Spectral phase shaping for high resolution CARS spectroscopy around 3000 cm 1

Transcription

1 Spectral phase shaping for high resolution CARS spectroscopy around 3 cm A.C.W. van Rhijn, S. Postma, J.P. Korterik, J.L. Herek, and H.L. Offerhaus Mesa + Research Institute for Nanotechnology, University of Twente, 75 AE, Enschede, The Netherlands ABSTRACT By spectral phase shaping of both the pump and probe pulses in coherent anti-stokes Raman scattering (CARS) spectroscopy we demonstrate the extraction of the frequencies of vibrational lines using an unamplified oscillator. Furthermore we demonstrate chemically selective broadband CARS microscopy on a mixed sample of 4 µm diameter polystyrene (PS) and poly(methyl methacrylate) (PMMA) beads. The CARS signal from either the PS or the PMMA beads is shown to be enhanced or suppressed, depending on the phase profile applied to the broadband spectrum. Using a combination of negative and positive (sloped) π-phase steps in the pump and probe spectrum the purely non-resonant background signal is removed. Keywords: Coherent anti-stokes Raman scattering, Ultrafast spectroscopy, Non-linear microscopy. INTRODUCTION Coherent anti-stokes Raman scattering (CARS) is a four-photon process, where a pump field at frequency ω p, a Stokes field at frequency ω s, and a probe field at frequency ω pr interact with a molecule to generate an anti-stokes photon of frequency ω as = ω p ω s + ω pr. This CARS signal is resonantly enhanced whenever the frequency difference between the pump and Stokes photons (ω p ω s ) equals the frequency Ω of a vibrational Raman active mode of the molecule.?,? In broadband CARS,?,?,?,? where one or several input pulses are broadband, the spectral phase profile of the laser pulses is also of importance, as the nonlinear interaction mixes different pathways in the output signal. Selective excitation of Raman levels,? improved resolution,? and non-resonant background rejection? have been demonstrated by spectral phase shaping of broadband laser pulses. Here, we demonstrate extraction of vibrational frequencies as well as chemically selective imaging in combination with purely non-resonant background removal by use of spectral phase shaping of both the pump and probe pulse. The applied phase shapes are based on the intrinsic (negative) phase step associated with a vibrational resonance. In our setup there are two broadband pulses the pump and probe pulse and multiple combinations of their frequency components can result in the same anti-stokes frequency, giving rise to interference. The phase of the pump and probe pulse and the phase of the molecular response determine whether the different pathways interfere constructively or destructively. Choosing a suitable phase profile will enhance or suppress CARS signals from certain vibrations, depending on their frequency and linewidth.. SETUP A representation of the setup is given in figure. A Nd:YVO laser with a center wavelength of 64.3 nm and a pulse length of 5 ps ( cm,. nm FWHM, Spectra Physics Vanguard) is used for the Stokes beam. The pump and probe beam is generated by a femtosecond unamplified Ti:Sapphire laser (KM-labs) that is tunable around 8 nm ( nm FWHM). We use the same pulse for the pump and probe fields. Both lasers have a repetition rate of 8 MHz. The two lasers are synchronized by an electronic locking system.? The shaper is a liquid crystal device (LCD) with an one-dimensional array of 496 pixels, spread over a length of 7.4 mm (Boulder Nonlinear Systems). There is an intentional crosstalk between adjacent pixels, which smears out the applied phase profile. This effect improves the filling factor at the expense of the number of Further author information: a.c.w.vanrhijn@tnw.utwente.nl

2 Nd:YVO 64.3 nm 5 ps Chopper Electronic locking Ti:Sapphire ~8 nm ~5 fs Shaper Photodiode.55 NA.65 NA PMT or photodiode Spectrometer x,y-piëzoscanner with sample Figure. Schematic representation of the setup. degrees of freedom (approximately 6), however the positioning accuracy of the phase profile is unaffected by this crosstalk and corresponds to pixel (about GHz or cm? ). A cylindrical mirror is used to focus the negative first order diffracted beam from the grating onto the LCD. The pump and probe beam from the Ti:Sapphire laser is directed through a prism compressor before entering the shaper to compensate for linear dispersion in the system. Any residual or higher order dispersion is compensated by the shaper. A derandomized adaptation (DR) algorithm? is used to optimize the second harmonic signal generated in a µm thick BiBO crystal which is placed at the sample location. The algorithm yields a 5th order polynomial phase shape which corrects for dispersion to create a Fourier limited pulse. The phase profiles used in the experiments are superimposed on the phase profile required for the Fourier limited pulse. The pump, probe and Stokes beams are focused in the sample by a reflective microscope objective with a numerical aperture of.65. A reflective objective has been chosen to prevent chromatic abberations. The CARS signal is collected by a long working distance microscope objective with a numerical aperture of.55. The images are obtained using a x,y-piëzo scanner (piezosystem jena) to scan the sample and a photomultiplier tube (Hamamatsu). A chopper in the Stokes beam in combination with a lock-in amplifier is used to reduce noise. Raman intensity SIMULATIONS Raman intensity Frequency [cm - ] Frequency [cm - ] Figure. Complex Lorentzian fit (red) of the Raman spectrum of PMMA (black). Complex Lorentzian fit (red) of the Raman spectrum of PS (black) Numerical simulations have been performed to understand the effect of different phase shapes on the CARS signal. For all simulations, the Stokes pulse is considered to be narrowband with a flat spectral phase. The pump and probe pulses are assumed to originate from the same beam and thus have the same bandwidth, intensity, and spectral phase with a Gaussian shaped spectral envelope. The sample, consisting of pure polystyrene (PS) or poly(methyl methacrylate) (PMMA), is described using equation, where the non-resonant contribution χ (3) NR is assumed to be constant for all frequencies. The strength A Rk, center-frequency Ω k, and linewidth Γ Rk of each

3 resonance is determined by a multiple complex Lorentzian fit on the spontaneous Raman spectrum, as shown in figure and. The phase step in the pump and probe spectrum is also described using the phase term of equation for a single resonance. The CARS response is calculated using equation. χ (3) = χ (3) NR + χ(3) R = χ(3) NR + k A Rk Ω k (ω p ω s ) i(ω p ω s )Γ Rk () I CARS P(ω + ω s ) e iφ(ω+ωs) (χ (3) R (ω) + χ(3) NR ) Pr(ω) eiφ(ωs) () Since the Stokes spectrum is very narrowband compared to the pump and probe spectrum, the mixing of the pump and Stokes beam is approximated as a spectral shift of the pump. The shifted pump is then multiplied by the spectral response of the molecule (with resonances around ω = ω p ω s ) and the result is convoluted with the probe spectrum. The function Φ indicates the spectral phase shape that is applied to the pump and probe spectrum.?,? 3. π-phase step Normalized intensity Intensity (x) 6 4 Pump and probe pulse 3 Phase.5 Intensity Resonant CARS Phase [rad] Normalized intensity Vibrational resonance [cm ] Intensity.5.8 Phase (d) Non resonant CARS 8 Intensity (x) Phase [rad] Figure 3. Spectral intensity and phase profile of the pump and probe pulse. Intensity and phase of a vibrational resonance. and (d) The resonant and non-resonant CARS contribution for the spectral phase in (black) and for a flat spectral phase (thin red). We analyze the simplest case of one transition in the center of the broad pump ( Stokes) pulse, that is shaped with a single positive π-phase step. The shaped pump and probe pulses are defined to have a FWHM of 7 THz (33 cm ), centered at 37 THz (4 cm, 86.5 nm). The FWHM of the vibrational band is chosen to be 3 GHz ( cm ). A positive π-phase step, located spectrally so that it compensates the phase of the vibrational resonance, is applied to the pump pulse. Since the probe is also shaped, the CARS spectrum shows a dip in the center due to destructive interference of the additional phase step. The normalized components of the laser pulse are shown in figure 3; the normalized intensity and phase profile of the vibrational resonance are shown in figure 3. Figure 3 and 3(d) show the resulting resonant and non-resonant CARS spectra. The black lines represent the calculated spectra for the shaped pulse (figure 3). The thin red (Gaussian) lines are for comparison and indicate the unshaped (flat phase) spectra. Here, the intensity reflects the chosen values for and χ(3) NR, and respectively. Note that the intensity of the vibrational band scales quadratically with the value of χ (3) R. From the spectrally resolved signals the positive phase step creates a dip in the resonant CARS-spectrum, whereas the non-resonant signal shows a maximum at the same spectral position (figures 3(c,d)). For a negative phase step, the non-resonant CARS-spectrum will be the same, because the non-resonant CARS-spectrum is independent of the sign of the phase step. However, interference in the resonant CARS-spectrum will cause an enhancement at that frequency due to the constructive interference of an effective π-phase step. The negative phase step yields a negligible effect on the integrated signal, in comparison with the positive phase step, but χ (3) R

4 Normalized intensity Intensity (x) Pump and probe pulse Phase Intensity Resonant CARS Normalized Phase [rad] intensity.8.6 Vibrational resonance [cm ] Phase. Intensity (d) Non resonant CARS Intensity (x) Phase [rad] Figure 4. Spectral intensity and phase profile of the pump and probe pulse. Intensity and phase of a vibrational resonance. and (d) The resonant and non-resonant CARS contribution for the spectral phase in (black) and for flat spectral phase (thin red). produces sharp spectral features that are easily identified in a spectrally resolved measurement. Figure 4 shows the effect of such a negative phase step when the resonance and phase step are located in the center of the pump ( Stokes) spectrum. The results for a simulation on PS are presented in a D spectrally resolved representation for both a negative and a positive π-phase step scan in figure 5 and figure 5. These spectra are dominated by the non-resonant response which is independent of the direction of the phase step. The resonant interaction however, includes a multiplication with the molecular response which contains a negative phase step for each resonance. Substracting the positive and negative spectra removes the purely non-resonant contributions and yields figure??(d). Thus by scanning a positive and negative π-phase step through the spectrum of the pump and probe pulse, detection of vibrational resonances is possible.? Integration over the spectrum provides a simple representation of the data, as shown in figure 5 together with the same calculation for PMMA. It can be seen that it is possible to obtain zero intensity for either of the two materials by choosing the correct frequency position for the π-phase steps Integrated difference intensity Figure 5. Negative phase step scan simulation on PS. Positive phase step scan simulation on PS. Integrated difference spectrum simulation for PS (solid) and PMMA (dashed) as a function of phase step location. (d) -D Difference spectrum simulation on PS (d)

5 3. π-phase step Figure 6. Negative π-phase step scan simulation on PS. Positive π-phase step scan simulation on PS. Integrated difference spectrum simulation for PS (solid) and PMMA (dashed) as a function of π-phase step location. (d) -D Difference spectrum simulation on PS for a π-phase step. Using a π-phase step the interference due to the π-phase difference between both sides of the phase step can be avoided. The π-phase step can be treated as a flat phase profile, except for the region where the phase step occurs, where there is a sloped phase profile. In this case the -D spectra, shown in figure 6(a-c), show the vibrational resonances in greater contrast. Furthermore, there is a different contrast in the integrated difference signal, shown in figure 6, when compared to the π-phase step. 4. Spectroscopy 4. MEASUREMENTS AND DISCUSSION Sweeping a π-phase step is first tested on a bulk PS sample. A π-phase step is scanned through the Ti:Sapphire (pump and probe) spectrum while measuring the CARS spectrum. Taking the difference of the spectra recorded with a positive and negative π-phase step removes the purely non-resonant contributions and reveals signals due to weaker resonances (see figure 7). Both the sloped and vertical contributions from the Raman vibration at 35 cm are visible at THz. A second, weaker resonance is also revealed. The two vertical lines indicate the crossing point of the resonant line features of both resonances with the non-resonant line. The second, weaker resonance is located at THz (or 93 cm ). This feature can be assigned to a Raman resonance of PS (9 cm ). Furthermore, there is signal evident from an even weaker resonance at THz (or 86 cm ). This signal is assigned to the weak resonance at 85 cm observed in the Raman spectrum of PS. 4. Microscopy A sample consisting of a mixture of 4 µm PS and PMMA beads dried on a glass surface is used for imaging. Selective imaging is demonstrated by applying a positive and negative π-phase step near the main resonance for PMMA or PS. By looking at the integrated CARS signals from a positive and negative π-phase step sweep, as presented in figure 7, a suitable phase step location can be chosen. Measurements with positive and negative π-phase steps at 37. THz, which is near the main resonance of PMMA, are shown in figures 8 and 8, respectively. For an image with a Fourier limited pulse, shown in figure 8, the difference in intensity between the PMMA and PS beads is largely due to the larger overall scattering cross-section of PS compared to PMMA. For the positive phase step at 37. THz an increase in relative intensity of the PMMA beads compared to

6 Integrated difference intensity Resonant Integrated difference intensity Non-resonant Resonant Figure 7. -D Difference spectrum simulation on PS Measured -D difference spectrum of PS Integrated difference spectrum simulation for PS (solid) and PMMA (dashed) as a function of phase step location. (d) Measured integrated difference spectrum of PS (d) the PS beads is observed when compared to the Fourier limited case. For the negative phase step the relative intensity of the PMMA beads is observed to be roughly the same as in the Fourier limited case. 7 x Figure 8. CARS images of 4µm PS and PMMA beads. Image size approximately 8x8 µm. For a positive π-phase step at 37. THz. For a negative π-phase step at 37. THz. For a Fourier limited pulse. Note that the same intensity scale bar applies to and, but has a different scale bar. By looking at the intensity difference between images obtained with positive and negative steps chemically selective imaging is possible. The intensity difference for the PMMA beads and PS beads varies greatly from positive to negative as a function of the step position (see figure 5). The difference intensity images for the π-phase step centered near the main resonance of PMMA (37. THz) and near the main resonance of PS (373. THz) are presented in figure 9. In the difference intensity image for the phase step positioned near the main resonance of PMMA (figure 9) the PMMA beads are visible, while the PS beads are suppressed. For the phase step positioned near the main resonance of PS (figure 9) the PS beads are visible, while the PMMA beads are mostly suppressed. The PMMA beads are still somewhat visible, but with a negative (difference) intensity as opposed to the positive intensity of the PS beads. 5. CONCLUSION AND OUTLOOK We show high resolution broadband CARS spectroscopy around 3 cm using π-phase steps in the pump and probe spectrum. Furthermore, broadband CARS imaging around 3 cm is demonstrated with phase shaped pump and probe pulses on a mixed sample of dried 4 µm PS and PMMA beads. We show suppression

7 3 - - Figure 9. (Chemically selective imaging. Difference intensity images for two phase step locations. Image size approximately 8x8 µm. Phase step at 37. THz, revealing the PMMA beads. Phase step at 373. THz, showing the PS beads. The same intensity scale bar applies to both images. -3 of either the PS or the PMMA signal, depending on the location of the π-phase step in the pump and probe pulses. Additionally we show non-resonant background removal in the images. No optical amplifiers are required for these measurements. Preliminary simulations on π-phase steps suggest even greater contrast when compared to π-phase steps. Furthermore the vibrational resonances may be more easily identified in the resulting CARS spectra. This topic will be the subject of further research. ACKNOWLEDGMENTS The authors would like to acknowledge L. Hartsuiker from the Biophysical Engineering group of the University of Twente for providing the Raman spectra of PS and PMMA. Furthermore the authors would like to acknowledge M. Jurna for his work on the Raman fits and his assistance. REFERENCES