Probing the Strong Near-IR Two-Photon Transition in Supramolecular Triphenylamine-based Polymers by Nonlinear Absorption Spectroscopy.

Due to their capability of film formation and remarkable optical features, semiconductor polymers with high two-photon absorption (2PA) have been studied as potential candidates for the development of organic photonic platforms. Furthermore, there is a high demand for photonic devices operating in the near-infrared (IR) region. However, the magnitude of the nonlinear optical response of random coil polymers in the IR region is weak due to the loss of molecular structure caused by increasing the π-conjugated backbone. Thus, herein we aim to investigate the molecular structure and 2PA features relationship for four polymers with supramolecular (helical) rodlike structure. Such polymers have a rigid core based on triphenylamine groups connected to the chiral binaphthalene units and a strong electron-withdrawing group (EWG). This kind of structure allows a very high chromophore density, which was responsible for generating 2PA cross-section between 305 GM and 565 GM in the near-IR (900-1300 nm), depending on the EWG strength. in light of the two-level model within the sum-overstates approach, we estimated the degree of intramolecular charge transfer induced by 2PA in the IR region, and values as high as 50-70% were found. Such a critical outcome allows the 2PA cross-section in the IR region to remain high even though the ratio between the visible/IR-band 2PA cross-section increases as a function of EWG strength.

Two-photon absorption properties of BODIPY-like compounds based on BF2-naphthyridine complexes.

Boron dipyrromethene type molecules (BODIPYs) are versatile molecules which have been used for applications ranging from photodynamic therapy to solar cells (DSSC). However, these molecules usually do not present high two-photon absorption cross-sections, limiting their use in nonlinear optical applications. Herein, we study a series of BF2-naphthyridine based boron-complexes with electron-donating and withdrawing groups to increase their two-photon absorption. We have found two-photon absorption cross-sections up to approximately 270 GM, which corresponds to an increase of approximately five times in comparison to the average cross-section value reported for molecules with similar conjugation length, indicating such compounds as potential materials for nonlinear applications in both the visible and infrared spectral regions.

Enhanced high-harmonic generation up to the soft X-ray region driven by mid-infrared pulses mixed with their third harmonic.

We systematically study the efficiency enhancement of high-harmonic generation (HHG) in an Ar gas cell up to the soft X-ray (SXR) range using a two-color laser field composed of 2.1 µm (ω) and 700 nm (3ω) with parallel linear polarization. Our experiment follows the recent theoretical investigations that determined two-color mid-infrared (IR) pulses, mixed with their third harmonic (ω + 3ω), to be close to optimal driving waveforms for enhancing HHG efficiency in the SXR region [Jin et al., Nature Comm. 5, 4003 (2014)]. We observed sub-optical-cycle-dependent efficiency enhancements of up to 8.2 of photon flux integrated between 20 - 70 eV, and up to 2.2 between 85 - 205 eV. Enhancement of HHG efficiency was most pronounced for the lowest tested backing pressure (≈ 140 mbar), and decreased monotonically as the pressure was increased. The single-color (ω)-driven HHG was optimal at the highest backing pressure tested in the experiment (≈ 375 mbar). Our numerical simulations based on single-atom response and 3D pulse propagation show good qualitative agreement with experimental observations. The lower enhancement at high pressure and higher photon energy indicates that phase matching of two-color-driven HHG is more sensitive to ionization rate and pulse propagation effects than the single-color case. We show that with further improvements to the relative phase jitter and the spatio-temporal overlap of the two beams, the efficiency enhancement could be further improved by at least a factor of ≈ 2.

Nonlinear optical spectrum of diamond at femtosecond regime.

Although diamond photonics has driven considerable interest and useful applications, as shown in frequency generation devices and single photon emitters, fundamental studies on the third-order optical nonlinearities of diamond are still scarce, stalling the development of an integrated platform for nonlinear and quantum optics. The purpose of this paper is to contribute to those studies by measuring the spectra of two-photon absorption coefficient (ß) and the nonlinear index of refraction (n2) of diamond using femtosecond laser pulses, in a wide spectral range. These measurements show the magnitude of ß increasing from 0.07 to 0.23 cm/GW, as it approaches the bandgap energy, in the region from 3.18 to 4.77 eV (390-260 nm), whereas the n2 varies from zero to 1.7 × 10-19 m2/W in the full measured range, from 0.83-4.77 eV (1500-260 nm). The experimental results are compared with theoretical models for nonlinear absorption and refraction in indirect gap semiconductors, indicating the two-photon absorption as the dominant effect in the dispersion of the third-order nonlinear susceptibility. These data, together with optical Kerr gate measurements, also provided here, are of foremost relevance to the understanding of ultrafast optical processes in diamond and its nonlinear optical properties.

Tetracarboxy-phthalocyanines: From excited state dynamics to photodynamic inactivation against Bovine herpesvirus type 1.

Herein we present the excited state dynamic of zinc and aluminum tetracarboxy-phthalocyanines (ZnPc and AlPc) and its application in the photodynamic inactivation (PDI) of Bovine herpesvirus type 1 (BoHV-1) in vitro. The excited state dynamic provides valuable data to describe the excited state properties of potential optical limiters and/or photosensitizers (PSs), such as: the excited state cross-sections, fluorescence lifetime and triplet state quantum yield. The excited state characterization was performed using three different Z-scan techniques: Single Pulse, White Light Continuum and Pulse Train. Considering the photodynamic inactivation of BoHV-1, an initial viral suspension containing 105.75TCID50/mL was incubated with the PSs for 1h at 37°C under agitation and protected from light. The samples were placed in microtiter plates and irradiated (180mW/cm2). During irradiation, a sample was taken every 15min and the viability of the virus was evaluated. The results show that both phthalocyanines were efficient against viruses. However, a higher photodynamic efficiency was observed by ZnPc, which can be attributed to its higher triplet and singlet quantum yields. The results presented here are important for animal health (treatment of BoHV-1) and also open up a field of studies to use AlPc and ZnPc as potential agents against a wide range of microorganisms of veterinary interest.

Multi-mJ, kHz, ps deep-ultraviolet source.

We demonstrate a 0.56-GW, 1-kHz, 4.2-ps, 2.74-mJ deep-ultraviolet (DUV) laser at ∼257.7 nm with a beam propagation factor (M2) of ∼2.54 from a frequency-quadrupled cryogenic multi-stage Yb-doped chirped-pulse amplifier. The frequency quadrupling is achieved using LiB3O5 and ß-BaB2O4 crystals for near-infrared (NIR)-to-green and green-to-DUV conversion, respectively. An overall NIR-to-DUV efficiency of ∼10% has been achieved, which is currently limited by the thermal-induced phase mismatching and the DUV-induced degradation of transmittance. To the best of our knowledge, this is the highest peak-power picosecond DUV source from a diode-pumped solid-state laser operating at kHz repetition rates.

Multi-mJ, kHz, 2.1 µm optical parametric chirped-pulse amplifier and high-flux soft x-ray high-harmonic generation.

We report on a multi-mJ 2.1 µm optical parametric chirped-pulse amplification (OPCPA) system operating at 1 kHz repetition rate, pumped by a picosecond cryogenic Yb:YAG laser, and the demonstration of soft x-ray high-harmonic generation (HHG) with a flux of ∼2×10(8) photon/s/1% bandwidth at 160 eV in Ar. The 1 kHz cryogenic Yb:YAG pump laser amplifies pulses up to 56 mJ and delivers compressed 42 mJ, 17 ps pulses to the 2.1 µm OPCPA system. In the three-stage OPCPA chain, we have obtained up to 2.6 mJ of output energies at 2.1 µm and pulses compressed to 40 fs with good beam quality. Finally, we show cut-off extension of HHG driven by this 2.1 µm source in Ar and N2 gas cells to 190 eV with high photon flux. Our 3D propagation simulation confirms the generation of soft x-ray attosecond pulses from the experiment with Ar.