Showing papers on "Laser published in 2014"

Temporal solitons in optical microresonators

Abstract: Temporal dissipative solitons are observed in a nonlinear, high-finesse, optical microresonator driven by a continuous-wave laser. This approach enables ultrashort pulses to be generated in spectral regimes lacking broadband laser gain media and saturable absorbers, making it potentially useful for applications in broadband spectroscopy, telecommunications, astronomy and low-phase-noise microwave generation.

Laser Material Processing

Abstract: In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Single-mode laser by parity-time symmetry breaking.

Abstract: Effective manipulation of cavity resonant modes is crucial for emission control in laser physics and applications. Using the concept of parity-time symmetry to exploit the interplay between gain and loss (i.e., light amplification and absorption), we demonstrate a parity-time symmetry-breaking laser with resonant modes that can be controlled at will. In contrast to conventional ring cavity lasers with multiple competing modes, our parity-time microring laser exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation with the selective whispering-gallery mode order. Exploration of parity-time symmetry in laser physics may open a door to next-generation optoelectronic devices for optical communications and computing.

Molybdenum disulfide (MoS_2) as a broadband saturable absorber for ultra-fast photonics

Abstract: The nonlinear optical property of few-layered MoS2 nanoplatelets synthesized by the hydrothermal exfoliation method was investigated from the visible to the near-infrared band using lasers. Both open-aperture Z-scan and balanced-detector measurement techniques were used to demonstrate the broadband saturable absorption property of few-layered MoS2. To explore its potential applications in ultrafast photonics, we fabricated a passive mode locker for ytterbium-doped fibre laser by depositing few-layered MoS2 onto the end facet of optical fiber by means of an optical trapping approach. Our laser experiment shows that few-layer MoS2-based mode locker allows for the generation of stable mode-locked laser pulse, centered at 1054.3 nm, with a 3-dB spectral bandwidth of 2.7 nm and a pulse duration of 800 ps. Our finding suggests that few-layered MoS2 nanoplatelets can be useful nonlinear optical material for laser photonics devices, such as passive laser mode locker, Q-switcher, optical limiter, optical switcher and so on.

Ultrafast lasers—reliable tools for advanced materials processing

Abstract: The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

Multi-GeV Electron Beams from Capillary-Discharge-Guided Subpetawatt Laser Pulses in the Self-Trapping Regime

Abstract: A laser-driven particle accelerator, delivering a beam of electrons with a record-breaking energy of 4.2 giga-electron-volts, could lead to compact x-ray lasers or high-energy colliders.

Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining

Abstract: Femtosecond-laser micromachining (also known as inscription or writing) has been developed as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. This paper focuses on crystalline dielectric materials, and is a review of the state-of-the-art in the fabrication, characterization and applications of femtosecond-laser micromachined waveguiding structures in optical crystals and ceramics. A brief outlook is presented by focusing on a few potential spotlights.

Reversing the pump dependence of a laser at an exceptional point

Abstract: When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of non-trivial physics in a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.

Attosecond band-gap dynamics in silicon

Abstract: Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling.

Watt-class high-power, high-beam-quality photonic-crystal lasers

Abstract: The applications of surface-emitting lasers, in particular vertical-cavity surface-emitting lasers (VCSELs), are currently being extended to various low-power fields including communications and interconnections. However, the fundamental difficulties in increasing their output power by more than several milliwatts while maintaining single-mode operation prevent their application in high-power fields such as material processing, laser medicine and nonlinear optics, despite their advantageous properties of circular beams, the absence of catastrophic optical damage, and their suitability for two-dimensional integration. Here, we demonstrate watt-class high-power, single-mode operation by a two-dimensional photonic-crystal surface-emitting laser under room-temperature, continuous-wave conditions. The two-dimensional band-edge resonant effect of a photonic crystal formed by metal–organic chemical vapour deposition enables a 1,000 times broader coherent-oscillation area, which results in a high beam quality of M2 ≤ 1.1, narrowing the focus spot by two orders of magnitude compared to VCSELs. Our demonstration promises to realize innovative high-power applications for surface-emitting lasers. Researchers demonstrate a watt-class high-power, single-mode photonic-crystal laser operating continuously at room temperature. A beam quality of M2 ≤ 1.1 is achieved.

Advances in small lasers

Abstract: The latest developments in laser miniaturization, including those based on metals and dielectrics, are reviewed and future challenges outlined. Small lasers have dimensions or modes sizes close to or smaller than the wavelength of emitted light. In recent years there has been significant progress towards reducing the size and improving the characteristics of these devices. This work has been led primarily by the innovative use of new materials and cavity designs. This Review summarizes some of the latest developments, particularly in metallic and plasmonic lasers, improvements in small dielectric lasers, and the emerging area of small bio-compatible or bio-derived lasers. We examine the different approaches employed to reduce size and how they result in significant differences in the final device, particularly between metal- and dielectric-cavity lasers. We also present potential applications for the various forms of small lasers, and indicate where further developments are required.

Terahertz laser frequency combs

Abstract: Frequency combs based on terahertz quantum cascade lasers, which combine the high power of lasers with the broadband capabilities of pulsed sources, are demonstrated. The frequency combs generate 5 mW of terahertz power covering a frequency range of almost 500 GHz and produce more than 70 lines at 3.5 THz.

Fabrication of β-Ga_2O_3 thin films and solar-blind photodetectors by laser MBE technology

Abstract: Laser molecular beam epitaxy technology has been employed to deposit β-gallium oxide (β-Ga2O3) on (0001) sapphire substrates. After optimizing the growth parameters, (2¯01)-oriented β-Ga2O3 thin film was obtained. Ultraviolet-visible absorption spectrum demonstrates that the prepared β-Ga2O3 thin film shows excellent solar-blind ultraviolet (UV) characteristic with a band gap of 5.02 eV. A prototype photodetector device with a metal-semiconductor-metal structure has been fabricated using high quality β-Ga2O3 film. The device exhibits obvious photoresponse under 254 nm UV light irradiation, suggesting a potential application in solar-blind photodetectors.

Adaptive real-time dual-comb spectroscopy

Abstract: The spectrum of a laser frequency comb consists of several hundred thousand equally spaced lines over a broad spectral bandwidth. Such frequency combs have revolutionized optical frequency metrology and they now hold much promise for significant advances in a growing number of applications including molecular spectroscopy. Despite an intriguing potential for the measurement of molecular spectra spanning tens of nanometres within tens of microseconds at Doppler-limited resolution, the development of dual-comb spectroscopy is hindered by the demanding stability requirements of the laser combs. Here we overcome this difficulty and experimentally demonstrate a concept of real-time dual-comb spectroscopy, which compensates for laser instabilities by electronic signal processing. It only uses free-running mode-locked lasers without any phase-lock electronics. We record spectra spanning the full bandwidth of near-infrared fibre lasers with Doppler-limited line profiles highly suitable for measurements of concentrations or line intensities. Our new technique of adaptive dual-comb spectroscopy offers a powerful transdisciplinary instrument for analytical sciences.

Femtosecond pulse erbium-doped fiber laser by a few-layer MoS 2 saturable absorber

Abstract: We report on the generation of a femtosecond pulse in a fiber ring laser by using a polyvinyl alcohol (PVA)-based molybdenum disulfide (MoS2) saturable absorber (SA). With a saturable optical intensity of 34 MW/cm2 and a modulation depth of ∼4.3%, the PVA-based MoS2 SA had been employed with an erbium-doped fiber ring laser as a mode locker. The mode-locking operation could be achieved at a low pump threshold of 22 mW. A ∼710 fs pulse centered at 1569.5 nm wavelength with a repetition rate of 12.09 MHz had been achieved with proper cavity dispersion. With the variation of net cavity dispersion, output pulses with durations from 0.71 to 1.46 ps were obtained. The achievement of a femtosecond pulse at 1.55 μm waveband demonstrates the broadband saturable absorption of MoS2, and also indicates that the filmy PVA-based MoS2 SA is indeed a good candidate for an ultrafast saturable absorption device.

Dielectric laser accelerators

Abstract: The use of infrared lasers to power optical-scale lithographically fabricated particle accelerators is a developing area of research that has garnered increasing interest in recent years The physics and technology of this approach is reviewed, which is referred to as dielectric laser acceleration (DLA) In the DLA scheme operating at typical laser pulse lengths of 01 to 1 ps, the laser damage fluences for robust dielectric materials correspond to peak surface electric fields in the $\mathrm{GV}/\mathrm{m}$ regime The corresponding accelerating field enhancement represents a potential reduction in active length of the accelerator between 1 and 2 orders of magnitude Power sources for DLA-based accelerators (lasers) are less costly than microwave sources (klystrons) for equivalent average power levels due to wider availability and private sector investment Because of the high laser-to-particle coupling efficiency, required pulse energies are consistent with tabletop microJoule class lasers Combined with the very high (MHz) repetition rates these lasers can provide, the DLA approach appears promising for a variety of applications, including future high-energy physics colliders, compact light sources, and portable medical scanners and radiative therapy machines

Fiber lasers and their applications [Invited]

Abstract: Fiber lasers have seen progressive developments in terms of spectral coverage and linewidth, output power, pulse energy, and ultrashort pulse width since the first demonstration of a glass fiber laser in 1964. Their applications have extended into a variety of fields accordingly. In this paper, the milestones of glass fiber laser development are briefly reviewed and recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.

Diamond nonlinear photonics

Abstract: An optical parametric oscillator in the telecom wavelength range is realized in a diamond system consisting of a ring resonator coupled to a diamond waveguide. Threshold powers as low as 20 mW are measured and up to 20 new wavelengths are generated from a single-frequency pump laser. Despite progress towards integrated diamond photonics1,2,3,4, studies of optical nonlinearities in diamond have been limited to Raman scattering in bulk samples5. Diamond nonlinear photonics, however, could enable efficient, in situ frequency conversion of single photons emitted by diamond's colour centres6,7, as well as stable and high-power frequency microcombs8 operating at new wavelengths. Both of these applications depend crucially on efficient four-wave mixing processes enabled by diamond's third-order nonlinearity. Here, we have realized a diamond nonlinear photonics platform by demonstrating optical parametric oscillation via four-wave mixing using single-crystal ultrahigh-quality-factor (1 × 106) diamond ring resonators operating at telecom wavelengths. Threshold powers as low as 20 mW are measured, and up to 20 new wavelengths are generated from a single-frequency pump laser. We also report the first measurement of the nonlinear refractive index due to the third-order nonlinearity in diamond at telecom wavelengths.

Femtosecond laser three-dimensional micro- and nanofabrication

Abstract: The rapid development of the femtosecond laser has revolutionized materials processing due to its unique characteristics of ultrashort pulse width and extremely high peak intensity. The short pulse width suppresses the formation of a heat-affected zone, which is vital for ultrahigh precision fabrication, whereas the high peak intensity allows nonlinear interactions such as multiphoton absorption and tunneling ionization to be induced in transparent materials, which provides versatility in terms of the materials that can be processed. More interestingly, irradiation with tightly focused femtosecond laser pulses inside transparent materials makes three-dimensional (3D) micro- and nanofabrication available due to efficient confinement of the nonlinear interactions within the focal volume. Additive manufacturing (stereolithography) based on multiphoton absorption (two-photon polymerization) enables the fabrication of 3D polymer micro- and nanostructures for photonic devices, micro- and nanomachines, and microfluidic devices, and has applications for biomedical and tissue engineering. Subtractive manufacturing based on internal modification and fabrication can realize the direct fabrication of 3D microfluidics, micromechanics, microelectronics, and photonic microcomponents in glass. These microcomponents can be easily integrated in a single glass microchip by a simple procedure using a femtosecond laser to realize more functional microdevices, such as optofluidics and integrated photonic microdevices. The highly localized multiphoton absorption of a tightly focused femtosecond laser in glass can also induce strong absorption only at the interface of two closely stacked glass substrates. Consequently, glass bonding can be performed based on fusion welding with femtosecond laser irradiation, which provides the potential for applications in electronics, optics, microelectromechanical systems, medical devices, microfluidic devices, and small satellites. This review paper describes the concepts and principles of femtosecond laser 3D micro- and nanofabrication and presents a comprehensive review on the state-of-the-art, applications, and the future prospects of this technology.

Quasi-monoenergetic and tunable X-rays from a laser-driven Compton light source

Abstract: Quasi-monoenergetic Compton X-rays tunable in the range ∼70 keV to >1 MeV are generated in a laser-driven scheme.

Ultrafast erbium-doped fiber laser mode-locked by a CVD-grown molybdenum disulfide (MoS2) saturable absorber.

Abstract: We demonstrate an erbium-doped fiber laser passively mode-locked by a multilayer molybdenum disulfide (MoS(2)) saturable absorber (SA). The multilayer MoS(2) is prepared by the chemical vapor deposition (CVD) method and transferred onto the end-face of a fiber connector. Taking advantage of the excellent saturable absorption of the fabricated MoS(2)-based SA, stable mode locking is obtained at a pump threshold of 31 mW. Resultant output soliton pulses have central wavelength, spectral width, pulse duration, and repetition rate of 1568.9 nm, 2.6 nm, 1.28 ps, and 8.288 MHz, respectively. The experimental results show that multilayer MoS(2) is a promising material for ultrafast laser systems.

A room temperature low-threshold ultraviolet plasmonic nanolaser

Abstract: Surface plasmon lasers could improve our ability to generate and use light at nanometre scales, but they regularly suffer from optical losses that make them impractical, particularly at ultraviolet wavelengths. Zhang et al. demonstrate a low-threshold surface plasmon laser that overcomes this problem.

Ultrafast plasmonic nanowire lasers near the surface plasmon frequency

Abstract: Light–matter interactions are inherently slow as the wavelengths of optical and electronic states differ greatly. Surface plasmon polaritons — electromagnetic excitations at metal–dielectric interfaces — have generated significant interest because their spatial scale is decoupled from the vacuum wavelength, promising accelerated light–matter interactions. Although recent reports suggest the possibility of accelerated dynamics in surface plasmon lasers, this remains to be verified. Here, we report the observation of pulses shorter than 800 fs from hybrid plasmonic zinc oxide (ZnO) nanowire lasers. Operating at room temperature, ZnO excitons lie near the surface plasmon frequency in such silver-based plasmonic lasers, leading to accelerated spontaneous recombination, gain switching and gain recovery compared with conventional ZnO nanowire lasers. Surprisingly, the laser dynamics can be as fast as gain thermalization in ZnO, which precludes lasing in the thinnest nanowires (diameter less than 120 nm). The capability to combine surface plasmon localization with ultrafast amplification provides the means for generating extremely intense optical fields, with applications in sensing, nonlinear optical switching, as well as in the physics of strong-field phenomena. Electron scattering limits the optical excitations produced by metal-based lasers to femtosecond timescales. But sub-picosecond pulsing can be achieved in a plasmonic nanowire laser by operating near the surface plasmon frequency.

Stimulated Raman Scattering Microscopy with a Robust Fibre Laser Source

Abstract: Stimulated Raman Scattering microscopy allows label-free chemical imaging and has enabled exciting applications in biology, material science, and medicine. It provides a major advantage in imaging speed over spontaneous Raman scattering and has improved image contrast and spectral fidelity compared to coherent anti-Stokes Raman. Wider adoption of the technique has, however, been hindered by the need for a costly and environmentally sensitive tunable ultra-fast dual-wavelength source. We present the development of an optimized all-fibre laser system based on the optical synchronization of two picosecond power amplifiers. To circumvent the high-frequency laser noise intrinsic to amplified fibre lasers, we have further developed a high-speed noise cancellation system based on voltage-subtraction autobalanced detection. We demonstrate uncompromised imaging performance of our fibre-laser based stimulated Raman scattering microscope with shot-noise limited sensitivity and an imaging speed up to 1 frame/s.

Generation of 0.9-mJ THz pulses in DSTMS pumped by a Cr:Mg₂SiO₄ laser

Abstract: We report on high-field terahertz transients with 0.9-mJ pulse energy produced in a 400 mm² partitioned organic crystal by optical rectification of a 30-mJ laser pulse centered at 1.25 μm wavelength. The phase-locked single-cycle terahertz pulses cover the hard-to-access low-frequency range between 0.1 and 5 THz and carry peak fields of more than 42 MV/cm and 14 Tesla with the potential to reach over 80 MV/cm by choosing appropriate focusing optics. The scheme based on a Cr:Mg₂SiO₄ laser offers a high conversion efficiency of 3% using uncooled organic crystal. The collimated pump laser configuration provides excellent terahertz focusing conditions.

Observation of soliton explosions in a passively mode-locked fiber laser

Abstract: Soliton explosions are among the most exotic dissipative phenomena studied in mode-locked lasers. In this regime, a dissipative soliton circulating in the laser cavity experiences an abrupt structural collapse, but within a few roundtrips returns to its original quasi-stable state. In this work we report on the first observation of such events in a fiber laser. Specifically, we identify clear explosion signatures in measurements of shot-to-shot spectra of an Yb-doped mode-locked fiber laser that is operating in a transition regime between stable and noise-like emission. The comparatively long, all-normal-dispersion cavity used in our experiments also permits direct time-domain measurements, and we show that the explosions manifest themselves as abrupt temporal shifts in the output pulse train. Our experimental results are in good agreement with realistic numerical simulations based on an iterative cavity map.

Femtosecond pulse generation from a topological insulator mode-locked fiber laser

Abstract: We reported on the generation of femtosecond pulse in a fiber ring laser by using a polyvinyl alcohol (PVA)-based topological insulator (TI), Bi2Se3 saturable absorber (SA). The PVA-TI composite has a low saturable optical intensity of 12 MW/cm2 and a modulation depth of ~3.9%. By incorporating the fabricated PVA-TISA into a fiber laser, mode-locking operation could be achieved at a low pump threshold of 25 mW. After an optimization of the cavity parameters, optical pulse with ~660 fs centered at 1557.5 nm wavelength had been generated. The experimental results demonstrate that the PVA could be an excellent host material for fabricating high-performance TISA, and also indicate that the filmy PVA-TISA is indeed a good candidate for ultrafast saturable absorption device.

Terahertz quantum cascade lasers with >1 W output powers

Abstract: Terahertz (THz) frequency quantum cascade lasers emitting peak powers of >1 W from a single facet in the pulsed mode are demonstrated. The active region is based on a bound-to-continuum transition with a one-well injector, and is embedded into a surface-plasmon waveguide. The lasers emit at a frequency of ∼3.4 THz and have a maximum operating temperature of 123 K. The maximum measured emitted powers are ∼1.01 W at 10 K and ∼420 mW at 77 K, with no correction made to allow for the optical collection efficiency of the apparatus.