Enhanced Femtosecond Nonlinearities and Multiphoton Absorptions in Discrete Bands of Porphyrins

Abstract

Optical nonlinearities of discrete absorption energy levels of one of the typical heterocyclic aromatic molecules, free-base porphyrins, have been probed over a broad spectral region (400−1600 nm) utilizing intense femtosecond pulses. A wide range of strong one-and multiphoton-induced nonlinear absorptions of both the blue-end Soret (B) band (a u → b 1g ) and red-end orbital mixing split quasi-allowed Q-bands (Q x (0,0; 0,1), Q y (0,0; 0,1), a u → e g ) are critically probed and reported. During the resonant excitation within B-(400 nm) and Q-bands (600−750 nm), the nonlinear absorption has become predominant by the saturation of absorption (SA) of the onephoton absorption (1PA) process due to ground-state bleaching. At nonresonant wavelengths, it is dominated by the reverse saturation of absorption (RSA), involving various nonlinear processes of two-, three-, and four-photon (2PA, 3PA, and 4PA) absorptions, either to B-or Q-bands (1100−1600 nm). The laser intensity-dependent nonresonant (2PA, 800 nm) excitations for the prominent B-band show a distinct cross-over from SA to RSA, contributed by the excited-state absorption (ESA) utilizing a three-photon induced (3PA) process, whereas resonant (1PA, 400 nm) excitation reveals a systematic strong SA process. Both wavelength-and intensity-dependent nonlinear refractive index studies exhibit positive electronic Kerr-based self-focusing effects, with prominent contributions of nonlinear absorption and higher-order effects. The spectrally discrete, highly intense laser probing of individual energy bands and the consequent variety of nonlinearities can be broadly generalized for many free-base porphyrins and metalloporphyrins. The present studies provide a strong foundation and new insight into the broad categories of macrocycles, such as porphyrins and phthalocyanines, for myriad applications in nonlinear optics and bio/optophotonics.

Ultrafast Dynamics, Optical Nonlinearities, and Chemical Sensing Application of Free-Standing Porous Silicon-Based Optical Microcavities

“…The involvement of higher-order nonlinear absorption (such as three-photon absorption) with the increasing excitation intensity leads to the decrease in the two-photon absorption, which is reflected in Figure 5d. 30,37 Some of the OA Z-scan data for the FS-PSi-based optical microcavity with cavity mode at 540 and 570 nm have been shown in the Supporting Information (Figure S16). Among all three samples, the highest value of β is 3.9 × 10 −10 mW −1 at an intensity of 1.5 × 10 15 W m −2 for the sample λ cav = 570 nm (Supporting Information Table S2).…”

Third-order nonlinear optical properties of highly electron deficient, nonplanar push–pull porphyrins: β-nitro-hexa-substituted porphyrins bearing bromo, phenyl, and phenylethynyl groups

“…Furthermore, porphyrin molecules offer architectural flexibility, π-conjugation, and an inner cavity to bind different metal ions, meso and β-positions for peripheral substitution, polarizability, and thermal and optical stability, which makes them exhibit tunable NLO properties such as reverse saturable absorption (RSA), saturable absorption (SA), and an intensity-dependent refractive index. [12][13][14][15] Third-order NLO materials have gained significant attention because of their potential applications in optical limiting, twophoton photodynamic therapy, 3-D optical memory, and aberration-corrected imaging. 16 If the excited state has a larger cross-section (σ ex ) than the ground state (σ g ) i.e.…”

“…Furthermore, porphyrin molecules offer architectural flexibility, π-conjugation, and an inner cavity to bind different metal ions, meso and β-positions for peripheral substitution, polarizability, and thermal and optical stability, which makes them exhibit tunable NLO properties such as reverse saturable absorption (RSA), saturable absorption (SA), and an intensity-dependent refractive index. 12–15…”

“…Usually, a decrease in transmission with an increase in light intensity is observed for materials exhibiting RSA. 14 The effective RSA region in the case of porphyrins lies between the Q and Soret absorption bands, where the ground state absorption is low. The RSA region can be extended to a longer wavelength by increasing the π-conjugation.…”