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3. Control of vibronic WPs in gas phase diatomic molecules

Iodine molecules have a small vibrational level spacing due to the heavy nuclear mass. Because of this, vibrational WPs can be generated easily by exciting electronic excited states with broadband fs laser pulse. As explained in the previous chapter, two Was generated in the electronic excited state interfere with each other. We need to shine another laser pulse (probe pulse) to retrieve the final WP state information. This kinds of pump-probe experiments are fundamental technique to observe ultrafast dynamics in a target system.

iodine.jpg

Fig.4 Experimental scheme of WP interferometry  in iodine molecule

Fig. 4 shows the scheme of such experiments. The WPs generated by the pump and control pulses are further excited to the upper electronic state (E state) by the probe pulse. The fluorescence from E state is measured by the photo-multiplier. 

We have performed two types of experiments, one with fs probe pulse and the other with nanosecond (ns) probe pulse. With ns probe pulse, we can selectively detect the population in one single eigenstate. On the other hand, we can visualize the temporal evolution of the WP by applying fs probe pulse. They are explained below.

3-1. Population read-out by ns probe pulse 

The bandwidth of ns probe pulse is much narrower than the energy spacing between neighboring vibrational levels of iodine. Thus, we can observe the fluorescence signal only when the wavelength of the ns probe pulse is resonant with particular vibronic transition between E and B states. If we scan the wavelength of the ns probe pulse, we can perform the state-selective detection of the eigenstate involved in the WP. Fig. 5 shows the modulation of the population in v=33 level as we scan the delay τ between the pump and control pulses. This kind of interferometric modulation is called "Ramsey fringe." During the scan, the wavelength of the probe pulse os fixed. The delay τ is scanned about ±2fs around 500 fs, which corresponds to the classical vibrational period of the WP. The quality of the interference is over 90%, demonstrating that high quality interference is realized. The oscillation period is 1.8fs, which corresponds to the transition energy between v=0 in X(ground) state and v=33 in B state. Actually, the main goal of original Ramsey fringe spectroscopy is to determine the transition frequency from the period of observed fringes.

Ramsey.jpg

Fig.5 Ramsey fringe observation in iodine

By scanning the probe wavelength, we can visualize the population distribution and the relative phases of each eigenstate involved in the WP. As advanced application of this technique, we have performed discrete Fourier transform using vibrational WP, and also actively control the relative phases by utilizing strong nonresonant laser pulses.

3-2. Visualization of WP motion by fs probe pulse

When we use fs probe pulse, it is possible to detect the probabolity density around the particular internuclear distance. This "window" position is determined by the wavelength of the probe pulse. Thus, by scanning the probe wavelength it is possible to visualize the spato-temporal WP probability density. (Strictly speaking, what is detected is the product of probability density and transition moment and electric field intensity.)

Fig. 6 shows the spatio-temporal plots of the WP density when pump and control pulses are shined with τ ~1.5 Tvib. The timing is tuned with ~0.45  fs step (90º phase difference). Although the full range of the spatial axis is only 6 pm, we can observe the modulating probability distribution as the relative phase changes. For example, the ridge (around 334pm) in the 0º scan is totally inverted to the valley in the 180º scan. 

carpet2.jpg

Fig.６ Observation of quantum carpets in iodine

3-3. Quantum interferometry with strong non-resonant laser pulse

​In preparation

The gas phase experiments described here are not continued in NAIST lab. If you want to know more details, see the publication list below.

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【関連論文】

  1. Visualizing picometric quantum ripples of ultrafast wave-packet interference
    H. Katsuki, H. Chiba, B. Girard, C. Meier, and K. Ohmori, Science 311, 1589-1592 (2006).

  2. Real-time observation of phase-controlled molecular wave-packet interference
    K. Ohmori, H. Katsuki, H. Chiba, M. Honda, Y. Hagihara, K. Fujiwara, Y. Sato, and K. Ueda, Phys. Rev. Lett. 96, 093002 (2006).

  3. READ and WRITE Amplitude and Phase Information by Using High-Precision Molecular Wave-Packet Interferometry
    H. Katsuki, K. Hosaka, H. Chiba, and K. Ohmori, Phys. Rev. A 76, 013403 (2007).

  4. Actively tailored spatiotemporal images of quantum interference on the picometer and femtosecond scales
    H. Katsuki, H. Chiba, C. Meier, B. Girard, and K. Ohmori, Phys. Rev. Lett. 102, 103602 (2009).

  5. Ultrafast Fourier transform with a femtosecond laser driven molecule
    K. Hosaka, H. Shimada, H. Chiba, H. Katsuki, Y. Teranishi, Y. Ohtsuki, and K. Ohmori, Phys. Rev. Lett. 104, 180501 (2010).

  6. Strong-Laser-Induced Quantum Interference
    H. Goto, H. Katsuki, H. Ibrahim, H. Chiba, and K. Ohmori, Nature Phys. 7, 383-385 (2011).

Last updated: Jun. 3 2022
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