Ultrahigh-resolution, frequency-resolved resonance fluorescence imaging with a monoisotopic mercury atom cell A. A. Podshivalov, M. R. Shepard, O. I. Matveev, B. W. Smith, and J. D. Winefordner Citation: J. Appl. Phys. 86, 5337 (1999); doi: 10.1063/1.371529 View online: http://dx.doi.org/10.1063/1.371529 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v86/i10 Published by the AIP Publishing LLC.
Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
Downloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
JOURNAL OF APPLIED PHYSICS
VOLUME 86, NUMBER 10
15 NOVEMBER 1999
Ultrahigh-resolution, frequency-resolved resonance fluorescence imaging with a monoisotopic mercury atom cell A. A. Podshivalov,a) M. R. Shepard, O. I. Matveev, B. W. Smith, and J. D. Winefordnerb) Department of Chemistry, P.O. Box 117200, University of Florida, Gainesville, Florida 32611
共Received 14 June 1999; accepted for publication 11 August 1999兲 A novel method of ultrahigh-resolution, frequency-resolved imaging using atomic vapor cells is proposed. The method is based on the accurate measurement of the fluorescence signal intensity distribution along the absorption path length when the signal frequency is tuned to a wing of the atomic absorption line. Two-step resonance fluorescence of 202Hg vapor was used for one-dimensional imaging of the Hg resonance radiation at 253.7 nm. An imaging signal with a frequency difference of 500 MHz could be easily distinguished visually and even a frequency difference of 80 MHz could be detected after appropriate processing of the fluorescence imaging signal. Several other novel methods of one- and two-dimensional multifrequency imaging are discussed. © 1999 American Institute of Physics. 关S0021-8979共99兲00922-6兴
I. INTRODUCTION
It is worth emphasizing that frequency information can be acquired not only within the Doppler broadened absorption wings, but also within the naturally broadened atomic absorption line. A resolution of ⬍1 MHz is feasible if the appropriate geometry of a Doppler-free two-photon resonance fluorescence excitation experiment is chosen.14
Because of the simple design and superior figures of merit, ultranarrowband atomic vapor imaging optical filters and detectors have many potential applications. These include moving object detection, aerodynamic velocity field measurement, Raman signal imaging, and detection of ultrasonic field intensity distributions.1–11 For most of these applications, it is important to obtain very high spectral resolution, approaching the kilohertz region, without any loss of luminosity. Furthermore, there is a need to make measurements without loss of multifrequency information in the kilohertz to gigahertz region and with nanosecond time resolution. For example, one practical application is to measure wind velocity distributions from several centimeter/second to several hundred meter/second with a wide angle of view. Using traditional interferometric10 and heterodyne12 methods, which have poor luminosity-resolving power products,8 such measurements can only be performed by spatially scanning a laser beam. Scanning a laser beam is a time consuming operation especially if spectral resolution of ⬍1 MHz is needed. In this article, a simple, novel method is suggested to measure multifrequency one-dimensional 关and in principle, two-dimensional 共2D兲兴 imaging signals with potential submegahertz spectral resolution.13 The method is based on the measurement of atomic fluorescence signal along the path length of light absorbed by an atomic vapor. For every frequency of light detected, the length of this path and the fluorescence intensity distribution will depend upon how close the excitation frequency is to the center of the atomic absorption line. By measuring the fluorescence intensity distribution, the frequency in the region between the center and far wing of the absorption line can be unambiguously measured.
II. FREQUENCY-RESOLVED ONE-DIMENSIONAL FLUORESCENCE IMAGING WITH A 202Hg CELL
The energy level scheme and experimental setup for the Hg two-photon resonance imaging are shown in Figs. 1 and 2, respectively. Two excimer pumped dye lasers 共Scanmate, Lambda Physik, and Molectron DL14, Cooper Laser Corp., with intracavity etalon and second harmonic generator兲 were used in the experiments. An evacuated quartz 202Hg atomic vapor cell was illuminated by two counterpropagating resonance laser beams at wavelengths of 共fixed兲 435.8 nm ( 202Hg transition 6 3 P 1 ⫺7 3 S 1 ) and 共tunable兲 253.7 nm, across the 6 1 S 0 ⫺6 3 P 1 transition. The linewidth of the UV 253.7 nm laser radiation was 0.5 GHz. The fluorescence imaging signal at 546.0 nm 共transition 7 3 S 1 ⫺6 3 P 2 ), 0.5–5 mm in length, was observed 共under magnification兲 from the side of the mercury cell by a CCTV camera with interference filters 共see Fig. 2兲. Naturally occurring Hg is a mixture of seven odd and even isotopes. Therefore, the absorption line at 253.7 nm has a complicated structure 共25 GHz isotopic and hyperfine splitting兲. To eliminate the influence of other mercury isotopic lines and to observe frequency-resolved images, we used an evacuated quartz cell 共Opthos Instruments, Inc., Rockville, MD兲 filled with 96% 202Hg. The intensity and length of the fluorescence trace depended upon the frequency shift of the 253.7 nm radiation with respect to the center of the atomic absorption line. The greater the frequency shift, the less the degree of absorption of the atomic line and, therefore, the greater the length of the fluorescence trace which is detected. Resonance fluorescence images 共546.0 nm兲 for several frequencies of 1 are shown in Fig. 3共a兲. The corresponding intensity distributions along the 202
a兲
Also at: International Laser Center of Moscow State University; electronic mail:
[email protected] b兲 Author to whom correspondence should be addressed; electronic mail:
[email protected] 0021-8979/99/86(10)/5337/5/$15.00
5337
© 1999 American Institute of Physics
Downloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
5338
J. Appl. Phys., Vol. 86, No. 10, 15 November 1999
FIG. 1. Energy level scheme for Hg.
fluorescence traces are shown in Fig. 3共b兲. One can visually differentiate between images with a frequency difference of only 500 MHz. Similar images were obtained by using a second excitation scheme 共Figs. 1 and 2兲. Here, we detected the 579 nm resonance fluorescence when the 6 3 D 2 and 6 1 D 2 levels were excited. In this case, the 202Hg vapor cell was illuminated by two counter-propagating resonance laser beams at wavelength 共fixed兲 313.18 nm 共transition 6 3 P 1 ⫺6 1 D 2 ) or 关312.57 nm 共transition 6 3 P 1 ⫺6 3 D 2 )] and tunable across the 6 1 S 0 ⫺6 3 P 1 transition. The fluorescence imaging signal at 579 nm was observed in the same manner as the previous scheme, when 435.84 nm excitation was used in the second step. III. ONE-DIMENSIONAL IMAGING WITH 80 MHZ SPECTRAL RESOLUTION
To verify the ability of our method to detect an imaging signal with a small frequency difference, we used the experimental setup shown in Fig. 4. One laser with a 10 mm beam
FIG. 2. Experimental setup for resonance imaging measurements.
Podshivalov et al.
width 共at 313.18 nm兲 and two laser beams with diameters of 1.5 mm 共at 253.7 nm with an 80 MHz frequency difference兲 were directed into the mercury atomic vapor cell. The 80 MHz frequency difference between the two 253.7 nm beams was produced by an acousto-optic 共AO兲 deflector 共Model DLM-40-7-V, Andersen Laboratories, Inc., Bloomfield, CT兲 and a frequency doubler. The AO deflector was placed in the output beam of a dye laser at 507.4 nm. The deflector produced two, nearly equal intensity beams with a frequency difference of 40 MHz. This frequency shift was doubled to 80 MHz after passing both laser beams through a second harmonic generator 共BBO crystal, oo-e type兲. Two yellow 共579 nm兲 Hg resonance fluorescence images were then observed simultaneously with every laser pulse. The first image was produced by the nondeflected beam at 253.7 nm, while the second image was produced by a deflected portion of the beam at the Bragg angle. Tuning of the 253.7 nm line in the vicinity of the Hg absorption line was performed and Hg atomic fluorescence images were detected by the CCTV camera 共see Fig. 2兲. The fluorescence intensity distribution plots along the images was obtained using SigmaScan Pro software. From theoretical predictions, and as is shown experimentally, the slopes 共see regression lines in Fig. 5兲 from these traces differed depending on tuning of the laser radiation frequency. When the laser frequency is tuned to the left wing 共higher frequency兲, the absorption coefficient is larger for the positive-shifted-frequency beam, because its frequency is closer to the center of absorption line. Conversely, when the frequency is tuned to the right wing 共lower frequency兲, the absorption coefficient of the frequency-shifted beam is smaller. Of course, if the frequency is tuned to the center of absorption line, the absorption coefficients for both beams are almost equal. Theoretical calculations15 of the absorption coefficient (k theor) for a 500 MHz laser bandwidth and for a purely Doppler broadened absorption line, and the experimentally obtained values of k exp for these three cases are in good agreement; the theoretical values (k theor) were found to be 7.6 and 9.6 cm⫺1 for frequency shifts of 500 and 420 MHz from the center of the line, respectively. The theoretical value for the line center was 14.9 cm⫺1 for the 500 MHz laser bandwidth 共for an infinitely narrow laser beam, k theor at v O ⫺ v L ⫽0 is 24.1 cm⫺1兲. A small deviation was observed due to the difficulties of tuning the laser radiation exactly 0.5 GHz from the line center. From the experimental results, it can be concluded that a frequency difference of 80 MHz was detected with a precision of approximately 15%. The differences in measured absorption coefficients, for both unshifted (k exp) and shifted (k ⬘ exp) frequencies, are shown in Fig. 5. It is important to emphasize that the spectral resolution of 80 MHz in this case was obtained with a laser having a much broader linewidth 共500 MHz兲. We consider this to be an important advantage over existing methods, because an expensive, narrowband laser is not needed for ultrahigh resolution. It should be noted that this method can also be used with a resonance ionization image detector 共RIID兲.5
Downloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
J. Appl. Phys., Vol. 86, No. 10, 15 November 1999
Podshivalov et al.
5339
FIG. 3. 共a兲 Resonance fluorescence images for different frequency shifts 共⫺1.3–1.2 GHz兲 of the 253.7 nm radiation; the x axis is the horizontal length of the fluorescence image and the y axis is the vertical length of the fluorescence image. 共b兲 Fluorescence intensity vs path length, experimentally measured from the images shown in Fig. 3共a兲.
IV. TWO-DIMENSIONAL FREQUENCY RESOLUTION IMAGING
Based on the one-dimensional results, we are confident that 2D frequency resolved imaging can be performed. Assume that an image on the surface of the RFIM 共or RIID兲 is created by a lens with a small numerical aperture. 共Also, assume the plane of the second-step resonance laser radiation ( 2 ) has a thickness much less than the absorption length of the first-step resonance radiation.兲 Using spatial scanning of the 2 laser radiation from the input window, as shown in
FIG. 4. Experimental observation of frequency shift in 253.7 nm laser radiation.
Downloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
5340
J. Appl. Phys., Vol. 86, No. 10, 15 November 1999
Podshivalov et al.
FIG. 5. Fluorescence intensity vs laser frequency tuning in the vicinity of the atomic line: Top: Experimental fluorescence from laser beams at shifted 共80 MHz兲 and unshifted frequencies. Bottom: Fluorescence intensity in log scale 共䊉-unshifted frequency, 䉱-80 MHz shifted frequency兲 and the corresponding ⬘ . experimental absorption coefficients, k exp and k exp
Fig. 6, and measuring the image signal intensity versus depth of 2 position, one would observe a fluorescence 共or ionization兲 trace at each point in an image. In turn, multifrequency imaging information would be extracted in a similar fashion as in the 1D case. Thus, every point would bear information about the image intensity and the frequency shift. From these measurements, one could construct a complete 3D picture in X – Y – Z coordinates, where Z is the frequency shift. The frequency range of the imaging signal measurements is limited only by the range of the descending or ascending parts of the absorption line. In the case of mercury, it would be between 650 and 750 MHz. To increase the frequency range, the cell could be placed in a magnetic field of variable field strength. The atomic line in the magnetic field would be split and the tunable range would be increased up to 20–30 GHz.
frame grabbers, allows one to obtain valuable imaging information about the frequency shift of detected laser radiation in real time. Furthermore, the RFIM technique allows one to
V. CONCLUSIONS
An important advantage of the suggested technique, which we call resonance fluorescence imaging is the ability to simultaneously acquire multifrequency information from images without scanning the illumination laser frequency. In addition, the design of the 202Hg RFIM cell is very simple. Two-photon laser-induced resonance images have high contrast levels which are impossible to achieve using practically all known narrowband detection techniques.8 The RFIM with modern computer image processing techniques, such as fast
FIG. 6. Principle of 2D frequency-resolved imaging of separate objects via two-color resonance fluorescence measurements.
Downloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
Podshivalov et al.
J. Appl. Phys., Vol. 86, No. 10, 15 November 1999
image these small frequency shifts in the UV and, in principle, the vacuum UV using conventional charge coupled device camera technology. ACKNOWLEDGMENT
This work was supported by the Defense Advanced Research Projects Agency. O. I. Matveev, J. Appl. Spectrosc. 46, 359 共1987兲. R. Miles and W. Lempert, Appl. Phys. B: Photophys. Laser Chem. 51, 1 共1990兲. 3 M. W. Smith, G. B. Northam, and J. P. Drumond, AIAA J. 34, 434 共1996兲. 4 N. D. Finkelstein, W. R. Lempert, and R. B. Miles, Opt. Lett. 22, 537 共1997兲. 5 O. I. Matveev, B. W. Smith, and J. D. Winefordner, Appl. Opt. 36, 8833 共1997兲. 1 2
5341
6
O. I. Matveev, B. W. Smith, and J. D. Winefordner, Opt. Lett. 23, 304 共1998兲. 7 O. I. Matveev, B. W. Smith, and J. D. Winefordner, Appl. Phys. Lett. 72, 1673 共1998兲. 8 O. I. Matveev, B. W. Smith, and J. D. Winefordner, Proc. SPIE 3380, 126 共1998兲. 9 H. Shimuzu, S. A. Lee, and C. Y. She, Appl. Opt. 22, 1373 共1983兲. 10 B. M. Gentry and C. L. Korb, Appl. Opt. 33, 5770 共1994兲. 11 J. H. Grinstead, N. D. Finkelstein, W. R. Lempert, and R. B. Miles, AIAA J. 96, 0302 共1996兲. 12 A. V. Jelalian, Laser Radar Systems 共Artech House, Boston, 1992兲. 13 O. I. Matveev, B. W. Smith, and J. D. Winefordner, Opt. Comm. 156, 259 共1998兲. 14 A. Mitchell and M. Zemansky, Resonance Radiation and Excited Atoms 共Cambridge University Press, London, 1971兲. 15 V. S. Letokhov and V. P. Chebotaev, Nonlinear Laser Spectroscopy, Springer Series in Optical Sciences, Vol. 4 共Springer, Berlin, 1977兲.
Downloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
