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Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids

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Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Tancogne-Dejean, Nicolas et al. “Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids.” Physical Review Letters 118.8 (2017): n. pag. © 2017 American Physical Society As Published http://dx.doi.org/10.1103/PhysRevLett.118.087403 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/107908 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. week ending PRL 118, 087403 (2017) PHYSICAL REVIEW LETTERS 24 FEBRUARY 2017 Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids † Nicolas Tancogne-Dejean,1,2,* Oliver D. Mücke,3,4 Franz X. Kärtner,3,4,5,6 and Angel Rubio1,2,3,5, 1Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany 2European Theoretical Spectroscopy Facility (ETSF), Luruper Chaussee 149, 22761 Hamburg, Germany 3Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany 4The Hamburg Center for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany 5Physics Department, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany 6Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (Received 29 September 2016; published 24 February 2017) An accurate analytic model describing the microscopic mechanism of high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that (i) the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals; (ii) the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential; the yield is increased for heavier atoms; and (iii) the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure. DOI: 10.1103/PhysRevLett.118.087403 Atoms and molecules interacting with strong laser pulses The process of HHG from gases is by now well under- emit high-order harmonics of the fundamental driving laser stood in terms of the three-step model [13–15] in which field. The high-harmonic generation (HHG) in gases is electrons are first promoted from the ground state of the routinely used nowadays to produce isolated attosecond atom (or molecule) to the continuum, then accelerated by pulses [1–4] and coherent radiation ranging from the visible the electric field, and finally recombine with the parent ion. to soft x rays [5]. Because of a higher electronic density, With this simple, intuitive model most of the observed solids are one promising route towards compact, brighter effects are well described, in particular, the dependence of HHG sources. The recent observation of nonperturbative the harmonic cutoff energy on driver wavelength and HHG in solids without damage [6–10], extending even intensity. In the case of solids, electrons are promoted to beyond the atomic limit [10], has opened the door to the discrete conduction bands, where they do not evolve freely. observation and control of attosecond electron dynamics in This leads, for instance, to a different linear field dependence solids [8,9,11], all-optical band-structure reconstruction for the cutoff energy [6], different time-frequency character- [12], and solid-state sources of isolated extreme-ultraviolet istics of the harmonic emission between atoms and solids pulses [9,11]. However, in contrast to HHG from gases, the [8,11,16], and a different ellipticity dependence [6,17]. microscopic mechanism underlying HHG from solids is Historically, HHG in solids was first discussed in terms of still controversially debated in the attoscience community, Bloch oscillations (i.e., pure intraband dynamics) [18–20], in some cases casting doubts on the validity of the proposed and more recently mainly analyzed using simplified models microscopic model and resulting in confusion about the based on numerical solutions of the semiconductor Bloch correct interpretation of experimental data. Various com- equations [21–23] treating the complex, coupled interband peting simplified models have been proposed but they often and intraband dynamics, with the exception of the ab initio are based on strong approximations and a priori assump- simulations of Ref. [24]. Even if these methods have been tions, often stating that there is a strong similarity with the successfully applied to some materials, such as GaSe [7,8,25] processes underlying atomic-gas HHG emission. However, or SiO2 [9], basic questions remain controversial and/or it is clear that many-body effects due to the crystalline unresolved, e.g., which bands are involved in the HHG structure of solids and the fermionic nature of interaction dynamics [26,27]. electrons play a decisive role that fundamentally distin- The first experimental observations of HHG from solids guishes the solid from the gas case. It is the scope of the were explained in terms of Bloch oscillations [6,7].A present work to unravel within an ab initio approach what competing model attributing the HHG mechanism to inter- the impact is of the underlying electronic band structure of band transitions (resembling the three-step model of gas the solids in the observed HHG emission. HHG [13,14]) was introduced [28,29]. For not too strong 0031-9007=17=118(8)=087403(6) 087403-1 © 2017 American Physical Society week ending PRL 118, 087403 (2017) PHYSICAL REVIEW LETTERS 24 FEBRUARY 2017 excitation in ZnO by 3.76 μm pulses, such that the electrons formula for the high-harmonic spectra. Using the current explore only the near-parabolic region of the Brillouin zone expressionR for the HHG spectra, namely, HHGðωÞ¼ 3 2 (BZ), it was found that the magnitude of the interband jFTf Ω d rð∂=∂tÞjðr;tÞgj , and plugging now Eq. (3),we contribution is larger than that of the intraband contribution obtain a general expression for the HHG spectra, [28]. Nevertheless, most theoretical works have used either a Z simplified two-band or five-band model, intrinsically ham- 3 HHGðωÞ ∝ FT d r nðr;tÞ∇v0ðrÞ pering the predictive power of the model and the full Ω microscopic understanding of the HHG process. j r;t B r;t 2 n r;t E r;t ð Þ × ð Þ ; In this Letter, using an ab initio approach based on time- þ ð Þ ð Þþ c ð4Þ dependent density-functional theory (TDDFT) [30,31],we study the microscopic origin of HHG in solids. Effects where the last two terms correspond to the Lorentz force stemming from the full electronic structure (valence and exerted by the external laser on the electronic system [34].If conduction bands) and the real crystal structure are properly we now make the dipole approximation, Eq. (4) further accounted for. We show that the nonperturbative emission of simplifies and we finally get harmonics in solids, arising from the interplay between Z 2 intraband and interband contributions, is enhanced when 3 HHGðωÞ ∝ FT d rnðr;tÞ∇v0ðrÞ þ NeEðωÞ ; ð5Þ the interband contribution is suppressed due to band- Ω structure effects. We identify that the joint density of states (JDOS) along the laser polarization is the key parameter which provides the first important physical result of the governing the weight of the interband contribution. In present work, shedding fundamental insights on the intrinsic addition, we address the still controversial question of the bulk contribution to the HHG spectra. Here, Ne is the number wavelength dependence of the cutoff energy in bulk crystals. of electrons contained in the volume Ω. Note that the HHG We start by presenting some exact analytical results (see spectra depend only on the electronic density. The second Supplemental Material [32] for details). We consider a term does not result in a nonperturbative nonlinearity and thus general interacting many-electron Hamiltonian Hˆ of the form cannot create a plateaulike HHG spectrum. The more ˆ ˆ ˆ ˆ interesting and relevant term for HHG is the first one in HðtÞ¼T þ VðtÞþW; ð1Þ Eq. (5). It shows that higher harmonics are generated by two Tˆ Vˆ t competing terms, the spatial variation of the total electronic where is the kinetic energy, ð Þ is the time-dependent density [nðr;tÞ] and the gradient of the electron-nuclei Wˆ external laser potential, and is the electron-electron potential [∇v0ðrÞ], the latter being time independent, as Coulomb interaction (the ionic motion is not considered we neglected ionic motion. In gases, the gradient of the here for the sake of simplicity). The exact equation of motion electron-nuclei potential is important, but the electronic for the total microscopic current, jðr;tÞ, can be rewritten density is low. In the case of solids, the electronic density as [33,34] is higher, but the potential is rather homogeneous, resulting in ∂ a smaller gradient of the potential than in the atomic case. In j r;t −n r;t ∇v r;t Πkin r;t Πint r;t ; fact, in the limit of a homogeneous electron gas, the gradient ∂t ð Þ¼ ð Þ ð Þþ ð Þþ ð Þ ð2Þ becomes 0, and no harmonics are generated, irrespective of where Πkinðr;tÞ and Πintðr;tÞ are the kinetic and the the value of the electronic density.
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