Integrated injection seeded terahertz source and amplifier for time-domain spectroscopy.

We used a terahertz (THz) quantum cascade laser (QCL) as an integrated injection seeded source and amplifier for THz time-domain spectroscopy. A THz input pulse is generated inside a QCL by illuminating the laser facet with a near-IR pulse from a femtosecond laser and amplified using gain switching. The THz output from the QCL is found to saturate upon increasing the amplitude of the THz input power, which indicates that the QCL is operating in an injection seeded regime

Measuring the sampling coherence of a terahertz quantum cascade laser

The emission of a quantum cascade laser can be synchronized to the repetition rate of a femtosecond laser through the use of coherent injection seeding. This synchronization defines a sampling coherence between the terahertz laser emission and the femtosecond laser which enables coherent field detection. In this letter the sampling coherence is measured in the time-domain through the use of coherent and incoherent detection. For large seed amplitudes the emission is synchronized, while for small seed amplitudes the emission is non-synchronized. For intermediate seed amplitudes the emission exhibits a partial sampling coherence that is time-dependent

Stochastic De-repression of Rhodopsins in Single Photoreceptors of the Fly Retina

The photoreceptors of the Drosophila compound eye are a classical model for studying cell fate specification. Photoreceptors (PRs) are organized in bundles of eight cells with two major types – inner PRs involved in color vision and outer PRs involved in motion detection. In wild type flies, most PRs express a single type of Rhodopsin (Rh): inner PRs express either Rh3, Rh4, Rh5 or Rh6 and outer PRs express Rh1. In outer PRs, the K50 homeodomain protein Dve is a key repressor that acts to ensure exclusive Rh expression. Loss of Dve results in de-repression of Rhodopsins in outer PRs, and leads to a wide distribution of expression levels. To quantify these effects, we introduce an automated image analysis method to measure Rhodopsin levels at the single cell level in 3D confocal stacks. Our sensitive methodology reveals cell-specific differences in Rhodopsin distributions among the outer PRs, observed over a developmental time course. We show that Rhodopsin distributions are consistent with a two-state model of gene expression, in which cells can be in either high or basal states of Rhodopsin production. Our model identifies a significant role of post-transcriptional regulation in establishing the two distinct states. The timescale for interconversion between basal and high states is shown to be on the order of days. Our results indicate that even in the absence of Dve, the Rhodopsin regulatory network can maintain highly stable states. We propose that the role of Dve in outer PRs is to buffer against rare fluctuations in this network

Terahertz radiation from magnetic excitations in diluted magnetic semiconductors

We probed, in the time domain, the THz electromagnetic radiation originating from spins in CdMnTe diluted magnetic semiconductor quantum wells containing high-mobility electron gas. Taking advantage of the efficient Raman generation process, the spin precession was induced by low power near-infrared pulses. We provide a full theoretical first-principles description of spin-wave generation, spin precession, and of emission of THz radiation. Our results open new perspectives for improved control of the direct coupling between spin and an electromagnetic field, e.g., by using semiconductor technology to insert the THz sources in cavities or pillars.Fil: Rungsawang, R.. Ecole Normale Supérieure; Francia. Universite Pierre et Marie Curie; Francia. Centre National de la Recherche Scientifique; FranciaFil: Perez, F.. Universite de Paris VI. Institut des Nanosciences de Paris; FranciaFil: Oustinov, D.. Ecole Normale Supérieure; Francia. Universite Pierre et Marie Curie; Francia. Centre National de la Recherche Scientifique; FranciaFil: Gomez, Javier Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universite de Paris VI. Institut des Nanosciences de Paris; . Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Kolkovsky, V.. Polish Academy of Sciences; PoloniaFil: Karczewski, G.. Polish Academy of Sciences; PoloniaFil: Wojtowicz, T.. Polish Academy of Sciences; PoloniaFil: Madéo, J.. Ecole Normale Supérieure; Francia. Universite Pierre et Marie Curie; Francia. Centre National de la Recherche Scientifique; FranciaFil: Jukam, N.. Centre National de la Recherche Scientifique; Francia. Ecole Normale Supérieure; Francia. Universite Pierre et Marie Curie;Fil: Dhillon, S.. Centre National de la Recherche Scientifique; Francia. Ecole Normale Supérieure; Francia. Universite Pierre et Marie Curie;Fil: Tignon, J.. Centre National de la Recherche Scientifique; Francia. Ecole Normale Supérieure; Francia. Universite Pierre et Marie Curie

Terahertz time domain spectroscopy of phonon-depopulation based quantum cascade lasers

The gain and loss in a terahertz quantum cascade laser based on a longitudinal optical phonon depopulation scheme is studied using terahertz time domain spectroscopy. At laser threshold the gain is found to be clamped at 25cm-1 for a 1mm long device and the full width at half maximum of the spectral gain during laser action is 0.6THz. At low biases, before laser action, significant absorption features are observed for frequencies lower and higher than the laser emission frequency. We assign their onset to the lower laser level being populated from a parasitic electronic channel. The absorption transitions are identified from bandstructure simulations and from the oscillator strengths of the subband transitions as a function of the hal-00401507, version 1- 3 Jul 2009 applied field. 2 Terahertz (THz) quantum cascade lasers (QCLs) based on longitudinal-optical (LO) phonon depopulation of the lower laser level 1 are currently attracting considerable interest, owing to their higher output powers 2 and operating temperatures 3 compared to bound-tocontinuum designs. 4 Recently time domain spectroscopy (TDS) has been used to study th

Broadband terahertz dispersion control in hybrid waveguides

Dispersion control is a key objective in the field of photonics and spectroscopy, since it enhances non-linear effects by both enabling phase matching and offering slow light generation. In addition, it is essential for frequency comb generation, which requires a phase-lock mechanism that is provided by broadband compensation of group velocity dispersion (GVD). At optical frequencies, there are several well-established concepts for dispersion control such as prism or grating pairs. However, terahertz dispersion control is still a challenge, thus hindering further progress in the field of terahertz science and technology. In this work, we present a hybrid waveguide with both broadband, tuneable positive and more than octave-spanning negative terahertz GVD on the order of $10^{−22}$ $s^{2}/m$, which is suitable for either intra- or extra cavity operation. This new terahertz device will enable ultra-short pulse compression, allow soliton propagation, improve frequency comb operation and foster the development of novel non-linear applications