Fundamental Laser Beam Research Articles

Flattop axial Bessel beam propagation with analytical form of the phase retardation function

This work focuses on a novel, to the best of our knowledge, analytical form of the phase retardation function for achieving a uniform axial intensity of Bessel beams. Traditional methods of generating Bessel beams often result in significant oscillations in the intensity along the beam’s axial path, which limits their practical applications. However, the proposed phase retardation function in this study overcomes these limitations by ensuring consistent beam creation regardless of factors such as the beam waist size, wavelength, or axicon angle. By implementing the proposed spatial phase function, a fundamental Gaussian laser beam, thereby generating a Bessel beam with an elongated and constant axial intensity profile, supports our theoretical predictions. The functionality of this new phase retardation function was further scrutinized using different wavelengths and beam waist sizes to confirm that the axial intensity remained uniform profile. Additionally, when contrasting our phase function with those from earlier researches, it was observed that our findings are consistent with both theoretical models and experimental outcomes.

Synthesis and study of nonlinear optical properties of an enaminone derived from dibenzoylmethane and N,N-diethylaminoaniline

The compound, (Z)-3-((4-(diethylamino)phenyl)amino)-1,3-diphenylprop-2-en-1-one, is synthesized by the reaction of dibenzoylmethane and 4-N,N-diethylaniline. The relative stabilities of the possible tautomers of the molecule are studied via the DFT B3LYP-D3BJ, CAM-B3LYP, M062X, and ωB97XD functionals in conjunction with the 6-311++G(d,p) basis set. The results showed that the enaminone tautomer with the intramolecularly hydrogen bonded chelated ring is the most stable. This is further confirmed by the Car–Parrinello MD calculations in the gas phase as well as the NCI analysis. The electronic spectrum is calculated by the TD DFT B3LYP/c-pVDZ level in ethanol, and the hole–electron analysis is carried out for the interpretation of the bands, which revealed that the longest one at 430 nm is of charge transfer origin while the others are of local transition origin. Atoms-in-molecules calculations in several media and levels of theory predicted that the ρBCP at the hydrogen bond in the gas phase to be 0.03791–0.04255 e/a0 3 which is a characteristic of a medium strong hydrogen bond. Researchers investigated the enaminone’s nonlinear optical (NLO) characteristics when it was exposed to a low power (<1 Watt), single fundamental transverse mode laser beam at 473 nm. By using diffraction patterns (DPs) and Z-scan methods, we calculated the nonlinear refractive index (NLRI) of the enaminone up to 4.597 × 10−11 m2 W−1 using DPs. The resulting DPs are numerically investigated using the Fraunhofer (F.) approximation and the Fresnel-Kirchhoff (F.K.) diffraction integral, showing excellent agreement with experimental findings. We successfully explored all-optical switching (AOS) in enaminone using two laser beams.

Quasi-Bessel beam generation by a diffractive axicon with an exponential phase function

A simple phase function for a diffractive axicon to allow generation of a quasi-Bessel beam with a uniform axial intensity profile is reported. By adding the new exponential phase function for the diffractive axicon, we theoretically show that a quasi-Bessel beam with a uniform axial intensity profile can be generated without amplitude oscillation or energy loss. We apply this spatial phase function to a spatial light modulator, illuminated by a fundamental Gaussian laser beam, and produce a quasi-Bessel beam with a long, uniform axial intensity profile, which is in very good agreement with theoretical calculation. Dependence of the phase parameters on the axial intensity profile and their optimization are discussed.

Intracavity frequency-doubled external-cavity surface-emitting blue laser with high conversion efficiency

Blue laser with high power and high beam quality has many applications such as in laser display, underwater communication and imaging, and non-ferrous metal processing. Optically pumped external-cavity surface-emitting laser combines the advantages of both surface-emitting semiconductor lasers and solid-state disk lasers, and can produce high output power and good beam quality simultaneously. Its high intracavity circulating power is more conducive to intracavity frequency doubling, achieving high-power and high beam quality blue light through fundamental laser in the near-infrared waveband. This paper reports an efficient intracavity frequency doubled 490 nm high power blue light by using a 980 nm fundamental laser in an external-cavity surface-emitting laser. The V-type resonant cavity is formed by the high reflectivity distributed Bragg reflector (DBR) at the bottom of gain chip, a folded flat concave mirror (high reflectivity coated for 980 nm and anti-reflectivity coated for 490 nm), and a flat concave end mirror (high reflectivity coated for 980 nm and 490 nm). By inserting a nonlinear crystal LBO into the cavity at the beam waist formed by the folded mirror and end mirror, and employing a birefringent filter (BRF) to polarize the fundamental laser and narrow the linewidth of the laser, a high power and high beam quality blue laser with high conversion efficiency is obtained. The effects of different factors including the length of nonlinear crystal, the linewidth of fundamental laser, and the compensation of walk off angle on the output power of the blue laser are studied experimentally. The length of the nonlinear crystal is optimized based on the size of the fundamental laser beam waist at the position of the crystal in the resonant cavity. Under the type-I phase matching condition of LBO, over 6 W output power at 491 nm wavelength is obtained when the crystal length is 5 mm and the BRF thickness is 1 mm. The beam quality &lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; factor in the &lt;i&gt;x&lt;/i&gt; direction and the &lt;i&gt;y&lt;/i&gt; direction are both 1.08, and the conversion efficiency of frequency doubling is 63%. The experimental results also show that symmetrically placed nonlinear crystals can compensate for the walk-off angle during frequency doubling to a certain extent, thereby clearly improving the conversion efficiency of the frequency doubled blue laser.

Controlling LIPSS formation on Ni surface using near-infrared laser beam and its low-order harmonics

Ultrafast laser ablation of metal surfaces is a common physical process used to generate periodic surface structures with tuneable multifunctionality for various applications in science and technology. In this work, the formation of femtosecond laser-induced periodic surface structures (LIPSSs) is investigated with 50 kHz repetition rate laser of 40 fs pulse duration and a central wavelength () along with its second () and third ) harmonic components. The variation in the LIPSS periodicity is demonstrated by changing the wavelength of the ablating laser pulses. LIPSS with periodicity ∼0.85 m at the fundamental ablating pulses, ∼0.42 m periodicity at the second harmonic, and ∼0.35 m periodicity at the third harmonic were observed. The third harmonic radiation was generated in an air-filament with minimal temporal delay with respect to the fundamental driving pulses to structure Nickel surfaces at ambient conditions. The approach demonstrates the potential for generating very fine LIPSS on materials’ surfaces using the efficient high-order harmonics of an infrared fundamental laser beam.

Simultaneous Second Harmonic Generation and multiphoton excited photoluminescence in anatase TiO2 nano powders

Second Harmonic Generation (SHG) and Multiphoton Excited Photoluminescence (MEPL) are observed in retro-reflection mode from a 22 nm diameter anatase TiO2 nanoparticle powder. Depth intensity profiles are obtained by scanning the fundamental beam focal point through the TiO2 powder surface into its volume for the energy of the incident photons tuned around the anatase TiO2 band-gap energy of 3.2 eV. Evidence for the appearance of both SHG and MEPL is presented, their relative contribution depending on the conditions of photo-excitation, in particular the fundamental wavelength and the focal point depth location. MEPL dominates at fundamental photon energies matching the band-gap energy whereas below and above the band-gap SHG is clearly seen. Depth intensity profiles exhibiting a rising edge determined by the fundamental laser beam focusing conditions and a decreasing edge determined by extinction and multiphoton scattering strongly differ between SHG and MEPL, SHG profiles being by far narrower than that of MEPL. Re-absorption of the emitted SHG and MEPL light occurs due to the spectral proximity with the TiO2 band gap energy but excitation with fundamental light below band-gap further suggests that the intrinsic difference between the two SHG and MEPL processes is responsible for the contrasted depth profiles. This combined study where both SHG and MEPL occur presents perspectives in the study of the photonic properties of powder materials.

Water-Based Coherent Detection of Broadband Terahertz Pulses.

Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1-18THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.

Rotational Doppler shift from a rotating rod.

This Letter reports on a rotational Doppler effect obtained from a rotating rod illuminated by a fundamental Gaussian laser beam. More specifically, we decompose the transmitted light behind the rotating rod into Laguerre-Gaussian modes and investigate the associated frequency shifts. The main contributing modes correspond to modes having the same rotational symmetry as the rotating object. Furthermore, their shifts equal the topological charge of the beam times the rotational frequency of the object. Potential applications in pattern recognition and rotation identification are then considered.

Noncollinear matched broadband third-harmonic generation based on angular dispersion

Broadband laser can effectively reduce the nonlinear effect in the process of laser plasma interaction. This paper proposes a noncollinear matching broadband third-harmonic generation based on angular dispersion, which uses the noncollinear sum frequency of the broadband fundamental wave and the narrowband second harmonic to generate broadband third harmonic, and the sum frequency process is realized by a specially designed gradient grating. The fundamental laser beams of different frequencies are incident at a specific angle, which compensates for the phase mismatch caused by the wavelength difference, so that the whole waveband meets the match condition of phase. Theoretical simulation shows that using KDP crystal type II phase matching, high efficiency broadband third-harmonic generation can be achieved by combining the broad-band fundamental wave (center wavelength 1058 nm, bandwidth 10 nm) and the second harmonic (526.5 nm).

Laser Beam Profile Shaping for Lidars With Diffraction Grating.(Dept.E)

A laser beam profile shaper was designed. It transforms a fundamental mode laser beam profile into flat top profile at a local plane. The shaper uses an ion-exchanged diffraction grating. The performance was tested and a good agreement between theoretical and experimental results was obtained.

The new single crystal dispersion interferometer installed on KSTAR and its first measurement.

Dispersion interferometers have been used to measure line integrated electron densities from many fusion devices. To optically suppress noise due to mechanical vibrations, a conventional dispersion interferometer typically uses two nonlinear crystals located before and after the plasma along the laser beam path. Due to the long beam path, it can be difficult to overlap the fundamental and second harmonic laser beams for a heterodyne dispersion interferometer and to focus the beams on the second nonlinear crystal located after the plasma, especially when the aperture of the nonlinear crystal is small, i.e., of the order of mm. To overcome such difficulties, a new concept of a heterodyne dispersion interferometer, a single crystal dispersion interferometer (SCDI), is developed and installed on KSTAR with the laser wavelength of 1064nm. The concept and the optical setup of the KSTAR SCDI are discussed, as well as its first measurement during a shattered pellet injection that produces abrupt and large changes in the electron density. To demonstrate feasibility, the KSTAR SCDI measurements are also compared with those from the existing two-color interferometer.

Intense high-order harmonic vector beams from relativistic plasma mirrors

Vector beam, with a spatial nonuniform polarization distribution, is important for many applications due to its unique field characteristics and novel effects when interacting with matter. Here through three-dimensional particle-in-cell simulations, we demonstrate that intense vector beams in the extreme-ultraviolet to x-ray spectral region can be generated by means of high harmonic generation (HHG) in the relativistic regime. The vector features of the fundamental laser beam can be transferred to the higher frequency emission coherently during the extreme nonlinear HHG dynamics from relativistic plasma mirrors. The vector harmonic beams can be synthesized into attosecond vector beams. It is also possible to generate vector harmonic beam carrying orbital angular momentum. Such bright vortices and vector light sources present new opportunities in various applications such as imaging with high spatial and temporal resolution, ultrafast magnetic spectroscopy, and particle manipulation.

Investigation of multiphoton pumped stimulated emission in semiconductor material using femtosecond two pulses induced stimulated emission technique

In this paper we introduce novel time-resolved photoluminescence spectroscopy called femtosecond two pulses induced stimulated emission spectroscopy, in which the generated nonlinear photoluminescence signal was used to identify the ultrafast dynamics of stimulated emission process in the semiconductor material. By employing two collinear fundamental femtosecond laser beams, we systematically investigated time-resolved multiphoton-induced amplified spontaneous emission in ZnO single crystal. Our results demonstrate the feasibility of this technique with the advantage of easy setup and high time resolution.

Non-linear optical study of hierarchical 3D Al doped ZnO nanosheet arrays deposited by successive ionic adsorption and reaction method

Successive ionic layer adsorption and reaction (SILAR) method is based on the adsorption and reaction of the ions in the cationic solution and the ionic solution, respectively. This method is simple, inexpensive, large-scale deposition, effective way for deposition on 3D substrates, low-temperature process and represents an easy way for the preparation of doped, composite and heterojunction materials. To take advantage of this method and the ZnO nanostructures, various parameters have been optimized.Undoped and Aluminum (Al) doped ZnO nanostructures were prepared by the SILAR technique. The characterization of the nanostructures prepared was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoemission spectroscopy (XPS), UV–Vis spectrometry and nonlinear optical analysis (NLO). The structural, compositional and optical properties confirm the introduction of Al3+ ions into the ZnO matrix. As a result, an enhancement of the crystallinity, enhancement of the light absorption and a change in the morphology of the nanostructures were observed.The laser stimulated nonlinear optical effects of the second and third harmonic generation were done using a fundamental laser beam. The laser stimulated NLO values obtained are at least 10% higher than the doped ZnO nanomaterials synthesized by other methods using the same set-up.

High Average Power Diode-Side-Pumped Intracavity Terahertz Parametric Source Based on Stimulated Polariton Scattering in RbTiOPO4 Crystal

This paper reports an intracavity terahertz parametric source based on the stimulated polariton scattering in RbTiOPO4 crystal with a high repetition rate and a high average power. The side pumping for the gain medium by laser diodes is adopted to get a higher fundamental power and a larger fundamental laser beam size than those in the diode-end-pumping in order to improve the THz output power. A non-collinear convex-plane fundamental cavity is adopted so that the thermal effect can be offset in some degree and the fundamental beam size is further increased. The obtained maximum average THz output power is 367 μW at 3.88 THz when the diode pump power is 105 W and the pulse repetition frequency (PRF) is 7 kHz. The terahertz power of 367 μW is the highest ever obtained in SPS sources.

Detection of voltage pulse width effect on charge accumulation in PSCs using EFISHG measurement

We report the effect of applied pulse width on charge accumulation in triple cation perovskite solar cells (PSCs) during the Electric Field Induced Second Harmonic Generation (EFISHG) measurements. The PSCs were prepared in the n-i-p structure using the layer-by-layer deposition technique. The triple cation perovskite precursor containing FAPbI3, MAPbBr3, and CsPbI3 was used to make the perovskite layer on the FTO/c-TiO2/mTiO2 substrates. Spiro OMeTAD was used as a hole transport material over the perovskite active layer, followed by the deposition of Au electrodes. The average power conversion efficiency (PCE) of the fabricated PSCs was 19.17 ± 0.02%. The electric field in the PSCs was selectively probed by the fundamental laser beam wavelength of 960 nm during the EFISHG measurements. Two different pulse widths (100 µs and 50 ms) of the applied potential were used in this study, with consideration of electrode charging and interface charging of PSCs. It has been observed that the pulse width of the applied electric field has a significant effect on the charge accumulation in the PSCs. Using the transient EFISHG measurements, it has also been found that the charging & discharging processes reversibly happen. However, the charging & discharging processes are influenced by remained charges at the interface in PSCs in the investigated triple cations PSCs. The EFISHG results can be well explained by using the Maxwell–Wagner model.

Optimization of a double-pass scheme for thermal de-phasing compensation in high-power second-harmonic generation

The performance of a double-pass scheme is theoretically investigated for the efficiency enhancement of a second-harmonic (SH) beam generated by using a high-power fundamental laser beam. Based on a modified version of coupled equations that include the possible effects leading to the thermal de-phasing, the Collins integral is used for optimizing the focusing optics to obtain maximum efficiency of SH conversion. We found through simulation that at a fundamental power of 40 W, when the focusing regime is set for the loose condition, a conversion efficiency of 74% can be reached. It is possible if the focusing points of the first and second passes are designed to be located at 19 mm and 6 mm away from the input face of the crystal, respectively.

Vectorizing the spatial structure of high-harmonic radiation from gas

Strong field laser physics has primarily been concerned with controlling beams in time while keeping their spatial profiles invariant. In the case of high harmonic generation, the harmonic beam is the result of the coherent superposition of atomic dipole emissions. Therefore, fundamental beams can be tailored in space, and their spatial characteristics will be imparted onto the harmonics. Here we produce high harmonics using a space-varying polarized fundamental laser beam, which we refer to as a vector beam. By exploiting the natural evolution of a vector beam as it propagates, we convert the fundamental beam into high harmonic radiation at its focus where the polarization is primarily linear. This evolution results in circularly polarized high harmonics in the far field. Such beams will be important for ultrafast probing of magnetic materials.

Comparative study on the paraxial approximation errors of tightly focused fundamental Gaussian beams

When the beam waist size is on the order of or smaller than a wavelength, the traditional Gaussian solution exhibits noticeable errors in describing a tightly focused laser beam of the fundamental mode. The spatial distributions of the paraxial approximate errors of the Gaussian solution to the paraxial Helmholtz equation, as compared with Sapozhnikov’s exact solution to the scalar Helmholtz equation, are illustrated, tabulated, and discussed systematically. The maximum and average errors of the Gaussian solution are presented for a list of different specified waist sizes of laser beams. We found that the maximum error distribution is of an annular structure and contains the largest error ring on the waist plane. The laser beam waist should be >1.2 wavelengths for a 1% error, while the beam waist should be >3.5 wavelengths for a 0.1% error. This study provides a useful criterion to ascertain whether the traditional Gaussian solution or the exact solution is to be used to describe the fundamental laser beam in optical engineering. The results exhibit potential applications in micronano technology, near-field optical technology, and other fields where tightly focused laser beams are utilized.

Stability in 3D and 2D/3D hybrid perovskite solar cells studied by EFISHG and IS techniques under light and heat soaking

Abstract We investigate the effect of temperature and light soaking on the stability and charge carrier behavior in triple cation 3-Dimensional (3D) and 2-Dimensional (2D)/3-Dimensional perovskite solar cells (PSCs) using electric field induced second harmonic generation (EFISHG) and impedance spectroscopy (IS) techniques. To prepare the 2D/3D perovskite structures, a layer-by-layer deposition method was employed. Spiro-OMeTAD based hole transport layer was deposited on the top of the perovskite layer using a spin coating procedure. The average power conversion efficiency (PCE) of the fresh fabricated cells was ∼18%. The electric fields in the PSCs were selectively probed by the EFISHG measurements, using the fundamental laser beam wavelength of 960 nm. The results show that the EFISHG technique could be potentially used to identify the degradation mechanism in the PSCs. Also, the IS technique was performed to investigate the electrical parameters affected under light and heat soaking environment. The 2D/3D hybrid PSCs show relatively good stability and better efficiency as compared to 3D PSCs.