Double Resonance Condition Research Articles

Raman enhancement via double optical resonances in all-dielectric photonic crystal slabs

All-dielectric photonic structures are an important class of substrates in surface-enhanced Raman spectroscopy (SERS), utilizing optical resonant modes to significantly enhance the electromagnetic field and amplify the Raman signals. In this study, we demonstrate the double-resonance approach to realize significant Raman enhancement using all-dielectric photonic crystal (PhC) slab. The double-resonance condition is satisfied by designing optical resonant modes in photonic bands to match frequencies of both excitation laser and Raman signals. By the fabricated PhC slab, the significant enhancement for the Raman signal of silicon is demonstrated. The enhanced Raman signals exhibit a uniform distribution on the PhC slab. The method of Raman enhancement via double optical resonances can advance the field of all-dielectric SERS and holds potential for future SERS applications.

Laser frequency stabilization for continuous-wave laser enhanced direct frequency comb spectroscopy system

We present the observation of high-precision direct frequency comb spectroscopy excited by an optical frequency comb and a diode laser when each of them drives one step of the two-photon transition in a rubidium vapor system. We demonstrate a stable and low noise system by directly locking the frequency of the continuous-wave laser to the rubidium two-photon transition. The frequency stability of a diode laser via the two-photon transition locking technique is 8 × 10−11 for a 1 s gate time and 3 × 10−12 for 1000 s. It proves to be a potential technique for locking the diode laser with high stability. We chose a suitable optical frequency comb pulse and the frequency of the diode laser to fulfill the double-resonance condition. These techniques eliminate spectrum line overlap and would benefit spectroscopy measurements.

Analysis of double-resonance crossing in adiabatic trapping phenomena for quasi-integrable area-preserving maps with time-dependent exciters.

In this paper we analyze the adiabatic crossing of a resonance for Hamiltonian systems when a double-resonance condition is satisfied by the linear frequency at an elliptic fixed point. We discuss in detail the phase-space structure on a class of Hamiltonians and area-preserving maps with an elliptic fixed point in the presence of a time-dependent exciter. Various regimes have been identified and carefully studied. This study extends results obtained recently for the trapping and transport phenomena for periodically perturbed Hamiltonian systems, and it could have relevant applications in the adiabatic beam splitting in accelerator physics.

Efficient Second-Harmonic Generation in Si-GaP Asymmetric Coupled-Quantum-Well Waveguides

We present a theoretical investigation of efficient second harmonic generation in the cubic lattice-matched N-doped Si/GsP multiple quantum well system integrated in a strip waveguide in the silicon-on-insulator platform. A “giant” second-order nonlinear optical susceptibility is obtained in an asymmetric coupled quantum well (ACQW) stack by engineering the 1-2 and 1-3 inter-subband spacings for resonance at both a ∼4 μm pump wavelength and a ∼2μm harmonic wavelength. Generation has been simulated as a function of the quantum well physical parameters, the infrared absorption losses, and the detuning from the double resonance condition. For TM pumps at 3.75 μm and 4.24 μm, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\boldsymbol{\chi }}_{{\bm{zzz}}}^{(2)}$</tex-math></inline-formula> value ranging from 7.73 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> pm/V to 1.13 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> pm/V has been calculated and a maximum conversion efficiency ranging between 1.23%/W and 1.68%/W has been obtained, where the waveguide coherence length was 5.57 μm and 6.16 μm.

SnP2S6: A Promising Infrared Nonlinear Optical Crystal with Strong Nonresonant Second Harmonic Generation and Phase-Matchability

High-power infrared laser systems with broadband tunability are of great importance due to their wide range of applications in spectroscopy and free-space communications. These systems require nonlinear optical (NLO) crystals for wavelength up/down conversion using sum/difference frequency generation, respectively. NLO crystals need to satisfy many competing criteria, including large nonlinear optical susceptibility, large laser induced damage threshold (LIDT), wide transparency range and phase-matchability. Here, we report bulk single crystals of SnP_2S_6 with a large non-resonant SHG coefficient of d33= 53 pm/V at 1550nm and a large LIDT of 350 GW/cm^2 for femtosecond laser pulses. It also exhibits a broad transparency range from 0.54 {\mu}m to 8.5{\mu}m (bandgap of ~2.3 eV) and can be both Type I and Type II phase-matched. The complete linear and SHG tensors are measured as well as predicted by first principles calculations, and they are in excellent agreement. A proximate double-resonance condition in the electronic band structure for both the fundamental and the SHG light is shown to enhance the non-resonant SHG response. Therefore, SnP2S6 is an outstanding candidate for infrared laser applications.

The role of nonlocal response in second harmonic generation at metasurfaces with triangular metaatoms

Metasurface consisting of square array of obtuse isosceles triangle metaatoms is found to exhibit much more efficient second harmonic generation (SHG) compared to that of split ring resonators and its heptagonal modification, experimentally as well as numerically. All of them are designed to lack inversion symmetry in horizontal direction but to have vertical symmetry, and to meet the double resonance condition for vertically-polarized fundamental and horizontally-polarized SHG waves. SHG efficiencies were estimated numerically for each structure by evaluating an overlap integral of second order nonlinear polarization and field distribution at the SHG wavelength. The large overlap integral for the triangle metasurface is ascribed to the nonlocal response at the SHG wavelength. Current induced at the center of the triangle near the obtuse angle by horizontally polarized light at SHG wavelength flows toward two corners of acute angles. As a result, polarization charges oscillate in time at the surfaces of two acute corners, which results in strong field oscillation away from the center where the electric field is applied to majority of free electrons. Experimentally observed wavelength dependence of SHG efficiencies for the three metasurfaces are reasonably reproduced by a numerical estimation.

Floquet state depletion in ac-driven circuit QED

We perform Floquet spectroscopy in a GaAs double quantum dot system coupled to a high-impedance superconducting resonator. By applying microwave induced consecutive passages under a double resonance condition, we observe novel Landau-Zener-St\"uckelberg-Majorana interference patterns that stem from a cavity-assisted interference pattern modified by the depletion of the ground state. Our experimental results reveal the stationary state behavior of a strongly driven two-level system, and are consistent with the simulations based on our theoretical model. This study provides an excellent platform for investigating the dynamics of Floquet states in the presence of strong driving.

Non-resonance and Double Resonance for a Planar System via Rotation Numbers

We consider a planar system $$z'=f(t,z)$$ under non-resonance or double resonance conditions and obtain the existence of $$2\uppi $$ -periodic solutions by combining a rotation number approach together with Poincare-Bohl theorem. Firstly, we allow that the angular velocity of solutions of $$z'=f(t,z)$$ is controlled by the angular velocity of solutions of two positively homogeneous system $$z'=L_i(t,z),i=1,2$$ , whose rotation numbers satisfy $$\rho (L_1)>n$$ and $$\rho (L_2)<n+1$$ , namely, nonresonance occurs in the sense of the rotation number. Secondly, we prove the existence of $$2\uppi $$ -periodic solutions when the nonlinearity is allowed to interact with two positively homogeneous system $$z'=L_i(t,z),i=1,2$$ , with $$\rho (L_1)\ge n$$ and $$\rho (L_2)\le n+1$$ , which gives rise to double resonance, and some kind of Landesman–Lazer conditions are assumed at both sides.

Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments.

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time.

Ionospheric HF Heating Experiment With Frequency Ramping‐up Sweep: Approach for Artificial Ionization Layer Generation

AbstractAn ionospheric high‐frequency (HF) heating experiment was conducted at the HAARP facility in Alaska by sweeping the heater frequency, near the third electron gyroharmonic, over a 145 kHz range with 4 min duration. A 446 MHz diagnostic radar was used to detect scattering from HF‐enhanced plasma waves corresponding to the HF‐enhanced ion lines (HFILs). Increasing frequency progressively moves the third gyroharmonic resonance layer downward, and this is evident in observations of both downgoing and upgoing HFILs dropping in height about 50 km. Artificial ionization was formed by the HF‐plasma interaction during this process and driven downward to about 150–155 km. Total electron content measurements oblique across the High Frequency Active Auroral Research Program transmit beam, above the altitude of primary ionization production, show variations that correspond to ionization production at lower heights. A theoretical interpretation of the experimental observations is presented to understand the HF‐plasma interaction process. It is shown that the double resonance condition enhances the excitation of the upper‐hybrid parametric decay instability (PDI). HF heater frequency ramping‐up sweep increases the spatial height span of the upper‐hybrid PDI generation region and the spectral width of generated upper‐hybrid waves, as well as provides a positive tuning frequency on the excited upper‐hybrid waves that keeps the continuous positive feedback on the Doppler‐shifted cyclotron harmonic resonance interaction. Resonant electrons gain energy from the upper‐hybrid waves and are generated in an extended spatial region; after a drift space, spatially distributed energetic electrons bunch together to build up a density structure.

Height Variation of Gaps in 150‐km Echoes and Whole Atmosphere Community Climate Model Electron Densities Suggest Link to Upper Hybrid Resonance

AbstractRadar echoes from the daytime lower region near the magnetic equator, so‐called 150‐km echoes, have been puzzling researchers for decades. Neither the mechanisms that generate the enhanced backscatter at very high frequencies (typically 30–50 MHz), the sharp lower cutoff height, the intricate layering with multiple echo layers separated by narrow gaps, nor the modulation of the echoes by short‐period gravity waves is well understood. Here we focus on the diurnal variation of the echo layers—specifically, certain wide gaps in the vertical structure—which apparently descend in the morning, reach their lowest altitude near local noon, and ascend in the afternoon, sometimes described as necklace structure based on the appearance of the layers in range‐time‐intensity diagrams. Analyzing high‐resolution data obtained with the Jicamarca radar between 2005 and 2017, spanning more than one solar cycle, we find that (a) wide gaps and narrow lines occur in vertically stacked, systematically repeating pattern; (b) the gap heights vary with season and solar cycle; and (c) the gap heights can be associated with specific contours of plasma frequencies or electron densities. The last two findings are supported by simultaneous observations of VIPIR ionosonde reflection heights and by comparison of gap heights with electron density contours obtained with the WACCM‐X 2.0 global model. Finally, the wide gaps appear to coincide with the double resonance condition, where the upper hybrid frequency equals integer multiples of the electron gyrofrequency. This may explain why field‐aligned plasma irregularities are suppressed and enhanced radar backscatter is not observed inside the gaps.

Nonlinear optical properties of cylindrical quantum dot with Kratzer confining potential in the presence of axial homogeneous electric field

Electronic states of an electron localized in a cylindrical quantum dot with Kratzer confining potential in the presence of axial electrostatic field are considered. The dependences of the dipole matrix elements on the geometrical parameters and values of the electric field are calculated. Nonlinear optical effects such as second harmonic generation and optical rectification in this structure are studied. The electric field values, at which double resonance condition are satisfied, have been found. Optical rectification and the second harmonic generation coefficients of energy dependence curves are obtained. Specificity of the Kratzer confinement potential profile leads to vanishing of the mean displacement of the electron in intraband transitions. This fact is reflected in the behavior of optical rectification coefficient, which has a deep dip around certain values of the electric field.

Novel single-mode narrow-band photon source of high brightness tuned to cesium D2 line

Cavity-enhanced Spontaneous parametric down-conversion (SPDC) is capable of efficient generation of single photons with suitable spectral properties for interfacing with the atoms. However, beside the remarkable progress of this technique, multi-mode longitudinal emission remains as major drawback. Here we demonstrate a bright source of single photons that overcomes this limitation by a novel mode-selection technique based on the introduction of an additional birefringent element to the cavity. This enables us to tune the double resonance condition independent of the phase matching, and thus to achieve single-mode operation without mode filters. Our source emits single-frequency-mode photons at 852 nm, which is compatible to the Cs D2 line, with a bandwidth of 10.9 MHz and a photon-pair generation rate exceeding 47 KHz at 10 mW of pump power, while maintaining a low $g^{\left(2\right)}(0) =$ 0.13. The efficiency of our source is further underlined by measuring a four-photon generation rate of 37 Hz at 20 mW of pump power. This brightness opens up a variety of new applications reaching from hybrid light-matter interactions to optical quantum information tasks based on long temporal coherence.

Second-Harmonic Generation in Mie-Resonant GaAs Nanowires

We investigate the enhancement of second-harmonic generation in cylindrical GaAs nanowires. Although these nanostructures confine light in two dimensions, power conversion efficiencies on the order of 10 − 5 with a pump peak intensity of ~ 1 GW / cm 2 are possible if the pump and the second-harmonic fields are coupled to the Mie-type resonances of the nanowire. We identify a large range of nanowire radii in which a double-resonance condition, i.e., both the pump and the second-harmonic fields excite normal modes of the nanowire, induces a high-quality-factor peak of conversion efficiency. We show that second-harmonic light can be scattered with large efficiency even if the second-harmonic photon energy is larger than 1.42 eV, i.e., the electronic bandgap of GaAs, above which the material is considered opaque. Finally, we evaluate the efficiency of one-photon absorption of second-harmonic light and find that resonant GaAs nanowires absorb second-harmonic light in the near-field region almost at the same rate at which they radiate second-harmonic light in the far-field region.

Effects of site\u2013city interaction and polarization of the incident S-wave on the transfer function and fundamental frequency of structures

The variation of 2D fundamental frequency of stand-alone structure on rock ({F}_{{02{text{D}}}}^{text{S}} ) and in a basin ({F}_{{02{text{D}}}}^{text{BS}} ) with the polarization of the incident S-wave is presented. This paper also presents the role of city density and polarization of the exciting S-wave in the site–city interaction effects on the response of structures in a city as well as free-field motion. The seismic responses of the various considered basin and site–city models are computed using SH-wave and P-SV-wave finite-difference programs. A considerable decrease in the value of {F}_{{02{text{D}}}}^{text{S}} of stand-alone structure on rock is obtained with the increase in shape ratio for the SV-wave but not for the SH-wave, as compared to the 1D fundamental frequency of structure ( {F}_{{01{text{D}}}}^{text{S}} ). However, a considerable decrease in the value of {F}_{{02{text{D}}}}^{text{BS}} of a stand-alone structure in basin under double resonance condition is obtained for the SH-wave and not for the SV-wave. The spectral amplification factor at the top of a structure at {F}_{{02{text{D}}}}^{text{BS}} of structure is larger for the SV-wave as compared to the SH-wave. A splitting of the bandwidth of the fundamental mode of vibration of structure and very large reduction in spectral amplification factor at {F}_{{02{text{D}}}}^{text{BS}} of structure is obtained due to the site–city interaction effects under double resonance condition for both the S-wave polarizations. Further, an increase in these inferred effects with an increase in city density and number of structures in a city with a constant city density is obtained. This finding raises the question concerning the validity of ground-motion prediction using soil–structure interaction for the design of structures in an urban environment. Further, the decrease in spectral amplification factor at {F}_{{02{text{D}}}}^{text{BS}} of structure due to the site–city interaction effects is larger in the case of SV-wave as compared to SH-wave. Furthermore, a considerable site–city interaction effects on free-field motion is obtained.

Quantification of Fundamental Frequencies of 3D Basins and Structures and Site–City Interaction Effects on Responses of Structures

Large-scale commercialization in Indian metrocities has made realtors develop new construction sites by filling land depressions with loose soil, which may behave as small three-dimensional (3D) basins. This study presents the development of empirical relations to predict the fundamental frequency of basins ( $$F_{{03{\text{D}}}}^{\text{B}}$$ ) and structures ( $$F_{{03{\text{D}}}}^{\text{S}}$$ ) on rock for different shape ratios to study the effects of site–city interactions (SCIs) on the response of structures under a double resonance condition. The S-wave responses of the various considered basins, structures, and site–city models are simulated using the finite-difference method. Analysis of the simulation results reveals that the $$F_{{03{\text{D}}}}^{\text{B}}$$ of the basin and the $$F_{{03{\text{D}}}}^{\text{S}}$$ of the structure depend strongly on the shape ratio, on average matching with those obtained using the SV-wave response of the section of the respective basin/structure. However, the spectral amplification factor (SAF) obtained at $$F_{{03{\text{D}}}}^{\text{B}}$$ / $$F_{{03{\text{D}}}}^{\text{S}}$$ for the basin/structure is much larger than that obtained using the SV-wave response of a section of the respective basin/structure. Empirical relations are developed to predict the $$F_{{03{\text{D}}}}^{\text{B}}$$ of basins and $$F_{{03{\text{D}}}}^{\text{S}}$$ of structures in terms of the shape ratio and one-dimensional (1D) fundamental frequency of the respective model. Analysis of 3D SCI effects reveals a reduction of the SAF at the fundamental frequency of the structure in the basin ( $$F_{{03{\text{D}}}}^{\text{SB}}$$ ) with an increase of the number of structures as compared with that at the fundamental frequency ( $$F_{{03{\text{D}}}}^{\text{SB}}$$ ) of a standalone structure in the basin, being on the order of about 60 % in the case of a city with 25 structures. This finding indicates the need for 3D SCI studies in urban environments for cost-effective earthquake engineering.

Resonant optical studies of GaAs/AlGaAs Multiple Quantum Well based Bragg Structures at excited states

ABSTRACTOptical reflectance (OR) and electro-reflectance (ER) spectroscopies were employed to study the resonant optical properties of GaAs/AlGaAs multiple quantum wells based Resonant Bragg Structures (RBS) at excited states. The RBS samples have 60 periods of GaAs/AlGaAs quantum well/barrier grown on semi-insulating GaAs substrates by molecular beam epitaxy with slightly different thicknesses of well/barrier. We observed enhanced OR and ER features when exciton energy coincides with the energy of the Bragg reflection peak, a double resonance condition. Bragg peak can significantly be tuned by changing the angle of incidence of the light. Exciton energies can be tuned by changing the temperature, external electric field and the thickness of the quantum wells. By tuning the Bragg peak for double resonance in the RBS samples of different thicknesses, we observed the electro-reflectance features related to the transitions of x(e2-hh2), x(e2-hh1), x(e2-lh1), x(e2-hh3) and x(e1-hh3) excitons along with the sharp features of x(e1-hh1) and x(e1-lh1) ground state exciton transitions from the ER experiments. The excitonic transitions x(e2-hh1), x(e2-lh1) and x(e2-hh3) which are prohibited at zero electric field, were also observed due to the increased overlap of the electron and hole wave functions caused by the electric field; built-in electric field or applied DC bias.

Enhanced Raman scattering of graphene using double resonance in silicon photonic crystal nanocavities

We demonstrate enhancements of Raman scattering from graphene on two-dimensional photonic crystals using double resonances, which originate from simultaneous enhancements by a localized guided mode and a cavity mode. By adjusting the photonic crystal cavity parameters, the double resonance can be tuned to the G′ Raman scattering. Excitation wavelength dependence measurements show a large Raman peak enhancement when the excitation and emission wavelengths meet the double resonance condition. Furthermore, spatial imaging measurements are performed to confirm that the enhancement is localized at the cavity, and we find that the enhanced Raman intensity is 60 times larger compared to the on-substrate Raman signal. The observed cavity enhancement of Raman scattering opens up new possibilities for the development of graphene-based light sources for silicon photonics.

Dynamic regimes of kinetic instabilities under conditions of double plasma resonance in mirror-confined plasma

The processes of plasma waves excitation under conditions of a double plasma resonance in a non-equilibrium plasma produced by high-power microwave radiation and confined in an open magnetic trap are studied. Two different dynamic regimes of kinetic instability have been experimentally observed: besides the generation of quasiperiodic series of short (∼50 ns) radiation pulses, a new scenario of instability in the form of generation of a single pulse of large (∼25 μs) duration with a slowly varying frequency near the second harmonic of the electron gyrofrequency in the center of the trap has been discovered. It is shown that the dynamic regimes of plasma wave generation observed in the experiment can arise when the processes of wave excitation and induced scattering compete, and the transition from quasi-continuous generation to pulse-periodic generation is determined by the ratio of the energy of non-equilibrium particles and the temperature of the background plasma. Hereinafter, the studied mechanisms will be used to explain similar phenomena in space, for example, sequences of pulses of different duration in the spectra of solar type-IV radio bursts.

Element Selectivity in Second-Harmonic Generation of GaFeO3 by a Soft-X-Ray Free-Electron Laser

Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.