Short-wavelength Laser Research Articles

Incoherent and Collective Thomson Scattering for the Determination of Electron and Ion Properties in Low-Temperature Plasma

In this lecture an overview of applications of incoherent Thomson scattering (TS) as well as collective Thomson scattering (CTS) will be given. These are the most accurate methods for measuring the electron and ion properties, because the method is direct and non-intrusive. A CTS system based on the fundamental wavelength of a seeded Nd:YAG laser, being developed for the high density, low-temperature plasma of the linear plasma generator Magnum-PSI will be described also. The small Debye length of dense low temperature plasma enables application of CTS at relatively short laser wavelength. The combination of this CTS system and existing incoherent TS system enables determination of electron density and temperature as well as ion temperature and plasma velocity of the near surface plasma. In this lecture, the theoretical background and experimental challenges of the work will be given along with some examples that demonstrate the capabilities of such systems.

Nested-ring doping for highly efficient 1907 nm short-wavelength cladding-pumped thulium fiber lasers.

Cladding-pumped Tm-doped fiber lasers operating below 1950 nm have difficulty matching the high-efficiency, power-scalable output that can be achieved at longer wavelengths. This challenge arises due to the strong three-level behavior at short wavelengths and strong competition from higher-gain long wavelength emission. In this Letter, we demonstrate a nested-ring fiber design in which a highly doped Tm ring is embedded within a larger undoped core. The fiber is specifically tailored for highly efficient and high power short-wavelength operation (<1950nm). The nested-ring Tm fiber laser has generated 62 W of single-mode 1907 nm output with up to 65% (70%) slope efficiency with respect to launched (absorbed) pump power.

Molecular Platform for Frequency Upconversion at the Single-Photon Level

Direct detection of single photons at wavelengths beyond 2 microns under ambient conditions remains an outstanding technological challenge. One promising approach is frequency upconversion into the visible (VIS) or near-infrared (NIR) domain, where single photon detectors are readily available. Here, we propose a nanoscale solution based on a molecular optomechanical platform to up-convert photons from the far and mid-infrared (covering part of the THz gap) into the VIS-NIR domain. We perform a detailed analysis of its outgoing noise spectral density and conversion efficiency with a full quantum model. Our platform consists in doubly resonant nanoantennas focusing both the incoming long-wavelength radiation and the short-wavelength pump laser field into the same active region. There, infrared active vibrational modes are resonantly excited and couple through their Raman polarizability to the pump field. This optomechanical interaction is enhanced by the antenna and leads to the coherent transfer of the incoming low-frequency signal onto the anti-Stokes sideband of the pump laser. Our calculations demonstrate that our scheme is realizable with current technology and that optimized platforms can reach single photon sensitivity in a spectral region where this capability remains unavailable to date.

Magneto-optical properties of wider gap semiconductors ZnMnTe and ZnMnSe films prepared by MBE

The II-VI based magnetic semiconductors with a direct and wide optical bandgap are expected to show high potential for optical applications utilizing short wavelength laser diodes (LDs), such as 532-nm green and 475-nm blue LDs. We have confirmed that the Faraday rotation θF in the ZnMnTe and ZnMnSe films deposited on quartz glass (QG) and sapphire (SA) substrates by using molecular beam epitaxy (MBE) is large near the absorption edge. This paper reports the magneto-optical properties of ZnMnTe and ZnMnSe films synthesized on the QG and SA substrates, and shows the result of a direct Faraday rotation observation successfully made for the ZnMnTe films under 1.28-kHz alternating magnetic fields. The optical absorption characteristics of the ZnMnTe films grown on the SA substrates by MBE are discussed by comparing them with the optical absorption properties and photoluminescence spectra of the II-VI ZnTe parent single crystals.

Controllable formation of plasmonic gold nanoparticles by pulsed laser–induced etching

This work discusses the preparation and formation of macro porous silicon (PSi) with silicon nano pillars; synthesized via laser–induced etching. The role of laser pulse duty cycle (10–100%) was investigated for silicon nano pillars samples creation using laser diode; characterized by its short wavelength (405 nm) and high laser intensity (500 mW/cm2). Morphological and spectroscopic aspects of the Si nanocrystallite sizes and plasmonic Au–NP, surface topography, roughness and thickness of the created layer were investigated by the scanning probe microscopy, photoluminescence measurements, the analysis of (FESEM) images and (XRD) patterns. The results revealed well-regulated Si nano pillars layers after adjusting the laser duty cycle under the same laser intensity and etching time. The histogram of Si nano pillars sizes, surface roughness and altitude of nano pillars within macro PSi created at 20% laser duty cycle showed specific characteristics; due to the minimal heat accumulation within the pillars and limited probability of damaging the pillars’ morphological aspects. The Si nano pillars were employed to synthesize and control the plasmonic features of Au-NPs. The histogram of hotspot regions and plasmonic Au-NPs sizes and their specific surface areas and grain size showed well-controlled structures after adjusting the laser pulse duty cycle.

Using Laser Duty Cycles for Modifying the Performance of Au-NPs/Si Nano Column Hot Spot SERS Sensors

In the current study, the Si nano column layer via pulsed photo chemical etching with different laser pulse duty cycle 30 and 60% using short laser wavelength (405 nm) and laser intensity (100 mW/cm2) was formed and studied. Two types of Si nano column-based plasmonic Au-NP hot spot layers were synthesized and examined successfully as an efficient SERS layer for the detection of the ultra-low concentration of amoxicillin. Si nano columns exposed a great effect on the performance of the Au-NP hot spot SERS sensor showing a strong dependence on the density of the hot spot gaps within the sensitive layer. Enhancement factor (EF) of the Raman signal improved considerably with increasing the density of the hot spot gaps due to the coupling efficiency among the plasmonic Au-NPs and the molecules of amoxicillin within the hot spot regions. EF increased by about four orders of magnitude with decreasing the laser duty cycles due to the increase of the integrated plasmonic Au-NPs into the Si nano column layer.

All-fiber short-wavelength tunable mode-locked fiber laser using normal dispersion thulium-doped fiber.

We report an all-fiber high pulse energy ultrafast laser and amplifier operating at the short wavelength side of the thulium (Tm) emission band. An in-house W-type normal dispersion Tm-doped fiber (NDTDF) exhibits a bending-induced distributed short-pass filtering effect that efficiently suppresses the otherwise dominant long wavelength emission. By changing the bending diameter of the fiber, we demonstrated a tunable mode-locked Tm-doped fiber laser with a very wide tunable range of 152 nm spanning from 1740 nm to 1892 nm. Pulses at a central wavelength of 1755 nm were able to be amplified in an all-fiber configuration using the W-type NDTDF, without the use of any artificial short-pass filter or pulse stretcher. The all-fiber amplifier delivers 2.76 ps pulses with an energy of ∼32.7 nJ without pulse break-up, due to the normal dispersion nature of the gain fiber, which marks so far, the highest energy amongst fiber lasers in the 1700 nm-1800 nm region.

Phase retrieval for short wavelength orbital angular momentum beams using knife-edge diffraction

Laser beams operating in high order transverse modes, especially orbital angular momentum (OAM) modes, have been used increasingly in many frontier research areas. The characterization of these laser modes and the related component analysis can be carried out using the retrieved optical phase. However, conventional iterative phase retrieval methods, when applied to pure OAM modes, may fail because their convergence depends on the choice of the initial phase. Here, we present an improved phase retrieval method which can incorporate additional phase information obtained using knife-edge diffraction. Using no additional optics or few optics available in a wide spectral range, this method is particularly suitable for short wavelength lasers such as free-electron lasers (FELs) in the VUV, EUV, and X-ray regimes. This method has been tested using simulated beams, and successfully applied to experimentally generated beams from a HeNe laser and an FEL.

A quantum model for photoemission from metal surfaces and its comparison with the three-step model and Fowler–DuBridge model

This paper studies a quantum mechanical model for photoemission from a metal surface due to the excitation of laser electric fields, which was developed by solving the time-dependent Schrödinger equation exactly. The quantum model includes the effects of laser fields (wavelength and intensity), properties of metals (Fermi energy and work function including Schottky effect), and the applied dc field on the cathode surface. Shorter wavelength lasers can induce more photoemission from electron initial energy levels further below the Fermi level and, therefore, yield larger quantum efficiency (QE). The dc field increases QE, but it is found to have a greater impact on lasers with wavelengths close to the threshold (i.e., the corresponding photon energy is the same as the cathode work function) than on shorter wavelength lasers. The quantum model is compared with existing classical models, i.e., the three-step model, the Fowler–DuBridge model, and the Monte Carlo simulation based on the three-step model. Even though with very different settings and assumptions, it is found that the scaling of QE of the quantum model agrees well with other models for low intensity laser fields. When the laser field increases, QE increases with the laser field strength in the longer laser wavelength range due to the increased contributions from multiphoton absorption processes.

Inverse bremsstrahlung absorption in magnetized plasmas

The Magneto-Inertial Fusion (MIF) approach has the benefits of both approaches of Inertial Confinement Fusion (ICF) and Magnetic Confinement Fusion (MCF). MIF is a complete approach, as compared to others. It has the advancement of the common ICF and some advantages over MCF. It also includes high-density plasma in the presence of strong magnetic fields. In this study, we investigate the inverse bremsstrahlung absorption (IBA) in magnetized plasma and the kinetic theory. Many researchers have calculated the inverse bremsstrahlung absorption coefficient through the ICF approach. In this investigation, however, the average of IBA is obtained that can be used in the MIF approach, the kinetic theory and the Krook collision frequency. In addition, the influences of some parameters such as laser wavelength, electron temperature, and static magnetic field are shown on the mean collisional absorption. A numerical solution is also used to calculate the dispersion function and the laser electric field. IBA is one of the basic heating processes in laser fusion plasma. The results of the study show that, in critical layers, the increase of absorption is proportional to short laser wavelength, low electron temperature, but IBA varies slowly with the static magnetic field.

Depth Dependence and Keyhole Stability at Threshold, for Different Laser Welding Regimes

Depending of the laser operating parameters, several characteristic regimes of laser welding can be observed. At low welding speeds, the aspect ratio of the keyhole can be rather large with a rather vertical cylindrical shape, whereas at high welding speeds, low aspect ratios result, where only the keyhole front is mainly irradiated. For these different regimes, the dependence of the keyhole (KH) depth or the keyhole threshold, as a function of the operating parameters and material properties, is derived and their resulting scaling laws are surprisingly very similar. This approach allows us to analyze the keyhole behavior for these welding regimes, around their keyhole generation thresholds. Specific experiments confirm the occurrence and the behavior of these unstable keyholes for these conditions. Furthermore, recent experimental results can be analyzed using these approaches. Finally, this analysis allows us to define the aspect ratio range for the occurrence of this unstable behavior and to highlight the importance of laser absorptivity for this mechanism. Consequently, the use of a short wavelength laser for the reduction of these keyhole stability issues and the corresponding improvement of weld seam quality is emphasized.

Five-photon absorption upconversion lasing from on-chip whispering gallery mode.

In the progress of ultrafast optics, nonlinear interactions between light and matter are very important in scientific and technical fields. In particular, the high-order nonlinear effect induced by multi-photon absorption (MPA) upconversion lasing has injected new impetus into the research on short-wavelength laser sources. Here, we report the realization of amplified spontaneous emission (ASE) by MPA simultaneously in an epitaxy thin film. In addition, by virtue of the excellent optical confinement of cylindrical microcavities with high Q (∼4 × 103) on-chip, we demonstrated, for the first time, low-threshold upconversion lasing of five-photon absorption enhanced by a microcavity at room temperature. The resonant whispering-gallery mode (WGM) distribution in cylindrical microcavities was simulated comprehensively by the finite difference time domain (FDTD) method. We found that the high-order nonlinear optical process could be significantly enhanced in the microcavity with an increase in the lifetime of radiation photons.

Highly efficient up-conversion in Nd:Y2O3 transparent ceramics

Up-conversion (UC) luminescence properties of Nd:Y2O3transparent ceramics are investigated. It is shown that, under a laser excitation of 808 nm, the samples exhibit significant UC emission. With an increase in the Nd3+ concentration, the intensity of green emission decreases, while the intensity of red emission first increases and then decreases. In particular, both green and red UC emissions are found to be two-photon processes, but with different quantum transition mechanisms. In addition, the dependence of UC emission intensity on the pumping power is studied. Our studies indicate that Nd:Y2O3 transparent ceramics are promising candidates as short-wavelength UC laser material.

Short-wavelength luminescence of Eu3+-doped KGd(WO4)2 crystals

Monoclinic 5 at% Eu3+ doped KGd(WO4)2 (KGW) single crystal was synthesized by top-seeded solution growth method. The spectroscopic properties and especially short-wavelength emission of Eu3+:KGW were investigated. Absorption, emission and excitation spectra were studied at low (10 K) and at room temperature. The strongest observed luminescence band corresponded to 5D0 → 7F2 transition at 610 nm but also intense ultraviolet and visible fluorescence was observed under one-photon excitation. The Ωλ parameters as well as radiative lifetimes of the 5D0,1,2,3 states were calculated based upon the Judd-Ofelt formalism. Branching ratios and electric dipole transition probabilities were also determined, based upon obtained experimental results. Stimulated emission cross sections of the strongest 5D1,2,3 → 7FJ transitions were evaluated for this system. The obtained results indicated that Eu:KGW crystal could potentially be a short wavelength laser material.

Calculation of the inverse bremsstrahlung absorption in unmagnetized plasmas

AbstractInverse bremsstrahlung is one important way to deliver laser energy to the plasma in inertial confinement fusion. In this article, we study the collisional absorption rate as obtained from the Fokker–Planck treatment of an unmagnetized plasma in harmonic laser field. The electron–ion collision rate is considered in the Krook approximation, and the electron distribution function is considered a Maxwellian function. We evaluate the inverse bremsstrahlung absorption near the irradiated plasma surface in the critical layer. We observe that absorption increases with shorter laser wavelength and lower electron temperatures. When the maximum electron velocity in the limit of q → 1 reaches infinity, the q‐non‐extensive distribution function reduces to the standard Maxwell‐Boltzmann distribution.

Soft X-ray spectral analysis of laser produced molybdenum plasmas using the fundamental and second harmonics of a Nd:YAG laser.

Our measurement of the soft X-ray emission of Mo plasmas produced by picosecond Nd:YAG lasers emitting on the fundamental (1064 nm, 150 ps) and second (532 nm, 130 ps) harmonics is presented. The contrast in intensity between spectral peaks and the intensity outside them is lower for the second harmonic produced plasmas probably due to the presence more intense satellite emission and higher optical thickness. The measured spectra are absolutely calibrated and the observed output photon flux was (7 - 9) × 1013 photons/sr in the water-window (2.3 - 4.4 nm) spectral range for a laser energy of 160 mJ independent of laser wavelength. However, in the short wavelength range 1.5 - 2 nm, the emission using the second harmonic is strongly enhanced and is even higher than for the maximum energy of 220 mJ of the fundamental wavelength, so despite inevitable energy losses, laser wavelength conversion may lead to emission enhancement in certain spectral ranges. This enhancement is attributed to higher absorption of short wavelength laser light and higher charge state generation in denser plasmas.

Phase-locking mechanism in non-sequential double ionization

Here, we identify a new mechanism where the early stage of electron ionization in the field of strong femtosecond laser pulse determines the final yield of non-sequential double ionization. By using simple trajectory methods we prove that the powerful short wavelength laser field causes an injection locking between its phase and the phases of the electron trajectories, which is next responsible for the enhanced or suppressed ionization for given intensities in the knee region. It is shown that both ionization and entanglement of the final electron state can be easily controlled by introducing frequency chirp of the laser field. Our methods allow one to quantify the quantum correlations due to the different mechanisms of strong field ionization.

Au nanoparticles for SERS: Temperature-controlled nanoparticle morphologies and their Raman enhancing properties

Quasi-fractal gold nanoparticles can be synthesized via a modified and temperature controlled procedure initially used for the synthesis of star-like gold nanoparticles. The surface features of nanoparticles lead to improved enhancement of Raman scattering intensity of analyte molecules due to the increased number of sharp surface features possessing numerous localized surface plasmon resonances (LSPR). The LSPR is affected by the size and shape of surface features as well as inter-nanoparticle interactions, as these affect the oscillation modes of electrons on the nanoparticle surfaces. The effect of the particle morphologies on the localized surface plasmon resonance (LSPR) and on the surface-enhancing capabilities of these nanoparticles is explored by comparing different nanoparticle morphologies and concentrations. We show that in a fixed nanoparticle concentration regime, quasi-fractal gold nanoparticles (gold nanocaltrop) provide the highest level of surface enhancement, whereas spherical nanoparticles provide the largest enhancement in a fixed gold concentration regime. The presence of highly branched features enables these nanoparticles to couple with a laser wavelength, despite having no strong absorption band and hence no single surface plasmon resonance. This cumulative LSPR may allow these nanoparticles to be used in a variety of applications in which laser wavelength flexibility is beneficial, such as in medical imaging applications where fluorescence at short laser wavelengths may be coupled with non-fluorescing long laser wavelengths for molecular sensing.

Filamentation of long-wave infrared pulses in the atmosphere [Invited

Filamentation of a high-power laser beam in air opens the possibility of the diffraction-compensated propagation of a laser beam over long distances and as such is being considered for remote stand-off detection, lightning control, free-space communications, and long-range projection of high-energy pulses. Switching to long-wave infrared (LWIR) range for filamentation, as shown in recent experiments, allows for generation of a single centimeter-diameter channel in air that, in comparison with a short-wavelength laser filament, has 4 orders of magnitude larger cross section and guides many joules of pulse energy over multiple Rayleigh distances at a clamped intensity of ∼1012W/cm2. Self-guiding of LWIR pulses in air arises from the balance between self-focusing, diffraction, and defocusing caused by free carriers generated via many-body Coulomb-induced ionization which effectively decrease the molecular polarizability during the long-wavelength laser pulse. Understanding where this new ionization regime plays a role, below the threshold of the well-adopted single-atom tunnel ionization in gases, could become a new frontier in strong-field interactions. This paper offers an overview of the atmospheric filamentation research at long-wave infrared wavelengths.

Core-shell Au@Ag nanodendrites supported on TiO2 nanowires for blue laser beam-excited SERS-based pH sensing

The blue laser beam-excited surface-enhanced Raman scattering (SERS)-based pH sensing holds great promise for avoiding undesired thermal heating effect on some special temperature-vulnerable molecules, as compared to the vast majority studies by exciting in the red or near-infrared (NIR). Herein, we report an ingenious approach to support core-shell Au@Ag nanodendrites (NDs) on TiO2 nanowires, which can possess enhanced SERS activity under 473 nm laser excitation, owing to the improved charge-transfer effect on modified TiO2 support by inserting plasmonic Au@Ag. By using pH-indicating 4-mercaptobenzoic acid (4-MBA), the obtained TiO2/Au@Ag NDs can not only exhibit high sensitive linear-responses of pH changes ranging from pH 4.0 to 9.0 in different solutions (deionized water, NaCl, CaCl2, and MgCl2) but also provide excellent temperature stability under 4°C, 25°C and 37°C temperatures as well as good time stability after storage for 10 days. The established SERS-pH sensing by using shorter wavelength laser excitation is highly desirable for understanding physiological process in temperature-vulnerable microenvironment.