Scientists have sought for over two decades to incorporate the necessary attributes of transparency, stability, and high nonlinear susceptibilities into optimized organic or organometallic chromophores for third-order nonlinear optical (NLO) applications. These investigations have provided an ever-expanding understanding of structure−function relationships for the second hyperpolarizability γ and the bulk third-order nonlinear optical susceptibility χ(3) in organic materials, which are reviewed herein. Contributing to this understanding are the studies of the third-order NLO properties displayed by an array of structurally related organic chromophores based on the conjugated carbon skeletons of hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs). A comprehensive series of donor (D) and/or acceptor (A) substituted derivatives of DEEs and TEEs has been measured by third-harmonic generation (THG) experiments, and the investigations on these one-...

Structure−Property Relationships in Third-Order Nonlinear Optical Chromophores

Two-photon absorption, the process by which an excited molecule or material is produced by the simultaneousabsorption of two photons, has been studied extensively in recent years, from both fundamental and applicationpoints of view. On one side, the field has been expanded with the measurement of two-photon absorptionspectra and cross sections for a wide range of conjugated molecules. In this contribution, we willreview the two-photon properties of some of these classes of materials, and we will discuss structure/propertyrelationships that have been developed from these investigations. On the other side, two-photon absorptionhas been exploited as a means to activate a variety of chemical and physical processes with sub-diffraction–limitedresolution, because three-dimensional spatial confinement of the excitation volume in a material irradiatedby a tightly focused laser beam can be achieved via two-photon or, more generally, multi-photon absorptionprocesses. This characteristic has led to applications in a number of technological fields, such asmicrofabrication and laser scanning fluorescence microscopy. Here, we will survey material systems thathave been developed to activate radical or cationic polymerization reactions, deprotection of functionalgroups, and singlet oxygen generation via two-photon excitation of one of the components in the system.

Two-Photon Absorbing Materials and Two-Photon-Induced Chemistry

Organic materials with large third-order nonlinearities in the near-infrared spectral regime are critical in the development of photonic devices to be utilized in all-optical signal processing. We have developed polymethine materials, specifically bisdioxaborine-terminated polymethine dyes, which possess large ultrafast third-order nonlinearities and low nonlinear loss all in the near-infrared spectral regime. An extended bisdioxaborine polymethine anion exhibited the largest value of γ (third-order microscopic nonlinearity) at 1.3 μm (|γ| = 5.7 × 10-32 esu) and showed no characteristics of symmetry breaking, unlike other polymethines of similar lengths. A neat film of this molecule maintained relatively low linear loss in the near-infrared and showed a large third-order macroscopic nonlinearity at 1.3 μm (|χ(3)| = 3.6 × 10-10 esu), with a temporal response of less than 8 ps. Furthermore, the real part of χ(3) was nearly an order-of-magnitude larger than the imaginary component. Consequently, this materia...

Bisdioxaborine Polymethines with Large Third-Order Nonlinearities for All-Optical Signal Processing

Organic materials possess many key attributes that make them suitable for exploitation in all-optical signal processing applications including facile tunability of their optical properties, strong and ultrafast nonlinear optical response, and potential for integration into device structures. In this perspective, we present molecular design guidelines for organic chromophores that could serve as the active constituents for such materials. Using a relatively simple model, a candidate class of chromophores, namely cyanine-like polymethines, is identified based on promising microscopic nonlinear optical properties in the near-IR spectral region. The challenges associated with translating these microscopic properties into materials with macroscopic properties suitable for device applications are presented along with molecular engineering approaches for overcoming these hurdles.

Design of Organic Chromophores for All-Optical Signal Processing Applications

All-optical switching—controlling light with light—has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π -conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a signifi cant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π -conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.

25th Anniversary Article: Design of Polymethine Dyes for All‐Optical Switching Applications: Guidance from Theoretical and Computational Studies

Third-order nonlinear polarizabilities of a homologous series of symmetric cyanines

The dispersion of the third-order susceptibility of the cyanine dye bis-(dimethylamino)-heptamethinium chloride was measured by Coherent Anti-Stokes Raman Scattering (CARS) over a wide wavelength range from 530 to 830 nm. Large negative values of the real part of the second-order hyperpolarizability are observed. The data are analyzed with the help of theoretical calculations based on a perturbative approach for the nonlinearities. The dispersion behavior of the third-order susceptibility is governed by the resonant enhancement due to vibrational transitions of the first excited electronic state and, to lesser extent, by an electronic two-photon resonance around 600 nm.

L. Dähne

Papers

Third-order nonlinear polarizabilities of a homologous series of symmetric cyanines

Dispersion of the third-order susceptibility of a cyanine dye measured by coherent anti-Stokes Raman scattering