Nonlinear Two-photon Absorption Research Articles

Deep-UV optical properties of KDP-family crystals: a comprehensive characterization

Comprehensive characterization of the transmittances, cut-off edges, band-gap energies, linear absorption, and nonlinear two-photon absorption properties of KDP-family crystals (ADP, KDP, 70% DKDP, and DKDP) in the deep-UV spectral region.

Power dependent nonlinear optical characteristics and two-photon absorption of NiO/PVA thin film

The NiO/PVA thin film was prepared by drop-casting the nickel oxide nanoparticles synthesized by the chemical precipitation method. The structural and optical characteristics of the film were investigated using XRD and UV-VIS absorption. The nonlinear optical property of the Nickel oxide thin film was studied by the open aperture Z-scan method at 532 nm. The two-photon absorption coefficient of the film was calculated with different optical intensities. With increasing input intensity, the value of the two-photon absorption coefficient has decreased. This is attributed to the saturation of two-photon absorption mechanism. The NiO/PVA film can be used for optical limiting applications.

On the application of Two-Photon Absorption for Laser Fault Injection attacks

Laser Fault Injection (LFI) is considered to be the most powerful semiinvasive fault injection method for implementation attacks on security devices. In this work we discuss for the first time the application of the nonlinear Two-Photon Absorption (TPA) effect for the purpose of LFI. Though TPA is an established technique in other areas, e.g. fluorescence microscopy, so far it did not receive any attention in the field of physical attack methods on integrated circuits. We show that TPA has several superior properties over the regular linear LFI method. The TPA effect allows to work on non-thinned devices without increasing the induced energy and hence the stress on the device. In contrast to regular LFI, the nonlinearity of the TPA effect leads to increased precision due to the steeper descent in intensity and also a vertically restricted photoelectric effect. By practical experiments, we demonstrate the general applicability of the method for a specific device and that unlike a regular LFI setup, TPA-LFI is capable to inject faults without triggering a latch-up effect. In addition we discuss the possible implications of TPA-LFI on various sensor-based countermeasures.

Large third-order optical nonlinearities of two-dimensional CsPbBr3 nanoplatelets

Metal halide perovskites show considerable optical nonlinearity and could be used for cost-effective nonlinear optical devices if their nonlinear susceptibilities can be improved. Here, we report large optical nonlinearity, including third-order nonlinear absorption, refraction, and two-photon absorption excited luminescence, of CsPbBr3 nanoplatelets with a thickness of two or three atomic layers and a plane size of about 60 nm. Specifically, the nonlinear absorption was mainly induced by two-photon absorption at low incident powers, and the nonlinear absorption cross section reached 2.15 × 107 GM. It is two orders of magnitude larger than that of CsPbBr3 nanocrystals, which makes them an ideal optical limiting material. Furthermore, the nanoplatelets exhibited large self-phase modulation-induced nonlinear refraction, and the figures of merit W and T satisfied W >1 and T <1, which allow for optical switching. The large optical nonlinearity of CsPbBr3 nanoplatelets provides a basis for multifunctional applications in nonlinear optical devices.

Spatial Narrowing of Two-Photon Imaging in a Silicon CCD Camera

Imaging based on nonlinear two-photon absorption can offer an attractive alternative for the traditional one-photon imaging scheme with many potential benefits of imaging resolution, speed, and ability to image through scattering media. Here we experimentally demonstrate a direct imaging scheme based on two-photon absorption in the conventional silicon CCD camera and observe a spatial narrowing effect in its imaging. This technique, compared with its one-photon counterpart, exhibits a better imaging resolution and signal-to-noise ratio, thanks to the nonlinear nature of the two-photon absorption process. This nonlinear imaging scheme opens up an avenue for rapid real-time and high-resolution imaging through scattering media like rain, fog, and biomedical samples.

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.

Studying the linear and nonlinear optical properties of trifluoroethoxy-coated zinc phthalocyanine ((4TFEO) 4-ZnPc) thin films

Trifluoroethoxy-coated zinc phthalocyanine (4TFEO) 4-ZnPc thin films were prepared using the spin coating process. The crystal structure of powder and different temperature annealed thin films was examined using X-ray Diffraction (XRD), which confirmed the amorphous character for the powder, as-deposited and annealed films. Raman spectroscopy analysis revealed that samples exhibited comparable active modes. Due to the strong carbon-fluorine bond and fluorine's high electronegativity, spectrophotometric studies in the UV–Vis–NIR region indicated that annealing temperatures of 373 and 473K had no discernible influence on the optical gap and optical constants of (4TFEO) 4-ZnPc thin films. The oscillator energy Eo and the dispersion energy Ed were obeyed the usual dispersion area (λ > 850 nm). Dielectric constants were determined to calculate optoelectronic characteristics such as the high-frequency dielectric constant ε∞, lattice dielectric constant εL, ratio of carrier concentration to the effective mass (N/m∗), and plasma frequency ωP. The third-order nonlinear susceptibility χ(3), nonlinear refractive index n2 and two-photon absorption coefficient βc of the (4TFEO) 4-ZnPc thin films were measured for the (4TFEO) 4-ZnPc thin films, which showed a good correlation with linear refractive index WDD parameters and optical gap. These findings will be a lead step to understand deeper linear and nonlinear optical properties of (4TFEO) 4-ZnPc thin films as a probable candidate for optoelectronic devices applications.

Dimensions of fluorescence kinetic concentration of doped morphology homologs synthesized by TCPP and UiO-66 MOF

Metal-organic frameworks (MOFs) have attracted significant attention thanks to their excellent physical and chemical properties. Metal-organic frames are widely used in optics. For example, metal-organic frameworks can be used in fluorescent probes to detect and treat diseases, and can also be used to make solid-state light-emitting devices. Considering the high stability and easy synthesis of Zr-based UiO-66 MOF as well as the strong fluorescent emission of porphyrin molecules, the TCPP-incorporated UiO frameworks would inherit these advantages and exhibit intriguing optical performance. Therefore, in this paper, TCPP was doped into UiO-66, and the optical properties of the resulting MOF structure were observed. To obtain the optimal doping condition, three doping concentrations of TCPP were used. Besides the structural and morphological characterization by X-ray diffraction and scanning electron microscopy, more attentions have paid on the optical testing based on absorption and fluorescence spectroscopy, two-photon fluorescence spectroscopy and time-resolved spectroscopy. As a result, the TCPP-doped MOFs exhibited good fluorescence properties and satisfied our previous expectations of nonlinear optical properties and two-photon absorption. This work would pave a new way in constructing various MOFs-based solid-state devices with widespread optical applications.

Two-Photon Fluorescence Imaging.

Two-photon fluorescence imaging is a powerful tool for observing the dynamics of cells in vivo in intact tissue and is well suitable for imaging neuronal activity for neuroscience research. Due to the nonlinear two-photon absorption, the optical sectioning ability is inherent, resulting in two-photon images with high signal contrast and signal-to-noise ratio with efficient illumination. In addition, the longer wavelength excitation light in two-photon imaging compared to one-photon imaging suffers less scattering and absorption by tissue, which allows deeper penetration. Today, two-photon microscopy is being rapidly developed to adapt to various biological applications for high-speed, high-resolution, large-volume, long-term imaging in freely behaving animals.

Enhanced optical power limiting and visible luminescence in colloidal dispersion of ultra-small Au nanoclusters synthesized by single-pot chemical technique

Here, we present a detailed investigation on the synthesis and nonlinear multiphoton absorption properties of the colloidal solution of Au nanoclusters (AuNCs) which contains the atomic clusters, with the number of atoms per cluster (NAC) of only one (Au1NC) and two (Au2NC). These AuNCs are synthesized by an easy single step one-pot simple chemical process and by using dimethylformamide (DMF) both as reducer and stabilizer. The presence of both Au1NC and Au2NC are found in the sample by their distinct signature in the UV–Vis. absorption spectrum as well as in the high-resolution mass spectrum. The synthesized material has been found to exhibits a strong and stable blue-luminescence with a moderately high quantum yield (QY) of 12.4% when excited with UV light. The nonlinear optical two-photon absorption (2PA) properties of Au1, Au2NC solutions are being reported here by Z-scan studies, for the first time, by using both 10 ns and 100 fs pulse laser radiations having wavelength of 532 nm. It is significantly noted here that the synthesized AuNCs are found to exhibit reverse saturable absorption (RSA) when excited either by ns or by fs laser pulses. A high third-order nonlinear susceptibility (ꭓ(3)) of the order of 10−13 (esu) of the synthesized materials are obtained under fs laser excitation and it is attributed to the 2PA through electronic band to band transition. In contrast, the variation of the 2PA coefficient (β) with input intensity (I0) represents the footprint of free carrier involvement in the enhanced nonlinear absorption in the case of ns excitation. Furthermore, through the non-saturable nonlinear multiphoton absorption, the synthesized AuNCs exhibit excellent optical power limiting phenomenon with the limiting threshold (Fth) of 9.1 mJ/cm2 (fs excitation) and 2.02 J/cm2 (ns excitation) owing to their enhanced 2PA coefficient. Therefore, we believe that our synthesized ultra-small colloidal AuNCs can be used as the promising candidate material for advanced photonics application in the future.

Vision with pulsed infrared light is mediated by nonlinear optical processes.

When the eye is exposed to pulsed infrared (IR) light, it is perceived as visible of the corresponding half wavelength. Previous studies have reported evidence that this is due to a non-linear two-photon absorption process. We have carried out a study which provides additional support to this nonlinear hypothesis. To this end, we have measured the spectral sensitivity at 2 different pulse repetition rates and have developed a theoretical model to account for the experimental observations. This model predicts a ratio between the minimum powers needed to detect the visual stimulus at the 2 pulse repetition rates employed of 0.45 if the stimulus were detected through a nonlinear effect and 1 if it were caused by a linear effect as in normal vision. The value experimentally found was 0.52 ± 0.07, which supports the hypothesis of a nonlinear origin of the two-photon vision phenomena.

2D polymer/metal structures for surface plasmon resonance

In this paper, we present a new type of polymer/metal structures for surface plasmon resonance (SPR) applications. The polymer structures with different periods were fabricated by a single-step 3D laser lithography using direct laser writing process based on non-linear two-photon absorption. Prepared polymer structures were coated by a thin film of gold. The morphological properties of final SPR structures were analyzed using a scanning electron microscope and atomic force microscope. The reflection spectra of the SPR structure with a defined period were simulated using a finite-difference time-domain method. The prepared structures show a sharp spectral dip due to the SPR effect in reflection measurements in a visible spectral range. The effect of SPR was also investigated in the angular dependence and confirmed the strong effect of 2D grating on SPR dip formation.

Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200–250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η2σ2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.

Linear and Nonlinear Absorption of Titanium Dioxide Films Produced by Plasma Ion-Assisted Electron Beam Evaporation: Modeling and Experiments

Titanium dioxide films were prepared by plasma ion-assisted electron beam evaporation. Linear optical properties were investigated in terms of spectrophotometry using the beta-distributed oscillator (ß_do) model as a parametrized dispersion law. The nonlinear two-photon absorption coefficient of titanium dioxide was determined by means of the laser-induced deflection technique at a wavelength of 800 nm. The obtained values of (2–5) × 10−11 cm/W were consistent with published experimental values for rutile as well as for simulations performed in the frames of the ß_do and Sheik–Bahae models.

Phthalocyanine-Cored Fluorophores with Fluorene-Containing Peripheral Two-Photon Antennae as Photosensitizers for Singlet Oxygen Generation.

A series of free base and Zn(II) phthalocyanines featuring fluorenyl antennae linked by methoxy or oxo bridges to the phthalocyanine core (Pc) were synthesized and characterized. Selected linear and nonlinear (two-photon absorption) optical properties of these new compounds were subsequently studied. As previously observed for related porphyrin dendrimers bearing 2-fluorenyl peripheral dendrons, an efficient energy transfer occurs from the peripheral antennae to the central phthalocyanine core following excitation in the fluorenyl-based π–π* absorption band of these chromophores. Once excited, these compounds relax to the ground state, mostly by emitting intense red light or by undergoing intersystem crossing. As a result, the tetrafunctionalized Zn(II) phthalocyanines are fluorescent, but can also efficiently photosensitize molecular oxygen in tetrahydrofurane (THF), forming singlet oxygen with nearly comparable yields to bare Zn(II) phthalocyanine (ZnPc). In comparison with the latter complex, the positive role of the fluorenyl-containing antennae on one- and two-photon brightness (2PA) is presently demonstrated when appended in peripheral (β) position to the phthalocyanine core. Furthermore, when compared to known porphyrin analogues, the interest in replacing the porphyrin by a phthalocyanine as the central core to obtain more fluorescent two-photon oxygen photosensitizers is clearly established. As such, this contribution paves the way for the future development of innovative biphotonic photosensitizers usable in theranostics.

Diffraction Properties and Application of 3D Polymer Woodpile Photonic Crystal Structure

We present a new technique for modification of diffraction and optical properties of photonic devices by surface application of polymer Three-Dimensional (3D) woodpile Photonic Crystal (PhC) structure. Woodpile structure based on IP-Dip polymer was designed and fabricated by Direct Laser Writing (DLW) lithography method based on nonlinear Two-Photon Absorption (TPA). At first, we investigated diffraction properties of woodpile structure with a period of 2 μm. The structure was placed on a glass substrate, and diffraction patterns were measured using laser sources with different wavelengths. After diffraction properties investigation, the fabricated structures were used in optoelectronic devices by their surface application. Our polymer 3D PhC woodpile structures were used for radiation properties modification of light emitting devices - optical fiber and Light Emitting Diode (LED) and for angular photoresponse modification of InGaAsN-based photodiode. The modification of the far-field radiation patterns of optical fiber and LED and spatial modulation of light coupling into photodiode chip with applied structures were measured by goniophotometer. Quality of fabricated structures was analyzed by a Scanning Electron Microscope (SEM).

Ultrafast All-Optical Modulation in 2D Hybrid Perovskites.

Two-dimensional (2D) hybrid organic-inorganic Ruddlesden-Popper perovskites (RPPs) have been recently shown to exhibit large nonlinear optical properties due to the strong excitonic effects present in their multiple quantum wells. In this work, we use nondegenerate pump-probe spectroscopy in the 600-1000 nm wavelength range to study the influence of nonlinear effects on the ultrafast dynamics of 2D RPP thin flakes. We find that, under sub-bandgap excitation, ∼100 nm thick perovskite sheets allow up to ∼2% reflectivity modulation within a 20 fs period, due to the nonlinear optical Kerr effect and two-photon absorption, surpassing by a factor of ∼5 the reported nonlinear performance of photonic metasurfaces and single nanoantennas. When the excitation is resonant with the excitonic absorption, the ultrafast nature of the nonlinear response is lost due to the presence of linear absorption creating long-lived free carriers. Our results suggest that 2D RPPs are potential nanoscale all-optical modulators in the visible/near-infrared waveband for applications such as ultrafast information processing, optical data transmission, and high-performance computing.

The effect of external applied fields on the third order nonlinear susceptibility and two-photon absorption cross-section of E5CN7@Fe3O4-CNT

The third order nonlinear optical properties and two-photon absorption(TPA) cross-section of nematic liquid crystals (NLCs) (E5CN7) doped with Fe3O4 nanoparticles, single wall carbon nanotubes (SWCNTs) and Fe3O4-CNTs were investigated at 632.8 nm. For this aim, the low power single beam was used in Z-scan technique. The study was performed with two different concentrations of dopant with and without the electric field (15 Volt) and magnetic field (0.5 Tesla). The functionalization of SWCNTs with Fe3O4 diminishes the CNT aggregation while the third order nonlinear susceptibility and TPA cross-section are increased. The results show that the best concentration and dopant in E5CN7 without any applying fields is 1% Fe3O4-CNT. After applying electric and magnetic fields and compare the results, the E5CN7@1% Fe3O4-CNT under electric field exhibits the largest nonlinear responses among all the samples. The direction of external fields with respect to the director axisn→ and then the type of initial alignment are crucial factors for changing the nonlinearity parameters. Furthermore, these results show that E5CN7@1% Fe3O4-CNT may be the promising candidate for the magneto-optical devices.

Theoretical study of multilayer coatingreflection taking into account third-order optical nonlinearities.

A theoretical approach is presented that allows calculating the reflectance of dielectric multilayer coatings taking third-order optical nonlinearities into account. The description is based on third-order optical susceptibility so that nonlinear refraction and two-photon absorption processes are automatically considered in terms of the real and imaginary parts of the susceptibility. Two model systems are calculated in order to demonstrate the physical validity of the approach: a mirror that is optimized for maximum reflectance at a given wavelength and a given incident intensity and a mirror that provides a rather flat dependence of the reflected intensity on the incident one. Model calculations are performed for laser wavelength values of 1064nm, 800nm, and 532nm.

Optical power limiting of ultrashort hyper-Gaussian pulses in cascade three-level system**Project supported by the National Natural Science Foundation of China (Grant No. 11574082) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2018MS050).

Propagation of strong femtosecond hyper-Gaussian pulses in a cascade three-level molecular system is studied by solving numerically the Maxwell–Bloch equations by the iterative predictor-corrector finite-difference time-domain method. Optical power limiting behavior induced by strong nonlinear two-photon absorption is observed for different orders of the femtosecond hyper-Gaussian pulses. Pulses of a higher order temporal profile are found to have a wider power range of optical limiting but a larger output saturation intensity. Both the output saturation value and the damage threshold of optical power limiting decrease with pulse duration increasing. The decrease of the pulse area along the pulse propagation is much slower than that obtained from the two-photon area theorem due to invalidity of the slowly varying amplitude approximation and the monochromatic field hypothesis.