Variation of two-photon absorption (2PA) of organic chromophores as a function of the environment such as different solvent polarity has important implications for practical applications of nonlinear optical (NLO) materials but could be also used as a sensitive probe of local inter- and intra-molecular interactions. We report measurement of the 2PA spectrum in the broad wavelength range 700-1000 nm of a benchmark NLO chromophore 4-Dimethylamino-4’-nitrostilbene (DANS) in a series of solvents with varying polarity by using a femtosecond nonlinear-optical transmission method with a nearly-collimated laser beam. The 2PA peak of DANS shifts systematically from 834 nm in a low dielectric constant solvent isobutyl isobutyrate (dielectric constant = 4) to 892 nm in a highly polar solvent DMSO (dielectric constant = 48), which quantitatively follows the corresponding solvatochromic shift of the S0→S1 transition peak in the linear absorption spectrum, and indicates that DANS has a significant ground state permanent electric dipole moment that interacts with the solvent environment. The corresponding change of the S0→S1 transition peak 2PA cross section in solvents with different polarity is quite large, ranging from 160 GM in isobutyl acetate to 240 GM in 1,3-Dioxolane. However, this variation exhibits no direct correlation with the solvent polarity. This effect may be tentatively attributed to the change of the excited state permanent electric dipole moment due to the instantaneous S0→S1 transition. Our results confirm that accurate measurement of 2PA spectra may serve as a probe of local molecular-level interactions.
Two-photon excitation of fluorescent proteins (FPs) is widely used in imaging whole organisms or living tissues. Many different FPs are now available but these proteins have only been optimized for their one-photon properties. We have developed a technique for screening entire libraries of E. coli colonies expressing FPs that utilizes multiple wavelengths of linear excitation as well as two-photon excitation. Single mutations in a particular protein that affect one or twophoton properties are easily identified, providing new views of structure/function relationships. An amplified femtosecond Ti:sapphire laser and a spectrally filtered lamp source are used to acquire the fluorescence signals of up to ~1000 E. coli colonies on a standard Petri dish. Automation of the analysis and acquisition of the fluorescent signals makes it feasible to rapidly screen tens of thousands of colonies. In a proof of principle experiment with the commonly used EGFP, we used two rounds of error prone PCR and selection to evolve new proteins with shifted absorption and increased two-photon cross sections at 790nm. This method of screening, coupled with careful measurements of photo bleaching dynamics and two-photon cross sections, should make it possible to optimize a wide variety of fluorescent proteins and biosensors for use in two-photon microscopes.
A fiber sensor array for subsurface CO2 concentration measurements was developed for monitoring geologic carbon sequestration sites. The fiber sensor array uses a single temperature-tunable distributed feedback (DFB) laser operating with a nominal wavelength of 2.004 μm. Light from this DFB laser is directed to one of the four probes via an inline 1×4 fiber optic switch. Each of the four probes is buried and allows the subsurface CO2 to enter the probe through Millipore filters that allow the soil gas to enter the probe but keeps out the soil and water. Light from the DFB laser interacts with the CO2 before it is directed back through the inline fiber optic switch. The DFB laser is tuned across two CO2 absorption features, where a transmission measurement is made allowing the CO2 concentration to be retrieved. The fiber optic switch then directs the light to the next probe where this process is repeated, allowing subsurface CO2 concentration measurements at each of the probes to be made as a function of time. The fiber sensor array was deployed for 58 days beginning from June 19, 2012 at the Zero Emission Research Technology field site, where subsurface CO2 concentrations were monitored. Background measurements indicate that the fiber sensor array can monitor background levels as low as 1000 parts per million (ppm). A 34-day subsurface release of 0.15 tones CO2/day began on July 10, 2012. The elevated subsurface CO2 concentration was easily detected by each of the four probes with values ranging over 60,000 ppm, a factor of greater than 6 higher than background measurements.
Organometallic complexes comprising of a platinum (II) acetylide core linked with different -conjugated chromophores are promising materials for applications requiring strong nonlinear-optical response. In dual-mode optical power limiting, the chromophore first undergoes ultrafast two-photon absorption (2PA) in singlet manifold, followed by efficient intersystem crossing (ISC) and subsequent T-T absorption. While the heavy atom facilitates efficient T-T absorption, achieving sufficiently high intrinsic 2PA cross-section value has remained an issue. Here we present a series of linear- and cross-conjugated p-phenylene vinylene platinum (II) acetylides (TPV1-Ph, TP01-TPV2, crossTPV1, crossTPV1-DPAF, crossTPV3) with extended π-conjugated chains and discuss their linear- and nonlinear photophysical properties, including comparison to the properties of the constituting ligand chromophores. Remarkably high femtosecond 2PA cross-section values (up to 10,000 GM) were obtained for several of the new complexes by both nonlinear transmission (NLT) and two-photon excited fluorescence (2PEF) method. The large 2PA values, especially in crossTPV1 and crossTPV3, span a broad range of wavelengths, 570 – 810 nm, which overlaps with maximum wavelength of strong T-T excited state absorption measured by nanosecond transient absorption method. This combination of properties renders these compounds efficient dual-mode nonlinear absorbers in the visible to near-IR region. Examination of the photophysical properties allows us to elucidate the structure-property relationships both in the solutions as well as in solid samples where the chromophores are incorporated into a polymer host.