Optical Saturable Absorption Research Articles

Nonlinear Optical Saturable Absorption Properties of 2D VP Nanosheets and Application as SA in a Passively Q-Switched Nd:YVO4 Laser.

Two-dimensional (2D) violet phosphorus (VP) plays a significant role in the applications of photonic and optoelectronic devices due to its unique optical and electrical properties. The ultrafast carrier dynamics and nonlinear optical absorption properties were systematically investigated here. The intra- and inter-band ultrafast relaxation times of 2D VP nanosheets were measured to be ~6.83 ps and ~62.91 ps using the pump-probe method with a probe laser operating at 1.03 μm. The nonlinear absorption coefficient βeff, the saturation intensity Is, the modulation depth ΔR, and the nonsaturable loss were determined to be -2.18 × 104 cm/MW, 329 kW/cm2, 6.3%, and 9.8%, respectively, by using the Z-scan and I-scan methods, indicating the tremendous saturable absorption property of 2D VP nanosheets. Furthermore, the passively Q-switched Nd:YVO4 laser was realized with the 2D VP nanosheet-based SA, in which the average output power of 700 mW and the pulse duration of 478 ns were obtained. These results effectively reveal the nonlinear optical absorption characteristics of VP nanosheets, demonstrating their outstanding light-manipulating capabilities and providing a basis for the applications of ultrafast optical devices. Our results verify the excellent saturable absorption properties of 2D VP, paving the way for its applications in pulsed laser generation.

Multi-Functional Applications of H-Glass Embedded with Stable Plasmonic Gold Nanoislands.

Metal nanoparticles (MNPs) are synthesized using various techniques on diverse substrates that significantly impact their properties. However, among the substrate materials investigated, the major challenge is the stability of MNPs due to their poor adhesion to the substrate. Herein,it isdemonstrated how a newly developed H-glass can concurrently stabilize plasmonic gold nanoislands (GNIs) and offer multifunctional applications. The GNIs on the H-glassaresynthesized using a simple yet, robust thermal dewetting process. The H-glass embedded with GNIs demonstrates versatility in its applications, such as i) acting as a room temperature chemiresistive gas sensor (70% response for NO2 gas); ii) serving as substrates for surface-enhanced Raman spectroscopy for the identifications of Nile blue (dye) and picric acid (explosive) analytes down to nanomolar concentrations with enhancement factors of 4.8 × 106 and 6.1 × 105 , respectively; and iii) functioning as a nonlinear optical saturable absorber with a saturation intensity of 18.36 × 1015 Wm-2 at 600nm, and the performance characteristics are on par with those of materials reported in the existing literature. This work establishes a facile strategy to develop advanced materials by depositing metal nanoislands on glass for various functional applications.

Effect of Cerium dopant on third-order nonlinear optical properties of CdS/PEG self-standing nanocomposite films

CdS/PEG, 4 and 7 mol% Ce doped CdS/PEG nanocomposite films were synthesized by in situ method. The XRD pattern confirmed the cubic structure of the CdS and the size of the CdS nanocrystallite is found to be smaller than the Bohr exciton radius for all the samples. SEM analysis shows that CdS and Ce doped CdS nanoparticles are well mono-dispersed in PEG polymer. FTIR spectra confirm the interaction of Ce-doped CdS nanoparticles with PEG polymer. UV–Vis optical absorption studies exhibit a red-shift of CdS nanoparticle with Ce doping. The photoluminescence spectra of all the films show a red emission due to the surface defects. The third-order nonlinear optical properties of all the films have been studied using the Z-scan technique. Closed and open aperture Z-scan plots show self-focusing optical nonlinearity and reverse saturable absorption with positive nonlinear coefficients. The 7 mol% Ce doped CdS/PEG nanocomposite film exhibits the highest value of nonlinear refractive index and nonlinear absorption coefficient which are about 1.649 × 10−9 m2/W and 7.149 × 10−3 m/W. The third-order nonlinear optical susceptibility and figure of merit have also been estimated for all the films and also increase with increase in Ce dopant concentrations. The maximum value of third-order nonlinear optical susceptibility and figure merit are found for 7 mol% Ce doped CdS/PEG nanocomposite film and are about 9.882 × 10−4 esu and 5.902 × 10−8 esu m. The present study strongly recommends that the Ce doped CdS/PEG nanocomposite films are worthy candidates for fabricating the future optical limiting devices.

Optical saturable absorption of conformal graphene directly synthesized on nonlinear device surfaces

Optical saturable absorption (SA) in graphene has been intensively studied during the last decades to achieve graphene-based ultrafast pulse generation. We experimentally investigate the optical nonlinearities of transfer-free graphene synthesized by the atomic carbon spraying (ACS) method that is invented for conformal and uniform graphene coating on non-planar device surfaces to enhance the interaction of lasers with the nanostructures. Its successful implementation of nonlinear operations in practical photonic devices is confirmed. The SAs of two other different graphene structures including graphene prepared by chemical vapor deposition (CVD) and reduced graphene oxides (rGOs) by electro-spraying (ES) are quantitatively evaluated as references. All three nanocrystal structures are elucidated by transmission electron microscopy and Raman analysis comparatively and systematically. We confirm in X-ray photoelectron spectroscopy analysis that the contents of sp2 bonds in ACS graphene as an indicator of the nonlinearity is 73.94 %. The measured value reaches the level of CVD graphene (76.70 %) and is much higher than 58.75 % of ES rGOs. Importantly, the ACS graphene displays a steep SA slope with saturation intensity of 32.14 MW cm−2, an order of magnitude lower than conventional ES rGOs, verifying unimpaired nonlinear properties of graphene prepared by the suggested ACS method.

Bi2Te3/Graphene Heterostructure as the Saturable Absorber for ~1.0 μm Passively Q-switched Solid State Pulsed Laser

Due to the tunable nonlinear optical properties of the Bi2Te3/graphene heterostructure, stable solid state pulsed lasers based on the Bi2Te3/graphene saturable absorber have attracted intensive attention. In this work, the Bi2Te3/graphene heterostructure with good nonlinear absorption characteristics was synthesized by a self-assembly solvothermal route, and the optical saturable absorption properties of the saturable absorber were investigated. Owing to the large modulation depth of Bi2Te3 nanosheets and the high thermal conductivity of graphene, the Bi2Te3/graphene heterostructure saturable absorber shown good nonlinear saturable absorber performance and contributed the improved passively Q-switched Yb3+: GdAl3(BO3)4 pulsed laser when compared with that of the pure Bi2Te3 based Yb3+: GdAl3(BO3)4 laser, no matter pulse width or pulse energy. Our work demonstrates that the Bi2Te3/graphene heterostructure was a promising saturable absorber in ~1 μm solid-state pulsed lasers.

Layer-Dependent Nonlinear Optical Properties of WS2, MoS2, and Bi2S3 Films Synthesized by Chemical Vapor Deposition.

Two-dimensional (2D) layered materials have shown layer-dependent optical properties in both linear optical and nonlinear optical (NLO) regimes due to prominent interlayer coupling and quantum confinement in an atomic scale. However, the NLO properties become more complicated as both saturable absorption (SA) and reverse saturable absorption (RSA) easily happen in 2D materials, which results in a significant challenge to understand the evolution of nonlinear absorption with layers. Motivated by this, chemical vapor-deposited chalcogenide compounds (WS2, MoS2, and Bi2S3) are used to investigate the pump intensity and layer number-dependent NLO properties. The values of nonlinear absorption coefficients of these chalcogenide compounds increase with the pump intensity by an 800 nm femtosecond laser, which can be described by an empirical power law function. The SA process due to the large transition probability of the ground state readily takes place in thick samples, while RSA occurs easily in thin samples due to the two-photon absorption (TPA). The transition from TPA to SA is deduced to occur at 13L-WS2, 15L-MoS2, and 5L-Bi2S3, which is related to the layer-dependent band gaps. Our results provide an efficient way to tune optical nonlinearities with a controlled layer number and to design corresponding NLO devices such as optical switches and saturable absorbers.

Few-layer ZrTe3 nanosheets for ultrashort pulse mode-locked laser in 1.55 μm region

As a new two-dimensional (2D) material, zirconium telluride (ZrTe3) is a typical charge density wave (CDW) material with strong electrical and optical anisotropy compared with graphene, transition metal disulfides (TMDs), topological insulators (TIs), and black phosphorus (BPs). To date, the nonlinear optical properties of ZrTe3 and its applications in information photonics aspect have not been reported. In this work, the nonlinear optical saturable absorption properties of ZrTe3 nanosheets and their application in passively mode-locked fiber laser are systematically investigated for the first time. The results show that the modulation depth and saturation intensity are 6.5% and 89.87 MW/cm2 at the optical fiber communication band (C band), respectively. In the erbium-doped fiber (EDF) ring laser with ZrTe3 nanosheets as saturable absorber (SA), a stable mode-locked pulse train is obtained with the central wavelength of 1562.12 nm, the 3 dB optical spectral width of 2.07 nm, the basic repetition rate of 3.377 MHz, and the pulse duration of 1.469 ps when the pump power is 65 mW. Moreover, the dynamic evolution process of the optical spectrum and pulse sequence under different pump power are illustrated. All results reveal that ZrTe3 has excellent potential for applying in many fields, including ultra-fast all-optical sampling, optical frequency comb, optical fiber communication, supercontinuum, etc, and provides an intuition of opening new avenues toward various advanced information photonic devices.

Temporal dissipative structures in optical Kerr resonators with transient loss fluctuation.

Dissipative structures are the result of spontaneous symmetry breaking in a dynamic open system, which is induced by either the nonlinear effect or loss fluctuations. While optical temporal dissipative solitons in nonlinear Kerr cavities has been widely studied, their operation is limited to the red-detuned regime. Here, we demonstrate an emergent dissipative soliton state in optical nonlinear cavities in the presence of loss fluctuations, which is accessible by self-evolution of the system on resonance. Based on a modified dissipative and Kerr-nonlinear cavity model, we numerically investigate the effect of the loss modulation on the intracavity field pattern, and in transmission observe a single and bright soliton pulse state at the zero detuning. The effect of the optical saturable absorption is also numerically investigated, which is recognized as an effective approach to the transient loss fluctuation in the cavity. The estimated power efficiency of the resonant bright soliton can be higher than that of the conventional dissipative Kerr soliton, which is determined by the loss modulation depth and the pump intensity. The self-starting soliton state on system's resonance is potentially of wide interest, which physically contributes to insights of the temporal structure formation in dissipative cavities. On application aspect, it may constitute a way to the generation of ultra-fast soliton pulse trains as well as the generation of soliton micro-combs.

SWCNT@BNNT With 1D Van Der Waals Heterostructure With a High Optical Damage Threshold for Laser Mode-Locking

Single-walled carbon nanotube encapsulated in boron nitrite nanotube (SWCNT@BNNT) is a novel nanomaterial with a one-dimensional van der Waals (1D-vdW) heterostructure. In this paper, we demonstrated that the SWCNT@BNNT has an enhanced optical power tolerance compared to that of the pristine SWCNT while exhibiting an optical saturable absorption properties. Under optical power intensity of 13kW/cm2, the lifetime of the SWCNT@BNNT is found to be 2,270 times longer than SWCNT for 1% degradation in absorbance. A short-cavity mode-locked laser with a high repetition rate of 1 GHz has been realized using the SWCNT@BNNT as the saturable absorber. We have shown that the technique of fabricating nanomaterial with a 1D-vdW heterostructure can modify and enhance the optical properties of the encapsulated nanomaterials.

Nonlinear Optical Response of SbSI Nanorods Dominated with Direct Band Gaps

Based on its excellent visible light absorption and photoelectric response capabilities, SbSI has attracted significant research attention for a wide range of applications. Despite the fact that interesting physical properties can be induced from the sophisticated band structures of SbSI, that is, either with direct or indirect band gaps, its nonlinear optical (NLO) properties are not yet fully explored. In the present work, the NLO response of SbSI nanorods is investigated by the combination of open-aperture (OA) Z-scan, pump–probe, and spatial self-phase modulation (SSPM). It is found that the synthesized SbSI nanorods possess a relatively large nonlinear absorption coefficient and a small saturable optical intensity at a wavelength of 400 nm, compared to those of other materials. More importantly, the OA Z-scan experiments suggest that the direct band gap of SbSI plays a dominant role on the NLO effects, which is further verified by the SSPM experiments. The present work suggests that the SbSI nanorods may have great potentials for optical switches and saturable absorbers.

Photonic device combined optical microfiber coupler with saturable-absorption materials and its application in mode-locked fiber laser.

We demonstrated a mode-locked fiber laser based on a novel photonic device that combined optical microfiber coupler (OMC) and saturable absorption materials. The stable ultrafast laser was formed based on the interaction between the deposited Indium Antimonide (InSb) and the evanescent field on OMC. Different from optical microfiber (OM), OMC can directly output the mode-locked laser without additional beam splitting devices, which further improves the integrated characteristics of the fiber laser. The pulse duration of the output pulse is 405 fs at the central wavelength of 1560 nm. To the best of our knowledge, this is the first time that optical microfiber coupler based saturable absorber (OMC-SA) for mode-locked fiber laser is demonstrated.

Highly enhanced nonlinear optical absorption with ultrafast charge transfer of reduced graphene oxide hybridized by an azobenzene derivative.

Graphene-based materials have been attracted many attentions due to their excellent properties and potential applications in many fields. Graphene also provides a flexible substrate to develop novel functional materials by hybridizing with other organic or inorganic components. Herein, we report the functionalization of reduced graphene oxide (RGO) with an azobenzene derivative (BNB-t8) containing the π-conjugated moiety and hydrogen bonding groups, to improve the optical and nonlinear optical properties of RGO. With the introducing of BNB-t8, a new absorption band is formed and dominates the absorption spectrum, clearly demonstrates that the BNB-t8 has been hybridized with RGO, by combining the analysis of Raman and XRD data. Femtosecond Z-scan results present a highly enhanced saturable optical absorption of BNB-t8/RGO hybrid compared with that of RGO. By optimizing the hybridization ratio of BNB-t8 to RGO, the saturable absorption coefficient of BNB-t8/RGO hybrid reaches to -237 m/W, 38 times larger than that of RGO (-6.2 m/W). In the meantime, the third-order susceptibility χ(3) of BNB-t8/RGO hybrid is aslo enhanced by 8 times to be 5.18×10-13 esu. These enhancements of nonlinear optical properties of BNB-t8/RGO hybrid mainly arise from the charge transfer from RGO to BNB-t8. Femtosecond transient absorption measurements reveal that the charge separation takes place in 0.28 ps and the charge recombination in 2.0 ps, indicating a strong electron coupling and thus an enhanced electron delocalization in BNB-t8/RGO hybrid compared with those in RGO. We suggest that the noncovalent π-π interaction plays the dominant role for enhancing the electron delocalization of RGO after hybridizing with BNB-t8, while the hydrogen bonding interaction reinforce the coupling interaction between BNB-t8 and RGO moieties in the hybrid. The as-prepared BNB-t8/RGO hybrid with high saturable absorption coefficient with an ultrafast response presents a potential candidate as saturable absorber of mode-locked laser.

Free-Space Optical Neural Network Based on Optical Nonlinearity and Pooling Operations

Despite various optical realizations of convolutional neural networks (CNNs), optical implementation of nonlinear activation functions and pooling operations are still challenging problems. In this regard, this paper proposes an optical saturable absorption nonlinearity and its atomic-level model, as well as two various optical pooling operations, namely optical average pooling and optical motion pooling, by means of 4f optical correlators. Proposing these optical building blocks not only speed up the neural networks due to negligible optical processing latency, but also facilitate the concatenation of optical convolutional layers with no optoelectrical conversions in-between, as the significant bottlenecks of implementing photonic CNNs. Furthermore, the proposed optical motion pooling layer increases the translation invariance property of CNNs, avoiding the inclusion of all corresponding translated images for the training procedure, and hence, increases the training speed of the neural network. The classification accuracy of the proposed optical convolutional layer is evaluated as the first layer of a customized version of AlexNet architecture, named as OP-AlexNet, for classification of Kaggle Cats and Dog challenge, CIFAR-10, and MNIST datasets, as 83.76%, 72.82%, and 99.25%, respectively, by using optical average pooling.

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.

MoO3-x as a wideband optical saturable absorber for passively Q-switching ytterbium-, erbium-, and thulium-doped fiber lasers

Wideband optical saturable absorbers (SA) are an ideal laser component for pulse laser generation in different wavelengths, which has attracted tremendous attention in recent years. Herein, a MoO3-x-based novel wideband optical SA has been demonstrated by utilizing it for ∼1/1.5/2 μm Q-switched pulses generation. After separately integrating the MoO3-x-SA into Yb-, Er- and Tm-doped fiber lasers, passively Q-switched pulses with pulse durations of ∼1.5/2.2/1.6 μs, repetition rates of several tens kilohertz at corresponding wavelengths of ∼1038/1562.9/1910 nm are obtained. Our work firstly reveals the remarkable wideband optical saturable absorption of the MoO3-x, which strongly implies the potential application in infrared photonics devices. It may also provide new opportunities for wideband optical SA based on oxygen-deficient metal oxides.

Charge transfer induced excitons and nonlinear optical properties of ZnO/PEDOT:PSS nanocomposite films.

In this current work, we have prepared zinc oxide (ZnO) nanorods by sol-gel method, and its composite films with a conducting polymer poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) also have been prepared by drop-casting method on the glass substrate. UV-Vis absorption and steady-state fluorescence studies revealed exciton dissociation and recombination at the interface of polymer chain and wide-bandgap semiconductor ZnO. Also, nonlinear optical properties of as-prepared nanocomposite films have been reported by employing an open aperture z-scan technique. A predominantly two-photon induced saturable absorption behavior, when excited with 532nm, 10ns laser pulses, appeared in nonlinear optical measurements. These results indicate that our as-synthesized composites can be useful in fabricating optical switch and saturable absorbers.

Emerging High-Performance SnS/CdS Nanoflower Heterojunction for Ultrafast Photonics.

Metal sulfide nanomaterials show many unique photoelectric properties when they are constructed as heterojunction materials, which have made them attractive in recent years. Among various applications of these heterojunction materials, nonlinear optical properties and related applications are promising research fields. Herein, a novel high performance SnS/CdS nanoflower heterostructure is successfully prepared by a water bath method. Scanning electron microscopy (SEM) images suggest the formation of a nanoheterojunction between SnS and CdS. In addition, a large modulation depth (13.6%) and a low saturation intensity (230.6 MW/cm2) of the SnS/CdS heterostructure are demonstrated, which indicates the outstanding potential of the SnS/CdS heterostructure in photonics among the other emerging novel nonlinear optical (NLO) materials. Meanwhile, the surface morphology, structures, and optical characteristics of the as-prepared SnS/CdS nanoflower sample are systemically analyzed. Furthermore, an ultrashort pulse laser with a fundamental repetition rate of 34.3 MHz, a pulse width of 558 fs, and a spectral width of 8.6 nm is realized at a central wavelength of 1560.8 nm. More importantly, we have successfully realized a soliton molecule with controllable pulse-pulse separation from 2.8 to 10.2 ps by controlling the phase difference of the cavity. This work reveals the excellent nonlinear optical properties of the SnS/CdS heterostructure and lays a foundation for its development in advanced optical modulators, saturable absorbers, and optical switching devices.

Size-dependent nonlinear optical absorption in SnO2 quantum dots

Using a Z-scan technique, we report size-dependent changeover from reverse saturable absorption to saturable nonlinear optical absorption properties of as-deposited and annealed stannic oxide semiconductor quantum dot thin films, prepared by a sol-gel technique on a quartz substrate. The average size of quantum dots, as obtained from grazing incident x-ray diffraction, was found to be in the range of the exciton Bohr radius. Atomic force microscopy data confirmed the spherical shape of quantum dots. The UV-Vis data of the films displayed bandgap enhancement due to the combined effect of Coulombic and confinement potentials. A density matrix analysis was conducted to obtain the transition energy-dependent nonlinear optical absorption coefficient in the cw excitation regime. We found a qualitative agreement between the experimental observations and theoretical calculations.

Emerging 2D Semiconducting Materials: Tin Diselenide for Ultrafast Photonics

AbstractSnSe2, a novel 2D semiconducting material with good photoelectric properties and tunable bandgaps structure, has been widely studied in high‐performance photodetectors and superconductivity applications, etc. Herein, another excellent property, nonlinear optical absorption, is applied to ultrafast photonics. The measured nonlinear optical absorption curve of SnSe2 with modulation depth of 5.52% is obtained. Q‐switching and mode‐locking in Er‐doped fiber lasers based on SnSe2 is reported. A Q‐switched fiber laser can be obtained with a 0.5‐m‐long gain fiber and a 50:50 optical coupler. With 0.75‐m‐long active fiber and a 40:60 optical coupler, mode locking can be obtained. Furthermore, a third‐order bound‐state soliton pulse with a pulse duration of 882 fs can be achieved. The experimental results show that SnSe2 is an excellent optical saturable absorber for the applications of ultrafast photonics.

Two-dimensional tellurene nanosheets as saturable absorber of passively Q-switched Nd:YAG solid-state laser

For the first time, a group-VI single element nanomaterial was used as the optical saturable absorber (SA) to generate laser pulses. With two-dimensional (2D) tellurene as a passive Q-switch, 1.06 μm and 1.3 μm pulse laser operations were realized from a diode-pumped Nd:YAG crystal. The shortest pulse widths were 98 ns and 178 ns, and the highest peak powers were 2.68 W and 2.45 W, respectively. Our research determines that tellurene is an excellent SA material in the near-infrared region.