Photography on the Femtosecond Scale

Visualizing Ultrafast Dynamics in a Single Shot

  • High-speed optical imaging is widely used for exploring and understanding fast dynamics, but the temporal resolution of conventional high-speed cameras is limited to about 1 ns due to their technical limitations.High-speed optical imaging is widely used for exploring and understanding fast dynamics, but the temporal resolution of conventional high-speed cameras is limited to about 1 ns due to their technical limitations.
  • High-speed optical imaging is widely used for exploring and understanding fast dynamics, but the temporal resolution of conventional high-speed cameras is limited to about 1 ns due to their technical limitations.
  • Fig. 1: Schematic of STAMP [5].
  • Fig. 2: Motion-picture photography of laser ablation with STAMP [5]. (A) Experimental setup. A femtosecond laser pulse is focused on the surface of a glass for ablating it. (B) Motion picture with STAMP. A plasma plume is generated by the laser pulse and expands in the radial direction rapidly.
  • Fig. 3: Motion-picture photography of phonon dynamics with STAMP [5]. (A) Experimental setup. A femtosecond laser pulse is cylindrically focused into a LiNbO3 crystal for generating lattice vibrational waves. (B) Motion picture with STAMP. A phonon-polariton wave-packet is formed and propagates in the upper direction. (C) Motion picture with STAMP with a higher frame rate.

High-speed optical imaging is widely used for exploring and understanding fast dynamics, but the temporal resolution of conventional high-speed cameras is limited to about 1 ns due to their technical limitations. Here we introduce a high-speed imaging method called sequentially timed all-optical mapping photography (STAMP) which enables motion-picture femtophotography at a frame rate of more than a trillion frames per second.

Introduction
High-speed optical imaging is a powerful tool for studying dynamic phenomena in scientific research and inspecting parts and devices in industrial settings [1]. Unfortunately conventional high-speed cameras are not fast enough to reach the sub-nanosecond regime due to fundamental limitations in their mechanical or electrical operation [2, 3]. While the pump-probe method [4] can provide time-resolved images of faster events, it falls short for imaging difficult-to-reproduce events owing to its requirement for repetitive measurements. A new type of fast imaging methods is clearly needed to fill the gap between time-resolved imaging and high-speed cameras and hence explore the uncharted area of nature which is mostly found in complex physical, chemical, and biological systems.

In this article, we introduce a unique ultrafast optical imaging method, known as sequentially timed all-optical mapping photography (STAMP) [5], which is capable of achieving motion-picture photography on the femtosecond-to-picosecond time scales. STAMP’s all-optical operation circumvents the mechanical and electrical limitations of the conventional high-speed cameras and hence provides approximately 1000 times higher frame rates than them. Since STAMP does not require repetitive measurements for movie acquisition, it allows us to observe fast difficult-to-reproduce processes in a single shot.

Principles of STAMP
STAMP is based on the projection of the target’s time-varying spatial profile onto the spatial domain. This is made possible by employing spatial and temporal dispersion to spatially and temporally separate the time-varying event, respectively. Since the spatial and temporal dispersion is an optical effect, it eliminates the speed bottleneck that exists due to the conventional camera’s mechanical and electrical operation in its movie acquisition process.

A specific embodiment of this concept is schematically shown in figure 1.

The STAMP camera is composed of five components: (1) ultrashort laser source, (2) temporal mapping device (TMD), (3) spatial mapping device (SMD), (4) image sensor, and (5) computer. First, an ultrashort laser pulse emitted from the laser source is temporally stretched and shaped by the TMD into several daughter pulses of different spectral bands. The daughter pulses are incident onto the time-varying target for sequential image recording. The image-encoded daughter pulses are spectrally separated by the SMD without causing image deformation so that they are detected on different areas of the image sensor. Based on the pre-calibrated relation between time and the optical wavelength and between the optical wavelength and the spatial coordinates on the detector, a sequential motion picture can be constructed from the recorded images.

To experimentally demonstrate motion-picture photography with STAMP, we developed the STAMP camera that can record six sequential images with variable temporal resolution. The laser source is a Ti:Sapphire femtosecond pulse laser with a chirped pulse amplifier that emits a pulse train with a repetition rate of 1 kHz, a center wavelength of 810 nm, and a pulse width of 70 fs. An optical chopper and mechanical shutter are used to pick a single pulse. The single pulse is split into a pulse for STAMP imaging and an excitation pulse for triggering an ultrafast event. The TMD consists of a pulse stretcher (e.g., a grating pair and glass rod) and a pulse shaper (e.g., a 4f lens system with a spatial light modulator) and tailors the temporal shape and intensity of each daughter pulse to be identical such that the successive “flashes” are the same except for the timing of their arrival at the target. The image-encoded daughter pulses are spatially separated by the SMD that consists a diffraction grating, periscope array, and a cylindrical lens while satisfying the requirement for image formation on the image sensor without image deformation. The recorded images on the image sensor are digitally processed and recombined, but in the time domain, for producing a motion picture out of the temporally stacked image frames. The frame rate and exposure time of the STAMP camera can be determined by tuning the temporal dispersion in the TMD.

Motion-Picture Photography with STAMP
To show STAMP’s utility on the picosecond scale, we used it to monitor the early-stage plasma dynamics of a femtosecond-laser-induced ablation process. Here an excitation pulse with a pulse energy of 100 µJ and pulse duration of 70 fs was focused on the surface of a glass plate for ablating it (fig. 2 (A)). As shown in figure (B), plasma dynamics was visualized with the STAMP camera in a shadowgraph mode with a frame interval of 15.3 ps (corresponding to 65.4 x 109 frames/s) and exposure time of 13.8 ps. As shown in the image sequence or movie, the plasma plume generated by femtosecond laser expands rapidly in the radial direction. From the movie, the speed of the plume front was found to be approximately 105 m/s.

To further show STAMP’s movie-shooting capability on the femtosecond scale, we observed phonon dynamics in a crystal with STAMP. Here an excitation pulse with a pulse energy of 40 µJ and pulse duration of 70 fs was line-focused into a LiNbO3 wafer to produce a phonon-polariton pulse via impulsive stimulated Raman scattering (fig. 3 (A)). The pulse formation was visualized in a polarization-gating mode and captured with a frame interval of 812 fs (corresponding to 1.23 x 1012 frames/s) and exposure time of 1020 fs. A STAMP movie of the phonon dynamics is shown in figure 3 (B). It indicates that the intense electromagnetic field of the laser pulse excites lattice vibrations in the crystal and then a phonon-polariton wave-packet is formed from a seemingly random response of the crystal and propagates away from the excitation region. Figure 3 (C) shows the wave-packet propagation with a finer frame interval of 229 fs (corresponding to 4.37 x 1012 frames/s). The speed of the phonon-polariton wave-packet was determined from acquired images to be 4.6 x 107 m/s (one sixth of the speed of light).

Conclusion
By overcoming the fundamental limitations that exist in the conventional high-speed cameras, STAMP enables motion-picture photography of ultrafast events on the femtosecond-to-picosecond time scales. By virtue of its ability to perform sequential image acquisition, STAMP can help us visualize fast dynamical processes of complex physical, chemical, and biological systems. We expect STAMP to be of use in a diverse range of applications including nuclear fusion, quantum chemistry, microfluidic biotechnology, shockwave therapy, and laser surgery.

Acknowledgements
This work was supported in part by the Translational Systems Biology and Medicine Initiative from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant-in-Aid from the Japan Society for the Promotion of Science Fellows, Photon Frontier Network Program of MEXT, and Burroughs Wellcome Foundation.

References
[1] Sidney F. Ray: High Speed Photography and Photonics, SPIE Publications, (2002)
[2] Erik C. Gelderblom, Hendrik J. Vos, Frits Mastik, Telli Faez, Ying Luan, Tom J. A. Kokhuis, Antonious F. W. van der Steen, Detlef Lohse, Nico de Jong, Michel Versluis: Brandaris 128 ultra-high-speed imaging facility: 10 years of operation, updates, and enhanced features, Review of Scientific Instruments 83, 103706 (2012), DOI: 10.1063/1.4758783
[3] Takeharu G. Etoh, Dao V. T. Son, Tetsuo Yamada, Edoardo Charbon: Toward One Giga Frames per Second — Evolution of in Situ Storage Image Sensors, Sensors 13, 4640-4658 (2013), DOI: :10.3390/s130404640
[4] Ahmed H. Zewail: Laser Femtochemistry, Science 242, 1645-1653 (1988), DOI: 10.1126/science.242.4886.1645
[5] K. Nakagawa, A. Iwasaki, Y. Oishi, R. Horisaki, A. Tsukamoto, A. Nakamura, K. Hirosawa, H. Liao, T. Ushida, K. Goda, F. Kannari, I. Sakuma: Sequentially timed all-optical mapping photography (STAMP), Nature Photonics 8, 695–700 (2014), DOI: 10.1038/nphoton.2014.163

Authors & Affiliations
Keisuke Goda1, Keiichi Nakagawa1, Fumihiko Kannari2, Ichiro Sakuma3
1The University of Tokyo, Japan
2Electronics and Electrical Engineering, Keio University, Japan
3Precision Engineering, Bio Engineering, Medical Device Development and Regulation Research Center, The University of Tokyo, Japan

Contact
Prof. Keisuke Goda

The University of Tokyo, Japan
www.u-tokyo.ac.jp

Dr. Keiichi Nakagawa
The University of Tokyo, Japan
https://sites.google.com/site/keinakagawa6/

Prof. Fumihiko Kannari
Keio University, Japan
Electronics and Electrical Engineering
http://www.kami.elec.keio.ac.jp/

Prof. Ichiro Sakuma
The University of Tokyo
Precision Engineering, Bio Engineering, Medical
Device Development and Regulation Research Center
Japan
http://www.bmpe.t.u-tokyo.ac.jp/index_e.html

More information on high-speed imaging: http://www.imaging-git.com/search/gitsearch/high-speed
Plus: Read more about femtosecond microscopy: http://www.imaging-git.com/search/gitsearch/femtosecond

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