J. A. Armstrong

MEASUREMENT OF PICOSECOND LASER PULSE WIDTHS

The special symmetry properties of second‐harmonic generation at the surface of a GaAs crystal are used in a technique which measures the shape of the fast pulses from a mode‐locked Nd‐glass laser. The pulses studied were found to have a full width at half power of between 4 and 6 picoseconds. The technique is capable of measuring pulse widths at least as short as 4 × 10−13 sec.

Theory of Interferometric Analysis of Laser Phase Noise

The line width of a well-stabilized laser operating far above threshold is determined by random fluctuations of the phase. This paper discusses several types of experiments which can give information about the details of this phase random process. In order to study the laser phase noise experimentally the laser signal (containing phase noise only) must be passed through some type of interferometer which will convert the phase noise to intensity noise. The various properties of this derived intensity noise which may then be determined are its probability density, first and second moments, autocorrelation function, and spectrum. These measurable quantities depend on two factors; the first and more fundamental is the joint probability distribution for the change in phase in a given time. The second factor is the manner of operation of the interferometer in changing phase to intensity noise. We discuss both two-beam and multiple-beam interferometers and derive theoretical expressions for the above-mentioned properties of the output intensity fluctuations. It is interesting that although in both cases the output intensity fluctuations are nongaussian random processes, it is nevertheless possible to derive a number of useful theoretical results.

Bound, odd-parity J = 1 spectra of the alkaline earths: Ca, Sr, and Ba

We have used multiphoton excitation via selected intermediate states to observe and identify triplet Rydberg series of the type msnp3P0 up to high n in Ca, Sr, and Ba. Previous efforts have failed to identify these series beyond n = 7. We present details of our experimental method, give tables of the newly determined energy levels, and describe their analysis by multi-channel quantum defect theory (MQDT). For the most part, the msnp3P0 series in Ca and Sr can be described by a nearly constant quantum defect δ with δCa = 19.7 and δSr = 2.90. The 6s n p3P0 series in Ba is more heavily affected by the configuration interaction, but to a reasonable approximation has δBa ≈ 3.81. For all three elements, we find that the msnp¹P10 series are much more strongly perturbed than the ³P10 series. Most of this paper treats Ba. In addition to the 3P0 states, 3F2 and 1G4 states are measured and tabulated. Further results in Ba include new and/or revised values for some ¹P10 states a revised ionization limit, evidence for an energy-dependent configuration interaction, and an MQDT prediction of the photoabsorption cross section at the ionization limit. This prediction agrees with the most recent experimental cross section. The MQDT analysis of Ba is described in great detail to make it useful as a guide to the reader who wishes to become a user of MQDT.

Saturable Optical Absorption in Phthalocyanine Dyes

Strong optical absorption lines have been almost totally saturated in two phthalocyanine molecules. A giant pulse ruby laser served as the source of intense resonance radiation. The qualitative behavior of the nonlinear absorption is in good agreement with theory.

RESONANTLY ENHANCED, NONLINEAR GENERATION OF TUNABLE, COHERENT, VACUUM ULTRAVIOLET (VUV) LIGHT IN ATOMIC VAPORS*

As is well known, atomic vapors, by virtue of their centrosymmetry, do not display quadratic optical nonlinearities. Thus, for example, second harmonic generation is ruled out in these media, provided there are no externally applied fields that destroy the symmetry. Third harmonic generation (THG) in vapors is allowed on the basis of symmetry. We briefly note that the earliest experiments were carried out with noble gas atomic vapors, and that the experiments themselves consisted in measuring the so-called hyperpolarizabilities.s2 These nonlinearities were weak compared t o those in crystalline solids by virtue of the much lower densities of the vapors and the nonresonant nature of the nonlinearities. The lowest lying excited states of noble gas atoms were far above the photon energies involved in these third harmonic generation experiments. The r e ~ e a r c h e r s ~ ~ a t Stanford University who pioneered the use of THG in gases as a practical source of coherent ultraviolet light studied atomic systems other than the noble gases-specifically, the alkali metals-using fixed-frequency input beams. They obtained for the first time practically useful third harmonic conversion efficiencies and showed generally that the large nonlinearities were due t o resonance enhancements. The advantage gained in using metal vapors stems from the fact of their generally low first ionization potentials. Consequently, various bound excited states lie in an energy range where resonance enhancement is possible with the visible input, or VUV third harmonic output beams. The work of Harris et al .3-5 stimulated the interest of various other groups,6-8 each of which then independently focused on the further advantages t o be gained in attaining exact two-photon resonance. These advantages are based upon the lack of linear absorption or dispersion associated with a twophoton resonance. Our own approach, which is characterized experimentally by the use of nitrogen-laser-pumped dye lasers t o provide the input beams, is perhaps best suited t o take advantage of two-photon and other resonance enhancements. It also enables the VUV output t o be tuned. Technical aspects of the dye lasers we employ are adequately described in Reference 9. In the present paper the emphasis will be on physical factors determining the relative VUV output in the various tuning ranges. Before actually discussing VUV generation, however, we shall describe at some length a multiphoton ionization experiment that graphically illustrates the importance of two-photon resonance in nonlinear phenomena in gases and also serves t o acquaint the reader with the arrange-

X‐RAY DIFFRACTION FROM PIEZOELECTRICALLY AMPLIFIED SHEAR WAVES IN THE 50‐GHz RANGE

X‐ray diffraction from piezoelectrically amplified acoustic shear waves in GaAs is reported. The spectrum of amplified acoustic flux is obtained in the microscopic limit, where the electron mean free path is much greater than the acoustic wavelength. The peak acoustic intensity occurs at 50 GHz, which corresponds to the frequency of maximum net gain according to the linear theory. The spectrum of piezoelectrically amplified acoustic waves is observed at frequencies an order of magnitude larger than may be studied by the previously reported Brillouin scattering technique.

Spectroscopy of 3 P 0 states of alkaline earths

Using multiphoton-ionization spectroscopy, we have observed transitions from the 1S0 ground states to the 4snp, 5snp, and 6snp3P0 states of the alkaline earth atoms Ca, Sr, and Ba, respectively. One-photon transitions from a 1S0 ground state to 3P0 states are spin-forbidden. We have identified these series in Ca and Sr up to n > 70, whereas previous efforts have failed to identify these series beyond n = 7. These series behave like nearly constant quantum-defect (δ) series with δCa = 1.97 and δSr = 2.90. Preliminary results for Ba, up to n = 60, indicate greater complexity.

An Industry Perspective on the Changing University

The globalization of high technology business is changing the industry/university interaction. Five important consequences are identified. First, the responsibility of U.S. industry for university support and the role of foreign-based corporations are not clear and require thoughtful discussion by those in government, industry, and academia. Second, universities should assess whether their role in technology transfer and national competitiveness has been correctly stated. Third, the university view of intellectual property rights and protection needs alteration, in many cases, so as to avoid impeding technology transfer and cooperative research. Fourth, many universities insist on a double standard of conduct when economic matters are at stake. Conflicts of interest arise as faculty accept public funding to invent and then involve themselves in enterprises that exploit these inventions in the marketplace. Fifth, given the unprecedented reliance of U.S. industry on foreign-born scientific and engineering talent, consideration should be given to enhancing the university graduate curricula to help prepare foreign students for managerial leadership.

Multiphoton Ionization Spectroscopy of the Alkaline Earths

Many aspects of the electronic structure of many-electron atoms are still not well understood. In many-electron atoms, the Coulomb repulsion and spin-orbit coupling effects between electrons strongly influence the nature of the electronic states. The one-electron picture, where each electron is viewed as moving in a well-defined orbit, independent of the motion of the other electrons, breaks down when these effects are strong enough. This has hindered efforts to identify and classify the states of many-electron atoms with one-electron labels. We have shown how multiphoton ionization spectroscopy provides us with new spectroscopic data allowing us to identify previously unknown states of alkaline earth atoms and how the Coulomb repulsion of the two outer electrons may be taken into account via a model based on multichannel quantum defect theory (MQDT).

University-industry relations: what do we do now?

I propose to discuss three topics that bear on the relations between industrial research labs and research universities. There are powerful forces of change at work on both of these parties; how we think about these changes, and how we react to them, will have a definite effect on the future course of university-industry relations.