Laser Physics Letters

When two electrons are emitted from a metal needle tip with the help of femtosecond laser pulses, they show a strong anticorrelation signal in the energy domain. Depending on the wavelength and intensity of the driving laser pulses, the electron emission process can be either in a perturbative regime, like single- or multi-photon photoemission, or in the strong-field regime, where emission is dominated by the instantaneous electric field of the laser pulse, or in the intermediate regime. Here, we report on the two-electron anticorrelation signal and how it evolves from the multiphoton toward the strong-field emission regime. We show that in both cases, the resulting anticorrelation signal can be well explained by semi-classical simulations using a point-particle model, thus the dynamics is dominated by the center-of-mass dynamics of the individual electrons. However, the actual emission process of multiple interacting electrons requires improved quantum mechanical models and therefore remains the subject of future work.

This paper is part of the Special Topic Collection: papers from the 31th Annual International Laser Physics Workshop 2023 (LPHYS 2023).

The simultaneous nonlinear spectral broadening and temporal cleaning of ultrashort pulses by the combination of the multipass cell (MPC) technique and nonlinear ellipse rotation are proposed and investigated with numerical simulations. The performance of the gas-filled MPC is studied at 800 and 1030 nm central wavelengths with mJ energy level. The results indicate that at least 10³ contrast enhancement is feasible with 50% internal efficiency while the beam quality is preserved during propagation. At the same time, nonlinear spectral broadening allows for a more than five-fold temporal compression. The technique is tested at 20 mJ energy and it is presumably suitable for the generation of high contrast, high energy few-cycle pulses, too.

We demonstrate the fabrication of surface nanoscale axial photonics bottle microresonators with angstrom precision using a flame. We observe strongly unscalable behavior of the whispering gallery mode cutoff wavelengths with different radial quantum numbers along the fibre length.

High pulse energy, low-alignment sensitivity master-oscillator power-amplifier (MOPA) systems enable portable long range laser devices. Comprehensive amplifier modelling is an essential tool in producing efficient, optimised amplfication capable of producing high pulse energies. This paper outlines the development of a large-mode, low-alignment sensitivity neodymium yttrium aluminium garnet (Nd:YAG) (MOPA) system, achieving a total output pulse energy of 265 mJ with an optical efficiency of 18%. A Q-switched diode-pumped Nd:YAG zig-zag oscillator is developed with an output pulse energy of 98 mJ and slope efficiency of 31%. Through the use of an intracavity aperture, the beam quality exhibited an M² of 4.3 and 4.6 and far field divergence of 1.3 mrad and 1.2 mrad in the horizontal and vertical, respectively. The oscillator output is amplified within a diode-pumped Nd:YAG zig-zag amplifier with a system amplification of 2.8. Comprehensive amplifier modelling based on a Frantz-Nodvik analysis is demonstrated, with the saturation characteristics suggesting a route to further energy enhancement and highlighting the necessity for amplifier modelling in high energy system design.

We report on a 976 nm pumped erbium doped fluoride (Er:ZBLAN) fiber laser emitting at 2.8 µm with nearly 70 W of output power in a quasi-continuous wave (Q-CW) regime with the slope efficiency versus launched pump power of ∼20%. It is believed to be the highest Q-CW power ever demonstrated from an Er:ZBLAN fiber laser . We also present the results of a study demonstrating that the Er:ZBLAN fiber laser slope efficiency versus absorbed pump power increases monotonically with the pump wavelength from 19% to 29% over the wavelength range of 960–985 nm. The achieved slope efficiency of 29% is, to the best of our knowledge, the highest efficiency ever reported for an Er:ZBLAN fiber non-cascaded laser.

A narrow spectral range between 2.3 nm and 4.4 nm wavelengths, the so-called 'water window' (WW), provides a unique opportunity for time-resolved imaging of proteins in their natural environment, as water is semi-transparent, while carbon is mostly opaque at these wavelengths. In this work we are presenting experimental developments toward high laser gain in CV ions at 4.03 nm, in a table-top device, and discuss possible application of such an x-ray laser to high-resolution microscopy, as well as generation of attosecond pulses.

This paper proposes a simple spectral radiance backward characterization model based on the key wavelength strategy. The color difference of spectral wavelength radiance amplitude loss under different conditions, the spectral amplitude change of primary color, and the monotonicity of spectral amplitude were investigated. The key wavelengths were selected based on these three factors. We innovatively introduced the selected wavelengths into a backward characterization interpolation table, which was tested using different color samples. The experimental results indicate that the proposed model is more suitable for spectral backward characterization than the existing methods.

Surface plasmon resonance (SPR) has become an important optical biosensing technology due to its real-time, label-free, and noninvasive nature. These techniques allow for rapid and ultra-sensitive detection of biological analytes, with applications in medical diagnostics, environmental monitoring, and agriculture. SPR is widely used in the detection of biomolecular interactions, and improvements are required for both sensitivity and in vivo uses for practical applications.

In this study, we developed an SPR biosensor to provide a highly sensitive and specific approach to early-stage detection of viral and malignant diseases, such as cancer tumors, for which biomarker detection is very important. A cancer cell line (HeLa cells) with biomarker Rodamine 6G was experimentally analyzed in vitro with our constructed SPR biosensor. It was observed that the biosensor can offer a potentially powerful solution for tumor screening with dominant angular shift. The angular shift for both regents is dominant with a time curve at a wavelength of 632.8 nm of a He–Ne laser. We have successfully captured and detected a biomarker in vitro for cancer diagnostics using the developed instrument.

In the past, laser propagation in a fluid with heat transfer has been modeled using simplistic conduction and convection conditions yielding inaccurate predictions. Here we present a detailed numerical study describing the thermal profile of the fluid and its interaction with the laser. Furthermore, we evaluate the diffraction field in the far field produced by a pump beam impinging in the fluid and the interferometric pattern obtained normal to the propagation direction to test our model. Direct comparison between experimental results and numerical simulation allows for a complete understanding of the energy transfer from the laser to the liquid and the subsequent effect on the laser propagation. Spatial self-phase modulation and propagation control from small-phase diffraction to aberration-controlled diffraction and up to diffraction oscillation are observed and explained with our modeling.

This paper investigates the dynamics of induced torque in Nitrogen-Vacancy (NV) centers interacting with two weak optical vortex beams as well as a strong control field, exploring the impact of different system parameters such as control field intensity, detuning, magnetic field, and vortex beam strength. We find a dispersive torque behavior, indicating the sensitivity of NV centers to control parameters. Magnetic field induces level splitting, leading to a transformative effect on torque, with notable enhancements observed at specific intensities. Additionally, non-resonant torque is explored, demonstrating the controllability of torque peaks through magnetic field manipulation. Unequal strengths of vortex beams is found to yield substantial enhancements in torque. These results provide crucial insights into the induced torque dynamics in NV centers, presenting opportunities for optimized torque-based applications in quantum systems.

In this paper, based on ghost imaging encryption, the preservation of Manhattan distance feature in ciphertext compared with plaintext is analyzed by utilizing the intraclass-interclass difference of image classification, and a classification method for image ciphertexts is proposed. After calculating Manhattan distance for both plaintexts and ciphertexts, respectively, the intraclass-interclass difference can be determined. The image that minimizes the intraclass-interclass difference is taken as the centroid to verify the consistency of the classification for various plaintext-ciphertext pairs under the same operation. The feasibility of proposed method is verified by numerical simulations, that the values of ACC and Weighted-F2 can be up to 90% when the MNIST is adopted as the test dataset. The whole process can be regarded as a kind of classification process by homomorphic encryption, however, different from the traditional homomorphic encryption methods based on mathematical model, the proposed method is accomplished based on the optical theory, and it does not require a lot of pre-training through models such as deep learning and neural networks, that means, reducing the computational expenses.

The polarization plane stimulated rotation angle of a probe signal in an intense laser field in plasma is calculated for arbitrary detunings of intense and weak laser waves compared with the resonant transition frequency of the medium. Estimates of the residual gas local density in a cesium plasma have been found based on the Faraday, Cotton–Mouton effects and on the effect of stimulated rotation of the polarization plane of the probe signal in an intense laser field. It is shown that the rotation in the medium has a complex structure consisting of the sum of only the influence of the magnetic field, only the influence of the intense laser field and the interfering part of the magnetic and intense laser fields.

A novel measurement-free mediated semi-quantum key distribution (MSQKD) protocol is proposed based on single-particle states. It enables two classical users to establish a secret key with the assistance of a third party. This protocol simplifies the third party's role to solely generating qubits in X-basis and conducting Bell measurements. A distinctive feature of this protocol is the efficient grouping and reordering of qubits by the classical users with a minimum of three delay lines. Security analyses demonstrate that the protocol can withstand various attack strategies, including collective attack, measurement attack, fake state attack, and modification attack. The noise tolerance is given by deriving a lower bound of the protocol's key rate in the asymptotic scenario. Simulations on the IBM Quantum Experience platform are conducted to illustrate the feasibility of this protocol. Compared with existing MSQKD protocols, the proposed protocol consumes fewer quantum resources and achieves a qubit efficiency of 1/8.

We demonstrate a compensation-free approach to the realization of multi-mode delay interferometers, mainly for use in phase-encoded quantum key distribution (QKD). High interference visibility of spatially multi-mode beams in unbalanced Michelson or Mach–Zehnder interferometers with a relatively wide range of delays is achieved by the appropriate choice of the transverse size of the beam. We provide a simple theoretical model that gives a direct connection between the visibility of interference, the delay and the beam parameters. The performed experimental study confirms our theoretical findings and demonstrates measured visibility of up to 0.95 for a delay of 2 ns. Our approach's simplicity and robust performance make it a practical choice for the implementation of QKD systems, where a quantum signal is received over a multi-mode fiber. The important application of such a configuration is an intermodal QKD system, where the free-space atmospheric communication channel is coupled into a span of the multi-mode fiber, delivering the spatially distorted beam to the remote receiver with minimal coupling loss.

A self-consistent quasi-classical generalization of the axiomatic quantization of the translational motion of a particle in a dissipative medium is proposed, which is modeled by a drag force ${F_{\text{d}}}$, which depends in a power-law manner on the velocity $v$: ${F_{\text{d}}}\sim {v^n}$, where $n$ is an arbitrary real number. A detailed analysis of the quasi-classical coherent state is carried out. It is shown that under condition $n < 3$ the dissipative medium suppresses the quantum properties of particle motion, performing the function of a classical measuring device. Otherwise, there is no suppression of quantum properties.

Photodynamic therapy (PDT) has been widely employed in clinical applications, healthcare, and public health (e.g. cancer research, microbiological control, vector control, etc). The photodynamic action is an advanced oxidation process based on the production of reactive oxygen species (ROS) and singlet oxygen by the excitation of a photosensitizer by specific wavelengths of light in the presence of molecular oxygen. The generation of ROS, which are highly reactive, encourages the use of PDT against recalcitrant pollutants and resistant parasites, a novel approach for PDT applications. Here, we explored recent research in PDT in water and wastewater treatment, elucidating operational conditions, main targets, potentials, and constraints, considering a collection of scientific papers curated by a well-defined research strategy. Retrieved records were filtered by subjects, and data was organized into a content network. Results showed that PDT is a timely alternative to deal with emerging chemical contaminants, resistant microorganisms, and other challenges, raising opportunities for versatile applications and sustainable solutions. Advances in environmental applications of PDT may help reach the Sustainable Development Goal 6 (SDG 6), but also positively impact other SDGs.

Energy deposition via laser-induced breakdown (LIB) in gases or other media and its accompanying secondary light and sound radiative processes are nowadays increasingly deployed in scientific and technological applications. The modeling and control of the breakdown and radiative processes occurring by the interactions of the free electrons with the heavy particles in the partially ionized medium, requires precise spatio-temporal description of the generated free electron density. This work presents an analysis of a free electron rate model describing the free electron density in air plasmas produced by nanosecond laser pulses. The model accounts for multiphoton and cascade ionization, and for electron diffusion, recombination, and attachment. A closed-form expression of the rate model is derived and validated by comparison with experimentally validated numerical solutions, showing very good agreement in a wide range of parameters. Simulation results are presented for different laser pulses and focal spot sizes and analysis is carried out regarding the dependence of the air plasma on the various laser radiation parameters. The presented approach is particularly useful for complex multi-scale models calculating the electron and ion temperature evolution, the thermoelastic expansion and the shock-wave following LIB of gases.

Pulsed laser ablation in liquid (PLAL) is an important method for synthesizing metal nanoparticles (NPs). Recently, it has garnered increasing interest as it is simple, rapid, and ecofriendly. Herein, PLAL is proposed as an approach to produce varied sizes of silver nanoparticles (Ag NPs) because NP size plays a vital role in their characteristics and several applications in the physical, chemical, biological, and medical fields. In PLAL, metal NP size could be controlled by either adjusting the laser parameters, such as wavelength, energy, fluence, reptation rate, ablation time, and focusing lens, or by modifying the ablation solvent properties. Herein, PLAL is proved as an effective and simple method for fabricating Ag NPs. This can provide guidance for synthesizing nanomaterials in diverse sizes, types, and shapes for applications in different fields.

We propose a scheme to describe magnon-mediated multi-channel high-order sideband generation in a cross cavity magnonic system. The high-order sideband generation is composed of equally spaced discrete output frequency components, which is essential tools for light communication, precision metrology, timing and spectroscopy. Beyond the generally linearized description by using the perturbative method, we deal with the Heisenberg–Langevin equations in the non-perturbative regime to obtain the output spectrum of high-order sidebands. Unlike conventional methods of using power that requires externally adjustable incident beams, here we demonstrate magnon-mediated high-order sideband generation in a cavity magnonic device by utilizing its intrinsically good tunability. Furthermore, until now most of the work in cavity magnonics is mainly restricted to the frequency/time domain, while we realize the modulation of sideband effects by the manipulation of cavity-magnon polaritons in real space, that is the external magnetic field with a tunable angle. By tuning the angle, we can obtain multi-channel high-order sideband generation, which may offer the potential for selectively transferring coherent information processing technologies.

Mixtures of quantum fluids, that is gases or liquids, are considered with the emphasis on the conditions characterizing the stability of the mixtures. The mixtures, that can be formed by cold atoms or molecules, are assumed to be quantum requiring the description using quantum techniques, but not so cold that to exhibit superfluidity or superconductivity. Reviewing the stability conditions of such normal quantum systems is important for the comparison of these conditions with the stability conditions of, e.g. Bose-condensed mixtures. The behavior of observable quantities under the stratification of quantum mixtures is discussed.

When two electrons are emitted from a metal needle tip with the help of femtosecond laser pulses, they show a strong anticorrelation signal in the energy domain. Depending on the wavelength and intensity of the driving laser pulses, the electron emission process can be either in a perturbative regime, like single- or multi-photon photoemission, or in the strong-field regime, where emission is dominated by the instantaneous electric field of the laser pulse, or in the intermediate regime. Here, we report on the two-electron anticorrelation signal and how it evolves from the multiphoton toward the strong-field emission regime. We show that in both cases, the resulting anticorrelation signal can be well explained by semi-classical simulations using a point-particle model, thus the dynamics is dominated by the center-of-mass dynamics of the individual electrons. However, the actual emission process of multiple interacting electrons requires improved quantum mechanical models and therefore remains the subject of future work.

This paper is part of the Special Topic Collection: papers from the 31th Annual International Laser Physics Workshop 2023 (LPHYS 2023).

In the past, laser propagation in a fluid with heat transfer has been modeled using simplistic conduction and convection conditions yielding inaccurate predictions. Here we present a detailed numerical study describing the thermal profile of the fluid and its interaction with the laser. Furthermore, we evaluate the diffraction field in the far field produced by a pump beam impinging in the fluid and the interferometric pattern obtained normal to the propagation direction to test our model. Direct comparison between experimental results and numerical simulation allows for a complete understanding of the energy transfer from the laser to the liquid and the subsequent effect on the laser propagation. Spatial self-phase modulation and propagation control from small-phase diffraction to aberration-controlled diffraction and up to diffraction oscillation are observed and explained with our modeling.

High pulse energy, low-alignment sensitivity master-oscillator power-amplifier (MOPA) systems enable portable long range laser devices. Comprehensive amplifier modelling is an essential tool in producing efficient, optimised amplfication capable of producing high pulse energies. This paper outlines the development of a large-mode, low-alignment sensitivity neodymium yttrium aluminium garnet (Nd:YAG) (MOPA) system, achieving a total output pulse energy of 265 mJ with an optical efficiency of 18%. A Q-switched diode-pumped Nd:YAG zig-zag oscillator is developed with an output pulse energy of 98 mJ and slope efficiency of 31%. Through the use of an intracavity aperture, the beam quality exhibited an M² of 4.3 and 4.6 and far field divergence of 1.3 mrad and 1.2 mrad in the horizontal and vertical, respectively. The oscillator output is amplified within a diode-pumped Nd:YAG zig-zag amplifier with a system amplification of 2.8. Comprehensive amplifier modelling based on a Frantz-Nodvik analysis is demonstrated, with the saturation characteristics suggesting a route to further energy enhancement and highlighting the necessity for amplifier modelling in high energy system design.

We demonstrate the fabrication of surface nanoscale axial photonics bottle microresonators with angstrom precision using a flame. We observe strongly unscalable behavior of the whispering gallery mode cutoff wavelengths with different radial quantum numbers along the fibre length.

A narrow spectral range between 2.3 nm and 4.4 nm wavelengths, the so-called 'water window' (WW), provides a unique opportunity for time-resolved imaging of proteins in their natural environment, as water is semi-transparent, while carbon is mostly opaque at these wavelengths. In this work we are presenting experimental developments toward high laser gain in CV ions at 4.03 nm, in a table-top device, and discuss possible application of such an x-ray laser to high-resolution microscopy, as well as generation of attosecond pulses.

We report on a 976 nm pumped erbium doped fluoride (Er:ZBLAN) fiber laser emitting at 2.8 µm with nearly 70 W of output power in a quasi-continuous wave (Q-CW) regime with the slope efficiency versus launched pump power of ∼20%. It is believed to be the highest Q-CW power ever demonstrated from an Er:ZBLAN fiber laser . We also present the results of a study demonstrating that the Er:ZBLAN fiber laser slope efficiency versus absorbed pump power increases monotonically with the pump wavelength from 19% to 29% over the wavelength range of 960–985 nm. The achieved slope efficiency of 29% is, to the best of our knowledge, the highest efficiency ever reported for an Er:ZBLAN fiber non-cascaded laser.

The simultaneous nonlinear spectral broadening and temporal cleaning of ultrashort pulses by the combination of the multipass cell (MPC) technique and nonlinear ellipse rotation are proposed and investigated with numerical simulations. The performance of the gas-filled MPC is studied at 800 and 1030 nm central wavelengths with mJ energy level. The results indicate that at least 10³ contrast enhancement is feasible with 50% internal efficiency while the beam quality is preserved during propagation. At the same time, nonlinear spectral broadening allows for a more than five-fold temporal compression. The technique is tested at 20 mJ energy and it is presumably suitable for the generation of high contrast, high energy few-cycle pulses, too.

This paper proposes a simple spectral radiance backward characterization model based on the key wavelength strategy. The color difference of spectral wavelength radiance amplitude loss under different conditions, the spectral amplitude change of primary color, and the monotonicity of spectral amplitude were investigated. The key wavelengths were selected based on these three factors. We innovatively introduced the selected wavelengths into a backward characterization interpolation table, which was tested using different color samples. The experimental results indicate that the proposed model is more suitable for spectral backward characterization than the existing methods.

We introduce a method of azimuthally invariant 3D Mueller-matrix (MM) layer-by-layer mapping of the phase and amplitude parameters of anisotropy of the partially depolarizing layers of benign (adenoma) and malignant (carcinoma) prostate tumours. The technique is based on the analysis of spatial variations of Mueller matrix invariant (MMI) of histological sections of benign (adenoma) and malignant (carcinoma) tissue samples. The phase dependence of magnitudes of the first-to-fourth order statistical moments is applied to characterize 3D spatial distributions of MMI of linear and circular birefringence and dichroism of prostate tumours. The high order statistical moments and phase sections of the optimal differentiation of the polycrystalline structure of tissue samples are revealed. The obtained results are compared with the results obtained by conventional methods utilizing polarized light, including 2D and 3D Mueller matrix imaging.

Coulomb-corrected strong-field approximation (CCSFA), which integrates Coulomb potential and the interference effects, is a semiclassical method successful in the study of atomic strong-field ionization. However, it is difficult to numerically solve the saddle-point equation describing tunnelling in the CCSFA, especially for complex laser fields such as elliptically or orthogonally polarized two-color laser fields. In this work, we propose an efficient method based on the genetic algorithm (CCSFA-GA) to overcome this difficulty. The accuracy of our method is verified by comparing our result with the calculation of the standard CCSFA on a hydrogen atom, subjected to an intense laser field. Moreover, we show that the result of the numerical solution of the time-dependent Schrödinger equation with an elliptically polarized laser field can be well reproduced by the CCSFA-GA.