Harmonic Laser Research Articles

Tunable optical nonlinearities and surface plasmon resonance in Fe3O4 nanoparticles-decorated Ti3C2 MXene composite for ultrafast fiber lasers

In this work, the Ti3C2 MXene was modified with Fe3O4 nanoparticles to obtain a composite with in-plane optical anisotropy and surface plasmon properties. Based on a side-polished D-shaped fiber (DSF), this Fe3O4/Ti3C2 MXene (F/M) composite exhibits a tunable nonlinear optical (NLO) absorption response under 1.5 μm laser excitation with different polarization states. The modulation depths can be obtained from 0.9% to 12.1%. As an application, the ultrafast erbium-doped fiber laser (EDFL) with multiple pulsed laser operations was demonstrated using the F/M-DSF device as a saturable absorber (SA). Both stable Q-switching and mode-locking operations were demonstrated in the same all-fiber laser cavity, which generated pulsed laser with microsecond and sub-picosecond durations respectively. In addition, the bound state (BS) of solitons in harmonic mode-locked lasers with the pulse time interval of 3 ps and the highest order of 30th was also realized. This versatile and simple-structured all-fiber laser has great potential in various applications, such as optical communication. With the numerical simulations, the dynamic of F/M composite-based soliton mode-locking was also explored, illustrating the mechanism of soliton formation and propagation. This work not only expands the advanced applications of F/M composite in ultrafast photonics but also provides a new perspective for designing and optimizing its ultrafast modulating performance.

Studying the Photocatalytic Characteristics of Pd Nanoparticles Created by Second Harmonic Laser Ablation

Studying the Photocatalytic Characteristics of Pd Nanoparticles Created by Second Harmonic Laser Ablation

Second harmonic generation laser scanning microscopy with intrinsic first-order modulation

The contrast or signal-to-noise ratio (SNR) in images is a crucial parameter that determines the quality of images in second harmonic generation laser scanning microscopy (SHG-LSM). With a better image contrast/SNR, SHG-LSM can be performed at a faster frame rate or deeper tissue locations without compromising image quality. In this work, we present a novel strategy based on the SHG signals of first-order modulation (1 M) to enhance the image contrast/SNR. Notably, a photodetector (i.e., photomultiplier tube) has a better signal SNR during the photon-to-electron conversion process at the frequency of 1 M, which is equivalent to the laser pulse repetition frequency. The improved signal SNR thus enhances the image quality, which agrees well with the results of the measured electrical spectra. A remarkable increment of image contrast by more than a factor of two has been obtained in images by using 1 M. In addition, the image acquisition time is also shortened by a factor of 2.5 compared to that for acquiring images with similar contrast taken without signal modulation. Furthermore, by analyzing the dependencies of image contrast/SNR on sample depth, we demonstrate that the images obtained with 1 M have better quality than those obtained without signal modulation at the same imaging depth.

Modeling of a secondharmonic spectrum in passive phase demodulation.

Spectral compression by passive phase demodulation provides an effective way to obtain a high-power single-frequency second harmonic (SH) laser. In this method, a single-frequency laser is broadened by (0, π) binary phase modulation for stimulated Brillouin scattering suppression in a high-power fiber amplifier and compressed to single frequency after frequency doubling. The effectiveness of compressing is determined by the properties of the phase modulation system, including the modulation depth, frequency response of modulation system, and modulation signal noise. A numerical model is developed to simulate the influence of these factors on the SH spectrum. The simulation results reproduce the experimental observation well, including the reduction of the compression rate at higher-frequency phase modulation, emergence of spectral sidebands and pedestal.

Angular Dispersion Boost of High Order Laser Harmonics Interacting with Dense Plasma Clusters

Angular Dispersion Boost of High Order Laser Harmonics Interacting with Dense Plasma Clusters

Single-step laser ablation synthesis of ZnO–Ag nanocomposites for broad-spectrum dye photodegradation and bacterial photoinactivation

We present the single-step production of zinc oxide (ZnO)–silver (Ag) nanocomposites using a second harmonic nanosecond laser. The Ag concentration was controlled using various concentration of AgNO3 solution as the medium. The evaluation of the structural and optical characteristics indicated the formation of ZnO–Ag nanocomposites. The resulting nanocomposites exhibited photocatalytic activity in broad-spectrum light, both UV and visible light, for the degradation of rhodamine 6G. Furthermore, the nanocomposites were assessed as antibacterial agents, and the addition of Ag improved the antibacterial activity of pure ZnO against Escherichia coli and Staphylococcus aureus. Under light exposure, Ag greatly increased S. aureus degradation through a simultaneous antibacterial process. Eventually, the nanocomposites could be anticipated as photoactivation antibacterial agent and water purification nanocatalyst applications.

Femtosecond laser Wavelength-Dependent formation of plasmonic gold nanostructures

Complex gold nanostructures fabricated on the surface of metal have a high potential for being used as plasmonic nanostructures. The precise shape and dimensions management of the structures enables accurate control of plasmonic properties. In this work, the formation of gold nanobumps, nanocones, and nanojets by laser direct writing technique was investigated using different laser wavelengths by changing harmonics. The used wavelength of laser irradiation impacts the size and shape of the formed structures. Additionally, the achievable minimal grating period, the layer modification fluence, and the formation fluence range change with the used laser harmonic. Experiments indicate that the reduction of the wavelength provides a more precise fabrication process and management of properties. The gratings of the wavelength-dependent nanostructures excite surface lattice resonances with high quality and have significant potential for practical uses.

Second harmonic in FELs: Generation, amplification and suppression

Theoretical analysis of the second harmonic generation in undulators shows physical reasons for their radiation in free electron lasers (FELs). The harmonic powers, the second in particular, are calculated with several different analytical approaches and compared with each other and with well documented experimental data for LEUTL and LCLS FELs. Various contributions to the harmonic powers are distinguished, their influence is explored. The possibility to suppress the second harmonic radiation by the second harmonic of the undulator magnetic field is shown in the example of LCLS FEL. The amplification and radiation of the second FEL harmonic in a FEL, using harmonic lasing self-seed (HLSS), is shown in the example of SwissFEL with separate harmonic buncher and amplifier.

206 nm deep ultraviolet laser generated from fifth harmonic of femtosecond fiber laser

Deep ultraviolet (DUV) femtosecond laser, which combines the advantages of high single-photon energy of DUV laser with high peak power of femtosecond laser, is widely used in scientific research, biomedicine, material processing and so on. However, in the process of generating DUV femtosecond laser based on nonlinear frequency conversion is encountered a problem that the group velocity mismatch caused by dispersion makes the temporal walk-off of the nonlinear frequency conversion larger than the pulse duration of the femtosecond laser, thus making the generation of the DUV femtosecond laser very difficult. In this work, based on a Yb-doped fiber femtosecond laser, the delay line is optimized to precisely compensate for the spatial and temporal walk-off, so DUV femtosecond laser possesses the following performances: the center wavelength is 206 nm, the repetition rate is 1 MHz, the maximum output power is 102 mW, the maximum conversion efficiency is 4.25% from near infrared to DUV, the root mean square (RMS) power stability is 0.88% within 3 h, and the peak-to-peak power stability is 3.75%. The evolution of laser spectra and beam quality in the process of second harmonic generation (SHG), fourth harmonic generation (FHG) and sum-frequency generation (SFG) are also systematically studied. The experimental results provide a basis for generating DUV femtosecond laser from femtosecond fiber lasers.

Repetition-rate-tunable second harmonic generation in mode-locked optically pumped semiconductor disk laser

We demonstrated a repetition-rate-tunable second harmonic mode-locked optically pumped semiconductor disk laser (OP-SDL), and the tuning range was from 1.168 GHz to 1.25 GHz. The entire tuning process was continuous, and OP-SDL was always in a mode-locked working state. To the best of our knowledge, this is the first work to obtain intracavity repetition rate tunable second harmonic laser in an OP-SDL. At same time, the repetition rate of fundamental wave can be continuously adjusted from 1.16 GHz to 1.66 GHz. The central wavelengths of the pulse trains were around 971 nm, and the highest repetition rate signal-to-noise ratio was more than 60 dB, indicating that the mode-locked pulses were stable.

Probing zinc oxide as a semiconductor photosensitizer of energetic materials to laser radiation

The development of methods for laser ignition of energetic materials opens up significant prospects for improving the safety of their application. The photochemical processes can increase the selective sensitivity of energetic material to laser radiation and make the most of its capabilities. The paper considers the application of zinc oxide as a photosensitizing additive capable of significantly reducing the laser ignition threshold of pentaerythritol tetranitrate (PETN). The dependencies of the laser initiation threshold by the fundamental frequency (1064 nm), second (532 nm), and third harmonic (355 nm) of an Nd:YAG laser on the concentration of zinc oxide were studied. The ignition threshold energy density is reduced by a factor from 2 to 5, depending on the oxide concentration and the laser radiation wavelength. The proposed initiation mechanisms based on the results of the several spectroscopy methods were discussed for each laser harmonic. Laser ignition of the PETN-ZnO composite through the ultraviolet laser radiation combined the thermal and photochemical mechanism. Visible laser harmonic (532 nm) drives a purely photochemical initiation of the PETN. The obtained results indicate that a mechanism of initiation by visible light should be caused by the charge transfer in the ZnO-PETN interface. The reported experimental result does not only serve as an example of the successful photochemical ignition of energetic material but also shows that processes, techniques, and materials of semiconductor photocatalysis are applicable to laser initiation.

Systematic analysis of semiconductor photoconductivity dynamics under different laser excitations: two- and three-level models

A numerical study of a model photoconductor under pulsed and continuous-wave laser excitation is presented. From the solution of a system of partial differential equations related to the populations of the conduction and valence bands, we analyze the time behavior of the photoexcited concentrations of electrons and holes where the diffusion phenomena and the presence of electric field are neglected. Additionally, for the three-level model, the population of trap levels is also introduced and analyzed as a third variable. The dynamics of electrons, holes and traps levels concentrations are studied in the presence of different types of pulse and harmonic laser photoexcitations. We show that an improved physical insight is obtained through a systematic analysis of the influence of the main physical parameters, such as generation and recombination rates, between the available energy levels.

Resonant two-photon ionization of helium atoms studied by attosecond interferometry

We study resonant two-photon ionization of helium atoms via the 1s3p, 1s4p and 1s5p1P1 states using the 15th harmonic of a titanium-sapphire laser for the excitation and a weak fraction of the laser field for the ionization. The phase of the photoelectron wavepackets is measured by an attosecond interferometric technique, using the 17th harmonic. We perform experiments with angular resolution using a velocity map imaging spectrometer and with high energy resolution using a magnetic bottle electron spectrometer. Our results are compared to calculations using the two-photon random phase approximation with exchange to account for electron correlation effects. We give an interpretation for the multiple π-rad phase jumps observed, both at and away from resonance, as well as their dependence on the emission angle.

Intensity Correlations in Quantum Cascade Laser Harmonic Frequency Combs

A novel study on harmonic frequency combs emitted by quantum cascade lasers (QCLs) is presented here, demonstrating the presence of intensity correlations between twin modes characterizing the emission spectra. These originate from a four‐wave mixing process driven by the active medium's third‐order nonlinearity. The study of such correlations is essential for the engineering of a new generation of semiconductor devices with the potential of becoming integrated emitters of light with quantum properties, such as squeezing and entanglement. Starting from experimental results, the limits of state‐of‐the‐art technology are discussed as well as the possible methodologies that could lead to the detection of nonclassical phenomena, or alternatively improve the design of QCLs, in the compelling perspective of generating quantum correlations in mid‐infrared light.

Imaging ultrafast evolution of subwavelength-sized topography using single-probe structured light microscopy

Imaging ultrafast processes in femtosecond (fs) laser–material interactions such as fs laser ablation is very important to understand the physical mechanisms involved. To achieve this goal with high resolutions in both spatial and temporal domains, a combination of optical pump–probe microscopy and structured illumination microscopy can be a promising approach, but suffers from the multiple-frame method with a phase shift that is inapplicable to irreversible ultrafast processes such as ablation. Here, we propose and build a wide-field single-probe structured light microscopy (SPSLM) to image the ultrafast three-dimensional topography evolution induced by fs lasers, where only a single imaging frame with a single structured probe pulse is required for topography reconstruction, benefiting from Fourier transform profilometry. The second harmonic of the fs laser is used as the structured probe light to improve spatial lateral resolution into the subwavelength region of ∼ 478 nm , and the spatial axial and temporal resolutions are estimated to be ∼ 22 nm and ∼ 256 fs , respectively. With SPSLM, we successfully image the ultrafast topography evolution of a silicon wafer surface impacted by single and multiple fs pulses. The variable formation and evolution of the laser induced periodic surface structures during an ultrashort time are visualized and analyzed. We believe that SPSLM will be a significant approach for revealing and understanding various ultrafast dynamics, especially in fs laser ablation and material science.

Coherent and ultrashort soft x-ray pulses from echo-enabled harmonic cascade free-electron lasers

It is a long-standing challenge for laser technologies to generate intense fully coherent pulses in the x-ray regime. Here, we demonstrate an external seeding mechanism, termed echo-enabled harmonic cascade (EEHC) for generating coherent and ultrashort soft x-ray pulses. The mechanism uses echo-enabled harmonic generation as the first stage, producing intense extreme ultraviolet pulses that seed the second stage x-ray free-electron laser (FEL) with the high-gain harmonic generation setup. Benefiting from the low sensitivity to electron-beam imperfections of EEHC, we generated high-power nearly transform-limited soft x-ray pulses. We have also demonstrated a unique feature of EEHC in generating isolated few-femtosecond-long x-ray pulses. The supreme up-frequency conversion efficiency and flexible pulse length control of this EEHC mechanism allow us to exceed the current limitations of seeded FELs while preserving the coherence of the seed. Our results are a step towards fully coherent and ultrashort x-ray lasers and could enable the extension of nonlinear optical techniques to shorter wavelengths.

GHz-level all-fiber harmonic mode-locked laser based on microfiber-assisted nonlinear multimode interference

Light-matter interaction can be strongly enhanced in tiny core fibers, and it holds significant potential for stable harmonic mode-locking at gigahertz and a higher repetition rate. In this paper, we propose and demonstrate a GHz harmonic mode-locked fiber laser based on a microfiber-assisted nonlinear multimode interference (NL-MMI), which is achieved by the enhanced mode-filtering of nonlinear self-imaging in multimode fiber with a waist diameter of 12μ m. A stable mode-locked is achieved with a repetition rate of 1.26 GHz at 1560.15 nm, corresponding to the 22nd harmonic of the fundamental frequency. Our results provide new insight into implementing high repetition frequency mode-locking fiber lasers.

Qualitative and quantitative discrimination of major elements in Chitosan (natural polymer) using laser induced breakdown spectroscopy

In this work, Laser-Induced Breakdown Spectroscopy (LIBS) is used for qualitative and quantitative analysis of Chitosan samples (natural polymer). For qualitative analysis, plasma parameters are determined, while for quantitative analysis, the initial concentration of chitosan samples is calculated. It developed a plasma plume using a second harmonic (532 nm) laser pulses, which makes the Q-Switched Nd: YAG laser compatible with delay generators and computing electronics. The component elements and their corresponding concentrations are calculated based on the strongest emission lines of the element using the line intensity ratio method and the calibration-free LIBS approach. The results obtained by both methods are compared for the effectiveness of the algorithm.In the chitosan monomer (C6H11NO4)n, four major elements (hydrogen, carbon, nitrogen and oxygen) were detected and their quantities were calculated. In addition to investigating the physical behavior of plasma in the interaction of laser beams with materials, the plasma parameters (electron number density and plasma temperature) are determined by stark-broadening profile fitting and Boltzmann plot method, respectively. The results obtained show that the LIBS can be exploited for quantitative and standard analysis of industrial grade materials for better results in applications.

Second harmonic generation, photoluminescence and laser damage threshold of L–Proline strontium chloride monohydrate: a prospective NLO tunable band gap for optoelectronic applications

Second harmonic generation, photoluminescence and laser damage threshold of L–Proline strontium chloride monohydrate: a prospective NLO tunable band gap for optoelectronic applications

Discrete Fourier Domain Mode Locked Laser for Simultaneous Dual Modal Swept Source OCT

In this paper, we propose and demonstrate a discrete Fourier domain mode locked (FDML) laser, which fully utilizes the high flexibility of temporal modulation. The FDML laser runs at the third harmonic frequency of the fiber cavity, which supports six independent swept signals in each cavity round trip. By manipulating the six swept signals, we demonstrate the ability to simultaneously generate multiple combinations of comb lines from a single FDML laser. The free spectral range (FSR) of each set of comb lines is 300 GHz but the six combs could be combined into combs with smaller FSRs. The discrete harmonic FDML laser enables a simultaneous dual modal swept source OCT with a fast imaging mode and a long imaging range mode obtained with the same set of data acquired. Such a swept source could also find applications in swept source LiDAR to realize interleaved line scanning.