Ultrafast Saturable Absorber Research Articles

Holographic Nano-Imaging of Terahertz Dirac Plasmon Polaritons in Topological Insulator Antenna Resonators.

Excitation of Dirac plasmon polaritons (DPPs) in bi-dimensional materials have attracted considerable interest in recent years, both from perspectives of understanding their physics and exploring their transformative potential for nanophotonic devices, including ultra-sensitive plasmonic sensors, ultrafast saturable absorbers, modulators, and switches. Topological insulators (TIs) represent an ideal technological platform in this respect because they can support plasmon polaritons formed by Dirac carriers in the topological surface states. Tracing propagation of DPPs is a very challenging task, particularly at terahertz (THz) frequencies, where the DPP wavelength becomes over one order of magnitude shorter than the free space photon wavelength. Furthermore, severe attenuation hinders the comprehensive analysis of their characteristics. Here, the properties of DPPs in real TI-based devices are revealed. Bi2Se3 rectangular antennas can efficiently confine the propagation of DPPs to a single dimension and, as a result, enhance the DPPs visibility despite the strong intrinsic attenuation. The plasmon dispersion and loss properties from plasmon profiles are experimentally determined, along the antennas, obtained using holographic near-field nano-imaging in a wide range of THz frequencies, from 2.05 to 4.3THz. The detailed investigation of the unveiled DPP properties can guide the design of novel topological quantum devices exploiting their directional propagation.

On-chip ultrafast pulse generation based on graphene-silicon hybrid waveguides

On-chip ultrafast mode-locking lasers are basic building blocks for the realization of a chip-based optical frequency comb. In this paper, an ultrafast saturable absorber made up of a graphene pad on top of a silicon waveguide is applied to implement an ultrafast pulse laser. Benefiting from the small mode area of the graphene/silicon hybrid waveguide, the saturable pulse energy is reduced by two orders of magnitude compared with the fiber. A mode-locked pulse with a duration of 542 fs and a repetition rate of 54.37 MHz is realized. Pump–probe measurement shows that the carrier relaxation process of free carrier recombination with atomic-thin graphene/silicon junctions is three orders of magnitude faster than silicon, which plays a fundamental role in pulse narrowing. The chip-scale silicon ultrafast laser lays a foundation for a new class of nonlinear devices, in which a combination with multiple functional silicon photonic circuits enables efficient nonlinear interaction at the micrometer scale and less than 1 W of power consumption.

Enhanced saturable absorption in the laser-treated free-standing carbon nanotube films.

We demonstrate an increase of optical transmittance and saturable absorption of laser-treated free-standing single-walled carbon nanotube (SWNT) films. The combined acid and low-power non-destructive laser treatment ensures an enhancement of linear transmittance across the visible range and double-digit increase of the saturable absorption of femtosecond laser radiation at 795 nm. The saturable absorption coefficient and the ratio of saturable to non-saturable losses increase by 26% and 35%, correspondingly, while the saturation intensity decreases by 20% because of the treatment. Our analysis indicates that with the performed treatment one can significantly improve the nonlinear optical properties of free-standing SWNT-based ultrafast saturable absorbers.

Ultrafast Saturable Absorbers: Fused Silica with Embedded 2D‐Like Ag Nanoparticle Monolayer: Tunable Saturable Absorbers by Interparticle Spacing Manipulation (Laser Photonics Rev. 14(2)/2020)

Ultrafast Saturable Absorbers: Fused Silica with Embedded 2D‐Like Ag Nanoparticle Monolayer: Tunable Saturable Absorbers by Interparticle Spacing Manipulation (Laser Photonics Rev. 14(2)/2020)

Self-starting mode-locked Cr:ZnS laser using single-walled carbon nanotubes with resonant absorption at 2.4 μm.

We develop a mode-locked Cr:ZnS polycrystalline laser using single-walled carbon nanotubes (SWCNTs) that have resonant absorption at the wavelength of 2.4μm. The laser generates ultrashort pulses of 49fs duration, a 2.4μm center wavelength, and a 9.2THz (176nm) spectral span at a repetition rate of 76MHz. We also confirm self-starting of the mode-locked operation. SWCNTs, if appropriately controlled in terms of their diameters, prove to be useful as ultrafast saturable absorbers in the mid-infrared region.

Liquid Exfoliation of Two-Dimensional PbI2 Nanosheets for Ultrafast Photonics

Two-dimensional (2D) materials are showing promise in next-generation optoelectronic devices. To this end, large-scale production of defect-free 2D nanosheets is vital. Herein, we demonstrate scalable exfoliation and preparation of high-quality 2D PbI2 nanosheets in cyclopentanone. The resulting dispersions can retain good stability over 30 days and can also sustain freezing treatment. The exfoliation kinetics was observed to fit an apparent pseudo-first-order process. Furthermore, the PbI2 nanosheets exhibit excellent saturable absorption properties both at femtosecond and nanosecond pulse excitations in the visible region, thus showing potential application in the field of ultrafast saturable absorbers.

Band Alignment of MoTe2/MoS2 Nanocomposite Films for Enhanced Nonlinear Optical Performance

AbstractBand alignment is a key issue for the optoelectronics based on 2D layered transition metal dichalcogenides (TMDs) heterostructures. Herein, band alignment of MoTe2/MoS2 mixed heterostructure is measured with high‐resolution X‐ray photoelectron spectroscopy. The MoTe2/MoS2 heterostructure belongs to type‐II heterostructure with the conduction band offset of 0.46 eV and the valence band offset of 0.9 eV. The stronger saturable absorption is observed in MoTe2/MoS2 heterostructure film compared with that of pure MoTe2 and MoS2 nanofilms at the same condition. An energy‐level model combined with Runge–Kutta algorithm is used to understand the enhancement mechanism. It is found that the interlayer transition from MoTe2/MoS2 heterojunction plays an important role in the nonlinear optical enhancement. Meanwhile, band structure of MoTe2/MoS2 heterostructure is calculated by the first principles. The contributions of the MoTe2 and MoS2 to the heterojunction are almost equal and the valence band maximum and conduction band minimum exist in MoTe2 and MoS2 separately. This structure can form the interlayer carriers easily. The results suggest that the band alignment of TMDs paves the way for the type‐II heterostructure for enhanced nonlinear response in the development of optical modulator, ultrafast laser mode lockers, saturable absorbers, and optical switches.

Few‐layer Bismuthene: Sonochemical Exfoliation, Nonlinear Optics and Applications for Ultrafast Photonics with Enhanced Stability

AbstractAs the last element in Group VA, bismuthene has garnered substantial interest for its unique electronic and mechanical properties and its enhanced stability. However, the mechanism that drives the light‐bismuthene interaction remains completely unclear. Herein, a sonochemical exfoliation approach is employed to deliver a successful synthesis of few‐layer bismuthene. The corresponding nonlinear optical response at the visible wavelength is investigated. The nonlinear refractive index is ∼10−6 cm2/W and was measured by spatial self‐phase modulation. Thanks to its direct energy band‐gap at 1550 nm, the saturable absorption property of bismuthene is experimentally illustrated at the telecommunication band with an optical modulation depth of ∼2.03% and a saturable intensity of ∼30 MW/cm2. The optimization of the laser parameters resulted in the generation of an ∼652‐femtosecond optical pulse centered at 1559.18 nm. This result indicates that the bismuthene‐based saturable absorber is indeed a new and excellent material for an ultrafast saturable absorber device. Our work highlights the promise of this material in ultrafast photonics and may be considered as an important step towards bismuthene‐based photonics devices (optical modulator, optical switcher, detector, etc.).

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

Effect of beam expansion loss in a carbon nanotube-doped PVA film on passively mode-locked erbium-doped fiber lasers with different feedback ratios

The effect of beam expansion induced divergent loss in a single-wall carbon nanotube (SWCNT) doped polyvinyl alcohol (PVA) based ultrafast saturable absorber (SA) film thickness on the passive mode-locking (PML) performances of erbium-doped fiber lasers are demonstrated. The variation on the PML pulsewidth of the EDFL is discussed by changing the SWCNT-PVA SA film thicknesses, together with adjusting the pumping power and the intra-cavity feedback ratio. An almost 6 dB increment of divergent loss when enlarging the SWCNT-PVA based SA film thickness from 30–130 µm is observed. When shrinking the SA thickness to 30 µm at the largest pumping power of 52.5 mW, the optical spectrum red-shifts to 1558.8 nm with its 3 dB spectral linewidth broadening up to 2.7 nm, while the pulse has already entered the soliton regime with multi-order Kelly sidebands aside the spectral shoulder. The soliton pulsewidth is as short as 790 fs, which is much shorter than those obtained with other thicker SWCNT doped PVA polymer film based SAs; therefore, the peak power from the output of the PML-EDFL is significantly enlarged accompanied by a completely suppressed residual continuous-wave level to achieve the largest on/off extinction ratio. The main mechanism of pulse shortening with reducing thickness of SWCNT doped PVA polymer film based SA is attributed to the limited beam expansion as well as the enlarged modulation depth, which results in shortened soliton pulsewidth with a clean dc background, and broadened spectrum with enriched Kelly sidebands. The increase of total SWCNT amount in the thicker SA inevitably causes a higher linear absorption; hence, the mode-locking threshold also rises accordingly. By enlarging pumping power from 38.5–52.5 mW, the highest ascent on pulse extinction of up to 32 dB is observed among all kinds of feedback conditions. Nevertheless, the enlargement on the extinction slightly decays with increasing the feedback ratio from 30–90%, as the ascent slope gradually decays due to the more severe gain saturation effect occurring when the mode-locking gradually completes at this stage.

Femtosecond single-mode diode-pumped Cr:LiSAF laser mode-locked with single-walled carbon nanotubes

A low threshold Cr:LiSAF laser pumped with an inexpensive single-mode laser diode emitting 120 mW was passively mode-locked with a novel ultrafast saturable absorber mirror based on single-walled carbon nanotubes (SWCNT-SAM). Pulses as short as 122 fs were achieved, tunable across 14 nm. A second pump diode coupled in polarization allowed to shorten the pulse duration to 106 fs, with up to 24-mW output power.

Nanostructured Plasmonic Medium for Terahertz Bandwidth All‐Optical Switching

Periodic nanostructuring can enhance the optical nonlinearity of plasmonic metals by several orders of magnitude. By patterning a gold film, the largest sub-100 femtosecond nonlinearity is achieved, which is suitable for terahertz rate all-optical data processing as well as ultrafast optical limiters and saturable absorbers.

Ultrafast saturable absorption devices incorporating efficiently electrosprayed carbon nanotubes

We demonstrate saturable absorption operation of single-walled carbon nanotubes (CNTs) deposited by an efficient electrospray process that provides target-localized deposition with a homogeneous dispersion of the nanomaterials. An improvement in deposition efficiency of 95% is achieved. CNT dispersion/deposition conditions are optimized for suppressed CNT agglomeration, thereby maximizing the nonlinear absorption of CNTs. An ultrafast saturable absorber is formed with side-polished fiber as a substrate for evanescent field interaction of propagated light with CNTs. A resultant fiber mode-locked laser is realized. The laser has output with an estimated pulse duration and repetition rate of 190.9 fs and 3.95 MHz, respectively.

Fabrication and characterization of ultrafast carbon nanotube saturable absorbers for solid-state laser mode locking near 1μm

Transmitting and reflecting ultrafast saturable absorbers based on single-walled carbon nanotubes are developed that are applicable for stable mode locking of bulk solid-state lasers operating near 1μm. For fabrication of these saturable absorbers, relatively simple spin coating and spray methods are employed. Parameters important for stable mode locking, such as transient nonlinear absorption, saturation fluence, and recovery time, are investigated by nonlinear transmission and time-resolved pump-probe measurements near 1μm. Typical modulation depths and recovery times amount to ∼0.21%–0.25% and <1ps, respectively.

Coherent and ultrafast optoelectronics in III–V semiconductor compounds

AbstractIII–V compound semiconductors offer a fascinating multitude of phenomena which have become accessible via ultrafast time‐resolved spectroscopy. Coherent vibronic and electronic dynamics are prepared by excitation with taylored femtosecond laser pulses. The analysis of their temporal dephasing or decay provides deep insights into the interaction between electronic and vibronic degrees of freedem and the surrounding bath in high purity quantum structures. In contrast to coherent electronic or vibronic states, deliberately introduced growth defects can be used to drastically shorten the lifetime of optically excited carriers. Sub‐picosecond carrier lifetimes open the possibility to realize ultrafast saturable absorbers and optoelectronic transducer elements. They are particularly important as key elements in THz technology, such as efficient THz emitters, detectors, and for on‐chip THz technology. This paper summarizes the most distinguished results relevant in the context of ultrafast optoelectronics and THz technology obtained in close collaboration with the Paul‐Drude‐Institute Berlin over the past decade. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast saturable absorbers based on carbon nanotubes and their applications to passively mode‐locked fiber lasers

AbstractThe authors propose third‐generation small and high‐speed saturable absorbers to replace semiconductor‐based saturable absorbers and optical‐fiber‐based nonlinear devices. The device is based on the carbon nanotube (CNT). This paper discusses the characteristics of CNT‐based saturable absorbers. The present status of research on passively mode‐locked fiber lasers using CNT‐based saturable absorbers is reported. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(2): 17– 24, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20272

Nonlinear Dynamics of Temporal Optical Soliton Molecules in Lasers

Recent experiments demonstrate that fiber laser cavities are able to support various multisoliton complexes, analogous to soliton molecules. These advances, which could have impact on optical information transmission or storage, are guided by the concept of dissipative soliton and supported by numerical simulations. DOI: 10.2529/PIERS060828120520 As passively mode-locked lasers rely strongly on nonlinear dissipation, there is a growing interest in understanding various pulse dynamics in terms of the dynamics of dissipative solitons [1]. In particular, the interaction between dissipative temporal solitons can lead to the formation of stable multi-soliton complexes. The stability of multi-soliton complexes arises from the existence of attractors that are able to bind stably several pulses in a way somehow analogous to the formation of molecules. The simplest of these complexes is the soliton pair, or stable bound state of two solitons. It was predicted in the frame of the Ginzburg-Landau propagation equation model and characterized by a fixed distance, as well as a fixed phase relationship of ±π/2 between the two solitons [2], then found experimentally in a passively mode-locked erbium-doped fiber ring laser [3]. A soliton pair uses the dissipative nonlinear dynamics of the active cavity in order to remain stable for hours without the need of external feed back stabilization. The formation of soliton molecules is not limited to diatomic molecules: triatomic and other polyatomic molecules were also reported [4]. Moreover, the ability to form soliton molecules is not limited to a very specific cavity design, it is rather a general feature that make them potentially observable in most passively mode-locked laser cavity schemes, since it relies mainly on the strength of nonlinear dissipation, which in turn provides attractor sets for soliton molecules formation. For instance, soliton molecules were found with negative and with positive path-averaged chromatic dispersions, as well as with low and high pumping powers [1, 4]. In the following, after having briefly presented the experimental setup used in the experiments, we shall discuss the latest issues discovered with soliton molecules in fiber lasers. They concern the possible control of formation and dissociation of soliton molecules, as well as vibrational motions. Figure 1: Sketch of the fiber laser experimental setup. PIERS ONLINE, VOL. 3, NO. 3, 2007 358 1. FIBER LASER EXPERIMENTAL SETUP As sketched in Fig. 1, we use a dispersion-managed fiber ring laser which series a 1.6-meter-long 1400-ppm erbium-doped fiber (EDF) in the normal dispersion regime (D = −35 ps.nm−1.km−1) which provides laser emission around 1.5μm, a polarization-insensitive optical isolator, a given length of standard telecom fiber (SMF) in the anomalous dispersion regime (D = ±17 ps. nm−1.km−1), and an open-air section that includes retarding wave plates and a polarizing beam splitter cube. Nonlinear polarization evolution that takes place in both fibers, along with polarization discrimination by the polarizing cube, provides the equivalent of an ultrafast saturable absorber. Under a 980-nm pumping power in the 100-300mW range, stable mode locking is achieved with appropriate adjustment of the retarding wave plates that precede the polarizing cube. According to the length of SMF chosen for a given experiment, the averaged cavity dispersion can be varied between anomalous or normal. Under adjustments of pump power and retarding wave plates, pulse durations are in the 100-600 fs range, with intracavity energies in the 50-500 pJ range. Using these adjustments of pump power and retarding wave plates, we are able to switch from single soliton to multiple soliton operation. Part of the multiple soliton operations lead to the formation of stable multisoliton complexes, such as the stable soliton pair [3].

Ultrafast monolithic semiconductor-polymer Kerr-lens saturable absorber

We propose an ultrafast Kerr-lens saturable absorber, which is suitable for incorporating monolithically into semiconductor waveguides. We propose suitable materials and geometries for device fabrication. We study the saturable absorption using numerical methods and consider the effect of such a hard Kerr-lens configuration on a monolithic passively mode-locked semiconductor diode device and the possibility of producing pulses of the order of hundreds of femtoseconds.

HIGHLY FE-DOPED InGaAs/InP SATURABLE ABSORBER MODE-LOCKING OF AN ERBIUM FIBER LASER

Erbium-doped unidirectional fiber ring laser has been passively mode-locked with a novel ultra-fast semiconductor saturable absorber used in transmission. The structure is based on InGaAs/InP multiple quantum wells grown on an InP substrate. Different structures with recovery time down to picosecond have been tested. Relaxation time reduction have been obtained by iron doping. With such structures, detrimental regimes such as Q-switch and Q-switch/ModeLock regimes have been avoided. Solitonic and slightly stretched-pulse dispersion regimes have been tested. The laser delivers for both, completely self-starting picosecond pulses of 8 mW maximum average output power at a repetition rate around 25 MHz.

Ultra-fast saturable absorber through spatial self-trapping and filtering in Ti:PPLN film waveguides

Numerical and experimental investigations on ultra-fast all-optical saturable absorber on picosecond optical pulses at 1547 nm using spatial self-trapped propagation in a quadratic nonlinear film waveguide combined with spatial filtering are reported. The influences of phase-mismatch, pulse intensity and spatial filtering on the temporal reshaping mechanism are discussed to derive the optimum parameters.