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Spintronic terahertz emitters (STEs), based on optical excitation of nanometer thick ferromagnetic/heavy metal (FM/HM) heterojunctions, have become important sources for the generation of terahertz (THz) pulses. However, the efficiency of the optical-to-THz conversion remains limited. Although optical techniques have been developed to enhance the optical absorption, no investigations have studied the application of THz cavities. Here, to enhance the THz efficiency of STEs in a selected THz spectral range, FM/HM structures are realized on ultra-thin sapphire layers with metallic mirrors to create λ/4 THz resonant cavities. THz emission time domain spectroscopy of these STE/sapphire/mirror heterostructures, with sapphire thicknesses ranging from 110 µm to 25 µm, shows enhancement of the emitted THz field that fits the λ/4 cavity resonance with up to a doubling of the field in the spectrum, and in agreement with temporal simulations of the emitted THz pulse. By taking advantage of birefringent materials, we further show the potential of control of the polarization state of the emitted THz pulse. This work shows the potential of enhancing and engineering THz emission from STEs using THz cavities that can be controlled over a broad spectral range, which can be easily combined with optical cavities.

In research and engineering, short laser pulses are fundamental for metrology and communication. The generation of pulses by passive mode-locking is especially desirable due to the compact setup dimensions, without the need for active modulation requiring dedicated external circuitry. However, well-established models do not cover regular self-pulsing in gain media that recover faster than the cavity round trip time. For quantum cascade lasers (QCLs), this marked a significant limitation in their operation, as they exhibit picosecond gain dynamics associated with intersubband transitions. We present a model that gives detailed insights into the pulse dynamics of the first passively mode-locked QCL that was recently demonstrated. The presence of an incoherent saturable absorber, exemplarily realized by multilayer graphene distributed along the cavity, drives the laser into a pulsed state by exhibiting a similarly fast recovery time as the gain medium. This previously unstudied state of laser operation reveals a remarkable response of the gain medium on unevenly distributed intracavity intensity. We show that in presence of strong spatial hole burning in the laser gain medium, the pulse stabilizes itself by suppressing counter-propagating light and getting shortened again at the cavity facets. Finally, we study the robustness of passive mode-locking with respect to the saturable absorber properties and identify strategies for generating even shorter pulses. The obtained results may also have implications for other nanostructured mode-locked laser sources, for example, based on quantum dots.

Over the past few decades, THz technology has made considerable progress, evidenced by the performance of current THz sources and detectors, as well as the emergence of several THz applications. However, in the realm of quantum technologies, the THz spectral domain is still in its infancy, unlike neighboring spectral domains that have flourished in recent years. Notably, in the microwave domain, superconducting qubits currently serve as the core of quantum computers, while quantum cryptography protocols have been successfully demonstrated in the visible and telecommunications domains through satellite links. The THz domain has lagged behind in these impressive advancements. Today, the current gap in the THz domain clearly concerns quantum technologies. Nonetheless, the emergence of quantum technologies operating at THz frequencies will potentially have a significant impact. Indeed, THz radiation holds significant promise for wireless communications with ultimate security owing to its low sensitivity to atmospheric disturbances. Moreover, it has the potential to raise the operating temperature of solid-state qubits, effectively addressing existing scalability issues. In addition, THz radiation can manipulate the quantum states of molecules, which are recognized as new platforms for quantum computation and simulation with long range interactions. Finally, its ability to penetrate generally opaque materials or its resistance to Rayleigh scattering are very appealing features for quantum sensing. In this perspective, we will discuss potential approaches that offer exciting prospects for generating and detecting non-classical states of THz light, thereby opening doors to significant breakthroughs in THz quantum technologies.

Prokaryotes, such as bacteria, have evolved defense mechanisms that protect them from foreign bodies invading and harming them.1 One of these mechanisms is the clustered regularly interspaced short palindromic repeats (CRISPR), alongside their accompanying CRISPR-associated (Cas) proteins.1 This system functions as an immune response that protects prokaryotes from viruses (and other harmful bodies) by detecting foreign genetic material invading them and disabling their functionality and ability to spread.2 Understanding the underlying mechanism of this immune response allowed scientists around the world to develop CRISPR and adapt it to various uses in gene editing, agriculture, and most recently, diagnosis of infectious and noninfectious diseases.2  The discovery of CRISPR as a biomedical disease detection tool has revolutionized modern day medicine and its accessibility. Infectious diseases are those that are caused by microorganisms and easily passed from one human to another. Currently, CRISPR--specifically the Cas13 protein system--is being utilized for disease detection and diagnosis through the detection and cleavage of specific ssRNA molecules. SHERLOCK, Specific High-sensitivity Enzymatic Reporter unLOCKing, is a specific breed of CRISPR Cas13a disease detection technology that has been deployed in West African countries to combat infectious disease such as Lassa Fever-- a disease caused by the zoonotic virus Lassa mammarenavirus that originates from the Mastomys natalensis rodent group.5 SHERLOCK’s ability to succeed in the detection and diagnosis of Lassa fever in West African countries that lack proper infrastructure relies on SHERLOCK’s accessibility and efficiency. Cas13a’s specificity and sensitivity in the detection of viral nucleic acids caters to the fact that Lassa Fever has high genetic diversity and requires highly sensitive tools to detect the disease.8 Moreover, genetic diseases are those caused by an error in the DNA of a person. The three main types include monogenic, complex, and chromosomal; the treatment and causes of the diseases are different for each type.17 There are still many obstacles when it comes to curing genetic diseases, yet scientists are using powerful tools such as CRISPR to help those who suffer from these diseases.18 The CRISPR Cas9 enzyme is one powerful system that can be used to disrupt, delete, or insert genes to help edit any errors in the genome and treat genetic diseases.23 It is comprised of the CRISPR-associated (Cas) enzyme and the guide RNA (gRNA), which work together to act as a scissor for the DNA.19 Adrenoleukodystrophy (ALD) is an X-linked genetic disease that is primarily due to a mutation within the ABCD1 gene. Currently, the Cas9 system provides a disease model in order to study the pathogenesis of ALD and target (and repair) the mutation causing this genetic disease.

Terahertz (THz) quantum cascade lasers (QCLs) are technologically important laser sources for the THz range but are complex to model. An efficient extended rate equation model is developed here by incorporating the resonant tunneling mechanism from the density matrix formalism, which permits to simulate THz QCLs with thick carrier injection barriers within the semi-classical formalism. A self-consistent solution is obtained by iteratively solving the Schrödinger–Poisson equation with this transport model. Carrier–light coupling is also included to simulate the current behavior arising from stimulated emission. As a quasi-ab initio model, intermediate parameters, such as pure dephasing time and optical linewidth, are dynamically calculated in the convergence process, and the only fitting parameters are the interface roughness correlation length and height. Good agreement has been achieved by comparing the simulation results of various designs with experiments, and other models such as density matrix Monte Carlo and non-equilibrium Green's function method that, unlike here, require important computational resources. The accuracy, compatibility, and computational efficiency of our model enable many application scenarios, such as design optimization and quantitative insights into THz QCLs. Finally, the source code of the model is also provided in the supplementary material of this article for readers to repeat the results presented here, investigate, and optimize new designs. [ABSTRACT FROM AUTHOR]

Unlike traditional group‐6 transition metal dichalcogenides (TMDs), group‐10 TMDs such as PtSe2 and PdTe2 possess highly tuneable indirect bandgaps, transitioning from semiconducting in the near‐infrared to semimetal behavior with a number of monolayers (MLs). This opens up the possibility of TMD‐based mid‐infrared and terahertz optoelectronics. Despite this large potential, the optical properties of such materials have shown an extremely large disparity between that predicted and measured. For example, simulations show that a few MLs is required for the semiconductor–semimetal transition, whilst tens of MLs is found experimentally. This is a result of widely used optical extrapolation methods to determine bandgaps, such as the Tauc plot approach, that are not adapted here owing to i) nearby direct transitions, ii) the material dimensionality and iii) large changes in the non‐parabolic bandstructure with MLs. Here, uniquely combining optical ellipsometry to determine the complex permittivity, terahertz time resolved spectroscopy for the complex conductivity and in‐depth density functional theory (DFT) simulations, it is shown that the optical properties and bandstructure can be determined reliably and demonstrate clearly that the semiconductor‐semimetal transition occurs for PtSe2 layers ≈5 MLs. The microscopic origins of the observed transitions and the crucial role of the Coulomb interaction for thin semiconducting layers, and that of interlayer van der Waals forces for multilayer semimetallic samples are also demonstrated. This work of combining complimentary experimental techniques and extensive simulations avoids the application of constrained extrapolation methods to determine the optical properties of group‐10 TMDs, and will be of importance for future mid‐infrared and terahertz applications. [ABSTRACT FROM AUTHOR]

Optoelectronic devices based on intraband or intersublevel transitions in semiconductors are important building blocks of the current THz technology. Large nanocrystals (NCs) of Mercury telluride (HgTe) are promising semiconductor candidates owing to their intraband absorption peak tunable from 60 THz to 4 THz. However, the physical nature of this THz absorption remains elusive as, in this spectral range, quantum confinement and Coulomb repulsion effects can coexist. Further, the carrier dynamics at low energy in HgTe NCs, which strongly impact the performances of THz optoelectronic devices, is still unexplored. Here, we demonstrate a broad THz absorption resonance centered at ≈4.5 THz and fully interpret its characteristics with a quantum model describing multiple intraband transitions of single carriers between quantized states. Our analysis reveals the absence of collective excitations in the THz optical response of these self-doped large NCs. Furthermore, using optical pump-THz probe experiments, we report on carrier dynamics at low energy as long as 6 ps in these self-doped THz HgTe NCs. We highlight evidence that Auger recombination is irrelevant in this system and attribute the main carrier recombination process to direct energy transfer from the electronic transition to the ligand vibrational modes and to nonradiative recombination assisted by surface traps. Our study opens interesting perspectives for the use of large HgTe NCs for the development of advanced THz optoelectronic devices such as emitters and detectors and for quantum engineering at THz frequencies.

In this work we show Frequency Comb (FC) and short pulsed operation of mid-infrared Interband Cascade Lasers (ICLs) in a single long section. This is through the use of an adapted ultrafast Quantum Well Infrared Photodetectors (QWIPs), and correlating the microwave beatnotes with high resolution spectra of the ICL. In particular, we will show active mode-locking (ML) of single -section ICL that does not require RF optimisation of the ICL device and highlight its temporal characteristics using Shifted Wave Infrared Fourier Transform Spectroscopy (SWIFTS) analysis to reconstruct the intensity in the time domain.

Lasers are usually described by their output frequency and intensity. However, laser operation is an inherently nonlinear process. Knowledge about the dynamic behaviour of lasers is thus of great importance [...]