Nonlinear Absorption Properties Research Articles

Non-linear photo-switching in molecular actuators through intra-molecular energy transfer from an electron donating core

Switching of molecular actuators by indirect biphotonic excitation of a centrosymmetric antenna, followed by intra-molecular energy transfer.

Nonlinear Optical Studies of [Pt17(CO)12(PPh3)8]n+ Metal Nanoclusters and Their Enhancement via All-Plastic Photonic Crystal Cavity

Metal nanoclusters of atomic precision have recently emerged as potential candidates for nonlinear optical applications owing to their strong quantum confinement effects. Herein, we report the nonlinear absorptive and refractive properties of platinum-17 (Pt17) metal nanoclusters stabilized by carbon monoxide and triphenyl phosphine ligands. The open aperture z-scan studies in the nanosecond regime reveal a transmittance dip at the focus characteristic of molecule-like behavior and discrete energy levels. The clusters also exhibited a significantly lower optical limiting threshold value and self-defocusing behavior which can be deployed to resist intense optical radiation. We also demonstrate a photonic crystal cavity-assisted enhancement in the absorptive nonlinearity of the synthesized clusters as a result of intense field localization in the defect layer. Our study throws light on the application of these ultrasmall structures in nanophotonic devices.

Carbon nanoparticles (CNPs) as a saturable absorber for a passively Q-switched erbium (Er3+) doped fiber laser

The carbon nanoparticles (CNPs) are demonstrated to be capable of acting as a saturable absorber (SA) for the Q-switching of an erbium doped fiber laser (EDFL). The Q-switched EDFL based on CNPs-SA emits at a central wavelength of 1566 nm with a 3 dB spectral bandwidth of 1.72 nm. The radio-frequency (RF) spectrum has a signal-to-noise ratio (SNR) of 42 dB at the fundamental repetiton rate, indicating a good pulse stability. The stable pulse operation can be maintained under the pump power of ranging from 28 to 272 mW. The pulse repetition frequency increases from 12.26 to 73.30 kHz, and the pulse narrows down from 23.32 to 6.27 μs, correspondingly. In addition, we investigate the nonlinear saturable absorption property of CNPs, and measure the SA’s damange threshold as well as the modulation depth. The synthesized CNPs are also characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Energy dispersive X-rays (EDX), Raman, Photoluminescence (PL), X-rays diffraction (XRD), and Fourier Transform Infrared (FTIR) spectroscopy. These experimental results reveal that CNPs-based SAs can be equivalent or even superior to the widely used nanoparticle SAs, and will have potential applications for pulsed laser sources.

Enhanced Ultrafast Broadband Reverse Saturable Absorption in Twistacenes with Enlarged π-Conjugated Central Bridge.

Optical nonlinearities of two all-carbon twistacenes, DPyA and DPyN, with the different π-conjugated central bridges were investigated. The nonlinear absorption properties of these compounds were measured using the femtosecond Z-scan with wavelengths between 650 and 900 nm. It has been found that the nonlinear absorption originated from two-photon absorption (TPA) and TPA-induced excited state absorption (ESA), wherein DPyA demonstrates higher performance than DPyN. The TPA cross section of DPyA (4300 GM) is nearly 4.3 times larger than that of DPyN at 650 nm. Moreover, the different central structures modulate the intensity of ESA at 532 nm, and DPyA exhibits an excellent ESA at 532 nm with multi-pulse excitation. Meanwhile, the result of data fitting and quantum chemistry calculation shows that the enhancement of nonlinear absorption in DPyA is due to the extended π- conjugated bridge and improved delocalization of π-electrons. These all-carbon twistacenes could yield potential applications in optical power limiting (OPL) technology.

Femtosecond laser-induced phase transition in VO2 films.

VO2 is a very promising material due to its semiconductor-metal phase transition, however, the research on fs laser-induced phase transition is still very controversial, which greatly limits its development in ultrafast optics. In this work, the fs laser-induced changes in the optical properties of VO2 films were studied with a variable-temperature Z-scan. At room temperature, VO2 consistently maintained nonlinear absorption properties at laser repetition frequencies below 10 kHz while laser-induced phase transition properties appeared at higher repetition frequencies. It was found by temperature variation experiments at 100 kHz that the modulation depth of the laser-induced VO2 phase transition was consistent with that of the ambient temperature-induced phase transition, which was increased linearly with thickness, further confirming that the phase transition was caused by the accumulation of thermal effects of a high-repetition-frequency laser. The phase transition process is reversible and causes substantial changes in optical properties of the film, which holds significant promise for all-optical switches and related applications.

InP/ZnSeS/ZnS Core–Shell Quantum Dots as Novel Saturable Absorbers in Mode‐Locked Fiber Lasers

AbstractIn this study, red light emitting InP/ZnSeS/ZnS quantum dots (QDs) are prepared by thermal injection method. The results show that the InP/ZnSeS/ZnS QDs saturable absorbers (SA) have good nonlinear saturable absorption properties with modulation depth of 24.2% and saturation intensity of 0.08 KW cm−2. Then the QD SAs are applied to the erbium‐doped fiber laser (EDFL) ring cavity system, and a mode‐locked laser pulse with a pulse width of 635 fs is generated. In addition, the pulse width is determined by the carrier recovery time, which is closely related to the defect density in the material. A large number of defects are eliminated in the QDs by hydrogen fluoride (HF) treatment, and the pulse width is reduced from 635 to 450 fs. Results of time‐resolved photoluminescence and ultrafast transient absorption spectroscopy (TAS) show that HF treatment indeed reduces defects in InP/ZnSeS/ZnS QDs, indicated by the decrease of the carrier recovery time. This is different from the reported 2D SA materials, which usually reduce the carrier recovery time by increasing the defect density. This study will inspire new applications of QDs in ultrafast photonics and nonlinear optics.

Mo4/3B2T2-based saturable absorber for generating conventional and bound-state solitons in erbium-doped fibre laser

As a new two-dimensional (2D) material, Mo4/3B2T2 has attracted great attention due to its characteristic electrical and optical properties. However, the nonlinear optical properties of Mo4/3B2T2 and its applications in the ultrafast photonics aspect have not been reported. In this work, we first demonstrate the nonlinear saturable absorption properties of 2D material Mo4/3B2T2. The modulation depth and saturation absorption intensity of Mo4/3B2T2 are 4.5 % and 534 MW/cm2, respectively. Herein, conventional soliton and bound-state solitons were generated in a fibre laser in the 1.5-μm region after inserting the 2D material Mo4/3B2T2 into the laser cavity as a saturable absorber (SA). A highly stable conventional soliton is observed at 1560.57 nm with a 3 dB bandwidth of 3.12 nm. The frequency rate, pulse width, and signal-to-noise ratio were 7.59 MHz, 1.08 ps, and 65.83 dB, respectively. In addition, the bound-state solitons are obtained at a pump power of 320 mW. The central wavelength, pulse interval and spectral modulation period of the bound-state soliton are 1560.9 nm, 6.1 ps and 1.33 nm, respectively. These results suggest that 2D material Mo4/3B2T2 can be used as a promising SA in ultrafast and ultra-narrow photonics.

Realization of harmonic mode-locking in thulium-doped fiber laser using In2(SO4)3-based saturable absorber

Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using indium sulfate (In2(SO4)3) as a saturable absorber (SA). The In2(SO4)3 has nonlinear saturable absorption properties with the nonlinear optical modulation depth of 13.3 % and the saturation power of 0.11 kW at 1930 nm. The fundamentally mode-locked pulses operate at 1964.44 nm have the 3-dB bandwidth of 4.70 nm, the pulse width of 1.266 ps, and the pulse repetition rate of 25.37 MHz. The maximum resonance frequency of 228 MHz is achieved, which corresponds to the 9th harmonic. The supermode suppression level (SMSL) up to 63 dB is realized. The spectral and temporal characteristics of the output pulses are investigated as a function of harmonic order. The experimental results demonstrated that In2(SO4)3 can be a promising mode-locker for generating harmonic ultrafast pulses in 2 μm waveband.

Bi2O2Te Nanosheets Saturable Absorber‐Based Passive Mode‐Locked Fiber Laser: From Soliton Molecules to Harmonic Soliton

AbstractAs a recent addition to the emerging 2D bismuth oxychalcogenides (Bi2O2X, where X = S, Se, and Te), atomically‐thin bismuth oxytellurium (Bi2O2Te) exhibits unique thermoelectric and photoelectric properties. In this study, high‐quality Bi2O2Te nanosheets (NSs)‐based saturable absorber (SA) with a modulation depth of 8.2% is developed using the liquid phase exfoliation (LPE) method and is successfully applied to a passively mode‐locked Er‐doped fiber laser (EDFL) for the first time (with reference to the available literature). Due to the excellent nonlinear saturable absorption property of Bi2O2Te NSs, various switchable and stable mode‐locking states can be realized in the same EDFL, including fundamental frequency conventional soliton (CS), soliton molecules, and harmonic mode‐locking (HML) states. Among them, the CS pulse at 7.48 MHz (fundamental frequency) with a high signal‐to‐noise ratio (SNR) of 67.25 dB is obtained. The 2nd, 3rd, and 4th‐order soliton molecules are observed with a soliton pulse separation of ≈6.2 ps. Moreover, for HML, a maximum repetition rate of 1.78 GHz (i.e., corresponding to the 238th‐order harmonic) is obtained. The results reveal that Bi2O2Te NSs demonstrate excellent nonlinear optical modulation properties and can serve as a fundamental basis for promotion of academic research and innovation of engineering applications involving ultrafast photonics.

The nonlinear optical property of germanium saturable absorber and its application in normal dispersion Er-doped fiber laser

Recently, germanium (Ge) has been considered as the two-dimensional (2D) material saturable absorber (SA) due to its excellent nonlinear saturable absorption properties. However, previous reports explored the nonlinear saturable absorption properties of Ge SA in anomalous dispersion Er-doped fiber (EDF) laser. To date, the mode-locked operation achieved by normal dispersion EDF laser based on Ge SA has not yet been reported until now. In this work, Ge SA with saturation intensity of 11.69 MW/cm2 and modulation depth of 6.17 % was used to demonstrate a normal dispersion EDF laser. Morphological and nonlinear saturable absorption properties of Ge were also investigated. To the best of our knowledge, the mode-locked pulse with repetition rate of 6.66 MHz were achieved in normal dispersion EDF laser based on Ge SA for the first time. Compared with previous work about Ge SA in anomalous dispersion regime, our result proves that Ge also has excellent optical performance in normal dispersion regime.

Experimental observation of enhanced reverse saturable absorption in Bi 2 Se 3 nanoplates doped PMMA thin film

By employing the ultrafast Z-scan technique, we characterize the nonlinear absorption property of PMMA/Bismuth Selenide (Bi 2 Se 3) composite with varying concentrations. We report the fabrication of bismuth selenide (Bi 2 Se 3) nanoplate (topological insulator (TI)) doped poly methyl methacrylate (PMMA) thin film with varying doping concentrations. The effect of Bi 2 Se 3 on structural and linear properties of PMMA thin film has been investigated through UV-Vis spectroscopy, scanning electron microscope (SEM), and Energy dispersive x-ray spectroscopy (EDS) elemental mapping techniques. Furthermore, the nonlinear optical absorption property of PMMA and PMMA/Bi 2 Se 3 composites have been performed employing a single beam open aperture z-scan technique under femtosecond laser excitation at 750 nm. The z-scan results exhibit an enhancement of reverse saturable absorption (RSA) property with an increased nonlinear absorption coefficient (β) of the PMMA/Bi 2 Se 3 composites compared to pure PMMA measured with intensity at 320 GW cm−2. The RSA response gets enhanced with the increase in doping concentration also. Our experimental observations reveal that PMMA/Bi 2 Se 3 composite can provide a promising platform to realize photonic devices such as optical limiters, optical switches, and efficient protectors from high power sources.

Defect assisted enhanced nonlinear optical performance and optical limiting of pure and doped BiVO4 powders and nanocomposite films

The linear and nonlinear absorption properties, photoluminescence and optical limiting properties of bismuth vanadate (BiVO4) powders and films were presented in this work. The structural, morphological and optical effects of different doping elements (M: Mo,W and Ni) were investigated (M: BiVO4). XRD and Raman results proved that BiVO4 and M: BiVO4 powders were synthesized in the monoclinic scheelite structure. Twhe powder morphology as found to be changed by Ni and W doping. The nonlinear optical responses of pure and M: BiVO4 were studied using the open aperture Z-scan experiment and a theoretical model was used to determine the nonlinear optical absorption performance. The nonlinear absorption coefficient (βeff) values were found to be 6.59 × 104, 8.86 × 104, 2.26 × 105 and 1.62 × 106 cm/GW for BiVO4, Mo: BiVO4, W: BiVO4 and Ni: BiVO4 nanocomposite films, respectively. The highest nonlinear absorption coefficients (βeff) and saturation intensity thresholds (ISAT) were obtained for Ni:BiVO4 film. Experimental results showed that the βeff value and band gap can be controlled by the dopant element, making doped BiVO4 a good candidate for nonlinear optical applications such as optical limiters, optical switches and modulators in integrated photonic devices.

Experimental study of saturable and reverse saturable absorption of Zn nanoparticles photodeposited onto etched optical fibers

In this paper, the non-linear optical response of photodeposited zinc nanoparticles onto an etched optical fiber is reported. Through chemical etching, 20 mm of the cladding of a standard single-mode optical fiber was removed to expose its core and subsequently, photodeposit zinc nanoparticles on it at different times (1, 5 and 10 min) by using a continuous wave laser at 1550 nm. The non-linear optical response was obtained by the P-scan technique using a high-gain pulsed amplifier with a pulse duration of 10 ns at a wavelength of 1550 nm. The experimental results showed that it is possible to get the effects of reverse saturable absorption (RSA) at irradiances less than 380 MW/cm 2 and saturable absorption (SA) for higher values. Consequently, the analysis of the results obtained shows that the third-order non-linear susceptibility x3 of the RSA is 2.2130 × 10−06 (esu) for 10 min of photodeposition, which decreases for shorter photodeposition times; On the other hand, the third-order non-linear susceptibility x3 of the SA is −5.8955 × 10−07 (esu) for 10 min of photodeposition, which increases for shorter exposure times. Finally, the experimental results obtained show that it is possible to make a photonic device with desired non-linear absorption properties by the photodeposition of metallic nanoparticles onto etched fibers, which makes it ideal for applications in optical communications.

Nonlinear optical properties of AIE doped PMMA thin film upon restriction of intramolecular motion

Aggregation Induced Emission (AIE) is an interesting Photophysical phenomenon in which a series of molecules non-emissive in solution state become fluorescent upon aggregation or by structural rigidification. Restriction of intramolecular motion (RIM) is the basic principle of this phenomenon. Nonlinear absorption properties of AIE dye named Tertraphenylethylene (TPE) in Anisole and doping it in PMMA matrix is studied using Open Apperture (OA) Z-scan technique. OA Z-scan trace of TPE in anisole showed RSA in SA behaviour due to two photon induced excited state absorption while TPE doped PMMA film showed RSA behaviour with two photon absorption (TPA) as an underlying mechanism. Five level energy model is used to understand this change in nonlinearity. Optical limiting threshold shows only a slight change upon RIM.

Linear and nonlinear absorption properties of hemicyanine dyes containing 5-(4H)-oxazolone as heterocyclic ring

A series of dyes containing electron-donating (D) and electron-accepting (A) groups connected through a π-conjugated linker were designed and their linear and nonlinear absorption properties were studied. The dyes combined 1,3-oxazol-5(4H)-one core with methoxy, methyl, phenyl, cyano, and nitro group in the para-position of a phenyl ring. Spectrally resolved two-photon absorption cross-section measurements carried out by the femtosecond Z-scan technique showed wide wavelength regions of nonlinear optical absorption. The studied compound containing the phenyl ring exhibits relatively high effective two-photon absorption cross-section equal to 772 ± 100 GM at 600 nm. The experimental optical characterization is supported by the results of electronic-structure calculations.

Impact of aluminum doping on nonlinear absorption and ultrafast carriers dynamics of Al:CdO thin films

Al:CdO thin films with different aluminum doping concentrations were prepared via magnetron sputtering technique with various sputtering power. It is found that the band gap Eg of Al:CdO films gradually increases with the increase of Al doping concentration, while the crystallinity became poor at high doping concentration. The nonlinear absorption properties of the film were characterized by nanosecond Z-scan measurements. The Z-scan results show that the nonlinear absorption coefficient β of Al:CdO thin films could be effectively modulated by Al concentration, and the maximum of β (2 × 10−6 m/W) is achieved around 20 W sputtering power. The carrier dynamics of Al:CdO thin films were studied by femtosecond pump–probe measurements. The results show that the carrier relaxation lifetime, carriers absorption cross section and carriers refraction volume are larger in the high Al concentration film. The optical limiting properties of Al: CdO films were also studied. These results demonstrate that the Al:CdO film is a promising candidate for applications in all optical switches and laser devices.

Ultrathin gold nanowires as broadband saturable absorbers for ultrashort pulsed lasers

Ultrathin gold nanowires (UGNWs) with a diameter of ⩽2 nm have attracted much attention because of their high aspect ratios, unusual physical properties and potential applications in nanoelectronics, sensors and photonics. Despite recent advances on UGNWs and the related devices, nonlinear optical properties of UGNWs and their application on photonic devices (e.g. ultrashort pulsed lasers) remain largely unexplored. Here we show that UGNWs can be used as broadband saturable absorbers (SAs) for constructing ultrashort pulsed lasers with an operating wavelength range from near-infrared to mid-infrared (MIR). UGNWs are prepared by using a silane-mediated approach, and then mixed with polyvinyl alcohol to form the UGNWs film. The UGNWs film not only show strong absorption from visible to MIR spectral region (over 25 μm), but also possess the nonlinear saturable absorption properties in the spectral region. As the UGNWs film SA is inserted into an erbium or thulium doped silica fiber laser cavity, stable passively mode-locked lasers at 1562.2 or 1970 nm with a pulse width of a few hundreds of femtoseconds is obtained, respectively. Especially, by inserting the UGNWs film SA into a holmium/praseodymium codoped fluoride fiber laser cavity, stable passively mode-locked laser at 2864 nm is achieved. Our results show that the UGNWs film are promising SAs for constructing broadband ultrashort pulsed lasers.

Linear and Nonlinear Optical Properties of Azobenzene Derivatives Modified with an (Amino)naphthalene Moiety.

The design of two-photon absorbing azobenzene (AB) derivatives has received much attention; however, the two-photon absorption (2PA) properties of bis-conjugated azobenzene systems are relatively less explored. Here, we present the synthesis of six azobenzene derivatives and three bis-azobenzenes substituted (or not) at para position(s) with one or two amino group(s). Their linear and nonlinear absorption properties are studied experimentally and theoretically. The switching behavior and thermal stability of the Z-isomer are studied for unsubstituted mono- (1a, 2a) and bis-azobenzene (3a) compounds, showing that when the length of the π system increases, the half-life of the Z-isomer decreases. Moreover, along with the increase of π-conjugation, the photochromic characteristics are impaired and the photostationary state (PSS) related to E–Z photoisomerization is composed of 89% of the Z-isomer for 2a and 26% of the Z-isomer for 3a. Importantly, the 2PA cross-section increases almost five-fold on extending the π-conjugation (2a vs 3a) and by about one order of magnitude when comparing two systems: the unsubstituted π-electron one (2a, 3a) with D-π-D (2c, 3c). This work clarifies the contribution of π-conjugation and substituent effects to the linear and nonlinear optical properties of mono- and bis-azobenzene compounds based on the experimental and theoretical approaches.

Nonlinear optical properties of tin telluride topological crystalline insulator at a telecommunication wavelength

Topological crystalline insulators (TCIs) have a new topological state where topological nature comes from crystal symmetries without necessitating spin-orbit coupling. Their superb electronic, magnetic, and superconducting properties have intensively been studied. Despite great potential, however, their nonlinear optical properties are rarely examined. For the first time, this study investigates the nonlinear refraction and absorption properties of SnTe TCIs using the Z-scan techniques. A high nonlinear refractive index of -0.111 ± 0.004 cm2/GW and a nonlinear absorption coefficient of -(2.02 ± 0.135)× 103 cm/GW are obtained from SnTe TCIs at the commercially important telecommunication wavelength, being superior and/or comparable to two dimensional (2D) materials and/or renowned photonic crystals. Ab-initio simulations show that SnTe TCIs are suitable for ultrabroad-band application and mechanically stable at high thermal conditions. Lastly, we demonstrate that 1550 nm mode-locked femtosecond lasers can readily be achieved by exploiting the conspicuous nonlinear optical absorption of SnTe TCIs without using any nano-engineered ones like quantum dots. This study strongly indicates a promising potential of SnTe TCIs in a wide range of nonlinear optics, opening an avenue toward advanced TCIs-based photonic technologies.

Interfacial charge transfers and ultrafast nonlinear optical response via constructing electronic structure-induced MoS2/ZnO heterostructure

The molybdenum disulfide (MoS 2 ) and ZnO composite system have reported as micro-nano photonic crystals in integrated optics. Based on interface engineering, their tunable nonlinear absorption (NLA) and photoelectric characteristics offer a flexible design strategies for saturation absorber and optical limiters. Nevertheless, the mechanism leading to the tunable NLA behavior has not been clearly elucidated, which were attribute to complex charge transfers and carrier regulation characteristics. In this article, MoS 2 /ZnO heterostructures with excellent nonlinear properties composed of ZnO nanorods (NRs) wrapped with MoS 2 nanosheets were prepared by magnetron sputtering (MS) synthesis of MoS 2 on highly stacked ZnO NRs. The MoS 2 /ZnO heterostructures exhibit an interesting brush-like morphology with abundant heterointerfaces base on optimizing sputtering time and temperature, which was beneficial to provide efficient charge transfer pathways. Due to the uneven distribution of the lowest nucleation free energy, MoS 2 nanosheets possess a hybrid mode of parallel (C // ) and perpendicular orientations (C ┴ ) on surface of ZnO NRs, which tends to form the bending and vertical growth of layered MoS 2 nanosheets. From electron-photon coupling effect, MoS 2 nanosheets, ZnO NRs and MoS 2 /ZnO heterostructures possess favorable saturation absorption (SA), reverse saturation absorption (RSA) and coexisting absorption behavior, respectively. In addition, a changeover from RSA to SA could be realized in MoS 2 /ZnO heterostructures by increasing the sputtering time. The NLA coefficient of MoS 2 /ZnO heterostructures are in the order of 10 −10 cm/W, which are about 2 times larger than those of the ZnO NRs. Thus, the MoS 2 /ZnO heterostructures with excellent NLA properties will be promising candidates in optoelectronic devices. • The interfacial charge transfers and carrier regulation characteristics of MoS 2 /ZnO composite have been studied. • The ultrafast nonlinear optical properties of MoS 2 /ZnO composite were analyzed. • The mechanism of growth and nonlinear absorption for MoS 2 /ZnO composite is analyzed.