaces supporting optical bound states in the continuum (BIC) are emerging as a promising platform for the study of light-matter interactions in the strong coupling regime. However, coupling of BIC to other optical excitations is generally weak, with the coupling strength limited in the range of several tens of milli electronvolts with short decoherence time. This greatly hinders their applications in solid-base quantum information devices that require a fast information exchange rate and long coherence time. Herein, a cavity-coupling strategy is used to extend the BIC-based coupling platforms. We propose a hybrid system containing an all-dielectric metasurface embedded in an optical microcavity, demonstrating the formation of BIC-cavity polaritons in the strong coupling regime with a surprisingly large Rabi splitting over 220 meV, far beyond the strong coupling regime. A flexible tuning of BIC-cavity coupling from weak to strong coupling regime is realized by controlling the shape of dielectric element of the metasurface. Importantly, a full quantum model based on Heisenberg-Langevin formalism reveals the temporal population dynamics of the coupling system, demonstrating the ultrafast Rabi socillation within 19 fs with a long decoherence time over 200 fs. Utilizing the multipolar expansion approach combined with full wave simulations, we also investigate the coupling-induced polariton characteristics in both the weak and strong coupling regimes. We find that the electric dipole component of the BIC mode dominates in the polariton states in weak coupling regime, which dramatically changes as the system is tuned into strong coupling regime. Such a cavity-coupling strategy in the present hybrid system is expected to be of great use for enhancing BIC-based light-matter interactions for more complex hybrid nanostructures such as three-mode systems with mixing BIC-cavity-exciton interactions.

We study Monte Carlo calculations of the effective potential for a scalar field theory using three techniques. One of these is a new method proposed and tested for the first time. In each case we extract the renormalised quantities of the theory. The system studied in our calculations is a one component $\phi^4$ model in two dimensions. We apply these methods to both the weak and strong coupling regime. In the weak coupling regime we compare our results for the renormalised quantities with those obtained from two-loop lattice perturbation theory. Our results are verified in the strong coupling regime through comparison with the strong coupling expansion. We conclude that effective potential methods, when suitably chosen, can be accurate tools in calculations of the renormalised parameters of scalar field theories.

We have studied dispersion of surface plasmon polaritons (SPPs) in the Kretcshmann’s geometry (prism/Ag/dye-doped polymer) in weak and strong exciton-plasmon coupling regimes. In reflection, the dispersion curves were in a good agreement with simple model predictions at small concentrations of the dye (R6G) in the polymer (PMMA). At the same time, highly unusual multi-segment ladder-like dispersion curves were observed at large dye concentrations. To describe this behavior, we upgraded the theoretical model taking into account strong exciton-plasmon coupling. The dispersion curves obtained in emission experiments were strongly different from those in reflection and did not seem to be affected by the strong coupling.

$J$-coupled nuclear magnetic resonance (NMR) spectroscopy in the strong coupling regime at low magnetic field ($10{}^{\ensuremath{-}7}$ T $lBl{10}^{\ensuremath{-}3}$ T) is more complex than at high field ($Bg{10}^{\ensuremath{-}3}$ T) and at ultralow field ($Bl{10}^{\ensuremath{-}7}$ T). We show that several upper and lower boundaries ${B}_{i}^{\mathrm{up}}$ and ${B}_{i}^{\mathrm{low}}$ of the magnetic field $B$ exist, where the complexity of $J$-coupled NMR spectra changes in terms of the number of lines. The index $i=1,2,\dots{}$ for ${B}_{i}^{\mathrm{up}}$ at high field specifies the perturbation order of the dominating Zeeman interaction and for ${B}_{i}^{\mathrm{low}}$ at ultralow field the perturbation order of the dominating $J$-coupling interaction. Mathematical expressions for these boundaries are derived for the case of a $J$-coupled $S$-$I$${}_{N}$ group where $S$ and $I$ are rare and abundant spins $\frac{1}{2}$ and $N$ counts the abundant spins $I$. The entire $B$-field range can further be delineated into two weak coupling regimes, one at high field with ${B}_{2}^{\mathrm{up}}lBl{B}_{1}^{\mathrm{up}}$ ($10{}^{\ensuremath{-}3}$ T $lBl{10}^{2}$ T), one at low field with ${B}_{1}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{low}}({10}^{\ensuremath{-}8}$ T $lBl{10}^{\ensuremath{-}7}$ T), and a strong coupling regime with ${B}_{2}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{up}}$ ($10{}^{\ensuremath{-}7}$ T $lBl{10}^{\ensuremath{-}3}$ T). The corresponding NMR spectra for the $S\ensuremath{-}{I}_{N}$ group are investigated by experiment and by simulation. In the strong coupling regime, the maximum number of lines is $(N+1){}^{2}$. In the weak coupling regime ${B}_{1}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{low}}$ at low field, symmetric multiplet structures group around the frequencies $0$, $J$, ($3/2$)$J$, $2J$, ($5/2$)$J$, etc. These spectra determine the structure of the $S\ensuremath{-}{I}_{N}$ group unambiguously and are in dual correspondence to the weakly coupled spectra at high field. High-resolution NMR spectroscopy at ultralow field may open up new ways for chemical analysis by small and mobile instruments with many applications in science and technology.

Summary form only given. The coherent synthesis of custom-tailored, intense, sub-cycle optical waveforms is promising for attosecond science and strong-field physics, e.g., for precision control of strong-field interactions in atoms, molecules and solids, for the generation of intense isolated attosecond pulses, and for attosecond pump-probe spectroscopy.Recently, coherent pulse synthesis based on supercontinuum generation in a hollow-core fiber compressor allowed the generation of sub-cycle -300-μJ optical pulses. However, in this case ionization losses in the gas medium prevent further scaling to the mJ level. In contrast, parametric synthesizers do not face an energy scaling limit, and allow for spectral extension into the particularly appealing MIR region. In previous works we demonstrated coherent pulse synthesis of two optical parametric chirped-pulse amplifiers and of two optical parametric amplifiers (OPAs) on the few-μJ level [3]. Here we present the ongoing development of a novel 3-channel parametric synthesizer for generating a 2-octave-wide spectrum, which can easily be upscaled to the mJ-level. We start from a cryogenically cooled Ti:sapphire chirped-pulse amplifier (150 fs, 22 mJ, 0.8 μm, 1 kHz) and generate a CEP-stable continuum (0.5-2.3 μm) [5], by white-light generation in a YAG crystal pumped by the second harmonic (1.06 μm) of the CEP-stable idler of a NIR OPA. The continuum is split with custom-designed dichroic beam splitters (which will also be used for the final beam recombination) and seeds three OPAs, a VIS noncollinear OPA (NOPA), a NIR and an IR degenerate OPA (DOPA), each composed of 2 (later 3) amplification stages, and pumped by the pulses at 0.8 μm (IR DOPA), and by its second harmonic at 0.4 μm (VIS NOPA, NIR DOPA). To synthesize a coherent ultrashort pulse from these three OPAs, the relative timing of the pulses will be tightly locked using feedback loops with balanced optical cross-correlators, that can achieve sub-cycle synchronization with s0-as RMS timing jitter [2,3]. Figure 1(a) shows the measured output spectra and the energies from the second amplification stages operating in parallel. The transform-limited (TL) pulse duration from the synthesis of these spectra is 1.9 fs FWHM, corresponding to 0.7 optical cycles at 785 nm center wavelength. Our current work aims to scale up the energies (by a third amplification stage) to -0.5 mJ for the VIS NOPA/NIR DOPA and -2 mJ for the IR DOPA using the 18.5 mJ of pump remaining after the first two amplification stages. State-of-the-art double-chirped mirror pairs (at present in fabrication) will allow for the final pulse compression with ultralow residual ripple in the resulting total group-delay dispersion over the full bandwidth (0.52-2.3μm). Temporal characterization of the synthesized two-octave-spanning optical waveforms will be performed by two-dimensional spectral shearing interferometry (2DSI). We foresee that our 3-channel synthesizer, when further scaled in energy to the mJ level, will become a versatile tool for controlling strong-field interactions in atoms, molecules and solids and for attosecond pumpprobe spectroscopy employing ultrashort pulses in the VIS/IR and XUV/soft-X-ray regions.

The stochastic mean-field (SMF) approach allows to treat correlations beyond mean-field using a set of independent mean-field trajectories with appropriate choice of fluctuating initial conditions. We show here, that this approach is equivalent to a simplified version of the Bogolyubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy between one-, two-, ..., N-body degrees of freedom. In this simplified version, one-body degrees of freedom are coupled to fluctuations to all orders while retaining only specific terms of the general BBGKY hierarchy. The use of the simplified BBGKY is illustrated with the Lipkin-Meshkov-Glick (LMG) model. We show that a truncated version of this hierarchy can be useful, as an alternative to the SMF, especially in the weak coupling regime to get physical insight in the effect beyond mean-field. In particular, it leads to approximate analytical expressions for the quantum fluctuations both in the weak and strong coupling regime. In the strong coupling regime, it can only be used for short time evolution. In that case, it gives information on the evolution time-scale close to a saddle point associated to a quantum phase-transition. For long time evolution and strong coupling, we observed that the simplified BBGKY hierarchy cannot be truncated and only the full SMF with initial sampling leads to reasonable results.

The spontaneous fluorescence background in optical parametric amplifiers is generally attributed to the zero-point fluctuations of the electromagnetic field. These are amplified in parallel to the seed light and lead to an uncompressible superfluorescence background that deteriorates the contrast in optical parametric chirped pulse amplifiers (OPCPA). The absolute level of the underlying parametric fluorescence has not been reported so far. Comparing the fluorescence to low level cw seed light and quantitatively monitoring the output of a noncollinear optical parametric amplifier for both sources, the level is now determined. In a situation of 50 nm visible output bandwidth and low Gaussian spatial modes about 58 photons are found in the signal direction within the femtosecond time window of the amplifier. The superfluorescence level is observed to be proportional to the pump area for constant signal amplification. The implications for the background in high power OPCPA are discussed.

In modern-day photonic integrated circuits, near-field Coulomb interaction plays an important role in device performance. In fact, harvesting the Coulomb effect intelligently can immensely help to realize advanced photonic devices on micro and nano length scales. In this context, electrically coupled terahertz metasurfaces are investigated for weak and strong near-field coupling regimes under the influence of variable photoexcitation. Our study demonstrates active tuning of metasurface resonances at relatively low pump powers (up to 10 mW (pump fluence of 12.7 µJ cm−2) and 20 mW (pump fluence of 25.4 µJ cm−2) for strong and weak near-field coupling regimes, respectively). We attribute photoinduced modifications of substrate electronic properties along with modifications of near-field interactions to be responsible for the resonance modulations. Furthermore, we demonstrate that the Coulomb effect parameter can be dynamically tunable with variable pump power. Our study reveals that the change in Coulomb parameter is more pronounced for the weak near-field coupling regime (more than six times enhancement in Coulomb parameter) in comparison to the strong near-field coupling regime. In addition, the optical switching of the fundamental resonance is validated utilizing numerically simulated patterns of surface current and electric field. We envisage that such active modulation of Coulomb effects in near-field coupled metasurfaces can lead to the realization of dynamically tunable terahertz metaphotonic devices.

A novel superstructure fiber Bragg grating (SFBG) based comb gain equalizer for fiber optical parametric amplifier (OPA) is proposed. This gain equalizer is characterized by its comb reflectivity spectrum. It can have two functions at least, one is flattening the gain in multi ITU-T regulated channels, the other is it can be used as multi-channel isolation filters taking full advantage of its comb reflectivity spectrum. In addition it can be dispersionless or have dispersion compensation function. A one-bump fiber OPA, 30dB bandwidth >15nm, is designed with one of its peak gains (41dB) located at the center of work wavelengths in a 16-channel 100GHz spaced WDM optical transmission system according to corresponding Chinese criterion (1548.51nm~ 1560.61nm). The object reflectivity spectra of gain equalizer are calculated in two parts independently. Inner channel part for every channel is designed according to the gain spectrum of fiber OPA. As a comb filter, its inter-channel part reflectivities are the smaller the better. But inter-channel part reflectivities can not be simply zeroed, for which would result in the rising and descending edges of reflectivity spectrum being too sharp to be performed by any filter. To split the difference, inter-channel part reflectivities are calculated with Gaussian hypergeometric function (-0.5dB bandwidth 0.1nm, -45dB bandwidth 0.3nm) to obtain slower rising and descending edges than the simply zeroed case, which is very necessary and important to be sure the object reflectivity spectrum not only has sharp enough edges to take the shape of comb but also is physically realizable. Adopting channel-by-channel windowing method to apodize the object reflectivity spectrum, a superstructure fiber Bragg grating is designed with lay-peeling inverse scattering technique. At last, according to the coupling coefficient calculated, reflection spectrum and group time delay are analyzed based on the method of transformation matrix. Calculation results show that this SFBG satisfies all requirements. Further calculation results show that, using this SFBG as comb gain equalizer, the gain of fiber OPA is flattened in every channel to within ±0.4dB among 16 channels, with channel isolation high up to -35dB. It needs to point out that this method can also be used to design gain equalizer for other optical amplifiers.

We have investigated the possibility of using optical parametric amplifiers (OPA) as optical parametric chirped pulse amplifiers (OPCPA), in which energy from a narrow bandwidth pump source is efficiently transferred to a chirped signal pulse prior to its recompression (1).

We have investigated the generation of new undesirable frequencies in a two-pump optical parametric amplifier, which are generated by the four-wave mixing of a signal or idler with one of the strong pump waves. The powers of the new frequencies impose upper bounds on the device gain and output power if cross talk is to be avoided. In the parametric amplifier we investigated, however, we found that, for 7.5 dB of gain and −3.5 dBm of output power, the ratio of power in the new frequencies and the signal is less than −30 dB. This demonstrates that cross talk is acceptable for low gain and output power applications of these devices such as frequency conversion. We present simple scaling relationships for the power in the new undesirable frequencies that can be used to help design optical parametric amplifiers.

The complementary optical properties of surface plasmon excitations of metal nanostructures and long-lived excitations of semiconductor quantum dots (QDs) make them excellent candidates for studies of optical coupling at the nanoscale level. Plasmonic devices confine light to nanometer-sized regions of space, which turns them into effective cavities for quantum emitters. QDs possess large oscillator strengths and high photostability, making them useful for studies down to the single-particle level. Depending on structure and energy scales, QD excitons and surface plasmons (SPs) can couple either weakly or strongly, resulting in different unique optical properties. While in the weak coupling regime plasmonic cavities (PCs) mostly enhance the radiative rate of an emitter, in the strong coupling regime the energy level of the two systems mix together, forming coupled matter-light states. The interaction of QD excitons with PCs has been widely investigated experimentally as well as theoretically, with an eye on potential applications ranging from sensing to quantum information technology. In this review we provide a comprehensive introduction to this exciting field of current research, and an overview of studies of QD-plasmon systems in the weak and strong coupling regimes.

Quantum tunneling of the apex oxygen atoms in YBa2Cu3O7− δ superconductors: Anharmonic effects on the out of plane electrical resistivity

At signal critical wavelength degeneracy (CWD), super-broad parametric gain bandwidths (SBGBs) can be obtained in nonlinear crystals. A 280 nm SBGB, for ∼60 small signal peak gain, has been calculated considering a collinear optical parametric chirped pulse amplification (OPCPA) at CWD in a 40 mm long, 58.3% partially deuterated KDP (P-DKDP) crystal, pumped by a frequency doubled nanosecond Nd:glass laser at 1 GW/cm2 pump intensity. This gain bandwidth in the 1 µm spectral range is significantly broader compared to the 150 nm gain bandwidth, calculated under similar conditions, for the broad-band non-collinear (NC) OPCPA in DKDP crystals in the 900 nm spectral range. It can support the amplification of significantly shorter femtosecond pulses, giving rise to a higher peak power at the same amplified pulse energy. The spectral profile of the gain bandwidth can be shaped by slightly adjusting the deuteration ratio (DR) of the P-DKDP crystals. A small-angle non-collinear OPCPA near the CWD can be used for the spatial separation of the signal and idler beams with the same polarization, keeping at the same time an ultra-broad gain bandwidth. A SBGB of 255 nm FWHM has been calculated for a non-collinear OPCPA stage with a 40 mm long P-DKDP crystal, 10.3 mrad signal-pump wavevectors internal angle, 57.8% DR, pumped by a few-ns 527 nm wavelength laser at 1 GW/cm2 pump intensity. The half-meter size P-DKDP crystals, pumped by frequency doubled few-ns multi-kJ Nd:glass lasers, are able to amplify super broad-band nanosecond stretched pulses up to the kJ energy level. They might be appropriate amplifying media for the development of single-beam 100-PW class femtosecond laser systems based on OPCPA at CWD.

For some cavity-quantum-electrodynamics systems, such as a single electron spin coupled to a passive cavity, it is challenging to reach the strong-coupling regime. In such a weak-coupling regime, the conventional dispersive readout technique cannot be used to resolve the quantum states of the spin. Here we propose an improved dispersive readout method to measure the quantum states of a weakly coupled qubit by harnessing either one or two auxiliary cavities linearly coupled to the passive cavity containing the qubit. With appropriate parameters in both cases, the system excluding the qubit can exhibit a parity-time-symmetric phase transition at the exceptional point (EP). Because the EP can amplify the perturbation induced by the qubit and the parity-time symmetry can narrow the linewidths of the peaks in the transmission spectrum of the passive cavity, we can measure the quantum states of the weakly coupled qubit via this transmission spectrum. Owing to the weak coupling between the qubit and the passive cavity, the backaction due to the measurement of the qubit can also be reduced in comparison with the conventional dispersive readout technique in the strong-coupling regime.