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 Biomedical Technology Resource 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 11 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. We have uncovered the key anharmonic resonance that "lights up" the most important Franck-Condon "dark states."
Professor Field, Dr. Adya Mishra, and associates have developed a suite of complementary spectroscopies (surface electron ejection by laser excited metastables and UV-laser induced fluorescence) and statistical pattern recognition schemes by which the detailed mechanism of intersystem crossing in small polyatomic molecules can be characterized. We have demonstrated that the dynamic range of our complementary spectroscopic techniques (UV-LIF and SEELEM) is sufficient to detect all vibrational levels of the excited singlet and triplet electronic states. The surprising result is that strong anharmonic and Coriolis mixing among the triplet states spreads transition intensity uniformly among all vibrational levels, yet the energy levels exhibit perturbation-free rotational structure.
Professor Field, Dr. Daniel Byun, and associates 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. All components of the experiment have been tested.
Professors Field and Steinfeld, in collaboration with Dr. Stephen Coy, have extended the IntraCavity Laser Absorption Spectroscopy 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. Jean-Michel Caruge continue research that has uncovered transient photoluminescence from multiple-exciton excited states of quantum dots (QDs) on subnanosecond timescales. This is the first known observation of emission from these 2- and 3-exciton states. Using single QD spectroscopy, Bawendi's group demonstrated that the excited-state lifetime of single QDs varies as a function of time. They also showed the novel relationship between the fluorescence intermittency in single QDs and the luminescence behavior of ensemble QD samples.
Professors Bawendi, Rubner, Jensen, Marc Kastner, Raymond Ashoori, and Vladimir Bulovic have continued research on the physics of electron conductivity and electroluminescence QD solids. Recent work on photoconductivity has demonstrated orders-of-magnitude gains in photocurrent of close-packed QD films. The increased photocurrent has revealed a previously unseen bimolecular recombination process. Highly efficient electroluminescence from hybrid organic-QD LEDs was demonstrated as well. The work highlighted the numerous advantages that these hybrid LEDs have over current LEDs, and the work has also suggested a novel mechanism for device function.
Professor Tokmakoff and his colleagues are investigating the hydrogen bond dynamics of liquid water using femtosecond infrared spectroscopy and molecular dynamics simulations. Vibrational echo peak shift spectroscopy has been used to follow time-dependent frequency shifts of the hydroxyl (OH) stretching vibration of hydrogen-oxygen-deuterium in deuterium oxide (heavy water), which reveal the timescales for intermolecular motion and hydrogen bond breaking. A model for the OH vibrational frequency shifts with different hydrogen bonding conformations, which draws on molecular dynamics simulations, has been developed in collaboration with Dr. Phillip Geissler.
Professor Nelson and his colleagues have determined heat transport properties of low-temperature glasses in connection with work aimed at direct observation of the high-wave vector acoustic phonons that mediate thermal diffusion. This work addresses fundamental questions concerning energy flow in disordered materials. In related work, Professor Nelson and his group members have refined a novel interferometric method for characterization of thermal and acoustic properties of complex materials. Finally, Professor Nelson and NSF postdoctoral fellow Dr. Rebecca Slayton have established an outreach laboratory aimed at high school students with a strong interest in science. The students will make photoacoustic measurements on thin films used in microelectronics manufacturing and will learn about modern optics and spectroscopy, advanced materials, and optical metrology.
Professor Mildred Dresselhaus and Drs. Gene Dresselhaus and Joseph Gardecki, using multiple laser excitation energies, have studied the inner and outer layers of double-wall nanotube samples synthesized from the catalytic decomposition of methane. They also studied aligned multiwall nanotube samples synthesized using oxidative surface decomposition of SiC. In addition, to further understand the deviation of experimental results from the commonly used tight-binding model for single-wall nanotubes, Raman studies were carried out on small-diameter (0.8-1.1 nm) single-walled nanotubes synthesized from the high-pressure carbon monoxide conversion.
Professors Green and Field, in collaboration with H. H. Carstensen (Colorado School of Mines), measured the visible spectrum and kinetics of the cyclohexadienyl radical and explained the surprising observation that its reaction rate with oxygen is 200 times faster in solution phase than it is in the gas phase.
Professor Feld and Dr. Christopher Fang-Yen have investigated the photon statistics properties of the cavity QED microlaser. They used a novel high-throughput photon correlation device to demonstrate that the microlaser output is in general antibunched, a uniquely quantum mechanical feature. From this measurement it can be inferred that the photon number distribution inside the microlaser cavity is strongly sub-Poissonian.
Professor Feld and Drs. Dasari, Gardecki, George Angheloiu, Kamran Badizadegan (Children's Hospital, Boston), Christopher Fang-Yen, Martin Hunter, Gabriel Popescu, Jason Motz, and James Tunnell have conducted basic and applied spectroscopic studies in biology and medicine. A combination of spectroscopic techniques including fluorescence, reflectance, Raman scattering, elastic light scattering, and low-coherence interferometry are employed for histological and biochemical analysis of tissues, diagnosis and imaging of disease, and cell biology applications. Clinical studies employing trimodal spectroscopy were conducted with researchers from the Cleveland Clinic Foundation, the Medical University of South Carolina, Brigham and Women's Hospital, and the Boston Medical Center. Real-time diagnostic capability has been incorporated into the FastEEM, a clinical instrument that collects a fluorescence EEM and a diffuse reflectance spectrum via an optical fiber probe. Successful diagnosis of dysplasia in the Barrett's esophagus, the urinary bladder, adenomatous polyps, the oral cavity, and the uterine cervix has been demonstrated. Drs. Popescu and Fang-Yan and associates have developed novel low-coherence interferometry techniques to measure precise optical phase shifts and obtained semiquantitative phase imaging of cells using an actively stabilized Mach-Zender interferometer with nanometer-scale phase sensitivity. A unique optical-fiber Raman probe, designed and constructed at MIT's Laser Biomedical Research Center, has been used in clinical trials at Metrowest Hospital to identify atherosclerotic lesions in carotid and femoral arteries.Professor Feld and colleagues have used near-infrared Raman spectroscopy to measure blood glucose concentrations in 20 volunteers without a needle stick. They have also developed a diagnostic algorithm for detecting breast cancer, with an overall classification accuracy of 87 percent.
Additional information about the George R. Harrison Spectroscopy Laboratory can be found on the web at http://web.mit.edu/spectroscopy/.