We report an improvement of three orders of magnitude in the spectral resolution of a recently pr... more We report an improvement of three orders of magnitude in the spectral resolution of a recently proposed single-comb terahertz spectroscopy [Opt. Lett.39, 5669 (2014)]. The improvement is achieved by using a femtosecond optical pulse train with a tunable repetition rate. Terahertz comb with tunable spectral line spacing generated by the train is detected via nonlinear mixing with a harmonic of a CW signal from a microwave frequency synthesizer. By applying this technique to the low-pressure gas spectroscopy, we achieved a 100 kHz spectral resolution in measuring separate absorption lines of the rotational manifold of fluoroform (CF<sub>3</sub>H).
ABSTRACT The dynamics of electromagnetic fields in open waveguiding structures whose dielectric p... more ABSTRACT The dynamics of electromagnetic fields in open waveguiding structures whose dielectric properties are changed in time due to rapid plasma creation is considered. The phenomenon of transient coupling between bulk and surface waves is found. The applicability of this phenomenon for input/output of electromagnetic energy is discussed for simple waveguiding structures with both semiconductor and gaseous plasmas. In particular, a component for transient input of an electromagnetic wave into a planar dielectric waveguide covered with a nonstationary semiconductor film is proposed. Using the example of standing waves we show that the transient dynamics is sensitive to the values of the electromagnetic fields at the moment of the plasma density growth. The knowledge of the transient dynamics in rapidly ionized material can be applied for the control of guided modes in open waveguiding structures of integrated optics and (sub)millimeter wave electronics, rapid manipulation by coupling between electromagnetic radiation and guided modes, and ultrafast transient spectroscopy of an electron-hole plasma in semiconductors.
The scattering of a surface plasmon polariton in a semiconductor slab by rapid plasma creation in... more The scattering of a surface plasmon polariton in a semiconductor slab by rapid plasma creation in the slab is explored. As a result of the scattering, the initial surface wave breaks up into two frequency-upshifted surface waves propagating in opposite directions along the slab and generates a static magnetic field and a dc inside the slab and transient radiation that escapes the slab. A part of the initial energy can also be trapped inside the slab owing to total internal reflection and forms frequency-upshifted guided modes of the slab. The scattering of a surface wave by a rapidly created plasma allows the analysis of basic processes that occur in nonstationary media in the presence of boundaries. The practical applications include the control of propagation of guided radiation in integrated optics devices, coupling the radiation out of waveguides, and ultrafast transient spectroscopy of an electron-hole plasma in semiconductors.
Journal of Electromagnetic Waves and Applications, 1996
ABSTRACT The results of theoretical investigation on transformation of the character of polarizat... more ABSTRACT The results of theoretical investigation on transformation of the character of polarization of electromagnetic wave under reflection from thin (comparing to the wavelength) solid-plasma (semiconductor) film are presented. The phenomenon of strong transformation is pointed out to occur when the wave frequency is close to the frequency of plasma resonance in the film. Various regimes of the transformation are studied. Influence of externally applied magnetostatic field and film inhomogeneity is investigated as well.
Mie resonances of dielectric spheres exhibit artificial dispersion in effective permeability and ... more Mie resonances of dielectric spheres exhibit artificial dispersion in effective permeability and permittivity. We propose a simple and possible way to construct a two-dimensional metamaterial with negative refractive index in the terahertz region. The dielectric spheres made of TiO2 are periodically arranged on a metal-mesh. The transmission peaks caused by the interaction between the TiO2 spheres and the metal-mesh are obtained. The transmission and the phase shift spectra suggest that the negative refractive index is realized by the combination of the negative permittivity of the metal-mesh and the negative permeability of TiO2 of the Mie resonance.
Strong electric and magnetic fields in the terahertz frequency range are required for many applic... more Strong electric and magnetic fields in the terahertz frequency range are required for many applications, including particle acceleration, molecular orientation, and terahertz streaking techniques. The strongest fields are currently generated by optical rectification of ultrashort laser pulses in nonlinear crystals. Multiphoton absorption is considered as one of the main factors that limits the optical pump intensity. It leads to a depletion of the pump pulse and photogeneration of free carriers that absorb terahertz radiation.
We put forward measuring Cherenkov-type terahertz emission from a moving pulse of magnetization a... more We put forward measuring Cherenkov-type terahertz emission from a moving pulse of magnetization as a method to explore ultrafast magnetic phenomena triggered in solids by femtosecond laser pulses. By applying the method to terbium-gallium-garnet, we discover the paramagnetic nature of ultrafast inverse Faraday effect in this material with the Verdet constant 3-10 times smaller than its table quasistatic value. Keywords—terahertz; spectroscopy; femtomagnetism Ultrafast magnetic phenomena triggered in solids by femtosecond laser pulses have attracted much interest in recent years both from the point of view of the physics of magnetism and due to their potential applications in information storage and processing technologies [1]. The observations of ultrafast demagnetization of ferromagnets and spin reorientation in antiferromagnets were interpreted in terms of laser-induced heating. Non-thermal optomagnetic phenomena, such as lasertriggered spin oscillations in rare-earth orthoferrite...
We explore the temporal dynamics of a surface plasmon on a graphene sheet after an abrupt increas... more We explore the temporal dynamics of a surface plasmon on a graphene sheet after an abrupt increase of graphene's carrier density. The plasmon is transformed into two frequency-upshifted surface plasmons propagating in the forward and backward directions, transient free-space radiation, and two-stream dc motion of carriers. The two-stream carrier motion carries zero net electric current and therefore does not generate any magnetic field. Nevertheless, it can consume a substantial fraction, up to a half, of the initial plasmon energy in the form of the kinetic energy of the carriers. We revisit the recent claim in the literature that graphene's nonstationarity can amplify plasmons.
It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A 95, 063817... more It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A 95, 063817 (2017)] that simultaneous optical rectification and multiphoton absorption of an ultrashort laser pulse in an electro-optic crystal can generate a quasistatic electromagnetic precursor propagating ahead of the laser pulse. We explore this phenomenon by finite-difference-time-domain modeling of Maxwell's equations supplemented with the constitutive relations of the optically ionized crystal. In particular, we elucidate the effects of the laser beam width and the pump depletion on the precursor generation. We show that focusing of the laser beam does not destroy the formation of the precursor, and the detrimental effect of the pump depletion can be eliminated by chirping the pump laser pulse.
We explore the effect of quasistatic field generation in a nonstationary medium. The nonstationar... more We explore the effect of quasistatic field generation in a nonstationary medium. The nonstationarity is provided by time-dependent density of free carriers created by an ultrashort laser pulse, a situation which is common in electro-optic schemes of THz generation. The quasistatic fields can serve as indicators of ultrafast carrier dynamics and can be used for particle acceleration, control of magnetic materials, streaking techniques, and spectroscopy.
We found theoretically that optical rectification of an ultrashort laser pulse in an electro-opti... more We found theoretically that optical rectification of an ultrashort laser pulse in an electro-optic crystal can produce a terahertz pulse with a quasistatic electromagnetic precursor propagating ahead of the pulse. The strengths of the electric and magnetic fields in the precursor can be comparable to those in the main terahertz pulse. This effect occurs if the pump optical pulse is intense enough to produce two-photon or multiphoton ionization in the crystal. The precursor is generated by the current of the optically created carriers, which are accelerated by the electric field of the terahertz pulse. The optically generated strong quasistatic precursors can be used for particle acceleration, control of magnetic materials, and in streaking techniques.
Non-collinear THz electro-optic (EO) sampling techniques based on Cherenkov phase-matching are pr... more Non-collinear THz electro-optic (EO) sampling techniques based on Cherenkov phase-matching are presented. The non-collinear phase-matching enables us to use EO crystal with a large EO coefficient but poor phase-matching property when used in collinear scheme.
We report a new approach to the terahertz frequency comb spectroscopy (TFCS) based on nonlinear m... more We report a new approach to the terahertz frequency comb spectroscopy (TFCS) based on nonlinear mixing of a photonically generated terahertz pulse train with a continuous wave signal from an electronic synthesizer. Unlike the standard TFCS technique, this approach does not require a complex double laser system. It still retains the advantages of TFCS - high spectral resolution and wide bandwidth.
We analyze the magnetic response of a dielectric ring of arbitrary width and thickness to an appl... more We analyze the magnetic response of a dielectric ring of arbitrary width and thickness to an applied timeharmonic magnetic field. The self-consistent polarization current distribution and multiple resonances of magnetic moment are studied.
We develop a theory of terahertz Cherenkov radiation from a femtosecond laser pulse propagating a... more We develop a theory of terahertz Cherenkov radiation from a femtosecond laser pulse propagating as a Gaussian beam in an electro-optic medium. We show that for a fixed energy of the laser pulse there is an optimal size of the beam waist, which maximizes the terahertz yield.
The model of a medium made of oscillators can describe various materials, including artificial on... more The model of a medium made of oscillators can describe various materials, including artificial ones made of meta-atoms. Here the transformation of an electromagnetic wave in such a medium with rapidly growing oscillator density is studied. The initial polariton is shown to be transformed not only into new polariton modes but also into natural oscillations of the existing and created oscillators. The oscillations produce zero net polarization but consume significant amount of the initial polariton energy. Although the boundary conditions at the temporal jump are sufficient to find the new polaritons, the description of the polarization oscillations requires integrating the material equations because the oscillations are uncoupled from the fields. Under some conditions, the spatial dependence of the amplitude of the oscillations can provide a snapshot of the electromagnetic field distribution at the moment of rapid density growth. Some subtle issues related to the continuity conditions at the density jump are also discussed.
Constitutive relations for Lorentz media with a time-varying density of oscillators and a time-va... more Constitutive relations for Lorentz media with a time-varying density of oscillators and a time-varying intrinsic frequency of oscillators are derived ab initio. For an electromagnetic wave propagating in a dynamic Lorentz medium, combinations of the wave energy and frequency (adiabatic invariants) that are conserved during slow time variations of the medium are obtained. If the oscillator density varies, the invariants differ from the known ones for nondispersive dielectrics and plasmas and have a different form for increasing and decreasing oscillator densities. In both cases, however, they predict that the frequency shift is accompanied by a decrease of the wave energy. If the intrinsic frequency varies, the invariant coincides with the one for a nondispersive dielectric and predicts the wave amplification and attenuation with the increase and decrease of the frequency, respectively. It is also shown that a medium with a decreasing density of oscillating dipoles cannot be described by a time-varying dielectric permittivity ε(t) even in the nondispersive limit.
We report an improvement of three orders of magnitude in the spectral resolution of a recently pr... more We report an improvement of three orders of magnitude in the spectral resolution of a recently proposed single-comb terahertz spectroscopy [Opt. Lett.39, 5669 (2014)]. The improvement is achieved by using a femtosecond optical pulse train with a tunable repetition rate. Terahertz comb with tunable spectral line spacing generated by the train is detected via nonlinear mixing with a harmonic of a CW signal from a microwave frequency synthesizer. By applying this technique to the low-pressure gas spectroscopy, we achieved a 100 kHz spectral resolution in measuring separate absorption lines of the rotational manifold of fluoroform (CF<sub>3</sub>H).
ABSTRACT The dynamics of electromagnetic fields in open waveguiding structures whose dielectric p... more ABSTRACT The dynamics of electromagnetic fields in open waveguiding structures whose dielectric properties are changed in time due to rapid plasma creation is considered. The phenomenon of transient coupling between bulk and surface waves is found. The applicability of this phenomenon for input/output of electromagnetic energy is discussed for simple waveguiding structures with both semiconductor and gaseous plasmas. In particular, a component for transient input of an electromagnetic wave into a planar dielectric waveguide covered with a nonstationary semiconductor film is proposed. Using the example of standing waves we show that the transient dynamics is sensitive to the values of the electromagnetic fields at the moment of the plasma density growth. The knowledge of the transient dynamics in rapidly ionized material can be applied for the control of guided modes in open waveguiding structures of integrated optics and (sub)millimeter wave electronics, rapid manipulation by coupling between electromagnetic radiation and guided modes, and ultrafast transient spectroscopy of an electron-hole plasma in semiconductors.
The scattering of a surface plasmon polariton in a semiconductor slab by rapid plasma creation in... more The scattering of a surface plasmon polariton in a semiconductor slab by rapid plasma creation in the slab is explored. As a result of the scattering, the initial surface wave breaks up into two frequency-upshifted surface waves propagating in opposite directions along the slab and generates a static magnetic field and a dc inside the slab and transient radiation that escapes the slab. A part of the initial energy can also be trapped inside the slab owing to total internal reflection and forms frequency-upshifted guided modes of the slab. The scattering of a surface wave by a rapidly created plasma allows the analysis of basic processes that occur in nonstationary media in the presence of boundaries. The practical applications include the control of propagation of guided radiation in integrated optics devices, coupling the radiation out of waveguides, and ultrafast transient spectroscopy of an electron-hole plasma in semiconductors.
Journal of Electromagnetic Waves and Applications, 1996
ABSTRACT The results of theoretical investigation on transformation of the character of polarizat... more ABSTRACT The results of theoretical investigation on transformation of the character of polarization of electromagnetic wave under reflection from thin (comparing to the wavelength) solid-plasma (semiconductor) film are presented. The phenomenon of strong transformation is pointed out to occur when the wave frequency is close to the frequency of plasma resonance in the film. Various regimes of the transformation are studied. Influence of externally applied magnetostatic field and film inhomogeneity is investigated as well.
Mie resonances of dielectric spheres exhibit artificial dispersion in effective permeability and ... more Mie resonances of dielectric spheres exhibit artificial dispersion in effective permeability and permittivity. We propose a simple and possible way to construct a two-dimensional metamaterial with negative refractive index in the terahertz region. The dielectric spheres made of TiO2 are periodically arranged on a metal-mesh. The transmission peaks caused by the interaction between the TiO2 spheres and the metal-mesh are obtained. The transmission and the phase shift spectra suggest that the negative refractive index is realized by the combination of the negative permittivity of the metal-mesh and the negative permeability of TiO2 of the Mie resonance.
Strong electric and magnetic fields in the terahertz frequency range are required for many applic... more Strong electric and magnetic fields in the terahertz frequency range are required for many applications, including particle acceleration, molecular orientation, and terahertz streaking techniques. The strongest fields are currently generated by optical rectification of ultrashort laser pulses in nonlinear crystals. Multiphoton absorption is considered as one of the main factors that limits the optical pump intensity. It leads to a depletion of the pump pulse and photogeneration of free carriers that absorb terahertz radiation.
We put forward measuring Cherenkov-type terahertz emission from a moving pulse of magnetization a... more We put forward measuring Cherenkov-type terahertz emission from a moving pulse of magnetization as a method to explore ultrafast magnetic phenomena triggered in solids by femtosecond laser pulses. By applying the method to terbium-gallium-garnet, we discover the paramagnetic nature of ultrafast inverse Faraday effect in this material with the Verdet constant 3-10 times smaller than its table quasistatic value. Keywords—terahertz; spectroscopy; femtomagnetism Ultrafast magnetic phenomena triggered in solids by femtosecond laser pulses have attracted much interest in recent years both from the point of view of the physics of magnetism and due to their potential applications in information storage and processing technologies [1]. The observations of ultrafast demagnetization of ferromagnets and spin reorientation in antiferromagnets were interpreted in terms of laser-induced heating. Non-thermal optomagnetic phenomena, such as lasertriggered spin oscillations in rare-earth orthoferrite...
We explore the temporal dynamics of a surface plasmon on a graphene sheet after an abrupt increas... more We explore the temporal dynamics of a surface plasmon on a graphene sheet after an abrupt increase of graphene's carrier density. The plasmon is transformed into two frequency-upshifted surface plasmons propagating in the forward and backward directions, transient free-space radiation, and two-stream dc motion of carriers. The two-stream carrier motion carries zero net electric current and therefore does not generate any magnetic field. Nevertheless, it can consume a substantial fraction, up to a half, of the initial plasmon energy in the form of the kinetic energy of the carriers. We revisit the recent claim in the literature that graphene's nonstationarity can amplify plasmons.
It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A 95, 063817... more It was recently predicted [M. I. Bakunov, A. V. Maslov, and M. V. Tsarev, Phys. Rev. A 95, 063817 (2017)] that simultaneous optical rectification and multiphoton absorption of an ultrashort laser pulse in an electro-optic crystal can generate a quasistatic electromagnetic precursor propagating ahead of the laser pulse. We explore this phenomenon by finite-difference-time-domain modeling of Maxwell's equations supplemented with the constitutive relations of the optically ionized crystal. In particular, we elucidate the effects of the laser beam width and the pump depletion on the precursor generation. We show that focusing of the laser beam does not destroy the formation of the precursor, and the detrimental effect of the pump depletion can be eliminated by chirping the pump laser pulse.
We explore the effect of quasistatic field generation in a nonstationary medium. The nonstationar... more We explore the effect of quasistatic field generation in a nonstationary medium. The nonstationarity is provided by time-dependent density of free carriers created by an ultrashort laser pulse, a situation which is common in electro-optic schemes of THz generation. The quasistatic fields can serve as indicators of ultrafast carrier dynamics and can be used for particle acceleration, control of magnetic materials, streaking techniques, and spectroscopy.
We found theoretically that optical rectification of an ultrashort laser pulse in an electro-opti... more We found theoretically that optical rectification of an ultrashort laser pulse in an electro-optic crystal can produce a terahertz pulse with a quasistatic electromagnetic precursor propagating ahead of the pulse. The strengths of the electric and magnetic fields in the precursor can be comparable to those in the main terahertz pulse. This effect occurs if the pump optical pulse is intense enough to produce two-photon or multiphoton ionization in the crystal. The precursor is generated by the current of the optically created carriers, which are accelerated by the electric field of the terahertz pulse. The optically generated strong quasistatic precursors can be used for particle acceleration, control of magnetic materials, and in streaking techniques.
Non-collinear THz electro-optic (EO) sampling techniques based on Cherenkov phase-matching are pr... more Non-collinear THz electro-optic (EO) sampling techniques based on Cherenkov phase-matching are presented. The non-collinear phase-matching enables us to use EO crystal with a large EO coefficient but poor phase-matching property when used in collinear scheme.
We report a new approach to the terahertz frequency comb spectroscopy (TFCS) based on nonlinear m... more We report a new approach to the terahertz frequency comb spectroscopy (TFCS) based on nonlinear mixing of a photonically generated terahertz pulse train with a continuous wave signal from an electronic synthesizer. Unlike the standard TFCS technique, this approach does not require a complex double laser system. It still retains the advantages of TFCS - high spectral resolution and wide bandwidth.
We analyze the magnetic response of a dielectric ring of arbitrary width and thickness to an appl... more We analyze the magnetic response of a dielectric ring of arbitrary width and thickness to an applied timeharmonic magnetic field. The self-consistent polarization current distribution and multiple resonances of magnetic moment are studied.
We develop a theory of terahertz Cherenkov radiation from a femtosecond laser pulse propagating a... more We develop a theory of terahertz Cherenkov radiation from a femtosecond laser pulse propagating as a Gaussian beam in an electro-optic medium. We show that for a fixed energy of the laser pulse there is an optimal size of the beam waist, which maximizes the terahertz yield.
The model of a medium made of oscillators can describe various materials, including artificial on... more The model of a medium made of oscillators can describe various materials, including artificial ones made of meta-atoms. Here the transformation of an electromagnetic wave in such a medium with rapidly growing oscillator density is studied. The initial polariton is shown to be transformed not only into new polariton modes but also into natural oscillations of the existing and created oscillators. The oscillations produce zero net polarization but consume significant amount of the initial polariton energy. Although the boundary conditions at the temporal jump are sufficient to find the new polaritons, the description of the polarization oscillations requires integrating the material equations because the oscillations are uncoupled from the fields. Under some conditions, the spatial dependence of the amplitude of the oscillations can provide a snapshot of the electromagnetic field distribution at the moment of rapid density growth. Some subtle issues related to the continuity conditions at the density jump are also discussed.
Constitutive relations for Lorentz media with a time-varying density of oscillators and a time-va... more Constitutive relations for Lorentz media with a time-varying density of oscillators and a time-varying intrinsic frequency of oscillators are derived ab initio. For an electromagnetic wave propagating in a dynamic Lorentz medium, combinations of the wave energy and frequency (adiabatic invariants) that are conserved during slow time variations of the medium are obtained. If the oscillator density varies, the invariants differ from the known ones for nondispersive dielectrics and plasmas and have a different form for increasing and decreasing oscillator densities. In both cases, however, they predict that the frequency shift is accompanied by a decrease of the wave energy. If the intrinsic frequency varies, the invariant coincides with the one for a nondispersive dielectric and predicts the wave amplification and attenuation with the increase and decrease of the frequency, respectively. It is also shown that a medium with a decreasing density of oscillating dipoles cannot be described by a time-varying dielectric permittivity ε(t) even in the nondispersive limit.
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Papers by M. Bakunov