The photophysics of bis(terpyridyl)osmium-(porphinato)zinc-bis(terpyridyl)osmium (OsPZnOs), a D-π-A-π-D symmetric supermolecule, were investigated in the femtosecond and nanosecond regimes. The supermolecule exhibits a two-photon absorption (δpeak ~900 GM) in the near IR (900-1300 nm) and optical pumping by two-photon absorption leads to a broad excited state absorption (σpeak ~1.1 × 10−16 cm2) in the same near IR region. Since the excited state has a long lifetime, OsPZnOs exhibits a strong nanosecond nonlinear absorption in this region. That nonlinear absorption is substantially enhanced when OsPZnOs is incorporated into a multimode waveguide. When two-photon pumping is the dominant mechanism, an additional enhancement of up to ~100 × in the nonlinear absorption is observed in a microchannel waveguide. OsPZnOs is a promising material for photonic applications such as optical noise suppression and optical limiting in the near IR.
© 2011 OSA
Enhancement of nonlinear absorption is of great interest due to the important roles played by nonlinear absorbers in photonic applications such as optical switching, noise suppression and sensor protection [1–10]. Molecular engineering of chromophores has been one of the major pathways that has been exploited to achieve stronger nonlinear absorption [1,9,11–18]. Recently we have shown that nonlinear absorption can also be enhanced via incorporation of nonlinear materials into multimode waveguides [2,19]. We show here that the combination of molecular engineering and material-device integration can lead to significant enhancement of nonlinear performance.
The enhancement achieved by incorporating an appropriate material into a waveguide arises from an increased interaction length and, for appropriate mechanisms, from the increased local intensity arising from partial mode filling within the waveguide core [2,19]. To take best advantage of the waveguide enhancement, it is desirable to have a multiphoton process as the initial excitation step in the mechanism for nonlinear absorption. Among the many known strong nonlinear absorbers, several possess strong two-photon absorption (TPA) in the near IR. If the direct excitation to a two-photon allowed state is followed by intersystem crossing to a long-lived charge-transfer, excited singlet and/or triplet state that absorbs in similar near IR wavelength regions, a strong nonlinear absorption is produced [4,12,13,15,18,20–22]. The simultaneous presence of TPA and excited state absorption (ESA) leads to strong nonlinear optical response in the near IR. This mechanism is advantageous because the linear transmission loss is small at the operating wavelength, a condition that can be crucial for integrating the material into a waveguide. Also, such two-photon accessed excited state absorbers can take advantage of the larger local intensities in waveguides that support only a relatively small number of modes.
In this study, we selected an engineered molecule [1,13,20,23], bis(terpyridyl)osmium-(porphinato)zinc-bis(terpyridyl)osmium (OsPZnOs), a porphyrin-based D-π-A-π-D symmetric supermolecule, as the candidate chromophore. The molecular structure is shown in Fig. 1 . First, an investigation of the photophysics and nonlinear optical properties of this chromophore is reported. Femtosecond nonlinear spectroscopy and transient pump-probe measurements showed that OsPZnOs possesses a strong excited state absorption that spans the near IR spectral region. Wavelength resolved femtosecond Z-scan measurements showed that the two-photon absorption spectrum coincides with the broad excited state absorption that results from the two-photon excitation. Following a discussion of the photophysical properties of OsPZnOs, the nonlinear transmission performance of OsPZnOs on a nanosecond time scale is presented. The nanosecond nonlinear transmission was measured in both free space and waveguide configurations. These studies demonstrate that OsPZnOs has a strong nonlinear absorption in the 900 nm to 1300 nm near IR region that is enhanced in a microchannel waveguide. Further we demonstrate that in the waveguide there is an additional ~100x enhancement in the nonlinear absorption threshold in the wavelength region where two-photon pumping is the dominant mechanism.
2. Bis(terpyridyl)osmium-(porphinato)zinc-bis(terpyridyl)osmium (OsPZnOs)
OsPZnOs is an organic supermolecule that possesses a symmetric donor-π-acceptor-π-donor (D-π-A-π-D)  structure with the bis(terpyridyl)osmium group as the donor and (porphinato)zinc macrocycle as the acceptor, as shown in Fig. 1. By molecular design, the rigid ethynyl π-linkers drive extensive mixing of the (porphinato)zinc-based π-π* and bis(terpyridyl)osmium charge-resonance states, giving rise to a variety of low-energy electronic transitions that feature significant absorption oscillator strengths [12,13,15,20,23], as seen in Fig. 2(a) . As a result, the ground state electronic spectrum of the supermolecule differs markedly from those characteristic of monomeric ethyne-elaborated (porphinato)zinc and bis(terpyridyl)osmium chromophores [21,24,25].
For these D-π-A-π-D symmetric molecules, the ground state electron density is largely localized on individual donor and acceptor moieties, while in the excited state the strong coupling between donor and acceptor gives rise to delocalized electronic structure and extended conjugation length [11,12]. In the excited state, the ethynyl linkage becomes cumulenic, leading to extensive delocalization which results in strong and spectrally broad ESA in the near IR . Furthermore, the electronic structure change between the ground and excited states gives rise to large transition dipole moments that result in strong TPA [11,12]. OsPZnOs has been previously reported to have broad and extraordinary long-lived (> 1 μs) excited state absorptions in the near IR , and its D-π-A-π-D structure leads to strong TPA , while both monomeric (porphinato)zinc and bis(terpyridyl)osmium exhibit rather small nonlinearities as stand-alone units [26,27]. In this study, femtosecond-pulsed open-aperture z-scan [28–30] and transient absorption, a pump-probe technique, were used to determine the dispersion of TPA and ESA of OsPZnOs in the near IR.
OsPZnOs was synthesized as described in a previous publication . For TPA and time-resolved transient absorption measurements, a concentrated solution (21 mM) of OsPZnOs in dimethyl sulfoxide (DMSO) (Sigma-Aldrich, spectroscopic grade) in a cell with path length of 32 μm was prepared. Time-resolved transient absorption measurements were performed in a pump-probe geometry. The probe white light continuum was generated by focusing a portion of the 772 nm output of an amplified Ti:Sapphire laser (Clark-MXR, CPA-2001) onto a sapphire plate. The repetition rate of the laser system was set at 100 Hz. For the transient absorption experiments, a pump wavelength of 695 nm was generated via a noncollinear optical parametric amplifier (Clark-MXR, NOPA) pumped by the 772 nm fundamental pulses. The polarization of the pump beam was set at the magic angle relative to the probe polarization. The pump beam was chopped at 50 Hz to obtain white light continuum spectra with and without the pump. The transient spectra, measured as a change in optical density, were calculated from the collected probe spectra and corrected on a pulse-to-pulse basis. The transient signals were collected via a computer-controlled imaging spectrometer (Roper Scientific, SpectraPro 2300i) and an InGaAs camera (Princeton Instruments, OMA V). The excited state cross-section spectra were extracted from the transient spectra by calculating the excited state density from the measured pulse energy and spot size.
The TPA spectra and cross sections were determined via the femtosecond-pulsed open-aperture z-scan technique [28–30], using the same sample (21 mM concentration with cell path length of 32 μm). The excitation wavelengths, from 850 to 1600 nm, were generated via a NOPA. The output wavelengths from the NOPA were measured with a near IR InGaAs spectrometer (StellarNet Inc., EPP-2000-NIR-InGaAs-1024) and a fiber optic silicon CCD spectrometer (Ocean Optics, USB2000). The excitation beam was spatially filtered and focused on the sample with beam radius of 23-28 μm, Rayleigh range of 1.3-2.8 mm, and beam quality factor (M2) of 1.03-1.15. The pulse width was measured using frequency-resolved optical gating based on second harmonic generation (SHG-FROG) . Z-scan measurements of the response of fused silica, zinc selenide, and gallium arsenide were used as references to calibrate the z-scan results [7,32–36]and agreed with literature values to better that 15%.
The nonlinear transmission of OsPZnOs was measured with nanosecond pulses both for the bulk material and for material incorporated into capillary waveguides. The measurements were made with an OPO laser system (Continuum, Panther EX) delivering 3 ns pulses between 800 and 1600 nm. For measurements between 800 and 1100 nm, a 2-channel energy meter (Laser Precision, RJ7620) was used to measure the signal and reference. Intensity was controlled via a half-wave plate polarizer pair. InGaAs energy probes were used at longer wavelengths (Spectrum Detector, DIJ-3). For the bulk measurement, a sample similar to that described above was used, and the focusing optics were set at f/5. For the measurement of material integrated into waveguides, a 2 cm long fused-silica capillary (n = 1.462) with an inner diameter of 10 μm was filled with a 0.2 mM solution of OsPZnOs in DMSO (n = 1.478). The excitation beam was coupled into the resulting multimode waveguide using a 10 × , 0.25 NA microscope objective and the transmitted light was collected with a 50 × long working range microscope objective.
4. Results and discussion
4.1 Excited-state photophysics of OsPZnOs
The ESA observed from a 21 mM solution of the OsPZnOs in DMSO measured with the excitation of 695 nm light is shown in blue in Fig. 2(a). As can be seen in the figure, the remarkably broad ESA spectrum is red-shifted by almost 400 nm from the ground state absorption and covers the near IR region above 850 nm. The transient spectra show few signs of spectral evolution after the instrument response and remain unchanged at delays ranging from 2 ps to the delay stage limit of 2.5 ns, as indicated in the kinetic measurement shown in Fig. 2(b). The derived ESA spectrum has peaks near 1000 and 1150 nm, with large cross sections exceeding 10−16 cm2 throughout much of the range. This ESA arises from an extensively delocalized triplet state that features substantial charge-separated character [12,13,15,20,23] and has been shown to exhibit long-lived decay kinetics (τ > 900 ns). The ultrafast rise time is shown in Fig. 2(b) to be ~680 fs and is attributed to intersystem crossing. The intensity dependence of the decay kinetics is negligible at excitation energies ranging from 50 to 360 nJ, suggesting that aggregation formation and excited-state quenching are effectively inhibited due to the steric hindrance of perpendicularly oriented bulky bis(alkoxy)phenyl substituents (see Fig. 1). The strong, broad near IR ESA in OsPZnOs appears rapidly, persists for nearly a microsecond, and has relaxation dynamics that are independent of excitation intensity.
4.2 Dispersion of nonlinear absorption of OsPZnOs in the near IR
The dispersion of the nonlinear absorption in OsPZnOs was examined by femtosecond z-scan studies. An example of the open aperture z-scan data at 1050 nm is shown with open circles in Fig. 3(a) . These measurements were repeated at a number of differing input energies at each of the wavelengths examined. Initially, the data were fit as though they arose from TPA alone. A plot of the resulting apparent two-photon absorption cross section as a function of intensity is shown as the red circles and line in Fig. 3(b). That there is an intensity dependence clearly shows that the nonlinear absorption is not due to a simple TPA. Further, Fig. 3(a) shows that the observed nonlinear absorption (black circles) is narrower than the fit of TPA (red curve), suggesting a significant contribution from ESA or higher-order nonlinear process (e.g. three-photon absorption).
Numerical simulations of nonlinear beam propagation [3,4,37] were used to model both the two-photon-accessed excited-state nonlinear process and the three-photon nonlinear absorption in order to better understand the contributing nonlinear process. For the simulations, most of the possible fitting parameters–beam profile, pulse width, concentration, thickness, and linear transmittance–were experimentally determined. The ESA cross sections and lifetimes were derived from the results of femtosecond transient measurements, so that the only variable parameter was the TPA cross-section (δ in GM). As shown in Fig. 3(a), the two-photon accessed excited state nonlinear process (blue line) gives a close fit to the z-scan data and a better correlation coefficient, while the three-photon absorption fit results in a narrower curve (green line) and a poorer correlation coefficient. The blue symbols and line in Fig. 3(b) show the TPA plus ESA model yields TPA cross sections that are satisfyingly independent of intensity.
This model was applied to all of the z-scan data with different excitation intensities in 900-1300 nm region. The spectral variations of the extracted TPA cross-sections are shown in Figs. 3(c) and 3(d). The two-photon accessed ESA model is a better fit to all z-scan curves within the 900-1300 nm spectral region and yields consistent extracted values of TPA cross-section with varying excitation intensities. The results of the simulations support the hypothesis that two-photon accessed excited state absorption is the major route for nonlinear absorption of OsPZnOs in the near IR.
The wavelength dependences of the extracted TPA cross sections and comparisons to the ground and excited state spectra are shown in Fig. 3(c) and 3(d). Figure 3(c) shows an overlay of the TPA spectrum plotted at half the wavelength with that of the ground state to demonstrate the energetic location of the two-photon states relative to the one-photon states. The TPA spectrum has a local peak near 1100 nm which coincides with the linear metal-to-ligand-charge-transfer(MLCT) absorption of bis(terpyridyl)osmium near 550 nm, suggesting that resonant enhancement from the charge transfer state may contribute significantly to the strong TPA . While the increase in TPA cross section as the experimental wavelength approaches 900 nm is probably due to the near-resonance with the tail of the one photon absorption, there may also be resonant contributions from higher vibronic members of the B-band. By contrast, the TPA spectrum is nearly flat in the spectral region associated with the porphyrin Q-band; the enhancement due to resonance with transitions that feature extensive Q-band character is quite small.
Figure 3(d) shows the ground state, excited state and two-photon absorption spectra plotted versus experimental wavelength. The TPA cross-section at 1100 nm is δ ~900 GM. This value is more than an order of magnitude larger than those reported for monomeric porphyrins . The large TPA cross-section is partially expected since the overall electronic structure of the OsPZnOs supermolecule is that of a symmetric D-π-A-π-D molecule, known to give rise to large TPA coefficients . The experimental results demonstrated in Fig. 3 show that the TPA and the ESA spectra are well overlapped. To demonstrate the potential photonic utility of the overlap and magnitude of the nonlinear coefficients, nanosecond nonlinear transmission measurements were performed.
4.3. Free-space and waveguide nanosecond nonlinear transmission of OsPZnOs
Butler et al. demonstrated that the threshold for nonlinear transmission in microcapillary waveguides is substantially lower than the threshold in free-space propagation . The enhancement of nonlinear optical response was attributed partly to the longer effective interaction path length. They also reported that in small-core multimode waveguides the light does not uniformly fill the waveguide core.
Rosenberg et al. later discussed the non-uniform intensity distribution of light within nonlinear multimode waveguides with cores designed to support a limited number of modes . In such a waveguide, the incident light couples to only a few modes, resulting in a nonuniform intensity distribution of light in the core. Therefore, the effective cross-sectional area of the propagating beam is smaller than the geometrical area of the waveguide core, resulting in locally higher intensities . This phenomenon is expected to enhance intensity dependent processes, such as two-photon absorption. A more efficient TPA response can increase the excited state pumping rate and, in turn, significantly reduce the threshold at which nonlinear absorption is observed. The configuration of these waveguides differs from those considered in other waveguide studies [38,39] where larger waveguides having uniform light distribution were investigated.
This waveguide mechanism for enhancing nonlinear absorption in OsPZnOs was explored by carrying out nanosecond nonlinear transmission studies of solutions using both bulk samples with free-space excitation geometries and material incorporated into capillary waveguides with 10 µm cores using the experimental arrangement described earlier [2,19]. The results of both bulk measurements (21 mM concentration with cell path length of 32 μm) and waveguide measurements (0.2 mM concentration incorporated into 2 cm long capillaries with an inner diameter of 10 μm) are shown in Fig. 4 .
Two features of the data in Fig. 4 are immediately apparent: First, at identical input fluences the waveguide yields larger changes in absorption than does the free-space configuration. Second, the enhancement in the waveguide over free-space depends on the wavelength; this is particularly striking when the changes in absorbance at 950 nm are compared to those at 1050 nm in Fig. 4(a) and 4(b). The first observation implies that there is an overall enhancement of the nonlinear response due to the longer interaction length in waveguides . The additional enhancement near 1050 nm in the waveguide can be attributed to the contribution of TPA to the optical pumping mechanism. The higher local intensities in the waveguide core enhance optical pumping by the intensity dependent TPA process.
In order to illustrate the magnitude of the waveguide enhancement, we compare the threshold for absorption observed in the two experiments. To do this, a threshold, F, is defined as the fluence at which the absorbance change reaches 0.2. This corresponds to the fluence at which the transmission is 63% of its initial value. Figure 5 shows a plot of the reciprocal of the measured thresholds on a log scale overlaid by the two-photon cross-section. 1/F is used in order to easily compare the wavelength dependence of the thresholds with that of the two-photon cross-section which is the red curve in Fig. 5.
The waveguide advantage is apparent from the data shown in Fig. 5. Despite the sample concentration being two orders of magnitude lower, 1/F is larger in the waveguide at all wavelengths measured. This means that the thresholds for nonlinear absorption are better in the waveguide than in free-space. Figure 5 also shows that the magnitude of the waveguide advantage increases near the local peak of the two-photon absorption. At 1100 nm, 1/F is nearly 100 × better in the waveguide than in free-space. The reciprocal threshold increases rapidly over the wavelength range where the TPA cross-section and the excited state absorption reach their peaks. This is the additional enhancement due to the increased local intensity on a nonlinear pumping step that was predicted by Rosenberg et al. .
The incorporation of this strongly two-photon absorbing supermolecule into a multimode waveguide results in two orders of magnitude better thresholds than in free space. The increase arises from two sources. First, the waveguide gives a much longer path length and second, in a region where the optical pumping is nonlinear, the pumping is more efficient.
The primary limit to the waveguide is the linear absorption. For this study, the OsPZnOs concentration was chosen to permit characterizing the nonlinear absorption over a wide wavelength range. The concentration of the supermolecule can easily be increased before the linear absorption becomes prohibitive. Experiments to verify that higher OsPZnOs concentration can provide even lower nonlinear absorption thresholds are underway. This will have important implications for noise suppression and optical limiting applications near the peak of the response.
In summary, femtosecond transient absorption and z-scan studies of the D-π-A-π-D symmetric supermolecule, bis(terpyridyl)osmium-(porphinato)zinc-bis(terpyridyl)osmium (OsPZnOs) showed that OsPZnOs exhibits both a two-photon absorption and a broad excited state absorption with sizable cross sections in the 900 to 1200 nm spectral region. The excited state is readily accessed by pumping into the two-photon absorption band. Therefore OsPZnOs also exhibits a strong nonlinear absorption in the near IR region where the mechanism is a two-photon initiated population of the excited state with a strong absorption. The nonlinear absorption was experimentally verified in a standard nanosecond nonlinear absorption experiment. Earlier work had suggested that the nanosecond nonlinear response of a chromophore using this mechanism could be substantially enhanced in small core multimode waveguides. This was confirmed. The threshold for nonlinear absorption was smaller in a ~10 μm diameter multimode waveguide than in a free-space experiment. Near 1100 nm, where both the two-photon and excited state cross-sections are peaked, the nonlinear absorption threshold improved by an additional ~100 × over the free-space configuration.
We acknowledge the United States Naval Research Laboratory, the Office of Naval Research, and the United States Naval Academy for providing funding for this work. M.J.T. acknowledges support from the National Science Foundation MRSEC Program Grant DMR0520020. S.-H. Chi thanks the National Research Council for support through the Research Associateship Program.
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