Nonlinear Light-matter Interactions Research Articles

Fringe-shifting interferometric laser lithography with optical nonlinearity for micro- and nanofabrications

The authors demonstrate an interferometric laser lithography process armed with a fringe-shifting technique, which utilizes nonlinear light-matter interaction. With this method, multiphoton-sensitive materials are ablated using ultrashort pulse interference fringes, enabling the ablated regions to be spaced by a fraction of the fringe pitch. For concept-proving experiments, confocal optics equipped with a diffractive beam splitter and a phase shifter are built to perform a fringe shift. This creates a phase difference between split beams that interfere with each other, producing high-contrast fringes. Precise control of beam intensity and beam phase is essential.

All-optical modulation by plasmonic excitation of CdSe quantum dots

Photonics is a promising candidate technology for information processing, communication and data storage. Essential building blocks, such as logic elements and modulators, have been demonstrated. However, because of weak nonlinear light–matter interactions, these components typically require high power densities and large interaction volumes, limiting their application in dense chip-based integration. A solution may be found in surface plasmon polaritons (SPPs), guided electromagnetic waves that propagate with high field confinement along a metal–dielectric interface. We demonstrate an all-optical modulator in which efficient interaction between two light beams at different wavelengths is achieved by converting them into co-propagating SPPs interacting by means of a thin layer of CdSe quantum dots (QDs). The high SPP field confinement and high QD-absorption cross-section enable optical modulation at low power densities (~10^2 W cm^(-2)) in micrometre-scale planar devices.

Plasmons lend a helping hand

Optical modulators typically rely on weak nonlinear light–matter interactions to modulate light with light. But using surface plasmons to excite quantum dots, researchers at the California Institute of Technology have now demonstrated an efficient approach to chip-based all-optical modulation.

Imaging cells and extracellular matrix in vivo by using second-harmonic generation and two-photon excited fluorescence.

Multiphoton microscopy relies on nonlinear light-matter interactions to provide contrast and optical sectioning capability for high-resolution imaging. Most multiphoton microscopy studies in biological systems have relied on two-photon excited fluorescence (TPEF) to produce images. With increasing applications of multiphoton microscopy to thick-tissue "intravital" imaging, second-harmonic generation (SHG) from structural proteins has emerged as a potentially important new contrast mechanism. However, SHG is typically detected in transmission mode, thus limiting TPEF/SHG coregistration and its practical utility for in vivo thick-tissue applications. In this study, we use a broad range of excitation wavelengths (730-880 nm) to demonstrate that TPEF/SHG coregistration can easily be achieved in unstained tissues by using a simple backscattering geometry. The combined TPEF/SHG technique was applied to imaging a three-dimensional organotypic tissue model (RAFT). The structural and molecular origin of the image-forming signal from the various tissue constituents was determined by simultaneous spectroscopic measurements and confirming immunofluorescence staining. Our results show that at shorter excitation wavelengths (<800 nm), the signal emitted from the extracellular matrix (ECM) is a combination of SHG and TPEF from collagen, whereas at longer excitation wavelengths the ECM signal is exclusively due to SHG. Endogenous cellular signals are consistent with TPEF spectra of cofactors NAD(P)H and FAD at all excitation wavelengths. The reflected SHG intensity follows a quadratic dependence on the excitation power, decays exponentially with depth, and exhibits a spectral dependence in accordance with previous theoretical studies. The use of SHG and TPEF in combination provides complementary information that allows noninvasive, spatially localized in vivo characterization of cell-ECM interactions in unstained thick tissues.

Laue soliton in a resonantly absorbing photonic crystal

A theory has been developed for coherent nonlinear light–matter interaction under condition of Bragg diffraction in a multidimensional resonantly absorbing photonic crystal. At the Laue scheme of diffraction in a two-wave approximation, a novel kind of the nonlinear solitary wave, the so-called Laue soliton of self-induced transparency, has been obtained as an analytical and numerical solution of corresponding Maxwell–Bloch equations.

Transform-Limited Pulses Are Not Optimal for Resonant Multiphoton Transitions.

Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.

Second-order nonlinear optical signatures of surface chirality

Second-harmonic generation can be used to probe chiral properties of surfaces and thin films with in-plane isotropy. We present a general formalism to analyse such chiral effects and apply it to review known effects and to introduce new effects. The formalism is based on expanding the fundamental and second-harmonic fields in terms of their p- and s-polarized components. Each second-harmonic signal can then be described in terms of only three nonlinear coefficients, which are associated with the quadratic combinations of the fundamental-field components. The coefficients can be classified as achiral (allowed for all isotropic surfaces) or chiral (allowed only for chiral surfaces) independent of the details of the nonlinear light-matter interaction. The basic signatures of chirality are intensity-difference effects in which the efficiencies of second-harmonic generation are different for left- and right-hand circularly-polarized light or two orthogonal linear polarizations, for example. These effects depend on proper phase relations between the achiral and chiral coefficients. Measurements in which the state of polarization of the fundamental beam is continuously varied by a rotating quarter-wave plate are shown to be sensitive to chirality independent of any particular phase relation between the coefficients. Such measurements can also be used to determine uniquely the relative complex values of the coefficients and thus to characterize completely the nonlinear chiral response. The ability of the techniques to distinguish absolutely between the enantiomers of a chiral sample is possible only if interference between the electric-dipole and higher-multipole parts of the coefficients dominates the chiral response.

Multipolar molecules and multipolar fields: probing and controlling the tensorial nature of nonlinear molecular media

A general class of multipolar molecules is introduced in the context of quadratic nonlinear optics by way of extension of the more specific cases of dipolar and octupolar molecules. An adequate irreducible tensor formalism permits us to define rotationally invariant molecular features that couple to corresponding field tensor components, thereby enabling us to account for a variety of coherent and noncoherent processes such as harmonic light (hyper-Rayleigh) scattering, coherent second-harmonic generation in electrically poled media, and the recently proposed optical poling scheme. Experiments in both harmonic light scattering in solution (for some multipolar molecules) and optical poling (in Disperse Red 1–methyl methacrylate thin films) are analyzed in light of this model. A general tensorial permutation lemma of broad validity allows nonlinear light–matter interactions to be condensed in a statistical medium in compact rotationally invariant expressions: The main tensorial symmetry features for both molecular susceptibility and read–write field polarization tensors that jointly drive these interactions are clearly revealed.

Quantitative comparison of excited state properties and intensity-dependent photosensitization by rose bengal

A quantitative (multistep) excitation-deactivation model of rose bengal (RB) has been developed which includes energy and electron transfer to oxygen and the possibility of photoproduct formation via higher excited triplet-singlet states. The model is based on previous measurements of non-linear absorption (NLA) and emission of RB with picosecond pulses at 532 nm and on NLA measurements with nanosecond pulses. A coupled rate equation and photon transport equation approach for non-linear light-matter interaction is used. The resulting term scheme with all relevant excited state parameters confirms that (i) in the first excited state of RB, relevant absorption at 532 nm takes place only in the triplet, and (ii) the previously reported intensity dependence of RB-sensitized enzyme inhibition is well modelled by the intensity-dependent RB-T 1 population and (as the main process) subsequent energy transfer to form singlet oxygen.

Instantaneous fluorescence quantum yield of organic molecular systems: information content of its intensity dependence

In pulse laser investigations of systems of fluorescent organic molecules (e.g. aggregates, light harvesting antennas) it is usual to derive from the intensity dependence of time-integrated fluorescence (fluorescence yield) conclusions on deactivation channels, bimolecular interaction and sizes of interacting units (“domains”), though the content of information of this type of measurement is restricted because the shape of this function is hardly structured. Especially when further processes take place (e.g. excited singlet absorption) problems of ambiguity arise. Based on analytical investigations of the adapted rate equation system of noncoherent nonlinear light-matter interaction, especially of the asymptotic behaviours of its solution with respect to the excitation intensity, we propose a new measuring quantity “instantaneous fluorescence quantum yield”, which in its intensity dependence reflects far more sensibly the several excited state processes and therefore should allow the characterization of absorption, relaxation, and annihilation quantitatively and in a unique way.

Nonlinear Behavior of Circularly Polarized Laser Beams Propagating through Sodium Vapor

Abstract A variety of surprising effects arise from the nonlinear light-matter interaction of circularly polarized laser light propagating through sodium vapor. We present experimental evidence for an asymmetry in the absorption of left hand and right hand circularly polarized light as well as for the creation of a collimated light beam of apposite polarization within the light-matter interaction region. Both effects are not yet explained by common theories.

Fluid formulation of high intensity laser beam propagation using lagrangian coordinates

The complete mathematical modeling of nonlinear light-matter interaction is presented in a hydrodynamic context. The field intensity and the phase gradient are the dependent variables of interest. The resulting governing equations are a generalization of the Navier-Stokes equations. This fluid formulation allows the insights and the methodologies which have been gained in solving hydrodynamics problems to be extended to nonlinear optics problems. To insure effective numerical treatment of the anticipated nonlinear self-lensing phenomena, a self-adjusted nonuniform redistribution, along the direction of propagation, of the computation points according to the actual local requirements of the physics must be used. As an alternative to the application of adaptive rezoning techniques in conjunction with Eulerian coordinates, Lagrangian variables are used to provide automatically the desired nonlinear mapping from the physical plane into the mathematical frame. In this paper we propose a method suitable for the solution of the described problemin one-dimensional cases as well as in two- dimensional cases with cylindrical symmetry. To overcome the numerical difficulties related to the inversion of the Jacobian, an analytical algorithm based on the paraxial approximation was developed.