City College has emerged over the last decade as a major center for research and scholarship. During 1989-90, for example, CCNY was awarded over $18.6 million in support for sponsored research, the largest total in our history and the largest amount for any CUNY college.

Polymers, long known for their mechanical properties, have recently been attracting attention for their conducting properties. Conductivity close to that of copper has been measured in polyacetylene to which non-metallic impurities (dopant) have been added.

We have used the nonlinear optical response of conjugated polymers to study the electronic structure of this class of one-dimensional semiconductors. These studies fall into two categories: Pump-probe experiments where a weak probe monitors the change in the linear optical response, and multiphoton processes such as third harmonic generation (X(3)) and two-photon absorption (TPA). In the first category of experiments we discuss a set of picosecond experiments where we measure the electronic response of the 1-D exciton in polydiacetylene thin films to a pump which is tuned to in, near and off resonance with respect to the excitonic absorption. These experiments provide information on the lifetime and the size of the 1-D exciton, and demonstrate how it couples to the optical field through the optical and phononmediated Stark effects. In the second category of experiment we discuss the consequences of intensity dependent change in the phase (n2) and amplitude (TPA) of optical field as high intensity electromagnetic field propagates through the nonlinear polydiacetylene waveguides. We find TPA is a sensitive measure of the two-photon accessible (covalent) states in a similar manner that the linear absorption is a sensitive measure of the one-photon accessible (ionic) states. Comparison of the two- and one-photon absorption spectra in polydiacetylenes indicate that the covalent gap is smaller than the optical gap and the optical nonlinearity is dominated by states well above the optical gap. These results are strikingly different from predictions of theoretical models that treat Coulomb interaction perturbatively. We conclude that the presence of an excitonic absorption in polydiacetylenes implies only a nonvanishing Coulomb interaction, and NLO spectroscopy is essential in uncovering the extent of many body effects in this class of 1-D semiconductors.

Earlier, we had shown how electron correlation effects in ?-electron virtual excitation processes determine the large, ultrafast nonresonant nonlinear optical responses of conjugated organic and polymeric materials1-3.

Nonlinear optical processes and electro-oplical effects are expected to have increasing importance as the information age matures and photonics augnnt electronics in various high density and high bandwidth technologies. Whereas for electronics the emphasis is in construction of smaller device structures from a few parent materials, for organic materials the direction of materials research has been reversed. For some time it's been known that some molecular structures engender exceptionally large molecular nonlinear-polarization responses. If such molecules could be assembled in convenient, versatile and reliable ways, the resulting materials would be very useful or even enabling in various photonics applications. The mature science and art of chemistry allows very good control over molecular composition and structure and, as will be illustrated in this talk, our knowledge of hyperpolarizability structureproperty relationships is advancing rapidly. However, the science of fabrication and arrangement in molecular ensembles and polymers is rather primitive. Thus the goal to develop the appropriately structured materials for utilization in nonlinear and electro-optics has fostered the widespread use of nonlinear optical processes to probe the nature of supramolecular order and assembly. Examples of intrinsic and artificially assembled structures of crystals, molecular aggregates, polymeric orientational electrets and molecular mono-and multi-layer thin films will be shown. Nonlinear optical processes primarily optical processes primarily optical second-harmonic generation, provide unique probes of these structures, their assembly and evolution.

The nonlinear optical properties of a series of thiophene based polymers have been investigated by studying the nonlinear optical excitations generated upon photoinjection of electron-hole pairs as well as the cubic susceptibility by degenerate four wave mixing (DFWM). The polymers have a similar electronic structure and slightly different molecular structures thus allowing to directly address the relation between the molecular structure and the nonlinear optical properties. Photoexcitation spectra show that all the systems give rise to bipolarons characterized by lattice distortions associated with intragap optical excitations. The DFWM study is performed at various laser frequencies encompassing the low energy transparent spectral range as well as the rising slope of the optical absorption due to the lowest ir -ir electronic transition. By doing so we measure both the nonresonant and resonant optical responses. The typical values of nonresonance X (3)(—w; , —w , w ) are in the range of 10-11 e.s.u. while in resonance are two orders of magnitude higher. The highest value of 1.13 x 10-8 e.s.u. is observed in polydithieno(3,2-b;2',3'-d)thiophene (PDTT); this value is among the highest of those observed in conjugated polymers. The most severe limitation to the use of these materials is in general due to the fact that the large optical nonlinear coefficients are accompanied by increasingly large optical absorption losses. The effect of photogenerated excited states on the nonlinear optical properties is investigated by a pump and probe DFWM experiment on polythiophene. The transient population of intrachain polarons gives rise to a substantial enhancement of the x(3)&-w ; , , w) indicating that the additional nonlinear processes which involve transient photoexcited states have a net positive contribution on the overall x(3)(—w ;w , , w ) process.

CW photomudalation technique has been applied to study the photoexcitation of a form of soluble polyacetylene with controlled conjugation length.We have been able to change the average conjugation length from few hundred units down to 20 units.A systematic variation of the photoinduced absorption spectra are observed upon decreasing the electronic delocalization.The linear dependence of the observed shift of the charged soliton photoinduced band on the average conjugation length is accounted for in terms of a first-order perturbative approach to the electronelectron interaction.

We report on the investigations of the response time, magnitude, and sign of the third order optical nonlinear coefficients of polythiophene and polysilane polymers using the Z-scan technique.

The scientific and technological importance of liquid interfaces in chemistry, physics, biology and engineering disciplines is due to the unique molecular properties of interfaces; properties that are a result of the interplay between the chemical composition, structure, and in general the reduced dimensionality and asymmetry of forces at interfaces.

Nonlinear gating of cw infrared lasers using ultrafast dye lasers permits a versatile infrared probe for investigating photochemical and vibrational dynamics of various molecules in condensed phases. Specific examples are presented for studies of optically excited bacteriorhodopsin, hemoglobin and myoglobin, as well as infrared excited ions and metal carbonyls.

The third order nonlinear optical susceptiblity, X(3)zzzz(-3?; ?, ?,?) of all-trans ?- carotene has been measured over three nearly contiguous spectral regions with 6,146 cm-1? ? ? 10,427 cm-1. Maker fringes generated by the third-harmonic signal, either from a thin film or from a thin solution, are analyzed. A strong three -photon resonance is found for the wellknown 1 1Bu <- 1 1Ag band in the blueireen. X(3) is vibronically interpreted within a simple 'essential' states model to produce parameters that successfully account for the very large magnitude of x(3)-1.6x10-10 esu - and its dispersion. A twophoton resonance is found, also in the same blue-green spectral region, with a peak located at 21 ,300 cm-1 and identified with the one n1Ag that is 'essential' to the optical nonlinearity. A 'giant' transition dipole of 31 D is found between this state and the essential' 11Bu state that is nearly degenerate with it. The two-photon cross-section for the 21Ag < -11Ag Thflsition is apparently very weak, for no (two-photon) resonance is seen anywhere in the green to the far red spectral region where the 21Ag is thought to lie.

We report absorption ("scattering depletion") spectra and Raman measurements on niobium containing matrix samples prepared by the mass-selected ion deposition technique. Niobium dimer ions, produced by sputtering, were mass-selected with a Wien filter and codeposited with Ar and electrons. Several new dimer absorptions are identified in regions which were previously obscured by intense atomic transitions. Two absorptions are also observed in the region 580 -720 nm, including a vibrational progression with ?e' = 426 (4) cm-1. The Raman excitation profile closely mimics the dimer absorption spectrum. Raman spectra give ?e" 420.5 (5) and ?exe" 0.5 (3) cm-1 ,in good agreement with previous measurements made in Kr matrices.

Near-infrared FT'-Raman spectroscopy has been used to obtain the Raman spectra from human cadaver specimens of calcified atherosclerotic aorta, normal, and fibrous atherosclerotic aortic tissues. The feature in the Raman spectra where compared with Raman spectra from protein such as clastin, and collagen, amino acids (tyrosin and tryptophan), ,and lipoprotein (cholesterol). Differences Ranian spectra in the different tissues offer a potentially new optical diagnostic approach to discriminate calcilied plaque from the other tissue types.

Time-resolved fluorescence measurements have been performed on benign and malignant breast tissue samples using 310 excitation of a 100 fs pulse. Fluorescence decay times were found to be double exponential. The 340nm band of benign and malignant tissues displayed differences in the slow components of lifetime and the ratio of the fast to the slow components.

The blue shift of the absorption spectrum of CdS clusters is calculated using pseudopotentials. The calculated blue shift is in exceptionally good agreement with experiment over a wide range of cluster sizes.

?-x carrier scattering in 35Å/35Å GaAs/AlAs superlattice at 5 K was investigated using time-resolved photoluminescence spectroscopy.

The anisotropy of the picosecond photoconductive response of trans-polyacetylene was measured for both above- (0.53?m) and below-gap (1.06?m) excitation. The anisotropy for below-gap excitation remain constant at a value of 2.4, independent of the laser intensity, while the anisoiropy for above-gap excitation decreased from a value of 7.4 to 4.5 as the intensity increased. In addition, the picosecond photocurrent was greatest for below-gap excitation. These results demonstrate the photoproduction of nonlinear charge carriers (solitons and polarons ) atenergies below the principle interband absorption edge.

Hot electrons in Al 0.6Ga 0.4 As were produced in the X 6 valley from a photoexcited indirect iransition by a 585nm femtosecond pump pulse. The time evolution of the population of electrons in the bottom of the X 6 valley was monitored by a femtosecond infrared probe pulse. The importance of the L 6?X 6 intervalley scattering for the relaxation process of hot carriers was investigated.

The primary process of visual pigment, rhodopsin, has been of interested for many years (1),There is a large body of evidence suggesting that the role of light is the photoexcitation of 11-cis retinyl, the chromophore found in rhodopsin, to the trans form, forming a photoproduct called bathorhodopsin.

The kinetics and energetics of photoinduced reactions may be determined using picosecond optical calorimetry (POC). The technique is a hybrid of thermal lensing and transient grating spectroscopies. The methods for acquisition of diffraction waveforms and their analysis are described herein. Application of POC in three chemical systems and a detailed examination of the results from one system are also presented.

As semiconductor technology continues to drive the scaling of electronic device dimensions into the ultrasubmicron, nanodimensional regime, many ultrasmall and ultrafast concepts and phenomena will continue to be put-forth for notional consideration. The stunning achievements of nanofabrication technology in the last decade now allow for band-engineering and atomiclevel structural tailoring not heretofore available or explorable except through naturally occurring atomic and molecular processes. Appropriately, this paper addresses several novel concepts, some new and some revisited, for consideration as interesting future directions. Five main themes will be put-forth and discussed as "food-for-thought" in considering novel directions in nanophysics, namely: coherent, electric-field assisted interference of closely spaced quantum states to provide long-lived energy trapping and novel optical properties; many-body effects relevant to single-electron transfer in Coulomb blockade phenomenology; coherent, defect-assisted tunneling as a related component of excess current during the Zener tunneling process; stimulated, resonant transfer of localized electrons in a periodic potential due to time varying electric fields; and confined-phonons in nanoscale systems.

With the current interest in ultrahigh speed electronic and optoelectronic devices, it is important to understand the relaxation dynamics of nonequilibrium carriers in GaAs and other compounds. When coupled with detailed experimental studies, this knowledge can provide significant insight into the physics of carrier transport in semiconductors

Raman spectroscopy with cw lasers in the visible, near ir and near uv is one of the most precise techniques to investigate low frequency elementary excitations in solids. It is particularly suitable for determining quasiparticle renormalizations due to small perturbations. The technique will be illustrated with examples for conventional semiconductors (effects of isotopic mass changes and fluctuations on phonon frequencies and line widths, effect of doping on same parameters) and high-Tc superconductors (effect of superconducting gaps on phonon frequencies and line widths, effect of crystal field transitions of the rare earth atoms on phonon frequencies and line shapes).

The first experiment [1] demonstrating the use of photoexcitation for creating as well as detecting non-equilibrium carrier distributions in semiconductors was performed more than 20 years ago.

The generation of a nonequilibrium population of optical phonons by photoexcited hot electrons in semiconductor quantum wells is investigated theoretically. The microscopic model of electron-phonon interaction proposed by Huang and Zhu has been used to compute the distributions of confined longitudinal optical phonons and interface modes in GaAs/AlAs quantum wells as a function of well width. Experimental tests of the calculated distributions by Raman scattering are discussed.

Room temperature excitonic absorption peaks have been difficult to observe in II-VI semiconductors, which has been attributed to strong exciton-phonon interactions. The first well-defined room teaperature excitonic absorption peaks, in II-VI semiconductors, were aeasured in CdxZn1-xTe/ZnTe multiple quantum wells (MQWs) grown by Rolecular bean epitaxy on GaAs substrates. Transmission, photoluinescence (PL), PL excitation and resonant Raman scattering experiRents reveal the important contributions to the exciton linewidth. The strong room temperature excitonic absorption was found to saturate at an incident optical intensity considerably higher than for III-V HQW5. The first visible wavelength waveguide intensity modulator based on the quantumconfined Stark effect was recently demonstrated. Featosecond time resolved measurements of room temperature exciton ionization by longitudinal optic (LO) phonon scattering, has resulted in an exciton ionization time of ? 125 fsec.

Ultrafast optical nonlinearities of semiconductor quantum wells and their application to high-speed switching is discussed. Both GaAs-A1GaAs type-I and GaAs-AlAs type-II quantum wells are considered. Optical Stark effect in these quantum wells and its use for fast photonic switching are covered. Application of optical Stark effect in demonstrating subpicosecond switching in a MQW directional coupler is discussed.

Semiconductor traveling wave amplifiers may play an important role in future photonic telecommunication switching networks. The present article discusses recent results on the generation of high power ultrashort optical pulses from semiconductor traveling wave amplifiers (TWA).

The physics and performance of narrow-linewidth dye lasers is reviewed. The physics is also applicable to gas and solid-state tunable laser oscillators and to the generation of femtosecond pulses. In addition, we discuss recent developments in the areas of ruggedized oscillators and high power dye lasers.

Spectroscopic and laser properties of chromium-doped forsterite (Cr:Mg2SiO4) are reviewed and future directions in development of other Cr4+-doped tunable solid-state lasers are discussed. The unique property of chromium-doped forsterite is that the lasing center was identified as tetravalent chromium (Cr4+) substituting for tetrahedrally coordinated Si4+. Development of chromium-doped forsterite laser may stimulate generation of a new class of tunable solid state lasers based on tetravalent chromium as a laser active ion for near infrared eyesafe spectral range. The criteria for the development of new Cr4+-based tunable solid-state laser crystals are discussed. We also present current research efforts to identify potential low field host crystals, such as silicates, titanites and germanates, that may be doped with Cr4+ as an active ion. These new laser materials are expected to cover the wavelength region from 1 -2 ?m, which may be of great technological importance.

Numerous methods have been demonstrated to produce tunable pulses from Ti:sapphire. The reported results have ranged from 600ps down to 50fs with varying tunability.

Recent advances in mode-locking techniques for Titanium Sapphire lasers will be presented covering: Active mode-locking, Coupled Cavity mode-locking (or Additive Pulse modelocking), self mode-locking, and Regenerative mode-locking. In addition, pulse compression of picosecond outputs to 50 femtoseconds will be discussed.

We review the history of time gated imaging, and summarize the method of Chrono-Coherent Imaging (CCI) that we have developed for imaging inside of scattering media such as tissue. We present experimental data on the time evolution of coherence properties in a scattering media that define some practical limitations of CCI and other coherence dependent imaging methods in transmission and reflection geometries. These data include results with well phased mode locked lasers and random phased microsecond dye lasers having femtosecond coherence. We also discuss the transformation of coherence in space and time in a scattering media, and use this analysis to suggest some new research directions.

During the past seven years, there have been remarkable advances in the frequency—domain method for measurement of time—resolved emission or light scattering. In this presentation we describe the recent extension of the frequency range to 10 GHz using a specially designed microchannel plate PMT. Experimental data will be shown for measurement of picosecond rotational diffusion and for sub—picosecond resolution of time delays. The resolution of PS to ns timescale processes is not obtained at the expense of sensitivity, or is shown by measurements on the intrinsic tryptophan emission from hemoglobin. We also describe a time—resolved reflectance imaging experiment on a scattering medium containing an absorbing object. Time—resolved imaging of the back—scattered light is realized by means of a RF—phase—sensitive camera, synchronized to the laser pulses. By processing the stored images, a final image can be created, the contrast of which is based only on time differences of the back—scattered photons. This image reveals the presence and position of the absorber within the scattering medium. And finally, we describe a new methodology, fluorescence lifetime imaging (FLIM) , in which the contrast depends on the fluorescence lifetime at each point in a two—dimensional image, and not the local concentration and/or intensity of the fluorophore. We used FLIM to create lifetime images of NADH when free in solution and when bound to malate dehydrogenase. FLIM has numerous potential applications in cell biology and imaging.

Various advanced streak cameras and their applications to science and engineering problems are discussed. The streak camera has permitted novel investigations to study some of the most important and fundamental processes in various fields. It is capable of performing two—dimensional analysis where the time domain and spatial or frequency domain are simultaneously measured; optical temporal profiles are determined directly with time resolution of better than 0.5 Ps and photodetection sensitivity in the single photon region.

Recently, the application of both time- and frequency- resolved fluorescence techniques for the determination of photon migration characteristics in strongly scattering media has been used to characterize the optical properties in strongly scattering media. Specifically, Chance and coworkers have utilized measurement of photon migration characteristics to determine tissue hemoglobin absorbance and ultimately oxygenation status in homogeneous tissues. In this study, we present simulation results and experimental measurements for both techniques to show the capacity of time-dependent photon migration characteristics to image optically obscure absorbers located in strongly scattering media. The applications of time-dependent photon imaging in the biomedical community include imaging of light absorbing hematomas, tumors, hypoxic tissue volumes, and other tissue abnormalities. Herein, we show that the time-resolved parameter of mean photon path length, <L<, and the frequency- resolved parameter of phase-shift, 0, can be used similarly to obtain three dimensional information of absorber position from two-dimensional measurements. Finally, we show that unlike imaging techniques that monitor the intensity of light without regard to the migration characteristics, the resolution of time-dependent photon migration measurements is enhanced by tissue scattering, further potentiating their use for biomedical imaging.

Today's medical professional is looking beyond the conventional procedures of X-rays, nuclear radiation, magnetic resonance, chemical analysis, and ultrasound to diagnose diseases ranging from cancer to heart ailments.

Much recent effort has been devoted toward developing clinically safe and effective laser angioplasty systems. Although it is possible to increase the luminal diameter of an artery by using laser energy to vaporize atherosclerotic plaque, these efforts have been complicated in early studies by an unacceptably high incidence of vessel perforation (Isner et al. 1972; Ginsburg et al. 1985; Deckelbaum et al. 1989).

Although lasers have traditionally been used in medicine as therapeutic devices, there has been considerable interest in using them as diagnostic tools.

Picosecond vibrational spectroscopy is used to address a model problem in protein dynamics, that of how carbon monoxide and dioxygen reach the functional group in myoglobin. The spectra indicate that there is a metastable binding position for CO at 300 K with 60 ns lifetime. We use polarization dependent measurements and molecular dynamics software to assign the ligand's metastable location to a particular pocket in the protein. This location does not coincide with that which is determined by freezing out intermediates at low temperatures. The implications for the entry and exit trajectories of ligands in myoglobin are considered.

The optical properties of tissues determine the penetration into tissue of the radiant energy from a laser (or other light) source. Subsequently, the laser energy is converted to chemical, thermal, or mechanical energy, and a variety of laser-tissue interactions are possible. The initial distribution of the radiant energy, however, affects the distribution and often the nature of the subsequent laser-tissue interactions. In this report, the manner in which optical penetration affects the subsequent photochemical, photothermal, and photomechanical mechanisms of laser-tissue interaction are presented. Understanding the optical dosimetry is an important step in developing protocols for clinical therapies.

To see an object in or behind a highly scattering random medium is one of the most challenging problems in imaging science. Various techniques have been introduced and proposed for this purpose.

Imaging of sub-millimeter phantoms in highly scauering diffusive media such as breast tissues was achieved using a picosecond optical Kerr gate imaging method. It was shown that the ballislic and snake parts of the gated transmitted light carried the least distorted image information with the highest spatial resolution and signal to noise ratio. The ultrafast time shutter rejected the late-arriving diffusive scattering light which blurred the image thus allowing us to see through an "opaque" scattering wall.

Locally absorbing microvolumes (10Å-10?m) much smaller than the radiation wavelength in size are characteristic of heterogeneous microstructure in cells and living systems and can be studied and controlled with ultrafast pulses of light. The ultrafast transient absorption and heating of local microvolumes absorbing through endogenous of exogenous chromophores at the radiation wavelength can be used to study size, structure and function of locally overheated microstructures. Pulse-heated microvolumes with altered refractive index and scattering and altered fluorescence are probed with a second light pulse. Also, the pulsed heating of the desired kind of microvolumes in cells and tissues with ultrafast laser pulses of a certain wavelength pulse duration and intensity opens up new possibilities for photothermotherapy. Ultrafast transient overheating of microvolumes may be substantial (?T=1-100 deg) while the time-and space-averaged heating of irradiated macrovolume is much lower. The fast transient perturbation of living systems with ultrafast, tunable laser pulses that significantly effect biological processes form the basis for new therapeutic applications. Ultrashort laser pulses (fs-ns) are shorter in duration than the time it takes for heat to diffuse from microregions even as small as 10-IOOA° across and coupled with their wide-band tunability make it possible to investigate local absorption microregions using endogenous or exogenous chromophores to determine optimum wavelength for spectroscopy and phototherapy. We have demonstrated remarkable effects on cell growth with femtosecond laser pulses (620nm) at an average intensity of 5.5x10-4 W/cm2 and dose of 0.33 J/cm2.

The use of near-infrared lasers and interstitial optical fibers for controlled heating of solid tumors is being developed both as an adjunct to photodynaniic therapy (interstitial laser hypertherrnia) and as a stand-alone modality (interstitial laser photocoagulation). Multiple optical fiber systems, and their use in vivo in muscle and liver tissue are presented, where interstitial thermocouple feedback has been used for direct temperature control as in laser hyperthermia. The tissue response during and after laser photocoagulation has been monitored by magnetic resonance imaging and ultrasound imaging in normal brain tissue and liver respectively, in order to assess the potential for imaging feedback and control. The general concepts of feedback monitoring and control in laser hyperthermia and photocoagulation are also discussed.

The laser field is rapidly creating improved ultraviolet laser, new wavelengths below 200 nm, new infrared lasers in the near infrared and some new delivery systems suitable for medical applications. Carefully assessment of the safety and effectiveness of these new advances for medical applications requires accurate measurement of exposure at the target tissue and careful correlation with the biological response.