Organic and polymeric materials have emerged in recent years as promising candidates for advanced device and system applications. This is due in large part to their extraordinary optical, structural and mechanical properties and from the success of molecular design in creating new kinds of materials and responses. Additionally, polymers can be fabricated into a wide range of structural shapes and sizes (fibers, films, molded parts), and possess large nonresonant nonlinearities and low dielectric constants. This unique combination of properties, coupled with the ability to alter the microscopic structure and the macroscopic orientation to produce entirely new materials, makes polymers prime candidates for inclusion in research programs addressing non linear devices.

The large third-order nonlinearity of the organometallic polymers can be utilized in the construction of optical devices. The presence of strong nonlinear absorption and low linear absorption in the visible wavelengths makes these polymers ideal material for use in nonlinear optical thresholding devices. We present here a study of such a device made of organometallic polymers using picosecond and nanosecond laser pulses.

We have developed a detailed theory for the two possible modes of switching at a nonlinear film bounded by two dielectric media. Because of the fact that the film is thin, and the high reflectivity of the boundary surfaces near the total internal reflection state, several factors have to be taken into account. These include reflection feedback, nonlinear Fabry Perot effects, nonlocal responses, intensity and angular dependences of time scales and switching threshold, optimal transmission, etc. Using liquid crystal as the nonlinear medium we have also performed experimental verifications of the theory.

In composite systems, optical fields may change the refractive index mismatch between components of the system, and thus affect the scattering of incident radiation by the bulk sample. We discuss this mechanism in polymer dispersed liquid crystal films. These composite materials consist of liquid crystal droplets dispersed in a polymer binder. Optical fields affect the orientational order of the liquid crystal in the inclusions, and hence alter the refractive index mismatch. We report observations of scattering in polymer dispersed liquid crystal films induced by a CW Ar+ laser. The scattering is due to both reorientation and thermal effects. An attempt is made to assess the relative contribution of these two mechanisms, and experimental results are compared with theoretical predictions.

Optical limiting measurements have been made on solutions of several metal containing phthalocyanines and naphthalocyanines. Measurements at 532nm using nanosecond pulses from a Q-switched Nd:YAG laser show limiting throughputs of 1-10 millijoules with mild focussing in alcohol solutions with nominal transmissions of 30-70%. Measurements on chloro-aluminum-phthalocyanine solutions utilizing individual 30 psec pulses or trains (spanning about 100nsec) of modelocked pulses have shown even lower limiting throughputs. Thus, the dynamic range of the limiting behavior has been shown to cover at least three orders of magnitude. Prompt limiting is attributed to strong singlet-singlet (S1-Sn) absorption, whereas the longer time limiting behavior is postulated to result from strong triplet-triplet (T1-Tn) absorption. In addition to these studies, efforts have been underway to identify materials with reduced limiting throughput and improved optical transmission characteristics.

Organic polymeric materials have advanced to the point where their nonlinear properties are sufficient for use in various designs for such device applications as tunable filters and photonic switches and shutters. These advances are a result of successful molecular and materials design aimed at improving the material nonlinear responses while maintaining superior optical, mechanical and processing properties for the bulk material. This paper will review recent progress in materials and device development at the Lockheed Research and Development Division. Section 2 provides a brief background on nonlinear organic materials, while the following sections will cover the different materials classes of resonant polymers, non-resonant third-order polymers, electro-optic materials and liquid crystal composites, and the appropriate device type for each material class.

We report the study of optical switching from total internal reflection to transmission at a liquid crystal interface. The laser induced phase transition to the isotropic state is responsible for the bistable behavior of this device. We show how the effect can be controlled by the angle of incidence of the light.

Laser-induced morphological and electrical changes to silicon CCD devices have been studied. The devices were poly-silicon gate Time Delay Integrating (TDI) CCD arrays of 2048x96 elements. The laser source for these experiments was a Q-switched Nd:YAG laser at 1.06 μm with 10 ns pulses at a 10 Hz repetition rate focused to an approximately 400 μm spot radius. Single pulse and multiple pulse damage behavior was studied. Both CCD arrays and diagnostic structures from the wafer periphery were tested. The additional diagnostic structures included poly-Si resistors and MOS-FET gates. Of the measurements made, it was found that drain-to-substrate and drain-to-source leakage currents and transconductance in FETs were the most sensitive parameters to laser-induced change. The onset of electrical parameter changes was observed as low as 0.2 J/cm2. Severe electrical parameter changes began at 0.5 J/cm2 and continued up to the onset of severe morphological damage at 1.0 J/cm2. Above this fluence, both poly-Si and aluminum interconnect lines were melted and broken.

It has been shown that a suspension of small, absorbing particles in a liquid demonstrates nonlinear transmission when exposed to high-energy laser pulses. Two experiments were performed to characterize the nonlinear phenomena that occur. First, an HeNe probe beam and fast photodetector were used to monitor the transmission of the suspension as high-intensity Nd:YAG pulses passed through. The sequence of events in the breakdown process was thus determined. Second, the spatial distribution of the transmitted Nd:YAG pulses was monitored with a CCD camera and Quantex QX-7 digital image processor. The distribution of the light scattered by the suspension was viewed for input pulses less than, near to, and greater than the threshold required to produce breakdown. The combined results of these two experiments gives a clearer understanding of the limiting process that occurs.

We have characterized the nonlinearities observed in suspensions of carbon black particles in liquids (CBS). We have developed a preliminary explanation of the optical limiting characteristic of the CBS that qualitatively explains the low thresholds, broad-band response and other limiting characteristics. In this model, the microscopic carbon particles are heated by linear absorption to a temperature at which a plasma can be created by the optical field. These microplasmas rapidly expand, thus scattering the incident light and limiting the transmittance. This model is consistent with our observations that nonlinear scattering dominates transmission losses. We find that limiting depends on the input fluence (J/cm2) rather than irradiance (W/cm2). Therefore, limiting works well (i.e., low limiting energy) for long pulses (≥ 10 nsec) but is less effective for short pulses (≈ psec). In addition, the CBS rapidly degrades with repetitive laser firings, thus, flowing or moving the liquid between firings is necessary.

The principles of operation of semiconductor optical limiters which utilize two-photon absorption and free-carrier induced defocusing are described. We present a review of early work using psec pulses at 532 nm in ZnSe, in which the problem of damage in solid state limiters is overcome by optimizing the focusing geometry. Limiting energies as loW as 10 nJ are seen, and a dynamic range (damage energy divided by limiting energy) in excess of 104 is demonstrated. The somewhat complicated propagation theory is simplified into a set of scaling rules which are used to predict operating characteristics of semiconductor limiters at longer wavelengths and for shorter pulses. We present new limiting data obtained with longer pulses in ZnSe, in CdTe at 1.06 μm and InSb at 10.6 μm, and we compare these results with the scaling rules.

There is an always growing interest in the study and characterization of new nonlinear optical materials with higher nonlinear coefficients and fast responses, especially for their potential applications in the telecommunications, in the optical computing, and in several other fields where the main issues are high bit rates, parallel processing and large mass storage. The semiconductor technology allowed the construction of very complicated structures, the multiple quantum wells (MPWs), with resonant nonlinearities far bigger than those available in semiconductor crystals, and pushing further the quantum confinement to comprise all the three space dimensions (quantum dots, QDs) is the next step toward even better performances.

We have performed analysis and measurements on a device which can selectively attenuate a coherent beam in the presence of an incoherent signal-bearing beam. Our calculations show that a rejection of 50 dB can be achieved for an optimum interaction geometry. Device measurements in the cw and pulsed regime are described.

External self-focusing and nonlinear interference phenomena can arise when a laser beam interacts with a nonlinear medium, and may be utilised to limit the transmitted beam power. The limiting mechanism may be complicated by the interplay of nonlinear absorption and refraction, and also by the shape of the incident beam profile. These effects have been investigated for Kerr-type media through analytic and numerical solutions of the nonlinear wave equation. Strong modulations in the transmitted irradiance can appear with optically thin nonlinear media in principle, even where very pronounced nonlinear absorption is present. This type of interference modulation does not necessarily decay as the incident power increases, depending upon the shape of the incident laser beam. Furthermore, the amplitude and periodicity of the modulations depend markedly upon nonlinear absorption. Structural features attributable to the nonlinearity appear at the vicinity of a focus in both the radial and axial directions, and increase in proportion to the magnitude of the nonlinear optical phase shift. Analytic techniques have established that relatively small changes in beam shape can markedly affect the transmitted irradiance. It is also shown that external self-focusing can be viewed, at least in terms of the incident power dependence of the nonlinear "focal shift", as a compromise between the opposing tendencies of nonlinear absorption and refraction.

Laser-induced grating spectroscopy was used to characterize the properties of three types of materials with potential applications for optical limiting. Figures of merit were determined from the magnitudes and response times of the observed signals. Three samples were chosen as baseline materials for different classes of nonlinear effects: CdTe, KNbO3, and Eu-doped phosphate glass. The first represents II-VI semiconductors, the second oxide crystals and the third nonlinear glasses. The physical processes underlying the nonlinear optical responses of these materials are very different leading to a wide range of properties relevant to optical limiting applications.

Passive optical limiting based on nonlinear refraction in a "thick" medium is analyzed using a simple model. In a tight focus geometry we found that the position of the sample with respect to the focal plane is an important parameter in the limiting characteristics of the device. In particular we have examined such characteristics in liquid CS2 using 300 ns pulses at 10.6 μm.

Optical limiters and switches can be fabricated in a variety of ways. The simplest devices are those which utilize materials that respond nonlinearly with incident intensity. We analyze and model five different passive limiter/switch concepts which could yield practical devices. These concepts include the total internal reflection, photorefractive beam-fanning, two-photon absorption, self-focusing, and self-defocusing. The analysis is carried out for typical optical materials used in the visible and infrared and the eye for a wide range of incident-pulse widths. The strength and speed of the nonlinearities required for the fundamental performance of each limiting device is provided. Factors considered in the analysis include dynamic range, transmission, response time, and the damage threshold.

We have demonstrated a novel optical power limiter which can be used to limit an unstable pulse peak power with temporally smoothing in shape for CO2 laser pulses. The peak power of pulses was limited with 1.1% fluctuation for a 30% change of incident pulse peak power and the frequency of subpulse oscillation on a single pulse was over 30 MHz.