LASERS & OPTICS 151 many cases non-destructively. A variety of beam manipulation schemes are avail­ able for the irradiation of samples either directly or after preparation. The unique properties of laser radiation in terms of coherence, intensity and directionality permit remote chemical sensing, where the analytical equipment and the sample are separated by large distances, even of several kilometres. Absorption, and in particular differential absorption, can be utilised in long-path measurements, whereas elastic and inelastic backscatter- ing as well as fluorescence can be used for range-resolved radar-like measure­ ments (LIDAR: light detection and rang­ ing). Laser light can also be efficiently transported in optical fibres to remotely located measurement sites. Various prop­ erties of the fibre itself influence the laser light propagating through the fibre, thus Applications of Laser forming a basis for fibre optic sensors. Applications of laser spectroscopy constitute a vast field, which is difficult to Spectroscopy cover comprehensively in a review. Rather than attempting such a review, examples from a variety of fields are cho­ Costas Fotakis sen for illustrating the power of applied University of Crete, Greece laser spectroscopy. Sune Svanberg Lund Institute of Technology, Sweden Applications in Analytical Chemistry Laser spectroscopy is making a major impact in many traditional fields of ana­ As the new millennium approaches the Above The Italian research vessel Urania, carrying a lytical spectroscopy. For instance, opto- know how in the field of laser-matter Swedish laser radar system, which has sailed under the galvanic spectroscopy on analytical smoky plumes of Sicilian volcanos in Italy measuring the interactions has matured to the stage of sulphur dioxide content flames increases the sensitivity of enabling several exciting real life applica­ absorption and emission flame spec­ tions. Several of these applications rely In recent years, laser sources have troscopy. Extremely sensitive direct on the narrow bandwidth of tuneable become more rugged, easy-to-use and absorption measurements with CW lasers lasers, which allows extreme selectivity in cheaper to allow real world applications can be performed using frequency-mod­ the interaction with free atoms and mole­ of laser spectroscopy. Semiconductor ulation spectroscopy with a t:107 absorp­ cules. In other applications, the high diode lasers have become very reliable tion sensitivity. A high sensitivity in spectral intensity available with pulsed as and easily accessible, and their wave­ absorption can also be achieved in intra­ well as continuous wave (CW) lasers may length range is extended higher and cavity experiments. saturate optical transitions or induce a higher up into the visible region. We can Traces of material can be detected variety of nonlinear effects, as for exam­ mention the emerging blue diode lasers using the REMPI (Resonance Enhanced ple multiphoton ionisation, thus provid­ based on gallium phosphide, and fre­ Multi-Photon Ionisation) or RIMS ing the basis for highly sensitive detec­ quency conversion of the output of reli­ (Resonance Ionisation Mass Spectros­ tion schemes. In fact, single-atom or mol­ able, high-power diode lasers into new copy) techniques, both employing pulsed, ecule detection is widely used, represent­ wavelength regions. New laser materials, tuneable lasers. In the first method, selec­ ing the ultimate sensitivity in analytical such as titanium-doped sapphire, allow tively produced ions of the element are chemistry. In a different trend, ultrafast all-solid-state tuneable systems with con­ detected through the ion current, where­ lasers have allowed detailed temporal siderable power to be constructed. as the second method employs a mass monitoring of chemical reaction dynam­ Diode-laser pumping of solid-state mate­ spectrometer for additional selectivity. ics at femtosecond timescales (femto- rials is making flash-lamp pumping grad­ The breadth of standard analytical- chemistry). In this way, significant insight ually obsolete and enables compact, reli­ chemistry techniques can be further into the basic steps of chemical processes able and energy-efficient laser sources, improved through laser spectroscopy, has been gained. Finally, in the form of especially when combined with current employed for enhanced detection sensi­ holographic techniques the coherence frequency conversion schemes (eg optical tivity and multiplexing. Thus, laser- properties of laser light are the basis of parametric amplifiers). induced fluorescence (LIF) can be ultrasensitive structural diagnostics Laser-based analysis and diagnostics utilised for detecting the separated frac­ applications. can readily be performed in situ and in tions passing the detector position in 1998 July/August News Europhysics 152 LASERS & OPTICS Fig 1 Left The Italian volcano Stromboli, seen from the research vessel Urania Below The volcanic plume of Mt Etna as recorded by laser radar techniques in September1994 during a ship-borne traverse under the plume, integrated vertical and horizontal profiles of sulphur dioxide are shown.The total flux was 44 tonnes/hour HPLC (High Performance Liquid Combustion Processes package by a laser pulse (repumping or Chromatography) and Capillary Combustion remains the most important dissociating the molecules) and then Electrophoresis. Further, a whole section means of energy conversion. Laser spec­ identify the package position at a later of the column can be illuminated and the troscopy provides a very powerful, non- time using a second laser pulse. fluorescence along the column can be intrusive measurement capability for the CARS (Coherent Anti-Stokes Raman imaged on a linear array or a CCD detec­ reactive and aggressive media constituted Scattering) is a powerful technique, espe­ tor, to provide simultaneous detection of by burning or exploding combustion cially for temperature measurements in all species undergoing separation. gases. A large number of techniques has strongly luminous media. A coherent, Differences in fluorescence spectra can provided new possibilities to measure the laser-like beam is obtained carrying the also be utilised for further discrimina­ concentration of species including short­ signal, making it largely immune to the tion. Fluorescence labelling can be used, lived radicals, and also the capability of background light. By single-shot CARS detecting simultaneously chromophores determining temperature and flow speed. recordings, PDFs (Probability Density with different fluorescence characteristics The measurements have allowed a realis­ Functions) for temperature can be deter­ and binding to different positions in the tic interplay with the advanced kinetic mined, yielding important information DNA strand. These developments are computer codes that are used for model­ on statistical fluctuations in turbulent very important, for example for the ling combustion. Important issues in combustion. Realistic measurements on Human Genome Project. combustion include NOx and soot forma­ test engines, jet engines and even coal- Laser ablation which occurs upon the tion, engine ignition and engine knock. fired power plants have been performed. interaction of intense laser radiation with LIF has been extensively used, also in Polarisation spectroscopy and DFWM solid targets has provided a number of imaging configurations, to monitor the (Degenerate Four-Wave Mixing) spec­ novel analytical schemes in combination distribution of molecules such as OH, C2, troscopy are further powerful techniques with mass spectrometric or optical detec­ CH and CN. Using a gated and image- in combustion diagnostics. tion. Along these lines, there are several intensified CCD detector the full distrib­ The methods developed for combus­ industrial applications, in which on-line ution can be captured using a single laser tion diagnostics are also applicable for monitoring may occur by laser-induced shot ( = 10-8s). To this end, a sheet of light the monitoring of other reactive media breakdown spectroscopy (LIBS). This tech­ is formed through the burning medium such as in plasma etching or MOCVD nique exploits changes in the emission using cylindrical optics. In a similar way, (Metal Organic Chemical Vapour spectra of laser-produced plasmas depend­ recordings using transitions starting in Deposition) for semiconductor process­ ing on the stage and quality of processing. states with a temperature-dependent ing. In a relevant recent application LIBS has population can obtain temperature dis­ been applied on polymer samples for the tributions. Flow velocities are evaluated Monitoring of the Atmosphere identification and separation of polymeric using Doppler shifts in the recorded sig­ The atmosphere can be monitored by materials for recycling purposes. nals. A further method is to mark a gas laser techniques employing absorption or Fig 2 Fluorescence imaging of the northern gate of the Lund Cathedral. Images were recorded through pass- bands centred at 438 and 682 nanometers using a scanning laser radar system positioned about 50 meters from the gate. A normal colour photo­ graph of the gate Is Europhysics News July/August 1998 July/August News Europhysics shown centre LESERS & OPTICS153 Fig 3 Above A restored area on a painting is differentiated with respect to the original and analysed by using quantitative fluorescence multispectral imaging— in visible light no difference is observed. The characterization of the image relies on a narrow