The Laboratory operates two laser resource facilities. The MIT Laser Biomedical Research Center (LBRC), a Biotechnology Resource Center of the National Institutes of Health, has the charter to develop the basic scientific understanding and the new techniques and technology required for advanced clinical applications of lasers. Core, collaborative and outside research projects are conducted at the LBRC. The second facility, the National Science Foundation-supported MIT Laser Research Facility (LRF), provides resources for core research programs in the physical sciences for thirteen MIT Chemistry and Physics faculty members. Information about the equipment and facilities of the LRF and the LBRC can be found in the Researcher's Guide, published by the Laboratory.
Professors Field and Robert J. Silbey have successfully recorded dispersed fluorescence (DF) spectra of acetylene from Ì state intermediates. Comparison from different intermediate states facilitates assignment and thereby can lead to a better understanding of ground state dynamics. Prof. Field is also conducting experiments with Dr. Stephen Drucker to locate the lowest lying triplet states of acetylene. Preliminary results indicate significant infrared fluorescence following excitation of three quanta of the trans-bending mode in the S1 state. This is consistent with T2 Æ T1 emission subsequent to S1 ~> T2 intersystem crossing.
Professors Field and Steinfeld are establishing a free radical spectroscopy facility based on pulse-amplified FM laser spectroscopy, to develop a highly sensitive, zero background, linear absorption technique. The system consists of a CW single mode ring dye laser, pulse amplified by a long pulse (20 ns) injection-seeded Nd:YAG laser.
Professor Bawendi is using the picosecond apparatus for time-correlated photon counting and the SPEX Fluorolog to study the electronic properties of semiconductor quantum dots and heterostructures containing those dots. A new mechanism for electronic relaxation in these nanostructures has been suggested to explain the data.
Professor Lippard and his associates have extended their investigations of the methane monooxygenase protein system and related model compounds by stopped-flow spectroscopy. The reactivity of these species with various small molecules has been explored. In addition, resonance Raman spectroscopy has been used to study these reactions and to characterize a metal-metal bond supported by a calixarene ligand.
Professor Nelson and his associates are studying ultrafast dynamics and inter molecular interactions of the liquid phase. They focus on the behavior of low frequency vibrational (hindered rotational) modes. Using an ultrafast transient grating technique, they can observe changes in the relaxation dynamics of liquids such as formamide, carbon disulfide, and acetonitrile.
Professor Ali S. Argon is studying laser pulsed probing of mechanical properties of homogeneous and heterogeneous solids. One study measures the tensile strength of planar interfaces characteristic of those encountered in composite materials between a fiber and its coating using a short wave length, high amplitude tension pulse. A second project measures the elastic properties of microporous ceramics using a pulse-echo method of sound velocity measurement in thin samples.
Professor Wrighton and Dr. Christoph Weder have designed and synthesized a series of novel poly(2,5-dialkoxy-p-phenyleneethynylene)s with identical conjugated backbone but different supramolecular orientations in the solid state. This experiment demonstrates high photoluminescence quantum yields, making some of the new polymers prospective materials for applications in electroluminescent devices, the fabrication of which is currently in progress.
Professors Klavs F. Jensen and Bawendi continue their work on spectroscopic characterization of CdSe-ZnSe quantum dot composite films deposited by electrospray organomettalic chemical vapor deposition. Photoluminescence spectroscopy was employed in combination with other characterization techniques to optimize the new processing technology and assess the materials performance for optoelectronic applications.
Professors Michael Rubner and Bawendi have been studying the photoluminescence of thin films of polymers containing various novel light emitting materials. They find that light emitting diodes can be successfully fabricated from thin films of CdSe and suitable hole transporting polymers such as poly(vinyl carbarzoyl). Measurements reveal that the emission of conjugated polymers such as poly(p-phenylene vinylene) can be tuned over a 100 nm range by simply changing the local molecular environment of polymer via the use of polyions.
Professor Tannenbaum and Drs. Paul L. Skipper Dasari and V. Bhaskaran Kartha have developed a cryogenic laser fluorescence method for sub-femtomole quantitation of protein and DNA adducts of the carcinogen B(a)P. Concentrations of histone adducts of B(a)P from lymphocytes and lung tissues have been found to be in the range of 0.01 to 1 pmol/mg histone, in good agreement with expectations based on BPDE-DNA adducts and ratio of DNA-histone binding.
Professor Alexander Rich and Drs. Yang Wang and Dasari are conducting experiments using Raman microspectroscopy to investigate the sequence dependent DNA conformations in crystalline and solution states. The current focus of their research is to probe the detailed conformation of DNA tetra-helix, d(CCCC)4.
Professor H. Gobind Khorana and Drs. Cheng Zhang and David Farrens studied the biological photoreceptor, rhodopsin, which is a model system for a large family of G-protein coupled biological receptors. Their aim is to understand how chromophore isomerization is translated into conformational changes at the exterior of the rhodopsin protein. Spectroscopically labeled rhodopsin is used to detect motion in the helices after retinal isomerization.
Professors Tanaka and Feld and Drs. Kartha and Dasari are conducting studies to determine the microscopic environment of heteropolymer gels that can selectively bind and release molecules using spectroscopic methods including Raman, infrared, and fluorescence spectroscopy. The polymer can form a complex with the target and also can reversibly stretch and shrink in response to environmental change. When the gel is in the collapsed phase it absorbs the target molecules, but release it when the gel is swollen.
Professor Kleppner and coworkers have conducted scaled-energy spectroscopy on lithium in parallel electric and magnetic fields. They observed the bifurcations of classical orbits of an atomic electron as the system evolves from an electric field to a magnetic field dominated region. The classical counterpart displays a transition to chaotic ionization.
Professor Feld and Drs. Dasari and Kyungwon An and their colleagues have recently developed a single atom laser, the "microlaser", in which laser oscillation is generated by the interaction of a single two-level atom coupled to a single mode optical resonator. Experimental results are in good agreement with quantum stochastic simulations. They have also studied the weak-field spectral response of a single atom in a resonator with a strong atom-field coupling, and have demonstrated unexpected structures in its spectra, originating from the fluctuations of atom number and positions in the resonator.
Professor Ali Javan and Dr. Michael Otteson continue to explore metal-oxide-superconductor junctions for optical frequency-mixing. Demonstration of direct transfer from a near-IR frequency to a near-UV frequency is underway.
Professor Feld and Drs. Dasari, Ramasamy Manoharan, Wang, Lev Perelman and Irving Itzkan are pursuing basic and applied aspects of applications of lasers to biology and medicine. Fluorescence and near infrared Raman spectroscopy are being used for biochemical analysis of tissues and blood, and diagnosis of dysplasia, cancer, atherosclerosis and other diseases. Clinical studies are being pursued with researchers from the Cleveland Clinic Foundation, and Boston's Brigham and Women's Hospital, MetroWest Hospital and New England Medical Center in the colon, bladder, breast, and coronary and peripheral arteries. UV resonance Raman spectroscopy is being explored to characterize of dysplastic alteration in tissue. Photon migration using ultrashort light pulses is being used to image small objects (lesions) imbedded in turbid biological tissue; a single-ended technique for three-dimensional imaging using time-resolved fluorescence emission or Raman scattering is under development. Finally, the mechanism of pulsed laser ablation of soft and hard tissue has been shown to be thermoelastic in origin. The experimental and theoretical work being conducted in this program will be important in developing new laser diagnostic and therapeutic technologies in the field of medicine.
Michael S. Feld
MIT Reports to the President 1994-95