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Giant Oscillator Strength
Giant oscillator strength is inherent in excitons that are weakly bound to impurities or defects in crystals. The spectrum of fundamental absorption of direct-gap semiconductors such as gallium arsenide (GaAs) and cadmium sulfide (CdS) is continuous and corresponds to band-to-band transitions. It begins with transitions at the center of the Brillouin zone, \boldsymbol=0. In a perfect crystal, this spectrum is preceded by a hydrogen-like series of the transitions to ''s''-states of Wannier-Mott excitons. In addition to the exciton lines, there are surprisingly strong additional absorption lines in the same spectral region. They belong to excitons weakly bound to impurities and defects and are termed 'impurity excitons'. Anomalously high intensity of the impurity-exciton lines indicate their giant oscillator strength of about f_i\sim10 per impurity center while the oscillator strength of free excitons is only of about f_\sim10^ per unit cell. Shallow impurity-exciton states are w ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Exciton
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge. An exciton can form when a material absorbs a photon of higher energy than its bandgap. This excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively charged electron hole (an abstraction for the location from which an electron was moved). The electron in the conduction band is then less attracted to this localized hole due to the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. These repulsive forces provide a stabilizing energy balance. Consequently, the exciton has slightly less energy than the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Polaritons
In physics, polaritons are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation. They are an expression of the common quantum phenomenon known as level repulsion, also known as the avoided crossing principle. Polaritons describe the crossing of the dispersion of light with any interacting resonance. To this extent polaritons can also be thought of as the new normal modes of a given material or structure arising from the strong coupling of the bare modes, which are the photon and the dipolar oscillation. The polariton is a bosonic quasiparticle, and should not be confused with the polaron (a fermionic quasiparticle), which is an electron plus an attached phonon cloud. Whenever the polariton picture is valid (i.e., when the weak coupling limit is an invalid approximation), the model of photons propagating freely in crystals is insufficient. A major feature of polaritons is a strong dependency of the propag ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quantum Yield
The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. Applications Fluorescence spectroscopy The fluorescence quantum yield is defined as the ratio of the number of photons emitted to the number of photons absorbed.Lakowicz, Joseph R. ''Principles of Fluorescence Spectroscopy'' (Kluwer Academic / Plenum Publishers 1999) p.10. Fluorescence quantum yield is measured on a scale from 0 to 1.0, but is often represented as a percentage. A quantum yield of 1.0 (100%) describes a process where each photon absorbed results in a photon emitted. Substances with the largest quantum yields, such as rhodamines, display the brightest emissions; however, compounds with quantum yields of 0.10 are still considered quite fluorescent. Quantum yield is defined by the fraction of excited state fluorophores that decay through fluorescence: where \Phi_ is the fluorescence quantum yield, k_ is the rate constant f ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oscillator Strength
In spectroscopy, oscillator strength is a dimensionless quantity that expresses the probability of Absorption (electromagnetic radiation), absorption or Emission spectrum, emission of electromagnetic radiation in transitions between energy levels of an atom or molecule. For example, if an emissive state has a small oscillator strength, Spontaneous emission#Radiative and nonradiative decay: the quantum efficiency, nonradiative decay will outpace Spontaneous emission#Radiative and nonradiative decay: the quantum efficiency, radiative decay. Conversely, "bright" transitions will have large oscillator strengths. The oscillator strength can be thought of as the ratio between the quantum mechanical transition rate and the classical absorption/emission rate of a single electron oscillator with the same frequency as the transition. Theory An atom or a molecule can absorb light and undergo a transition from one quantum state to another. The oscillator strength f_ of a transition from a low ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Polariton
In physics, polaritons are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation. They are an expression of the common quantum phenomenon known as level repulsion, also known as the avoided crossing principle. Polaritons describe the crossing of the dispersion of light with any interacting resonance. To this extent polaritons can also be thought of as the new normal modes of a given material or structure arising from the strong coupling of the bare modes, which are the photon and the dipolar oscillation. The polariton is a bosonic quasiparticle, and should not be confused with the polaron (a fermionic quasiparticle), which is an electron plus an attached phonon cloud. Whenever the polariton picture is valid (i.e., when the weak coupling limit is an invalid approximation), the model of photons propagating freely in crystals is insufficient. A major feature of polaritons is a strong dependency of the propag ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Exciton
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge. An exciton can form when a material absorbs a photon of higher energy than its bandgap. This excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively charged electron hole (an abstraction for the location from which an electron was moved). The electron in the conduction band is then less attracted to this localized hole due to the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. These repulsive forces provide a stabilizing energy balance. Consequently, the exciton has slightly less energy than the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Vladimir Broude
Vladimir Lvovich Broude (December 1, 1924, Moscow, Soviet Union – June 22, 1978, Moscow), was a Soviet and Russian experimental physicist of Jewish descent. His father was a Professor of biochemistry and his mother was a medical doctor. His elder brother Yevgeny was conscripted soon after beginning of the Nazi invasion in June 1941 and lost his life. Career In 1947, Broude graduated from the Moscow Institute of Chemical Engineering and was directed to Kyiv, to the Institute of Physics of the National Academy of Sciences of Ukraine, for developing and installing equipment for low-temperature optical spectroscopy. Here he developed interest in the low-temperature spectroscopy, studied quantum mechanics and group theory, and very soon became an active and inventive experimental physicist. In his first break-through papers Broude discovered in the low-temperature spectra of crystalline benzine a triplet of absorption bands strongly polarized along the crystallographic axes. M ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Antonina Prikhot'ko
Antonina Fedorivna Prykhotko (; 26 April 1906, Pyatigorsk – 29 September 1995, Kyiv), was a Soviet and Ukrainian experimental physicist. She was an Academician of the National Academy of Sciences of Ukraine and is known for her fundamental contributions to the condensed matter spectroscopy. Career Prykhotko was accepted to the Leningrad Polytechnical Institute in 1923 and graduated in 1929. Still as a Junior, she started research on spectroscopy under the supervision of Ivan V. Obreimov, and finally earned a PhD in this field under his guidance. In 1930, she moved to the newly established Kharkiv Institute of Physics and Technology for working with Obreimov on low-temperature spectroscopy of molecular crystals, a field pioneered by him, in the first in the USSR cryogenic laboratory created by Lev Shubnikov. This research was primary focused on comparing the spectra of vapors and crystals of weakly interacting molecules that at low temperature manifest a number of narrow band ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Resonance Fluorescence
Resonance fluorescence is the process in which a two-level atom system interacts with the quantum electromagnetic field if the field is driven at a frequency near to the natural frequency of the atom. General theory Typically the photon contained electromagnetic field is applied to the two-level atom through the use of a monochromatic laser. A two-level atom is a specific type of two-state system in which the atom can be found in the two possible states. The two possible states are if an electron is found in its ground state or the excited state. In many experiments an atom of lithium is used because it can be closely modeled to a two-level atom as the excited states of the singular electron are separated by large enough energy gaps to significantly reduce the possibility of the electron jumping to a higher excited state. Thus it allows for easier frequency tuning of the applied laser as frequencies further off resonance can be used while still driving the electron to jump to on ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Direct And Indirect Band Gaps
In semiconductor physics, the band gap of a semiconductor can be of two basic types, a direct band gap or an indirect band gap. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone. If the k-vectors are different, the material has an "indirect gap". The band gap is called "direct" if the crystal momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice. Examples of direct bandgap materials include amorphous silicon and some III-V materials such as InAs and GaAs. Indirect bandgap materials include crystalline silicon and Ge. Some III-V materials are indirect bandgap as well, for example AlSb. Implications for radiative ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quantum Yield
The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. Applications Fluorescence spectroscopy The fluorescence quantum yield is defined as the ratio of the number of photons emitted to the number of photons absorbed.Lakowicz, Joseph R. ''Principles of Fluorescence Spectroscopy'' (Kluwer Academic / Plenum Publishers 1999) p.10. Fluorescence quantum yield is measured on a scale from 0 to 1.0, but is often represented as a percentage. A quantum yield of 1.0 (100%) describes a process where each photon absorbed results in a photon emitted. Substances with the largest quantum yields, such as rhodamines, display the brightest emissions; however, compounds with quantum yields of 0.10 are still considered quite fluorescent. Quantum yield is defined by the fraction of excited state fluorophores that decay through fluorescence: where \Phi_ is the fluorescence quantum yield, k_ is the rate constant f ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Emmanuel Rashba
Emmanuel I. Rashba (born October 30, 1927, Kyiv) is a Soviet-American theoretical physicist of Jewish origin who worked in Ukraine, Russia and in the United States. Rashba is known for his contributions to different areas of condensed matter physics and spintronics, especially the Rashba effect in spin physics, and also for the prediction of electric dipole spin resonance (EDSR),E. I. Rashba, Properties of semiconductors with a loop of extrema, I. Cyclotron and combined resonances in a perpendicular field, Sov. Phys. Solid State 2, 1109 (1960). that was widely investigated and became a regular tool for operating electron spins in nanostructures, phase transitions in spin-orbit coupled systems driven by change of the Fermi surface topology, Giant oscillator strength of impurity excitons, and coexistence of free and self-trapped excitons. The principal subject of spintronics is all-electric operation of electron spins, and EDSR was the first phenomenon predicted and experimentally ob ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
