Effective Interaction Length Research Articles

Magnetic interactions in anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer magnets

The magnetic properties and the possible interaction mechanisms of anisotropic soft- and hard-magnetic multilayers have been investigated by altering the thickness of different kinds of spacer layers. The metal Ta and the insulating oxides MgO, Cr2O3 have been chosen as spacer layers to investigate the characteristics of the interactions between soft- and hard-magnetic layers in the anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer system. The dipolar and exchange interaction between hard and soft phases are evaluated with the help of the first order reversal curve method. The onset of the nucleation field and the magnetization reversal by domain wall movement are also evident from the first-order-reversal-curve measurements. Reversible/irreversible distributions reveal the natures of the soft- and hard-magnetic components. Incoherent switching fields are observed and the calculations show the semiquantitative contributions of hard and soft components to the system. An antiferromagnetic spacer layer will weaken the interaction between ferromagnetic layers and the effective interaction length decreases. As a consequence, the dipolar magnetostatic interaction may play an important role in the long-range interaction in anisotropic multilayer magnets.

Generating Entangled State Between the Signal Modes in Forward Three-Wave Mixing

We present a scheme for generating entangled state with three-wave mixing. By analytic derivation and numerical calculation, we see that the entangled state can be generated by adjusting the phase of the pump field, the effective interaction length and the total number of photons.

Role of geometry in the superfluid flow of nonlocal photon fluids

Recent work has unveiled a new class of optical systems that can exhibit the characteristic features of superfluidity. One such system relies on the repulsive photon-photon interaction that is mediated by a thermal optical nonlinearity and is therefore inherently nonlocal due to thermal diffusion. Here we investigate how such a nonlocal interaction, which at a first inspection would not be expected to lead to superfluid behavior, may be tailored by acting upon the geometry of the photon fluid itself. Our models and measurements show that restricting the laser profile and hence the photon fluid to a strongly elliptical geometry modifies thermal diffusion along the major beam axis and reduces the effective nonlocal interaction length by two orders of magnitude. This in turn enables the system to display a characteristic trait of superfluid flow: the nucleation of quantized vortices in the flow past an extended physical obstacle. These results are general and apply to other nonlocal fluids, such as dipolar Bose-Einstein condensates, and show that "thermal" photon superfluids provide an exciting and novel experimental environment for probing the nature of superfluidity, with applications to the study of quantum turbulence and analogue gravity.

Super-Resonant Intracavity Coherent Absorption.

The capability of optical resonators to extend the effective radiation-matter interaction length originates from a multipass effect, hence is intrinsically limited by the resonator’s quality factor. Here, we show that this constraint can be overcome by combining the concepts of resonant interaction and coherent perfect absorption (CPA). We demonstrate and investigate super-resonant coherent absorption in a coupled Fabry-Perot (FP)/ring cavity structure. At the FP resonant wavelengths, the described phenomenon gives rise to split modes with a nearly-transparent peak and a peak whose transmission is exceptionally sensitive to the intracavity loss. For small losses, the effective interaction pathlength of these modes is proportional respectively to the ratio and the product of the individual finesse coefficients of the two resonators. The results presented extend the conventional definition of resonant absorption and point to a way of circumventing the technological limitations of ultrahigh-quality resonators in spectroscopy and optical sensing schemes.

Quantum simulators based on the global collective light-matter interaction

We show that coupling ultracold atoms in optical lattices to quantized modes of an optical cavity leads to quantum phases of matter, which at the same time posses properties of systems with both short- and long-range interactions. This opens perspectives for novel quantum simulators of finite-range interacting systems, even though the light-induced interaction is global (i.e. infinitely long range). This is achieved by spatial structuring of the global light-matter coupling at a microscopic scale. Such simulators can directly benefit from the collective enhancement of the global light-matter interaction and constitute an alternative to standard approaches using Rydberg atoms or polar molecules. The system in the steady state of light induces effective many-body interactions that change the landscape of the phase diagram of the typical Bose-Hubbard model. Therefore, the system can support non-trivial superfluid states, bosonic dimer, trimers, etc. states and supersolid phases depending on the choice of the wavelength and pattern of the light with respect to the classical optical lattice potential. We find that by carefully choosing the system parameters one can investigate diverse strongly correlated physics with the same setup, i.e., modifying the geometry of light beams. In particular, we present the interplay between the density and bond (or matter-wave coherence) interactions. We show how to tune the effective interaction length in such a hybrid system with both short-range and global interactions.

Thick-crystal regime in photon pair sources

The workhorse technique for generating correlated pairs of photons is based on spontaneous parametric downconversion in nonlinear crystals. These photon pair sources are usually designed with relatively short crystal lengths, in the belief that this is necessary to attain good performance. We show, contrary to common practice, that concurrent high brightness and efficiency are also available to longer crystals. We present comprehensive measurement data on the pump and collection beam parameters necessary to achieve high collection efficiency (89.0%±1.7% and 81.9%±3.7% for signal and idler) together with high brightness when a single thick β-barium borate crystal (15.76 mm) is pumped with a narrow linewidth laser. Spectral analysis of the collinear, nondegenerate photons suggests that the effective interaction length within the crystal is dominated by the collection beam mode, and the use of longer crystals with increased spatial walk-off does not necessarily lead to reduced collection efficiency. This surprising result is an important consideration for optical designers who seek to develop practical photon pair sources.

Cavity-based mid-IR fiber gas laser pumped by a diode laser

Mid-infrared (IR) lasers are currently an area of rapid development, with several competing technologies. In traditional gas lasers, the effective interaction length is limited and the system as a whole is bulky and inflexible, limiting their applications. Standard gain fibers cannot be used in the mid-IR because the glass forming the fiber core is not transparent at these longer wavelengths. In this Letter, we report the demonstration of a mid-IR fiber gas laser using feedback in an optical cavity. The laser uses acetylene gas in a high-performance silica hollow-core fiber as the gain medium, and lases either continuous wave or synchronously pumped when pumped by telecom-wavelength diode lasers. We have demonstrated lasing on a number of transitions in the spectral band of 3.1–3.2 μm. The system could be extended to other selected molecular species to generate output in the spectral band up to 5 μm, and it has excellent potential for power scaling.

Widely tunable and monochromatic terahertz difference frequency generation with organic crystal 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile

We report an experimental study on widely tunable terahertz (THz) wave difference frequency generation (DFG) with hydrogen-bonded crystals 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile (OH1). The organic crystals were pumped by a ∼1.3 μm double-pass KTiOPO4 optical parametric oscillator. A tuning range of 0.02–20 THz was achieved. OH1 crystals offer a long effective interaction length (also high output) for the generation below 3 THz, owing to the low absorption and favorable phase-matching. The highest energy of 507 nJ/pulse was generated at 1.92 THz with a 1.89-mm-thick crystal. Comprehensive explanations were provided, on the basis of theoretical calculations. Cascading phenomenon during the DFG process was demonstrated. The photon conversion efficiency could reach 2.9%.

Broadband Continuous Frequency Tuning in a Terahertz Gyrotron With Tapered Cavity

Broadband continuous frequency tuning (CFT) in a terahertz gyrotron is promising for advanced terahertz applications. However, it is challenging to realize broadband CFT in a conventional open cavity, because a long cavity is helpful to expand the bandwidth but is generally difficult to suppress the high $Q$ -factor gyromonotron competition. In this paper, a tapered cavity with a long effective interaction length is proposed to expand the CFT bandwidth. The tapered circuit can reduce the $Q$ -factor of the first-order axial mode and accordingly suppress the gyromonotron competition. By selecting a reasonable $Q$ -factor cavity, a gyrotron could generate effective radiation sequentially under gyromonotron and gyrobackward-wave oscillator (BWO) states during the magnetic field tuning. In gyromonotron range, the bandwidth is expanded because of the cutoff frequency shifting. On the other hand, in gyro-BWO range, the bandwidth is expanded because of the axial mode transition. The CFT bandwidth of 4 GHz is realized in a tapered 330-GHz TE12,4 mode low-voltage gyrotron. The principle is important for developing broadband CFT terahertz gyrotrons.

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range.

Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of -0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals.

High-power, high-repetition-rate performance characteristics of β-BaB₂O₄ for single-pass picosecond ultraviolet generation at 266 nm.

We report a systematic study on the performance characteristics of a high-power, high-repetition-rate, picosecond ultraviolet (UV) source at 266 nm based on β-BaB2O4 (BBO). The source, based on single-pass fourth harmonic generation (FHG) of a compact Yb-fiber laser in a two-crystal spatial walk-off compensation scheme, generates up to 2.9 W of average power at 266 nm at a pulse repetition rate of ~80 MHz with a single-pass FHG efficiency of 35% from the green to UV. Detrimental issues such as thermal effects have been studied and confirmed by performing relevant measurements. Angular and temperature acceptance bandwidths in BBO for FHG to 266 nm are experimentally determined, indicating that the effective interaction length is limited by spatial walk-off and thermal gradients under high-power operation. The origin of dynamic color center formation due to two-photon absorption in BBO is investigated by measurements of intensity-dependent transmission at 266 nm. Using a suitable theoretical model, two-photon absorption coefficients as well as the color center densities have been estimated at different temperatures. The measurements show that the two-photon absorption coefficient in BBO at 266 nm is ~3.5 times lower at 200°C compared to that at room temperature. The long-term power stability as well as beam pointing stability is analyzed at different output power levels and focusing conditions. Using cylindrical optics, we have circularized the generated elliptic UV beam to a circularity of >90%. To our knowledge, this is the first time such high average powers and temperature-dependent two-photon absorption measurements at 266 nm are reported at repetition rates as high as ~80 MHz.

Nonlinear distortion of intense THz beams

Near- and far-field beam profiles were measured for THz pulses generated in LiNbO3 by optical rectification of 200 fs pulses with a tilted pulse front. The variation of the THz beam size and a dramatically increasing divergence angle with increasing pump fluence were observed in the (horizontal) plane of the pulse front tilt. No significant variation was observed in the vertical direction. The reason for the observed nonlinear beam distortion is the shortening of the effective interaction length for THz generation caused by the combined effect of pump spectral broadening and angular dispersion in the tilted pulse front geometry. Our results indicate that nonlinear THz beam distortion effects have to be taken into account when designing intense THz sources and related experiments.

Direct observation of DNA overwinding by reverse gyrase

Reverse gyrase, found in hyperthermophiles, is the only enzyme known to overwind (introduce positive supercoils into) DNA. The ATP-dependent activity, detected at >70 °C, has so far been studied solely by gel electrophoresis; thus, the reaction dynamics remain obscure. Here, we image the overwinding reaction at 71 °C under a microscope, using DNA containing consecutive 30 mismatched base pairs that serve as a well-defined substrate site. A single reverse gyrase molecule processively winds the DNA for >100 turns. Bound enzyme shows moderate temperature dependence, retaining significant activity down to 50 °C. The unloaded reaction rate at 71 °C exceeds five turns per second, which is >10(2)-fold higher than hitherto indicated but lower than the measured ATPase rate of 20 s(-1), indicating loose coupling. The overwinding reaction sharply slows down as the torsional stress accumulates in DNA and ceases at stress of mere ∼ 5 pN ⋅ nm, where one more turn would cost only sixfold the thermal energy. The enzyme would thus keep DNA in a slightly overwound state to protect, but not overprotect, the genome of hyperthermophiles against thermal melting. Overwinding activity is also highly sensitive to DNA tension, with an effective interaction length exceeding the size of reverse gyrase, implying requirement for slack DNA. All results point to the mechanism where strand passage relying on thermal motions, as in topoisomerase IA, is actively but loosely biased toward overwinding.

Numerical study on the mean velocity distribution law of air backflow and the effective interaction length of airflow in forced ventilated tunnels

The effective interaction length of airflow (Le) is the key factor in the determination of the distance between an air duct nozzle and the working face in dead-end tunnels. At present, the current calculation methods of Le consider only the tunnel cross section dimension and are therefore not suitable for large underground cavern groups. Because air backflow is the direct promoting factor in the removal of pollutants and the construction specifications in underground space require a minimum wind speed, this paper proposed that Le is the distance between the nozzle and the critical cross section, where the mean velocity of the air backflow equals the minimum wind speed required. For this purpose, a computational study was conducted to investigate the flow pattern of air backflow in ventilated tunnels. Semi-empirical equations of the mean velocity distribution law of air backflow were derived based on the numerical results. By solving the equations, the location of the critical cross section can then be acquired. Accordingly, the value of Le could be obtained. The proposed method is related to the tunnel width, the tunnel height, the air duct diameter and the efflux velocity. Thus, it is more comprehensive and applicable compared to the current methods.

Effect of Coulomb screening length on nuclear “pasta” simulations

We study the role of the effective Coulomb interaction strength and length on the dynamics of nucleons in conditions according to those in a neutron star's crust. Calculations were made with a semi-classical molecular dynamics model, studying isospin symmetric matter at sub-saturation densities and low temperatures. The electrostatic interaction between protons interaction is included in the form of a screened Coulomb potential in the spirit of the Thomas-Fermi approximation, but the screening length is artificially varied to explore its effect on the formation of the non-homogeneous nuclear structures known as "nuclear pasta". As the screening length increases, we can a transition from a one-per-cell pasta regime (due exclusively to finite size effects) to a more appealing multiple pasta per simulation box. This shows qualitative difference in the structure of neutron star matter at low temperatures, and therefore, special caution should be taken when the screening length is estimated for numerical simulations.

Measurement of nonlinear refraction of thick samples using nonlinear-imaging technique with a phase object

Abstract The third-order nonlinear refractive index of carbon disulfide (CS2) with different thicknesses is measured by a nonlinear-imaging technique with a phase object (NIT-PO). The propagating characteristics of the laser beam within thick medium are analyzed by numerically solving the electromagnetic wave equation. By theoretically calculating, we study the relationship of the phase shift induced by nonlinear refraction versus medium thicknesses. Fitting the output image with the help of the effective interaction length, the nonlinear refractive index n2 of CS2 can be determined, which suggests a one-single-shot method for the investigation of optical nonlinearity in thick medium.

Deflection of a monochromatic THz beam by acousto-optic methods

The possibility of controlled deflection of an electromagnetic THz beam of a free-electron laser by acousto-optic (AO) methods has been demonstrated for the first time. The material of the AO deflector was chosen to be single-crystal germanium, which has a fairly large refractive index and a relatively low absorption coefficient for electromagnetic waves. The absorption coefficient in germanium is at a wavelength . The diffracted beam intensity is shown to be maximum at an effective AO interaction length . A diffraction efficiency of 0.05 % at a travelling acoustic wave power of is experimentally obtained. It is established that a change in the ultrasonic frequency from 25 to leads to variation in the external Bragg angle in the range from to . At a fixed Bragg angle the frequency band of diffraction is and the angular range of laser beam scanning reaches . The results obtained indicate that AO interaction can be used for controlled deflection of electromagnetic THz beams.

Extending the Sensing Range of Brillouin Optical Time-Domain Analysis Combining Frequency-Division Multiplexing and In-Line EDFAs

We demonstrate a high-performance Brillouin optical time-domain analysis (BOTDA) system with an extended sensing range by combining frequency-division multiplexing and in-line Erbium doped fiber amplifiers (EDFAs). The frequency-division multiplexing BOTDA features multiple sections of fibers with different Brillouin frequency shifts, and it reduces the effective Brillouin interaction length to one resonant Brillouin frequency section rather than the entire length of the sensing fiber, so that the power of CW probe of BOTDA can be increased to enhance the Brillouin signal within individual sections and consequently extend the sensing range combined with high strain or temperature resolution with negligible pump depletion. In addition, in-line EDFAs placed between spans are used to compensate the fiber loss for similar Brillouin gains in each span. In experiment, a 150-km sensing range is achieved by dividing the sensing fibers into two spans of equal length and using two types of fibers in each span. Using the differential pulse-width pair technique, a 100/120 ns pulse pair is used to realize a 2-m spatial resolution and a measurement accuracy of 1.5°C/30 μe at the end of the sensing fibers.

Measuring Nanolayer Profiles of Various Materials by Evanescent Light Technique

The evanescent light photon extraction efficiency of insulator, semiconductor and conductor amorphous nanolayers deposited on glass waveguides was evaluated from Differential Evanescent Light Intensity measurements. The Differential Evanescent Light Intensity technique uses the evanescent field scattered by the deposited nanolayer, enabling nanometer thickness profiling due to the high inherent dark background contrast. The results show that the effective evanescent photon penetration depth increases from metal to semiconductor and then to insulating layers, establishing thus the effective photon-material interaction length for the various materials classes.

Threshold Analysis of a THz-Wave Parametric Oscillator

The parametric gain of a terahertz wave parametric oscillator (TPO) is analyzed. Meanwhile the expression of TPO threshold pump intensity is derived and theoretically analyzed with different factors. The effective interaction length between the pump wave and Stokes wave is calculated, and particular attention is paid to the coupling efficiency of the pump wave and Stokes wave. Such an analysis is useful for the experiments of TPO.