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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editors: Andrew Dunn and Anthony Durkin
  • Vol. 7, Iss. 10 — Oct. 5, 2012
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Photolytic-interference-free, femtosecond two-photon fluorescence imaging of atomic hydrogen

Waruna D. Kulatilaka, James R. Gord, Viswanath R. Katta, and Sukesh Roy  »View Author Affiliations


Optics Letters, Vol. 37, Issue 15, pp. 3051-3053 (2012)
http://dx.doi.org/10.1364/OL.37.003051


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Abstract

We discuss photolytic-interference-free, high-repetition-rate imaging of reaction intermediates in flames and plasmas using femtosecond (fs) multiphoton excitation. The high peak power of fs pulses enables efficient nonlinear excitation, while the low energy nearly eliminates interfering single-photon photodissociation processes. We demonstrate proof-of-principle, interference-free, two-photon laser-induced fluorescence line imaging of atomic hydrogen in hydrocarbon flames and discuss the method’s implications for certain other atomic and molecular species.

© 2012 Optical Society of America

Spatially and temporally resolved, concentration measurements of important intermediate species in chemically reacting flows, such as flames and plasmas, provide a key insight into the physical and chemical nature of such systems. Noninvasive, laser-based spectroscopic techniques such as laser-induced fluorescence (LIF) are widely used for such measurements [1

1. R. P. Lucht, J. T. Salmon, G. B. King, D. W. Sweeney, and N. M. Laurendeau, Opt. Lett. 8, 365 (1983). [CrossRef]

,2

2. K. Niemi, V. Schulz-von der Gathen, and H. F. Döbele, Plasma Sources Sci. Technol. 14, 375 (2005). [CrossRef]

]. In many situations, LIF offers sensitive detection as well as the ability to extend to two-dimensional (2D) imaging by using planar LIF. LIF-based measurements of key atomic species such as H, O, and N as well as molecular species such as CO require multiphoton excitation, as the frequencies of single-photon electronic transitions fall in the vacuum ultraviolet region, for which the medium becomes optically thick in most practical devices [3

3. W. D. Kulatilaka, B. D. Patterson, J. H. Frank, and T. B. Settersten, Appl. Opt. 47, 4672 (2008). [CrossRef]

]. The relatively weaker multiphoton excitation cross-sections necessitate the use of high-energy UV pulses. Such high-energy UV photons can photodissociate certain other molecules in the medium, generating the species being probed. For example, in two-photon excitation LIF (TPLIF) detection of atomic hydrogen, a substantial amount of additional H can be produced via photodissociation of vibrationally excited water vapor and methyl (CH3) radicals [4

4. W. D. Kulatilaka, J. H. Frank, B. D. Patterson, and T. B. Settersten, Appl. Phys. B 97, 227 (2009). [CrossRef]

]. In earlier work, improved TPLIF detection has been reported in H and O using picosecond (ps) duration pulses as opposed to the traditionally used nanosecond (ns) duration pulses [3

3. W. D. Kulatilaka, B. D. Patterson, J. H. Frank, and T. B. Settersten, Appl. Opt. 47, 4672 (2008). [CrossRef]

,5

5. J. H. Frank, X. L. Chen, B. D. Patterson, and T. B. Settersten, Appl. Opt. 43, 2588 (2004). [CrossRef]

].

In the present study, for the first time to our knowledge, we demonstrate nearly photolytic-interference-free femtosecond (fs) TPLIF line imaging of atomic hydrogen in flames. By using high-peak-power but low-energy fs pulses, the photolytic interferences are virtually eliminated while substantially increasing the two-photon excitation efficiency. Additionally, by using amplified Ti:sapphire-based laser systems, the measurement bandwidths can be increased to the 1–10 kHz regime, enabling the study of the spatiotemporal dynamics of turbulent reacting flows. Hydrogen is a key intermediate element in hydrocarbon combustion because of its high reactivity and diffusivity, playing a major role in ignition/extinction and heat release [3

3. W. D. Kulatilaka, B. D. Patterson, J. H. Frank, and T. B. Settersten, Appl. Opt. 47, 4672 (2008). [CrossRef]

,6

6. R. Sivaramakrishnan, J. V. Michael, L. B. Harding, and S. J. Klippenstein, J. Phys. Chem. A 116, 5981 (2012). [CrossRef]

,7

7. W. D. Kulatilaka, J. H. Frank, and T. B. Settersten, Proc. Combust. Inst. 32, 955 (2009). [CrossRef]

]. In another example, understanding the spatial distribution of H and O atoms is critical for effective use of biomedical plasmas in the rapidly growing area of plasma medicine [8

8. M. G. Kong, G. Kroesen, G. Morfill, T. Nosenko, T. Shimizu, J. van Dijk, and J. L. Zimmermann, New J. Phys. 11, 115012 (2009). [CrossRef]

].

Fig. 1. (a) Two-photon-excited LIF detection scheme for atomic hydrogen and (b) efficient two-photon excitation of H using multiple pairs of femtosecond photons.

The experimental apparatus consists of a 1 kHz repetition rate, amplified fs Ti:sapphire laser system (Spectra-Physics Solstice) pumping an optical parametric amplifier (OPA) (Coherent OPerA Solo). The peak pulse energy of the pump laser was 2.5 mJ at 800 nm; parametric conversion of the pump beam followed by frequency mixing and then upconversion of the idler beam resulted in up to 8 μJ of UV radiation at 205 nm. The 205 nm beam was collimated to a diameter of approximately 1 mm and transmitted through a Φ=1.2, CH4/O2/N2 Bunsen flame stabilized over a 5 mm diameter underexpanded jet. Two +50mm focal length f/1.2 camera lenses (Nikon Nikkor AIS) were used in the conjugate configuration to collect the fluorescence signal and focus it back onto the detection system. A 50 mm diameter Hα bandpass filter (Semrock FF01-655/40-50) was mounted in between the two camera lenses.

Two types of camera systems were used on either side of the flame to record the LIF line image orthogonal to the direction of propagation of the laser beam. An intensified charge-coupled device (ICCD) camera (Princeton Instruments PI-MAX2) recorded single-laser-shot images at a rate of 30 Hz, while a dual-stage high-speed intensifier (LaVision High Speed IRO with S25 visible-enhanced photocathode) coupled to a complementary metal-oxide semiconductor (CMOS) camera system (Photron FastCAM SA5) recorded single-shot images at 1 kHz. The CCD-based camera system was used for initial characterization of the system and for power dependence studies, as well as to ensure that the CMOS-based system was free of nonlinear response issues at the configurations used. The corresponding spatial resolutions were 30μm/pixel (ICCD) and 35μm/pixel (CMOS) respectively, and were uniform in both horizontal and vertical directions. For both camera systems, the spatial response functions across the image plane were determined by recording a fixed LIF signal while translating the burner along the direction of propagation of the excitation laser beam. It was experimentally verified that—for a given flame condition—after proper calibrations, the LIF line profiles recorded using both camera systems were nearly identical.

Fig. 2. Peak-normalized TPLIF line profiles of H from the right half of the flame for various values of laser pulse energy. A flame-luminosity image is shown in the inset.

Interference-free, H-atom TPLIF profiles were recorded in the same flame at four different heights above the nozzle exit using 4 μJ of pump energy. Atomic hydrogen line profiles derived from single-shot images at 1 kHz were compared with 256-shot averaged line profiles obtained using the ICCD camera system as well as with numerical calculations. Spatially resolved temperature and species mole-fraction calculations were performed using the UNICORN code, a time-dependent, 2D mathematical model developed for the simulation of unsteady reacting flows [12

12. V. R. Katta, L. P. Goss, and W. M. Roquemore, AIAA J. 32, 84 (1994). [CrossRef]

]. The experimental profiles were corrected for quenching by N2, H2, O2, H2O, CO, CO2, CH4, and C2H2 using the calculated species number densities and using the quenching cross-sections given in [13

13. J. Bittner, K. Kohse-Höinghaus, U. Meier, and T. Just, Chem. Phys. Lett. 143, 571 (1988). [CrossRef]

]. The quenching correction procedure is described in detail in [7

7. W. D. Kulatilaka, J. H. Frank, and T. B. Settersten, Proc. Combust. Inst. 32, 955 (2009). [CrossRef]

]. The total quenching rate increases by up to a factor of 7 from the hot flame front to cold reactants or products. As can be seen in Fig. 3, an excellent match is obtained between the shapes of the experimental and calculated H atom profiles. In a separate experiment, we also obtained H-atom TPLIF line images in a CH4/H2/air turbulent diffusion flame. Such measurements, especially when extended to full 2D imaging, can provide critical experimental data for validating complex turbulent combustion models.

Fig. 3. Experimentally measured and calculated H-atom profiles at four different heights above the nozzle.

Funding for this research was provided by the United States Air Force Research Laboratory under Contract No. FA8650-10-C-2008 and by the United States Air Force Office of Scientific Research (Drs. Enrique Parra and Chipping Li, Program Managers).

References

1.

R. P. Lucht, J. T. Salmon, G. B. King, D. W. Sweeney, and N. M. Laurendeau, Opt. Lett. 8, 365 (1983). [CrossRef]

2.

K. Niemi, V. Schulz-von der Gathen, and H. F. Döbele, Plasma Sources Sci. Technol. 14, 375 (2005). [CrossRef]

3.

W. D. Kulatilaka, B. D. Patterson, J. H. Frank, and T. B. Settersten, Appl. Opt. 47, 4672 (2008). [CrossRef]

4.

W. D. Kulatilaka, J. H. Frank, B. D. Patterson, and T. B. Settersten, Appl. Phys. B 97, 227 (2009). [CrossRef]

5.

J. H. Frank, X. L. Chen, B. D. Patterson, and T. B. Settersten, Appl. Opt. 43, 2588 (2004). [CrossRef]

6.

R. Sivaramakrishnan, J. V. Michael, L. B. Harding, and S. J. Klippenstein, J. Phys. Chem. A 116, 5981 (2012). [CrossRef]

7.

W. D. Kulatilaka, J. H. Frank, and T. B. Settersten, Proc. Combust. Inst. 32, 955 (2009). [CrossRef]

8.

M. G. Kong, G. Kroesen, G. Morfill, T. Nosenko, T. Shimizu, J. van Dijk, and J. L. Zimmermann, New J. Phys. 11, 115012 (2009). [CrossRef]

9.

L. Gasnot, P. Desgroux, J. F. Pauwels, and L. R. Sochet, Appl. Phys. B 65, 639 (1997). [CrossRef]

10.

S. Roy, W. D. Kulatilaka, D. Richardson, R. P. Lucht, and J. R. Gord, Opt. Lett. 34, 3857 (2009). [CrossRef]

11.

J. E. M. Goldsmith, Opt. Lett. 11, 416 (1986). [CrossRef]

12.

V. R. Katta, L. P. Goss, and W. M. Roquemore, AIAA J. 32, 84 (1994). [CrossRef]

13.

J. Bittner, K. Kohse-Höinghaus, U. Meier, and T. Just, Chem. Phys. Lett. 143, 571 (1988). [CrossRef]

14.

A. G. Hsu, V. Narayanaswamy, N. T. Clemens, and J. H. Frank, Proc. Combust. Inst. 33, 759 (2011). [CrossRef]

OCIS Codes
(120.1740) Instrumentation, measurement, and metrology : Combustion diagnostics
(300.2530) Spectroscopy : Fluorescence, laser-induced
(320.7150) Ultrafast optics : Ultrafast spectroscopy
(350.3450) Other areas of optics : Laser-induced chemistry

ToC Category:
Spectroscopy

History
Original Manuscript: May 29, 2012
Manuscript Accepted: June 7, 2012
Published: July 16, 2012

Virtual Issues
Vol. 7, Iss. 10 Virtual Journal for Biomedical Optics

Citation
Waruna D. Kulatilaka, James R. Gord, Viswanath R. Katta, and Sukesh Roy, "Photolytic-interference-free, femtosecond two-photon fluorescence imaging of atomic hydrogen," Opt. Lett. 37, 3051-3053 (2012)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=ol-37-15-3051


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References

  1. R. P. Lucht, J. T. Salmon, G. B. King, D. W. Sweeney, and N. M. Laurendeau, Opt. Lett. 8, 365 (1983). [CrossRef]
  2. K. Niemi, V. Schulz-von der Gathen, and H. F. Döbele, Plasma Sources Sci. Technol. 14, 375 (2005). [CrossRef]
  3. W. D. Kulatilaka, B. D. Patterson, J. H. Frank, and T. B. Settersten, Appl. Opt. 47, 4672 (2008). [CrossRef]
  4. W. D. Kulatilaka, J. H. Frank, B. D. Patterson, and T. B. Settersten, Appl. Phys. B 97, 227 (2009). [CrossRef]
  5. J. H. Frank, X. L. Chen, B. D. Patterson, and T. B. Settersten, Appl. Opt. 43, 2588 (2004). [CrossRef]
  6. R. Sivaramakrishnan, J. V. Michael, L. B. Harding, and S. J. Klippenstein, J. Phys. Chem. A 116, 5981 (2012). [CrossRef]
  7. W. D. Kulatilaka, J. H. Frank, and T. B. Settersten, Proc. Combust. Inst. 32, 955 (2009). [CrossRef]
  8. M. G. Kong, G. Kroesen, G. Morfill, T. Nosenko, T. Shimizu, J. van Dijk, and J. L. Zimmermann, New J. Phys. 11, 115012 (2009). [CrossRef]
  9. L. Gasnot, P. Desgroux, J. F. Pauwels, and L. R. Sochet, Appl. Phys. B 65, 639 (1997). [CrossRef]
  10. S. Roy, W. D. Kulatilaka, D. Richardson, R. P. Lucht, and J. R. Gord, Opt. Lett. 34, 3857 (2009). [CrossRef]
  11. J. E. M. Goldsmith, Opt. Lett. 11, 416 (1986). [CrossRef]
  12. V. R. Katta, L. P. Goss, and W. M. Roquemore, AIAA J. 32, 84 (1994). [CrossRef]
  13. J. Bittner, K. Kohse-Höinghaus, U. Meier, and T. Just, Chem. Phys. Lett. 143, 571 (1988). [CrossRef]
  14. A. G. Hsu, V. Narayanaswamy, N. T. Clemens, and J. H. Frank, Proc. Combust. Inst. 33, 759 (2011). [CrossRef]

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