The nature and properties of surface-enhanced vibrational spectra of molecules adsorbed on surfaces that can enhance the absorption and the emission of electromagnetic radiation are discussed. Examples of surface-enhanced resonant Raman scattering, surface-enhanced Raman scattering in the near-infrared and surface-enhanced infrared are given. The rough surfaces used are metal island films coated with a nanometric organic film using the Langmuir-Blodgett technique or deposited by vacuum evaporation.

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Surface enhanced vibrational spectroscopy

The current status of organic low-molecular weight and polymeric materials for third-order nonlinear optics is reviewed. The importance of organic materials lies in their promise of large nonlinear optical figure of merit, high optical damage thresholds, ultrafast optical responses, architectural flexibility, and ease of fabrication. Organic materials exhibiting interesting third-order nonlinear optical properties are discussed to illustrate the importance of structure–property correlations. Results on emerging organic materials that include liquids, dyes, fullerenes, charge-transfer complexes, π-conjugated polymers, dye-grafted polymers, organometallic compounds, composites, and liquid crystals are presented. Organic nonlinear optical materials seem promising for a wide range of applications and their potential for integrated optics should be further explored.

Organic Materials for Third‐Order Nonlinear Optics

This paper describes the electronic energy levels of semiconductor heterostructures within the envelope function scheme. Quantum well and superlattice electronic states are calculated and discussed, especially the in-plane dispersion relations. The Coulombic bound states in heterostructures (impurities, excitons) are then discussed. Finally, we present a brief overview of the effect of a static electric field on the carrier and exciton energy levels in semiconductor quantum wells.

Electronic states in semiconductor heterostructures

It is now possible to grow high-quality strained-layer superlattices, in which individual layers are composed of semiconductor materials which would normally have significantly different lattice constants. Strained structures open new possibilities for band-structure engineering and applications, but place even higher demands on crystal growth techniques. The authors first describe some of the growth that has been achieved and the theoretical and experimental limits on good quality growth. They then discuss in detail the electronic structure of strained quantum wells, and in particular the possibility of valence band engineering in strained layers. Axial strain splits the degeneracy of the light- and heavy-hole zone centre states, accessing a range of subband structures, including the possibility of the highest band being light-hole-like, and of holes with electron-like zone centre masses. The authors next describe the experimental evidence confirming such subband structures. This strain-based band engineering suggests new device applications, including high hole mobility transistors and low threshold current, long-wavelength lasers. The authors review what has been achieved in respect of the main proposed device applications to date. Strained layers are also of interest for the new materials combinations which they allow. They briefly overview some of the materials-based advantages, and conclude that strained layers will continue to attract significant attention.

https://iopscience.iop.org/article/10.1088/0268-1242/4/3/001/pdf

Valence band engineering in strained-layer structures

We provide an in-depth treatment of the various mechanisms by which an incident light beam can produce an intensity- or flux-dependent change in the refractive index and absorption coefficient of different materials. Whenever possible, the mechanisms are initially traced to single-atom and -molecule effects in order to provide physical understanding. Representative values are given for the various mechanisms. Nine different mechanisms are discussed, starting with the Kerr effect due to atoms and/or molecules with discrete states, including organic materials such as molecules and conjugated polymers. Simplified two and/or three-level models provide useful information, and these are summarized. The nonlinear optics of semiconductors is reviewed for both bulk and quantum-confined semiconductors, focusing on the most common types II–VI and III–V. Also discussed in some detail are the different nonlinear mechanisms that occur in liquid crystals and photorefractive media. Additional nonlinear material systems and mechanisms such as glasses, molecular reorientation of single molecules, the electrostrictive effect, the nuclear effect (vibrational contributions), cascading, and the ever-present thermal effects are quantified, and representative tables of values are given.

Nonlinear refraction and absorption: mechanisms and magnitudes

The temporal response of the third‐order nonlinear optical susceptibility in poly bis( p‐toluene sulfonate) of 2,4‐hexadiyne‐1,6 diol was determined by time‐resolved degenerate four‐wave mixing using tunable dye laser pulses with 300 fs duration. In the material’s absorption region these experiments were combined with saturated absorption techniques yielding a measurement for the excited state lifetime of 1.8 ps. Off resonance the nonlinear response was determined to be optical pulse width limited.

Nonlinear optical processes in a polydiacetylene measured with femtosecond duration laser pulses

Because of the large variety of materials and material forms available, organic materials are attractive candidates for ultrafast nonlinear optical signal processing systems based on the intensity-dependent index of refraction, i.e., the change of index of refraction with a change in applied optical intensity. As an example, we consider an organic polymeric system, the polydi-acetylenes. Through molecular engineering (organic synthesis and Langmuir-Blodgett film growth combined with monolayer deposition techniques), the polydiacetylene was prepared in the technologically important format of a planar waveguide on a grating. By measuring the change in the angle that the incident laser radiation couples into the guided-wave mode (via the grating) with the change in intensity of the laser radiation, we determined the intensity-dependent index of refraction to be 10-6 (MW/cm2)-1 in the transparent region of the material. Implications of this reseach for future work related to signal processing applications are discussed.

Intensity Dependent Index Of Refraction In Organic Materials

We report the first observation of the effect of two-dimensional (2D) confinement on the uniaxial stress dependence of excitons in GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells. For uniaxial stress X\ensuremath{\parallel}[100], perpendicular to the quantization axis (z^\ensuremath{\parallel}[001]) of the exciton Hamiltonian without external stress, the exciton energy consists of two components. The first, containing hydrostatic strain terms, is the same for bulk and 2D excitons, in agreement, to within experimental error, with hydrostatic photoluminescence data. The second component contains shear strain terms and is observed to be dependent on quantum-well width. This dependence is interpreted in terms of a model based on the overlap of the light-hole (LH) and heavy-hole (HH) wave functions and on the energy differences between the LH and HH excitons.

Uniaxial stress dependence of spatially confined excitons

Study of Langmuir-Blodgett polydiacetylene polymer films by surface enhanced Raman scattering

Photoluminescence excitation (PLE) and temperature‐dependent photoluminescence (PL) spectroscopies have been employed to interpret fine structure observed in the low‐temperature PL spectra of GaAs/AlGaAs single quantum wells grown by molecular beam epitaxy under conditions of interrupted growth. Multiple peaks observed in the low‐temperature PL spectra of similar samples grown by others have been held to be proof of monolayer steps in exceptionally smooth interfaces. We show that similar structure in the low‐temperature PL spectra of our samples is not intrinsic. However, PLE or higher temperature PL spectroscopy yields unambiguous evidence for the model of interface smoothing due to growth interruption.

Y. J. Chen

Papers

Nonlinear optical processes in a polydiacetylene measured with femtosecond duration laser pulses

Intensity Dependent Index Of Refraction In Organic Materials

Uniaxial stress dependence of spatially confined excitons

Study of Langmuir-Blodgett polydiacetylene polymer films by surface enhanced Raman scattering

Temperature‐dependent optical spectra of single quantum wells fabricated using interrupted molecular beam epitaxial growth