Ab InitioSimulation of Optical Limiting: The Case of Metal-Free Phthalocyanine

Abstract

We present a fully ab initio, non-perturbative description of the optical limiting properties of a metal-free phthalocyanine, by simulating the effects of a broadband electric field of increasing intensity. The results confirm reverse saturable absorption as leading mechanism for optical limiting phenomena in this system and reveal that a number of dipole-forbidden excitations are populated by excited-state absorption, at more intense external fields. The excellent agreement with the experimental data supports our approach as a powerful tool to predict optical limiting, in view of applications.PACS numbers: 82.37. Vb, 71.15.Mb, 78.40.Me Optical limiting (OL), a strong attenuation of the light transmittance through a material at increasing input intensity, is a prototype nonlinear phenomenon of relevance for an entire class of devices [1]. It has received rising attention in view of the need to protect light-sensitive elements, including the human eye, from optical damage e.g. due to novel intense and tunable light sources [2]. Organic compounds are good candidates as OL materials [3,4], and among them macrocyclic molecules are particularly effective, due to their large and delocalized π-electron network [5]. Phthalocyanines and porphyrins present several advantages in this field [6,7]: in addition to their excellent chemical processability and versatility, they are characterized by a complex spectrum in the UV -visible range, which can be tuned by chemical modifications, either by decorating the organic network or by inserting a metal atom [8][9][10]. A number of experimental studies in the last decades provided a general picture of the main physical mechanisms driving optical limiting in these molecules [11][12][13]. Previous theoretical studies (see e.g. Ref.[14]), however, are mostly limited to a phenomenological level of description, and are therefore unable to clarify the microscopic origin of OL phenomena and to predict the behavior of different systems.Within the framework of real-time time-dependent density functional theory (TDDFT), we perform a fully ab initio investigation of the OL properties of a metalfree phthalocyanine. The calculated optical spectra are the key ingredient to determine the OL curve in the frequency region of interest, upon increasing irradiance. A large excited-state absorption around the green visible region emerges as the driving mechanism for reverse saturable absorption (RSA) and hence for OL effects in these systems. Our results reproduce the nonlinear behavior of the intensity-dependent absorption coefficient, which is a fingerprint of optical limiting, and confirms the validity of our approach to characterize the response of macromolecules to strong electromagnetic fields. The insight 2 ), in agreement with the experimental spectrum (dashed line, from Ref. [7]). The visible (∼ 2 eV, Qband) and the near UV region (∼ 3-4 eV, B-band) are separated by a large absorption valley (2.3 -2.9 eV, OL).into the microscopic origin of these phenomena, provided by ab initio calculation...

Understanding real-time time-dependent density-functional theory simulations of ultrafast laser-induced dynamics in organic molecules

“…If κ is sufficiently small, only the linear response is obtained, while stronger perturbations excite in higher order. 3 In order to calculate the response of the material to an ultrafast laser pulse, we consider electric fields of the form…”

“…1 Being intrinsically non-perturbative, this approach can access on the same footing both the linear and the nonlinear regime of excitations. 2,3 Recent developments of RT-TDDFT have added the possibility to explicitly include a time-dependent electric field, [4][5][6][7][8][9][10][11][12] paving the way for RT-TDDFT to simulate pump-probe experiments and thus to investigate quantitatively the dynamics of charge carriers on their natural femtosecond timescale. Merging of RT-TDDFT with the Ehrenfest molecular dynamics scheme [13][14][15][16] additionally allows us to describe the coupled electron-nuclear dynamics in the sub-picosecond time window, adopting a mixed quantum-classical treatment.…”

Facile one-step fabrication of upconversion fluorescence carbon quantum dots anchored on graphene with enhanced nonlinear optical responses

“…Materials with this property can be useful to fabricate devices for pulse shaping, 1 passive mode locking, 2 and eye protection against powerful lasers. 3 Extensive research has been performed on optical limiters in the past 20 years, and the strong OL properties of a wide range of materials have been intensively studied, from organic dyes (phthalocyanine, porphyrin, fullerene), [4][5][6][7][8][9] carbon nanomaterials (carbon black, carbon nanotubes, graphene) [10][11][12][13][14][15] to noble metal nanoparticles (NPs) [16][17][18] and quantum dots. [19][20][21] Typically, the occurrence of OL effects in materials arises from three main classes of mechanisms: nonlinear absorption (NLA), nonlinear scattering (NLS), and nonlinear refraction (NLR).…”

Laser-Induced Electronic and Vibronic Dynamics in the Pyrene Molecule and its Cation

“…The linear absorption spectrum is proportional to the imaginary component of the Fourier transform of d(t), if a sufficiently small value of κ is set. 70 Large values of κ are known to lead to optical nonlinearities, 71,72 but this aspect will not be considered in the present work.…”