Our analysis based on calculations carried out with different methods but with the data taken fro... more Our analysis based on calculations carried out with different methods but with the data taken from [Optica, 3, 351 (2016)] lead to the conclusion that results obtained in [Optica, 3, 351 (2016)] are questionable in several respects.
Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating ... more Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating wave the energy locally propagates backward. In this study the energy backflow has been examined in connection with relatively simple causal unidirectional finite-energy solutions of the wave equation which are derived from a factorization of the so-called basic splash mode. Specific results are given for the energy backflow arising in known azimuthally symmetric unidirectional wavepackets, as well as in novel azimuthally asymmetric extensions. Using the Bateman-Whittaker technique, a novel finite-energy unidirectional null localized wave has been constructed that is devoid of energy backflow and has some of the topological properties of the basic Hopfion.
Frontiers in Optics 2009/Laser Science XXV/Fall 2009 OSA Optics & Photonics Technical Digest, 2009
Using SEA TADPOLE, we directly measure the spatio-temporal field of diffracting ultrashort pulses... more Using SEA TADPOLE, we directly measure the spatio-temporal field of diffracting ultrashort pulses with fs-temporal and μm-spatial resolutions. Using a circular aperture and an opaque disk, we observe boundary wave pulses including their superluminal speeds.
One of the most promising fields for picosecond spectroscopy is resonant secondary emission, as b... more One of the most promising fields for picosecond spectroscopy is resonant secondary emission, as by making use of this optical response there is no need to apply strong excitation or additional probing on the sample, which may disturb the processes under study. The introduction of a sensitive spectrochronography method [1] has made the temporal resolution of hot luminescence spectrum possible [2], providing thus a direct measure of the vibrational relaxation processes in the excited electronic state of an impurity centre. The physical situation is more intriguing when light-matter interaction cannot be considered in the perturbation approach and one has to use polaritons to describe the properties of the secondary spectrum [3]. Some special features in the temporal behaviour of the polariton luminescence spectrum of anthracene were revealed already in a nanosecond study [4]. Since the first report on picosecond luminescence kinetics [5] we have carried out a systematic study of polariton dynamics in anthracene crystals at low temperatures in order to distinct between the scattering/luminescence processes and to elucidate the effective decay channels of the primary excitations.
The phenomenon of stimulated photon echo [1] has already been investigated in a variety of molecu... more The phenomenon of stimulated photon echo [1] has already been investigated in a variety of molecular systems but in all cases the magnitude of echo signals has been limited to a small fraction of the pumping pulses intensity. The main reason for that lies in a rapid relaxation of nonequilibrium population of molecular states, which hinder the formation of spectral population gratings responsible for the stimulation of retarded optical pulses.
The relativistic aberration of a wavevector and the corresponding Doppler shift are examined in c... more The relativistic aberration of a wavevector and the corresponding Doppler shift are examined in connection with superluminal and subluminal spatiotemporally localized pulsed optical waves. The requirement of a null Doppler shift is shown to give rise to a speed associated with the relativistic velocity composition law of a double (two-step) Lorentz transformation. The effects of such a transformation are examined both in terms of four-coordinates and in the spectral domain. It is established that a subluminal pulse reverses its direction. In addition to a change in direction, the propagation term of a superluminal pulse becomes negative. The aberration due to a double Lorentz transformation is examined in detail for propagation invariant superluminal waves (X wave, Bessel X wave), as well as intensity-invariant superluminal and subluminal waves. Detailed symmetry considerations are provided for the superluminal focus X wave and the subluminal MacKinnon wavepacket.
Our analysis based on calculations carried out with different methods but with the data taken fro... more Our analysis based on calculations carried out with different methods but with the data taken from Optica 3, 351 (2016) led to the conclusion that the results obtained in Optica 3, 351 (2016) are questionable in several respects.
The Bateman-Whittaker theory, which was developed a century ago, is shown to be a comprehensive b... more The Bateman-Whittaker theory, which was developed a century ago, is shown to be a comprehensive basis for deriving a large class of null spatiotemporally localized electromagnetic waves characterized by intriguing vortical structures. In addition, it provides the modeling for studying topological structures dealing with linked and knotted electromagnetic waves.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
SUMMARY Referring to experiment, some particular problems of the distinction are discussed in thi... more SUMMARY Referring to experiment, some particular problems of the distinction are discussed in this paper. 1. If the vibronic spectra of an impurity crystal exhihits clear-cut quasi-line structure scattering, hot and ordinary luminescence can be definitely distin guished with steady state monochromatic excitation in both theory and experiment. 2. Resonant scattering does not dominate nonresonant scattering for a condensed system with rapid relaxa tional processes. Even though the excitation line is within the absorption band of a given electronic tran sition, the scattering spectrum may have significant contributions from other nonresonant electronic terms. Thus for a proper interpretation, polarization and other additional measurements are required. 3. Although invoking T2-processes generally results in conversion of a part of the scattering into lumi nescence-like emission, it will not suffice to intro duce (frequency-independent) relaxation constants to describe properly the resonant secondary emission of a crystal, but rather a detailed dynamic model is required. Using the former approach, the decomposi tion of the entire flux of resonant emission into components may turn out to be impossible since the contributions are of a mixed nature, even though the relaxation criterion generally works well (especially in nonlinear spectroscopy). 4. While its applicability to the study of more complicated systems is beyond doubt, the experimentally realizable time resolution does not provide any 315
Hot luminescence (HL) in rapidly relaxing impurity centres has been experimentally and theoretica... more Hot luminescence (HL) in rapidly relaxing impurity centres has been experimentally and theoretically studied in about ten publications (1–7). In (8) the conception of HL has been applied to exciton states of crystals. In the present paper we shall give a short survey of the basic, already published results; here we also present some new experimental data and dwell upon some general questions.
Journal of the Optical Society of America, Jul 22, 2022
Highly focused space-time wavepackets in free space have already been achieved by means of suitab... more Highly focused space-time wavepackets in free space have already been achieved by means of suitable superpositions of nondiffracting and almost undistorted spatiotemporally localized pulses. Here, we present analytically individual autofocusing luminal and superluminal localized waves that can attain high-intensity peaks and spatiotemporal localization at prespecified positions along the path of their propagation.
Using SEA TADPOLE with m-range spatial and fs-range temporal resolution, we report the first dir... more Using SEA TADPOLE with m-range spatial and fs-range temporal resolution, we report the first direct spatiotemporal measurements of ultrashort Bessel-X pulses. We demonstrate their propagation invariance and superluminal velocity and verify our results with simulations.
The wave-field produced by a ∼30 fs duration Ti:sapphire oscillator pulse behind a circular apert... more The wave-field produced by a ∼30 fs duration Ti:sapphire oscillator pulse behind a circular aperture and circular opaque disk is measured using the ultrashort-laser-pulse measurement technique, scanning SEA TADPOLE. The high spatial and temporal resolution of the measuring technique enables us to fully image the diffracted field behind the apertures and record the interference pattern produced by the so-called boundary diffraction wave pulses.
Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating ... more Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating wave the energy locally propagates backward. In this study the energy backflow has been examined in connection with relatively simple causal unidirectional finite-energy solutions of the wave equation which are derived from a factorization of the so-called basic splash mode. Specific results are given for the energy backflow arising in known azimuthally symmetric unidirectional wavepackets, as well as in novel azimuthally asymmetric extensions. Using the Bateman-Whittaker technique, a novel finite-energy unidirectional null localized wave has been constructed that is devoid of energy backflow and has some of the topological properties of the basic Hopfion.
Electromagnetic energy backflow is a phenomenon occurring in regions where the direction of the P... more Electromagnetic energy backflow is a phenomenon occurring in regions where the direction of the Poynting vector is opposite to that of the propagation of the wave field. It is particularly remarkable in the nonparaxial regime and has been exhibited in the focal region of sharply focused beams, for vector Bessel beams, and vector-valued spatiotemporally localized waves. A detailed study is undertaken of this phenomenon and the conditions for its appearance are examined in detail in the case of a superposition of four plane waves in free space, the simplest electromagnetic arrangement for the observation of negative energy flow, as well as its comprehensive and transparent physical interpretation. It is shown that the state of polarization of the constituent components of the electromagnetic plane wave quartet determines whether energy backflow takes place or not and what values the energy flow velocity assumes. Depending on the polarization angles, the latter can assume any value from c (the speed of light in vacuum) to −c in certain spatiotemporal regions of the field.
It is well known that although the group velocity of structured light pulses propagating in vacuu... more It is well known that although the group velocity of structured light pulses propagating in vacuum can be subluminal or superluminal, the upper limit of the energy flow velocity is c, the speed of light in vacuum. This inequality can be explained in terms of the reactive energy left behind by the fields. Energy and reactive energy densities have been calculated for vector-valued two-dimensional (light sheet) superluminal electromagnetic nondiffractive pulses, as well as scalar-valued and TM three-dimensional superluminal and subluminal spatiotemporally localized electromagnetic waves. Emphasis is placed on the physical formation of the reactive energy due to interference of the plane-wave constituents of the structured light waves.
We demonstrate persistent storage, recall, and conjugation of picosecond light signals from vario... more We demonstrate persistent storage, recall, and conjugation of picosecond light signals from various model objects, including a coin,-by making use of coherent optical responses in photochemically active media. A simple linear theory of holographic storage and playback of both the spatial and the temporal behavior of the signal field is shown to describe well the experimental results obtained by utilizing octaethylporphin-doped polystyrene at 1.8 K as a spectrally selective recording material.
Our analysis based on calculations carried out with different methods but with the data taken fro... more Our analysis based on calculations carried out with different methods but with the data taken from [Optica, 3, 351 (2016)] lead to the conclusion that results obtained in [Optica, 3, 351 (2016)] are questionable in several respects.
Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating ... more Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating wave the energy locally propagates backward. In this study the energy backflow has been examined in connection with relatively simple causal unidirectional finite-energy solutions of the wave equation which are derived from a factorization of the so-called basic splash mode. Specific results are given for the energy backflow arising in known azimuthally symmetric unidirectional wavepackets, as well as in novel azimuthally asymmetric extensions. Using the Bateman-Whittaker technique, a novel finite-energy unidirectional null localized wave has been constructed that is devoid of energy backflow and has some of the topological properties of the basic Hopfion.
Frontiers in Optics 2009/Laser Science XXV/Fall 2009 OSA Optics & Photonics Technical Digest, 2009
Using SEA TADPOLE, we directly measure the spatio-temporal field of diffracting ultrashort pulses... more Using SEA TADPOLE, we directly measure the spatio-temporal field of diffracting ultrashort pulses with fs-temporal and μm-spatial resolutions. Using a circular aperture and an opaque disk, we observe boundary wave pulses including their superluminal speeds.
One of the most promising fields for picosecond spectroscopy is resonant secondary emission, as b... more One of the most promising fields for picosecond spectroscopy is resonant secondary emission, as by making use of this optical response there is no need to apply strong excitation or additional probing on the sample, which may disturb the processes under study. The introduction of a sensitive spectrochronography method [1] has made the temporal resolution of hot luminescence spectrum possible [2], providing thus a direct measure of the vibrational relaxation processes in the excited electronic state of an impurity centre. The physical situation is more intriguing when light-matter interaction cannot be considered in the perturbation approach and one has to use polaritons to describe the properties of the secondary spectrum [3]. Some special features in the temporal behaviour of the polariton luminescence spectrum of anthracene were revealed already in a nanosecond study [4]. Since the first report on picosecond luminescence kinetics [5] we have carried out a systematic study of polariton dynamics in anthracene crystals at low temperatures in order to distinct between the scattering/luminescence processes and to elucidate the effective decay channels of the primary excitations.
The phenomenon of stimulated photon echo [1] has already been investigated in a variety of molecu... more The phenomenon of stimulated photon echo [1] has already been investigated in a variety of molecular systems but in all cases the magnitude of echo signals has been limited to a small fraction of the pumping pulses intensity. The main reason for that lies in a rapid relaxation of nonequilibrium population of molecular states, which hinder the formation of spectral population gratings responsible for the stimulation of retarded optical pulses.
The relativistic aberration of a wavevector and the corresponding Doppler shift are examined in c... more The relativistic aberration of a wavevector and the corresponding Doppler shift are examined in connection with superluminal and subluminal spatiotemporally localized pulsed optical waves. The requirement of a null Doppler shift is shown to give rise to a speed associated with the relativistic velocity composition law of a double (two-step) Lorentz transformation. The effects of such a transformation are examined both in terms of four-coordinates and in the spectral domain. It is established that a subluminal pulse reverses its direction. In addition to a change in direction, the propagation term of a superluminal pulse becomes negative. The aberration due to a double Lorentz transformation is examined in detail for propagation invariant superluminal waves (X wave, Bessel X wave), as well as intensity-invariant superluminal and subluminal waves. Detailed symmetry considerations are provided for the superluminal focus X wave and the subluminal MacKinnon wavepacket.
Our analysis based on calculations carried out with different methods but with the data taken fro... more Our analysis based on calculations carried out with different methods but with the data taken from Optica 3, 351 (2016) led to the conclusion that the results obtained in Optica 3, 351 (2016) are questionable in several respects.
The Bateman-Whittaker theory, which was developed a century ago, is shown to be a comprehensive b... more The Bateman-Whittaker theory, which was developed a century ago, is shown to be a comprehensive basis for deriving a large class of null spatiotemporally localized electromagnetic waves characterized by intriguing vortical structures. In addition, it provides the modeling for studying topological structures dealing with linked and knotted electromagnetic waves.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
SUMMARY Referring to experiment, some particular problems of the distinction are discussed in thi... more SUMMARY Referring to experiment, some particular problems of the distinction are discussed in this paper. 1. If the vibronic spectra of an impurity crystal exhihits clear-cut quasi-line structure scattering, hot and ordinary luminescence can be definitely distin guished with steady state monochromatic excitation in both theory and experiment. 2. Resonant scattering does not dominate nonresonant scattering for a condensed system with rapid relaxa tional processes. Even though the excitation line is within the absorption band of a given electronic tran sition, the scattering spectrum may have significant contributions from other nonresonant electronic terms. Thus for a proper interpretation, polarization and other additional measurements are required. 3. Although invoking T2-processes generally results in conversion of a part of the scattering into lumi nescence-like emission, it will not suffice to intro duce (frequency-independent) relaxation constants to describe properly the resonant secondary emission of a crystal, but rather a detailed dynamic model is required. Using the former approach, the decomposi tion of the entire flux of resonant emission into components may turn out to be impossible since the contributions are of a mixed nature, even though the relaxation criterion generally works well (especially in nonlinear spectroscopy). 4. While its applicability to the study of more complicated systems is beyond doubt, the experimentally realizable time resolution does not provide any 315
Hot luminescence (HL) in rapidly relaxing impurity centres has been experimentally and theoretica... more Hot luminescence (HL) in rapidly relaxing impurity centres has been experimentally and theoretically studied in about ten publications (1–7). In (8) the conception of HL has been applied to exciton states of crystals. In the present paper we shall give a short survey of the basic, already published results; here we also present some new experimental data and dwell upon some general questions.
Journal of the Optical Society of America, Jul 22, 2022
Highly focused space-time wavepackets in free space have already been achieved by means of suitab... more Highly focused space-time wavepackets in free space have already been achieved by means of suitable superpositions of nondiffracting and almost undistorted spatiotemporally localized pulses. Here, we present analytically individual autofocusing luminal and superluminal localized waves that can attain high-intensity peaks and spatiotemporal localization at prespecified positions along the path of their propagation.
Using SEA TADPOLE with m-range spatial and fs-range temporal resolution, we report the first dir... more Using SEA TADPOLE with m-range spatial and fs-range temporal resolution, we report the first direct spatiotemporal measurements of ultrashort Bessel-X pulses. We demonstrate their propagation invariance and superluminal velocity and verify our results with simulations.
The wave-field produced by a ∼30 fs duration Ti:sapphire oscillator pulse behind a circular apert... more The wave-field produced by a ∼30 fs duration Ti:sapphire oscillator pulse behind a circular aperture and circular opaque disk is measured using the ultrashort-laser-pulse measurement technique, scanning SEA TADPOLE. The high spatial and temporal resolution of the measuring technique enables us to fully image the diffracted field behind the apertures and record the interference pattern produced by the so-called boundary diffraction wave pulses.
Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating ... more Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating wave the energy locally propagates backward. In this study the energy backflow has been examined in connection with relatively simple causal unidirectional finite-energy solutions of the wave equation which are derived from a factorization of the so-called basic splash mode. Specific results are given for the energy backflow arising in known azimuthally symmetric unidirectional wavepackets, as well as in novel azimuthally asymmetric extensions. Using the Bateman-Whittaker technique, a novel finite-energy unidirectional null localized wave has been constructed that is devoid of energy backflow and has some of the topological properties of the basic Hopfion.
Electromagnetic energy backflow is a phenomenon occurring in regions where the direction of the P... more Electromagnetic energy backflow is a phenomenon occurring in regions where the direction of the Poynting vector is opposite to that of the propagation of the wave field. It is particularly remarkable in the nonparaxial regime and has been exhibited in the focal region of sharply focused beams, for vector Bessel beams, and vector-valued spatiotemporally localized waves. A detailed study is undertaken of this phenomenon and the conditions for its appearance are examined in detail in the case of a superposition of four plane waves in free space, the simplest electromagnetic arrangement for the observation of negative energy flow, as well as its comprehensive and transparent physical interpretation. It is shown that the state of polarization of the constituent components of the electromagnetic plane wave quartet determines whether energy backflow takes place or not and what values the energy flow velocity assumes. Depending on the polarization angles, the latter can assume any value from c (the speed of light in vacuum) to −c in certain spatiotemporal regions of the field.
It is well known that although the group velocity of structured light pulses propagating in vacuu... more It is well known that although the group velocity of structured light pulses propagating in vacuum can be subluminal or superluminal, the upper limit of the energy flow velocity is c, the speed of light in vacuum. This inequality can be explained in terms of the reactive energy left behind by the fields. Energy and reactive energy densities have been calculated for vector-valued two-dimensional (light sheet) superluminal electromagnetic nondiffractive pulses, as well as scalar-valued and TM three-dimensional superluminal and subluminal spatiotemporally localized electromagnetic waves. Emphasis is placed on the physical formation of the reactive energy due to interference of the plane-wave constituents of the structured light waves.
We demonstrate persistent storage, recall, and conjugation of picosecond light signals from vario... more We demonstrate persistent storage, recall, and conjugation of picosecond light signals from various model objects, including a coin,-by making use of coherent optical responses in photochemically active media. A simple linear theory of holographic storage and playback of both the spatial and the temporal behavior of the signal field is shown to describe well the experimental results obtained by utilizing octaethylporphin-doped polystyrene at 1.8 K as a spectrally selective recording material.
Uploads
Papers by Peeter Saari