2PA Spectra Research Articles

Effects of disorder on two-photon absorption in amorphous semiconductors.

Structural disorder inherent to amorphous materials affords them unique, tailorable properties desirable for diverse applications, but our ability to exploit these phenomena is limited by a lack of understanding of complex structure-property relationships. Here we focus on nonlinear optical absorption and derive a relationship between disorder and the two-photon absorption (2PA) coefficient. We employ an open-aperture Z-scan to measure the 2PA spectra of arsenic (III) sulfide (As2S3) chalcogenide glass films processed with two solvents that impart different levels of structural disorder. We find that the effect of solvent choice on 2PA depends on the energy of the exciting photons and explain this as a consequence of bonding disorder and electron state localization. Our results demonstrate how optical nonlinearities in As2S3 can be enhanced through informed processing and present a fundamental relationship between disorder and 2PA for a generalized amorphous solid.

Arylphthalimidoporphyrins: New Approaches to Imaging pH and Temperature Simultaneously with Oxygen

We will discuss a family of recently developed tetraarylphalimidoporphyrins (TAPIP) and diarylphalimidoporphyrins (DAPIP), which possess unique photophysical properties and suggest new approaches to the design of superior biological sensors for pH and temperature in addition to their already established ability as sensors for oxygen. The synthesis of TAPIP has already been developed by our group previously. Here we present a synthetic route to asymmetric diarylphthalimidoporphyrins (DAPIP) based on [2+2] condensation of the corresponding dipyrromethanes, followed by oxidative aromatization. We report the properties of free-base (H2), Zn(II), Pt(II) and Pd(II) DAPIP's, including their two-photon absorption (2PA) spectra. Using a combination of phosphorescent Pt complexes of TAPIP and DAPIP we are designing a phosphorescent sensor for pH and oxygen, termed pHOx, which operates by measuring ratios of phosphorescence lifetimes. Unlike all existing optical pH sensors, measurements by pHOx are not affected by optical heterogeneities of the medium and provide unbiased pH readings in vivo simultaneously with pO2. At the same time, Pd complexes of TAPIP have been shown to emit both phosphorescence and thermally activated E-type delayed fluorescence, providing a functional element for optical sensing of temperature. However, to make the temperature readings unobstructed by optical heterogeneities of biological tissue, a PdTAPIP-based probe is supplemented by another probe system, based on H2DAPIP, which emits prompt fluorescence at the same wavelengths and can serve as a correction standard for unbiased temperature measurements.

Benzenesulfonyl incorporated chalcones: Synthesis, structural and optical properties

Although the optical properties and applicability of chalcones as potential nonlinear optical(NLO) materials is well known, hybrids having both chalcone and sulfonamide portions are relatively scarce. In this sense, we are seeking to show if combining both into a double functionalized compound will still have similar or better NLO responses. For this, we have synthetized three sulfonamide-chalcone analogues by changing the substituent bonded to the sulfonamide ring (I = ethoxy; II = Cl and III = Br). These compounds were characterized by spectroscopic methods (NMR, IR and HRMS) and thermal methods (HSM and DSC/TGA). Their crystal structures were determined by Single Crystal X-ray Diffraction (SCXRD) and their molecular structures were compared. Crystallographic results showed that one compound crystallizes in a triclinic system whereas the others crystallize in a monoclinic crystal system. Moreover, their crystal packing is dominated by C–H⋯O interactions. In addition to this study and to first characterize these compounds, linear and nonlinear optical (NLO) properties were performed on all three compounds dissolved in dimethyl sulfoxide. One-photon and two-photon absorption (2PA) spectra and incoherent second harmonic generation were obtained by employing different spectroscopic techniques. The interesting results observed in the linear and NLO measurements showed that different groups bounded to the same main backbone did not significantly modify their optical properties, although they strongly affect their crystal structures. σ2PA spectra reveal that the first excited state is allowed by both 1PA and 2PA, which indicates asymmetry in the charge distribution along the π-conjugated molecule structure. NLO properties of compounds I-III agree with other chalcones previously studied, which indicates that adding the benzenesulfonyl group does not influence these properties. Considering the similar experimental values for different substituents, the results further motivate investigation on substituent-based optical properties for other sulfonamide-chalcone hybrids.

New Insights into the Charge-Transfer-to-Solvent Spectrum of Aqueous Iodide: Surface versus Bulk.

Liquid phase charge-transfer-to-solvent (CTTS) transitions are important, as they serve as photochemical routes to solvated electrons. In this work, broadband deep-ultraviolet electronic sum frequency generation (DUV-ESFG) and two-photon absorption (2PA) spectroscopic techniques were used to assign and compare the nature of the aqueous iodide CTTS excitations at the air/water interface and in bulk solution. In the one-photon absorption (1PA) spectrum, excitation to the 6s Rydberg-like orbital (5p → 6s) gives rise to a pair of spin-orbit split iodine states, 2P3/2 and 2P1/2. In the 2PA spectra, the lower-energy 2P3/2 peak is absent and the observed 2PA peak, which is ∼0.14 eV blue-shifted relative to the upper 2P1/2 CTTS peak seen in 1PA, arises from 5p → 6p electronic promotion. The band observed in the ESFG spectrum is attributed to mixing of excited states involving 5p → 6p and 5p → 6s promotions caused by both vibronic coupling and the external electric field generated by asymmetric interfacial solvation.

Femtosecond-laser induced two-photon absorption of GaN and AlxGa1-xN thin films: Tuning the nonlinear optical response by alloying and doping

Semiconductors thin films are the foundation of modern technology. While the nonlinear optical (NLO) properties of bulk semiconductors have been systematically studied in the last three decades, it is still a great challenge to obtain them for semiconductors thin films, as the high laser irradiance in NLO experiments tends to irreversibly damage the thin films. In addition, tuning the NLO response of semiconductor thin films by alloying and doping has not been explored yet. Here, we study the influence of the Aluminum content in AlxGa1-xN thin films and the n-type doping concentration in GaN thin films on their two-photon absorption (2PA) coefficients. For this, we investigate five different GaN-based thin films: an unintentionally doped one with no Al as a reference, two n-type doped GaN films with distinct concentrations of silicon impurities, and two AlxGa1-xN alloys with an aluminum content of 5.5% and 9.0%, respectively. The femtosecond 2PA spectra reveal that doping impurities reduce the non-linear coefficients (∼10%), while alloying with Al enhances the 2PA coefficient up to 30%. We use the model of Brandi and Araujo to determine Kane’s energy parameter related to the transition matrix element for each sample and compare them with recent theoretical studies based on the k·p theory where an excellent agreement is found.

Characterizing the Two-photon Absorption Properties of Fluorescent Molecules in the 680-1300 nm Spectral Range.

Two-photon laser scanning microscopy (2PLSM) is a state-of-the-art technique used for non-invasive imaging deep inside the tissue, with high 3D resolution, minimal out-of-focus photodamage, and minimal autofluorescence background. For optimal application of fluorescent probes in 2PLSM, their two-photon absorption (2PA) spectra, expressed in absolute cross sections must be characterized. Excitation at optimum wavelength will make it possible to reduce the laser power and therefore minimize photodamage. Obtaining 2PA spectra and cross sections requires correcting the two-photon excited fluorescence signals for a combination of laser properties, including the beam spatial profile, pulse duration, and absolute power, at each wavelength of the tuning range. To avoid such tedious day-to-day laser characterization required in the absolute measurement method, a relative method based on independently characterized 2PA reference standards is often used. By carefully analyzing the available literature data, we selected the most reliable standards for both the 2PA spectral shape and cross section measurements. Here we describe a protocol for measuring the 2PA spectral shapes and cross sections of fluorescent proteins and other fluorophores with the relative fluorescence method using these reference standards. Our protocol first describes how to build an optical system and then how to perform the measurements. In our protocol, we use Coumarin 540A in dimethyl sulfoxide and LDS 798 in chloroform for the spectral shape measurements to cover the range from 680 to 1300 nm, and Rhodamine 590 in methanol and Fluorescein in alkaline water (pH 11) for the absolute two-photon cross section measurements.

Electronic Nature of Neutral and Charged Two-Photon Absorbing Squaraines for Fluorescence Bioimaging Application

The electronic properties of neutral 2,4-bis(4-bis(2-hydroxyethyl) amino-2-hydroxy-6-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)squaraine (1) and charged 2-((3-octadecylbenzothiazol-2(3H)-ylidene)methyl)-3-oxo-4-((3-(4-(pyridinium-1-yl)butyl)benzo-thiazol-3-ium-2-yl)methylene)cyclobut-1-enolate iodide (2) squaraine derivatives were analyzed based on comprehensive linear photophysical, photochemical, nonlinear optical studies (including two-photon absorption (2PA) and femtosecond transient absorption spectroscopy measurements), and quantum chemical calculations. The steady-state absorption, fluorescence, and excitation anisotropy spectra of these new squaraines revealed the values and mutual orientations of the main transition dipoles of 1 and 2 in solvents of different polarity, while their role in specific nonlinear optical properties was shown. The degenerate 2PA spectra of 1 and 2 exhibited similar shapes, with maximum cross sections of ∼300–400 GM, which were determined by the open aperture Z-scan method over a broad spectral range. The nature of the time-resolved excited-state absorption spectra of 1 and 2 was analyzed using a femtosecond transient absorption pump–probe technique and the characteristic relaxation times of 4–5 ps were revealed. Quantum chemical analyses of the electronic properties of 1 and 2 were performed using the ZINDO/S//DFT theory level, affording good agreement with experimental data. To demonstrate the potential of squaraines 1 and 2 as fluorescent probes for bioimaging, laser scanning fluorescence microscopy images of HeLa cells incubated with new squaraines were obtained.

Electronic Structure of Liquid Methanol and Ethanol from Polarization-Dependent Two-Photon Absorption Spectroscopy.

Two-photon absorption (2PA) spectra of liquid methanol and ethanol are reported for the energy range 7-10 eV from the first electronic excitation to close to the liquid-phase ionization potential. The spectra give detailed information on the electronic structures of these alcohols in the bulk liquid. The focus of this Article is to examine the electronic structure change compared with water on substitution of a hydrogen by an alkyl group. Continuous 2PA spectra are recorded in the broadband pump-probe fashion, with a fixed pump pulse in the UV region and a white-light continuum as a probe. Pump pulses of two different energies, 4.6 and 6.2 eV, are used to cover the spectral range up to 10 eV. In addition, theoretical 2PA cross sections for both molecules isolated in the gas phase are computed by the equation-of-motion coupled-cluster method with single and double substitutions (EOM-CCSD). These computational results are used to assign both the experimental 2PA and literature one-photon linear absorption spectra. The most intense spectral features are due to transitions to the Rydberg states, and the 2PA spectra are dominated by the totally symmetric 3pz ← 2pz transition in both alcohols. The experimental 2PA spectra are compared with the simulated 2PA spectra based on ab initio calculations that reveal a general blue shift of the excited transitions upon solvation. The effective 2PA thresholds in methanol and ethanol decrease to 6.9 eV compared with 7.8 eV for water. The analysis of the 2PA polarization ratio leads us to conclude that the excited states of ethanol deviate more markedly from water in the lower energy region compared with methanol. The polarization dependence of the 2PA spectra reveal the symmetries of the excited states within the measured energy range. Natural transition orbital calculations are performed to visualize the nature of the transitions and the orbitals participating during electronic excitation.

Far‐Red‐ to NIR‐Emitting Adamantyl‐Functionalized Squaraine Dye: J‐Aggregation, Dissociation, and Cell Imaging

Squaraine dyes AM‐1′ and AM‐1 bearing adamantyl termini were synthesized and spectroscopically characterized. AM‐1′ exhibited intense absorption and sharp emission in the far‐red to near‐infrared (NIR) spectral region. The degenerate two‐photon absorption (2PA) spectrum of AM‐1′ was obtained over a broad spectral range with a maximum cross section of ca. 360 GM. In aqueous solutions, AM‐1′ displayed a strong tendency for J‐aggregation and the reference AM‐1 exhibited entirely different aggregation behavior. We conclude that both the terminal pendant groups and substituents adjacent to the squaraine core play important roles in determining the aggregation behaviors and optical properties of the resultant squaraine dyes. Further an AM‐1′/β‐cyclodextrin complex dramatically suppressed squaraine π–π stacking and dissociated the J‐aggregate. Fluorescence microscopy imaging of HCT 116 cells incubated with the complex was accomplished, suggesting the potential application of this complex in fluorescence bioimaging.

Dispersive white light continuum single Z-scan for rapid determination of degenerate two-photon absorption spectra

We present an experimental technique to determine the degenerate two-photon absorption (2PA) spectra by performing a single Z-scan using a high-spectral-irradiance white light continuum (WLC) generated by a hollow core fiber. The hollow fiber was filled with Argon (Ar) gas at a pressure of 0.6 bar and was pumped with 500 mJ, 30 fs, and 800 nm pulses. The broadband WLC pulses with 350 nm bandwidth in the range of 600–950 nm were compressed to sub-8 fs pulses. To characterize and interpret the data obtained from this method, the spectral, temporal and spatial characteristics of the WLC were first analyzed. The WLC emerging from the compressor was dispersed using a prism pair and then focused into the sample by a cylindrical lens. Since different spectral components are spatially separated, any part of the sample in the beam cross section is irradiated with almost single wavelength pulses leading to only a degenerate 2PA process. The nonlinear transmittance was then measured by a charge-coupled-device (CCD) line camera as a function of the sample position while the sample was moved along the beam direction by a motorized translation stage. In this way the Z-scans at different wavelengths in the WLC spectral range can be measured and thus the wavelength-resolved degenerate 2PA spectra can be obtained by performing a single scan using dispersive WLC. This method was verified on a well-characterized dye Rhodamine B and yield a reasonable agreement with the data found in the literature. We used this method to determine the 2PA spectra of some two-photon initiators (2PIs) developed for two-photon polymerization (2PP) based 3D micro-structuring.

Dual Emissive Multinuclear Iridium(III) Complexes in Solutions: Linear Photophysical Properties, Two-Photon Absorption Spectra, and Photostability

Linear steady-state and time-resolved spectroscopic properties, degenerate two-photon absorption (2PA) spectra, and photochemical stability of IrIII complexes TCQ[IrIII(ppz)2]n, (TCQ = tricycloquinazoline; ppz = 1-phenylpyrazole; n = 1 (1), n = 2 (2), and n = 3 (3)) are presented for liquid solutions. The analysis of the linear photophysical properties revealed the nature of the observed dual-component fluorescence–phosphorescence emission of 1–3 at room temperature. The values of 2PA cross sections were determined by open aperture Z-scans using a 1 kHz femtosecond laser system. The specific dependence of the 2PA efficiency on the number of ppz ligand units in 1–3 was determined. The quantum yields, Φph, for photochemical decomposition of TCQ[IrIII(ppz)2]n complexes were obtained for the first time using the absorption method [Corredor, C. C.; J. Photoch. Photobio. A 2006, 184, 105−112] with continuous wave laser irradiation, and the highest stability of Φph ≈ 4 × 10–6 was shown for 2 in toluene.

Two-photon absorption spectra of fluorescent isomorphic DNA base analogs.

Fluorescent DNA base analogs and intrinsic fluorophores are gaining importance for multiphoton microscopy and imaging, however, their quantitative nonlinear excitation properties have been poorly documented. Here we present the two-photon absorption (2PA) spectra of 2-aminopurine (2AP), 7-methyl guanosine (7MG), isoxanthopterin (IXP), 6-methyl isoxanthopterin (6MI), as well as L-tryptophan (L-trp) and 3-methylindole (3MI) in aqueous solution and some organic solvents measured in the wavelength range 550 - 810 nm using femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods. The peak 2PA cross section values range from 0.1 GM (1 GM = 10-50 cm4 s photon-1) for 2AP to 2.0 GM for IXP and 7MG. Assuming typical excitation conditions for a scanning 2PEF microscope, we estimate a maximum image frame rate of ~175 frames per second (FPS).

Quantitative description of MLCT transitions in ruthenium polypyridyl complexes probed by two-photon absorption spectroscopy

Quantitative two-photon absorption (2PA) cross section and 2PA spectrum measurements were used to determine the molecular electric dipole change in the metal-to-ligand charge-transfer transition of ruthenium(II) tris-complexes of 2,2’-bipyridine ([Ru(bpy)3]2+) and 1,10-phenanthroline ([Ru(phen)3]2+) in several solvents.

Electronic Nature of New Ir(III) Complexes: Linear Spectroscopic and Nonlinear Optical Properties

Comprehensive investigations of the linear and nonlinear optical properties of new Ir(III)-complexes [Ir(pbt)2(dbm)] (1), [Ir(pbt)2(dmac)] (2), and [Ir(pbt)2(minc)] (3) (pbt=2-phenylbenzothiazole; dbm=dibenzoyl methane; dmac=(1E,4Z,6E)-1,7-bis(4-(dimethylamino)phenyl)-5-hydroxyhepta-1,4,6-trien-3-one; minc=(1E,4Z,6E)-5-hydroxy-1,7-bis(1-methyl-1H-indol-3-yl)hepta-1,4,6-trien-3-one) are reported, including, photostability, two-photon absorption, and femtosecond transient absorption spectroscopy. The steady-state and time-resolved spectral properties of 1-3 revealed the electronic nature of the absorption bands, and photoluminescence emission of 2 and 3 shows both fluorescence and phosphorescence processes occurring simultaneously in liquid solution at room temperature. This unusual behavior of 2 and 3 can be explained by a dual-minimum potential surface of the excited electronic state resulting in two independent fluorescence and phosphorescence emission channels. The degenerate 2PA spectra of 1-3 were obt...

Measurement of degenerate two-photon absorption spectra of a series of developed two-photon initiators using a dispersive white light continuum Z-scan

To achieve efficient micro- and nanostructuring based on two-photon polymerization (2PP), the development and evaluation of specialized two-photon initiators (2PIs) are essential. Hence, a reliable method to determine the two-photon absorption (2PA) spectra of the synthesized 2PIs used for 2PP structuring is crucial. A technique by which absolute visible-to-near-infrared 2PA spectra of degenerate nature can be determined via performing a single dispersive white-light continuum (WLC) Z-scan has been realized. Using a dispersed white light beam containing 8 fs pulses at wavelengths ranging from 650 nm to 950 nm, the nonlinear transmittance as a function of the sample position can be measured for all spectral components by performing a single scan along the laser beam propagation direction. In this work, the 2PA spectrum of three different 2PIs was determined using this technique. 2PP structuring was also accomplished using the developed 2PIs at different wavelengths. Tuning the wavelength of the laser to match the peak of the 2PA spectra of the developed 2PIs resulted in lower intensity thresholds and facilitated higher structuring speeds. As an example, using M2CMK 2PI for 2PP, the scanning speed can be increased up to 5 folds when the laser wavelength is tuned to 760 nm (i.e., 2PA maximum) instead of the conventionally used 800 nm.

Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism and Two-photon Absorption.

Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ2PA ) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ2PA =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S0 →S1 transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.

Femtosecond Two-Photon Absorption Spectroscopy of Poly(fluorene) Derivatives Containing Benzoselenadiazole and Benzothiadiazole.

We have investigated the molecular structure and two-photon absorption (2PA) properties relationship of two push–pull poly(fluorene) derivatives containing benzoselenadiazole and benzothiadiazole units. For that, we have used the femtosecond wavelength-tunable Z-scan technique with a low repetition rate (1 kHz) and an energy per pulse on the order of nJ. Our results show that both 2PA spectra present a strong 2PA (around 600 GM (1 GM = 1 × 10−50 cm4·s·photon−1)) band at around 720 nm (transition energy 3.45 eV) ascribed to the strongly 2PA-allowed 1Ag-like → mAg-like transition, characteristic of poly(fluorene) derivatives. Another 2PA band related to the intramolecular charge transfer was also observed at around 900 nm (transition energy 2.75 eV). In both 2PA bands, we found higher 2PA cross-section values for the poly(fluorene) containing benzothiadiazole unit. This outcome was explained through the higher charge redistribution at the excited state caused by the benzothiadiazole group as compared to the benzoselenadiazole and confirmed by means of solvatochromic Stokes shift measurements. To shed more light on these results, we employed the sum-over-states approach within the two-energy level model to estimate the maximum permanent dipole moment change related to the intramolecular charge transfer transition.

Two-photon absorption spectroscopy of stilbene and phenanthrene: Excited-state analysis and comparison with ethylene and toluene.

Two-photon absorption (2PA) spectra of several prototypical molecules (ethylene, toluene, trans- and cis-stilbene, and phenanthrene) are computed using the equation-of-motion coupled-cluster method with single and double substitutions. The states giving rise to the largest 2PA cross sections are analyzed in terms of their orbital character and symmetry-based selection rules. The brightest 2PA transitions correspond to Rydberg-like states from fully symmetric irreducible representations. Symmetry selection rules dictate that totally symmetric transitions typically have the largest 2PA cross sections for an orientationally averaged sample when there is no resonance enhancement via one-photon accessible intermediate states. Transition dipole arguments suggest that the strongest transitions also involve the most delocalized orbitals, including Rydberg states, for which the relative transition intensities can be rationalized in terms of atomic selection rules. Analysis of the 2PA transitions provides a foundation for predicting relative 2PA cross sections of conjugated molecules based on simple symmetry and molecular orbital arguments.

Two-photon absorption spectroscopy of trans-stilbene, cis-stilbene, and phenanthrene: Theory and experiment.

Two-photon absorption (2PA) spectroscopy provides complementary, and sometimes more detailed, information about the electronic structure of a molecule relative to one-photon absorption (1PA) spectroscopy. However, our understanding of the 2PA processes is rather limited due to technical difficulties in measuring experimental 2PA spectra and theoretical challenges in computing higher-order molecular properties. This paper examines the 2PA spectroscopy of trans-stilbene, cis-stilbene, and phenanthrene by a combined experimental and theoretical approach. The broadband 2PA spectra of all three compounds are measured under identical conditions in order to facilitate a direct comparison of the absolute 2PA cross sections in the range 3.5-6.0 eV. For comparison, the theoretical 2PA cross sections are computed using the equation-of-motion coupled-cluster method with single and double substitutions. Simulated 2PA spectra based on the calculations reproduce the main features of the experimental spectra in solution, although the quantitative comparison is complicated by a number of uncertainties, including limitations of the theoretical model, vibronic structure, broadening of the experimental spectra, and solvent effects. The systematic comparison of experimental and theoretical spectra for this series of structurally similar compounds provides valuable insight into the nature of 2PA transitions in conjugated molecules. Notably, the orbital character and symmetry-based selection rules provide a foundation for interpreting the features of the experimental 2PA spectra in unprecedented detail.

Observation of Distinct Two-Photon Transition Channels in CdTe Quantum Dots in a Regime of Very Strong Confinement.

We report here on the direct observation of distinct two-photon transition channels in glutathione-capped (GSH) CdTe quantum dots (QDs) in a very strong confinement regime. CdTe-GSH QDs with different average diameters (2.5, 3.0, and 3.3 nm) were synthesized through the one-pot method and their two-photon absorption (2PA) spectrum determined by a femtosecond wavelength-tunable Z-scan. Our results show that the two lower-energy one-photon-allowed excitonic transitions, 1S3/2(h) → 1S(e) and 2S3/2(h) → 1S(e), are also accessed via 2PA. These results were ascribed to the relaxation of the parity selection rules due to the noncentrosymmetric structure of the CdTe QDs (zinc-blended structure), whose magnitude are determined by surface defects and structural irregularities present in CdTe-GSH QDs, in the strong confinement regime.