Optical limiting devices based on nonlinear optical (NLO) materials exhibit a throughput radiation level that is related to the magnitude of the nonlinear material parameters. In order to determine the upper bound on optical limiting performance in a variety of configurations it is necessary to establish the limitations of the NLO materials themselves. Various physical mechanisms responsible for NLO behavior in the visible spectral region are examined. Some of the largest effective nonlinearities are derived. These large nonlinear values are then compared to the nonlinearities required to protect the human eye from Q-switched visible laser pulses in a 0° field-of-view (FOV) "thin" cell limiter.
The so-called "defocusing limiter" is one of the best configurations for achieving laser radiation protection over a broadband spectrum in a low fnumber system. Available nonlinear materials, however, can not provide maximum permissible exposure levels over a large dynamic range. On the other hand, we believe that novel nonlinear optical engineering techniques can be used to overcome these difficulties. We have recently discovered a rich class of nonlinear optical phenomena in defocusing media, which opens new opportunities to enhance the performance of defocusing limiters and to explore completely new limiting schemes. Here we show that a nonlinear mask can be used to improve the device performance by reducing the throughput power by 90% in an ff5 optical system. We have numerically modeled a generic nonlinear optical limiter to account for these effects. The code allows us to vary the parameters of the optical system and the nonlinçar medium. In addition to obtaining the transmission data, we also determine the intensity distribution in the final focal plane.
This paper summarizes recent results obtained on CdTe polarization rotation switches and optical limiter based on lead glasses. The polarization rotation switch is based on picosecond pulse four wave mixing with two beams and the switching characteristics are dependent on beam crossing angle, relative polarization directions, and pulse timing. Material enhancement of the nonlinear scattering is demonstrated by using Mn-doped crystals. Optical limiting by thermal lensing on the nanosecond time scale was characterized in a variety of lead based glasses and it was found that the main contribution to the thermo-optic coefficient is due to thermally induced changes in the electronic polarizability of the glass components (especially oxygen) and is effected by the polarizing power of the network former ions. Random network structures have greater thermo—optic coefficients than ring and chain structures and modifier ions affect the properties through changes in the absorption coefficient of the material.
An increase in the dynamic range of an optical limiting device is achieved by using two different nonlinear elements cascaded within a single limiting device. A comparison between the results of optical limiting experiments for a single nonlinear element limiter and a two element limiter shows a significant increase in the damage threshold of the latter with only a small increase of the limiting threshold.
We have investigated self-switching mechanisms in carbon microparticle suspensions. A dominant nonlinear switching mechanismin the majority ofthese samples is laserinduced cavitation which leads to total internal reflection (TIR) switching. Nonlinear reflectivities up to 90% were measured using frequency doubled, single 5ns Nd:YAG laser pulses. The fraction of energy that is not switched via 'HR undergoes additional nonlinear attenuation by nonlinear plasma scattering. We have also developed a technique for avoiding laser induced degradation of the nonlinear interface at high laser pulse energies. A simple model which may be used in the optimization of this technique is discussed.
The effects of optical beam shape variations upon the near- field transmission features of a thin nonlinear medium located at a beam waist have been investigated. This work is particularly relevant to the z- scan techniques, as well as to the aperture -scan techniques, for characterising nonlinear media. In the model that has been explored, a family of incident transverse beam shapes F(p,v) characterised by a shape parameter v has been introduced at the focusing lens. For v <2, these transform to a diffraction pattern containing circular rings and expressible in terms of an ordinary Bessel function. For v < 2 there is only a central spot, expressible in terms of a modified Bessel function. The functions F(p ,v) conserve both the incident peak irradiance and the beam power as V changes. A simple cubic (Kerr) nonlinearity has been studied. It is shown that a basic trade-off exists between the width and height of the peak-valley separation in the normalised transmission curve.
Using an excite-i,robe two-co1or Zscan technique, we investigate the dynamics of optical nonlinearites in semiconductors with picosecond pulses at 1.06 and 0.532 pm. We use the technique to obtain a direct timeresolved measurement of the nondegenerate nonlinear refraction in polycrystalline ZnSe and show how it can resolve the bound electronic and free carrier components.
In many sensor protection applications, it is useful to selectively attenuate a strong coherent beam in the presence of a collinear, weak white light image. We call a device which can perform this function a "coherent beam excisor." One convenient approach for sensing coherence is to divide the input beam and recombine it in a pho torefractive material (see Figure la). The portion of the input beam which is coherent will write a refractive index grating, resulting in energy exchange between the two beams. Inparticular, the sample can be oriented so that a large fraction of the energy in the coherent beam is diverted away from the direction of the incoherent white light. Put differently, we wish to optimize the depletion of the signal beam, accompanied by strong amplification of the probe beam. Several recent studiesl.2 have shown that photorefractive two-wave mixing is a viable means of implementing a coherent beam excisor. The only disadvantage of this approach is that portions of the optical train are not coaxial; a beamsplitter and at least one turning mirror are required to form the auxiliary probe beam. One obvious simplification is to use a single beam (Figure 1b), with the role of the second beam now taken by scattered light inside the crystal. This beam fanning approach was actually the first photorefractive limiter reported in the literature.3 Recently, Salamo, et al.4 have proposed an intermediate solution consisting of a single inci dent beam, with a grating in contact with the entrance face of the crystal (see Figure le). The grating can be blazed so as to produce a single diffracted order, with a controlled diffraction efficiency. The diffracted beam can be looked upon as the probe beam in a two-wave mixing approach, or an auxiliary seed beam in the fan ning approach. Inany case, the diffracted beam will be amplified, leading to depletion of the signal beam. Until recently,5 no systematic study of excisor performance using a grating for probe generation has been undertaken. In particular, the effect of walkoff of the probe beam has not been considered. The ideal beam crossing posi tion is at the center of the sample, but the use of a separate grating places the effective beam cross ing position at the entrance face of the crystal. In this paper, we have simulated the operation of a grating by using a separately generated probe beam with a variable intensity, which can be set to cross the signal beam at a number of positions. We measured both the rejection and the activation energy. We will show that a factor of -10 improvement in activation energy can be obtained (compared to a pure fanning geometry), even when the beams cross at the entrance face.
Two types of optical limiting devices which have been demonstrated using photorefractive crystals are the 'beam fanning' limiter and the 'two-beam coupling' limiter. Experimental demonstrations of these two devices have revealed an interesting difference in behavior between those two limiter types. The two-beam coupling limiter is consistently faster than the beam fanning limiter for the same incident intensity. This is somewhat surprising since the beam fanning limiter is based on two-beam coupling phenomena. Our recent experiments show, however, that the relative speed of the two devices can be made more alike using a phase grating at the entrance face of the beam fanning limiter. While the phase grating scatters only about 1% to 5% of the incident light is apparently provides sufficient seeding to significantly enhance the beam fanning response time. Since the diffracted light is only about 1% the distortion to vision through the limiter due to dispersion is minimal. In the presentation we will present results from an experimental investigation of the effect of seeding on the 'beam fanning' limiter response time. In particular, we have examined the response time dependence on the intensity and direction of the seed beam. Comparison with theory will also be presented.
A technique for real time direct measurement of both the amplitude and phase of photorefractive space charge fields during grating formation is presented. Photorefractive gratings are formed by the interference pattern of two intensity modulated beams. The two beams are single sideband modulated at the same RF frequency but are of opposite frequency shift. A detector tuned to the modulation frequency is used to monitor the time development of both the amplitude and phase of the photorefractive grating using a heterodyne detection scheme. This technique provides substantial dynamic range and the necessary sensitivity for the detection of refractive index changes as small as 10-8 and phase changes as small as 1 degree(s). Data is presented for the build-up of the amplitude and phase of the photorefractive space-charge fields. The photorefractive physical properties of these crystals including diffusion transport length, photovoltaic transport length, screening length, and the mobility free carrier lifetime product are determined.
We report the characteristics of a strong 3rd order negative nonlinear effect in the nematic and smectic A liquid crystal phases as well as in the isotropic phase. Limiting is achieved as a result of a strong light scattering effect with some contribution from nonlinear absorption. We used an optical limiting setup to measure the limiting threshold and clamped output parameters. We present data regarding the dependence of these parameters on polarization direction, liquid crystalline order and alignment, depth of focus into the liquid crystal layer, cell thickness and temperature. We also discuss the temporal behavior of the limited pulses.
In order to study third order optical nonlinearities of nematic liquid crystals on the nanosecond timescale, we have carried out Z-scan measurements using a Q-switched Nd:YAG laser at 532 nm. The nonlinear refractive indices and absorption coefficients were determined for a number of liquid crystals at different temperatures. For the cyano-biphenyl 5CB, we used a two-pulse technique to estimate the response time, and we studied the intensity and wavelength dependence of the response to determine the dominant contributing mechanism. The correlation between the observed nonlinearities and the molecular structures is discussed.
In this paper we describe measurements of the third order nonlinear susceptibility and the response time for a large number of novel liquid crystal materials, in their isotropic phase, at temperatures close to the isotropic to nematic phase transition. By examining the dependence of the length of the alkyl chain part of the molecule, for the cyanobiphenyl and alkoxy- cyanobiphenyl homologous series, it is shown that the small alkyl chain materials have a superior nonlinear response. Unfortunately in these materials the large nonlinear effects close to the nematic phase are inaccessible because the solid phase is formed directly on cooling the isotropic liquid. An alternative route to large effects is to increase the molecular anisotropy in polarizability, while maintaining approximately the same molecular dimensions. It is shown that a significant increase in the size of the nonlinear response can be achieved by changing the various chemical constituents.
Studies have been carried out across the visible on liquid crystal electro-optic (EO) and nonlinear optical devices. Two geometries were studied, a variable birefringence filter (VBF) and a Fabry Perot (FP) etalon, with a view to using them as (1) EO modulators and (2) EO tunable filters. Restrictions on device design and performance imposed by LC technology are analyzed and discussed. It is shown that for modulators the VBF yields better performance characteristics when employing LC technology, whereas for tunable filters the FP etalon geometry offers advantages. A hybrid spacer layer FP device and a double element VBF, designed to improve the wavelength selectivity of tunable filters, are described. Finally, results on bistable switching in the FP etalon are presented.
We present results from some preliminary experimental studies of two electro-optical phenomena in nematic liquid crystal cells: dynamic scattering and field induced alignment for optical modulation and switching applications, using liquid crystal cells with a variety of surface alignment. Broadband, good extinction ratio, non-polarization selective modulators/switches with on-off response times on the order of a millisecond or so are demonstrated.
Using a Q-switched CO2 laser pulse induced dynamic grating technique, the basic mechanisms for the optical nonlinearities of nematic and isotropic liquid crystals in the infrared (10.6 micrometers ) region are reexamined. The principle mechanisms are the density and temperature effects, which are characterized by relaxation times in the nano- and micro-second time scales, respectively. Optical self-limiting effects and optical switching using isotropic liquid crystals are also demonstrated for visible nanosecond (Q-switch Nd:YAG at 0.53 micrometers ) and for infrared microsecond laser pulses, respectively.
Diacetylenic monomers have been synthesized in order to obtain long conjugation to provide high nonlinear optical response. We report the preparation of nonadeca-10:12-diynoic acid and used as precursor for the following compounds: (1) 4-cyanobiphenyl-4'-(nonadeca-10:12- diynoate), and (2) 2,3,6,7,10,11-triphenylene-hexa(nonadeca-10:12-diynoate). The polymerizations of these monomers were carried out by UV-irradiation of the monomers in bulk and in 25 (mu) thickness of quartz cells. The cyanobiphenyl-containing diacetylenic monomer exhibits monotropic nematic phase and the polymer displays enantiotropic nematic phase. The diacetylenic discotic monomer is not polymerizable and exhibit no mesophase. The nonlinear absorption properties of the diacetylenic discotic monomer was studied by using 532 nm frequency-doubling output from a Q-switched Nd:YAG laser with a pulse width of 7 ns.
We have measured the photodynamics of reverse saturable absorption (RSA) in solutions of cyclopentadienyliron carbonyl tetramer (King's complex) using picosecond pump-probe techniques. Similar preliminary measurements in solutions of synthesized variations of the King's complex indicate that the excited state transition responsible for the observed RSA is most likely a second d-d transition within the metal core of the molecule. On time scales of hundreds of picoseconds, the observed RSA in the King's complex is well characterized by a three-level rate-equation, singlet-state absorption model, where the excited-state cross section is greater than that of the ground state. On nanosecond timescales and at fluences above 200 mJ.cm-2, however, we observe the onset of a response that is consistent with a thermally induced scattering process. Further evidence of this scattering is provided by angularly-resolved measurements of the transmitted and back-scattered signals for nanosecond excitation. When the King's complex is incorporated in a solid host negligible scatter was observed and the response is completely described by the singlet parameters extracted from the picosecond measurements. The observation of, scatter from solution, together with a time- resolved decay to the ground state that is rapid (approximately 120 ps) and largely nonradiative in this molecule, indicate that solutions of King's complex may provide a mechanism for efficiently generating thermal nonlinearities on a subnanosecond timescale.
The nonlinear characteristics of InSb and Hg1-xCdxTe were compared at room temperature with a CO2 laser operating at 10.6 micrometers . Overall, undoped InSb was found to be more promising than Hg1-xCdxTe (0.226
We demonstrate a proof-of-principle, GaP optical energy limiter for 532 nm, 25 ps, pulsed radiation that exhibits an output limiting level of less than 1 (mu) j/cm2. Optical limiting at this level is significant for use in systems designed to protect the human eye from laser radiation damage. The device employs a standard configuration that is realized by placing the GaP at the intermediate focal plane of a 1-to-1 inverting telescope, which is followed by an aperture set to clip the linearly transmitted beam at the 1/e point of the irradiance profile. The linear (indirect) absorption in GaP at this wavelength results in the optical generation of very large densities of free carriers. We therefore anticipate the performance of this device to be highly dependent on free carrier nonlinearities. This is confirmed by performing both two beam and single beam measurements on the material itself in order to determine both the absorptive and refractive parameters at this wavelength. We find that the free carrier absorption cross section is is congruent to 2 X 10-18 cm2, and the change in index per photogenerated electron hole pair is is congruent to -2.4 X 10-22 cm3.
In this paper, we describe the development (which includes theoretical modeling, structure design, material growth, and sample characterizations) of SiGe superlattices (SL) for nonlinear optical applications in infrared. Our theoretical model predicts that the two-photon absorption coefficient in SiGe (110) SL structures can be on the order of 1 cm/KW, with very fast response (approximately 1 ps). In addition, we also show that quantum well materials with enhanced nonlinear refraction and moderate absorption offer potential advantages for applications in a lowest order Fabry-Perot etalon.
Organically modified silicates (Ormosils) have been applied as matrices in the preparation of CdS-doped glassy nanocomposites via a low temperature route. EDX spectra show that a CdS concentration up to about 20 wt.% has been obtained. The CdS microcrystallites of hexagonal wurtzite structure with average particle size in the range of 20 to 120 angstroms were formed within Ormosils matrices. The quantum confinement effects were clearly observed in samples with smaller particle sizes from absorption and photoluminescence excitation spectra. High- quality CdS-doped films with enough thickness have been prepared by spin-coating for waveguide device application. The Ormosils used are thermally stable up to 360 degree(s)C which is higher than the processing temperature for normal integrated circuits. The nonlinear optical properties of these nanocomposites measured by degenerated four wave mixing technique on picosecond time scales using a Nd+3:YAG laser at 532 nm are described.