Development of a Multiwavelength Thermal Lens Spectrophotometer Based on an Acousto-Optic Tunable Filter as a Polychromator CHIEU D. TRAN,* RICARDO J. FURLAN, and JIAN LU Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233 Instrumentation development of a novel multiwavelength thermal lens spectrophotometer which has the capability of achieving truly multi- wavelength excitation is described. The spectrophotometer is based on a new concept by which the sample is excited by multiwavelength ex- citation beams simultaneously, not sequentially as in previously reported multiwavelength thermal lens apparatus. This was accomplished by use of the acousto-optic tunable filter (AOTF) as a polychromator. Specif- ically, four different rf signals were simultaneously applied to the filter to enable it to diffract incident multiline laser light into a beam which contained four different wavelengths. This multiwavelength beam was then used to excite the sample, and the corresponding thermal lens signal was measured by a He-Ne probe laser. Compared with other multiwave- length thermal lens instruments, this all-solid-state thermal lens spec- trophotometer has advantages that include its ability to simultaneously analyze multicomponent samples in microsecond times scale, without the need for any prior sample preparation. With this apparatus and with the use of a 12-mW multiwavelength excitation beam, the limit of de- tection for four-component (lanthanide ions) samples is estimated to be 10 -6 cm -~, which is similar to the detection limit obtained for one- component samples with the use of a single-wavelength system. Index Headings: Acousto-optic tunable filter; Laser; Multicomponent sample analysis; Thermal lens. INTRODUCTION Recent advances in electronics, quantum electronics, electro-optics, and acousto-optics have renewed interest in techniques which are based on the photothermal effect. The effect is based on the temperature increase that is produced in a sample by nonradiative relaxation of the energy absorbed from a laser. Because the temperature increase (i.e., the amount of heat generated) is directly proportional to the intensity of the excitation laser light, the effect has been successfully used to develop a variety of sensitive techniques including the thermal lens, pho- tothermal deflection, and photothermal refraction. 1-11 These techniques are so sensitive that they can be used for the determination of absorbances as low as 10-7. l-ll Due to their inherent ultra-sensitivity, recent activities in the field are centered mainly on enhancing the selec- tivity and improving the thermal physical properties of the sample medium. Selectivity enhancement has re- portedly been achieved by increasing the number of ex- citation wavelengths, i.e., the instrumental development of the multiwavelength thermal lens spectrophotome- ters. 7,1°,12 The most notable development is the instru- ment based on the acousto-optic tunable filter (AOTF).12 The AOTF is an electronically driven dispersive device Received 8 June 1993; revision received 6 August 1993. * Author to whom correspondence should be sent. which operates on the principle of acousto-optic inter- action in an anisotropic medium. 12-17 Incident white light is diffracted by the AOTF into a specific wavelength when a specific rf is applied to it. 12-17Additionally, the AOTF can also provide amplitude modulation of the diffracted monochromatic light as well as maintaining its intensity constant. 12,15 These advantages were synergistically ex- ploited to develop the AOTF-based multiwavelength thermal lens spectrophotometer to enable it to have such unique capabilities as being all solid state, fast-scanning, and wide tuning and having no moving parts. The in- strument has been successfully used for the simultaneous determination of multicomponent samples with a limit of detection of 1.0 x 10 -l° M. 12However, it is important to point out that, in this instrument, the AOTF was used as a fast-scanned electronic grating; that is, at any given time, only a single rf signal was applied to the filter, there- by providing a diffracted beam with only a single wave- length. Multiwavelength excitation was accomplished by changing the frequency of the applied rf, i.e., sequentially exciting the sample with a different wavelength. There was always a dark period between two consecutive (dif- ferent) excitation wavelengths to enable the sample to return to its original thermal state. 12Strictly speaking this is not a truly "simultaneous" but rather a sequential tech- nique, and while it has the other aforementioned advan- tages, it also suffers from limitations such as having a long measurement time. As a consequence, the instrument may not be suited for the measurement of short-lived samples such as photochemical and/or chemical transient species. It is possible to use the AOTF in a slightly different manner to provide truly simultaneous excitation. This possibility, as pointed out in our recent publications, 13,17 is based on the fact that several different rf signals can be simultaneously applied to the filter. The filter, in turn, will simultaneously diffract more than one wavelength. Each diffracted wavelength can be differentiated from the others by amplitude-modulation (AM);13.17that is, by AM of each applied rf at different AM frequency, each wave- length of the multiwavelength beam diffracted from the AOTF will be amplitude-modulated at a (corresponding) different frequency. 13,17 It is, therefore, possible to achieve truly simultaneous thermal lens measurement, because the thermal lens signal of a sample will be modulated at those AM frequencies when the sample is excited by this multiwavelength beam. Such a possibility prompted us to initiate this work, which seeks to explore the use of the AOTF in this novel configuration to develop a truly simultaneous multiwavelength thermal lens spectropho- tometer. Preliminary results on the instrumentation de- Volume 48, Number 1, 1994 0003-7028/94/4801-01o152.00/0 APPLIED SPECTROSCOPY 101 © 1994 Societyfor Applied Spectroscopy