Waters Symposium: Lasers in Chemistry
Nonlinear Optical Instrumentation N. Bloembergen Division of Applied Science and Department of Physics, Pierce Hall 231, Harvard University, Cambridge MA 02138
At high light intensities the optical response of materials can no longer be described by a frequency-dependent linear susceptibility or linear index refraction. Often a power series expansion of the polarization in terms of the macroscopic electric field, occurring in Maxwell’s equations P = χ(1)E + χ(2)E 2 + χ(3)E 3 +…
(1)
is sufficient to describe nonlinear optical phenomena that occur at high intensity. In eq 1, χ(1) is the linear susceptibility, a second-rank tensor. The lowest-order nonlinear susceptibility is χ(2), a third-rank tensor. It vanishes in media with inversion symmetry. If the incident field is a monochromatic wave at frequency f, it leads to an electric polarization at the second harmonic frequency 2f and also at zero frequency. This polarization describes second harmonic generation and rectification of light. If the incident field consists of two waves with different frequencies f1, and f 2, it leads to polarization terms at the sum frequency f 1 + f 2. Thus the lowest order quadratic nonlinear response causes the generation of new frequencies. One may choose f1 at an optical frequency and f 2 at a microwave frequency. In this case one obtains a description of microwave modulation of light. For f 2 = 0, the electro-optic Pockels effect is recovered. This effect, known
since the previous century, describes the linear change in index of refraction with an applied dc electric field. The third nonlinear susceptibility χ(3) , a fourth-rank tensor, gives rise to a large number of nonlinear optical phenomena, including third harmonic generation, twophoton absorption and intensity dependent index refraction. Every material, including noble gases, has a nonvanishing value of χ(3). For molecular liquids and solids the nonlinear susceptibilities are related to the nonlinear polarizabilities of the constituent molecules. The local field corrections due to molecular interactions are, however, very significant in most cases. The field of nonlinear optics developed rapidly following the realization of lasers. Various nonlinear optical effects are readily observed with available laser beams and can be used in the experimental determination of χ(2) and χ(3) and their dispersion and absorption in two- or three-dimensional frequency spaces. This leads, in turn, to new information about the electronic structure of various materials. Conversely, certain materials with large values of χ(2) and χ (3) may be selected to create nonlinear optical instruments and devices. In the following sections a few examples are selected to illustrate these general principles, which have been described in many textbooks (1–4).
The Annual James L. Waters Symposia at Pittcon The objectives of the annual James L. Waters Symposia at Pittcon are different from those of other symposia at either Pittcon or other conferences. Waters, founder of the well-known Waters Associates, Inc., and currently president of Waters Business Systems, Inc., arranged with the Society for Analytical Chemists of Pittsburgh (SACP) in 1989 to offer annual symposia at Pittcon to explore the origin, development, implementation, and commercialization of scientific instrumentation of established and major significance. The main goals were and still are to ensure that the early history of this cooperative process be preserved, to stress the importance of contributions of workers with diverse backgrounds, objectives, and perspectives, and to recognize some of the pioneers and leaders in the field. Important benefits of these symposia are creation of awareness of the way in which important new instruments and, through them, new fields, are created, and promotion of interchange among inventor, development engineer, entrepreneur, and marketing organization. The top-
ics of the first seven Waters Symposia, beginning in 1990, were gas chromatography, atomic absorption spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry, high-performance liquid chromatography, and ion selective electrodes. Publication of the papers presented at the Waters Symposia is a high priority of the SACP. The papers of the first symposium were published in LC.GC Magazine and those of the next four symposia were in Analytical Chemistry. The next two Waters Symposia were published in this Journal: the sixth, on highperformance liquid chromatography, appeared in the January 1997 issue (pages 37–48) and the seventh, on ion selective electrodes, appeared in the February 1997 issue (pages 159–182). Lasers in chemistry was the topic for the eighth Waters Symposium, held in March of 1997, and is featured in this issue of the Journal. Administration of the Symposium, including selection of the topic and speakers, is handled by the SACP. J. F. Coetzee University of Pittsburgh Waters Symposium Coordinator
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Waters Symposium: Lasers in Chemistry
Applications to Surface Chemistry Since χ vanishes in centrosymmetric media, second harmonic generation (SHG) and sum frequency generation (SFG) can be used to specifically probe interfaces and surfaces. SHG by a laser pulse on the surface of a liquid arises in the electric dipole approximation only from molecules in the surface layer. The SH signal may be observed in reflection. The signal is weak, as only molecules at the surface contribute. Typically only a few SH photons are generated during an incident focused pulse of a few millijoules of picosecond duration, which contains about 1015 incident photons. A short focused pulse is necessary to observe the weak nonlinear signal without causing significant variation in temperature or breakdown of the surface layer. Shen and coworkers (5) have probed molecular monolayer adsorbates and observed an orientational transition in a liquid crystal monolayer (6 ). In some cases corrections have to be made for a quadrupolar nonlinear contribution from the bulk material by detailed analysis of the orientational symmetry of the SH signal. More specific information may be obtained from SFG. A tunable infrared pulse at frequency f 2 is generated in an optical parametric downverter oscillator (OPO), where a pump pulse at f1 is split into an idler beam at f1 – f 2 and the infrared beam at f 2. The frequency f 2 is tuned by varying the temperature or orientation of the OPO crystal, for example, lithium niobate. The pulses at f 1 and f 2 are synchronized and focused onto the same spot at the surface. Signal photons at f1 + f 2 are collected in a CCD detector. The frequency f 2 may be tuned through the vibrational frequency of molecular normal modes at the surface, and the polarization of the beams at f1, f 2, and f 1 + f 2 may be varied. In this manner Y. R. Shen and coworkers obtained information about dangling O–H bonds at the ultrapure water liquid–vapor interface. A sharp resonance at 3600 cm ᎑1 corresponding to the stretch mode of free OH is found. This resonance disappears when the surface is covered by a single monolayer of sterile alcohol. The vibrational spectrum then assumes a form that is also found for the surface of ice. The free OH resonance is independent of temperature between 20 and 80 °C. From polarization studies Shen et al. deduce that the angle between the OH bond and the surface normal is less than 38°. About one quarter of the water molecules in the surface layer have a dangling OH bond. Another original application is the study of chirality of surface configurations. Persoons and coworkers (8) have observed the difference in surface SHG for left and right circular light polarizations in various geometries. (2)
Organic Nonlinear Optics During the past two decades there has been a large-scale effort by many research groups, combining the talents of chemists, physicists, and optical engineers, to achieve nonlinear optical devices based on organic materials. These efforts started with the discovery by Ducuing and associates (9, 10) that molecules with long chains of conjugated π-electron bonds exhibit very large nonlinear polarizabilities. Polymers can be designed that combine desirable nonlinear optical properties with low-cost fabrication techniques. The materials can be spin-coated and etched to form optical wave guides 556
of small dimensions. The dielectric constant at microwave frequencies is much lower than that of anorganic nonlinear optical crystals. This reduces the power requirements for optical microwave modulators. The progress in organic nonlinear optics has been described in several comprehensive publications (4, 11–13). A brief introduction to this vast and rapidly growing field was also recently published (14 ). SHG has been demonstrated in quasi-phase matched, periodically poled, polymeric films. However, such devices cannot compete with the more efficient SH conversion obtainable with anorganic materials. The usefulness of such devices, designed to convert infrared semiconductor laser output to visible wavelengths, is further compromised by the development of wide bandgap II–VI semiconductor lasers, which can emit directly at blue and green wavelengths. Very thin polymeric targets are however useful to analyze the structure of femtosecond pulses by time-resolved spectral analysis of an SH signal created by the fundamental pulse that is split into two parts, which may be delayed with respect to each other (15). Polymeric photorefractive materials have been developed for the purpose of holographic optical storage. These materials incorporate various constituent moieties, either in the main chain or side-chains of polymeric backbones, to achieve the optimum combination of the electro-optic, photoconductive, and carrier trap response. These developments were reviewed by Kippelen et al. (16 ). A polymer-based device that has entered the stage of commercial production is the electro-optic microwave modulator. One version, which resulted from the collaboration between several academic and industrial research groups, is described as follows (17 ). The polymeric optical guide is split into two arms, which are recombined after a certain distance. This guided structure acts like a Mach– Zehnder interferometer. If the propagation in the two arms produces a phase shift of π radians, there will be destructive interference in the output channel. The polymeric optical guide consists of a poled polymer PUR-DRIg with index of refraction n = 1.650, sandwiched between cladding layers with n = 1.533. The layers are deposited on a quartz substrate plated with a gold film. Another gold film is deposited on top of the polymeric guide structure. The gold electrodes on one arm are connected to a variable dc bias source; the electrodes on the other arm of the Mach–Zehnder interferometer are connected to standard coaxial microwave connectors. In the absence of a microwave signal, the dc bias can be adjusted so that the electro-optic effect in the poled polymer guide produces a π-phase shift and therefore destructive interference at the output. The output can be switched by a microwave pulse along the other arm. Optical connections to the polymeric guides are made by tapers that couple into standard optical fiber connectors. This optical modulator has a flat response to 60 GHz, a long lifetime, and acceptable low power consumption. It can transmit 80 CATV channels with a 53 dB carrier-to-noise ratio. It may find a large commercial market in optical fiber communications. Organic nonlinear optics devices may yet economically justify the large R&D investments of the past two decades. The large values of both real and imaginary parts of χ(3) in several organic materials have stimulated a search for practical devices based on the χ(3) nonlinearity. Two-photon absorption and enhanced excited-state absorption may be
Journal of Chemical Education • Vol. 75 No. 5 May 1998 • JChemEd.chem.wisc.edu
Waters Symposium: Lasers in Chemistry
used in optical limiters, which could protect sensor and eye retinas from damage (18). Limiting action may also be achieved by nonabsorbing structures. Total reflection in an optical prism could be frustrated if the index of an adjacent fluent layer is increased at high intensity. The larger value of the intensity dependent index of refraction n2I could produce an intensity-dependent phase shift π over a distance l, provided that the absorption losses over this length remain small, (∂1 + ∂2 I )l
