Ultrafast dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon investigated with a midinfrared continuum probe

We employ ultra-broadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities (10^{15}-10^{19} cm^{-3}) at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of approx. 200 fs at low densities, reaching approx. 20 fs for densities of approx. 10^{19} cm^{-3}, where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial sub-picosecond temporal development of the Drude response, and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.Comment: 9 pages, 4 figure

600-GHz Fourier Imaging Based on Heterodyne Detection at the 2nd Sub-harmonic

Fourier imaging is an indirect imaging method which records the diffraction pattern of the object scene coherently in the focal plane of the imaging system and reconstructs the image using computational resources. The spatial resolution, which can be reached, depends on one hand on the wavelength of the radiation, but also on the capability to measure - in the focal plane - Fourier components with high spatial wave-vectors. This leads to a conflicting situation at THz frequencies, because choosing a shorter wavelength for better resolution usually comes at the cost of less radiation power, concomitant with a loss of dynamic range, which limits the detection of higher Fourier components. Here, aiming at maintaining a high dynamic range and limiting the system costs, we adopt heterodyne detection at the 2nd sub-harmonic, working with continuous-wave (CW) radiation for object illumination at 600 GHz and local-oscillator (LO) radiation at 300 GHz. The detector is a single-pixel broad-band Si CMOS TeraFET equipped with substrate lenses on both the front- and backside for separate in-coupling of the waves. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of the single detector unit through the focal plane. With only 56 uW of power of the 600-GHz radiation, a dynamic range of 60 dB is reached, sufficient to detect the entire accessible Fourier space spectrum in the test measurements. A lateral spatial resolution of better than 0.5 mm, at the diffraction limit, is reached

Развитие системы управления теплоэнергетической отраслью в регионе

Объектом исследования является – система управления теплоэнергетической отраслью в регионе. Цель работы - разработка подходов к выявлению, анализу и разрешению проблем, препятствующих эффективному развитию системы управления в теплоэнергетике. В процессе исследования проводился анализ зарубежных и отечественных систем управления и практического опыта реализации моделей управления в энергетической и теплоэнергетической отрасли.The object of study is the control system of thermal power industry in the region. The aim of this work is to develop approaches to the identification, analysis and resolution of problems impeding the effective development of the management system in power. In the process of investigation the analysis of foreign and domestic control systems and practical experience in the implementation of management models in the energy and power industry

Terahertz Nano-Imaging with s-SNOM

Spectroscopy and imaging with terahertz radiation propagating in free space suffer from the poor spatial resolution which is a consequence of the comparatively large wavelength of the radiation (300 μm at 1 THz in vacuum) in combination with the Abbe diffraction limit of focusing. A way to overcome this limitation is the application of near-field techniques. In this chapter, we focus on one of them, scattering-type Scanning Near-field Optical Microscopy (s-SNOM) which − due to its versatility − has come to prominence in recent years. This technique enables a spatial resolution on the sub-100-nm length scale independent of the wavelength. We provide an overview of the state-of-the-art of this imaging and spectroscopy modality, and describe a few selected application examples in more detail

Combined investigation of collective amplitude and phase modes in a quasi-one-dimensional charge-density-wave system over a wide spectral range

We investigate experimentally both the amplitude and phase channels of the collective modes in the quasi-1D charge-density-wave (CDW) system, K0.3MoO3, by combining (i) optical impulsive-Raman pump-probe and (ii) terahertz time-domain spectroscopy (THz-TDS), with high resolution and a detailed analysis of the full complex-valued spectra in both cases. This allows an unequivocal assignment of the observed bands to CDW modes across the THz range up to 9 THz. We revise and extend a time-dependent Ginzburg-Landau model to account for the observed temperature dependence of the modes, where the combination of both amplitude and phase modes allows one to robustly determine the bare-phonon and electron-phonon coupling parameters. While the coupling is indeed strongest for the lowest-energy phonon, dropping sharply for the immediately subsequent phonons, it grows back significantly for the higher-energy phonons, demonstrating their important role in driving the CDW formation. We also include a reassessment of our previous analysis of the lowest-lying phase modes, whereby assuming weaker electronic damping for the phase channel results in a qualitative picture more consistent with quantum-mechanical treatments of the collective modes, with a strongly coupled amplitudon and phason as the lowest modes

Strong coupling of plasmonic bright and dark modes with two eigenmodes of a photonic crystal cavity

Dark modes represent a class of forbidden transitions or transitions with weak dipole moments between energy states. Due to their low transition probability, it is difficult to realize their interaction with light, let alone achieve the strong interaction of the modes with the photons in a cavity. However, by mutual coupling with a bright mode, the strong interaction of dark modes with photons is possible. This type of mediated interaction is widely investigated in the metamaterials community and is known under the term electromagnetically induced transparency (EIT). Here, we report strong coupling between a plasmonic dark mode of an EIT-like metamaterial with the photons of a 1D photonic crystal cavity in the terahertz frequency range. The coupling between the dark mode and the cavity photons is mediated by a plasmonic bright mode, which is proven by the observation of a frequency splitting which depends on the strength of the inductive interaction between the plasmon bright and dark modes of the EIT-like metamaterial. In addition, since the plasmonic dark mode strongly couples with the cavity dark mode, we observes four polariton modes. The frequency splitting by interaction of the four modes (plasmonic bright and dark mode and the two eigenmodes of the photonic cavity) can be reproduced in the framework of a model of four coupled harmonic oscillators

Обогащение углей гравитационным методом на УОФ, г. Ангрен

Исследования опыты по обогащению угля гравитационным методом.Research experiments on coal preparation by gravitational method

High-power even- and odd mode emission from linear arrays of resonant-tunneling-diode (RTD) oscillators in the 0.4- to 0.8-THz frequency range

Resonant tunneling diode (RTD) oscillators possess the highest oscillation frequency among all electronic THz emitters. However, the emitted power from RTDs remains limited. Here, we propose linear RTD-oscillator arrays capable of supporting coherent emission from both odd and even coupled modes. Both modes exhibit constructive interference in the far field, enabling high power emission. Experimental demonstrations of coherent emission from 11-RTD-oscillator linear arrays are presented. The odd mode oscillates at approximately 450 GHz, emitting about 0.5 mW, while the even mode oscillates at around 750 GHz, emitting about 1 mW. Moreover, certain RTD-oscillator arrays demonstrate dual-band oscillation under different biases, allowing for controllable switching between two coupled modes. In addition, during bias sweeping in both directions, a notable hysteresis feature is observed in the switching bias for the odd and even modes. Our linear RTD-oscillator array represents a significant step forward in the realization of high-power large RTD-oscillator arrays and enables large-scale applications of RTD devices.Comment: 7 pages, 6 figure