We have developed a high-sensitivity, low-noise femtosecond imaging technique based on pump-probe time-resolved measurements with a standard CCD camera. The approach used in the experiment is based on lock-in acquisitions of images generated by a femtosecond laser probe synchronized to modulation of a femtosecond laser pump at the same rate. This technique allows time-resolved imaging of laser-excited phenomena with femtosecond time resolution. We illustrate the technique by time-resolved imaging of the nonlinear reshaping of a laser-excited picosecond acoustic pulse after propagation through a thin gold layer. Image analysis reveals the direct 2D visualization of the nonlinear acoustic propagation of the picosecond acoustic pulse. Many ultrafast pump-probe investigations can profit from this technique because of the wealth of information it provides over a typical single diode and lock-in amplifier setup, for example it can be used to image ultrasonic echoes in biological samples.
© 2014 Optical Society of America
Time-resolved optical experiments based on femtosecond lasers have become extremely popular and of routine use for investigating ultrafast phenomena in condensed matter, chemical, and biological systems. Independent of the phenomena investigated, the pump-probe femtosecond technique behind the time-resolved optical experiment is typically used in conjunction to lock-in amplifiers fed by photodiodes which restrict the possibility of ultrafast imaging. It is only in the disciplines involving high power lasers (shock waves , polaritons ...) that cameras are extensively used. The reason behind this is the necessity of high power lasers to achieve sufficient detectable pump-probe signal magnitudes. Although the pulse repetition rate from low-power femtosecond laser oscillators is extremely high, the pulse energies are low compared to high-power lasers, and therefore the signal levels are difficult to detect by cameras. Since standard CCD cameras are not suitable to detect modulated signals, the advantage of high-repetition-rate femtosecond laser oscillators to detect high signal-to-noise ratio modulated pump-probe signals becomes less of a benefit. To address this, we present in this paper an imaging technique with low-noise and high-sensitivity performances (in the range of 10−4) based on pump-probe lock-in acquisition of images with a standard CCD.
In the discipline of picosecond ultrasonics, femtosecond lasers enable the generation and detection of ultrashort acoustic pulses on picosecond time scales . In order to go beyond the conventional picosecond ultrasonics experiments, we illustrate the potential of our technique by imaging picosecond time scale ultrasonic pulses propagating in a thin gold film. Usually, ultrasonic pulses are exploited for thin film mechanical diagnostics and for the measurement of linear acoustic properties of materials [4,5]. In the present case, we demonstrate through direct visualization of ultrasonic pulse nonlinear acoustic reshaping during propagation which can be used in order to estimate the acoustic nonlinear properties of the material. Our results are a step forward in picosecond ultrasonic imaging over pump-probe scanning of the sample surface [6–11], far-field acoustic imaging [12–14], multi-channel detection [15–18] or asynchronous lasers [19–21].
2. Experimental set-up and methodology
A Ti-sapphire oscillator with an intra-cavity dumper running at a repetition rate of 500 kHz was used to generate laser pulses with a central wavelength of 800 nm and duration of 150 fs. The laser output was split into separate pump and probe beams. The probe beam was frequency-doubled by a second harmonic generation BBO crystal in order to obtain a probe wavelength of 400 nm. For imaging the sample surface onto the CCD, we used a ×50 Mitutoyo microscope objective, and a 20 cm lens placed before the probe beam-splitter in order to illuminate the whole CCD sensor, see Fig. 1(a). The pump beam of 15 nJ single pulse energy was focused onto the back of the sample with a ×10 Mitutoyo microscope objective producing a 2D gaussian laser profile at the focal spot with a FWHM (Full-Width at Half-Maximum) of about 10 μm, see Fig. 1(b). In order to avoid unwanted drift of the probe pointing during image acquisition, the pump beam is time delayed rather than the probe beam. The delay-line used in the experiment is composed of a Newport ILS-LM high speed linear motor stage of maximum speed of 500 mm/s controlled by a Newport XPS controller as shown in Fig. 1(a). This stage controller is a key aspect of the setup ensuring the electronic synchronization of pump modulation and image acquisition. The optical modulator is an acousto-optic (AO) deflector cell which deflects the pump beam with a deflection efficiency of about 90%. The disadvantage of using an AO deflector is the fact that it alters the beam quality of the deflected beam; thus, the 0th order transmitted pump beam is directed to the sample rather than the deflected 1st order pump beam. The TTL trigger sent by the XPS controller at a predefined rate (lower than the maximum frame rate of the camera) during the continuous motion of the delay-line alternatively switches the AO modulation on and off while the image acquisition occurs at each trigger. When the TTL trigger rate is set at f0, the image frame rate is locked at f0, while the AO modulation rate is locked at f0/2. This means that two successively recorded images do not carry the same information since one of them is recorded when the pump beam is present (the signal image) while the second is recorded without pump beam (the reference image). The reference image is recorded in order to sample the laser noise. Typically, laser fluctuations with the laser noise amplitude inversely proportional to the frequency dominate over electrical noise of the light detector. As a result, balancing the reference image with the signal image is used to drastically reduce laser noise. As in the conventional point pump-probe measurements with a lock-in amplifier, the signal and reference images should be acquired as close together in time as possible i.e. at the highest possible frequency. Thus, the ideal camera would have extremely low electrical noise and a high dynamic range in order to detect weak optical signals on top of a significant background (set close to pixel saturation for maximum detection capability), and fast frame rate for noise reduction. The camera we use is a Hamamatsu EM-CCD 9100-02, with high dynamic range (full well capacity of 70 000 electrons, 10 electrons of readout noise, 14 bit A/D converter) and maximum frame rate of 30 Hz at full image resolution of 1000×1000 pixels. Compared to CMOS cameras, CCD cameras have the advantage of low-noise electrical detectors, however, their frame rate at full image resolution is always lower than for CMOS . Given the fact that the full image resolution of 1000×1000 pixels is not necessary in our experiment, a good compromise between frame rate and image resolution is given by an image acquisition at setting the image acquisition at 256×256 pixels for the image resolution while running the CCD at 101 Hz. The exposure time is set below 1/101 s and the probe light intensity is adjusted close to the pixel saturation for optimum sensitivity.
All the recorded images were processed by the following basic image treatment,
3. Results and discussion
For the experiment, a hybrid multilayer sample structure composed of a 210 nm gold / 30 nm cobalt / sapphire substrate, depicted in Fig. 1(c), has been manufactured by RF magnetron sputtering (details can be found elsewhere ). The femtosecond pump beam is focused through the transparent sapphire substrate on the back of the sample at the cobalt layer. As a results of the strong electron-phonon coupling in cobalt, the laser excited electrons rapidly transfer their excess energy to the lattice such that the cobalt layer thermally expands and generates an ultra-short acoustic strain pulse propagating both into the gold layer and into the sapphire substrate. The excellent acoustic impedance matching between the cobalt opto-acoustic transducer and gold or sapphire minimizes the acoustic reflections at the surrounding interfaces (only about 10% of acoustic strain gets reflected). As a consequence, the initial shape of the acoustic pulse transmitted into gold follows the spatial profile of the laser deposited heat in the cobalt layer. Since the optical skin depth in cobalt at 800 nm wavelength is about 13 nm, the acoustic pulse profile follows the same spatial extension. After propagation through the 210 nm gold layer at the gold acoustic speed of ∼ 3.3 nm/ps , the laser excited acoustic pulse reaches the free surface of gold where it slightly modifies the optical reflectivity R through the acousto-optic effect and produces an image sampled by the imaging femtosecond probe pulse. Figure 2 shows the transient reflectivity at five different time delays τ. The complete sequence with a total number of images ni = 150 has been averaged for N = 1000 times which corresponds to a total acquisition time of about 30 minutes. As a comparison, for an equivalent image resolution of 250×250, conventional pump-probe scanning of the surface sample would take about 250×250×5 s ∼ 5000 minutes (if we assume that one single acquisition takes about 5 s each), which is roughly 100 times longer than offered by our experimental technique. To further improve image quality and to diminish the uncorrelated pixel noise, a standard numerical filtering procedure using a 3×3 circular averaging filter has been applied to each individual image of the sequence. This numerical filter does not alter the image resolution (which is close to the diffraction limit) but only removes high frequency spatial noise and improves the pixel standard deviation (STD) from 5.1×10−4 to 2.1×10−4, see Fig. 2 at τ = 56 ps. The value of the STD (in the range ∼10−4) gives a reliable estimate of the detection sensitivity of the experiment, which is significantly better than required for this measurement. Increasing the number of averages N would improve the detection sensitivity by , however it would critically augment the total acquisition time.
The acousto-optic detection process at the free sample surface works as long as the propagating acoustic pulse remains in the probe optical penetration depth area of about ξ = 32 nm typical size at 400 nm probe wavelength. Because of the reflection of the propagative acoustic pulse at the free surface with a change in sign (the pressure strain is converted into a tensile strain), the detected reflectivity signal goes through a minimum when the pressure and tensile strain upon reflection balance each other (i.e. when the first half of the reflected strain overlaps with the other half of the incoming strain) and the sign of the detected reflectivity variation changes due to the inversion of the sign of the strain. As a consequence of the acoustic reflection at the free surface the acoustic pulse appears to have a bipolar reflectivity signal even if the acoustic pulse has unipolar shape. This artifact of acousto-optic detection can be seen in Fig. 2 which displays a selection of reflectivity images at different time delays τ. The resulting minimum in the reflected signal at τ = 65.5 ps can be directly seen in the corresponding image and previous and subsequent images, at τ = 63 ps and 72.7 ps respectively, illustrating the non-symmetrical shape (i.e. bipolar shape) of the acousto-optic signal. The relevant acousto-optic detection process is described by,3]) weights the influence of the propagative acoustic strain η(z, τ) on the change in reflectivity at different depths z below the free surface of the film. From Eq. (2) it is obvious that the measured change in reflectivity does not directly describe the acoustic strain, but is a convolution of the optical sensitivity function s(z) with the propagative acoustic strain η(z, τ). The inability to optically detect the exact acoustic pulse shape  is problematic. Different detection techniques have been developed to circumvent this limitation [25–27], however, in a situation as the one described here it has recently been demonstrated  that an appropriate algorithm can retrieve the actual profile of the acoustic pulse from transient reflectivity measurements. We will see in the following that in case of transient reflectivity detection of shorter-than-skin-depth acoustic pulses in a gold film, the situation can be simplified. At the moment, we will emphasize the direct observation of the nonlinear reshaping of the propagating acoustic pulse.
The foremost observation in nonlinear laser acoustics (high-power laser excitation of weak to strong shock waves [1, 22, 29, 30] or solitons [31–33]) is the increase in sound velocity (over the speed of sound of the material) of large pressure acoustic pulses. As a matter of fact, any experimental evidence of the acoustic velocity being dependent on the acoustic pressure implies the involvement of nonlinear acoustics. The sequence of reflectivity images of Fig. 2, in particular the orange edge portion whose maximum has a time offset as compared to the blue center part (compare the images at τ = 72.7 ps and 83 ps in Fig. 2) is a direct demonstration of the nonlinear propagation of the inhomogeneous acoustic pulse. Since the laser pump is focused as an inhomogeneous 2D gaussian profile, see Fig. 1(a), the non-uniform amplitude of the laser pump will give rise to an inhomogeneous acoustic pulse that does not travel at a constant speed. As a consequence, the acoustic speed will be non-uniform along the in-plane position of the acoustic pulse. This is the reason for the center blue portion of the acoustic pulse with higher acoustic pressure arriving before the orange edge portion with lower acoustic pressure. A second step of image analysis provides deeper insight into the observation of the governing nonlinear acoustic phenomena. In this step, each image of the sequence has been processed with a predefined image mask - adapted to the 2D Gaussian excitation profile, with four plus one different areas in order to isolate five different subimages. A selected raw image and the adapted mask are shown in Figs. 3(a) and 3(b). The mask has a central part, the first area with a disk-like shape centered at the in-plane position of the maximum transient reflectivity signal, and corresponds to the maximum in pump light intensity. Finally, the last region, Area 5, serves as a reference area with negligible pump induced transient reflectivity signal to quantify the measurement noise. After applying the mask, the mean values of each of the five resulting subimages are calculated. From all the images constituting the sequence, we obtain the picosecond time evolution of the mean values for each of the 4 areas - as displayed in Figs. 3(c) and 3(d). While Fig. 3(c) shows the extracted mean values of Areas 1 through 4, Fig. 3(d) shows the same results after subtractions of the mean value of the reference area, Area 5, which diminishes the noise level further. Comparison of the results of Area 4 shown in Figs. 3(c) with 3(d) demonstrates that the subtraction of the reference area removes noise by about a factor of 3 – a substantial improvement of data quality. More quantitatively, the STD value of Area 4 decreases from 8.9×10−5 to 3.4×10−5 when the reference Area is subtracted. In case of gold and a probe wavelength of 400 nm , the time derivative of the transient reflectivity signal approximates very well the acoustic strain profile η in Eq. (2). As a final step, we perform the time derivation of the reflectivity signals displayed in Fig. 3(d), which yields the approximate acoustic pulse profiles for each of the four different areas. The results of this procedure in Fig. 3(e) highlight that the acoustic pulse arrival is different for the 4 different areas with different acoustic pressures. From the acoustic pulse with higher pressure (Area 1) arriving before the acoustic pulse with lower pressure (Area 4), we conclude the nonlinear acoustic propagation through gold. Between Area 1 and Area 4, we measure a time jitter of about 1.6 ps in arrival time of the acoustic pulse, consistent with what has been measured with femtosecond surface plasmon interferometry . This time jitter only depends on the third order elastic parameter of gold and on the peak amplitude of the generated acoustic pulse. In case of gold, since this nonlinear elastic coefficient is already known , our results displayed in Fig. 3, directly scaled in absolute units, can be used to estimate the absolute value of the photoelastic coefficients in the sensitivity function s(z) of Eq. (2). From the comparison of the measured intensity-dependence of the time jitter with the solutions of the nonlinear Korteveg-de Vries (KdV) theory [22, 31], we obtain an estimate of the photoelastic coefficients in gold at 400 nm probe wavelength, dn/dη = 2.0 ± 0.7 and dk/dη = 1.0 ± 0.3. Vice versa when a material is studied with unknown nonlinear elastic coefficient, the analysis of the time jitter and the nonlinear reshaping of ultrashort acoustic pulses can be exploited to extract the values of elastic nonlinearities.
Through this paper we introduced an ultrafast pump-probe experiment based on lock-in acquisition of images. The technique enables direct 2D visualization of femtosecond laser induced phenomena. To illustrate the high-sensitivity, low-noise performance of the developed setup, we present the direct visualization of the nonlinear propagation and reshaping of an ultrashort acoustic pulse in gold. Potentially, this experimental technique could lead to a simple and routine measurement of nonlinear elastic parameters of many materials. To facilitate the detection of even smaller features in time resolved images it could be possible to further increase the image resolution close to the diffraction limit. The detection sensitivity could be improved further with a new type of low noise CMOS sensors with a frame rate of ∼ 800 Hz at 256×512 pixels resolution (Hamamatsu OrcaFlash), and by using different detection techniques [35–37]. Beyond that, we envision to use this new technique for imaging longitudinal or transverse ultrasonic echoes in biological or liquid samples [38–42] or to adapt the experiment for Magneto-optical Kerr measurements in order to image magnetic domains subjected to ultrashort acoustic pulses . The latest results can be found online .
This work has been financially supported by Région des Pays de la Loire under contract ECHOPICO, and sponsored by Réseau-Femto CNRS. We acknowledge Brice Villier from Hamamatsu Photonics for technical support and C. Krien (IFW Dresden) for deposition of metal films.
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