Strong Two-photon Absorption Research Articles

Highly efficient one-component benzylidene cyclopentanone initiators for visible light polymerization

Visible light polymerization is a development trend in the field of photocuring. However, most of the reported visible light photoinitiators (PIs) are two-component systems that display limited initiation efficiency due to diffusion restriction and back electron transfer effect. In this study, two one-component visible light PIs (BDNPG-1 and BDNPG-2) are synthesized through combining N-phenylglycine (NPG) moiety with cyclopentanone moiety together. Upon irradiation of blue or green light, they can undergo intermolecular electron transfer followed by proton transfer plus a decarboxylation reaction to produce carbon dioxide and aminoalkyl radicals. This decarboxylation reaction is irreversible so that the intermolecular back electron transfer can be effectively blocked, resulting in a significantly improved photoinitiation efficiency of BDNPG-1 and BDNPG-2 compared with their prototype PI (BDEA) containing no NPG group and the two-component system of BDEA+NPG. Furthermore, both of them have strong two-photon absorption capability and show excellent performance in two-photon polymerization.

Superior optical limiting performance of lead-free metal halide perovskites nano-crystals embedded chalcogenide glass

The nonlinear optical properties of metal halide perovskites (MHP) are highly attractive, making them an emerging hotspot in the field of perovskite photonics. In this study, the third-order optical nonlinear properties of lead-free MHP (CsSnCl3) nano-crystals (NCs) embedded in a chalcogenide glass (GeS2-Sb2S3) substrate were characterized using the Z-scan technique. The spectral range examined was from 750 to 900 nm. Experimental results revealed that the incorporation of CsSnCl3 NCs enhanced the nonlinear refraction and nonlinear absorption behavior of the GeS2-Sb2S3 glass. Furthermore, the magnitude of these effects increased with the growth size of the CsSnCl3 NCs. Importantly, the MHP glasses exhibited strong two-photon absorption due to the large multiple-absorption cross-section of MHP crystals and the intrinsic high optical nonlinearity of the chalcogenide glass. The superior optical limiting performance induced by this process was successfully demonstrated.

Two-photon phototriggering of ROS storm in ruthenium(II) coordinated carbon nitride for robust cancer immunotherapy

The use of Ru(II) polypyridine complexes for photodynamic therapy is receiving much attention. Despite their promising properties, these compounds are associated with a poor excitation in the biological spectral window, a highly reduced or complete loss of the therapeutic effect under hypoxic conditions, and no immune response, limiting their application. To overcome all these limitations, herein, the coordination of Ru(II) polypyridine complexes to graphitic carbon nitride nanosheets for oxygen-self-sufficient two-photon photodynamic immunotherapy is presented. The conjugates were found with an exceptionally strong two-photon absorption and could self-sufficiently produce an ROS storm consisting of •O2-, •OH, and 1O2. Upon two-photon irradiation, the nanomaterial induced cell death in hypoxic monolayer cancer cells and multicellular tumor spheroids by a combination of apoptosis, paraptosis, ferroptosis, and immunogenic cell death. The nanomaterial was found with a strong tumor growth inhibition against the primary and secondary distant tumor in a melanoma-bearing mouse model through activation of the immune system.

Optical properties of inverted type-I InP quantum dots with near-infrared emission

Unlike conventional type-I InP-based quantum dots (QDs), inverted type-I QDs can push the emission wavelength to the near-infrared (NIR) range. However, the fundamental optical properties of such kinds of QDs have yet to be studied in depth. In this work, we report the optical properties of inverted type-I ZnSe/InP/ZnS QDs with photoluminescence (PL) emission in the first biological window. Using femtosecond ultrafast transient absorption, the electron-injection time, Auger lifetime, and hot-carrier cooling time of the inverted type-I QDs are determined. Their temperature-dependent PL spectrum reveals that these QDs possess a higher exciton-binding energy and weaker carrier-phonon coupling effect than conventional type-I InP-based QDs. Interestingly, the QDs have strong two-photon absorption cross sections in the second biological window, up to 5120 GM at 1200 nm. These results suggest that ZnSe/InP/ZnS QDs have potential applications for light emitting devices and deep-tissue bioimaging.

Anisotropic Two-Photon Excited Photoluminescence and Optical Constants of InGaN Nanowires

Gallium nitride (GaN) and its derivatives are important materials for optoelectronic and electronic applications, such as light-emitting diodes, solid-state lasers, optical modulators, artificial photosynthesis, etc. Controlled growth and manipulation of these materials at nanoscale dimensions and understanding their linear and nonlinear optical properties lay the foundation for developing next-generation optoelectronic and electronic devices. In this context, herein, we report on the two-photon absorption and anisotropic two-photon excited photoluminescence of InGaN (IGN) nanowires grown by molecular beam epitaxy. The IGN nanowires were imaged using a home-built two-photon photoluminescence microscope with a tunable femtosecond laser to excite over the near-IR range (690–950 nm). In addition, the polarization angle of the excitation electric field was also rotated, and the emitted photoluminescence was analyzed with a polarizer. Results showed polarized photoluminescence with a stronger signal in the direction perpendicular to the IGN nanowires’ axis. Also, we observed strong two-photon absorption above the bandgap, with several excitation spectrum peaks near 740, 790, and 870 nm. These peaks showed anisotropic response to the excitation polarization angle, which may be due to the anisotropy of the IGN nanowires’ wurtzite phase. The polarized light absorption and emission nature of IGN nanowires, as revealed in this work, may be useful for optimizing materials for future optoelectronic device applications.

Ultrahigh thermal-efficient all-optical silicon photonic crystal nanobeam cavity modulator with TPA-induced thermo-optic effect.

We propose and experimentally demonstrate an on-chip all-optical silicon photonic crystal nanobeam cavity (PCNBC) modulator. With the advantages of the strong two-photon absorption (TPA)-induced thermo-optic (TO) effect, ultrahigh thermal-efficient tuning with π phase shift temperature difference ΔTπ of 0.77°C and power Pπ of 0.26 mW is implemented. Moreover, the all-optical modulation is carried out by a pulsed pump light with an average switching power of 0.11 mW. The response times for the rising and falling edges are 7.6 µs and 7.4 µs, respectively. Such a thermal-efficient modulator is poised to be the enabling device for large-scale integration optical signal control systems.

Molecular and Aggregate Synergistic Engineering of Aggregation-Induced Emission Luminogens to Manipulate Optical/Electronic Properties for Efficient and Diversified Functions.

The optical/electronic properties of organic luminescent materials can be regulated by molecular structure modification, which not only requires sophisticated and time-consuming synthesis but also is unable to accurately afford the optical properties of materials in the aggregate state. Herein, a facile strategy of molecular and aggregate synergistic engineering is proposed to manipulate the optical/electronic properties of a luminogen, ACIK, in the solid state for efficient and diversified functions. ACIK is facilely synthesized and exhibits three polymorphic states (ACIK-Y, ACIK-R, and ACIK-N) with a large emission difference of 102 nm from yellow to near-infrared (NIR). Their structure-property relationships were investigated by crystallographic analyses and computational studies. ACIK-Y, with the most twisted structure, exhibits an intriguing color-tuned fluorescence between yellow and NIR in the solid state in response to multiple stimuli. Shuttle-like ACIK-R microcrystals exhibit an optical waveguide property with a low optical loss coefficient of 19 dB mm-1. ACIK dots display bright NIR-I emission, large Stokes shift, and strong NIR-II two-photon absorption. ACIK dots show specific lipid droplets-targeting capability and can be successfully applied for two-photon fluorescence imaging of mouse brain vasculature with deep penetration and high spatial resolution. This study will inspire more insights in developing advanced optical/electronic materials based on a single chromophore for practical applications.

Engineering the nonlinear optical properties of barium molybdate by doping Sn4+ ions for optical limiting device applications

Materials exhibiting strong two-photon absorption and low photo-damage threshold are highly desirable owing to their wide application in optoelectronic and photonic devices. Metal molybdates have gained significant attention for optoelectronic applications due to their excellent optical and luminescent properties. Herein, we report the nonlinear properties of the unexplored tin (Sn) doped barium molybdate (BaMoO4) nanostructures for tunable optical limiting applications. XRD and Raman Spectroscopy extensively studied structural properties. The UV–Vis absorption spectra clearly reveal the strong absorption in the blue region and suggest the bandgap reduction with increased Sn concentration in the host lattice. Open aperture Z scan mode, exhibits strong reverse saturable absorption owing to two-photon absorption. In addition, we observed that the nonlinear absorption coefficient and optical limiting threshold depend extensively on the excitation laser energy and can be altered by varying the concentration of Sn ions. The observed nonlinearity confirms that the synthesized Tin (Sn) doped Barium Molybdate (BaMoO4) nanostructures are potential for tunable optical limiting applications.

Interpretable Machine Learning of Two-Photon Absorption.

Molecules with strong two-photon absorption (TPA) are important in many advanced applications such as upconverted laser and photodynamic therapy, but their design is hampered by the high cost of experimental screening and accurate quantum chemical (QC) calculations. Here a systematic study is performed by collecting an experimental TPA database with ≈900 molecules, analyzing with interpretable machine learning (ML) the key molecular features explaining TPA magnitudes, and building a fast ML model for predictions. The ML model has prediction errors of similar magnitude compared to experimental and affordable QC methods errors and has the potential for high-throughput screening as additionally validated with the new experimental measurements. ML feature analysis is generally consistent with common beliefs which is quantified and rectified. The most important feature is conjugation length followed by features reflecting the effects of donor and acceptor substitution and coplanarity.

Rapid preparation of N,B-codoped carbon quantum dot based films with strong two-photon absorption and optical limiting effect

Strong two-photon absorption N,B-CQDs with high crystallinity prepared via a quick microwave method and their films have shown important application value in the optical limiting fields.

Growth, characterizations and nonlinear optical studies of dimethylamine substituted anthracene chalcone single crystals

We report the structural, thermal, linear and nonlinear optical (NLO) properties of a newly grown anthracene based chalcone single crystal, (2E)-1-(anthracen-9-yl)-3-[4-(dimethylamino)phenyl]prop-2-en-1-one (abbreviated as DA-ANC). The chalcone was synthesized by Claisen-Schmidt condensation reaction and the single crystals were grown by slow evaporation of solution growth technique. Single-crystal XRD, NMR (1H & 13C), FT-IR, UV-Vis-NIR spectroscopies, TG/DTA, and computational DFT studies were performed to characterize the crystals. The intermolecular interactions were described by Hirshfeld surface analysis (HSA). The third-order NLO properties of DA-ANC were studied by Z-scan technique using CW-DPSS laser at 532 nm. Open aperture Z-scan exhibits strong two-photon assisted reverse saturation absorption (RSA) with two-photon absorption (TPA) coefficient (β) 6.9 × 10−5 cmW−1. Closed aperture Z-scan reveals the negative nonlinear refraction arising due to self-defocusing effect with nonlinear refractive index (n2) coefficient -1.31 × 10−8 cm2W−1. The optical limiting behavior based on TPA was performed and limiting-threshold was evaluated to be 6.47 kJ cm−2. The obtained one-photon and two-photon figures of merit satisfied the conditions of all-optical switching. The results show that DA-ANC can be a potential candidate for NLO applications.

Minimized thermal effects and strong two-photon absorption of aluminum, silver doped platinum bi-metal nanoparticles

Herein, for the first time, we have investigated the strong nonlinear optical (NLO) properties aided with minimal thermo-optical effects in silver-platinum (Ag-Pt) nanoparticles (NPs) using Z-scan technique with high repetition rate femtosecond pulses at 700 nm. The NPs were synthesized using ultrafast laser ablation of Ag/Al targets immersed in the Pt colloidal solutions in isopropyl alcohol (IPA). We observed reverse saturable absorption (RSA)/two-photon absorption (2PA) behavior in Ag-Pt NPs (with 0.5 mM concentration of Pt), W-shaped switching behavior in Ag-Pt NPs (with 1 mM concentration of Pt), and RSA/2PA behavior in Ag-Pt NPs (with 3 mM concentration of Pt). The multiple-stage nonlinear absorption phenomenon is attributed to the interchange of pure 2PA or excited-state absorption, and 2PA saturation effects. Additionally, the broadband third-order NLO properties of aluminum-doped Pt (Al-Pt) bi-metallic NPs were obtained using spectral-dependent Z-scan studies at 700 nm, 800 nm, and 900 nm. The results revealed that Al-Pt bi-metallic NPs exhibited strong saturable absorption (SA), 2PA, and switching from SA to RSA. It is also revealed from the closed aperture Z-scan data that Ag-Pt and Al-Pt bi-metallic NPS exceed thermal contribution with strong 2PA coefficients unprecedentedfor single metal NPs. In the case of Ag-Pt NPs, strong 2PA saturation effects are attributed to the filling of Ag-O anti-bonding states by the photo-excited electrons and then charge transfer to the Pt plane. In the case of Al-Pt NPs, strong 2PA phenomena are ascribed to the enhancement of charge carriers in the conduction band and, hence, the strong surface plasmon resonances. The 2PA coefficients were evaluated to be ∼10−7 cm/W. The superior and switching NLO effects indicate that these materials might be utilized for potential applications, including optical modulators, and all-optical switching devices.

Ultrafast Dynamics and Strong Two-Photon Absorption Properties of Nonplanar β-Functionalized “Push–Pull” Copper Corroles with a Mixed Substituent Pattern

A new series of nonplanar and unsymmetrically β-functionalized "push-pull" copper corroles, CuTPC(CHO)R7 [R = H, Br, Ph, Me, or 2-thienyl (Th)], were synthesized and characterized to elucidate the effect of β-functionalization and nonplanarity on the photophysical, redox, and nonlinear optical (NLO) properties on the corrole ring. The synthetic route to unsymmetrically β-octasubstituted copper corroles includes bromination of CuTPC(CHO) to get CuTPC(CHO)Br7 in 80% yield, which was further subjected to the Pd-catalyzed Suzuki reaction. CuTPC(CHO)Br7 exhibited a large red shift in the Soret band (Δλmax = 35-40 nm) and both the Q bands (Δλmax = 10-50 nm), as compared to CuTPC and CuTPC(CHO). CuTPC(CHO)Br7 was 510 and 290 mV anodically shifted in the first oxidation and the first reduction compared to CuTPC owing to the strong -I effect of CHO and Br groups. Density functional theory studies revealed that all the β-octasubstituted copper corroles exhibited highly nonplanar saddle-shape conformation of the corrole ring. Very high torsional saddling was observed for CuTPC(CHO)Th7 (79-83°) than that for CuTPC (49-53°), even larger than that for CuTPCBr8 (67-70°). Femtosecond laser-induced third-order NLO studies from these copper corroles showed strong two-photon absorption cross-sections (0.48-6.98 × 104 GM) and self-focusing-type positive nonlinear refraction behavior. The observed structure-dependent two-photon absorption coefficients (β) are in the range of ∼2.7-20.9 × 10-12 m/W, and the n2 values are in the range of ∼0.64-6.45 × 10-18 m2/W. The present results may facilitate a new window for these copper corroles in nonlinear optical devices, femtosecond optical limiters, and many other ultrafast photonic applications.

Strong two-photon absorption induced by energy funneling in chiral quasi-2D perovskites.

Quasi-2D Ruddlesden-Popper-type perovskites (RPPs) exhibit excellent nonlinear optical properties due to their multiple quantum well structures with large exciton binding energy. Herein, we introduce chiral organic molecules into RPPs and investigate their optical properties. It is found that the chiral RPPs possess effective circular dichroism in the ultraviolet to visible wavelengths. Two-photon absorption (TPA)-induced efficient energy funneling from small- to large-n domains is observed in the chiral RPP films, which induces strong TPA with a coefficient up to 4.98 cm MW-1. This work will broaden the application of quasi-2D RPPs in chirality-related nonlinear photonic devices.

Strategic Design of Luminescent Rhenium(I), Ruthenium(II), and Iridium(III) Complexes as Activity-Based Probes for Bioimaging and Biosensing.

The rapid development of responsive fluorescent probes has advanced optical imaging for biological research and biomedical applications. Among different sensing strategies, activity-based sensing, which exploits the unique reactivity of the target chemical species to achieve high chemoselectivity, has emerged as a promising paradigm for the development of responsive probes for selective molecular imaging. Luminescent transition metal complexes have received considerable attention for bioimaging and biosensing applications over the last decade due to their remarkable photophysical behavior including intense emission with large Stokes' shifts, long emission lifetimes, strong two-photon absorption, and high photostability. In this Review, we summarize the design strategies and applications of luminescent complexes of rhenium(I), ruthenium(II), and iridium(III) polypyridines as activity-based probes for the detection of various chemical species and bioactive molecules in live cells and organisms. The current challenges and future prospects of these complexes as activatable reagents for disease diagnosis and treatment are also discussed.

A comparative analysis on electrical and nonlinear optical properties of pure and Co–Ni co-doped SnO2 nanoparticles

Compared to bulk materials, nanostructured materials possess ultrafast optical response and strong two-photon absorption which leads to the utility of materials in various laser related applications. Hence, Cobalt (Co)-Nickel (Ni) co-doped SnO2 nanoparticles were synthesized via co-precipitation technique and its optical limiting behavior was studied. PXRD confirms the formation of tetragonal rutile SnO2with space group P42/mnm with decrease in crystallite size due to co-doping. Shift in PXRD peak position towards higher angle and suppression of Sn–O–Sn vibrational peaks confirm the intervention of Co2+ and Ni2+ ions into the SnO2 lattice. Upon co-doping, the band gap decreases (3.35, 3.28 and 3.20 eV) and the PL intensity of blue-green emission increases due to the increase in density of oxygen vacancies on the surface of SnO2. Low dielectric constant, dielectric loss and ac conductivity indicates the prepared materials are having lesser defects. Nonlinear optical (NLO) absorption and optical limiting behavior of developed NPs were recorded by Z-scan method (open aperture) using nano pulsed green laser excitation. Open aperture Z-scan curves with a valley-like pattern indicate the presence of reverse saturable nonlinear absorption due to the sequential two-photon absorption (2PA) process. The 2PA coefficient increases (0.27–0.56 ×10−10W/m) and onset optical limiting threshold decreases (4.5 –1.7 ×10−12W/m2) with increasing dopant concentration of Co–Ni (2 & 4 wt%) in SnO2 NPs. These results demonstrate that Co–Ni (4 wt%) NPs have virtuous dielectric and efficient NLO absorption and limiting properties which suggest that it could be an eminent candidate for the potential applications such as high-performance dielectric material and optical limiting components in optical devices.

Enhancement of Two-Photon Absorption in Boron-Dipyrromethene (BODIPY) Derivatives

The linear and nonlinear optical properties of two BODIPY derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)-boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenylvinyl)phenyl)-boron-diuoride-azadipyrromethene (ZL-22), were comprehensively investigated based on experimental and theoretical studies. It was found that both compounds show a strong two-photon absorption response in the near-infrared regime, and the two-photon-absorption cross-section values of ZL-61 and ZL-22 were determined to be 8321 GM and 1864 GM at 800 nm, respectively. The improvement of the two-photon absorption cross section in ZL-61 was attributed to the enhancement of the donor group, which was confirmed by transient absorption measurements and DFT calculation. Our results indicate that these BODIPY derivatives are a promising candidate for optical limiting and two-photon imaging applications.

Large third-order optical nonlinear susceptibility in natural dye derived from Clitoria ternatea petal

The third-order nonlinear optical response of Clitoria ternatea (CT) petal’s dye in various solvents were characterized via Z-scan technique with a diode pumped solid state (DPSS) laser at 532 nm. The observations from the closed aperture z-scan showed the dye possess a self-defocusing effect with a negative nonlinear refractive index, n2 with a moderate magnitude of real part of the third-order nonlinear optical susceptibilities, Re χ(3) of the order of 10−10 esu. Open z-scan results showed that the CT dyes samples demonstrate a reverse saturable absorption (RSA) with strong two-photon absorption (TPA) coefficients, β ~ 10−3 cm/W, corresponds to large imaginary parts, Im χ(3) in the order of 10−5 esu. The highest values of n2 and β was observed in CT dye in ethanol and dimethyl sulphoxide, respectively. Not only the linear absorption spectra depends on the type of solvent used, but also the nonlinear characteristics are drastically influenced by it. On the whole, large χ(3) of the dye samples were found to be in order of 10−5 esu. Based on the measured value of nonlinear susceptibility, χ(3), the natural dye derived from CT petals can be a candidate for applications such as optical limiting and switching.

A femtosecond optical switch using molecular two-photon absorption with multi-step charge dissociation

An ultrafast optical switch is reported with a response time shorter than 300 fs in a single-layer PFB, where the unique band structure of PFB that allows strong transient two-photon absorption with high contrast is the main responsible mechanism.

D-π-A type conjugated indandione derivatives: ultrafast broadband nonlinear absorption responses and transient dynamics.

The ultrafast nonlinear optical response of two 1,3-indandione derivatives (INB3 and INT3) was systematically investigated by the femtosecond Z-scan and pump-probe technique at multiple visible and near infrared wavelengths. Both compounds show strong broadband nonlinear absorption (NLA) and different wavelength-dependent two-photon absorption (TPA) characteristics in the range of 650–1100 nm. The TPA cross section of trithiophene-based compound INT3 was found to be larger than that of triphenylamine-based compound INB3 in the red region (650–800 nm), which is attributed to its longer π-conjugated structure and better molecular planarity. Interestingly, the effective NLA of INB3 was found to be larger than INT3 in the NIR region (800–1100 nm), which is related to the excited state absorption (ESA) induced by TPA. The ultrafast dynamics of both compounds were investigated by femtosecond transient absorption spectroscopy, which revealed a broadband ESA including several relaxation processes. This work extends nonlinear optical research on indandione derivatives, and the results suggest that these derivatives are promising candidates for optical limiting applications.