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 Fano resonances induced by combining metamaterial modes of the same symmetry

Fano resonances are observed in a composite metamaterial that consists of an electric split ring resonator eSRR and an I-shaped resonator ISR. By adjusting the length of the ISR the degree of asymmetry in the line shape of the composite metamaterial can be controlled and even made to be symmetric. In contrast to other methods to create Fano resonances, the individual modes of the eSRR and ISR have the same symmetry and are not evanescently coupled to each other. The transmission is simulated using the finite difference time domain method and a coupled oscillator model is used to obtain nominal values of the Fano asymmetry factor q. Composite metamaterials and individual eSRR and ISR metamaterials are fabricated, and their transmission is measured with terahertz time-domain spectroscopy.</jats:p

Electrical Characterization of Cu-Doped PEDOT:PSS Polymeric Thin Films

In this study we investigate the electrical properties in the transparent conductive polymer poly(3,4-ethylendioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and Cu-doped polymer (PEDOT:PSS 1.3% + CuCl2) using Hall measurements under the Van der Pauw configuration. The influence of doping on the electrical conductivity, Hall mobility, and carrier concentration is demonstrated in Cu-PEDOT:PSS and compared with pure PEDOT:PSS. Through this, we figured out there are difficulties in preparing Cu-doped (PEDOT:PSS 1.3% + CuCl2) polymeric thin films. The sheet resistance was measured to determine the electrical conductivity, aided by knowledge of the thin film thickness. Temperature dependency was evaluated using a closed cycle cryostat, covering temperatures ranging from 8K to 300 K. These comprehensive measurements provide valuable insights into the electrical behavior and temperature characteristics of PEDOT:PSS and PEDOT:PSS 1.3% + CuCl2, facilitating the development of high-performance organic electronic devices