George R. Harrison Spectroscopy Laboratory
The George Russell Harrison Spectroscopy Laboratory conducts research in modern optics and spectroscopy for the purpose of furthering fundamental knowledge of atoms and molecules and pursuing advanced engineering and biomedical applications. Professor Michael S. Feld is director; Professor Jeffrey I. Steinfeld and Dr. Ramachandra R. Dasari are associate directors. As an interdepartmental laboratory, the Spectroscopy Laboratory encourages participation and collaboration among researchers in various disciplines of science and engineering. Professors Steinfeld and Moungi G. Bawendi, Robert W. Field, Stephen J. Lippard, Keith A. Nelson, Andrei Tokmakoff of the MIT Chemistry Department, Professors Feld and Alexander Van Oudenaarden of the Physics Department, Professor William H. Green of the Chemical Engineering Department, and Dr. Dasari are core investigators.
The laboratory operates two laser resource facilities. The MIT Laser Biomedical Research Center (LBRC), a Biotechnology Resource Center of the National Institutes of Health, develops basic scientific understanding, new techniques and technology for advanced biomedical applications of lasers; core, collaborative and outside research are conducted. The National Science Foundation-supported MIT Laser Research Facility (LRF) provides resources for core research programs in the physical sciences for eleven MIT Chemistry, Physics and Chemical Engineering faculty. Information about the facilities of the LRF and the LBRC can be found in the Spectroscopy Laboratory Researcher's Guide.
Professor Field and collaborators Anthony Merer (University of British Columbia), Soji Tsuchiya and Nami Yamakita (Japan Women's University), John Stanton (University of Texas), and Fleming Crim and Sarah Henton (University of Wisconsin) have assembled a complete experimental picture of the normal modes, their anharmonic and Coriolis couplings, and the way the trans-cis isomerization barrier is encoded in the spectrum of the first electronically excited singlet state of acetylene. A wide variety of types of spectra were assigned and interpreted within a unified model, which is unique among all other four-atom and larger molecules. One goal of this project is to devise the optimal local-bender "pluck" by which the acetylene<->vinylidene transition state region on the electronic ground state potential surface can be systematically characterized: "transition state spectroscopy."
Professor Field, Dr. Adya Mishra and associates have developed a suite of complementary spectroscopies (surface electron ejection by laser excited metastables, UV-laser induced fluorescence, and photofragment IR-laser-induced fluorescence) and statistical pattern recognition schemes by which the detailed mechanism of intersystem crossing (ISC) in small polyatomic molecules can be characterized. When the ISC is "doorway mediated" a wide range of possibilities for controlling the early time dynamics exist. In a related experiment, a two-photon excitation scheme to produce metastable, electronically excited Hg atoms, which have been used by organic photochemists to populate triplet states of polyatomic molecules, is being developed.
Professor Field and associates, in collaboration with Andrei Tokmakoff (MIT), Dr. Merer, Christian Jungen (University of Paris, Sud), and Frederic Merkt (ETH, Zurich), have developed a one-color femtosecond pump/probe scheme to examine the mechanism of coupling between a molecular ion and an electron in a remote Rydberg orbital. The experiment consists of two ns laser excitation steps to prepare the "launch state", a pair of phase coherent 100 fs pulses to initiate and probe the dynamics, and a combination of pulsed-field ionization and near-infrared photoionization to measure the ratio of populations in the "launch" and "target" state energy regions.
Professors Field and Steinfeld, in collaboration with Dr. Stephen Coy, have extended the IntraCavity Laser Absorption Spectroscopy (ICLAS) instrument to carry out time-resolved measurements. This capability enables kinetic measurements to be carried out on transient weakly absorbing species such as atmospheric free radicals. Preliminary measurements on the HNO radical reacting with molecular oxygen in a discharge flow system were conducted.
Professor Bawendi and Dr. Hans-Jurgen Eisler, collaborating with a group at Los Alamos, have shown that nanocrystal quantum dots are capable of stimulated emission. Furthermore, in collaboration with Professor Henry Smith of the Department of Electrical Engineering and Computer Science, they demonstrated nanocrystal quantum dot lasing, using microcavity resonators. This achievement is a milestone in nanocrystal quantum dot research. Lasing in such systems had been discussed for the last decade, but this was the first demonstration.
Professors Bawendi, Rubner, Jensen, Marc Kastner, and Raymond Ashoori have been investigating the physics of electron conductivity and the effects of charge on nanocrystal quantum dot solids. They discovered a Coulomb glass behavior, resulting in a power law decay of the conductivity of these thin films. They also discovered that charging the dots can control the intensity of the photoluminescence of a film of dots. This discovery is consistent with a previous speculation that dots that contain one electron or one hole are prevented from emitting a photon because of a fast competing Auger process.
Professor Tokmakoff and his colleagues have installed the Spectroscopy Laboratory's new 30 fs titanium-sapphire amplifier, and constructed a mid-infrared optical parametric amplifier for this system. The mid-infrared pulses will be used to study the molecular dynamics of the hydrogen bond network of liquid water using two-dimensional infrared spectroscopy. Additionally, new methods for acquiring two-dimensional infrared spectra were developed, which apply the phase-cycling methods used in NMR to nonlinear spectroscopic techniques.
Professor Feld and Dr. Christopher Fang-Yen have investigated the many-atom behavior of the cavity QED microlaser. The microlaser was found to display second and third thresholds, which result from an oscillatory gain function and are analogous to first-order phase transitions of the cavity field. Lineshape asymmetries and hysteresis were observed and explained by means of a semiclassical theory including bichromatic interaction with two Doppler-shifted cavity fields.
Professor Nelson and Dr. Christ Glorieux, working collaboratively with Professor Michael D. Fayer and Dr. Gerald Hinze of Stanford University, have completed the development of a novel method for photoacoustic observation of coupling between flow and molecular orientational motion in viscoelastic fluids. In related work, Professor Nelson and his group members used the same photoacoustic measurement method to study mechanical and thermal transport properties of complex materials under high pressure, in diamond or sapphire anvil cells. The method permits direct determination of the equations of state of such materials, of particular interest where structural change including vitrification may occur at elevated pressures.
Professor Katrin Kneipp, together with Doctors Dasari, Harald Kneipp, Kamran Badizadegan, and Charles Boone, and Professor Feld have applied surface enhanced Raman scattering (SERS) inside living cells. Colloidal gold particles 60 nm in diameter were deposited inside cells as "SERS-active nanostructures," resulting in strongly enhanced Raman signals of the chemical constituents of the cells. The new spectroscopic method provides a tool for ultrasensitive and structurally selective detection of chemicals inside a cell, and for monitoring their intracellular distributions. This opens exciting opportunities for cell biology and biomedical studies.
Professor Feld and Doctors Dasari, Charles W. Boone, Annika Enejder, Joseph Gardecki, Irene Georgakoudi, Martin Hunter, and Adam Wax have pursued basic and applied applications of lasers and spectroscopy in biology and medicine at the LBRC. Fluorescence, reflectance, Raman, light scattering spectroscopy, and low coherence interferometry were used for histological and biochemical analysis of tissues, diagnosis and imaging of disease and cell biology applications. Clinical studies were conducted with researchers from the Cleveland Clinic Foundation, the Medical University of South Carolina, Brigham and Women's Hospital, Metrowest Hospital, Beth Israel/Deaconness Medical Center and Boston University Medical Center. Clinical studies using tri-modal spectroscopy, the combined application of intrinsic fluorescence, diffuse reflectance and light scattering spectroscopies, demonstrated successful diagnosis of dysplasia in Barrett's esophagus, the urinary bladder, adenomatous polyps, the oral cavity and the uterine cervix. Light-scattering spectroscopy was used to measure and image sub-cellular structures much smaller than an optical wavelength. Novel low coherence interferometry techniques used light at two harmonically-related wavelengths to measure optical phase. Exceedingly small refractive index and length changes, tomographically mapped, were used to study structure and dynamics of cellular organelles. Raman spectroscopy was used to measure blood analytes with clinical accuracy and identify morphology of breast lesions. This experimental and theoretical work is advancing new laser diagnostic technologies in the fields of medicine and cell biology.
More information about the Spectroscopy Laboratory can be found online at http://web.mit.edu/spectroscopy/.