Optimal Focusing Conditions Research Articles

The influence of laser focusing conditions on the direct laser acceleration of electrons

Direct laser acceleration of electrons during a high-energy, picosecond laser interaction with an underdense plasma has been demonstrated to be substantially enhanced by controlling the laser focusing geometry. Experiments using the OMEGA EP facility measured electrons accelerated to maximum energies exceeding 120 times the ponderomotive energy under certain laser focusing, pulse energy, and plasma density conditions. Two-dimensional particle-in-cell simulations show that the laser focusing conditions alter the laser field evolution, channel fields generation, and electron oscillation, all of which contribute to the final electron energies. The optimal laser focusing condition occurs when the transverse oscillation amplitude of the accelerated electron in the channel fields matches the laser beam width, resulting in efficient energy gain. Through this observation, a simple model was developed to calculate the optimal laser focal spot size in more general conditions and is validated by experimental data.

A high resolution reaction microscope with universal two-region time-focusing method.

This paper presents a novel reaction microscope designed for ion-atom collision investigations, established at the Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China. Its time-of-flight (TOF) spectrometer employs an innovative flight-time focusing method consisting of two acceleration regions, providing optimal time focusing conditions for charged fragments with diverse initial velocities. The TOF spectrometer's axis intentionally tilts by 12° relative to the ion beam direction, preventing potential obstructions from the TOF grid electrodes. The introduced focusing method allows for a flexible time-focusing TOF spectrometer design without restricting the length ratio of the two regions. In addition, this configuration in our case significantly suppresses noise on the recoil ion detector produced by residual gas in the ion beam trajectory, which is a considerable challenge in longitudinal spectrometers. In a test experiment on the single electron capture reaction involving 62.5 keV/u He2+ ions and a helium atomic beam, the recoil longitudinal momentum resolution achieved 0.068 atomic units. This novel configuration and successful test run show excellent precision for ion-atom collision studies.

Optimal focusing conditions for bright spontaneous parametric down-conversion sources

Optimal focusing conditions for bright spontaneous parametric down-conversion sources

Goos–Hänchen shift of a light beam tunable by graphene in the resonant optical tunneling structure

The structure for implementing resonant optical tunneling effect is a simple layered system of dielectrics that provides full light transmission for resonance condition, despite the presence of barrier layers partially locking light. The presence of a sharp resonant peak both for the intensity and for the spatial shift of the transmitted light beam makes such a structure promising for the creation of sensors and light control devices. This paper focuses on the spatial shift called the Goos–Hänchen shift of such a structure with interfaces of the waveguide layer coated by graphene. The effect of Goos–Hänchen shift near the resonance in this case may be controlled by small changes in the chemical potential or the Fermi energy of graphene, which can be controlled both chemically and by electrical bias. The characteristics of transmitted light beam strongly depend on the beam width for the selected optimal focusing condition.

Study on real-time z-scanning of multiple-pulse laser ablation of metal applied in roll-printed electronics

The interaction between a metal surface and multiple pulses was investigated to achieve optimal focusing conditions during ultrashort laser ablation. We report a simple theoretical expression of morphological changes in the ablated channel and demonstrate its advantage in positioning the interactive surface at the focus in real time during multiple-pulse laser ablation of a metallic material. Experimental results on the ablation depth for zinc, nickel, and copper show that the combination of a dynamic focusing system and a theoretical formula of ablation-cycle-dependent ablation depth enables one to control the shape of ablated channels. This model can be applied to a variety of high-efficiency ablation systems and may play an essential role in the development of a high-precision ablation system for the curved surfaces in highly scaled metal gravures used in printed electronics, which currently present challenges for engineers and technicians.

Ring and unimodal angular-frequency distribution of THz emission from two-color femtosecond plasma spark.

We study angular and frequency-angular distributions of the terahertz (THz) emission of the low-frequency region (0.3-3 THz) from a two-color femtosecond plasma spark experimentally and in three-dimensional numerical simulations. We investigate the dependence of the angular shapes of the THz radiation on focusing conditions and pulse durations by using two laser facilities (pulse durations 35 and 150 fs) for different focusing geometries. Our experiments and simulations show that decrease in the numerical aperture from NA ≈0.2 to NA ≈0.02 results simultaneously in (I) squeezing of the THz angular distribution and (II) formation of the bright conical emission in the THz range. The moderate focusing NA ≈0.05, which forms the relatively narrow unimodal THz angular distribution, is identified as optimal in terms of angular divergence. Numerical simulations with carrier wave resolved show that bright THz ring structures appear at the frequencies ≥2 THz for longer focuses (NA ≈0.02), while for optimal focusing conditions NA ≈0.05 the conical emission develops at THz frequencies higher than 10 THz.

Synthesis by picosecond laser ablation of ligand-free Ag and Au nanoparticles for SERS applications

The morphological and optical properties of noble metal nanoparticles prepared by picosecond laser generated plasmas in water were investigated. First, the ablation efficiency was maximized searching the optimal focusing conditions. The nanoparticle size, measured by Scanning Transmission Electron Microscopy, strongly depends on the laser fluence, keeping fixed the other deposition parameters such as the target to scanner objective distance and laser repetition frequency. STEM images indicate narrow gradients of NP sizes. Hence the optimization of ablation parameters favours a fine tuning of nanoparticles. UV-Visible spectroscopy helped to determine the appropriate laser wavelength to resonantly excite the localized surface plasmon to carry out Surface Enhanced Raman Scattering (SERS) measurements. The SERS activity of Ag and Au substrates, obtained spraying the colloids synthesized in water, was tested using crystal violet as a probe molecule. The good SERS performance, observed at excitation wavelength 785 nm, is attributed to aggregation phenomena of nanoparticles sprayed on the support.

High power, higher order ultrafast hollow Gaussian beams

We report on linear and nonlinear generation of ultrafast hollow Gaussian beams (HGBs). Using only two spiral phase plates (SPPs) having phase variation corresponding to vortex orders, l = 1 and 2, and an experimental scheme, we have generated high power, ultrafast HGBs of orders up to 3 at 1064 nm. Based on single-pass, frequency doubling of the HGBs in a 5 mm long, MgO doped, periodically poled LiNbO3 (MgO:PPLN) crystal, we have produced HGBs of average output power in excess of 250 mW at 532 nm and order as high as 6. Experimentally, we verified that the frequency doubled HGBs have orders twice those of the pump HGBs. Like the Gaussian beams, the HGBs of all orders have an optimum focusing condition for the highest conversion efficiency. On the contrary to previous reports, we observed that the propagation of the vortex beam of order, l, through a SPP corresponding to the vortex order of, –l, results in HGBs of the same order, |l|.

Frequency-Doubling of Femtosecond Pulses in “Thick” Nonlinear Crystals With Different Temporal and Spatial Walk-Off Parameters

We present a comparative study on frequency-doubling characteristics of femtosecond laser pulses in thick nonlinear crystals with different temporal and spatial walk-off parameters. Using single-pass second harmonic generation (SHG) of 260 fs pulses at 1064 nm from a high-average-power femtosecond Yb-fiber laser in 5-mm-long crystals of β-BaB 2O4 (BBO) and BiB3O6 (BIBO), we find that for comparable values of temporal and spatial walk-off parameters in each crystal, the optimum focusing condition for SHG is more strongly influenced by spatial walk-off than temporal walk-off. It is also observed that under such conditions, the Boyd and Kleinman theory commonly used to define the optimum focusing condition for frequency-doubling of cw and long-pulse lasers is also valid for SHG of ultrafast lasers. We also investigate the effect of focusing on the spectral, temporal, and spatial characteristics of the second harmonic (SH) radiation, as well as angular acceptance bandwidth for the SHG process, under different temporal and spatial walk-off conditions in the two crystals.

Kagome fiber based ultrafast laser microsurgery probe delivering micro-Joule pulse energies.

We present the development of a 5 mm, piezo-actuated, ultrafast laser scalpel for fast tissue microsurgery. Delivery of micro-Joules level energies to the tissue was made possible by a large, 31 μm, air-cored inhibited-coupling Kagome fiber. We overcome the fiber's low NA by using lenses made of high refractive index ZnS, which produced an optimal focusing condition with 0.23 NA objective. The optical design achieved a focused laser spot size of 4.5 μm diameter covering a 75 × 75 μm2 scan area in a miniaturized setting. The probe could deliver the maximum available laser power, achieving an average fluence of 7.8 J/cm2 on the tissue surface at 62% transmission efficiency. Such fluences could produce uninterrupted, 40 μm deep cuts at translational speeds of up to 5 mm/s along the tissue. We predicted that the best combination of speed and coverage exists at 8 mm/s for our conditions. The onset of nonlinear absorption in ZnS, however, limited the probe's energy delivery capabilities to 1.4 μJ for linear operation at 1.5 picosecond pulse-widths of our fiber laser. Alternatives like broadband CaF2 crystals should mitigate such nonlinear limiting behavior. Improved opto-mechanical design and appropriate material selection should allow substantially higher fluence delivery and propel such Kagome fiber-based scalpels towards clinical translation.

Optimal focusing conditions of lenses using Gaussian beams

By using the analytical equations of the propagation of Gaussian beams in which truncation exhibits negligible consequences, we describe a method that uses the value of the focal length of a focusing lens to classify its focusing performance. We show that for different distances between a laser and a focusing lens there are different planes where best focusing conditions can be obtained and we demonstrate how the value of the focal length impacts the lens focusing properties. To perform the classification we introduce the term delimiting focal length. As the value of the focal length used in wave propagation theory is nominal and difficult to measure accurately, we describe an experimental approach to calculate its value matching our analytical description. Finally, we describe possible applications of the results for characterizing Gaussian sources, for measuring focal lengths and/or alternatively for characterizing piston-like movements.

Design and Optimization of the Monochromator for the New Residual Stress Diffractometer

The monochromator for the new residual stress diffractometer at China Advanced Research Reactor (CARR) was simulated and optimized using SIMRES software. Objected the Fe (211) diffraction peak, the effect of horizontal and vertical curvature of Si (400) monochromator on Figure of Merit was studied to optimize the focusing parameters. With the optimal focusing condition, the influence of wafer thickness, number of wafers, packet height and gap of packets on performance of instrument was calculated consequently and the corresponding geometrical size was fixed. Finally, the design of the monochromator for the new residual stress diffractometer at CARR was completed.

Frequency-doubling characteristics of high-power, ultrafast vortex beams.

We report on frequency-doubling characteristics of high-power, ultrafast optical vortex beams in a nonlinear medium. Based on single-pass second-harmonic generation (SHG) of optical vortices in 1.2 mm long bismuth triborate (BIBO) crystal, we studied the effect of different parameters influencing the SHG process in generating high-power and higher-order vortices. We observed a decrease in SHG efficiency with the order, which can be attributed to the increase of the vortex beam area with order. Like a Gaussian beam, optical vortices show focusing-dependent conversion efficiency. However, under similar experimental conditions, the optimum focusing condition for optical vortices is reached at tighter focusing with orders. We observed higher angular acceptance bandwidth in the case of optical vortices than that of a Gaussian beam; however, there is no substantial change in angular acceptance bandwidth with vortex order. We also observed that in the frequency-doubling process, the topological charge has negligible or no effect in temporal and spectral properties of the beams. We have generated ultrafast vortices at 532 nm with power as much as 900 mW and order as high as 12. In addition, we have devised a novel scheme based on linear optical elements to double the order of any optical vortex at the same wavelength.

Yb-fiber-laser-based, 1.8 W average power, picosecond ultraviolet source at 266 nm.

We report a compact, stable, high-power, picosecond ultraviolet (UV) source at 266 nm based on simple single-pass two-step fourth-harmonic generation (FHG) of a mode-locked Yb-fiber laser at 79.5 MHz in LiB3O5 (LBO) and β-BaB2O4. Using a 30-mm-long LBO crystal for single-pass second-harmonic generation, we achieve up to 9.1 W of average green power at 532 nm for 16.8 W of Yb-fiber power at a conversion efficiency of 54% in 16.2 ps pulses with a TEM00 spatial profile and passive power stability better than 0.5% rms over 16 h. The generated green radiation is then used for single-pass FHG into the UV, providing as much as 1.8 W of average power at 266 nm under the optimum focusing condition in the presence of spatial walk-off, at an overall FHG conversion efficiency of ∼11%. The generated UV output exhibits passive power stability better than 4.6% rms over 1.5 h and beam pointing stability better than 84 μrad over 1 h. The UV output beam has a circularity of >80% in high beam quality with the TEM00 mode profile. To the best of our knowledge, this is the first report of picosecond UV generation at 266 nm at megahertz repetition rates.

Type-I frequency-doubling characteristics of high-power, ultrafast fiber laser in thick BIBO crystal

We report on experimental realization of optimum focusing condition for type-I second-harmonic generation (SHG) of high-power, ultrafast laser in "thick" nonlinear crystal. Using single-pass, frequency doubling of a 5 W Yb-fiber laser of pulse width ~260 fs at repetition rate of 78 MHz in a 5-mm-long bismuth triborate (BIBO) crystal we observed that the optimum focusing condition is more dependent on the birefringence of the crystal than its group-velocity mismatch (GVM). A theoretical fit to our experimental results reveals that even in the presence of GVM, the optimum focusing condition matches the theoretical model of Boyd and Kleinman, predicted for continuous-wave and long-pulse SHG. Using a focusing factor of ξ=1.16 close to the estimated optimum value of ξ=1.72 for our experimental conditions, we generated 2.25 W of green radiation of pulse width 176 fs with single-pass conversion efficiency as high as 46.5%. Our study also verifies the effect of pulse narrowing and broadening of angular phase-matching bandwidth of SHG at tighter focusing. This study signifies the advantage of SHG in "thick" crystal in controlling SH-pulse width by changing the focusing lens while accessing high conversion efficiency and broad angular phase-matching bandwidth.

On-Line Preconcentration Method Using Normal Stacking Mode and Dynamic pH Junction for the Quality Control of Herbal Medicines Containing Related Phenolic Compounds

An on-line preconcentration capillary electrophoresis (CE) methodology, which combines a normal stacking mode (NSM) with a dynamic pH junction technique, is introduced in this paper. A systematic study of the parameters (concentration and pH value of background electrolyte, composition of sample matrix, injection pressure and time, separation voltage and temperature) affecting on-line concentration of seven related phenolic compounds was investigated and optimized. Under the optimum focusing conditions, about 54-fold improvement in the detection sensitivity was obtained compared with usual hydrodynamic sample injection (0.5 psi, 5 s) without detriment in separation efficiency. In particular, the concentration limits of detection (LOD) (S/N = 3) for the phenolic compounds obtained after preconcentration were from 6.0 to 11.0 ng mL-1 with UV detection without any pretreatment procedure. The proposed method has been validated with RSD values between 1.13% and 1.98% for migration times and between 1.28% and 4.65% for peak areas. Developed NSM-dynamic pH junction method was applied for determination of related phenolic compounds in labeled M. officinalis medicinal herbal products commercialized in our country. Keywords: Capillary electrophoresis, dynamic pH junction, normal stacking mode, on-line preconcentration, phenolic compounds.

Optimal Focusing and Scaling Law for Uniform Photo-Polymerization in a Thick Medium Using a Focused UV Laser

We present a modeling study of photoinitiated polymerization in a thick polymer-absorbing medium using a focused UV laser. Transient profiles of the initiator concentration at various focusing conditions are analyzed to define the polymerization boundary. Furthermore, we demonstrate the optimal focusing conditions that yield more uniform polymerization over a larger volume than the collimated or non-optimal cases. Too much focusing with the focal length f < f* (an optimal focal length) yields a fast process; however, it provides a smaller polymerization volume at a given time than in the optimal focusing case. Finally, a scaling law is derived and shows that f* is inverse proportional to the product of the extinction coefficient and the initiator initial concentration. The scaling law provides useful guidance for the prediction of optimal conditions for photoinitiated polymerization under a focused UV laser irradiation. The focusing technique also provides a novel and unique means for obtaining uniform photo-polymerization within a limited irradiation time.

A new adaptive light beam focusing principle for scanning light stimulation systems

In this article a novel principle to achieve optimal focusing conditions or rather the smallest possible beam diameter for scanning light stimulation systems is presented. It is based on the following methodology: First, a reference point on a camera sensor is introduced where optimal focusing conditions are adjusted and the distance between the light focusing optic and the reference point is determined using a laser displacement sensor. In a second step, this displacement sensor is used to map the topography of the sample under investigation. Finally, the actual measurement is conducted, using optimal focusing conditions in each measurement point at the sample surface, that are determined by the height difference between camera sensor and the sample topography. This principle is independent of the measurement values, the optical or electrical properties of the sample, the used light source, or the selected wavelength. Furthermore, the samples can be tilted, rough, bent, or of different surface materials. In the following the principle is implemented using an optical beam induced current system, but basically it can be applied to any other scanning light stimulation system. Measurements to demonstrate its operation are shown, using a polycrystalline silicon solar cell.

Effective object planes for aberration-corrected transmission electron microscopy

In aberration-corrected transmission electron microscopy, the image contrast depends sensitively on the focus value. With the point resolution extended to an information limit of below 0.1nm, even a focus change of as small as one nanometer could give a significant change in image contrast. Therefore, it is necessary to consider in detail the optimum focus condition in order to take full advantage of aberration-correction. In this study, the thickness dependence of the minimum contrast focus has been investigated by dynamical image simulations for amorphous model structures of carbon, germanium, and tungsten, which were constructed by molecular dynamics simulations. The calculation results show that the minimum contrast focus varies with the object thickness, supporting the use of an effective object plane close to the midplane instead of the exit plane of a sample, as suggested by Bonhomme and Beorchia [J. Phys. D: Appl. Phys. 16, 705 (1983)] and Lentzen [Microscopy and Microanalysis 12, 191 (2006)]. Thus supported particles and wedge-shaped crystals with symmetrical top and bottom surfaces could be imaged at a focus condition independent of the uneven bottom face. Image simulations of crystalline samples as a function of focus and thickness show: for an object thickness of less than 10nm, the optimum focus condition is matched better if the midplane of the object, instead of the exit plane, is chosen as reference plane.

Transfer-Matrix Method for Efficient Ablation by Pulsed Laser Ablation and Nanoparticle Generation in Liquids

Comparable low nanoparticle production is a weakness of femtosecond-pulsed laser ablation in liquids, but the process ablation rate can be maximized at optimal focusing conditions and liquid levels. Refraction at the air−liquid boundary, vaporization of the liquid, self-focusing, and optical breakdown in the liquid complicate the determination of these optimal parameters. A semiempirical method has been developed, allowing an a priori determination of the appropriate experimental setup (liquid layer over the target, focal length, and lens position) for efficient ablation. The presented work can be applied with high accuracy for tightly focused beams, whereas loosely focused ultrashort lasers should be avoided to induce effective fabrication of colloids via laser ablation in liquids.