Small Detuning Research Articles

Sharp Focusing of an Atomic Beam with the Doppler and Sub-Doppler Laser Cooling Mechanisms in a Two-Dimensional Magneto-Optical Trap

The focusing of an atomic beam with the use of a two-dimensional magneto-optical trap in order to increase the number of atoms in the region of their laser cooling and localization near an atom chip is discussed. Two regimes of the interaction of atoms with a focusing laser field are considered: (i) the Doppler interaction regime, which occurs at small detunings of the laser field from the atomic resonance, and (ii) the sub-Doppler interaction regime, which occurs at large detunings of the laser field from the atomic resonance. The efficiency of focusing in the first case is low because of the momentum diffusion. It has been shown that the momentum diffusion in the sub-Doppler cooling mechanism is insignificant and, as a result, the broadening of the transverse velocity distribution of atoms is small. The sharp focusing of the atomic beam is possible in this interaction regime.

Single site-controlled inverted pyramidal InGaAs QD–nanocavity operating at the onset of the strong coupling regime

Precise positioning of single site-controlled inverted pyramidal InGaAs quantum dots (QDs) at the antinode of a GaAs photonic crystal cavity with nanometer-scale accuracy holds unique advantages compared to self-assembled QDs and offers great promise for practical on-chip photonic quantum information processing. However, the strong coupling regime in this geometry has not yet been achieved due to the low cavity Q-factor based on the (111)B-oriented membrane structures. Here, we reveal the onset of phonon-mediated coherent exciton–photon interaction on our tailored single site-controlled InGaAs QD–photonic crystal cavity. Our results present the Rabi-like oscillation of luminescence intensity between excitonic and photonic components correlated with their energy splitting pronounced at small detuning. Such Rabi-like oscillation is well reproduced by modeling the coherent exchange of the exciton-photon population. The modeling further reveals an oscillatory two-time covariance at QD-cavity resonance, which indicates that the system operates at the onset of the strong coupling regime. Moreover, by using the cavity mode as a probe of the virtual state of the QD induced by phonon scattering, it reveals an increase in phonon scattering rates near the QD–cavity resonance and asymmetric phonon emission and absorption rate of even around 50 K.

Analysis of intermittency near the boundary of the phase synchronization in systems with complex topology of attractor

Intermittent behavior near the boundary of phase synchronization in systems with a complex topology of attractor is studied. Using the calculation of statistical characteristics of the laminar phase lengths it is shown that in the case of a relatively small detuning of the control parameters of interacting systems near the phase synchronization boundary the eyelet intermittency takes place.

Sub-wavelength effects in a free electron laser oscillator.

Previous simulation studies of cavity based free electron lasers (FELs) have utilised models which average the optical field in the FEL interaction over an integer number of radiation wavelengths. In this paper, two unaveraged simulation codes, OPC and Puffin, are combined to enable modelling, for the first time, of a cavity based FEL at the sub-wavelength scale. This enables modelling of effects such as coherent spontaneous emission from the electron beam and sub-wavelength cavity length detuning. A cavity FEL operating in the mid-infrared is modelled and it is shown that, for small sub-wavelength cavity detunings, the FEL can preferentially lase at the third harmonic of the fundamental FEL wavelength. This novel result suggests other modes of operation may be possible and opens up cavity-based FEL operation to investigation of further, potentially useful, modes of operation.

Sharp Focusing of an Atomic Beam with the Doppler and Sub-Doppler Laser Cooling Mechanisms in a Two-Dimensional Magneto-Optical Trap

The focusing of an atomic beam with the use of a two-dimensional magneto-optical trap in order to increase the number of atoms in the region of their laser cooling and localization near an atom chip is discussed. Two regimes of the interaction of atoms with a focusing laser field are considered: (i) the Doppler interaction regime, which occurs at small detunings of the laser field from the atomic resonance, and (ii) the sub-Doppler interaction regime, which occurs at large detunings of the laser field from the atomic resonance. The efficiency of focusing in the first case is low because of the momentum diffusion. It has been shown that the momentum diffusion in the sub-Doppler cooling mechanism is insignificant and, as a result, the broadening of the transverse velocity distribution of atoms is small. The sharp focusing of the atomic beam is possible in this interaction regime.

Detection of HCl molecules by resonantly enhanced sum-frequency mixing of mid- and near-infrared laser pulses

We perform experimental studies of resonantly enhanced sum-frequency mixing (SFM), driven by tunable, spectrally narrowband mid-infrared and fixed-frequency nanosecond laser pulses, aiming at applications in molecular gas detection. The mid-infrared pulses are tuned in the vicinity of two-photon rovibrational transitions in the electronic ground state to provide strong resonance enhancements of the nonlinear susceptibility, while a probe laser at shorter wavelength uses an off-resonant single-photon coupling to excited electronic states. This SFM approach benefits from the advantageous combination of typically small detunings among the mid-infrared, vibrational transitions and the typically large transition dipole moment for couplings of electronic states. Moreover, compared to resonantly enhanced third harmonic generation (THG), a signal wave at much shorter wavelength permits simple and efficient detection. We demonstrate resonantly enhanced SFM via rovibrational states in gaseous hydrogen chloride molecules and compare its features to THG. The SFM spectra offer a large signal-to-noise ratio of 4 orders of magnitude and a detection limit down to a pressure of 0.1 mbar, corresponding to a particle density of $0.35\ifmmode\times\else\texttimes\fi{}{10}^{15}$ per ${\mathrm{cm}}^{3}$.

Investigation of the possibility of the existence of multistability in the regime of intermittent phase synchronization in ensemble of unidirectionally coupled Rössler systems

The question of the possibility of the existence of multistability near the boundary of phase synchronization in unidirectionally coupled chaotic systems is studied. The cases of relatively large and relatively small detuning between them are considered. The multistability is shown to be observed in both cases. A quantitative characteristic of the degree of multistability is introduced and it is shown that its time dependence in the regime of intermittent phase synchronization can be used to determine the lengths of the characteristic phases of the system behavior.

Frequency combs with multiple offsets in THz-rate microresonators.

Octave-wide frequency combs in microresonators are essential for self-referencing. However, it is difficult for the small-size and high-repetition-rate microresonators to achieve perfect soliton modelocking over the broad frequency range due to the detrimental impact of dispersion. Here we examine the stability of the soliton states consisting of one hundred modes in silicon-nitride microresonators with the one-THz free spectral range. We report the coexistence of fast and slow solitons in a narrow detuning range, which is surrounded on either side by the breather states. We decompose the breather combs into a sequence of sub-combs with different carrier-envelope offset frequencies. The large detuning breathers have a high frequency of oscillations associated with the perturbation extending across the whole microresonator. The small detuning breathers create oscillations localised on the soliton core and can undergo the period-doubling bifurcation, which triggers a sequence of intense sub-combs.

Wave mixing efficiency in InAs/GaAs semiconductor quantum dot optical amplifiers and lasers

The nonlinear features of both semiconductor optical amplifiers (SOAs) and semiconductor lasers, which are made from the same InAs/GaAs quantum dot (QD) wafers, are investigated in detail. By employing pump-probe driven four-wave mixing as an experimental tool, the wave conversion process shows notably different profiles for the two types of devices. Due to the contributions of ultrafast, sub-picosecond mechanisms, such as carrier heating and spectral hole burning, the pump-probe frequency can be easily tuned to the THz range. SOAs generally benefit more from sub-picosecond carrier dynamics, hence exhibiting a higher conversion efficiency (CE) in the THz range, compared to their laser diode counterparts. The discrepancy even exceeds 10 dB. In addition, laser experiments yield some differences from the amplifier ones, hence leading to a higher nonlinear CE at small detuning ranges. These results strongly improve our insight into the fundamental nonlinear properties of InAs/GaAs QD material, and contribute to the conception of novel devices for future on-chip applications in all-optical communication networks, such as signal wavelength conversion, mode-locking, and optical frequency comb generation.

High Sensitivity Hollow-Core Fiber Strain Sensor Based on Signal Processing Assisted Vernier Effect

A fiber strain sensor with improved sensitivity is proposed and experimentally demonstrated, based on a single Fabry–Perot interferometer (FPI) using a signal processing assisted Vernier effect. The FPI structure is simply fabricated by splicing a segment of hollow-core fiber (HCF) between two pieces of single-mode fiber (SMF) with a commercial fusion splicer. Through data processing, we do not need to precisely manufacture two interferometers with very small detuning between their optical path differences (OPDs), as in the traditional schemes based on Vernier effect, since it is difficult to control the detuning strictly. The optical spectra of the reference interferometer (RIM) are replaced by an artificial reference spectrum (ARS), which is produced by spatial frequency down-conversion of the interference spectra of the sensing interferometer (SIM) in the initial state. Then, the optical spectrum envelope of Vernier-effect can be obtained by multiplying the SIM intensity with the constructed ARS intensity. When the length of HCF is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1201\mu \text{m}$ </tex-math></inline-formula> , the sensitivity can reach 315pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> with the modulation length of 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> m. Compared with the sensitivity without signal processing, the sensitivity magnification is 188.6 times. The proposed scheme to realize sensitivity-enhancement using a single FPI based on the HCF, could simplify the sensor structure and device fabrication.

Low-Field Feshbach Resonances and Three-Body Losses in a Fermionic Quantum Gas of 161Dy

We report on the high-resolution Feshbach spectroscopy on a degenerate, spin-polarized Fermi gas of 161Dy atoms, measuring three-body recombination losses at a low magnetic field. For field strengths up to 1 G, we identify as much as 44 resonance features and observe the plateaus of very low losses. For four selected typical resonances, we study the dependence of the threebody recombination rate coefficient on the magnetic resonance detuning and on the temperature. We observe a strong suppression of losses with decreasing temperature already for small detunings from the resonance. The characterization of complex behavior of the three-body losses of fermionic 161Dy is important for future applications of this peculiar species in research on atomic quantum gases.

Non-resonant exceptional points as enablers of noise-resilient sensors

Exceptional point degeneracies (EPDs) in the resonant spectrum of non-Hermitian systems have been recently employed for sensing due to the sublinear response of the resonance splitting when a perturbant interacts with the sensor. The sublinear response provides high sensitivity to small perturbations and a large dynamic range. However, the resonant-based EPD sensing abides to the resolution limit imposed by the resonant quality factors and by the signal-to-noise ratio reduction due to gain-elements. Moreover, it is susceptible to local mechanical disturbances and imperfections. Here, we propose a passive non-resonant (NR) EPD-sensor that is resilient to losses, local cavity variations, and noise. The NR-EPD describes the coalescence of Bloch eigenmodes associated with the spectrum of transfer matrices of periodic structures. This coalescence enables scattering cross-section cusps with a sublinear response to small detunings away from an NR-EPD. We show that these cusps can be utilized for enhanced noise-resilient sensing.

Magneto-optical rotation: accurate approximated analytical solutions for single-probe atomic magnetometers.

We report an approximated analytical solution for a single-probe four-state atomic magnetometer where no analytical solution exists. This approximated analytical solution demonstrates excellent accuracy in broad probe power and detuning ranges when compared with the numerical solution obtained using a 4th order Runge-Kutta differential equation solver on MATLAB. The theoretical framework and results also encompass widely applied single-probe three-state atomic magnetometers for which no analytical solution, even approximated, is available to date in small detuning regions.

Asynchronous and synchronous quenching of a globally unstable jet via axisymmetry breaking

We explore experimentally whether axisymmetry breaking can be exploited for the open-loop control of a prototypical hydrodynamic oscillator, namely a low-density inertial jet exhibiting global self-excited axisymmetric oscillations. We find that when forced transversely or axially at a low amplitude, the jet always transitions first from a period-1 limit cycle to $\mathbb {T}^2$ quasiperiodicity via a Neimark–Sacker bifurcation. However, we find that the subsequent transition, from $\mathbb {T}^2$ quasiperiodicity to $1:1$ lock-in, depends on the spatial symmetry of the applied perturbations: axial forcing induces a saddle-node bifurcation at small detuning but an inverse Neimark–Sacker bifurcation at large detuning, whereas transverse forcing always induces an inverse Neimark–Sacker bifurcation irrespective of the detuning. Crucially, we find that only transverse forcing can enable both asynchronous and synchronous quenching of the natural mode to occur without resonant or non-resonant amplification of the forced mode, resulting in substantially lower values of the overall response amplitude across all detuning values. From this, we conclude that breaking the jet axisymmetry via transverse forcing is a more effective control strategy than preserving the jet axisymmetry via axial forcing. Finally, we show that the observed synchronization phenomena can be modelled qualitatively with just two forced coupled Van der Pol oscillators. The success of such a simple low-dimensional model in capturing the complex synchronization dynamics of a multi-modal hydrodynamic system opens up new opportunities for axisymmetry breaking to be exploited for the open-loop control of other globally unstable flows.

Wavelet resolved coherence beating in the Overhauser field of a thermal nuclear spin ensemble

This work introduces the so-called synchrosqueezed wavelet transform, to shed light on the dipolar fluctuations of a thermal ensemble of nuclear spins in a diamond crystal structure, hyperfine-coupled to a central spin. The raw time series of the nuclear spin bath coherent dynamics is acquired through the two-point correlation function computed using the cluster correlation expansion method. The dynamics can be conveniently analyzed according to zero-, single-, and double-quantum transitions derived from the dipolar pairwise spin flips. We show that in the early-time behavior when the coherence is preserved in the spin ensemble, the Overhauser field fluctuations are modulated by dipole-dipole-induced small inhomogeneous detunings of nearly resonant transitions within the bath. The resulting beating extending over relatively longer time intervals is featured on the scalograms where both temporal and spectral behaviors of nuclear spin noise are unveiled simultaneously. Moreover, a second kind of beating that affects faster dynamics is readily discernible, originating from the inhomogeneous spread of the hyperfine coupling of each nucleus with the central spin. Additionally, any quadrupolar nuclei within the bath imprint as beating residing in the zero-quantum channel. The nuclear spin environment can be directionally probed by orienting the hyperfine axis. Thereby, crucial spatial information about the closely separated spin clusters surrounding the central spin are accessible. Thus, a wavelet-based post processing can facilitate the identification of proximal nuclear spins as revealed by their unique beating patterns on the scalograms. Finally, when these features are overwhelmed by either weakly or strongly coupled classical noise sources, we demonstrate the efficacy of thresholding techniques in the wavelet domain in denoising contaminated scalograms.

Production of good quality holograms by the THz pulsed vortex beams

In this paper, we discuss the quality of holograms based on the calculation of the modulation depth. It’s shown that the terahertz (THz) pulsed vortex beams play a vital role in holography filed, where two lasers with frequency difference have used. The THz vortex beams give new regions of larger frequency detuning and an important value of the modulation depth and fringe contrast (MDFC ratio) for obtaining the best holograms contrary to the Gaussian beams for which the best holograms are realized for small frequency detuning. The particular cases such as, Gaussian beam and single cycle pulses are deduced from our result. Numerical simulations are also presented to study the dependence of the MDFC ratio on the frequency detuning for THz vortex beams and Gaussian beams. This research could be beneficial in holographic interferometry, and it will firmly establish as a tool for scientific and engineering studies.

Coherent and incoherent laser pump on a five-level atom in a strongly coupled cavity-QED system

We propose a cavity quantum electrodynamic system consisting of a five-level atom coupled to a single mode of the cavity electromagnetic field. The study is focused on the regime of strong coupling between the cavity and atom. Pump laser fields and cavity fields connect the split energy levels of the atom. Instead of the well-known two-level Dicke model obtained by adiabatic elimination of the high-energy levels, we consider the pump lasers' detunings to the atomic transitions to be very small such that we can examine the influence of the higher-energy states. We have studied the effect of an external coherent drive and incoherent pumping on these higher-energy levels and observed the enhancement of intracavity photon numbers due to quantum coherence effects. The amplification of intracavity photons is achieved even without a population inversion. However, the effect of the coherent and incoherent drive is negligible for very large detunings when the higher-energy states are adiabatically eliminated. At zero and small detunings, the system reaches the steady state at an earlier instant of time for higher incoherent pumping. We find an almost agreeable steady-state behavior of the system's exact full quantum dynamics model and its semiclassical approximation. Our model tries to accurately simulate the open system by considering the cavity decay, spontaneous decay, and dephasing of the system.

Role of intense laser-excited dressed states via electromagnetically induced transparency on the Fresnel-Fizeau photon drag through an asymmetric double quantum dot molecule (GaAs/AlGaAs) in the Λ-type configuration

Hereby, we first report theoretical demonstration on the role of intense laser-excited dressed states on the superluminal-assisted propagation, optical properties, and lateral and rotary photon drag, via electromagnetically induced transparency (EIT). For this, we employed a scheme of an asymmetric double quantum dot molecule GaAs/AlGaAs in the Λ − type configuration. In particular, using the compact density-matrix formulism, we present a detailed analytical solution for the required eigenvalues of the system in view of the dressed states. The dressed-states played a vital role on the formation of EIT and hence the modification of dispersion and absorption spectra of the system via quantum interference. The calculated theoretical results on the optical properties such as absorption and dispersion spectrum, group index, group velocity , group delay, phase delay and photon drag are further elaborated with the aid of numerical simulation. Switching ON the control field, the OFF-field single ON-resonance absorption peak split into an OFF-resonance absorption doublet accompanied by an ON-resonance absorption-less dip in the form of EIT window. Interestingly, the absorption doublet split further into an absorption peak and a gain singlet well below and just above the resonance, at a small control field detuning. Further increase in the control-field detuning resulted in a near-resonance gain singlet. Alongside, by tuning various spectral parameters, we observed an interesting switching from luminal to subluminal and superluminal propagation. The optimal calculated values of positive and negative group velocity are 6 × 10 11 m/s and − 2.5 × 10 11 m/s, respectively. The predicted lateral and rotary photon drag are of order ± 6 × 10 − 8 rad. • Role of dressed sates in view of EIT coherence on the light propagation, optical properties and photon drag. • The nature of dressed states influence largely the optical properties such as group index, group velocity, group delay, and photon drag. • The optimal values of positive and negative group velocity are 6 × 10 11 m/s and − 2.5 × 10 11 m/s, respectively. • The calculated lateral and rotary photon drag are of order ± 6 × 10 − 8 rad.

Analogue of electromagnetically-induced transparency with ultra-narrow bandwidth in a silicon terahertz metasurface

Achieving an ultra-narrow bandwidth analogue of electromagnetically induced transparency (EIT) in bright–bright mode coupling metasurface requires a large contrast of the Q factor and small wavelength detuning between the two coupled modes. Here, by coupling a toroidal dipole (TD) high-Q Fano resonance and a low-Q magnetic dipole (MD) mode, we numerically demonstrated a high Q factor analogue of EIT on an all-silicon metasurface in the terahertz regime. The Q factor of Fano resonance and consequent EIT can be easily adjusted by the spacing between the air holes. By adjusting the radii of the air holes, the thickness of the silicon wafer, or the lattice constant of the metasurface, EIT-like response exhibiting a very high group refractive index and a large group delay was achieved. The proposed EIT metasurface is easy to fabricate and has potential applications in the fields of narrowband filtering and slow-light based devices.

Morris-Shore transformation for nondegenerate systems

The Morris-Shore (MS) transformation is a powerful tool for decomposition of the dynamics of multistate quantum systems to a set of two-state systems and uncoupled single states. It assumes two sets of states wherein any state in the first set can be coupled to any state in the second set but the states within each set are not coupled between themselves. Another important condition is the degeneracy of the states in each set, although all couplings between the states from different sets can be detuned from resonance by the same detuning. The degeneracy condition limits the application of the MS transformation in various physically interesting situations, e.g. in the presence of electric and/or magnetic fields or light shifts, which lift the degeneracy in each set of states, e.g. when these sets comprise the magnetic sublevels of levels with nonzero angular momentum. This paper extends the MS transformation to such situations, in which the states in each of the two sets are nondegenerate. To this end, we develop an alternative way for the derivation of Morris-Shore transformation, which can be applied to non-degenerate sets of states. We present a generalized eigenvalue approach, by which, in the limit of small detunings from degeneracy, we are able to generate an effective Hamiltonian that is dynamically equivalent to the non-degenerate Hamiltonian. The effective Hamiltonian can be mapped to the Morris-Shore basis with a two-step similarity transformation. After the derivation of the general framework, we demonstrate the application of this technique to the popular Lambda three-state system, and the four-state tripod, double-Lambda and diamond systems. In all of these systems, our formalism allows us to reduce their quantum dynamics to simpler two-state systems even in the presence of various detunings, e.g. generated by external fields of frequency drifts.