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Quantum control of high harmonic generation in anharmonic potential Muhamad Koyimatu a,b , Kiyoshi Nishikawa b a Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung 40132 Indonesia, E-mail: s105koyimatu@mail.chem.itb.ac.id b Institute of Science and Engineering, Kanazawa University, Kakuma, Kanazawa 920-1192 Japan, E-mail: kiyoshi@wriron1.s.kanazawa-u.ac.jp Abstract. In this work, we first aim to realize the high harmonic generation(HHG) spectrum in a classical and quantum treatment, and then to achieve the quantum control of the HHG spectrum. Here, we consider the HHG from the Duffing oscillator well-known as a typical anharmonic system. The HHG induced by the intense laser is a nonlinear optical process, where a molecular system absorbs n photons with energy �ω and emits one high energy photon with energy n�ω. The light generated by HHG is a promising light source with an attosecond ultrashort pulse, which could be used to analyze the electronic structure and motion in molecule. First, we perform a classical simulation to explain the structure of the HHG spectrum. Then, we investigate quantum-mechanically the mechanism of HHG, and compare the classical and quantum result. In quantum treatment, we apply the Fourier Grid Method (FGM) to obtain the eigenvalues and wavefuntion of the Duffing oscillator and use the Split Operator Method (SOM) to solve the time-dependent Schrödinger equation. Here we study the laser intensity dependence of the HHG with a single laser pulse, and then from the interference point of view, we focus on the role of the relative phase of two laser pulses and simulate to find out the phase dependence of the HHG. Finally we investigate the laser control of the HHG by changing the initial quantum state, which is a kind of a wave packet due to the superimposition of few eigenstates. Keywords: HHG, SOM, anharmonic oscillator, quantum control 1 Introduction Nonlinear optics process (NLOP) describes the response of the molecular system to the strong electromagnetic wave, and is usually induced only by a strong light such as a laser with high intensity. If the induced dipole moment of the material respond instantaneously to an applied electric field E(t), the induced polarization P (t) can be expressed in terms of the power of the electric field E(t), P (t) = χ (1) E(t) + χ (2) E 2 (t) + χ (3) E 3 (t) + · · · . (1) Above perturbation treatment of the nonlinear optics could describe accurately the characteristics of the ordinary nonlinear optical phenomenon. With sufficiently intense laser field, the higher order terms play an very important role, and give rise to Fourier components of the polarization with high harmonics of the laser frequency, emitting the electromagnetic wave with the same frequency as the polarization. The harmonic generation is an interesting phenomenon, which describe a nonlinear process of light matter interaction, and a typical example of the process is a second-order harmonic generation. 47
ISCS 2011 Selected Papers Vol.2 M. Koyimatu, K. Nishikawa Due to the great progress of the laser technique, an intense laser with the intensity 10 8 < I < 10 12 W/cm 2 have become available, and we have discovered the noble interesting multiphoton processes in molecule [1], such as multi-photon absorption (MPA), multi-photon ionization (ATI), multi-photon dissociation (MPD), above threshold dissociation (ATD), the high harmonic generation (HHG), and so on, which are nonlinear optical processes arising from the higher-order effect of the external laser field. Here we focus on the HHG, where a molecule absorbs n photons (with energy �ω ) and emits one high photon with energy n�ω. HHG is experimentally observed by irradiating a noble gas with a near-IR laser pulse with a ultra high intensity 10 14 W/cm 2 , and the energy of the radiating electromagnetic wave is in the region of the extreme-ultraviolet (XUV) [2]. It have been known that the emitted photon energy is found to be the odd harmonics of the laser frequency (2m+1)�ω, and in some cases, the harmonic order n = 2m + 1 becomes larger than 100. The HHG spectrum has a characteristic structure, namely in a low energy region of HHG spectrum, the intensity of HHG rapidly decreases as the harmonic order increases, and then the typical harmonic shows a plateau, followed by a sharp cut-off. Thus the HHG provides an attractive light source of ultrashort coherent radiation in the XUV and soft-X-ray range, and it is expected that using the attosecond laser pulses by HHG, we could observe the electronic structure in molecule and control directly the motion of electron in molecule. On the other hand, several theoretical methods have been developed to understand the basic mechanism of the HHG. Most familiar model is the sp-called three-step model by P.B.Corkum to explain semiclassically the basic generation mechanism of higher-order harmonics. Usually the potential induced by the ultrahigh intensity of the external oscillating field is comparable with the Coulomb potential due to the nucleus, so that electron moves in the deformed oscillating Coulomb potential, being a superposition of the Coloumb potential and the time dependent potential due to the laser field. When the intensity of the external field becomes close to the maximum, the electron go through the potential barrier by the tunneling ionization. Then the ionized electron is accelerated by the strong laser electric field, and acquire a lot of energy form the external field. Finally this accelerated ionized electron return back to the nucleus to recombine with it, emitting the high energy photon. Namely an electron emits the electromagnetic wave by three steps, i.e., (1) field induced tunnel ionization, (2) acceleration in the laser field, and (3) recombination and photoemission. Thus the energy of the emitted photon depends on the ionization potential of the atom and on the kinetic energy of the electron upon its return to its parent ion. The paper is organized in the following way. In section 2,we develop the theoretical treatment, where we introduce the split operator method (SOM) to solve the time-dependent Scrödinger equation, and the Fourier grid Hamiltonian (FGH) method to solve the time-independent Scrödinger equation. In SOM, the time development operator U(t) to generate the wavefunction |Ψ(t)〉 from the initial state |Ψ(0〉) is an unitary transformation so that the norm of the wavefunction is always kept constant. Furthermore it uses the fast Fourier transformation (FFT), so that it solves the dynamics of the system in a very efficient way. On the other hand, FGH method is a effective way to solve the eigenvalue problem of the Hamiltonian matrix, and yields the eigenvalues and eigenvectors, which is required to construct the initial wavepacket for the quantum control of HHG. In section 3, we describe the details of the target Duffing oscillator, which is a typical anharmonic oscillator. The anharmonic term plays an fundamental role to generate the HHG. In section 4, we perform the classical simulation of the HHG, From the numerical solution of the Newton’s equation, we estimate the dependence of the HHG spectra on the field intensity. In section 5, we develop the quantum mechanical treatment of the HHG, and calculate the HHG spectrum by changing the field intensity, and moreover investigate the relative phase dependence in the case of 48
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