Low Saturation Intensity Research Articles

Multiplasmon Absorption in Graphene.

We show that graphene possesses a strong nonlinear optical response in the form of multiplasmon absorption, with exciting implications in classical and quantum nonlinear optics. Specifically, we predict that graphene nanoribbons can be used as saturable absorbers with low saturation intensity in the far-infrared and terahertz spectrum. Moreover, we predict that two-plasmon absorption and extreme localization of plasmon fields in graphene nanodisks can lead to a plasmon blockade effect, in which a single quantized plasmon strongly suppresses the possibility of exciting a second plasmon.

Carrier-wave Rabi-flopping signatures in high-order harmonic generation for alkali atoms.

We present a theoretical investigation of carrier-wave Rabi flopping in real atoms by employing numerical simulations of high-order harmonic generation (HHG) in alkali species. Given the short HHG cutoff, related to the low saturation intensity, we concentrate on the features of the third harmonic of sodium (Na) and potassium (K) atoms. For pulse areas of 2π and Na atoms, a characteristic unique peak appears, which, after analyzing the ground state population, we correlate with the conventional Rabi flopping. On the other hand, for larger pulse areas, carrier-wave Rabi flopping occurs, and is associated with a more complex structure in the third harmonic. These characteristics observed in K atoms indicate the breakdown of the area theorem, as was already demonstrated under similar circumstances in narrow band gap semiconductors.

Wide spectral and wavelength-tunable dissipative soliton fiber laser with topological insulator nano-sheets self-assembly films sandwiched by PMMA polymer.

Topological insulators have been theoretically predicted as promising candidates for broadband photonics devices due to its large bulk band gap states in association with the spin-momentum-locked mass-less Dirac edge/surface states. Unlike the bulk counterpart, few-layer topological insulators possess some intrinsic optical advantages, such as low optical loss, low saturation intensity and high concentration of surface state. Herein, we use a solvothermal method to prepare few-layer Bi₂Te₃ flakes. By sandwiching few-layer Bi₂Te₃ flakes with polymethyl methacrylate (PMMA) polymer, a novel light modulation device had been successfully fabricated with high chemical and thermal stabilities as well as excellent mechanical durability, originating from the contribution of PMMA acting as buffer layers that counteract excessive mechanical bending within the fragile Bi₂Te₃ flakes. The incorporation of the as-fabricated PMMA-TI-PMMA as saturable absorber, which could bear long-term mechanical loadings, into the fiber laser cavity generated the stable dissipative soliton mode-locking with a 3-dB spectral bandwidth up to 51.62 nm and tunable wavelength range of 22 nm. Our work provides a new way of fabricating PMMA-TI-PMMA sandwiched composite structure as saturable absorber with promising applications for laser operation.

Saturable absorption behavior of free-standing graphene polymer composite films over broad wavelength and time ranges.

A comparative research on saturable absorption (SA) behavior dependence on wavelength and pulse duration was performed for graphene polymer composites. Free-standing graphene-polyvinyl alcohol (PVA) composite films were fabricated by using solution cast method in combination of liquid phase exfoliation. SA responses were observed by using an open-aperture Z-scan technique for 340 fs pulses at 1030 nm and 515 nm from a mode-locked fiber laser, and 6 ns pulses at 1064 nm and 532 nm from a Q-switched Nd:YAG laser. The graphene films possess better SA property, i.e., larger SA coefficient and figure of merit (FOM), and lower saturation intensity I(s), for ns pulses than that for fs pulses at the similar near infrared (NIR) wavelength. For fs pulses, the films show better SA response at 1030 nm than that at 515 nm. By employing slow and fast SA modelling, the excited state and ground state absorption cross sections were estimated to be ~10(-17) cm(2), and the ratio was ~0.6 at NIR for both fs and ns pulses.

Progress in Mid-IR Lasers Based on Cr and Fe-Doped II–VI Chalcogenides

Transition metal (TM) doped II-VI chalcogenide laser materials offer a unique blend of physical, spectroscopic, and technological parameters that make them the gain media of choice for cost effective broadly tunable lasing in the Mid-IR. The II-VI semiconductor hosts provide a low phonon cut-off, broad IR transparency, and high thermal conductivity. When doped with transition metal ions, these materials feature ultrabroadband gain, low saturation intensities, and large pump absorption coefficients. This combined with the low-cost mass production technology of crystal fabrication by postgrowth thermal diffusion, as well as broad availability of convenient pump sources, make these materials ideal candidates for broadly tunable mid-IR lasing in CW, gain-switched, free running, and mode-locked regimes of operation. This review summarizes experimental results on optically pumped lasers based on Cr and Fe doped II-VI wide band semiconductors providing access to the 1.9-6 μm spectral range with a high (exceeding 60%) efficiency, multi-Watt-level (18 W in gain switch and 30 W in pure CW) output powers, tunability in excess of 1000 nm, short-pulse (<;50 fs) multi-watt oscillation, multi-Joule long-pulse output energy, and narrow spectral linewidth (<;100 kHz).

Comprehensive evaluation of the structural, absorption, energy transfer, luminescent properties and near-infrared applications of the neodymium doped germanate glass

Nd3+ doped germanate glass with good thermal stability was investigated through absorption, near-infrared emission spectrum and lifetime measurement. Intensity parameters (Ω2,4,6) and spectroscopic quality factor (χ=Ω4/Ω6) were analyzed in detail on the basis of the Judd–Ofelt theory. The calculations of bonding parameters and the degree of ionic or covalent bonding character demonstrate the nature of ionic bonding of the prepared sample. It is found that the present glass possesses large spectroscopic quality factor of 93%, high stimulated emission cross section, fluorescence branching ratio and optical gain parameters as well as lower saturation intensity at 1.05μm. Thus, the excellent radiative properties owing to 4F3/2→4I11/2 transition reveal the favorable laser action at 1.05μm. Nd3+ doped germanate glass might have potential advantages for near-infrared applications.

Q-switched mode-locked erbium-doped fiber laser based on topological insulator Bi(2)Se(3) deposited fiber taper.

We have demonstrated the passive Q-switching mode-locking operation in an erbium-doped fiber (EDF) laser by using topological insulator Bi(2)Se(3) deposited on fiber taper, whose damage threshold can be further increased by the large evanescent field interacting length. Due to the low saturation intensity, stable Q-switched mode-locked fiber lasers centered at 1562 nm can be generated at a pump power of 10 mW. The temporal and spectral characteristics for different pump strengths have also been investigated. To the best of our knowledge, it is the first time a Q-switched mode-locked EDF laser based on the fiber taper deposited by Bi(2)Se(3) was generated.

Low saturable intensity

Low saturable intensity

High-repetition-rate Q-switched fiber laser with high quality topological insulator Bi₂Se₃ film.

We demonstrated a high-repetition-rate Q-switched fiber laser with topological insulator Bi₂Se₃ absorber. The absorber was made into a film structure by spin-coating method using few-layer Bi₂Se₃ nano-platelets which had regular shape. The uniform film had a low saturable optical intensity of 11 MW/cm(2), which is the lowest saturable optical intensity in the saturable absorbers made by topological insulator till now. By inserting the absorber film into an Erbium-doped fiber laser, a high-repetition Q-switched laser with the repetition rates from 459 kHz to 940 kHz was achieved. The maximum output power was 22.35 mW with the shortest pulse duration of 1.9 μs. To the best of our knowledge, both of the repetition rate and the output power were the highest values among the Q-switched fiber lasers with topological insulator absorber.

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

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.

106μm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi_2Se_3 as a saturable absorber

Passive Q-switching of an ytterbium-doped fiber (YDF) laser with few-layer topological insulator (TI) is, to the best of our knowledge, experimentally demonstrated for the first time. The few-layer TI: Bi2Se3 (2-4 layer thickness) is fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm2 measured by the Z-scan technique. The optical deposition technique is used to induce the few-layer TI in the solution onto a fiber ferrule for successfully constructing the fiber-integrated TI-based saturable absorber (SA). By inserting this SA into the YDF laser cavity, stable Q-switching operation at 1.06 {\mu}m is achieved. The Q-switched pulses have the shortest pulse duration of 1.95 {\mu}s, the maximum pulse energy of 17.9 nJ and a tunable pulse-repetition-rate from 8.3 to 29.1 kHz. Our results indicate that the TI as a SA is also available at 1 {\mu}m waveband, revealing its potential as another wavelength-independent SA (like graphene).

Optimizing the signal-to-noise ratio of a beam-deflection measurement with interferometric weak values

The amplification obtained using weak values is quantified through a detailed investigation of the signal-to-noise ratio for an optical beam-deflection measurement. We show that for a given deflection, input power and beam radius, the use of interferometric weak values allows one to obtain the optimum signal-to-noise ratio using a coherent beam. This method has the advantage of reduced technical noise and allows for the use of detectors with a low saturation intensity. We report on an experiment which improves the signal-to-noise ratio for a beam-deflection measurement by a factor of 54 when compared to a measurement using the same beam size and a quantum-limited detector.

Atomic‐Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers

AbstractThe optical conductance of monolayer graphene is defined solely by the fine structure constant, α = $e^2 /\hbar c$ (where e is the electron charge, $\hbar $ is Dirac's constant and c is the speed of light). The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero‐gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, use of atomic layer graphene as saturable absorber in a mode‐locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth, and wideband tunability.

Wideband-tuneable, nanotube mode-locked, fibre laser

Ultrashort-pulse lasers with spectral tuning capability have widespread applications in fields such as spectroscopy, biomedical research and telecommunications. Mode-locked fibre lasers are convenient and powerful sources of ultrashort pulses, and the inclusion of a broadband saturable absorber as a passive optical switch inside the laser cavity may offer tuneability over a range of wavelengths. Semiconductor saturable absorber mirrors are widely used in fibre lasers, but their operating range is typically limited to a few tens of nanometres, and their fabrication can be challenging in the 1.3-1.5 microm wavelength region used for optical communications. Single-walled carbon nanotubes are excellent saturable absorbers because of their subpicosecond recovery time, low saturation intensity, polarization insensitivity, and mechanical and environmental robustness. Here, we engineer a nanotube-polycarbonate film with a wide bandwidth (>300 nm) around 1.55 microm, and then use it to demonstrate a 2.4 ps Er(3+)-doped fibre laser that is tuneable from 1,518 to 1,558 nm. In principle, different diameters and chiralities of nanotubes could be combined to enable compact, mode-locked fibre lasers that are tuneable over a much broader range of wavelengths than other systems.

Soliton fiber laser mode‐locked by a single‐wall carbon nanotube‐polymer composite

AbstractWe present a detailed investigation of pulse width and spectral dynamics of an Er3+‐doped soliton fiber ring laser, mode‐locked by a carbon‐nanotube‐polymer composite. The laser is stable and produces ∼ 650 fs pulses at a fundamental repetition rate of 20.8 MHz. The carbon nanotube saturable absorber has a low saturation intensity of ∼ 18.9 MW/cm2 and a 16.9% reduction in absorption (due to saturation) at high pulse intensity (∼ 270 MW/cm2). Spectral and transient effects confirm the soliton operation of the laser. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Single‐Molecule Spectroscopy of Uniaxially Oriented Terrylene in Polyethylene

Single terrylene molecules doped into linear low-density polyethylene can be oriented by tensile deformation of the matrix. In measurements on ensembles at ambient and on single terrylene molecules at cryogenic temperature, strong orientation along the stretching direction was observed by polarization-resolved confocal microscopy. At cryogenic temperatures narrow and spectrally stable zero-phonon lines were found. The low saturation intensity of 0.07 W cm(-2) is consistent with an uniaxial orientation of terrylene in the sample plane.

Passive mode locking of a Nd:YAG laser with co-doped Nd, Cr:YAG as saturable absorber

We demonstrate the characteristics of relatively low saturation intensity using co-doped Nd, Cr:YAG as saturable absorber for passively mode locking the Nd:YAG laser. The difference of the saturation intensity between Q-switched and mode-locked operation in co-doped Nd, Cr:YAG was only one to two orders of magnitude, while Cr:YAG was generally reported at a difference of five orders of magnitude. More than 80% mode locking modulation depth was achieved at an incident pump power of 4.4W, corresponding to an intracavity intensity of 6×104W/cm2, using a 68cm long plano-concave cavity.

Low mode-locking saturation intensity of co-doped Nd3+,Cr4+:YAG crystal as saturable absorber

We report the characteristics of low saturation intensity of Nd,Cr:YAG as saturable absorber both in a diode pumped Nd:YAG laser and a self-mode-locking Nd,Cr:YAG laser. The difference of saturation intensity between Q-switched and mode-locked operation in co-doped Nd,Cr:YAG was only one to two orders of magnitude while the reported difference in Cr:YAG was five orders of magnitude. More than 80% passive mode-locking modulation depth was achieved at an intracavity intensity of 6 x 10(4) W/cm(2). Self-mode-locking phenomenon was observed in a fiber-coupled laser diode pumped Nd,Cr:YAG laser at an intracavity intensity of 1.0 MW/cm(2). The modulation depth is approximately 40%. (C) 2002 Published by Elsevier Science B.V.

Erratum: Diode-pumped erbium-ytterbium-glass laser passively Q-switched with a PbS semiconductor quantum-dot doped glass

Q-switched and cw operation of different diode-pumped erbium-ytterbium doped glasses at 1.5 μm has been studied in a compact microlaser setup. For Q-switching we used a novel PbS semiconductor quantum-dot doped glass which offers low saturation intensity compared with typical absorbers used and a fast time response. The cw laser delivered output powers of 35 mW with slope efficiencies of 16%. In Q-switched operation pulse energies of 1 μJ at repetition rates of 1–2 kHz and pulse durations of about 30–50 ns, depending on absorber thickness were obtained.

The effect of Coulomb explosion ion heating in optical-field ionised nitrogen X-ray laser schemes

This paper examines the effect of Coulomb ion heating in various nitrogen targets produced by optical-field ionisation from plane polarised high intensity laser radiation. Methods for controlling the explosion, such that plasma suitable for recombination X-ray lasers may be produced are devised. A numerical model of breakdown by high intensity pulses is used to calculate optical-field ionisation, above-threshold ionisation and inverse bremsstrahlung electron heating, and Coulomb ion heating mechanisms. A 1D Lagrangian hydrodynamics and atomic kinetics model is used as a postprocessor to simulate the recombination phase. Both optimised nitrogen and nitrogen doped with hydrogen mixtures are examined. Electron temperatures are found to be significantly lower in mixtures, yielding higher gains. Single and double (pre- and main) pulses are investigated with double pulses found to give good control of the ion temperature resulting from the Coulomb ion repulsion. An optimised nitrogen hydrogen mixture ionised by a double pulse produce a gain of 60 cm −1 at 247 Å with a low saturation intensity of ∼1×10 7 W cm −2. A two-dimensional pulse propagation model, which solves the paraxial wave equation and includes optical-field ionisation, refraction and diffraction effects, is used to study nitrogen hydrogen mixtures with double pulse configurations. A loosely focused pre-pulse of vacuum peak intensity 4×10 14 W cm −2 is applied to produce a wide and 4 mm long channel of singly ionised dissociated gas mixture. The plasma is further ionised to the He-like N state by a tighter main pulse of peak intensity 6×10 16 W cm −2, which is suitable for achieving the gain referred to above.