Ultrafast carrier dynamics in terahertz photoconductors and photomixers: beyond short-carrier-lifetime semiconductors

Efficient terahertz generation and detection are a key prerequisite for high performance terahertz systems. Major advancements in realizing efficient terahertz emitters and detectors were enabled through photonics-driven semiconductor devices, thanks to the extremely wide bandwidth available at optical frequencies. Through the efficient generation and ultrafast transport of charge carriers within a photo-absorbing semiconductor material, terahertz frequency components are created from the mixing products of the optical frequency components that drive the terahertz device – a process usually referred to as photomixing. The created terahertz frequency components, which are in the physical form of oscillating carrier concentrations, can feed a terahertz antenna and get radiated in case of a terahertz emitter, or mix with an incoming terahertz wave to down-convert to DC or to a low frequency photocurrent in case of a terahertz detector. Realizing terahertz photoconductors typically relies on short-carrier-lifetime semiconductors as the photo-absorbing material, where photocarriers are quickly trapped within one picosecond or less after generation, leading to ultrafast carrier dynamics that facilitates high-frequency device operation. However, while enabling broadband operation, a sub-picosecond lifetime of the photocarriers results in a substantial loss of photoconductive gain and optical responsivity. In addition, growth of short-carrier-lifetime semiconductors in many cases relies on the use of rare elements and non-standard processes with limited accessibility. Therefore, there is a strong motivation to explore and develop alternative techniques for realizing terahertz photomixers that do not rely on these defect-introduced short-carrier-lifetime semiconductors. This review will provide an overview of several promising approaches to realize terahertz emitters and detectors without short-carrier-lifetime semiconductors. These novel approaches utilize p-i-n diode junctions, plasmonic nanostructures, ultrafast spintronics, and low-dimensional materials to offer ultrafast carrier response. These innovative directions have great potentials for extending the applicability and accessibility of the terahertz spectrum for a wide range of applications

Ultra-narrow linewidth CW sub-THz generation using GS based OFCG and n-i-pn-i-p superlattice photomixers

A report is presented on the photonic synthesis of ultra-narrow line-width continuous-wave (CW) sub-THz signals using a gain-switching (GS) based optical frequency comb generator (OFCG), selective optical filtering and a n-i-pn-i-p superlattice photomixer. This setup provides continuous tunability with a tuning resolution in the range of 0.1 Hz at 120 GHz and full width at half maximum of the generated signals below the limits of the measurement setup (< 10 Hz). The advantages of this system make it a very good candidate for applications requiring extremely low phase noise and continuous tunability, such as high resolution spectroscopy in the sub-THz and THz range.Work supported by the Spanish Ministry of Science and Technology through the project TEC2009-14525-C02-02. The work by A.R. Criado has been supported by the Spanish Ministry of Science and Technology under the FPI Program, Grant# BES2010-030290.Publicad

Fully Integrated THz Schottky Detectors Using Metallic Nanowires as Bridge Contacts

This paper investigates fully integrated Terahertz (THz) Schottky detectors using silver (Ag) metallic nanowires (NWs) with 120 nm diameter as bridge contacts for zero-bias operating THz detectors based on highly doped Gallium Arsenide (GaAs) and Indium Gallium Arsenide (InGaAs) layers. The combination of InGaAs and metallic NWs shows improved performance at zero-bias than a GaAs based detector with a simulated capacitance of 0.5 fF and a series resistance of 29.7 Ω . Thus, the calculated maximum cut-off frequency of 2.6 THz was obtained for a NW contacted vertical InGaAs THz detector. Initial THz measurements were carried out using a common THz setup for frequencies up to 1.2 THz. A responsivity of 0.81 A/W and a low noise-equivalent power (NEP) value of 7 pW/√Hz at 1 THz were estimated using the measured IV-characteristics of the zero-bias NW-InGaAs based THz Schottky detector

Continuous wave sub-THz photonic generation with ultra-narrow linewidth, ultra-high resolution, full frequency range coverage and high long-term frequency stability

We report on a photonic system for generation of high quality continuous-wave (CW) sub-THz signals. The system consists on a gain-switching-based optical frequency comb generator (GS-OFCG), a two-optical-modes selection mechanism and a n-i-pn-i-p superlattice photomixer. As mode selection mechanism, both selective tunable optical filtering using Fabry&-Pérot tunable filters (FPTFs) and Optical Injection Locking (OIL) are evaluated. The performance of the reported system surpasses in orders of magnitude the performance of any commercially available optical mm-wave and sub-THz generation system in a great number of parameters. It matches and even overcomes those of the best commercially available electronic THz generation systems. The performance parameters featured by our system are: linewidth $<$10 Hz at 120 GHz, complete frequency range coverage (60&-140 GHz) with a resolution in the order of 0.1 Hz at 120 GHz ({hbox{10}} -12} of generated frequency), high long term frequency stability (5 Hz deviation over one hour). Most of these values are limited by the measurement instrumentation accuracy and resolution, thus the actual values of the system could be better than the reported ones. The frequency can be extended straightforwardly up to 1 THz extending the OFCG frequency span. This system is compact, robust, reliable, offers a very high performance, especially suited for sub-THz photonic local oscillators and high resolution spectroscopy.This work was supported by the Spanish Ministry of Science and Technology through the Project TEC2009-14525-C02-02. The work of Á. R. Criado has been supported by the Spanish Ministry of Science and Technology under the FPI Program, Grant BES2010-030290

Recruitment of a splicing factor to the nuclear lamina for its inactivation

Precursor messenger RNA splicing is a highly regulated process, mediated by a complex RNA protein machinery, the spliceosome, that encompasses several hundred proteins and five small nuclear RNAs in humans. Emerging evidence suggests that the spatial organization of splicing factors and their spatio temporal dynamics participate in the regulation of splicing. So far, methods to manipulate the spatial distribution of splicing factors in a temporally defined manner in living cells are missing. Here, we describe such an approach that takes advantage of a reversible chemical dimerizer, and outline the requirements for efficient, reversible re localization of splicing factors to selected sub nuclear compartments. In a proof of principle study, the partial re localization of the PRPF38A protein to the nuclear lamina in HEK293T cells induced a moderate increase in intron retention. Our approach allows fast and reversible re localization of splicing factors, has few side effects and can be applied to many splicing factors by fusion of a protein tag through genome engineering. Apart from the systematic analysis of the spatio temporal aspects of splicing regulation, the approach has a large potential for the fast induction and reversal of splicing switches and can reveal mechanisms of splicing regulation in native nuclear environment

A Indissociabilidade entre Ensino, Pesquisa e Extensão: um Relato de Experiência da Integração entre a Universidade e a Rede de Atenção Integral à Saúde Mental em Volta Redonda – RJ

O objetivo deste relato partilhar nossa experincia como docentes e alunos do Curso de Psicologia da Universidade Federal Fluminense Volta Redonda, em projetos de extenso desenvolvidos em parceria com a rede de sade mental da regio em que a Universidade est sediada. As atividades desenvolvidas situam-se em torno do tema dos desafios do trabalho em sade mental, considerando trs eixos fundamentais: educao permanente, integrao ensino-servio de sade e indissociabilidade entre ensino, pesquisa e extenso