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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 5, Iss. 1 — Jan. 4, 2010
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A route to sub-diffraction-limited CARS Microscopy

Willem P. Beeker, Petra Groß, Chris J. Lee, Carsten Cleff, Herman L. Offerhaus, Carsten Fallnich, Jennifer L. Herek, and Klaus-Jochen Boller  »View Author Affiliations


Optics Express, Vol. 17, Issue 25, pp. 22632-22638 (2009)
http://dx.doi.org/10.1364/OE.17.022632


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Abstract

We theoretically investigate a scheme to obtain sub-diffraction-limited resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We find using density matrix calculations that the rise of vibrational (Raman) coherence can be strongly suppressed, and thereby the emission of CARS signals can be significantly reduced, when pre-populating the corresponding vibrational state through an incoherent process. The effectiveness of pre-populating the vibrational state of interest is investigated by considering the excitation of a neighbouring vibrational (control) state through an intense, mid-infrared control laser. We observe that, similar to the processes employed in stimulated emission depletion microscopy, the CARS signal exhibits saturation behaviour if the transition rate between the vibrational and the control state is large. Our approach opens up the possibility of achieving chemically selectivity sub-diffraction-limited spatially resolved imaging.

© 2009 OSA

1. Introduction

Optical microscopy is the workhorse of biology, providing high contrast images, often in real time, of biological processes at sub-cellular distance scales. Ultimately, it is desirable to observe interactions at the single molecule or even single functional group level. This requires resolution in the nanometer range, which is well beyond the resolution of standard confocal microscopy.

Near-field techniques, such as scanning near-field optical microscopy (SNOM) [1

1. H. Heinzelmann and D. W. Pohl, “Scanning near-field optical microscopy,” Appl. Phys., A Mater. Sci. Process. 59(2), 89–101 ( 1994). [CrossRef]

], have been shown to provide sub-diffraction-limited resolution images. However, these require an optical aperture, usually in the form of a tapered fiber, be placed within a few tens of nanometers of the region of interest, which typically limits the technique to surface mapping.

Recently, far-field sub-diffraction-limited resolution has been achieved by a few imaging techniques, such as: stimulated emission depletion (STED) microscopy [2

2. S. W. Hell and J. Wichmann, “Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy,” Opt. Lett. 19(11), 780–782 ( 1994). [CrossRef] [PubMed]

, 3

3. T. A. Klar, S. Jakobs, M. Dyba, A. Egner, and S. W. Hell, “Fluorescence microscopy with diffraction resolution barrier broken by stimulated emission,” Proc. Natl. Acad. Sci. U.S.A. 97(15), 8206–8210 ( 2000). [CrossRef] [PubMed]

], photoactivated localization microscopy (PALM) [4

4. E. Betzig, G. H. Patterson, R. Sougrat, O. W. Lindwasser, S. Olenych, J. S. Bonifacino, M. W. Davidson, J. Lippincott-Schwartz, and H. F. Hess, “Imaging Intracellular Fluorescent Proteins at Nanometer Resolution,” Science 313(5793), 1642–1645 ( 2006). [CrossRef] [PubMed]

], and stochastic optical reconstruction microscopy (STORM) [5

5. M. J. Rust, M. Bates, and X. Zhuang, “Sub-diffraction-limit imaging by stochastic optical reconstruction microscopy (STORM),” Nat. Methods 3(10), 793–796 ( 2006). [CrossRef] [PubMed]

, 6

6. B. Huang, S. A. Jones, B. Brandenburg, and X. Zhuang, “Whole-cell 3D STORM reveals interactions between cellular structures with nanometer-scale resolution,” Nat. Methods 5(12), 1047–1052 ( 2008). [CrossRef] [PubMed]

]. In the latter method the build up of photon counting statistics is used to compute emitter locations within the diffraction limited spot. STORM therefore requires an extended time to construct an image. Furthermore, STORM requires rather specialized fluorescent labels [7

7. R. Zenobi, “Analytical tools for the nano world,” Anal. Bioanal. Chem. 390(1), 215–221 ( 2008). [CrossRef] [PubMed]

].

The STED technique requires fluorescent labels as well but exploits their saturable light emission; an excited state, generated by a light field, is partly depleted by an additional light field that causes stimulated emission from that state. Thereby the amount of spontaneous fluorescence is suppressed. If the additional light field possesses a node at its centre, the fluorescence process is suppressed except for a small volume around the node where the intensity is lower than the saturation intensity of the fluorescent label. Thus, resolution can be increased by increasing the intensity of the additional laser beam. Recent results show that STED can provide images with a lateral resolution of 6 nm in solids [8

8. E. Rittweger, K. Y. Han, S. E. Irvine, C. Eggeling, and S. W. Hell, “STED microscopy reveals crystal colour centres with nanometric resolution,” Nat. Photonics 3(3), 144–147 ( 2009). [CrossRef]

] or 33 nm in liquids [9

9. M. Dyba and S. W. Hell, “Focal Spots of Size λ/23 Open Up Far-Field Fluorescence Microscopy at 33 nm Axial Resolution,” Phys. Rev. Lett. 88(16), 163901 ( 2002). [CrossRef] [PubMed]

].

A disadvantage of fluorescence microscopy techniques is that labels have to be introduced to the cell. The labels may influence metabolic processes in the cell, or perhaps lead to cytotoxicity [10

10. H. Ikagawa, M. Yoneda, M. Iwaki, Z. Isogai, K. Tsujii, R. Yamazaki, T. Kamiya, and M. Zako, “Chemical Toxicity of Indocyanine Green Damages Retinal Pigment Epithelium,” Invest. Ophthalmol. Vis. Sci. 46(7), 2531–2539 ( 2005). [CrossRef] [PubMed]

, 11

11. H. H. Szeto, P. W. Schiller, K. Zhao, and G. Luo, “Fluorescent dyes alter intracellular targeting and function of cell-penetrating tetrapeptides,” The FASEB Journal, 04–1982fje (2004).

]. Furthermore, they may not attach to the correct molecule or functional group of interest [12

12. C. J. Daly and J. C. McGrath, “Fluorescent ligands, antibodies, and proteins for the study of receptors,” Pharmacol. Ther. 100(2), 101–118 ( 2003). [CrossRef] [PubMed]

]. There is, therefore, great interest in developing new label-free imaging techniques.

Label-free imaging modalities, such as CARS microscopy [13

13. M. D. Duncan, J. Reintjes, and T. J. Manuccia, “Scanning coherent anti-Stokes Raman microscope,” Opt. Lett. 7(8), 350–352 ( 1982). [CrossRef] [PubMed]

, 14

14. A. Zumbusch, G. R. Holtom, and X. S. Xie, “Three-Dimensional Vibrational Imaging by Coherent Anti-Stokes Raman Scattering,” Phys. Rev. Lett. 82(20), 4142–4145 ( 1999). [CrossRef]

], Stimulated Raman Scattering (SRS) microscopy [15

15. C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-Free Biomedical Imaging with High Sensitivity by Stimulated Raman Scattering Microscopy,” Science 322(5909), 1857–1861 ( 2008). [CrossRef] [PubMed]

], second harmonic generation microscopy and third harmonic generation microscopy, have demonstrated high contrast levels and video rate image acquisition times [16

16. C. L. Evans, E. O. Potma, M. Puoris’haag, D. Côté, C. P. Lin, and X. S. Xie, “Chemical imaging of tissue in vivo with video-rate coherent anti-Stokes Raman scattering microscopy,” Proc. Natl. Acad. Sci. U.S.A. 102(46), 16807–16812 ( 2005). [CrossRef] [PubMed]

]. However, current implementations are unable to achieve sub-diffraction-limited lateral resolution. Unlike the STORM process, which relies on switchable fluorophores, there are no labels to be switched on or off and, unlike STED, there is no excited state to be selectively depopulated. Recently it was shown that the CARS emission can be suppressed through an interference technique [17

17. A. Nikolaenko, V. V. Krishnamachari, and E. O. Potma, “Interferometric switching of coherent anti-Stokes Raman scattering signals in microscopy,” Physical Review A (Atomic, Molecular, and Optical Physics) 79(1), 013823–013827 ( 2009). [CrossRef]

], however, due to the linear nature of interference, this cannot be used to obtain sub-diffraction-limited resolution. This leads in to an important point: sub-diffraction-limited resolution requires nonlinearity in the optical emission process. For instance, STED works because spontaneous emission is reduced by the additional light field in a nonlinear way, which is provided by the saturation process.

To introduce sub-diffraction-limited resolution to CARS, a similar nonlinear saturation process must be identified. However, this appears to be difficult because the physical process does not involve any population inversion. A more promising approach could lie in manipulating the coherence between the ground state and the vibrational state (the so-called vibrational coherence), the presence of which is necessary for CARS emission. The analogy between CARS and STED would lie within a nonlinear process that reduces the vibrational coherence in a saturable process.

Instead of coupled-wave equations [17

17. A. Nikolaenko, V. V. Krishnamachari, and E. O. Potma, “Interferometric switching of coherent anti-Stokes Raman scattering signals in microscopy,” Physical Review A (Atomic, Molecular, and Optical Physics) 79(1), 013823–013827 ( 2009). [CrossRef]

], we use a density matrix approach [18

18. P. W. Milonni, and J. H. Eberly, Lasers (John Wiley & Sons, New York, 1988).

] to model the CARS emission process, because this traces the excitation separately from coherences, which can then be used to find intensity dependent effects, like saturation. We consider CARS emission in a configuration where two vibrational states are coupled and used to selectively inhibit CARS emission. For this situation we identified a mechanism that should enable sub-diffraction-limited resolution CARS: incoherently populating the CARS vibrational state suppresses the build-up of the vibrational coherence and reduces the CARS emission. This pre-population process is saturable and results in system behavior that is analogous to STED.

2. Theoretical framework

The level scheme of the medium and the driving light fields used in our calculations are shown in Fig. 1
Fig. 1 Energy level diagram for CARS extended with an additional level |4〉. Level |1〉 is the ground level and initially fully occupied, |2〉 is a vibrational level of the medium, |3〉 is the excited level and level |4〉 the control level which has a fast population exchange with level |2〉. The vertical arrows between levels indicate possible transitions induced by the corresponding laser fields, which are shown far detuned from the transition |1〉 - |3〉 (ωp, ωS and ωpr), or on resonance with the transition |1〉 - |4〉 (ωctrl). Through the nonlinear process the medium obtains a polarization at the additional ωcars frequency, which is radiated by the medium.
. The four-level system contains a ground state (|1〉), a vibrational state (|2〉), an excited state (|3〉) and a control state (|4〉). Transitions between the ground and excited states, the vibrational and excited states as well as the ground and control states are dipole allowed, while all other transitions are dipole forbidden.

First let us recall the standard description of a CARS process. The medium is illuminated by a set of three pulsed laser fields with frequencies ωp, ωS, ωpr, called the pump, Stokes and probe, respectively. All fields are far detuned from the |1〉 - |3〉 and |2〉 - |3〉 transition frequencies. The pump and Stokes lasers are tuned such that there is a two-photon resonance between levels |1〉 and |2〉, through which |2〉 is populated. The population difference between the states |1〉 and |2〉 in combination with the pump and Stokes fields builds up the vibrational coherence. This coherence then induces two optical sidebands on the probe light frequency, one of which is the CARS emission frequency (ωcars = ωp - ωS + ωpr). The laser fields ωp and ωpr can be degenerate in frequency. However, for clarity, we use two different laser fields for ωp and ωpr.

To understand how the additional laser (called the control laser ωctrl), resonant with the |1〉 - |4〉 transition, influences CARS emission, we calculate the dynamics of the envelopes of the density matrix elements with the Liouville equation [19

19. Y. R. Shen, The Principles of Nonlinear Optics (John Wiley and Sons, New York, 1984).

] dρ/dt = i/[H,ρ](Γijρij) (see Eqs. (1-10) in the appendix for the full list of density matrix equations). The Hamiltonian, H (in dipole approximation), contains the pump, probe and Stokes laser fields, plus the control laser, with frequency ωctrl, chosen to be resonant with the |1〉 - |4〉 transition. We study the following general case (described in section 3): the incoherent population exchange rate between levels |4〉 and |2〉 is considered to be high (0.1 THz), resulting in an equal distribution of population between those two levels. Although this case seems to be special, the vibrational modes of complex organic molecules often cross-relax on a relatively short time scale [20

20. H. Okamoto and K. Yoshihara, “Femtosecond time-resolved coherent Raman scattering from β-carotene in solution. Ultrahigh frequency (11 THz) beating phenomenon and sub-picosecond vibrational relaxation,” Chem. Phys. Lett. 177(6), 568–572 ( 1991). [CrossRef]

] and therefore meet the conditions. Furthermore, we have included the effect of additional vibrational states and found that a part of the total population is transferred to these additional states, which reduces the total amount of population involved in the CARS process. However, this only affects the main result insofar that it introduces a secondary suppression of the CARS process.

The incident light fields Eij are the sum of the pulse envelopes from the pump, Stokes, probe, and control light fields. Eij=nAn(t)eiΔijtwhere n = {p, S, pr, ctrl}. An(t) is the Gaussian shaped pulse envelope, Δij = ωn-ωij, with ωij being the transition frequency. We have assumed that there is no temporal delay between the light pulses of the pump, probe and Stokes fields. In order to prepare the state of the medium, the control pulse arrives and terminates before the onset of the other pulses.

The off-diagonal density matrix elements ρ14(t), ρ24(t), ρ13(t) and ρ23(t) act as the source terms of radiation in the Maxwell equations [18

18. P. W. Milonni, and J. H. Eberly, Lasers (John Wiley & Sons, New York, 1988).

]. We obtain the full temporal development of these elements by multiplying the envelopes ρij(t) with their respective transition frequencies. The Fourier transform of this provides the spectrum of the emitted radiation.

We choose energy levels, detunings and pulse durations that are typical for CARS emission processes from molecules (specifically, we do not choose values for a particular molecule so that our results are quite general) and in view of the complexity of the system, we use numerical solutions (fourth-order Runge-Kutta algorithm with a fixed step size). The |1〉 - |3〉 transition frequency is set to 1000 THz (~300 nm, ~33,000 cm−1), and the |1〉 - |2〉 frequency to 47 THz (6.4 μm or 1550 cm−1). Likewise, we choose a |1〉 - |4〉 transition frequency of 97 THz (3.1 μm or 3200 cm−1), which is close enough to |2〉 to allow for a high nonradiative population transfer rate of 0.1 THz. The detuning for the pump, Δp, and Stokes, ΔS, light fields from the |1〉 - |3〉 and |2〉 - |3〉 transition, respectively, are both taken as −353 THz to provide two-photon resonance with the |1〉 - |2〉 transition. The detuning for the probe Δpr is taken as −200 THz on the |1〉 - |3〉 transition. To allow for coherent population transfer between |1〉 and |2〉, the total lifetime of state |3〉 is taken to be in the order of picoseconds, the lifetime of |2〉 and |4〉 is taken to be in the order of nanoseconds, while the coherence lifetimes between states is of the order of picoseconds (see e.g., refs [21

21. R. de Vivie-Riedle and U. Troppmann, “Femtosecond Lasers for Quantum Information Technology,” Chem. Rev. 107(11), 5082–5100 ( 2007). [CrossRef] [PubMed]

23

23. A. J. Wurzer, T. Wilhelm, J. Piel, and E. Riedle, “Comprehensive measurement of the S1 azulene relaxation dynamics and observation of vibrational wavepacket motion,” Chem. Phys. Lett. 299(3-4), 296–302 ( 1999). [CrossRef]

].). All the laser pulse durations τ are set to 2 ps (1/e 2) except for the control pulse, which is set to 35 ps and arrives 60 ps in advance. The simulations extend over 100 ps in steps of 2.5 attoseconds.

First, the stability and accuracy of the numerical solutions were confirmed by reproducing well-known results (see Fig. 2
Fig. 2 Calculated emission spectrum. The features are (a) the Stokes shifted field of ωS, (b,c,f) Rayleigh scattering of ωS, ωp and ωpr, respectively, (d) the anti-Stokes shifted field of ωp, (e) Coherent-Stokes-Raman scattering (CSRS) and (g) Coherent Anti-Stokes Raman Scattering (CARS) of ωpr.
) such as the various Raman shifted and Rayleigh scattered fields. The population dynamics found in these initial simulations confirm that the two-photon resonance of the pump and Stokes light fields generates vibrational coherence, which radiates CARS emission as a sideband of the probe field. The nature of this process also indicates how to suppress CARS emission: if the system is prepared with equal initial populations in the |1〉 and |2〉 states, this will suppress the build-up of coherence on that transition, thus preventing CARS emission as well. In this approach, it is the purpose of the control laser to achieve this by first populating state |4〉, which is chosen such that |2〉 is rapidly populated via non-radiative mechanisms, before the arrival of the pump, Stokes, and probe pulses.

3. Results and discussion

We found that the saturation intensity, which we define as the intensity of the control beam at which the CARS signal drops to one half of its maximum value [25

25. B. Hein, K. I. Willig, and S. W. Hell, “Stimulated emission depletion (STED) nanoscopy of a fluorescent protein-labeled organelle inside a living cell,” Proc. Natl. Acad. Sci. U.S.A. 105(38), 14271–14276 ( 2008). [CrossRef] [PubMed]

], corresponds to a 25% reduction of the ground state population density achieved by the control pulse prior to the CARS process. For comparison with required values for the absolute intensities, a ground state depletion of 50% and population inversion of vibrational states of a liquid biological sample has been demonstrated by Ventalon et al. [26

26. C. Ventalon, J. M. Fraser, M. H. Vos, A. Alexandrou, J.-L. Martin, and M. Joffre, “Coherent vibrational climbing in carboxyhemoglobin,” Proc. Natl. Acad. Sci. U.S.A. 101(36), 13216–13220 ( 2004). [CrossRef] [PubMed]

], through direct excitation by a mid-IR laser. Considering the reported pulse duration, pulse energy, and the bandwidths of the vibrational states, we estimate that the populations of their ground state and first vibrational state become equalized at intensities in the order of 70 GW/cm2 during a 100 fs pulse. However, it should be noted that the pulse area of the control laser is more important than the intensity, thus, in our calculations, where a 35 ps pulse duration is considered, the intensity is just 200 MW/cm2. This is well below the threshold for multiphoton ionization and excitation in the mid-infrared. A considerable advantage of this excitation scheme is that the molecule remains in its electronic ground state.

The resolution can be estimated by considering an experimental setup wherein the control laser illuminates the sample with a donut mode, focused to a diffraction-limited spot. The CARS signal from the node with an area defined by where the control laser’s intensity is lower than the saturation intensity is not suppressed. Emission from outside the node is suppressed by 99.8%. The resolution can then be considered to be defined by the radius of the node when the signal contribution from within the node is greater than the signal contribution from outside the node. From our numerical results, we derive an upper limit to the improved resolution of approximately λ/(22*NA) where NA is the numerical aperture of the imaging system and λ is the wavelength of the probe beam.

We conclude from our calculations that sub-diffraction-limited resolution images can be obtained by extending the standard CARS microscopy setup with a control laser beam with a node at its center (a donut shaped beam), resonant with the |1〉 - |4〉 transition and with an intensity above the saturation level of the vibrational transition of interest. Then, the node at the center leaves the CARS process unaffected in a sub-diffraction-limited space around the center of the common focus, whereas CARS emission from that area outside the node is suppressed. The measured CARS signal is then attributed to this sub-diffraction-limited region, where the intensity is less than the saturation intensity.

4. Conclusion

We have demonstrated a route to obtain sub-diffraction-limited resolution in CARS microscopy, for which we have presented density matrix calculations for a four-level system. An intense control laser beam can be used to suppress CARS emission, when the control laser pre-populates an additional vibrational state that is non-radiatively coupled to the vibrational state and probed by the CARS process. For neighboring vibrational states that are coupled with high transitional probabilities, as can routinely be found for aqueous biological samples, a saturation process analogous to that of STED is observed. Our calculations, based on typical parameters for molecular transitions used in CARS microscopy, showed that high saturation levels of up to 99.8% could be obtained, which provides an upper limit on the improved resolution of ~λ/(22*NA) of the probe beam. The required intensities for the control beam, which depend on the pulse area, span a range from 200 MW/cm2 through to 100 GW/cm2, depending on the pulse duration, seem tolerable because the molecules remain in their electronic ground state.

5. Appendix

The differential equations of all density matrix elements are:

ρ˙11=i2(χ13ρ13χ13*ρ13*+χ14ρ14χ14*ρ14*)+ρ22R21+ρ33R31+ρ44R41
(1)
ρ˙22=i2(χ23ρ23χ23*ρ23*+χ24ρ24χ24*ρ24*)+ρ33R32+ρ44R41ρ22R24ρ22R21
(2)
ρ˙33=i2(χ13ρ13χ13*ρ13*+χ23ρ23χ23*ρ23*)(R31+R32+R34)ρ33
(3)
ρ˙44=i2(χ14ρ14χ14*ρ14*+χ24ρ24χ24*ρ24*)+ρ33R34ρ44R41ρ44R42+ρ22R24
(4)
ρ˙12=i2(χ13*ρ23*χ23ρ13+χ14*ρ24*χ24ρ14)Γ12ρ12
(5)
ρ˙13=i2(χ13*(ρ33ρ11)+χ14*ρ34*χ23*ρ12)Γ13ρ13
(6)
ρ˙14=i2(χ14*(ρ44ρ11)+χ13*ρ34χ24*ρ12)Γ14ρ14
(7)
ρ˙23=i2(χ23*(ρ33ρ22)+χ24*ρ34*χ13*ρ12*)Γ23ρ23
(8)
ρ˙24=i2(χ24*(ρ44ρ22)+χ23*ρ34χ14*ρ12*)Γ24ρ24
(9)
ρ˙34=i2(χ13ρ14+χ23ρ24χ14*ρ13*χ24*ρ23*)Γ34ρ34
(10)

With Rij being the decay rate through spontaneous emission from state i to j and Γij the decoherence rate between states i and j.

References and links

1.

H. Heinzelmann and D. W. Pohl, “Scanning near-field optical microscopy,” Appl. Phys., A Mater. Sci. Process. 59(2), 89–101 ( 1994). [CrossRef]

2.

S. W. Hell and J. Wichmann, “Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy,” Opt. Lett. 19(11), 780–782 ( 1994). [CrossRef] [PubMed]

3.

T. A. Klar, S. Jakobs, M. Dyba, A. Egner, and S. W. Hell, “Fluorescence microscopy with diffraction resolution barrier broken by stimulated emission,” Proc. Natl. Acad. Sci. U.S.A. 97(15), 8206–8210 ( 2000). [CrossRef] [PubMed]

4.

E. Betzig, G. H. Patterson, R. Sougrat, O. W. Lindwasser, S. Olenych, J. S. Bonifacino, M. W. Davidson, J. Lippincott-Schwartz, and H. F. Hess, “Imaging Intracellular Fluorescent Proteins at Nanometer Resolution,” Science 313(5793), 1642–1645 ( 2006). [CrossRef] [PubMed]

5.

M. J. Rust, M. Bates, and X. Zhuang, “Sub-diffraction-limit imaging by stochastic optical reconstruction microscopy (STORM),” Nat. Methods 3(10), 793–796 ( 2006). [CrossRef] [PubMed]

6.

B. Huang, S. A. Jones, B. Brandenburg, and X. Zhuang, “Whole-cell 3D STORM reveals interactions between cellular structures with nanometer-scale resolution,” Nat. Methods 5(12), 1047–1052 ( 2008). [CrossRef] [PubMed]

7.

R. Zenobi, “Analytical tools for the nano world,” Anal. Bioanal. Chem. 390(1), 215–221 ( 2008). [CrossRef] [PubMed]

8.

E. Rittweger, K. Y. Han, S. E. Irvine, C. Eggeling, and S. W. Hell, “STED microscopy reveals crystal colour centres with nanometric resolution,” Nat. Photonics 3(3), 144–147 ( 2009). [CrossRef]

9.

M. Dyba and S. W. Hell, “Focal Spots of Size λ/23 Open Up Far-Field Fluorescence Microscopy at 33 nm Axial Resolution,” Phys. Rev. Lett. 88(16), 163901 ( 2002). [CrossRef] [PubMed]

10.

H. Ikagawa, M. Yoneda, M. Iwaki, Z. Isogai, K. Tsujii, R. Yamazaki, T. Kamiya, and M. Zako, “Chemical Toxicity of Indocyanine Green Damages Retinal Pigment Epithelium,” Invest. Ophthalmol. Vis. Sci. 46(7), 2531–2539 ( 2005). [CrossRef] [PubMed]

11.

H. H. Szeto, P. W. Schiller, K. Zhao, and G. Luo, “Fluorescent dyes alter intracellular targeting and function of cell-penetrating tetrapeptides,” The FASEB Journal, 04–1982fje (2004).

12.

C. J. Daly and J. C. McGrath, “Fluorescent ligands, antibodies, and proteins for the study of receptors,” Pharmacol. Ther. 100(2), 101–118 ( 2003). [CrossRef] [PubMed]

13.

M. D. Duncan, J. Reintjes, and T. J. Manuccia, “Scanning coherent anti-Stokes Raman microscope,” Opt. Lett. 7(8), 350–352 ( 1982). [CrossRef] [PubMed]

14.

A. Zumbusch, G. R. Holtom, and X. S. Xie, “Three-Dimensional Vibrational Imaging by Coherent Anti-Stokes Raman Scattering,” Phys. Rev. Lett. 82(20), 4142–4145 ( 1999). [CrossRef]

15.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-Free Biomedical Imaging with High Sensitivity by Stimulated Raman Scattering Microscopy,” Science 322(5909), 1857–1861 ( 2008). [CrossRef] [PubMed]

16.

C. L. Evans, E. O. Potma, M. Puoris’haag, D. Côté, C. P. Lin, and X. S. Xie, “Chemical imaging of tissue in vivo with video-rate coherent anti-Stokes Raman scattering microscopy,” Proc. Natl. Acad. Sci. U.S.A. 102(46), 16807–16812 ( 2005). [CrossRef] [PubMed]

17.

A. Nikolaenko, V. V. Krishnamachari, and E. O. Potma, “Interferometric switching of coherent anti-Stokes Raman scattering signals in microscopy,” Physical Review A (Atomic, Molecular, and Optical Physics) 79(1), 013823–013827 ( 2009). [CrossRef]

18.

P. W. Milonni, and J. H. Eberly, Lasers (John Wiley & Sons, New York, 1988).

19.

Y. R. Shen, The Principles of Nonlinear Optics (John Wiley and Sons, New York, 1984).

20.

H. Okamoto and K. Yoshihara, “Femtosecond time-resolved coherent Raman scattering from β-carotene in solution. Ultrahigh frequency (11 THz) beating phenomenon and sub-picosecond vibrational relaxation,” Chem. Phys. Lett. 177(6), 568–572 ( 1991). [CrossRef]

21.

R. de Vivie-Riedle and U. Troppmann, “Femtosecond Lasers for Quantum Information Technology,” Chem. Rev. 107(11), 5082–5100 ( 2007). [CrossRef] [PubMed]

22.

J. B. Asbury, T. Steinel, C. Stromberg, K. J. Gaffney, I. R. Piletic, A. Goun, and M. D. Fayer, “Hydrogen Bond Dynamics Probed with Ultrafast Infrared Heterodyne-Detected Multidimensional Vibrational Stimulated Echoes,” Phys. Rev. Lett. 91(23), 237402 ( 2003). [CrossRef] [PubMed]

23.

A. J. Wurzer, T. Wilhelm, J. Piel, and E. Riedle, “Comprehensive measurement of the S1 azulene relaxation dynamics and observation of vibrational wavepacket motion,” Chem. Phys. Lett. 299(3-4), 296–302 ( 1999). [CrossRef]

24.

S. L. McCall and E. L. Hahn, “Self-Induced Transparency by Pulsed Coherent Light,” Phys. Rev. Lett. 18(21), 908–911 ( 1967). [CrossRef]

25.

B. Hein, K. I. Willig, and S. W. Hell, “Stimulated emission depletion (STED) nanoscopy of a fluorescent protein-labeled organelle inside a living cell,” Proc. Natl. Acad. Sci. U.S.A. 105(38), 14271–14276 ( 2008). [CrossRef] [PubMed]

26.

C. Ventalon, J. M. Fraser, M. H. Vos, A. Alexandrou, J.-L. Martin, and M. Joffre, “Coherent vibrational climbing in carboxyhemoglobin,” Proc. Natl. Acad. Sci. U.S.A. 101(36), 13216–13220 ( 2004). [CrossRef] [PubMed]

OCIS Codes
(300.6230) Spectroscopy : Spectroscopy, coherent anti-Stokes Raman scattering
(350.5730) Other areas of optics : Resolution

ToC Category:
Spectroscopy

History
Original Manuscript: July 1, 2009
Revised Manuscript: September 17, 2009
Manuscript Accepted: November 9, 2009
Published: November 25, 2009

Virtual Issues
Vol. 5, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Willem P. Beeker, Petra Groß, Chris J. Lee, Carsten Cleff, Herman L. Offerhaus, Carsten Fallnich, Jennifer L. Herek, and Klaus-Jochen Boller, "A route to sub-diffraction-limited 
CARS Microscopy," Opt. Express 17, 22632-22638 (2009)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-17-25-22632


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