Takashi Nakajima

Transverse-mode dependence of femtosecond filamentation

We theoretically investigate the transverse-mode dependence of femtosecond filamentation in Ar gas. Three different transverse modes, Bessel, Gaussian, and Laguerre modes, are considered for incident laser pulses. By solving the extended nonlinear Schrödinger equation coupled with the electron density equation, we find that the lengths of the filament and the plasma channel induced by the Bessel incident beam is much longer than the other transverse modes with the same peak intensity, pulse duration, and beam diameter. Moreover we find that the temporal profile of the pulse with the Bessel incident mode is nearly undistorted during the propagation. Since the pulse energy that the Bessel beam can carry is more than one order of magnitude larger than the other modes for the same peak intensity, pulse duration, and beam diameter, the Bessel beam can be a very powerful tool in ultrafast nonlinear optics involving propagation in a Kerr medium.

Formation of filament and plasma channel by the Bessel incident beam in Ar gas: role of the outer part of the beam

We theoretically investigate the formation of filament and plasma channel in Ar gas by intense femtosecond pulses in the Bessel, truncated Bessel, and combination of two Gaussian modes. Through the numerical results obtained by solving the generalized nonlinear Schrödinger equation coupled with the electron density evolution equation, we find that there is a radial energy flow during the propagation, which implies that the outer part of the Bessel beam serves as an energy reservoir for the filament formed around the central peak. The results we obtain for the Bessel and truncated Bessel incident beams are consistent in that we can obtain a longer filament and plasma channel if more energy is reserved in the outer part of the Bessel incident beam. More interestingly we show that the combined use of two Gaussian beams with different beam diameters increases the energy stored in the outer part of the beam, and as a result the lengths of the filament and plasma channel become remarkably longer. This can be a practical choice to improve the propagation properties.

A scheme to polarize nuclear-spin of atoms by a sequence of short laser pulses: application to the muonium

We theoretically show that a sequence of short laser pulses can efficiently polarize nuclear-spin of atoms/ions. This is a variant of optical pumping with an important difference that a sequence of short laser pulses is used instead of a continuous-wave laser. Such a replacement is particularly useful if the pumping wavelength is in the ultraviolet or vacuum-ultraviolet region where obtaining a continuous-wave light source with a sufficient intensity is very difficult. Because of the use of short laser pulses neither hyperfine transitions nor fine structure transitions are spectrally resolved, which is quite in contrast to the standard optical pumping scheme by a continuous-wave laser. As an example we apply the scheme to polarize the muonium (μ(+)e(-), lifetime 2.2 μs), for which the pumping wavelength is 122 nm. From numerical solutions of a set of density matrix equations, we find that the use of only a single, two, and five pulses with a ps duration at the peak intensity of 2×10(8) W/cm(2) and a 5 ns time interval results in the degrees of spin-polarization of 33, 50, and 80 %, respectively, within the time scale of a few tens of ns.

Characterization of attosecond XUV pulses utilizing a broadband UV~VUV pumping

We propose a simple scheme to characterize attosecond extreme ultraviolet (XUV) pulses. A broadband ultraviolet (UV) approximately vacuum ultraviolet (VUV) pump pulse creates a coherent superposition of atomic bound states, from which photoionization takes place by the time-delayed attosecond XUV probe pulse. Information on the spectral phase of the XUV pulse can be extracted from the phase offset of the interference beating in the photoelectron spectra using a standard SPIDER (spectral phase interferometry for direct electric-field reconstruction) algorithm. We further discuss the influence of the chirp and polychromaticity of the pump pulse, and show that they do not spoil the reconstruction process. Since our scheme is applicable for various simple atoms such as H, He, and Cs, etc., and capable of characterizing attosecond XUV pulses with a pulse duration of a few hundred attoseconds or even less, it can be an alternative technique to characterize attosecond XUV pulses. Specific numerical examples are presented for the H atom utilizing the 2p and 3p states.

Characterization of attosecond XUV pulses from photoelectron spectra of atoms

A method to characterize attosecond extreme ultra violet (XUV) pulses from photoelectron spectra of atoms is presented. A pump pulse prepares a coherent superposition of two atomic bound states, from which photoionization takes place after variable time delays by the attosecond XUV pulse. Information on the spectral phase of the attosecond XUV pulse is extracted from the analysis of photoelectron spectra as a function of photoelectron energy and time delay. Together with information on the spectral intensity obtained from a separate optical measurement, a temporal shape of the attosecond XUV pulse can be precisely reconstructed. After the theoretical formulation of the problem, we present numerical examples for H atom and show that, depending on the choice of energy separation of two bound states, a different accuracy is reached to characterize attosecond XUV pulses.

Ultrafast nuclear spin polarization for isotopes with large nuclear spin

We theoretically investigate the temporal dynamics of nuclear spin induced by short laser pulses. To realize ultrafast nuclear spin polarization, we coherently excite the hyperfine manifolds and let it evolve in time. An appropriate choice of the state with large hyperfine splittings allows us to realize ultrafast nuclear spin polarization. As specific examples, we consider the various isotopes of Mg and Ca with different values of nuclear spin (1/2, 3/2, 5/2, and 7/2) and show that 89%, 76%, 49%, and 36% of nuclear spin polarization can be achieved, respectively, within a few nanoseconds, which is at least 2 to 3 orders of magnitude shorter than the time needed for any known optical methods. Because of its ultrafast nature, our scheme would be very effective not only for stable nuclei but also unstable nuclei with a lifetime as short as 1 μs.

Polarizing nuclear-spin by a sequence of short-laser pulses: Application to polarize muonium

We propose a new and simple scheme to polarize nuclear-spin by a sequence of short-laser pulses. This is a variant of optical pumping, but completely free from a complicated optimization procedure which is usually required to match the laser spectrum to that of the hyperfine transition lines of the target. Moreover the time needed to complete the polarization is in the order of tens of ns, which is more than a few orders of magnitude shorter than that usually needed to polarize nuclei with a continuous-wave (CW) laser. As a specific example we apply the idea to polarize muonium (μ+e−, lifetime 2.2 μ s).

Strong-field ionization of a heteronuclear diatomic molecule

We theoretically study strong-field ionization of a heteronuclear diatomic molecule, CO, by calculating the photoelectron angular distributions (PADs) and the total ionization rates using linearly and circularly polarized laser fields. We find that, although the PADs of CO generally do not have inversion symmetry, they become more inversion symmetric as the photoelectron energy increases. Heteronuclear features of CO upon ionization are better understood by comparing the results with those of a representative of homonuclear molecules, N{sub 2}, in that, although there are some similarities between CO and N{sub 2} due to the same orbital symmetry, {sigma}{sub g}, there are some differences between them in terms of the ionization suppression and orientation dependence of the total ionization yield. Namely, CO behaves more like an atom in the low-intensity range in a sense that ionization takes place mainly from the neighborhood of the C core, while it behaves more like a double-core molecule in the high-intensity range since ionization takes place from the neighborhood of both C and O cores. This explains why ionization suppression of CO is not seen at the low intensity but it becomes more visible at the high intensity range.

Multiphoton ionization of the calcium atom by linearly and circularly polarized laser fields

We theoretically study multiphoton ionization of the Ca atom irradiated by the second (photon energy 3.1 eV) and third (photon energy 4.65 eV) harmonics of Ti:sapphire laser pulses (photon energy 1.55 eV). Because of the dense energy level structure the second and third harmonics of a Ti:sapphire laser are nearly single-photon resonant with the 4s4p {sup 1}P{sup o} and 4s5p {sup 1}P{sup o} states, respectively. Although two-photon ionization takes place through the near-resonant intermediate states with the same symmetry in both cases, it turns out that there are significant differences between them. The photoelectron energy spectra exhibit the absence or presence of substructures. More interestingly, the photoelectron angular distributions clearly show that the main contribution to the ionization processes by the third harmonic arises from the far-off-resonant 4s4p {sup 1}P{sup o} state rather than the near-resonant 4s5p {sup 1}P{sup o} state. These findings can be attributed to the fact that the dipole moment for the 4s{sup 2} {sup 1}S{sup e}-4s5p {sup 1}P{sup o} transition is much smaller than that for the 4s{sup 2} {sup 1}S{sup e}-4s4p {sup 1}P{sup o} transition.

First Step Toward Ultrafast Nuclear‐Spin Polarization: All‐optical Control and Direct Detection of Ultrafast Electron‐Spin Polarization Using Femtosecond Laser Pulses

We demonstrate the first experimental observation of ultrafast electron‐spin polarization upon photoionization using femtosecond and nanosecond lasers. For the optical detection of electron‐spin polarization we measure the laser‐induced fluorescence of photoions. The experimental results agree well with our theoretical results obtained by solving the time‐dependent Schrodinger equations.