Wavelength-versatile graphene-gold film saturable absorber mirror for ultra-broadband mode-locking of bulk lasers

VTe2: Broadband Saturable Absorber for Passively Q-Switched Lasers in the Near- and Mid-Infrared Regions

In this study, we demonstrate the fabrication of a novel 2D transition metal dichalcogenide, VTe2, into a saturable absorber (SA) by using the liquid phase exfoliation method. Furthermore, the first-principles calculations were conducted to elucidate the electronic band structures and absorption spectrum. The nonlinear optical absorption properties of VTe2 at 1.0, 2.0, and 3.0 μm were measured using open-aperture Z-scan and P-scan methods, which showed saturation intensities and modulation depths of 95.57 GW/cm2 and 9.24%, 3.11 GW/cm2 and 7.26%, and 15.8 MW/cm2 and 17.1%, respectively. Furthermore, in the realm of practical implementation, the achievement of stable passively Q-switched (PQS) lasers employing SA composed of few-layered VTe2 nanosheets has manifested itself with broadband operating wavelengths from 1.0 to ∼3.0 μm. Specifically, PQS laser operations from near-infrared to mid-infrared with pulse durations of 195 and 563 ns for 1.0 and 2.0 μm solid-state lasers, respectively, and 749 ns for an Er3+-doped fluoride fiber laser at 3.0 μm were obtained. Our experimental results demonstrate that VTe2 is a potential broadband SA device for achieving PQS lasers. To the best of our knowledge, this is the first demonstration of using VTe2 as an SA in PQS lasers in the near- and mid-infrared regions, which highlights the potential of VTe2 for future research and applications in optoelectronic devices.

Nickel-vanadium layered double hydroxide for a mid-infrared 2 µm Tm:YAG ceramic ultrafast laser

In this study, a nickel-vanadium layered double hydroxide (NiV-LDH) nanosheet was prepared as a saturable absorber (SA) by liquid phase exfoliation and a drop-coating method. The microstructure and optical transmission properties of the obtained NiV-LDH nanosheet were then systematically studied. An "X"-type fold cavity was designed to evaluate the ultrafast laser modulation performance of the NiV-LDH nanosheet with a Tm:YAG ceramic gain medium. A stable passively Q-switched mode-locked (QML) pulse centered at 2011.6 nm has successfully been realized, with a repetition frequency of 145 MHz and a pulse duration of 320 ps. To the best of our knowledge, this is the first time that the LDH has been used as an SA in a mid-infrared range ultrafast laser.

Electronic and optical properties of metal decorated nitrogen-doped vacancy defects in graphene

We present a first principles study of the stability, and of the electronic and optical properties of graphene with nitrogen doped vacancies. Moreover, we use the vacancies as anchoring sites for Mg, Zn, Pd al Pt atoms and vary the concentration of defects. Decoration of the defects with metal atoms produces semi-metallic systems for any studied size of the cell, with linear bands crossing at the Fermi level. The peculiar electronic properties of massless Dirac fermions in graphene are hence kept, although with anisotropic Fermi velocities. New sharp peaks appear in the optical conductivity in the visible range, thus strongly enhancing the optical response of graphene.

67-fs pulse generation from a mode-locked Tm,Ho:CLNGG laser at 2083 nm

We report on a mode-locked Tm,Ho:CLNGG laser emitting in the 2 µm spectral range using single-walled carbon nanotubes (SWCNTs) as a saturable absorber (SA). Pulses with duration of 98 fs are generated at 99.28 MHz repetition rate with an average output power of 123 mW, yielding a pulse energy of 1.24 nJ. Using a 0.5% output coupling, pulses as short as 67 fs, i.e., 10 optical cycles, are produced after extracavity compression with a 3-mm-thick ZnS plate.

Sub-80 fs mode-locked Tm,Ho-codoped disordered garnet crystal oscillator operating at 2081 nm

We demonstrate a mode-locked (ML) femtosecond laser based on the disordered garnet crystal Tm,Ho:CNGG. Employing a single-walled carbon nanotube saturable absorber, pulses as short as 83 and 76 fs at 2081 nm are achieved without and with external compression, respectively. The latter represents, to the best of our knowledge, the shortest pulse duration obtained from any Ho-doped or Tm,Ho-codoped laser. The average power amounts to 67 mW at a repetition rate of 102 MHz. By analyzing the soliton ML regime, the nonlinear refractive index of Tm,Ho:CNGG is estimated to be ∼1.1×10^-19  m²/W.

78 fs SWCNT-SA mode-locked Tm:CLNGG disordered garnet crystal laser at 2017 nm

A passively mode-locked Tm:CLNGG laser using single-walled carbon nanotubes (SWCNT) as a saturable absorber (SA) is demonstrated at 2017 nm. Pulses as short as 78 fs are generated at an 86 MHz repetition rate with an average output power of 54 mW. By increasing the output coupling from 0.5% to 1.5%, a higher power of 100 mW is achieved for slightly longer pulses with a duration of 105 fs at 1996 nm.

Sub-100 fs Tm:MgWO4 laser at 2017 nm mode locked by a graphene saturable absorber

We present the first sub-100 fs bulk solid-state laser in the 2-μm spectral range employing the monoclinic Tm³⁺-dopedMgWO₄ crystal as an active medium. By applying a graphene-based saturable absorber and chirped mirrors for dispersion management, stable self-starting mode-locked operation at 2017 nm was achieved. Nearly Fourier-limited pulses as short as 86 fs featuring a bandwidth of 53 nm were generated at a repetition rate of 76 MHz. A pulse energy of 1.1 nJ was achieved at 87 MHz for a pulse duration of 96 fs. The mode-locked Tm³⁺:MgWO₄ laser exhibits excellent stability with a fundamental beat note extinction ratio of 80 dBc above noise level.

Infrared absorber based on sandwiched two-dimensional black phosphorus metamaterials

As a direct band gap two-dimensional (2D) semiconductor material, black phosphorus (BP) bridges the characteristics of graphene, with a zero or near-zero band gap, and transition metal dichalcogenides, with a wide band gap. In the infrared (IR) regime, 2D BP materials can harvest electromagnetic energy due to losses derived from its surface conductivity. In this paper, we propose an IR absorber design comprising 2D BP metamaterials sandwiched between dielectric layers. The multilayered sandwich-like absorber structure is mounted on a full reflective gold mirror, which forms a Fabry-Perot resonator to strengthen light-matter interactions. Harvested surface plasmons are excited around the 2D BP metamaterial edges, and the incident IR light can be efficiently dissipated by increasing the number of layers of the sandwich-like structure (NLSS). The physical absorption mechanism can be attributed to the destructive interference from the metamaterials, which can be enhanced with increasing NLSS. Here, a phase difference of about 180° is obtained between the directly reflected wave from the first interface and the emergent wave derived from the superposition of the multiple reflections among the resonator, and the amplitude of the emergent wave is steadily reduced to a value close to that of the directly reflected wave with increasing NLSS for incident transverse-magnetic polarized IR illumination.

Universal Near-Infrared and Mid-Infrared Optical Modulation for Ultrafast Pulse Generation Enabled by Colloidal Plasmonic Semiconductor Nanocrystals

Field effect relies on the nonlinear current-voltage relation in semiconductors; analogously, materials that respond nonlinearly to an optical field can be utilized for optical modulation. For instance, nonlinear optical (NLO) materials bearing a saturable absorption (SA) feature an on-off switching behavior at the critical pumping power, thus enabling ultrafast laser pulse generation with high peak power. SA has been observed in diverse materials preferably in its nanoscale form, including both gaped semiconductor nanostructures and gapless materials like graphene; while the presence of optical bandgap and small carrier density have limited the active spectral range and intensity. We show here that solution-processed plasmonic semiconductor nanocrystals exhibit superbroadband (over 400 THz) SA, meanwhile with large modulation depth (∼7 dB) and ultrafast recovery (∼315 fs). Optical modulators fabricated using these plasmonic nanocrystals enable mode-locking and Q-switching operation across the near-infrared and mid-infrared spectral region, as exemplified here by the pulsed lasers realized at 1.0, 1.5, and 2.8 μm bands with minimal pulse duration down to a few hundreds of femtoseconds. The facile accessibility and superbroadband optical nonlinearity offered by these nonconventional plasmonic nanocrystals may stimulate a growing interest in the exploiting of relevant NLO and photonic applications.

Graphene mode-locked femtosecond Cr2+:ZnS laser with ~300 nm tuning range

Graphene has proved to be an excellent broadband saturable absorber for mode-locked operation of ultrafast lasers. However, for the mid-infrared (mid-IR) range where broadly tunable sources are in great needs, graphene-based broadly tunable ultrafast mid-IR lasers have not been demonstrated so far. Here, we report on passive mode-locking of a mid-IR Cr:ZnS laser by utilizing a transmission-type monolayer graphene saturable absorber and broad spectral tunability between 2120 nm and 2408 nm, which is the broadest tuning bandwidth ever reported for graphene mode-locked mid-IR solid-state lasers. The recovery time of the saturable absorber is measured to be ~2.4 ps by pump-probe technique at a wavelength of 2350 nm. Stably mode-locked Cr:ZnS laser delivers Fourier transform-limited 220-fs pulses with a pulse energy of up to 7.8 nJ.

MoS2-clad microfibre laser delivering conventional, dispersion-managed and dissipative solitons

Molybdenum disulfide (MoS₂), whose monolayer possesses a direct band gap, displays promising applications in optoelectronics, photonics, and lasers. Recent researches have demonstrated that MoS₂ has not only a significant broadband saturable absorption performance, but also a higher optical nonlinear response than graphene. However, MoS₂ shows much lower optical damage threshold owing to the poorer thermal conductivity and mechanical property. Here, we exploit a MoS₂-clad microfibre (MCM) as the saturable absorber (SA) for the generation of ultrashort pulses under different dispersion conditions. The improved evanescent field interaction scheme can overcome the laser-induced thermal damage, as well as take full advantage of the strong nonlinear effect of MoS₂. With the MCM SA, conventional, dispersion-managed, and dissipative solitons are generated around 1600 nm in Er-doped fibre lasers with anomalous, near-zero, and normal cavity dispersions, respectively. Our work paves the way for applications of 2D layered materials in photonics, especially in laser sources.

Phase Conjugated and Transparent Wavelength Conversions of Nyquist 16-QAM Signals Employing a Single-Layer Graphene Coated Fiber Device

We fabricate a nonlinear optical device based on a fiber pigtail cross-section coated with a single-layer graphene grown by chemical vapor deposition (CVD) method. Using the fabricated graphene-assisted nonlinear optical device and employing Nyquist 16-ary quadrature amplitude modulation (16-QAM) signal, we experimentally demonstrate phase conjugated wavelength conversion by degenerate four-wave mixing (FWM) and transparent wavelength conversion by non-degenerate FWM in graphene. We study the conversion efficiency as functions of the pump power and pump wavelength and evaluate the bit-error rate (BER) performance. We also compare the time-varying symbol sequence for graphene-assisted phase conjugated and transparent wavelength conversions of Nyquist 16-QAM signal.

Generation of 30 fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser

Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode-locked 8.43 optical-cycle pulses have a spectral bandwidth of ∼50  nm and a pulse repetition frequency of ∼113.5  MHz. To the best of our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique for directly generating few-cycle optical pulses from a laser oscillator.

Black phosphorus as saturable absorber for the Q-switched Er:ZBLAN fiber laser at 2.8 μm

Black phosphorus, a newly emerged two-dimensional material, has attracted wide attention as novel photonic material. Here, multilayer black phosphorus is successfully fabricated by liquid phase exfoliation method. By employing black phosphorus as saturable absorber, we demonstrate a passively Q-switched Er-doped ZBLAN fiber laser at the wavelength of 2.8 μm. The modulation depth and saturation fluence of the black phosphorus saturable absorber are measured to be 15% and 9 μJ/cm(2), respectively. The Q-switched fiber laser delivers a maximum average power of 485 mW with corresponding pulse energy of 7.7 μJ and pulse width of 1.18 μs at repetition rate of 63 kHz. To the best of our knowledge, this is the first time to demonstrate that black phosphorus can realize Q-switching of 2.8-μm fiber laser. Our research results show that black phosphorus is a promising saturable absorber for mid-infrared pulsed lasers.

Dissolution-and-reduction CVD synthesis of few-layer graphene on ultra-thin nickel film lifted off for mode-locking fiber lasers

The in-situ dissolution-and-reduction CVD synthesized few-layer graphene on ultra-thin nickel catalyst film is demonstrated at temperature as low as 550 °C, which can be employed to form transmission-type or reflection-type saturable absorber (SA) for mode-locking the erbium-doped fiber lasers (EDFLs). With transmission-type graphene SA, the EDFL shortens its pulsewidth from 483 to 441 fs and broadens its spectral linewidth from 4.2 to 6.1 nm with enlarging the pumping current from 200 to 900 mA. In contrast, the reflection-type SA only compresses the pulsewidth from 875 to 796 fs with corresponding spectral linewidth broadened from 2.2 to 3.3 nm. The reflection-type graphene mode-locker increases twice of its equivalent layer number to cause more insertion loss than the transmission-type one. Nevertheless, the reflection-type based saturable absorber system can generate stabilized soliton-like pulse easier than that of transmission-type system, because the nonlinearity induced self-amplitude modulation depth is simultaneously enlarged when passing through the graphene twice under the retro-reflector design.

Flexible pulse-controlled fiber laser

Controlled flexible pulses have widespread applications in the fields of fiber telecommunication, optical sensing, metrology, and microscopy. Here, we report a compact pulse-controlled all-fiber laser by exploiting an intracavity fiber Bragg grating (FBG) system as a flexible filter. The width and wavelength of pulses can be tuned independently by vertically and horizontally translating a cantilever beam, respectively. The pulse width of the laser can be tuned flexibly and accurately from ~7 to ~150 ps by controlling the bandwidth of FBG. The wavelength of pulse can be tuned precisely with the range of >20 nm. The flexible laser is precisely controlled and insensitive to environmental perturbations. This fiber-based laser is a simple, stable, and low-cost source for various applications where the width-tunable and/or wavelength-tunable pulses are necessary.

Mode-locked Tm,Ho:KLu(WO(4))(2) laser at 2060 nm using InGaSb-based SESAMs

Passive mode-locking of a Tm,Ho:KLu(WO(4))(2) laser operating at 2060 nm using different designs of InGaAsSb quantum-well based semiconductor saturable absorber mirrors (SESAMs) is demonstrated. The self-starting mode-locked laser delivers pulse durations between 4 and 8 ps at a repetition rate of 93 MHz with maximum average output power of 155 mW. Mode-locking performance of a Tm,Ho:KLu(WO(4))(2) laser is compared for usage of a SESAM to a single-walled carbon nanotube saturable absorber.