金属/介质薄膜中声子热辐射的空间和各向异性研究

基于理论、实验和仿真相结合的方式,着重研究了金属/介质(MD)薄膜中声子热辐射的空间特性和各向异性。声子是由于晶格振动产生的元激发,是物质的内在属性。尽管声子不易调控,但是声子与其他光学激发的耦合会产生奇异的光学现象。特别是红外到太赫兹范围内的光子与极性介质中的声子强耦合产生表面声子激元（SPhP）。SPhP具有强局域、低损耗等特点,与等离子体（plasmon polaritons）形成互补,使得深亚波长光学成为可能。为了进一步了解声子吸收的内在理论基础,首先通过黄昆方程和超晶格连续介电模型在理论上分析了声子吸收。实验上,主要以SiO₂声子作为研究对象,利用等离子体增强化学气相沉积(PECVD)方法,分别在Si/Al（150 nm）薄膜和Si衬底上制备出500 nm厚的SiO₂薄膜。基于傅里叶红外光谱仪(FTIR),在垂直入射下得到热辐射光谱,通过热辐射光谱分析,并结合由时域有限差分算法（finite-difference time-domain,FDTD）计算得出的仿真光谱图,对比了MD薄膜结构和非MD薄膜结构中声子的热辐射,发现MD薄膜结构更能够有利于声子和SPhP的激发。根据Berreman效应,纵光学波（LO）声子只在倾斜入射时产生。光谱线没有呈现洛伦兹线型,因此,虽然LO声子在垂直入射时测得的热辐射图中不辐射,但同样影响横光学波（TO）声子辐射谱的线型。另外,利用FTIR对金属（Si/Al）/介质（SiO₂薄膜）进行热辐射转角测试,对热辐射转角图分析证明,Si/Al/SiO₂薄膜中SiO₂声子遵循LST(lyddano-sachs-teller)关系,纵横声子成对出现,且两种声子的空间辐射特性不同。改变偏振,发现在S偏振和P偏振下,声子热辐射呈现不同的模式, 体现出声子的空间各向异性。并且,声子与光子耦合可以激发SPhP,反过来,SPhP可以增强声子的吸收。基于MD结构,能够激发并调控SPhP和声子辐射行为,为红外器件的实现奠定了基础。     

Forward coherent inelastic Mössbauer scattering of synchrotron radiation

A theoretical analysis is presented of the possibilities of satisfying the conditions of phase matching (PM) in the coherent inelastic Mössbauer scattering (CIMS, which is scattering accompanied by the creation or annihilation of phonons) of synchrotron radiation (SR) and the conditions under which the maximum intensity of CIMS obtains are studied. Of the two types of CIMS — 1) participation of phonons in a scattering event only in the stage of absorption or only during re-emission of a photon, and 2) participation of phonons in both stages of scattering — PM is possible only in the first. The process in which phonons participate only at the photon absorption stage leads to efficient conversion of the SR from a wide spectral line into a narrow line, determined by the width of the Mössbauer transition. Photons of this type of CIMS effectively possess a higher penetrating power than the standard Mössbauer radiation, and their spectral distribution is shifted somewhat in the direction of low frequencies.     

Generation of high-frequency phonons by defect-induced one-phonon absorption

By defect-induced one-phonon absorption of pulsed infrared radiation, we excited in diamond phonons at 28 THz. Their anharmonic decay leads to nonthermal frequency distributions of acoustic phonons which we investigated in a frequency range between 0.5 THz and 6 THz by use of vibronic sideband spectroscopy.     

Heating and cooling of a two-dimensional electron gas by terahertz radiation

The absorption of terahertz radiation by free charge carriers in n-type semiconductor quantum wells accompanied by the interaction of electrons with acoustic and optical phonons is studied. It is shown that intrasubband optical transitions can cause both heating and cooling of the electron gas. The cooling of charge carriers occurs in a certain temperature and radiation frequency region where light is most efficiently absorbed due to intrasubband transitions with emission of optical phonons. In GaAs quantum wells, the optical cooling of electrons occurs most efficiently at liquid nitrogen temperatures, while cooling is possible even at room temperature in GaN heterostructures.     

Intersubband radiation in weakly bound superlattice structures with wide quantum wells in a transverse electric field

Long-wavelength IR radiation was detected in a long-period superlattice with wide quantum wells under conditions of electrical injection of charge carriers into lower size-quantization subbands, which was explained by intersubband transitions. The detected radiation probably suggests that there is a strongly nonequilibrium carrier distribution in subbands, caused by the difference between scattering processes into lower subbands with and without involving optical phonons.     

Theory of nonlinear effects in semiconductors with a nondiffusion approximation for acoustic phonons

Momentum and energy balance equations in impurity semi-metals and degenerate semiconductors are derived and investigated in a nondiffusion approximation for the acoustic phonons with arbitrary heating and entrainment of electrons and phonons taken into account. It was shown that the diffusion approximation is not satisfied even for relatively weak electrical fields. In cases of thermal entrainment and no heating of the phonons, cases are possible when the electron temperature becomes equal to and less than the lattice temperature, which is associated with radiation obtained from the energy field by electrons in the form of acoustic phonons at a point of acoustic instability. If mutual entrainment and heating of electrons and phonons occurs then the crystal boundaries are the main channel of energy and momentum relaxation. Necessary conditions delimiting the strong changes of all the galvano- and thermomagnetic effects at a point of acoustic instability are found in every specific case.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 11–16, November, 1990.     

Gamma-ray tuning by stimulated emission of recoilphonons

Tuning of gamma radiation by exciting the ultrasonic vibrations of different coherency degree in crystals is discussed. The quantum approach based on the formalism of coherent quantum states is used to take into account statistical properties of the stimulating ultrasonic wave. Multi-quantum transitions with emission of gamma quanta and simultaneous stimulated emission of recoil phonons or absorption of one or several phonons from an external ultrasonic wave are considered as an example of multi-quantum transitions with quanta coupled to different degrees of freedom of the radiating nuclei. The spectrum of gamma radiation is determined for a partially coherent stimulating ultrasonic wave with fixed amplitude and uncertain phase, and completely incoherent casual acoustic oscillations. A comparison is made with some known results obtained by both classical and quantum methods. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nonequilibrium phonons in SiGe quantum-well nanostructures upon excitation by picosecond laser pulses

The phonon pulses initiated by photoexcitation of structures containing Si_0.8Ge_0.2 double quantum wells under picosecond radiation of a MIRA 900P titanium-sapphire laser (λ = 760 nm) are studied. The propagation of nonequilibrium acoustic phonons is detected with a superconducting bolometer. The recorded bolometer response is found to differ substantially from that observed in photoexcitation of the same structure by nanosecond pulses of a nitrogen laser (λ = 337 nm). The generation of coherent acoustic phonons is suggested as an explanation.     

Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 10¹³ W/cm² in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF₂ crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF₂ and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.     

Generation and detection of very high frequency acoustic phonons in diamond

Generation and detection of very high frequency acoustic phonons in diamond is reported. We generate phonons at a frequency of 28 THz by defect-induced one-phonon absorption of CO₂ laser radiation and observe, after pulsed excitation, phonon decay products in the frequency range from 1 THz to 7 THz. For detection vibronic sideband spectroscopy is used. We find strongly frequency dependent lifetimes for frequencies above 4 THz which we attribute to spontaneous phonon decay.Dedicated to K. Dransfeld on occasion of his 60th birthday     

Direct measurement of Raman tensor in solids

It is argued that the measurement of absorption of electromagnetic radiation at optical frequencies induced by an electric field at a different frequency provides a valid and convenient way to obtain an absolute measure of the Raman scattering tensor of optical phonons in solids.     

Far-infrared laser spectroscopy of phonon difference band absorption in TlCl

Far-infrared laser radiation is used to study the lattice absorption of a TlCl crystal over a large temperature range. We found a strong absorption band caused by phonon difference processes where longitudinal and transverse acoustic phonons with wave vectors at the Brillouin zone boundary are involved.     

Coherent phonons, nanoseismology and THz radiation in InGaN/GaN heterostructures

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. We discuss the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also discuss the THz radiation emitted from these phonons.     

Electron energy losses by excitation of transition radiation

Inelastic scattering of KeV electrons in thin films with energy losses corresponding to the infrared spectral region is discussed taking retardation into account, that is allowing for radiative modes. For sufficiently thin target films Cerenkov radiation does not play any role. Rather it turned out even for nonrelativistic electrons, that in the spectral region of negligible absorption, where for example excitation of phonons or surface phonons does not occur, the energy loss spectrum is exclusively due to the creation of transition radiation. By means of high resolution spectroscopy of electrons inelastically scattered from TiO₂ films it is shown experimentally that this conclusion is indeed correct.     

Time delay in formation of a γ-phase electron-hole drop in germanium

An explanation is given for the long time delay in the formation of gamma phase EHD in germanium. The Cerenkov radiation of phonons by moving alpha drops at velocity mach 0.2–0.3 is calculated to be the controlling process in the time delay.     

On the possibility of ‘Bose condensation’ in the excitation of coherent modes in biological systems

The phenomenon of Bose-Einstein condensation of phonons in biological structure, predicted by Fröhlich on the basis of rate equations, is approached here from a microscopic point of view. Rough estimates are compared with the recent experimental evidence of the action of coherent millimeter electromagnetic radiation on biological systems.     

Electrodynamic response of a quantum nanotube in a parallel magnetic field

This paper reports on a theoretical study of intraband resonances arising in the absorption of electromagnetic radiation by a quantum nanotube both with and without involvement of optical phonons. Explicit relations are derived for the absorption coefficients. The relative intensities of the resonance peaks are analyzed.     

Generation of millimeter radiation due to electric-field-induced electron-transit-time resonance in indium phosphide

The stimulated millimeter radiation from n-type indium phosphide was observed at T=4.2 K in a strong electric field. The generation is due to the appearance of negative differential conductivity near the electron-transit-time resonance and electron bundling in momentum space in a strong electric field under conditions of low-temperature scattering from optical phonons. The dependence of the radiation frequency and intensity on the electric field was experimentally measured and the radiation spectrum is presented. The experimental data satisfactorily agree with the results of numerical simulations obtained previously by the Monte Carlo method.     

Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.     

Spatial correlation of non-equilibrium effects in superconductors: Optical excitation

Laser radiation impinging on a thin film produces markedly different effects in a spatially separated second superconducting film if the first film is normal rather than superconducting. This phenomenon is postulated to be due to the dominant role of 2Δ phonons in sustaining the non-equilibrium condition of superconductors.