We recorded and observed, for the first time, three-dimensional image of femtosecond light pulse propagation as continuous moving picture using light-in-flight recording by holography. We present the moving pictures of collimated and converging light pulses and some images extracted from them. We also discussed inherent feature appearing in the images. Such a discussion is essential to determine the actual shape of the propagating light pulse. This technique provides the means for observation of a temporally and spatially continuous moving picture of light itself and also enables the analysis of various kinds of ultrafast phenomena.
©2007 Optical Society of America
At the frontiers of recent researches in natural sciences, there has been a great deal of interest in ultrafast phenomena in the femtosecond time domains. There have been many discoveries of new phenomena in the fields of natural sciences . Observation of propagating light is important and valuable, not only to verify numerical results, but also to observe and analyze ultrafast phenomena. However, there have only been a few studies investigating propagating light pulse. One example of the techniques used in these studies is femtosecond time-resolved optical polarigraphy (FTOP) [2–5], and another one is a technique using a photon scanning tunneling microscope (PSTM) and its related techniques [6–9]. In the FTOP, the pulses are indirectly observed by way of a nonlinear optical phenomenon. The PSTM allows observation of evanescent wave of the pulses by near-field optical microscopy. Since these techniques require repeating femtosecond pulses to acquire a moving picture of the propagating femtosecond light pulses, they are not always able to correctly observe the pulses when the repetition characteristics of pulses are not uniform. Furthermore, these techniques cannot obtain a moving picture of the propagating light pulses that is both spatially and temporally continuous. One powerful technique capable of overcoming these problems is light-in-flight recording by holography, which can observe propagating ultrashort light pulses as a spatially and temporally continuous moving picture at a desired speed [10–18]. This technique requires neither a nonlinear optical phenomenon nor repeating femtosecond pulses in principle. However, in the experiments conducted until now, a section of a light pulse is projected on a ground glass plate, and the picture is restricted to a two-dimensional one.
Here we present, for the first time, the recording and observation of a three-dimensional image of femtosecond light pulse propagation as a spatially and temporally continuous moving picture using light-in-flight recording by holography. In order to record the image using this technique, a medium that scatters the light is usually required. For this purpose, we adopted gelatin as a three-dimensional scattering medium, which provides the following advantages:
- Since gelatin exhibits moderate light scattering, it is possible to visualize the path of light propagates inside of it; and
- The refractive index can be adjusted by mixing gum syrup and sugar with the gelatin.
Figure 1(a) shows the arrangement for recording the hologram used in observing the propagation of collimated light pulses in the gelatin. A light pulse from a femtosecond pulsed laser was divided into two light pulses by a beam splitter and each pulse was collimated by a microscope objective and a collimator lens. One collimated light pulse was introduced to the recording material at 45° to the surface as a reference beam. The other collimated light pulse was introduced to the glass container in which the gelatin was set, at 50° to the surface of the recording material. The size of the glass container was 20 cm (length) × 8 cm (width) × 7 cm (height). To observe the pulse front of the collimated femtosecond light easily, a 2 cm × 2 cm mask was attached to the entrance face of the glass container. A Japanese character meaning “light” was printed on the mask (Fig. 1(b)). The light passed through the transparent part of the mask. Since the mask was attached so that the character could be correctly read from the side of the beam splitter, the right-to-left reversed character was seen from the side of the recording material, as shown in Fig. 1(a). The scattered light pulse from the gelatin was used as the object wave. The reference pulse and a part of the scattered pulse proceed to the recording material. When an ultrashort pulsed laser is used in the hologram recording, interference fringes are recorded only where the scattered object pulse and the reference pulse simultaneously arrive in the recording material. Since the reference pulse obliquely illuminates the recording material in our arrangement, the arrival time of the reference pulse at each point in the recording material is different. Thus, a time-evolving image of the object pulse is recorded along the lateral direction of the recording material. In this technique, the reference pulse plays the role of an ultrafast optical gate in recording moving picture and the propagation of a single-shot pulse can be recorded. A mode-locked Ti:Sapphire laser (Mira 900-D, Coherent Inc.) was used as the ultrashort pulsed laser. The pulse width of the femtosecond laser pulses was 224 fs and the center wavelength was 720 nm. Agfa Holotest 8E75HD plate was used as the recording material.
When the hologram was illuminated with a continuous light wave, each different portion of the hologram reconstructed the light pulse at a different point in time. Therefore, it was possible to observe temporally continuous moving picture of the image of ultrashort light pulse propagation by moving the point of observation along the lateral direction of the hologram. The reconstruction speed of the moving picture is determined by the moving speed. When a 100-ps phenomenon was recorded by a 10-cm-long hologram and observed over 1 s, an effect which we term “temporal microscope”, with 1010 magnification, was achieved. The time resolution of the system depends on the pulse width and it becomes better as pulse width becomes short because the width of the light pulse recorded on a hologram becomes short. The incidence angle of the hologram also affects the resolution for the same reason.
Figure 2 shows the moving picture of the observed three-dimensional image of the collimated femtosecond light pulse. Four scenes extracted from the moving picture are shown in Fig. 3. By moving the observation position from right to left, which is the same direction as that of the reference light on the surface of the recording material, the reconstructed image moved from right to left. The time interval between each picture was 14 ps.
As seen from these scenes, the observed image differed from what was expected. That is, although the right-to-left reversed character of “light” was expected to be seen from the observation side, actually the observed character was correctly read. In other words, it appears that the back side of the character can be seen. This strange phenomenon can be explained in terms of the optical path length under the following assumption. The pulse scattered from each point composing the character does not reach a single point in the recording material at the same time. The departing time of the object pulse scattered from each point differs depending on the recording geometry. The scattered light from a portion near the recording material has a short optical path length to the recording material. The reconstructed image is thus observed as the locus of scattered light whose optical path length is equal. As a result, the plane of the observed pulse front is distorted, and right-to-left reversed image is observed. The phenomenon can be explained graphically using the idea of holodiagram  by drawing a diagram of parabola whose focal point is located on the observation point and the axis is parallel to the incident beams in the gelatin.
The shape of the observed reconstructed image was numerically simulated using the parameters meeting the experimental arrangement and analyzed under the above assumptions. The result of the simulation is shown in Fig. 4. The x-y plane was set on the surface of the recording material and the z-axis was set perpendicular to the plane. The image was observed at x = 8.1 cm and y = 0 in the hologram plane. The points A and B were the positions of the corner of the femtosecond light pulse plane propagating in the gelatin at a certain time, as shown in Fig. 1; the positions of A and B were x = 5.35 cm, y = 1 cm, and z = 19.1 cm, and x = 5.35 cm, y = -1 cm, and z = 19.1 cm, respectively. The green lines show the outline of the path of the light pulse. As shown in Fig. 3, the result indicates that the character “light” was correctly read, indicating that the image was horizontally flipped and reversed right-to-left. The result agrees well with the experiment and explains the above-mentioned effect appearing in the reconstructed image. Thus, the reconstructed image was observed as the locus of the scattered light of the object light whose optical path length was equal. In other words, the image was composed of a set of scattered light beams whose departing times were different. The time at points C and D in the image were 192 ps before A and B. The actual shape of the propagating light pulse can be determined from the reconstructed image by image processing based on the above analysis. In addition to the processing, the recording of the hologram from two or more directions is an effective method to get more information about the actual shape.
As another example of moving picture, the observed three-dimensional image of femtosecond light pulse converging and diverging through a convex lens is shown in Fig.5.
The arrangement used for the experiment was basically the same as that shown in Fig. 1(a). In order to record the converging femtosecond light pulse, a convex lens with a diameter of 4 cm and a focal length of 5 cm was placed just before the mask attached to the entrance face of the glass container. The pulse surface was spatially modulated by the Japanese character “light”. Eight scenes extracted from the obtained moving picture are shown in Fig. 6. The light pulse front was converging in Figs. 6(a)–(e). The pulse was just focused in Fig. 6(f) and diverging in Figs. 6(g)–(h). The time interval between adjacent images was 15 ps. The reconstructed image from the hologram recorded with a continuous laser beam was overlaid in the picture to identify the propagation path. It is clear from the inversion of the character that the pulse surface was inverted before or after the focus.
In summary, we recorded and observed three-dimensional image of femtosecond light pulse propagation as continuous moving picture using light-in-flight recording by holography. We presented some moving pictures of the reconstructed images of collimated and converging light pulses. The shape of the reconstructed image was different from what we expected. It was possible to explain the phenomenon by considering the optical path length and the inherent feature appearing in the images was discussed. Such a discussion is essential to determine the actual shape of the propagating light pulse. This technique achieved a “temporal microscope” with a magnification of more than 1010 and will open the possibility of observing and analyzing various kinds of ultrafast phenomena. The proposed technique is expected to be a powerful tool for directly observing ultrafast dynamics of atoms and molecules when applied to electron holography or X-ray holography.
References and links
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