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technique are not only limited to its ultrasensitivity but Thermal Lens Spectroscopy also include other unique characteristics including small- volume sample capability and dependency on thermo- Mladen Franko optical properties of solvents. In this overview, the theory of the technique is initially described. The main focus is, Laboratory of Environmental Research, University however, on the instrumentation and applications based of Nova Gorica, Nova Gorica, Slovenia on unique characteristics of the technique. Specifically, the discussion begins with a description of different types of Chieu D. Tran thermal lens apparatuses (e.g. single-beam and double- Department of Chemistry, Marquette University, beam instruments, differential, multiwavelength, thermal Milwaukee, WI, USA lens-circular dichroism instruments, and thermal lens microscope). A detailed description of various applica- tions including applications based on its ultratrasensitivity 1 Introduction 1 (e.g. applications in environment, agriculture and food 2 Theory 2 science, biochemistry and biomedicine, measurements in the near- and middle-infrared region, and kinetic 3 Instrumentation 4 determination), applications based on its small-volume 3.1 Single-beam Instruments 4 capability (microfluidic devices, detection for capillary 3.2 Dual-beam Instruments 5 electrophoresis), and applications based on exploitation of 3.3 Differential Thermal Lens Instruments 7 its dependency on thermal physical properties of solvents 3.4 Multiwavelength and Tunable Thermal to either determine physical properties of the solvent or Lens Spectrometers 8 to further enhance the sensitivity of the technique follows. 3.5 Circular Dichroism TLS Instruments 9 Finally, the future of the technique is forecasted. 3.6 Miniaturization of Thermal Lens Instruments 9 4 Unique Properties and Capabilities 1 INTRODUCTION of Thermal Lens Techniques 10 4.1 High Sensitivity: Trace Chemical The availability of lasers makes it possible to observe Characterization 10 and measure a variety of phenomena, which would not 4.2 Applications Based on Small-volume otherwise be feasible using other light sources. One such Characteristic of Thermal Lens phenomenon is the photothermal effect.(1) In the case Techniques 16 of the thermal lens,(1–10) which is a type of photother- 4.3 Determination of Thermal and Physical mal effect, a sample is excited by a laser beam, which Properties of Solvents 20 has a symmetrical intensity distribution (TEMoo). The 4.4 Dependency of Thermal Lens Signal on nonradiative relaxation releases the absorbed energy in Thermo-Optical Properties of Sample the form of heat. The heat generated is strongest at the Matrix and Its Utilization to Enhance center of the beam because the beam intensity is strongest Sensitivity 22 at that point. Consequently, a lenslike optical element is 5 Conclusions 25 formed in the sample owing to the temperature gradient between the center of the beam and the bulk material. Abbreviations and Acronyms 25 The thermal lens effect is observable for laser beams Related Articles 26 in the power range of only microwatts in samples normally References 26 thought to be transparent, and is thus suitable for the low- absorption measurement of nonfluorescent samples. Its sensitivity is relatively higher than that of the conventional transmission or reflection measurements because, in this The thermal lens technique is based on measurement of technique, the absorbed energy is measured directly. the temperature rise that is produced in an illuminated The first measurement of the thermal lens effect sample as a result of nonradiative relaxation of the energy was performed by Gordon et al. in 1965 using a absorbed from a laser. Because the technique is based simple single-beam apparatus.(1) Theories have since on direct measurement of the absorbed optical energy, been derived to explain the effect and to facilitate its its sensitivity is higher than conventional absorption applications to chemical analyses and characterization. techniques. However, advantages of the thermal lens Experimentally, recent advances in optics, electronics, Encyclopedia of Analytical Chemistry R.A. Meyers (Ed.) Copyright 2010 John Wiley & Sons Ltd 2 ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPY quantum electronics, and material science have been small-volume capability (microfluidic devices, capillary synergistically exploited to develop novel instruments electrophoresis), its use for the determination of thermal that have lower background noise, higher sensitivity physical properties, and methods based on its dependency and selectivity, and wider applications (by expanding its on thermo-optical properties of sample matrix to enhance measurement capability from the conventional visible its sensitivity. Finally, possible future aspects of the region to the ultraviolet, near-infrared, and infrared technique are also discussed. regions). The unique characteristics of lasers, namely Finally, it is important to emphasize that this article low-beam divergence, pure polarization, high spectral is restricted to the thermal lens technique, which is and spatial resolution, and its ability to be focussed to a one of the many photothermal techniques. Others include diffraction-limited spot, have been fully exploited to use photothermal beam deflection, photoacoustic spec- the thermal lens as a detection technique for microfluidic troscopy, photothermal interferometry, and photopyro- devices. As a consequence of these developments, the electric techniques. Reviews on these other techniques thermal lens technique has been established as a highly may be found elsewhere in the literature.(2–4,6) sensitive technique for trace chemical characterization, including single molecule detection. In addition to its ultrasensitive and small-volume capabilities, the thermal lens technique has other features 2 THEORY that cannot be rivalled by other techniques namely, the thermal lens signal is dependent not only on sample It is generally accepted that the thermal lens effect orig- concentration and excitation laser power but also on inates from nonradiative relaxation of excited species the position and thermophysical properties of the sample. in a sample irradiated by a laser beam with a Gaussian These unique features have been exploited either to intensity profile. During such radiationless relaxation further increase its sensitivity or to use it for sensitive and processes, which include vibrational relaxation, inter- accurate determination of thermal physical properties of system crossing, and external conversion, the absorbed a variety of substances including solids, liquids, and gases. energy from the excitation light is converted to heat. As a Owing to its advantages and unique features, the result, the temperature of the irradiated sample changes thermal lens technique has been extensively revie- and a refractive index gradient is formed in the sample, wed.(2–10) However, most of these reviews are restricted resulting in a change in the laser beam radius, and a to relatively narrow topics and/or applications (e.g. instru- related change in the intensity of the beam on its axis. The mentation, applications in separations, agriculture, envi- magnitude of the thermal lens effect is proportional to the ronmental, and microfluidics) or are a part of a broader amount of heat generated in the irradiated sample, and is topic such as photothermal techniques, which include not consequently dependent on the power P of the excitation only the thermal lens techniques but also photothermal light, absorbance A of the sample, as well as on the fluo- deflection, photopyroelectric, photothermal interferom- rescence efficiency of absorbing species, which decreases etry, as well as photoacoustic spectroscopy. the thermal lens effect. Thermo-optical properties of the This review is written as a self-contained, stand- medium (i.e. temperature coefficient of refractive index – alone article on the thermal lens technique. It is not ∂n/∂T and thermal diffusivity D k/ρc , where k, ρ, = p only intended for analytical spectroscopists who are and cp represent thermal conductivity, density, and heat knowledgeable in this field but also for readers who capacity, respectively) also have an important impact on are not specialists on this topic. To accomplish this goal, the magnitude of the thermal lens effect. the first part of this review is a theoretical treatment of Different theoretical descriptions of the thermal lens the technique to provide a fundamental understanding effect under a variety of experimental conditions can of this approach as well as to enable readers to follow be found in the literature and have been reviewed discussion in the subsequent chapter on instrumentation. extensively in several books and review articles.(6–8,11–14) Various types of thermal lens apparatuses are discussed These models cover thermal lens effects generated in detail in chronological order, commencing with under pulsed and continuous wave (CW) excitation, the historical single-beam instrument developed by and under different pump/probe geometries, i.e. single Gordon back in 1965, and ending with microfludics and dual beams as well as collinear and crossed-beam and near-field devices with multiwavelength capability. configurations. Furthermore, steady and flowing sample Subsequent sections are devoted to applications based conditions have been considered. on unique features and capabilities of the thermal lens For illustration, the thermal lens theory for a collinear technique,

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