Control of electronic transport in graphene by electromagnetic dressing.

We demonstrated theoretically that the renormalization of the electron energy spectrum near the Dirac point of graphene by a strong high-frequency electromagnetic field (dressing field) drastically depends on polarization of the field. Namely, linear polarization results in an anisotropic gapless energy spectrum, whereas circular polarization leads to an isotropic gapped one. As a consequence, the stationary (dc) electronic transport in graphene strongly depends on parameters of the dressing field: A circularly polarized field monotonically decreases the isotropic conductivity of graphene, whereas a linearly polarized one results in both giant anisotropy of conductivity (which can reach thousands of percents) and the oscillating behavior of the conductivity as a function of the field intensity. Since the predicted phenomena can be observed in a graphene layer irradiated by a monochromatic electromagnetic wave, the elaborated theory opens a substantially new way to control electronic properties of graphene with light.

Since the discovery of graphene1, it has attracted the persistent interest of the scientific community. Particularly, the influence of an electromagnetic field on the electronic properties of graphene is in the focus of attention23. Usually, the electron-field interaction is considered within the regime of weak light-matter coupling, where the electron energy spectrum is assumed to be unperturbed by photons. However, a lot of interesting physical effects can be expected within the regime of strong light-matter coupling, where the electron energy spectrum is strongly modified by a high-frequency electromagnetic field. Following the conventional classification, this regime is jurisdictional to quantum optics which is an established part of modern physics45. Therefore, the developing of interdisciplinary research at the border between graphene physics and quantum optics is on the scientific agenda.

The methodology of quantum optics lies at the basis of various exciting fields of modern physics, including quantum information6, polaritonics7, quantum teleportation89, quantum cryptography1011, etc. Particularly, it allows to describe fundamental physical effects (e.g., Bose-Einstein condensation of polaritons12 and optical bistability13) and creates a basis of modern technological applications (e.g., optical logic circuits14, novel sources of terahertz emission15, and novel types of lasers1617). Within the quantum optics approach, the system “electron + strong electromagnetic field” should be considered as a whole. Such a bound electron-field system, which was called “electron dressed by field” (dressed electron), became a commonly used model in modern physics45. The field-induced modification of the energy spectrum and wave functions of dressed electrons was discovered many years ago and has been studied in detail in various atomic systems181920212223 and condensed matter24252627282930313233. In graphene-related research, the attention has been paid to the field-induced modification of energy spectrum of dressed electrons3435363738394041, optical response of dressed electrons42, transport of dressed electrons in graphene-based p-n junctions43 and electronic transport through dressed edge states in graphene4445464748. As to stationary (dc) transport properties of a spatially homogeneous graphene layer dressed by light, they still await detailed analysis. The present Report is aimed to fill partially this gap at the border between graphene physics and quantum optics.

Model

For definiteness, we will restrict our consideration to the case of electron states near the Dirac point of a single graphene sheet subjected to an electromagnetic wave propagating perpendicularly to the graphene plane. Let the graphene sheet lie in the plane at , and the wave propagate along the z axis [see Fig. 1]. Then electronic properties of the graphene are described by the Hamiltonian23

Figure 1

Sketch of the electron-field system under consideration.

The graphene sheet dressed by (a) circularly polarized electromagnetic wave with the amplitude and (b) linearly polarized one.

where is the Pauli matrix vector, is the electron wave vector in the graphene plane, v is the electron velocity in graphene near the Dirac point, e is the electron charge, and is the vector potential of the electromagnetic wave in the graphene plane. In what follows, we will be to assume that the wave frequency, ω, lies far from the resonant frequencies of graphene, . Solving the non-stationary Schrödinger equation with the Hamiltonian (1),

we can obtain both the energy spectrum of electrons dressed by the electromagnetic field, , and their wave functions (see technical details of the solving within the Supplementary Information attached to the Report).

For the case of the circularly polarized electromagnetic field with the vector potential

we arrive at the energy spectrum of the dressed electrons,

where signs “+” and “−” correspond to the conduction band and valence band of graphene, respectively,

is the field-induced band gap in graphene, is the amplitude of electric field of the electromagnetic wave, and the field frequency ω is assumed to satisfy the condition of . Corresponding wave functions of electrons dressed by the circularly-polarized field read as

where is the electron radius-vector in the graphene plane, are the known basic functions of the graphene Hamiltonian (the periodical functions arisen from atomic π-orbitals of the two crystal sublattices of graphene)2, is the plane electron wave, S is the graphene area, and θ is the azimuth angle of electron in the space of wave vector, .

In the case of linearly polarized electromagnetic field with the vector potential

directed along the x axis, the energy spectrum of the dressed electrons reads as

and the corresponding wave functions of dressed electrons are

where

is the Bessel function of the first kind, and the field frequency is assumed to satisfy the condition of .

The energy spectra of dressed electrons, (2) and (5), are pictured schematically in Fig. 2. As to a consistent derivation of Eqs (2, 3, 4, 5, 6, 7), it can be found within the Supplementary Information attached to the Report. In order to verify the derived expressions, it should be stressed that the energy spectrum of electrons dressed by a classical circularly polarized field, which is given by Eq. (2), exactly coincides with the energy spectrum of electrons dressed by a quantized field in the limit of large photon occupation numbers36. This can serve as a proof of physical correctness of the presented approach elaborated for a classical dressing field.

Figure 2

The energy spectrum of dressed electrons in graphene for the dressing field with different polarizations: (a) circularly polarized dressing field; (b) dressing field polarized along the x axis. The energy spectrum of electrons in absence of the dressing field is plotted by the dotted lines and is the Fermi energy.

In order to calculate transport properties of dressed electrons, we have to solve the scattering problem for nonstationary electron states (4) and (6). Following the scattering theory for dressed conduction electrons32, the problem comes to substituting the wave functions of dressed electrons (4) and (6) into the conventional expression for the Born scattering probability49. Assuming a total scattering potential in a graphene sheet, , to be smooth within an elementary crystal cell of graphene, we can write its matrix elements as

where , and is the Kronecker delta. As a result, the Born scattering probability for dressed electronic states in graphene takes the form

where

for the case of circularly polarized dressing field, and

for the case of linearly polarized dressing field.

In what follows, we will assume that the wave frequency, ω, meets the condition

where is the electron relaxation time in an unirradiated graphene, which should be considered as a phenomenological parameter taken from experiments. It is well-known that the intraband (collisional) absorption of wave energy by conduction electrons is negligibly small under condition (11) (see, e.g., Refs 32,50,51). Thus, the considered electromagnetic wave can be treated as a purely dressing field which can be neither absorbed nor emitted by conduction electrons. As a consequence, the field does not heat the electron gas and, correspondingly, the electrons are in thermodynamic equilibrium with a thermostat. Therefore, electron distribution under the condition (11) can be described by the conventional Fermi-Dirac function, where the energy of “bare” electron should be replaced with the energy of dressed electron (2),(5). Substituting both this Fermi-Dirac function and the scattering probability (8) into the conventional kinetic Boltzmann equation, we can analyze the stationary (dc) transport properties of dressed electrons in graphene. Within this approach, we take into account the two key physical factors arisen from a dressing field: (i) modification of the electron energy spectra (2) and (5) by the dressing field; (ii) renormalization of the electron scattering probability (8–10) by the dressing field.

Results and Discussion

Let us focus our attention on the dc conductivity of the dressed electrons. Generally, the density of the conduction electrons can be tuned by applying a bias voltage which fixes the Fermi energy, , of electron gas2. Assuming the Fermi energy to be in the conduction band and the temperature to be zero, let us apply a stationary (dc) electric field to the graphene sheet. It follows from the conventional Boltzmann equation for conduction electrons (see, e.g., Refs 2,52) that the electric current density, J, is given by the expression

where is the electron velocity, and is the relaxation time. In the most general case of anisotropic electron scattering, this relaxation time is given by the equation53

Substituting the scattering probability of dressed electron (8) into Eq. (13), we can obtain from Eqs (12,13) the conductivity of dressed graphene, .

To simplify calculations, let us consider the electron scattering within the s-wave approximation49, where the matrix elements do not depend on the angle . Substituting the probability (8) into Eqs (12,13), we arrive at the isotropic conductivity of a graphene dressed by a circularly polarized field, , which is given by the expression

where . It is seen in Fig. 3a that the conductivity (14) monotonically decreases with increasing field intensity . Physically, this behavior is a consequence of decreasing Fermi velocity, , with increasing field amplitude (see Fig. 2a). For the case of a dressing field linearly polarized along the x axis, the conductivity is plotted in Fig. 3b,c. There are the two main features of the conductivity as a function of the dressing field intensity: Firstly, the conductivity oscillates, and, secondly, the giant anisotropy of the conductivity, appears (see the insert in Fig. 3b). The oscillating behavior arises from the Bessel functions which take place in both the energy spectrum (5) and the scattering probability (8). As to the conductivity anisotropy, it is caused by the field-induced anisotropy of the energy spectrum (5). Namely, the linearly polarized dressing field turns the round (isotropic) Fermi line of unperturbed graphene into the strongly anisotropic ellipse line (see Fig. 2b). As a result, the Fermi velocities of dressed electrons along the axes are strongly different and the discussed anisotropic conductivity appears. It should be stressed that the aforementioned features of electronic properties are typical exclusively for linear electron dispersion and, correspondingly, do not take place in a dressed electron gas with parabolic dispersion33. To avoid misunderstandings, it should be noted also that the zeros of conductivity in Fig. 3b lie within physically irrelevant areas pictured by dashed lines. Formally, these irrelevant areas correspond to the broken condition , which is crucial for the correctness of the energy spectrum (5) at the Fermi energy. Thus, the zeros have no physical meaning and should be ignored.

Figure 3

The conductivity of dressed electrons in graphene for the dressing field with the different polarizations: (a) circularly polarized dressing field; (b,c) dressing field polarized along the x axis. Physically relevant regions of the field parameters, where the developed theory is applicable, correspond to the solid lines in the plot (b) and wide areas between the dashed lines in the plot (c).

It is seen in Fig. 3 that the behavior of conductivity is qualitatively different for the dressing field with different polarizations. Physically, the strong polarization dependence of electronic transport follows directly from the strong polarization dependence of energy spectrum of dressed electrons. Namely, the energy spectrum of electrons dressed by a circularly polarized field (2) is isotropic and has the field-induced gap (3) at the Dirac point. In contrast, the energy spectrum of electrons dressed by the linearly polarized field (5) is gapless and has the field-induced anisotropy arisen from the Bessel function in Eq. (7). These differences in the spectra (2) and (5) lead to the discussed difference of transport for electrons dressed by circularly polarized light and linearly polarized one. It should be noted that an electromagnetic field can open energy gaps within conduction and valence bands at electron wave vectors (see, e.g., Refs 38, 39, 40, 41). These gaps arise from the optical (ac) Stark effect and take place at resonant points of the Brillouin zone, where the condition of is satisfied. Certainly, the basic expressions (2–7) are not applicable near the Stark gaps. However, these gaps lie far from the Dirac point in the case of high-frequency dressing field. Therefore, they do not influence on low-energy electronic transport under consideration.

Conclusions

We have shown that the transport properties of electrons in graphene are strongly affected by a dressing field. Namely, a circularly polarized dressing field monotonically decreases the isotropic conductivity of graphene, whereas a linearly polarized dressing field results in the oscillating behavior of the conductivity and its giant anisotropy. As a result, the dc transport properties of graphene can be effectively controlled by a strong high-frequency electromagnetic field. From the viewpoint of possible applications, the discussed effect can make graphene more tunable. Particularly, the switching times for conductivity of graphene controlled by a high-frequency field are expected to be shorter then for the case of conventional electrostatic control of conductivity by gate electrodes. This can create physical prerequisites for novel graphene-based optoelectronic devices.

Additional Information

How to cite this article: Kristinsson, K. et al. Control of electronic transport in graphene by electromagnetic dressing. Sci. Rep. 6, 20082; doi: 10.1038/srep20082 (2016).