A variety of experiments are underway in the Quantum Optics Group at Caltech which investigate the quantum nature of atom-field interactions at the level of individual atoms and quanta. A recent technical advance in support of this research is the observation of cooling and trapping of single neutral cesium atoms in magneto-optical trap. Discrete steps are recorded in the fluorescence signal from the trap and are associated with the arrival and departure of individual trapped atoms. Such a spatially localized sample of a single atom with small kinetic energy is an enabling advance for diverse studies in quantum optics, including the possibility of spectroscopy with squeezed and other forms of nonclassical light and cavity quantum electrodynamics with strong coupling of an atom to the field of an optical cavity.
We have studied the user of vapor-cell magneto-optical traps for the collection and investigation of trace elements of radioactive isotopes. The fundamental processes governing efficient collection of neutral isotopes in a trap have been identified and integrated into a model that accurately describes the collection process. The model has been verified experimentally; a 6% capture efficiency has been demonstrated for stable Cs isotopes and we predict a simple change in trapping cell geometry will allow a 50% capture efficiency. This paper discussed the issues involved in efficient capture, such as the trapping laser properties and wall-coatings. It describes a new cell design for efficient capture and discusses current work towards trapping small numbers of 221Fr (T1/2 equals 4.8 minutes) atoms.
Using sources of laser trapped radioactive atoms in beta-decay experiments could lead to more precise measurements of decay correlations that can be used to test the Standard Model of electroweak interactions. We discuss a recent demonstration of the feasibility of slowing and trapping significant numbers of accelerator produced short lived atoms.
Small samples of radioactive atoms can now be concentrated with magneto- optic laser traps into a very small space and cooled to microdegree temperatures. The first application of these new techniques to atoms with radioactive nuclei offers new possibilities for nuclear physics measurements. The general applicability of these techniques of minute quantities of other materials is discussed.
The use of an optical trap for the quantitative detection of small numbers of radioactive atoms is featured. Presently under construction is a system consisting of a high-efficiency magneto-optical trap (MOT) coupled to a mass separator. Mass-separated samples will be implanted into a thin platinum foil, subsequently released from the foil by heating, and resonantly trapped in the MOT. The number of atoms in the trap will be quantitatively deduced from the amplitude of the fluorescence signal. Measurements for different isotopes are planned by appropriately shifting the trapping and repumping laser frequencies of the optical trap. Our initial investigations will concentrate on the determination of trace amounts of 135Cs and 137Cs.
Focusing powerful laser pulses on a material produces microplasmas that vaporize and excite a small amount of the sample. By spectrally resolving the plasma emission, the elemental composition of the material can be determined. This method, termed laser-induced breakdown spectroscopy (LIBS), has many advantages that make it particularly suited for field-based monitoring. These include: simplicity, multielement detection capability, minimal sample preparation, and remote analysis capability. The remote elemental analysis capability of LIBS is unique compared to other conventional analysis methods. Remote analysis can be provided either by direct focusing of laser pulses on a distant object or by fiber optic delivery of the laser energy to the sample. To date, useful spectra of rock samples have been obtained by projecting the laser pulses out to a distance of 24 meters and collecting the plasma light with a simple lens system. Elements at major and minor concentrations were easily detected. Using fiber optic delivery of the laser pulses, LIBS spectra can be obtained from samples in relatively inaccessible locations (e.g. down a borehole, in a reactor). Laser pulses of 80 mJ at 10 Hz repetition rate have been used to remotely generate the laser plasmas and to collect the plasma light using a single fiber optic.
In this manuscript we review briefly the history of Resonant Laser Ablation (RLA), and discuss some current ideas regarding sample preparation, laser parameters, and mechanisms. We also discuss current applications including spectral analysis of trace components, depth profiling of thin films and multilayer structures, and the use of RLA with the Ion Trap Mass Spectrometer (ITMS).
An application of an ion trap mass spectrometer (ITMS) for real time chemical analysis of airborne microparticles is described. Fragmentation analysis of organic chemicals condensed on a microparticle's surface has been demonstrated using laser induced desorption ionization and MS/MS techniques with ITMS. An on-line atmospheric particle sampling system has been designed and interfaced to the ITMS. A dual particle detector system using optical fibers has been designed and timing electronics have been developed for the particle speed measurements required in order for the desorption laser to intercept the particles.
The chemistry at surfaces is playing an increasing role in understanding the fate of chemicals in the environment and in technologies that utilized surface coatings and thin films. Understanding the chemistry of the very top monolayer has been hampered by the difficulty in probing only this region (without penetrating into the bulk of the solid). We have developed two unique technologies that enable static secondary ion mass spectrometry to be used in this application. Pulsed extraction sample neutralization overcomes the sample charging problems commonly encountered with electrically nonconducting samples, and our molecular anion primary beam heightens sensitivity for molecular compounds. We are also combining these with ion trap mass spectrometry to improve sensitivity and permit multiple stages of mass spectrometry to be performed. This provides increased specificity and the ability to gain a more complete understanding of the chemical environment at the surface. As one example, we have shown that the oxidation/reduction chemistry of the surface of mineral samples can be characterized using probe molecules and static SIMS. The technique is also being applied to detection of trace levels of herbicides and pesticides, for detection and characterization of hazardous wastes, and as a possible method of air sampling via chemically selective surfaces.
A hot cell-based solid sampling system has been developed for application to the analyses of irradiated materials such as metallic reactor fuel from a nuclear reactor, and radioactive waste forms. This system employs one of two sampling techniques, glow discharge or laser ablation, for introduction of material into one of two instruments for analysis, either a time of flight mass spectrometer or an inductively coupled argon plasma-atomic emission spectrometer (ICP-AES). This paper will discuss using ICP-AES for the analysis of major, minor and trace constituents in an unirradiated U-10 wt % Zr metallic reactor fuel. The behavior of highly refractory elements, such as Zr, in the laser ablation process will be examined in detail. Additional corroborative data provided via scanning electron microscopy-energy dispersive x-ray fluorescence spectroscopy (SEM-EDS) will be presented.
The principles of the single stage and dual stage liquid crystal Fabry- Perot interferometer (FPI) are discussed. A dual stage liquid crystal FPI which has 11-12 cm-1 pass band and is electronically tunable is described. The filter is characterized as a component of a light microscope. Its application to Raman microscopy of materials and to fluorescence microscopy of nucleic acid dynamics is demonstrated with examples from the author's laboratory.
Ultrasensitive fluorescence spectroscopic imaging has been employed to study the interaction of adhesion promoting thin films on polymer substrates. We have employed microscopic and macroscopic imaging to characterize and monitor, respectively, films that have been doped with fluorescent contrast agents to enhance their detectability. The microscopic spectral imaging systems employ an electronically controllable liquid crystal tunable filter (LCTF) that has been integrated with imaging optics and multichannel detection to afford efficient visualization of tagged film/substrate interactions. The macroscopic imaging system is optimized to monitor thin film coverage uniformity.
The high spatial resolution and sensitivity of near-field fluorescence microscopy allows one to study spectroscopic and dynamical properties of individual molecules at room temperature. Time-resolved experiments which probe the dynamical behavior of single molecules is discussed. Ground rules for applying near-field spectroscopy and the effect of the aluminum coated near-field probe on spectroscopic measurements are presented.
Fluorescence excitation and vibrationally resolved dispersed fluorescence spectra of single molecules of terrylene in a polyethylene matrix at 1.5 K are presented. Single molecules are selected from the inhomogeneous ensemble by tuning a narrow-band dye laser into resonance with the sharp electronic origin features in highly dilute samples. Total fluorescence is detected with a photomultiplier and photon counting electronics, while vibrationally resolved fluorescence spectra are obtained by dispersing the emission onto a CCD detector. The excitation spectra reveal a variety of spectral diffusion effects on a wide range of time scales, including both continuous wandering over a range of frequencies and jumping among two or more discrete resonance frequencies. The dispersed fluorescence spectra show two distinct types of vibrational frequency and intensity patterns, possibly arising from terrylene molecules in the amorphous and crystalline regions of the polyethylene matrix, respectively. The description of the emission as resonance Raman or fluorescence is discussed.
Non-Photochemical Hole Burning, NPHB, is one of a number of phenomena universally observed in glasses, and termed anomalous because they are not observed in crystalline solids. All of these phenomena are explainable by postulating the existence of a set of nearly isoenergetic configurations of the atoms or molecules of the glass. These configurations, first proposed independently by Anderson eta!.' and by Phillips2, are modelled as a double well potential and are called two level systems, 115. If there is a broad distribution of the parameters characterizing these potentials, all of the anomalous properties are explainable in terms of tunneling and phonon-assisted tunneling between the potential minima. NPHB is unique among these anomalous glass properties in that it requires the presence of a dilute probe species in the glass. For such systems, electronic or vibrational transitions of the probe are inhomogeneously broadened and monochromatic excitation, at w, of these transitions can cause a loss of absorption at the excitation frequency. This is the hole burning phenomenon. The presence of the probe molecule can cause some TLS strongly coupled to the probe to differ from their intrinsic properties. Thus these TLS have been called TLSCx to indicate their interaction with the extrinsic probe species. The other TLS are refered to as TLSmt, with the subscript indicating their intrinsic nature. In a model of NPHB proposed by Small and Shu4, it is the rate of conversion of ThS that is the rate determining step for hole formation. For short bum times, however, the hole widths are determined by ThS. Thus NPHB can be used to probe both short range (TLS) and spatiallyaveraged (TLS) glass properties. In this paper, we report on the use of NPHB (kinetics and hole widths) to investigate structural details of various materials. We first present data on NPHB of the dye oxazine 720 in two different forms of glassy ethanol. Next, we show how NPHB is sensitive to the configurational relaxation that occurs in glassy water at temperatures between 100 K and the glass to crystal transition temperature. Finally, we present some preliminary data on NPHB of the DNA binding dye, TO-PRO-3.
The Laser Interferometer Gravitational-Wave Observatory (LIGO), currently under construction, will observe cosmic-gravitational waves as small apparent displacements induced between suspended test masses. Laser interferometers with arm lengths of 4 kilometers, that are capable of resolving 10-18 meter changes in armlength, are being developed for the observatory. A test-bed interferometer with 40 meter long arms has been constructed and is being used to develop detection techniques. Sensitivity to displacements of order 10-17 meter and strains of order 10-19 have already been demonstrated with this interferometer. This talk will discuss the technical challenges involved in this effort and review progress toward observatory and detector development.
Progress is reported on the development of a new technique for measurement of trace levels of radioisotopes which is based on fluorescence detection of output from a mass spectrometer. Significant achievements include the observation of fluorescence and burst signals from Kr isotopes, including enriched samples of 85Kr with a 4- collector system. An isotopic abundance sensitivity of about 10-8 is demonstrates with 83Kr and 85Kr.
Fluorescence detection of single molecules in room temperature liquids is usually plagued by many difficulties such as finite saturated absorption rate, finite laser-analyte interaction time, and analyte diffusion. Our approach to single molecule detection in liquids has been to use microdroplets as the sample medium which has many advantages over conventional 'bulk' techniques such as extended laser-analyte interaction time, a probe volume defined by the droplet, and modified radiative properties of molecules due to interactions with droplet resonances. We have demonstrated detection of single rhodamine molecules with signal-to-noise ratios on the order or 30 using levitated microdroplets as the sample medium. The focus of current work is on single molecule detection in falling droplet streams with analysis rates on the order of 5-10 kHz.
A new technique for the detection of trace concentrations of molecules in solution has been developed. This system utilizes the amplification characteristics of a bubble chamber in which energy deposition from laser absorption is monitored. In the experimental set-up, a trace quantity of solute is introduced into liquid propane that is contained in a small (10 ml) stainless steel cell at 120 psi. The propane is superheated by sudden reduction of the cell pressure. Before wall nucleated boiling occurs, target solute molecules are energized by a laser pulse. Absorption of pump laser energy results in the formation of nucleation centers which develop into bubbles and which in turn are detected by CCD camera. Preliminary experiments with crystal violet used as a test absorber have demonstrated detection sensitivity of 10 parts per trillion (ppt).
Resonant degenerate four-wave mixing is presented as an unusually sensitive nonlinear laser method that yields Doppler-free spectral resolution at trace-concentration levels even when using low laser power levels. Using a non-planar four-wave mixing optical setup, one can extract the signal beam from the input beams more easily, and hence, suppress the background noise more effectively and improve signal-to- noise ratios. Optical alignment is simple and convenient for this multiphoton setup. Sub-Doppler spectral resolution allows reliable measurement of many isotope and hyperfine lines using room-pressure flame atomizers, low-pressure discharge atomizers or room-pressure graphite furnace atomizers. While maintaining sub-Doppler spectral resolution, four-wave mixing still yields parts-per-trillion level detection sensitivity using these popular analytical atomizers. While flame atomizers offer convenient and fast sample introduction, low- pressure discharge atomizers offer better spectral resolution (i.e., sub-Doppler plus sub-Lorentzian), and graphite furnace atomizers yield lower atomizer background noise. Since laser power requirements are low (e.g., mW for CW lasers and nJ for pulsed lasers), many compact lasers (e.g., solid-state lasers) could be used in this simple yet sensitive nonlinear laser method.
We have used the inherent surface sensitivity of second harmonic generation to develop an instrument for nonlinear optical microscopy of surfaces and interfaces. We have demonstrated the use of several nonlinear optical responses for imaging thin films. The second harmonic response of a thin film of C60 has been used to image patterned films. Two photon absorption light induced fluorescence has been used to image patterned thin films of Rhodamine 6G. Applications of nonlinear optical microscopy include the imaging of charge injection and photoinduced charge transfer between layers in semiconductor heterojunction devices as well as across membranes in biological systems.