Showing papers on "Laser published in 2018"

Topological insulator laser: Theory

Abstract: INTRODUCTION Topological insulators emerged in condensed matter physics and constitute a new phase of matter, with insulating bulk and robust edge conductance that is immune to imperfections and disorder To date, topological protection is known to be a ubiquitous phenomenon, occurring in many physical settings, ranging from photonics and cold atoms to acoustic, mechanical, and elastic systems So far, however, most of these studies were carried out in entirely passive, linear, and conservative settings RATIONALE We propose topological insulator lasers: lasers whose lasing mode exhibits topologically protected transport without magnetic fields Extending topological physics to lasers is far from natural In fact, lasers are built on foundations that are seemingly inconsistent with the essence of topological insulators: They require gain (and thus are non-Hermitian), they are nonlinear entities because the gain must be saturable, and they are open systems because they emit light These properties, common to all lasers, cast major doubts on the possibility of harnessing topological features to make a topological insulator laser Despite this common mindset, we show that the use of topological properties leads to highly efficient lasers, robust to defects and disorder, with single-mode lasing even at conditions high above the laser threshold RESULTS We demonstrate that topological insulator lasers are theoretically possible and experimentally feasible We consider two configurations involving planar arrays of coupled active resonators The first is based on the Haldane model, archetypical for topological systems The second model, geared toward experiment, constitutes an aperiodic array architecture creating an artificial magnetic field We show that by introducing saturable gain and loss, it is possible to make these systems lase in a topological edge state In this way, the lasing mode exhibits topologically protected transport; the light propagates unidirectionally along the edges of the cavity, immune to scattering and disorder, unaffected by the shape of the edges Moreover, we show that the underlying topological properties not only make the system robust to fabrication and operational disorder and defects, they also lead to a highly efficient single-mode lasing that remains single-mode even at gain values high above the laser threshold The figure describes the geometry and features of a topological insulator laser based on the Haldane model while adding saturable gain, loss, and an output port The cavity is a planar honeycomb lattice of coupled microring resonators, pumped at the perimeter with a lossy interior We show that under these conditions, lasing occurs at the topological edge mode, which has unidirectional flux and is extended around the perimeter with almost-uniform intensity The topological cavities exhibit higher efficiency than the trivial cavity, even under strong disorder For the topological laser with a small gap, the topological protection holds as long as the disorder level is smaller than the gap size DISCUSSION The concept of the topological insulator laser alters current understanding of the interplay between disorder and lasing, and opens exciting possibilities at the interface of topological physics and laser science, such as topologically protected transport in systems with gain We show here that the laser system based on the archetypal Haldane model exhibits topologically protected transport, with features similar to those of its passive counterpart This behavior means that this system is likely to have topological invariants, despite the nonhermiticity Technologically, the topological insulator laser offers an avenue to make many semiconductor lasers operate as one single-mode high-power laser The topological insulator laser constructed from an aperiodic array of resonators was realized experimentally in an all-dielectric platform, as described in the accompanying experimental paper by Bandres et al

Battery-operated integrated frequency comb generator.

Abstract: Optical frequency combs are broadband sources that offer mutually coherent, equidistant spectral lines with unprecedented precision in frequency and timing for an array of applications1. Frequency combs generated in microresonators through the Kerr nonlinearity require a single-frequency pump laser and have the potential to provide highly compact, scalable and power-efficient devices2,3. Here we demonstrate a device—a laser-integrated Kerr frequency comb generator—that fulfils this potential through use of extremely low-loss silicon nitride waveguides that form both the microresonator and an integrated laser cavity. Our device generates low-noise soliton-mode-locked combs with a repetition rate of 194 gigahertz at wavelengths near 1,550 nanometres using only 98 milliwatts of electrical pump power. The dual-cavity configuration that we use combines the laser and microresonator, demonstrating the flexibility afforded by close integration of these components, and together with the ultra low power consumption should enable production of highly portable and robust frequency and timing references, sensors and signal sources. This chip-based integration of microresonators and lasers should also provide tools with which to investigate the dynamics of comb and soliton generation. Integrating an optical Kerr frequency comb source with an electronically excited laser pump produces a battery-powered comb generator that does not require external lasers, moveable optics or laboratory set-ups.

Soliton microcomb range measurement.

Abstract: Laser-based range measurement systems are important in many application areas, including autonomous vehicles, robotics, manufacturing, formation flying of satellites, and basic science. Coherent laser ranging systems using dual-frequency combs provide an unprecedented combination of long range, high precision, and fast update rate. We report dual-comb distance measurement using chip-based soliton microcombs. A single pump laser was used to generate dual-frequency combs within a single microresonator as counterpropagating solitons. We demonstrated time-of-flight measurement with 200-nanometer precision at an averaging time of 500 milliseconds within a range ambiguity of 16 millimeters. Measurements at distances up to 25 meters with much lower precision were also performed. Our chip-based source is an important step toward miniature dual-comb laser ranging systems that are suitable for photonic integration.

Near-100 MeV protons via a laser-driven transparency-enhanced hybrid acceleration scheme

Abstract: The range of potential applications of compact laser-plasma ion sources motivates the development of new acceleration schemes to increase achievable ion energies and conversion efficiencies. Whilst the evolving nature of laser-plasma interactions can limit the effectiveness of individual acceleration mechanisms, it can also enable the development of hybrid schemes, allowing additional degrees of control on the properties of the resulting ion beam. Here we report on an experimental demonstration of efficient proton acceleration to energies exceeding 94 MeV via a hybrid scheme of radiation pressure-sheath acceleration in an ultrathin foil irradiated by a linearly polarised laser pulse. This occurs via a double-peaked electrostatic field structure, which, at an optimum foil thickness, is significantly enhanced by relativistic transparency and an associated jet of super-thermal electrons. The range of parameters over which this hybrid scenario occurs is discussed and implications for ion acceleration driven by next-generation, multi-petawatt laser facilities are explored.

Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam

Abstract: The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today's lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (epsilon > 500 MeV) with an intense laser pulse (a(0) > 10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (gamma rays), consistent with a quantum description of radiation reaction. The generated gamma rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy epsilon(crit) > 30 MeV.

Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

Abstract: During femtosecond laser fabrication, photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions is of picosecond order. Hence, lattice motion is negligible within the femtosecond pulse duration, whereas femtosecond photon-electron interactions dominate the entire fabrication process. Therefore, femtosecond laser fabrication must be improved by controlling localized transient electron dynamics, which poses a challenge for measuring and controlling at the electron level during fabrication processes. Pump-probe spectroscopy presents a viable solution, which can be used to observe electron dynamics during a chemical reaction. In fact, femtosecond pulse durations are shorter than many physical/chemical characteristic times, which permits manipulating, adjusting, or interfering with electron dynamics. Hence, we proposed to control localized transient electron dynamics by temporally or spatially shaping femtosecond pulses, and further to modify localized transient materials properties, and then to adjust material phase change, and eventually to implement a novel fabrication method. This review covers our progresses over the past decade regarding electrons dynamics control (EDC) by shaping femtosecond laser pulses in micro/nanomanufacturing: (1) Theoretical models were developed to prove EDC feasibility and reveal its mechanisms; (2) on the basis of the theoretical predictions, many experiments are conducted to validate our EDC-based femtosecond laser fabrication method. Seven examples are reported, which proves that the proposed method can significantly improve fabrication precision, quality, throughput and repeatability and effectively control micro/nanoscale structures; (3) a multiscale measurement system was proposed and developed to study the fundamentals of EDC from the femtosecond scale to the nanosecond scale and to the millisecond scale; and (4) As an example of practical applications, our method was employed to fabricate some key structures in one of the 16 Chinese National S&T Major Projects, for which electron dynamics were measured using our multiscale measurement system.

Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser

Abstract: Substantial energy loss in an electron beam passing through a high-intensity laser provides clear evidence of the radiation reaction, shedding light on how electrons interact with extreme electromagnetic fields.

Mid-infrared frequency comb via coherent dispersive wave generation in silicon nitride nanophotonic waveguides

Abstract: Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on the one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Here, we demonstrate a simple, yet effective, method for the direct generation of mid-infrared optical frequency combs in the region from 2.5 to 4.0 μm (that is, 2,500–4,000 cm−1), covering a large fraction of the functional group region, from a conventional and compact erbium-fibre-based femtosecond laser in the telecommunication band (that is, 1.55 μm). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in an unprecedented large-cross-section silicon nitride (Si3N4) waveguide with the dispersion lithographically engineered. The long-wavelength dispersive wave can perform as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such an approach can be considered as an alternative option to mid-infrared frequency comb generation. Moreover, it has the potential to realize compact dual-comb spectrometers. The generated combs also have a fine teeth-spacing, making them suitable for gas-phase analysis. The direct generation of mid-infrared optical frequency combs in the region 2.5–4.0 μm from a 1.55-μm conventional and compact erbium-fibre-based femtosecond laser is demonstrated via coherent dispersive wave generation in silicon nitride nanophotonic waveguides.

Experimental demonstration of a three-dimensional lithium niobate nonlinear photonic crystal

Abstract: A nonlinear photonic crystal (NPC)1 possesses space-dependent second-order nonlinear coefficients, which can effectively control nonlinear optical interactions through quasi-phase matching2. Lithium niobate (LiNbO3) crystal is one of the most popular materials from which to fabricate NPC structures because of its excellent nonlinear optical properties3–5. One- and two-dimensional LiNbO3 NPCs have been widely utilized in laser frequency conversion6,7, spatial light modulation8–12 and nonlinear optical imaging13,14. However, limited by traditional poling methods, the experimental realization of three-dimensional (3D) NPCs remains one of the greatest challenges in the field of nonlinear optics1,15. Here, we present an experimental demonstration of a 3D LiNbO3 NPC by using a femtosecond laser to selectively erase the nonlinear coefficients in a LiNbO3 crystal16,17. The effective conversion efficiency is comparable to that of typical quasi-phase-matching processes. Such a 3D LiNbO3 NPC provides a promising platform for future nonlinear optical studies based on its unique ability to control nonlinear interacting waves in 3D configuration. By selectively erasing the nonlinear coefficients in a lithium niobate crystal using a femtosecond laser, a 3D nonlinear photonic crystal, with an effective conversion efficiency comparable to that of the typical quasi-phase-matching processes, is demonstrated.

A silicon Brillouin laser.

Abstract: Brillouin laser oscillators offer powerful and flexible dynamics as the basis for mode-locked lasers, microwave oscillators, and optical gyroscopes in a variety of optical systems However, Brillouin interactions are markedly weak in conventional silicon photonic waveguides, stifling progress toward silicon-based Brillouin lasers The recent advent of hybrid photonic-phononic waveguides has revealed Brillouin interactions to be one of the strongest and most tailorable nonlinearities in silicon In this study, we have harnessed these engineered nonlinearities to demonstrate Brillouin lasing in silicon Moreover, we show that this silicon-based Brillouin laser enters a regime of dynamics in which optical self-oscillation produces phonon linewidth narrowing Our results provide a platform to develop a range of applications for monolithic integration within silicon photonic circuits

2D Ruddlesden-Popper Perovskites Microring Laser Array

Abstract: 3D organic-inorganic hybrid perovskites have featured high gain coefficients through the electron-hole plasma stimulated emission mechanism, while their 2D counterparts of Ruddlesden-Popper perovskites (RPPs) exhibit strongly bound electron-hole pairs (excitons) at room temperature. High-performance solar cells and light-emitting diodes (LEDs) are reported based on 2D RPPs, whereas light-amplification devices remain largely unexplored. Here, it is demonstrated that ultrafast energy transfer along cascade quantum well (QW) structures in 2D RPPs concentrates photogenerated carriers on the lowest-bandgap QW state, at which population inversion can be readily established enabling room-temperature amplified spontaneous emission and lasing. Gain coefficients measured for 2D RPP thin-films (≈100 nm in thickness) are found about at least four times larger than those for their 3D counterparts. High-density large-area microring arrays of 2D RPPs are fabricated as whispering-gallery-mode lasers, which exhibit high quality factor (Q ≈ 2600), identical optical modes, and similarly low lasing thresholds, allowing them to be ignited simultaneously as a laser array. The findings reveal that 2D RPPs are excellent solution-processed gain materials potentially for achieving electrically driven lasers and ideally for on-chip integration of nanophotonics.

Frontiers of Mid-IR Lasers Based on Transition Metal Doped Chalcogenides

Abstract: Enabling broad tunability, high peak and average power, ultrashort pulse duration, and all known modes of laser operation—transition-metal (TM)-doped II–VI chalcogenides are the materials of choice for direct lasing in the mid-IR. The host materials feature broad infrared transparency, high thermal conductivity, low phonon cutoff, low optical losses, and are available as either single crystals or polycrystalline ceramics. Doped with TM ions, these media exhibit a four-level energy structure, the absence of excited state absorption, as well as broad absorption and emission bands. Doped single-crystals of high optical quality are difficult to grow; however, the advent of postgrowth diffusion doped ceramics has resulted in significant progress in laser development. Here, we summarize recent experimental laser results on Cr and Fe doped II–VI chalcogenides providing access to the 1.8–6 μm spectral range with a high (>60%) efficiency, multi-Watt-level [140 W in continuous wave (CW)] output powers, tunability of >1000 nm, short-pulse (<16 fs) multi-Watt oscillation, and multi-Joule output energies in free running and gain-switched regimes. We also review recent results on hybrid fiber-bulk (Er-fiber/Er:YAG, Tm-fiber: Ho:YAG/YLF) systems combining high efficiency of CW fiber lasers with high pulse energies of bulk materials and serving as pump sources of gain-switched Cr:II-VI lasers.

Molecular fingerprinting with bright, broadband infrared frequency combs

Abstract: Spectroscopy in the molecular fingerprint spectral region (6.7–20 µm) yields critical information on material structure for physical, chemical, and biological sciences. Despite decades of interest and effort, this portion of the electromagnetic spectrum remains challenging to cover with conventional laser technologies. In this paper, we present a simple and robust method for generating super-octave, optical frequency combs in the fingerprint region through intra-pulse difference frequency generation in an orientation-patterned gallium phosphide crystal. The attainable brightness from this tabletop source reaches the same level achievable by infrared synchrotron radiation with a bandwidth spanning from 4 to 12 µm. We demonstrate the utility of this unique coherent light source for high-precision, dual-frequency-comb spectroscopy of methanol and ethanol vapor. These results highlight the potential of laser frequency combs for a wide range of infrared molecular sensing applications from basic molecular spectroscopy to nanoscopic imaging.

Narrow-linewidth lasing and soliton Kerr microcombs with ordinary laser diodes

Abstract: Narrow-linewidth lasers and optical frequency combs generated with mode-locked lasers have revolutionized optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators has opened new horizons in academic research and industrial applications. These combs, as was naturally assumed, however, require narrow-linewidth, single-frequency pump lasers. We demonstrate that an ordinary cost-effective broadband Fabry–Perot laser diode at the hundreds of milliwatts level, self-injection-locked to a microresonator, can be efficiently transformed to a powerful single-frequency, ultra-narrow-linewidth light source with further transformation to a coherent soliton comb oscillator. Our findings pave the way to the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production. A broadband multi-frequency Fabry–Perot laser diode, when coupled to a high-Q microresonator, can be efficiently transformed to an ~100 mW narrow-linewidth single-frequency light source, and subsequently, to a coherent soliton Kerr comb oscillator.

Spatiotemporal control of laser intensity

Abstract: The controlled coupling of a laser to plasma has the potential to address grand scientific challenges1–6, but many applications have limited flexibility and poor control over the laser focal volume. Here, we present an advanced focusing scheme called a ‘flying focus’, where a chromatic focusing system combined with chirped laser pulses enables a small-diameter laser focus to propagate nearly 100 times its Rayleigh length. Furthermore, the speed at which the focus moves (and hence the peak intensity) is decoupled from the group velocity of the laser. It can co- or counter-propagate along the laser axis at any velocity. Experiments validating the concept measured subluminal (−0.09c) to superluminal (39c) focal-spot velocities, generating a nearly constant peak intensity over 4.5 mm. Among possible applications, the flying focus could be applied to a photon accelerator 7 to mitigate dephasing, facilitating the production of tunable XUV sources. By combining a chromatic focusing system with chirped laser pulses, the spatiotemporal distribution of the laser pulse is controlled in the focal region. The focal spot propagates over nearly 100 times its Rayleigh length at any velocity.

Unique Color Converter Architecture Enabling Phosphor-in-Glass (PiG) Films Suitable for High-Power and High-Luminance Laser-Driven White Lighting.

Abstract: As a next-generation high-power lighting technology, laser lighting has attracted great attention in high-luminance applications. However, thermally robust and highly efficient color converters suitable for high-quality laser lighting are scarce. Despite its versatility, the phosphor-in-glass (PiG) has been seldom applied in laser lighting because of its low thermal conductivity. In this work, we develop a unique architecture in which a phosphor-in-glass (PiG) film was directly sintered on a high thermally conductive sapphire substrate coated by one-dimensional photonic crystals. The designed color converter with the composite architecture exhibits a high internal quantum efficiency close to that of the original phosphor powders and an excellent packaging efficiency up to 90%. Furthermore, the PiG film can even be survived under the 11.2 W mm-2 blue laser excitation. Combining blue laser diodes with the YAG-PiG-on-sapphire plate, a uniform white light with a high luminance of 845 Mcd m-2(luminous flux: 1839 lm), luminous efficacy of 210 lm W-1, and correlated color temperature of 6504 K was obtained. A high color rendering index of 74 was attained by adding a robust orange or red phosphor layer to the architecture. These outstanding properties meet the standards of vehicle regulations, enabling the PiG films with the composite architecture to be applied in automotive lighting or other high-power and high-luminance laser lighting.

High-order coherent communications using mode-locked dark-pulse Kerr combs from microresonators.

Abstract: Microresonator frequency combs harness the nonlinear Kerr effect in an integrated optical cavity to generate a multitude of phase-locked frequency lines. The line spacing can reach values in the order of 100 GHz, making it an attractive multi-wavelength light source for applications in fiber-optic communications. Depending on the dispersion of the microresonator, different physical dynamics have been observed. A recently discovered comb state corresponds to the formation of mode-locked dark pulses in a normal-dispersion microcavity. Such dark-pulse combs are particularly compelling for advanced coherent communications since they display unusually high power-conversion efficiency. Here, we report the first coherent-transmission experiments using 64-quadrature amplitude modulation encoded onto the frequency lines of a dark-pulse comb. The high conversion efficiency of the comb enables transmitted optical signal-to-noise ratios above 33 dB, while maintaining a laser pump power level compatible with state-of-the-art hybrid silicon lasers.

Sub-Hz Linewidth Photonic-Integrated Brillouin Laser

Abstract: Photonic systems and technologies traditionally relegated to table-top experiments are poised to make the leap from the laboratory to real-world applications through integration. Stimulated Brillouin scattering (SBS) lasers, through their unique linewidth narrowing properties, are an ideal candidate to create highly-coherent waveguide integrated sources. In particular, cascaded-order Brillouin lasers show promise for multi-line emission, low-noise microwave generation and other optical comb applications. Photonic integration of these lasers can dramatically improve their stability to environmental and mechanical disturbances, simplify their packaging, and lower cost. While single-order silicon and cascade-order chalcogenide waveguide SBS lasers have been demonstrated, these lasers produce modest emission linewidths of 10-100 kHz. We report the first demonstration of a sub-Hz (~0.7 Hz) fundamental linewidth photonic-integrated Brillouin cascaded-order laser, representing a significant advancement in the state-of-the-art in integrated waveguide SBS lasers. This laser is comprised of a bus-ring resonator fabricated using an ultra-low loss Si3N4 waveguide platform. To achieve a sub-Hz linewidth, we leverage a high-Q, large mode volume, single polarization mode resonator that produces photon generated acoustic waves without phonon guiding. This approach greatly relaxes phase matching conditions between polarization modes, and optical and acoustic modes. Using a theory for cascaded-order Brillouin laser dynamics, we determine the fundamental emission linewidth of the first Stokes order by measuring the beat-note linewidth between and the relative powers of the first and third Stokes orders. Extension to the visible and near-IR wavebands is possible due to the low optical loss from 405 nm to 2350 nm, paving the way to photonic-integrated sub-Hz lasers for visible-light applications.

Light Stops at Exceptional Points.

Abstract: Almost twenty years ago, light was slowed down to less than 10^{-7} of its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden turn-off of the coupling laser, a slow light pulse can be imprinted on cold atoms such that it can be read out and converted into a photon again. In this process, the light is stopped by absorbing it and storing its shape within the atomic ensemble. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides, where the group speed vanishes. Here, we extend the phenomenon of stopped light to the new field of parity-time (PT) symmetric systems. We show that zero group speed in PT symmetric optical waveguides can be achieved if the system is prepared at an exceptional point, where two optical modes coalesce. This effect can be tuned for optical pulses in a wide range of frequencies and bandwidths, as we demonstrate in a system of coupled waveguides with gain and loss.

Multi-photon near-infrared emission saturation nanoscopy using upconversion nanoparticles.

Abstract: Multiphoton fluorescence microscopy (MPM), using near infrared excitation light, provides increased penetration depth, decreased detection background, and reduced phototoxicity. Using stimulated emission depletion (STED) approach, MPM can bypass the diffraction limitation, but it requires both spatial alignment and temporal synchronization of high power (femtosecond) lasers, which is limited by the inefficiency of the probes. Here, we report that upconversion nanoparticles (UCNPs) can unlock a new mode of near-infrared emission saturation (NIRES) nanoscopy for deep tissue super-resolution imaging with excitation intensity several orders of magnitude lower than that required by conventional MPM dyes. Using a doughnut beam excitation from a 980 nm diode laser and detecting at 800 nm, we achieve a resolution of sub 50 nm, 1/20th of the excitation wavelength, in imaging of single UCNP through 93 μm thick liver tissue. This method offers a simple solution for deep tissue super resolution imaging and single molecule tracking.

Attosecond time–energy structure of X-ray free-electron laser pulses

Abstract: The time–energy information of ultrashort X-ray free-electron laser pulses generated by the Linac Coherent Light Source is measured with attosecond resolution via angular streaking of neon 1s photoelectrons. The X-ray pulses promote electrons from the neon core level into an ionization continuum, where they are dressed with the electric field of a circularly polarized infrared laser. This induces characteristic modulations of the resulting photoelectron energy and angular distribution. From these modulations we recover the single-shot attosecond intensity structure and chirp of arbitrary X-ray pulses based on self-amplified spontaneous emission, which have eluded direct measurement so far. We characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties, thus paving the way for X-ray pump/X-ray probe attosecond free-electron laser science.

High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses.

Abstract: Laser-driven high-harmonic generation provides the only demonstrated route to generating stable, tabletop attosecond x-ray pulses but has low flux compared to other x-ray technologies. We show that high-harmonic generation can produce higher photon energies and flux by using higher laser intensities than are typical, strongly ionizing the medium and creating plasma that reshapes the driving laser field. We obtain high harmonics capable of supporting attosecond pulses up to photon energies of 600 eV and a photon flux inside the water window (284 to 540 eV) 10 times higher than previous attosecond sources. We demonstrate that operating in this regime is key for attosecond pulse generation in the x-ray range and will become increasingly important as harmonic generation moves to fields that drive even longer wavelengths.

Robust Subwavelength Single-Mode Perovskite Nanocuboid Laser

Abstract: On-chip photonic information processing systems require great research efforts toward miniaturization of the optical components. However, when approaching the classical diffraction limit, conventional dielectric lasers with all dimensions in nanoscale are difficult to realize due to the ultimate miniaturization limit of the cavity length and the extremely high requirement of optical gain to overcome the cavity loss. Herein, we have succeeded in reducing the laser size to subwavelength scale in three dimensions using an individual CsPbBr3 perovskite nanocuboid. Even though the side length of the nanocuboid laser is only ∼400 nm, single-mode Fabry–Perot lasing at room temperature with laser thresholds of 40.2 and 374 μJ/cm2 for one- and two-photon excitation has been achieved, respectively, with the corresponding quality factors of 2075 and 1859. In addition, temperature-insensitive properties from 180 to 380 K have been demonstrated. The physical volume of a CsPbBr3 nanocuboid laser is only ∼0.49λ3 (where ...

Passively Q-Switched and Mode-Locked Fiber Laser Based on an ReS2 Saturable Absorber

Abstract: Transition metal dichalcogenides, a family of two-dimensional material with unusual electronic, optical, mechanical, and electrochemical properties, have received much research attention in recent years. Here we demonstrate that, another type of few-layer transition metal dichalcogenides, rhenium disulfide (ReS2) nanosheets display saturable absorption property at 1.55 μm. By incorporating the ReS2 nanosheets with the polyvinyl alcohol (PVA), a film-type ReS2-PVA saturable absorber is fabricated to realize Q-switching and mode locking of erbium-doped fiber lasers. The repetition rate of the Q-switched laser pulses varies from 12.6 to 19 KHz while the duration changes from 23 to 5.496 μs by tuning the pump from 45 to 120 mW. By optimizing the polarization state, the mode-locked operation is also obtained, emitting a train of pulses centered at 1558.6 nm with the duration of 1.6 ps and the fundamental repetition rate of 5.48 MHz. It is demonstrated that ReS2 nanosheets have the similar saturable absorption property as that of MoS2 and WS2, and may find potential applications in pulsed laser, optical modulators, and sensors.

On the influence of laser defocusing in Selective Laser Melting of 316L

Abstract: Despite its many benefits, Selective Laser Melting's (SLM) relatively low productivity compared to deposition-based additive manufacturing techniques is a major drawback. Increasing the laser beam diameter improves SLM's build rate, but causes loss of precision. The aim of this study is to investigate laser beam focus shift, or “defocus”, using a dynamic focusing unit, in order to increase the laser spot size. When applied to the SLM process, focus shift can be integrated into a “hull-core” strategy. This involves scanning the core with a high productivity parameter set using defocus while enabling return to the focused smaller spot size position for hull scanning. To assess the process stability, single line scans were made from 316L stainless steel powder. The consolidated melt pool morphology was analyzed and correlated with the process parameters comprising laser power, scanning speed and defocus distance. In order to link the melt pool morphology with the heat input, Volumetric Energy Density, Normalized Enthalpy and Rosenthal equation were considered. The suitability of using the Normalized Enthalpy as a design parameter to predict the melt pool depth and Rosenthal equation to predict its width was highlighted. This study shows that within a single laser setup, implementing defocus can lead to a potential productivity increase by 840%, i.e. to 18.8 mm3/s.

Ponderomotive Generation and Detection of Attosecond Free-Electron Pulse Trains.

Abstract: Atomic motion dynamics during structural changes or chemical reactions have been visualized by pico- and femtosecond pulsed electron beams via ultrafast electron diffraction and microscopy. Imaging the even faster dynamics of electrons in atoms, molecules, and solids requires electron pulses with subfemtosecond durations. We demonstrate here the all-optical generation of trains of attosecond free-electron pulses. The concept is based on the periodic energy modulation of a pulsed electron beam via an inelastic interaction, with the ponderomotive potential of an optical traveling wave generated by two femtosecond laser pulses at different frequencies in vacuum. The subsequent dispersive propagation leads to a compression of the electrons and the formation of ultrashort pulses. The longitudinal phase space evolution of the electrons after compression is mapped by a second phase-locked interaction. The comparison of measured and calculated spectrograms reveals the attosecond temporal structure of the compressed electron pulse trains with individual pulse durations of less than 300 as. This technique can be utilized for tailoring and initial characterization of suboptical-cycle free-electron pulses at high repetition rates for stroboscopic time-resolved experiments with subfemtosecond time resolution.

Several new directions for ultrafast fiber lasers [Invited].

Abstract: Ultrafast fiber lasers have the potential to make applications of ultrashort pulses widespread – techniques not only for scientists, but also for doctors, manufacturing engineers, and more. Today, this potential is only realized in refractive surgery and some femtosecond micromachining. The existing market for ultrafast lasers remains dominated by solid-state lasers, primarily Ti:sapphire, due to their superior performance. Recent advances show routes to ultrafast fiber sources that provide performance and capabilities equal to, and in some cases beyond, those of Ti:sapphire, in compact, versatile, low-cost devices. In this paper, we discuss the prospects for future ultrafast fiber lasers built on new kinds of pulse generation that capitalize on nonlinear dynamics. We focus primarily on three promising directions: mode-locked oscillators that use nonlinearity to enhance performance; systems that use nonlinear pulse propagation to achieve ultrashort pulses without a mode-locked oscillator; and multimode fiber lasers that exploit nonlinearities in space and time to obtain unparalleled control over an electric field.

Optical spectroscopy of laser-produced plasmas for standoff isotopic analysis

Abstract: Rapid, in-field, and non-contact isotopic analysis of solid materials is extremely important to a large number of applications, such as nuclear nonproliferation monitoring and forensics, geochemistry, archaeology, and biochemistry. Presently, isotopic measurements for these and many other fields are performed in laboratory settings. Rapid, in-field, and non-contact isotopic analysis of solid material is possible with optical spectroscopy tools when combined with laser ablation. Laser ablation generates a transient vapor of any solid material when a powerful laser interacts with a sample of interest. Analysis of atoms, ions, and molecules in a laser-produced plasma using optical spectroscopy tools can provide isotopic information with the advantages of real-time analysis, standoff capability, and no sample preparation requirement. Both emission and absorption spectroscopy methods can be used for isotopic analysis of solid materials. However, applying optical spectroscopy to the measurement of isotope ratios from solid materials presents numerous challenges. Isotope shifts arise primarily due to variation in nuclear charge distribution caused by different numbers of neutrons, but the small proportional nuclear mass differences between nuclei of various isotopes lead to correspondingly small differences in optical transition wavelengths. Along with this, various line broadening mechanisms in laser-produced plasmas and instrumental broadening generated by the detection system are technical challenges frequently encountered with emission-based optical diagnostics. These challenges can be overcome by measuring the isotope shifts associated with the vibronic emission bands from molecules or by using the techniques of laser-based absorption/fluorescence spectroscopy to marginalize the effect of instrumental broadening. Absorption and fluorescence spectroscopy probe the ground state atoms existing in the plasma when it is cooler, which inherently provides narrower lineshapes, as opposed to emission spectroscopy which requires higher plasma temperatures to be able to detect thermally excited emission. Improvements in laser and detection systems and spectroscopic techniques have allowed for isotopic measurements to be carried out at standoff distances under ambient atmospheric conditions, which have expanded the applicability of optical spectroscopy-based isotopic measurements to a variety of scientific fields. These technological advances offer an in-situ measurement capability that was previously not available. This review will focus on isotope detection through emission, absorption, and fluorescence spectroscopy of atoms and molecules in a laser-produced plasma formed from a solid sample. A description of the physics behind isotope shifts in atoms and molecules is presented, followed by the physics behind solid sampling of laser ablation plumes, optical methods for isotope measurements, the suitable physical conditions of laser-produced plasma plumes for isotopic analysis, and the current status. Finally, concluding remarks will be made on the existing knowledge/technological gaps identified from the current literature and suggestions for the future work.