Maximum Two-photon Absorption Research Articles

Effect of weak doping with cobalt over the optical magnitudes of tin oxide thin films

This paper aims to investigate the different properties of cobalt-doped and undoped tin oxide thin films. The tin oxide thin films were doped with different atomic percentages of cobalt, ranging from 0% (undoped) to 2% and 4%, using a cost-effective and easily accessible technique known as spray pyrolysis with a moving nozzle at 490 °C. The phase identification of the different thin layers was carried out using X-ray diffraction, which indicated that the compound is homogeneous has a polycrystalline structure and is, with preferential orientation growth at the (211) level. The optical properties of the synthesized thin films were investigated using UV-visible spectroscopy in the range from 300 to 900 nm, which allowed the estimation of both linear and nonlinear optical properties. The results of the investigations into the linear optical properties indicated that the samples have a direct band gap energy between 3.75 to 3.81 eV, permittivity between 2.67 to 3.266, photoelectrons between 0.2 to 3.35 = 1025, and refractive index between 1.778 to 1.875. The results also indicated that the absorption cross section changes with photon energy and the number of photoelectrons, where it has an approximately constant value at low energies. Moreover, the nonlinear optical properties were determined using the Sheik-Bahae model, which revealed that the samples have the same maximum two-photon absorption (TPA) process coefficient of 3.63 cm/W and the same maximum nonlinear refractive index (NRI) value of 6.91 × 10−8 cm2/W.

Single and two-photo excited violet-blue fluorescence from aza [6]helicenes and the plannar precursors based-on indole, carbazole, and fluorene

Organic π-conjugated materials exhibiting violet-blue emissions are of great interest for diverse applications. Helicenes and stilbenes hold great potential as compounds capable of achieving violet-blue fluorescence. In this study, aza[6]helicenes THNI and DHNC, along with their planar precursors DFVHI and HHIVC, were successfully synthesized using indole, carbazole, and fluorene units as building blocks. The molecular structures were verified through the utilization of 1H and 13C nuclear magnetic resonance (NMR), high resolution mass spectroscopies (HRMS), and the crystal structure of THNI was further elucidated via X-ray analysis. The synthesized material exhibited exceptional thermal stability with a Td value of ≥350 °C. The HOMO and LUMO energy levels were determined through cyclic voltammetry analysis, while density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the B3LYP/6-31G(d) level were employed to investigate ground-state geometries and vertical electronic transitions. Optical properties were characterized using UV–vis absorption spectroscopy as well as single- and two-photon excited photoluminescence. The UV–vis absorption spectra indicate that helicenes possess superior light-harvesting capabilities compared to their planar precursors, owing to the former's higher molar extinction coefficients. These compounds exhibit violet-blue emissions in various solvents and solid-state films. In addition, aza[6]helicenes exhibit higher photoluminescence quantum yields (PLQYs) and shorter maximum emission peaks compared to their planar precursors. However, the maximum two-photon absorption (TPA) cross section (δ) values of aza[6]helicenes are lower than those of their planar counterparts. Therefore, THNI and DHNC may be promising candidates for violet-blue emitters in OLEDs due to their curved confined π-electron systems, high thermal and optical stability, and high PLQYs. Meanwhile, DFVHI and HHIVC exhibit potential as two-photon fluorescent probes for bioimaging.

Theoretical study on a series of naphthalimide-contained two-photon fluorescent hypochlorite probe targeting endoplasmic reticulum: response mechanism and receptor effect.

The development of detecting hypochlorous acid (HClO) in living endoplasmic reticulum has attracted much attention in the fields of biology, medicine, and pharmacy. In the present work, the one-photon absorption (OPA), one-photon emission (OPE), and two-photon absorption (TPA) properties of a series newly synthesized chemosensors with naphthalimide as the fluorophore were systematically investigated using time-dependent density functional theory in combination with response theory. Special emphasis is placed on evolution of the probes' optical properties in the presence of HClO. These compounds show drastic changes in their photoabsorption and photoemission properties when they react with HClO, indicating them to be excellent candidates as fluorescent chemosensors. To further understand the mechanisms of the two probes, we have employed the hole and electron analysis to investigate the charge transfer process for the photoemission of the molecules. The receptor effect is found to play a dominant role in the sensing performance of these probes. Specifically, two-photon absorption properties of the molecules are calculated. We have found that all probes show significant two-photon responses in the near-infrared light region. And the maximum two-photon absorption cross section of probe 2 is greatly enhanced with the presence of HClO, indicating that probe 2 can act as a potential two-photon excited fluorescent HClO probe. The theoretical investigations would be helpful to build the structure-property relationships for the naphthalimide-contained probes, providing information on the design of efficient two-photon fluorescent sensors that can be used for biological imaging of HClO in endoplasmic reticulum.

Nonlinear optical properties and optimization strategies of D-π-A type phenylamine derivatives in the near-infrared region

Modifying simple molecular structures to significantly improve nonlinear optical (NLO) performance is a primary prerequisite for scientific research. Based on the four phenylamine derivatives reported in previous studies, we designed four organic nonlinear molecules by changing the acceptor group and π-linker. (Time-dependent) density functional theory (DFT/TD-DFT) was performed on molecular geometry optimization, the contribution of π electrons to the bond order, linear and two-photon absorption (TPA) spectra, the intra-molecular charge transfer matrix (CTM), and NLO coefficients. These aspects were considered to analyze in detail how the structural modification of acceptors and π-linkers affects NLO characteristics. The three modification methods were: adding a carbonyl group at the junction of the π-linker and the acceptor group, adding a carbonyl group and a nitrogen atom to the acceptor group, and replacing the quinolinone with a pyrenyl group as the π-linker. The latter two methods can significantly reduce the excitation energy and enhance the intensity of intra-molecular charge transfer during the two-photon transition. The maximum TPA cross-sections and wavelengths of the designed molecules are DPPM (84722.6 GM, 815.7 nm) and DDPM (21600.6 GM, 781.3 nm). These two molecules have large TPA cross-sections in the near-infrared region, which renders them as possible NLO materials with broad application prospects.

Modification of crystallization behavior, mechanical strength and optical property of Ge–S binary chalcogenide glass ceramics by trace CsCl incorporation

In present work, we discovered that nano-crystallization behavior of Ge–S binary chalcogenide glass can be remarkably improved by incorporating trace amount of CsCl as nucleating agent. After implementing an annealing process, the resultant Ge-S-CsCl chalcogenide glass ceramics (GSC ChGCs) possess fine distribution of crystallites belonging to GeS2 and GeS mixture phase which could enhance mechanical strength and narrow bandgap energy of the original glass. By utilizing femtosecond Z-scan method, nonlinear optical properties of the GSC ChGCs were investigated in a spectral range from 750 to 900 nm. The results showed that the nonlinear absorption attribute of the GSC ChGCs can be enhanced by the crystallinity increase which led to the maximum two-photon absorption coefficient of 19.57 cm/GW at excitation wavelength of 750 nm, increased by 78% as compared to the original glass. This indicates the current GSC ChGCs a promising candidate for the optical limiting devices.

New triazine bridged triads based on BODIPY-porphyrin systems: Extended absorption, efficient energy transfer and upconverted emission

Two novel triads connecting a BODIPY to ethylenediamine substituted porphyrins via triazine linker have been synthesized and characterized. One of the triads is a linear D-A structure with one BODIPY (D) and one porphyrin (A) bridged by the triazine linker and the other one is a branched A-D4 structure with the porphyrin core linked to four BODIPY units. The triads show extended absorption in the visible region with contributions from both porphyrin (Soret band centred at 410–430 nm) and BODIPY units (strong absorption at ≈ 502 nm) in good agreement with the expected molar ratio. Both triads exhibit linear and nonlinear optical properties featuring an efficient energy transfer from the BODIPY donor to the porphyrin acceptor. The nonlinear upconverted emission properties of the triads were studied by two-photon excitation in the Near-infrared (NIR, 710–930 nm). The maximum two-photon absorption cross-section values for the triads (40–70 GM) are larger than those typically reported in this wavelength range for porphyrins and BODIPY. Both the green emission of BODIPY (≈514 nm) and the red emission of porphyrins (650–750 nm) were observed under NIR excitation at 930 nm. The distinct features of triads, namely i) an extended absorption; ii) an efficient energy transfer and iii) the nonlinear upconverted emission featuring a large separation between the excitation and emission wavelengths could be beneficial for application in sensing and imaging procedures.

Bright Aggregation-Induced Emission Nanoparticles for Two-Photon Imaging and Localized Compound Therapy of Cancers.

Photodynamic therapy (PDT), a noninvasive therapeutic strategy for cancer treatment, which always suffers from the low reactive oxygen species (ROS) yield of traditional organic dyes. Herein, we present lipid-encapsulated aggregation-induced emission nanoparticles (AIE NPs) that have a high quantum yield (23%) and a maximum two-photon absorption (TPA) cross-section of 560 GM irradiated by near-infrared light (800 nm). The AIE NPs can serve as imaging agents for spatiotemporal imaging of tumor tissues with a penetration depth up to 505 μm on mice melanoma model. Importantly, the AIE NPs can simultaneously generate singlet oxygen (1O2) and highly toxic hydroxyl radicals (•OH) upon irradiation with 800 nm irradiation for photodynamic tumor ablation. In addition, the AIE NPs can be effectively cleared from the mouse body after the imaging and therapy. This study provides a strategy to develop theranostic agents for cancer image-guided PDT with high brightness, superior photostability, and high biosafety.

Linear and nonlinear photophysical properties of ZnSe/CdS/ZnS core/shell/shell type II nanocrystals

In this work, one kind of type II ZnSe/CdS/ZnS core/shell/shell nanocrystals (NCs) is synthesized, and their linear and nonlinear photophysical properties are investigated. Through measurements of the temperature-dependent photoluminescence spectra of NCs, their excitonic properties, including the coefficient of the bandgap change, coupling strength of the exciton acoustic phonons, exciton longitudinal optical (LO) phonons, and LO–phonon energy are revealed. Femtosecond transient absorption spectroscopy was employed to obtain insight into ultrafast processes occurring at the interface of ZnSe and CdS, such as those involving the injection of photo-induced electrons into the CdS shell, interfacial state bleaching, and charge separation time. At the end, their multiphoton absorption spectra were determined by using the z-scan technique, which yielded a maximum two-photon absorption cross section of 3717 GM at 820 nm and three-photon absorption cross section up to 3.9 × 10 − 77 cm 6 · s 2 · photon − 2 at 1220 nm, respectively. The photophysical properties presented here may be important for exploiting their relevant applications in optoelectronic devices and deep-tissue bioimaging.

Indole-Based Aza[n]helicenes (n=5, 6) with Violet-Blue Fluorescence and Two-Photon Absorption (TPA).

A series of indole-based helicenes, namely, 15-hexyl-15H-tetraphenyl[1,2-e]indole (HTPI), 14-hexyl-14H-benzo[4',5']thieno [2',3':7,8]naphtha[1,2-e]indole (HBTNI), 7-hexyl-7H-indolo[5,4-k] phenanthridine (HIPD) and 3-hexyl-3H-phenanthro[4,3-e]indole (HPI) were successfully synthesized by three- or four-step reactions. They exhibited good solubility and high thermal stability of Td =247, 388, 294 and 251 °C, respectively. These compounds emit violet-blue light with maximum emission peaks at 415, 397, 397 and 391 nm in hexane. Among them, HBTNI had excellent thermal stability, narrow and sharp emission peaks, and highest photoluminescence quantum yield (0.32). Thus, HBTNI was an ideal candidate for the violet-blue emitters of OLEDs. Furthermore, the four compounds had two-photon absorption and two-photon excited fluorescence. HTPI showed the maximum TPA with the TPA cross-section (δ) of 171.5 GM at 770 nm. They were rare examples of helicenes with both violet-blue emission and TPA.

Two-photon fluorescent Zn2+ probe for ratiometric imaging and biosensing of Zn2+ in living cells and larval zebrafish

As an essential transition-metal ion in organism, zinc ion (Zn2+) plays a significant role in cellular metabolism, apoptosis and neurotransmission. Herein, we reported a new intramolecular charge transfer-based two-photon probe, (E)-1-(6-(4-(2-(2-(2-methoxyethoxy)ethoxy)eth-oxy)-3,5-dimethylstyryl)quinolin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine (TEO-MPVQ), for ratiometric imaging and biosensing of Zn2+ in living cells and larval zebrafish. The maximum two-photon absorption cross-section values (δmax) of the developed probe increased from 115 ± 16 GM to 188 ± 27 GM, after the probe was chelated with Zn2+. The emission spectrum of TEO-MPVQ probe increased and red-shifted about 60 nm, resulting in the ratiometric determination of Zn2+ with high sensitivity and selectivity. The present fluorescent probe also demonstrated good biocompatibility, high spatial resolution and deep penetration depth for Zn2+ detection. Accordingly, TEO-MPVQ was able to track the endogenous Zn2+ release in neurons and also successfully applied for larval zebrafish imaging. By using of this effective probe, it was found that the endogenous Zn2+ was rapidly released from metallothionein under the stimulation of NO in neurons.

Femtosecond nonlinear absorption and optical limiting action in nanoplatelet CuFe2O4-decorated rGO nanocomposites

Using 800 nm infrared, 150 femtosecond (fs), 80 MHz high repetition rate laser pulse excitation, intensity-dependent (Io = 245–735 MW/cm2) measurements on the nonlinear absorption of copper ferrite hetero-architecture functionalized with three rGO contents (15, 25 and 40 wt%) were performed. Copper ferrite–rGO nanocomposite demonstrated a peculiar W-pattern [a peak (saturable absorption) with two valleys [reverse saturable absorption (RSA)] at extreme] in the open aperture Z-scan data for the decorated (15 wt%) rGO systems recorded at a low peak intensity (Io = 245 MW/cm2). The Z-scan traces were dominated by RSA characteristics due to two-photon absorption process at high peak intensities. The variation in intensity-dependent nonlinear absorption coefficient and ground-state absorption at laser excitation wavelength revealed the presence of excited-state absorption (ESA). The maximum two-photon absorption coefficient was recorded for CuFe2O4–(40 wt%) rGO at 735 MW/cm2 excitation, which is attributed to the combined contribution of rGO and CuFe2O4 that is four times higher than pure rGO and CuFe2O4. The presence of layer like CuFe2O4 nanoplatelets upon the layer-structured rGO offers an added advantage in enhancing the nonlinear absorption coefficient. Near-resonant ESA-based optical limiters comprising CuFe2O4–rGO can possibly be used as a versatile optical limiting material for eye safety against intense, high repetition rate fs laser pulses.

Influence of thiophene spacer and auxiliary acceptor on the optical properties of 4-(Diethylamino)-2-hydroxybenzaldehyde based D-π-A-π-D Colorants with N-alkyl donors: Experimental, DFT and Z-scan study

We report here design and synthesis of four new D-π-A-π-D type dyes (4 and 5) with triphenylamine/N-ethyl carbazole as fixed rotor/donor and N, N-diethyl amine as a secondary donor having dicyanovinylene as a common acceptor. The Influence of π-linker (thiophene) along with an auxillary cyano group on the nonlinear and linear optical properties is investigated. Molecules with thiophene spacer (4b and 5b) exhibit progressive variation in their absorption and emission profiles reflecting the effect of π-linker, compared to those devoid of thiophene spacer (4a and 5a). The dyes display solvent dependent emission [e.g. 4b (toluene = 689 nm) and 5b (DMSO = 795 nm)] suggesting a more polarized photo-excited state arising as a result of enhanced intramolecular charge transfer in the excited state. The solvatochromic nature is supportive of a general solvent effect, which is confirmed by solvent polarity plots (Weller’s and Rettig’s plot). The trends in the optical properties were also validated by time dependent density functional theory computations which further reveals charge transfer (from secondary donor to acceptor) for the long wavelength absorption. Dye with the highest charge transfer dipole moment relatively has the maximum two-photon absorption cross-section area (4b = 246–399 GM) which was established using theoretical two-level model. Non-linear optical properties were investigated using solvatochromic two-level model and computationally using global and range separated hybrid functionals. The dyes show high first-order hyperpolarizability (β0) in the range of 191–1162 × 10−30 e.s.u. and second-order hyperpolarizability (γ0) in the range of 491–6,704 × 10−36 e.s.u. Z-scan results show 4a (3.79 × 10−13 e.s.u.) possesses a higher value of third order nonlinear susceptibility in DMSO. Thus, the dyes can be considered as promising candidates for NLO application in material science.

Two-photon absorption spectrum of solid solution nanocrystals of ZnS-AgInS2

Two-photon absorption behavior of dispersion of solid solution nanocrystals of ZnS-AgInS2 was investigated, the measured maximum two-photon absorption cross-section of this nanoparticle being 3900 ...

Ultrafast dynamics of a triphenylamine-based tri-branched derivative with potential application as two-photon polymerization initiator

A triphenylamine-based tri-branched derivative named 1,3,5-tris[p-(4-N,N-diphenylamino)vinylbenzene]benzene (TPAVBB) with intense two-photon excited fluorescence (TPF) emission was investigated by means of UV–Vis, one-photon/two-photon fluorescence. The maximum two-photon absorption (TPA) cross-section of derivative TPAVBB in DMF solution was measured to be 546 GM at wavelength of 800 nm, which is the most commonly used wavelength in two-photon photopolymerization (TPP) and TPF cell imaging fields. Pump–probe and time-resolved fluorescence spectroscopy were employed to investigate the ultrafast responses of the tri-branched derivative. The formation and evolution processes of the intra-molecular charge transfer state were determined to be approximately picoseconds and several 100 picoseconds domain. The results indicate potential applications in TPP and TPF imaging fields.

Viscosity induced emission of red-emitting NLOphoric coumarin morpholine-thiazole hybrid styryl dyes as FMRs: Consolidated experimental and theoretical approach

Four new coumarin hybrid styryl dyes are synthesized by condensing 4-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-2-morpholinothiazole-5-carbaldehyde with dicyanovinylene containing active methylene intermediates and their linear and non-linear optical properties are studied. The dye having dicyanovinylene-isophorone acceptor displayed a large Stokes shift of 3702–4795 cm−1 in non-polar to polar solvent respectively. The dyes exhibit a good charge transfer characteristics and positive emission solvatochromism (∼50 nm–72 nm) in non-polar to a polar solvent which is well supported by multi-linear regression analysis. Viscosity induced enhancement study in ethanol/polyethylene glycol-400 system shows 2.71–6.78 fold increase in emission intensity. The intra and twisted-intramolecular charge transfer (ICT-TICT) characteristics were established using emission solvatochromism, polarity plots, generalised Mullikan-Hush (GMH) analysis and optimized geometry. A dye having the highest charge transfer dipole moment relatively possess the maximum two-photon absorption cross-section area (KK-1 = 165–207 GM) which was established using theoretical two-level model. The NLO properties have been investigated employing solvatochromic and computational methods and were found to be directly proportional to the polarity of the solvent. Z-scan results reveal that the dyes KK-1 and KK-2 possesses reverse saturable kind of behaviour whereas KK-3 and KK-4 show saturable kind of behaviour. From the experimental and theoretical data, these coumarin thiazole hybrid dyes can be considered as promising candidates for FMR and NLOphores.

Nonlinear optical property and ultrafast response of a two-photon photopolymerization initiator based on dibenzothiophene

A dibenzothiophene-based two-photon photopolymerization (TPP) initiator named E,E-1,4-bis[3-vinyl-dibenzothiophene]-2,5-dimethoxybenzene (VBPMOB) with intense two-photon fluorescence (TPF) was investigated by UV–visible, fluorescence, pump–probe and time-resolved fluorescence spectroscopies. The maximum two-photon absorption (TPA) cross-section of VBPMOB in DMF was determined to be 274[Formula: see text]GM. In addition, the TPA cross-section at 800[Formula: see text]nm was 230[Formula: see text]GM, which is the most commonly used wavelength for TPP. The ultrafast spectra result indicated that after excitation, the VBPMOB molecule will first undergo a fast internal conversion process to form an intramolecular charge transfer (ICT) state with a certain width followed by relaxation of the ICT state with fluorescence emission.

Theoretical Studies on Two-Photon Fluorescent Hg2+ Probes Based on the Coumarin-Rhodamine System.

The development of fluorescent sensors for Hg2+ has attracted much attention due to the well-known adverse effects of mercury on biological health. In the present work, the optical properties of two newly-synthesized Hg2+ chemosensors based on the coumarin-rhodamine system (named Pro1 and Pro2) were systematically investigated using time-dependent density functional theory. It is shown that Pro1 and Pro2 are effective ratiometric fluorescent Hg2+ probes, which recognize Hg2+ by Förster resonance energy transfer and through bond energy transfer mechanisms, respectively. To further understand the mechanisms of the two probes, we have developed an approach to predict the energy transfer rate between the donor and acceptor. Using this approach, it can be inferred that Pro1 has a six times higher energy transfer rate than Pro2. Thus the influence of spacer group between the donor and acceptor on the sensing performance of the probe is demonstrated. Specifically, two-photon absorption properties of these two probes are calculated. We have found that both probes show significant two-photon responses in the near-infrared light region. However, only the maximum two-photon absorption cross section of Pro1 is greatly enhanced with the presence of Hg2+, indicating that Pro1 can act as a potential two-photon excited fluorescent probe for Hg2+. The theoretical investigations would be helpful to build a relationship between the structure and the optical properties of the probes, providing information on the design of efficient two-photon fluorescent sensors that can be used for biological imaging of Hg2+ in vivo.

Study on a series of water-soluble photoinitiators for fabrication of 3D hydrogels by two-photon polymerization

A series of water-soluble benzylidene cyclanone dyes T1-T3 were synthesized. Their photo-physical properties were investigated using UV–Vis spectra, fluorescence spectra and two-photon absorption spectra. The maximum two-photon absorption cross-sections (σ2) of T1-T3 in deionized water were determined as 567 GM, 808 GM and 231 GM. Using T1-T3 as photoinitiators (PIs) directly, 2D and 3D nano patterns based on two-photon crosslinking or polymerization of bovine serum albumin (BSA), water-soluble acrylic ester monomer (SR610) and hyaluronic acid derivative (HAGM) were successfully fabricated, respectively. Much lower threshold energies and wider fabrication windows were obtained in the formulations containing these PIs compared with commercial ones. Furthermore, the cytotoxicity study showed a favorable biocompatibility of these PIs, which indicated T1-T3 would have application prospects in 3D fabrication of biomaterials.

Molecular-Based Fluorescent Nanoparticles Built from Dedicated Dipolar Thienothiophene Dyes as Ultra-Bright Green to NIR Nanoemitters.

Fluorescent Organic Nanoparticles (FONs), prepared by self-aggregation of dedicated dyes in water, represent a promising green alternative to the toxic quantum dots (QDs) for bioimaging purposes. In the present paper, we describe the synthesis and photophysical properties of new dipolar push-pull derivatives built from thieno[3,2-b]thiophene as a π-conjugated bridge that connects a triphenylamine moiety bearing various bulky substituents as electron-releasing moiety to acceptor end-groups of increasing strength (i.e., aldehyde, dicyanovinyl and diethylthiobarbiturate). All dyes display fluorescence properties in chloroform, which shifts from the green to the NIR range depending on the molecular polarization (i.e., strength of the end-groups) as well as a large two-photon absorption (TPA) band response in the biological spectral window (700–1000 nm). The TPA bands show a bathochromic shift and hyperchromic effect with increasing polarization of the dyes with maximum TPA cross-section reaching 2000 GM for small size chromophore. All dyes are found to form stable and deeply colored nanoparticles (20–45 nm in diameter) upon nanoprecipitation in water. Although their fluorescence is strongly reduced upon aggregation, all nanoparticles show large one-photon (up to 108 M−1·cm−1 in the visible region) and two-photon (up to 106 GM in the NIR) brightness. Interestingly, both linear and non-linear optical properties are significantly affected by interchromophoric interactions, which are promoted by the molecular confinement and modulated by both the dipolar strength and the presence of the bulky groups. Finally, we exploited the photophysical properties of the FONs to design optimized core-shell nanoparticles built from a pair of complementary dipolar dyes that promotes an efficient core-to-shell FRET process. The resulting molecular-based core-shell nanoparticles combine large two-photon absorption and enhanced emission both located in the NIR spectral region, thanks to a major amplification (by a factor of 20) of the core fluorescence quantum yield. These novel nanoparticles, which combine huge one-and two-photon brightness, hold major promise for in vivo optical bioimaging.

Third-Order Nonlinear Optical Properties of Infrared Emitting PbS and PbSe Quantum Dots

The optical properties of small band gap, colloidal quantum dots are presented, with the special emphasis put on the measurements of their nonlinear optical properties in the infrared region of spectra. In particular, two types of colloidal quantum dots (PbS and PbSe), with the first exciton absorption band maxima in the near-infrared region of spectra, were investigated using a tunable femtosecond laser system and the Z-scan technique. The measurements of closed- and open-aperture Z-scan traces allowed for the calculation of real and imaginary parts of cubic nonlinearity, which were presented as appropriate cross sections used to characterize the nonlinear refractive and absorptive properties of the studied quantum dots. The maximum two-photon absorption cross section values taken for a single quantum dot were found to be ∼2400 GM (Goeppert-Mayer units) at 1300 nm and ∼15 500 GM at 1700 nm, for PbS and PbSe QDs, respectively. PbS quantum dots showed two-photon induced emission upon infrared excitation. T...