Using an optical novelty filter to enhance contrast in photothermal refraction spectrometry

Stephen E. Bialkowski

Department of Chemistry and Biochemistry
Utah State University
Logan, UT 84322-0300
USA

Area 10, New instrumentation and methodology

An all-optical adaptive spatial filter is used for signal contrast enhancement and imaging in pulsed laser excited photothermal spectrometry. The apparatus is based on the use of a nonlinear optical element that serves to reject the space-time averaged probe laser light along an extraordinary ray path. The extraordinary ray path is formed through a dynamic refractive optical element recorded into the nonlinear medium. A 45^o poled BaTiO₃ crystal, operated in the optical beam-fanning limiter configuration, is used for this purpose due to the high photorefractive efficiency. This configuration redirects the Ar⁺ laser beam used to record a spatial rejection filter away from the normal ray path. The Ar⁺ laser is also used as a probe for pulsed laser excited photothermal refraction spectrometry. A matched, spatial rejection filter for the Ar⁺ laser beam is thus recorded and continuously updated. Recording time, and thus novelty response time, can be adjusted by changing the Ar⁺ laser irradiance.

When placed in the conjugate plane of a 2F optical correlator, the beam fanning limiter only passes novel spatial information. The 2F correlator is constructed by placing the object one focal length in front of the lens. In this case the spatial Fourier transform of the object if constructed one focal length beyond the lens. Novel information is that which changes on time scales less than that required to write the matched spatial rejection filter. Since phase changes in the object plane produce spatial changes in the conjugate, image plane, rapid phase shifts due to the pulsed laser excited photothermal effects are coupled out of the optical novel filter along the normal ray path.

To test this, a pulsed, nominal, 10.6 mm infrared carbon-dioxide laser is tuned to an absorption of a gas sample located in the object plane of the correlator. The absorbed infrared radiation rapidly heats the gas sample, changing the refractive index through the photothermal refraction effect. This, in turn, produces a phase shift in the Ar⁺ beam. The phase shift results in a change in the Ar⁺ laser beam profile at the BaTiO₃ limiter and the phase-shifted light is transmitted through the limiter along the normal ray path. The pulsed excitation source is focused in to the sample cell resulting in a beam waist radius, which is much smaller than that of the Ar⁺ probe laser. This serves three purposes. First, the small excitation beam produced a higher irradiance in the sample. Since the phase shift produced by the photothermal effect is proportional to the inverse fourth power of the beam radius, enhanced sensitivity to small sample absorbance results. Second, the small index perturbation produces a large change in the spatial dimension of the image at the optical novelty filter since the object and image are related by spatial Fourier transforms. This, in turn, also enhances sensitivity to analyte absorbance. Third, the relatively small excitation laser beam reduces pointing and mode noise due to the excitation laser. As a consequence, the photothermal refraction signals are easily reproducible, being adaptive to slow changes in environment, and robust to pointing and mode-stability noise in the excitation and probe lasers.

The beam fanning limiter method has been tested for pulsed laser excited photothermal refraction of gas phase analyte species. Photothermal signal contrasts ratio on the order of 10² have been obtained from samples with total absorbance of 10^-3 have been obtained using the 2F optical correlator apparatus. This constitutes a 5 order of magnitude signal to noise ratio enhancement over conventional absorption spectrophotometry. Though sensitive to phase shift, this novelty filter based interferometer is robust to environmental interference and yields understandable results. The most useful feature of beam fanning is that the average signal of the probe laser is effectively blocked. Monotonous (versus novel) signal rejection of 98% or better are easily obtained. When used for pulsed infrared laser excited photothermal spectrometry, small infrared optical absorption is measured by observing copious numbers of visible photons with almost no background. In addition, scattering due to imperfections in the salt widows does not contribute to the background due to the adaptive nature of the filter.

An image of the photothermal perturbation can be obtained when the BaTiO₃ optical limiter is placed in the conjugate plane of a 4F optical correlator. In this configuration, a lens placed one focal length beyond the optical beam-fanning limiter produces a second conjugate plane. This conjugate plane is again related to the object by spatial Fourier transforms. In addition, the two-lens system images the primary object, in this case the sample, one focal length beyond the second lens. Magnification of the primary object is obtained using different focal lengths for the two lenses. There is no reason why this concept could not be extended to photothermal microscopy.

The actual image produced by the optical novelty filter apparatus is that of the phase shift that occurs upon pulsed irradiation of the sample. The photothermal effect produces a space-dependent temperature change, resulting in a novel, space-dependent phase shift in the sample. The novel (both spatial and temporal) phase shift produces a real image in the second conjugate plane. The 4F optical correlator based optical novelty apparatus is thus an imaging interferometer. The subtle advantage to using optical novelty filtering is that the images are not subject to slow time scale phase shifts that are normally difficult to compensate for. At present, the images are those of photothermal perturbations produced in homogeneous samples. However, the apparatus may also be used to obtain images of heterogeneous (scattering) samples.

Stephen E. Bialkowski "Application of the BaTiO3 Beam Fanning Limiter as an Adaptive Spatial Filter for Signal Enhancement in Pulsed Laser Excited Photothermal Spectroscopy" Optics Letters 14 1020 1989

Shashi D. Kalaskar and Stephen E. Bialkowski "Comparison of BaTiO3 Optical Novelty Filter and Photothermal Lensing Configuration in Photothermal Experiments" Analytical Chemistry 64 1824 1992