Helically twisted nonlinear photonic crystals.

Nonlinear photonic crystals with a helical structure in the second-order nonlinear coefficient (χ(2)) are fabricated using infrared femtosecond laser poling in ferroelectric Sr0.61Ba0.39Nb2O6 crystals. The quasi-orbital angular momentum of the helical χ(2) structure can be imprinted on the interacting photons during nonlinear optical processes, allowing the topological charge of the generated photons at new frequencies to be controlled. Here we study the case of a double-helix nonlinear photonic structure for the generation of a second-harmonic vortex beam from a Gaussian pump beam without phase singularity. The conservation law for orbital angular momentum in the second-harmonic process is also verified, with the topological charge of the pump photons being fully compensated by the double-helix structure. The flexible control of light carrying orbital angular momentum (OAM) at new frequencies will find important applications in both classical and quantum photonics, such as nonlinear wavefront shaping and multidimensional entanglement of photons.

Ferroelectric domain engineering with femtosecond pulses of different wavelengths.

Direct femtosecond laser writing of ferroelectric domain structures has been an indispensable technique for engineering the second-order optical nonlinearity of materials in three dimensions. It utilizes localized thermoelectric field motivated by nonlinear absorption at the position of laser focus to manipulate domains. However, the impact of laser wavelengths, which is pivotal in nonlinear absorption, on the inverted domains is still sketchy. Herein, the light-induced ferroelectric domain inversion is experimentally studied. It is shown that the domain inversions can be achieved over a broad spectral range, but the optical threshold for domain inversion varies dramatically with the laser wavelength, which can be explained by considering the physical mechanism of femtosecond laser poling and nonlinear absorption properties of the crystal. Meanwhile, the effects of other laser processing parameters are also experimentally investigated. Our findings are useful to guide the fabrication of high-performance optical and electronic devices based on ferroelectric domains.

Nonlinear generation of an optical bottle beam in domain-engineered ferroelectric crystals.

Nonlinear wavefront shaping in periodically poled ferroelectric crystals has received great attention because it offers a convenient way to generate a structured light beam at new frequencies. In contrast to structurally uniform beams like Laguerre-Gaussian or Hermite-Gaussian modes, here we demonstrate the possibility to generate a spatially varied optical bottle beam via a frequency doubling process in a domain-engineered Sr0.61Ba0.39Nb2O6 (SBN) crystal. The nonlinear holography method was employed to design the modulation pattern of the second-order nonlinear coefficient χ(2), and the femtosecond laser poling was used to imprint the χ(2) pattern into the SBN crystal via ferroelectric domain inversion. The second harmonic bottle beam with zero intensity in its center that is surrounded in all three dimensions by light was observed with the incidence of a fundamental Gaussian beam. These results are useful for nonlinear generation and control of structured light at new frequencies, which has important applications in nonlinear photonics and quantum optics.

Effect of spatial variation of the duty cycle in transverse second-harmonic generation.

Transverse second-harmonic generation, in which the emission angles of the second harmonic are determined by the spatial modulation of the quadratic nonlinearity, has important applications in nonlinear optical imaging, holography, and beam shaping. Here we study the role of the local duty cycle of the nonlinearity on the light intensity distribution in transverse second-harmonic generation, taking the generation of perfect vortices in periodically poled ferroelectric crystal as an example. We show, theoretically and experimentally, that spatial variations of the nonlinearity modulation must be accompanied by the corresponding changes of the width of inverted ferroelectric domains, to ensure uniformity of the light intensity distribution in the generated second harmonic. This work provides a fundamental way to achieve high-quality transverse second-harmonic generation and, hence, opens more possibilities in applications based on harmonic generation and its control.

Generation of watt-level supercontinuum covering 2-6.5 µm in an all-fiber structured infrared nonlinear transmission system.

We demonstrate a watt-level mid-infrared supercontinuum source, with the spectrum covering the infrared region from 2 to 6.5 µm, in an all-fiber structured laser transmission system. To further improve the SC spectral bandwidth, power and system compactness in the follow-up As2S3 fiber, we theoretically and experimentally explored some knotty problems that would potentially result in the As2S3 fiber end-facet failure and low SC output power during the high-power butt-coupling process and proposed an optimal coupling distance on the premise of the safety of As2S3 fiber end face. In addition, we also built a multi-pulse pumping model for the first time to more precisely estimate the SC spectral evolution in As2S3 fiber. This work will give an important reference to someone who is working on the all-fiber structured, high-power mid- and far-infrared supercontinuum source.

Metallic 2H-Tantalum Selenide Nanomaterials as Saturable Absorber for Dual-Wavelength Q-Switched Fiber Laser.

A novel 2H-phase transition metal dichalcogenide (TMD)-tantalum selenide (TaSe2) with metallic bandgap structure is a potential photoelectric material. A band structure simulation of TaSe2 via ab initio method indicated its metallic property. An effective multilayered TaSe2 saturable absorber (SA) was fabricated using liquid-phase exfoliation and optically driven deposition. The prepared 2H-TaSe2 SA was successfully used for a dual-wavelength Q-switched fiber laser with the minimum pulse width of 2.95 µs and the maximum peak power of 64 W. The repetition rate of the maximum pulse energy of 89.9 kHz was at the level of 188.9 nJ. The metallic 2H-TaSe2 with satisfactory saturable absorbing capability is a promising candidate for pulsed laser applications.

Generation of a square-shaped pulse in mode-locked fiber lasers with a microfiber-based few-layer Nb2C saturable absorber.

Niobium carbide (Nb2C), a novel two-dimensional MXene material, has attracted much attention due to its outstanding electronic and optical properties. In this work, a microfiber-based few-layer Nb2C saturable absorber (SA) is fabricated by the magnetron sputtering deposition technique. The reverse saturable absorption (RSA) response of few-layer Nb2C nanosheets is observed with I-scan measurements. The square-wave pulses (SWPs) are generated by using the as-prepared microfiber-based few-layer Nb2C SA in an erbium-doped fiber laser. The SWP width increases from 0.33 to 2.061 ns with the single pulse energy increases linearly up to 0.89 nJ while the amplitude remains as a constant. In addition, nonlinear polarization rotation mode-locking fiber lasers with different cavity lengths are constructed to explore the formation conditions of SWP. Our results indicate that the RSA effect of the few-layer Nb2C nanosheets plays a decisive role in the formation of the SWP.

Multi-wavelength bright-dark pulse pair fiber laser based on rhenium disulfide.

As a new member of transition metal dichalcogenides (TMDs), rhenium disulfide (ReS2), with a nearly unchanged direct bandgap from bulk to monolayer form, is attractive in physics and material fields. By using the optically driving deposition method, the ReS2 saturable absorber (SA) has been fabricated with a modulation depth and saturation fluence of 6.9% and 27.5 µJ/cm2, respectively. Based on the ReS2-SA, a multi-wavelength bright-dark pulse pair from a mode-locked fiber laser has been observed experimentally for the first time, to the best of our knowledge. The saturable absorbing ability of the ReS2 is attributed to the formation of the bright pulses and the dark pulses. The cross-phase modulation (XPM) caused by different wavebands of bright pulse and dark pulse support the coexisting of the bright-dark pulse pair.

Passively mode-locked 1.34 µm bulk laser based on few-layer black phosphorus saturable absorber.

By using few-layer black phosphorus (BP) as saturable absorber, an efficient mode-locked Nd:GdVO4 bulk laser operating at 1.34 µm was realized. An average output power of 350 mW was achieved with a slope efficiency of 15%. The corresponding mode-locking pulse repetition rate, pulse duration and pulse energy were 58.14 MHz, 9.24 ps and 3.0 nJ, respectively. To the best of our knowledge, the pulse width is the shortest among the mode-locked 1.34 µm neodymium lasers ever obtained with other two-dimensional materials saturable absorber. The results clearly indicate the few-layered BP is a kind of promising saturable absorber for ultrafast 1.34 µm lasers.

Multi-layered black phosphorus as saturable absorber for pulsed Cr:ZnSe laser at 2.4 µm.

A high-quality black phosphorus (BP) saturable-absorber mirror (SAM) was successfully fabricated with the multi-layered BP, prepared by liquid-phase exfoliation (LPE) method. The modulation depth and saturation power intensity of BP absorber were measured to be 10.7% and 0.96 MW/cm(2), respectively. Using the BP-SAM, we experimentally demonstrated the mid-infrared (mid-IR) pulse generation from a BP Q-switched Cr:ZnSe laser for the first time to our best knowledge. Stable Q-switched pulse as short as 189 ns with an average output power of 36 mW was realized at 2.4 µm, corresponding to a repetition rate of 176 kHz and a single pulse energy of 205 nJ. Our work sufficiently validated that multi-layer BP could be used as an optical modulator for mid-IR pulse laser sources.

Femtosecond solid-state laser based on a few-layered black phosphorus saturable absorber.

In this Letter, a high-quality, few-layered black phosphorus (BP) saturable absorber (SA) was fabricated successfully, and a femtosecond solid-state laser modulated by BP-SA was experimentally demonstrated for the first time, to the best of our knowledge. Pulses as short as 272 fs were achieved with an average output power of 0.82 W, corresponding to the pulse energy of 6.48 nJ and peak power of 23.8 MW. So far, these represent the shortest pulse duration and highest output power ever obtained with a BP-based mode-locked solid-state laser. The results indicate the promising potential of few-layered BP-SA for applications in solid-state femtosecond mode-locked lasers.

759 fs pulse generation with Nd3+-doped CNGS ordered crystal based on a semiconductor saturable absorber mirror.

A diode-pumped passively continuous wave mode-locked laser at 1064.2 nm based on an ordered Nd:CNGS crystal has been experimentally investigated (for the first time, to our knowledge). Stable mode-locked pulses with a duration of 759 fs were produced at a repetition rate of 43.2 MHz. It is the shortest pulse generation of mode-locked lasers based on Nd3+-doped ordered crystal, as far as we know. A maximum average mode-locked output power of 133 mW was obtained at the absorbed pumped power of 6.7 W, and corresponding single-pulse energy and peak power were determined to be 3.1 nJ and 4.1 kW, respectively. The results indicate that the Nd:CNGS as an ordered crystal is indeed a potential candidate as a femtosecond laser gain medium.

Exfoliated layers of black phosphorus as saturable absorber for ultrafast solid-state laser.

High-quality black phosphorus (BP) saturable absorber mirror (SAM) was successfully fabricated with few-layered BP (phosphorene). By employing the prepared phosphorene SAM, we have demonstrated ultrafast pulse generation from a BP mode-locked bulk laser for the first time to our best knowledge. Pulses as short as 6.1 ps with an average power of 460 mW were obtained at the central wavelength of 1064.1 nm. Considering the direct and flexible band gap for different layers of phosphorene, this work may provide a possible method for fabricating BP SAM to achieve ultrafast solid-state lasers in IR and mid-IR wavelength region.

High-power passively mode-locked laser at 1062.4 nm based on Nd:LaGGG disordered crystal.

A diode-pumped passively continuous-wave mode-locked Nd:(La(x)Gd(1-x))3Ga5O12 (Nd:LaGGG) laser at 1062.4 nm with a semiconductor saturable absorber mirror was demonstrated for the first time, to the best of our knowledge. Pulses with duration of 12.78 ps were produced at a repetition rate of 59.8 MHz. A maximum average mode-locked output power of 3.18 W was obtained at the absorbed pumped power of 10.12 W, corresponding to a slope efficiency of 35.7% and a peak power of 4.2 kW. As far as we know, this is the highest power obtained in the passively mode-locking operation with Nd3+-doped disordered garnet crystals.

760 fs diode-pumped mode-locked laser with Yb:LuAG crystal at 1032 nm.

In this study, we experimentally demonstrate the generation of 760 fs pulse duration from a diode-pumped Yb:LuAG mode-locked laser at 1032 nm. At the repetition rate of 58.6 MHz, the maximum average power of 1.07 W was obtained, corresponding to the peak power of 24 kW. To our knowledge, these results represent the shortest pulse duration and highest peak power ever obtained for a 1032 nm mode-locked laser with Yb:LuAG crystal.

Passively Q-switched 2 µm Tm:YAP laser based on graphene saturable absorber mirror.

Using high-quality single-layer graphene as a saturable absorber, Tm:YAlO3 (Tm:YAP) crystal as the gain medium, we demonstrated a laser-diode-pumped, compact, passively Q-switched (PQS) solid-state laser in the 2 µm region. The maximum average output power was 362 mW, with the corresponding largest pulse repetition rate and pulse energy of 42.4 kHz and 8.5 µJ, respectively. Under the same pump power, the pulse width of 735 ns was obtained, which is, to our best knowledge, the shortest pulse width among Tm-doped solid-state PQS lasers using graphene saturable absorber mirrors.

Optical modulation of microfibers and application to ultrafast fiber lasers.

Microfibers with different waist diameters were prepared successfully by a flame-brushing technique. Their saturable absorption properties were investigated. The non-saturable loss and modulation depth both decreased with the increase of the diameter. According to the mode distribution of microfibers with a waist diameter of 25 µm, it could be supposed that the evanescent field effect may be useful for microfibers being saturable absorbers (SAs). Based on the 25 µm-diameter microfiber, an all-fiber-structure mode-locked fiber laser was achieved successfully with long term stability in the span of a week. The results indicated that microfibers with suitable diameters were excellent SAs. To the best of our knowledge, this is first report on the usage of microfibers as a SA for building ultrafast fiber lasers.