Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Preface 1. Planck's radiation law and the Einstein coefficients 2. Quantum mechanics of the atom-radiation interaction 3. Classical theory of optical fluctuations and coherence 4. Quantization of the radiation field 5. Single-mode quantum optics 6. Multimode and continuous-mode quantum optics 7. Optical generation, attenuation and amplification 8. Resonance fluorescence and light scattering 9. Nonlinear quantum optics Index

The quantum theory of light.

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Solid Surfaces Interfaces And Thin Films

We report the first observation of two-photon excitation of fluorescence using the evanescent wave from total internal reflectance (TIR). The evanescent wave at 770 nm from a fs Ti:Sapphire laser was used to excite the calcium probe Indo-1 at a quartz–water interface. The emission intensity of Indo-1 depended quadratically on the incident power at 770 nm, when incidence angles were above and below the critical angle (θc) for TIR. The time-resolved intensity and anisotropy decays with TIR at 770 nm demonstrated the origin of the signals as Indo-1 and eliminated the possibility of scattered light contributing to the signal. The emission from Indo-1 was further demonstrated to be due to two-photon excitation by the increased anisotropy observed both from the steady-state and time-resolved data. Comparison of the intensities for one-photon and two-photon evanescent wave excitation revealed a smaller effective excited volume for two-photon excitation, indicating that the excited fluorophores are located closer to the interface with two-photon excitation. These results suggest that total internal reflectance can be combined with two- or multiphoton excitation for studies of surface absorption, immunoassays, or pattern photobleaching.

Two-photon excitation by the evanescent wave from total internal reflection

We report the fabrication of ionic liquid (IL)-gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS2. Our transport measurements indicate that the electron mobility μ ≈ 60 cm(2) V(-1) s(-1) at 250 K in IL-gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from ≈ 100 cm(2) V(-1) s(-1) at 180 K to ≈ 220 cm(2) V(-1) s(-1) at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio >10(7) (10(4)) for electrons (holes), and a near ideal subthreshold swing of ≈ 50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin IL dielectric layer.

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Improved Carrier Mobility in Few-Layer MoS$_{2}$ Field-Effect Transistors with Ionic-Liquid Gating

The investigation of surfaces and thin films is of particular interest in current research as it provides a basis for a multiplicity of applications such as waveguides, sensors, solar cells and optoelectronics. The origins of complex phenomena on surfaces and in thin films can be revealed by applying angle-resolved spectroscopy in two dimensions: the angle of incidence is scanned while analyzing the full emission spectrum in terms of exit angle and emission energy. The interface of a metal layer and a dielectric can support collective electron plasma resonances, i.e. surface plasmon polaritons, which are accompanied by giant field enhancement while propagating along the interface. We characterize the Kretschmann and the Otto coupling configuration in terms of their coupling efficiency and their impact on the surface plasmon resonance as a function of wave-vector. Although being commonly considered as equivalent in terms of plasmonic coupling, we identify differing dependencies of their respective coupling efficiency on the coupling layer thickness and the excitation wavelength which is fundamental for sensing applications. Provided that a metal layer is embedded in a symmetric cladding in terms of its dielectric function and the film thickness is reduced to the order of λ/10, modes from both interfaces can couple and propagate as long-range surface waves. Surprisingly, even intrinsically absorbing films support low-loss surface waves, whose propagation length can become arbitrarily long in the limit of vanishing film thickness. This phenomenon requires only that the material’s dielectric function be predominantly imaginary over that particular range of optical frequencies. Furthermore we show that the orientation of transition dipole moments inside thin monolayer films of effective media that contain oriented CdSe nano-platelets can be determined by applying k-space spectroscopy. Thus we determine electronic and dielectric contributions to the emission anisotropy and reveal the intrinsic nature of the directionality in the emission. We show that this phenomenon is related to the anisotropy of the electronic Bloch states that govern the transition dipole moment of the exciton. Beyond the linear investigation of surfaces and thin films, 2D-k-space spectroscopy can provide an insight into the principles of nonlinear wave-mixing interactions. The role of surface plasmons in second harmonic generation, whether they act as field-enhancing catalysts or as quasiparticles converted in the interaction can be revealed by k-space spectroscopy: by way of the signature in k-space, we identify a nonlinear interaction where two surface plasmons annihilate to create a second-harmonic photon as well as the interaction of a plasmon and a photon by virtue of a degenerate three-wave mixing process. We analyze the intrinsic origin of surface plasmon enhanced second harmonic generation in metal films by comparing the absolute nonlinear yield in attenuated internal reflection configurations to theoretical calculations based on the hydrodynamic model. A first estimation of the nonlinear parameters in the hydrodynamic model is given and the contributions of the bulk and surface source are determined, showing that the in-plane surface source is stronger than predicted. For absorbing thin films however, we report the first evidence of field enhancement and long-range surface wave enhanced second harmonic generation. Here, we identify the out-of-plane surface source to have the strongest contribution to the second harmonic yield.As the nonlinear susceptibility of a material can greatly increase if the probing frequency approaches an absorption resonance, absorbing materials can indeed be considered as low-loss optical media for doing surface-wave optics in the nonlinear regime. We show further, that, in contrast to the isotropic linear absorption, the two-photon absorption in oriented nano-platelets is highly anisotropic. This transition dipole orientation is dependent on the probabilities of the involved processes and their selection rules. We demonstrate that an additional silver layer covered with SiO2 enables surface plasmon enhanced excitation of oriented nano-platelets, boosting the photoluminescent emission which is highly directed through coupling to the plasmonic mode. The combination of TPA and the plasmonic resonance even leads to further concentration of the absorption range as a function of excitation wave-vector. In summary, this work has shown that 2D-k-space spectroscopy –as applied to solid surfaces, thin films and nano-particles– provides insight into the intrinsic material properties, as well as the surfacewave and radiation phenomena supported by these structures.

Jan Heckmann

Papers

Investigations of nonlinear surface-wave phenomena and anisotropic photoluminescence of nano-film structures via two-dimensional k-space spectroscopy