Laser & Photonics Reviews

A train of optical pulses are injected into a quantum dot semiconductor optical waveguide, where the velocity is slowed down and the pulsewidth is reduced.

Anchoring proteins to surfaces for single-molecule optical-trapping experiments. Biotinylated-BSA, bovine serum albumin (grey) with an attached biotin molecule, can be adsorbed to glass. When the biotin (red dot) is correctly positioned, it is bound by streptavidin (green). Since streptavidin has four binding sites, it can also bind to the biotinylated helicase. Polyethylene glycol (PEG) (yellow) is covalently linked to a cover glass. A fraction of the PEG molecules are terminated in biotin which allows for a controlled density of streptavidin and thus the helicase.

In the fabrication of high-resolution 3D microstructures, Two-photon stereolithography (TPS) has significant advantages over conventional microelectromechanical system (MEMS) processing, which involves time-consuming multistep indirect fabrication processes. Many studies have recently been made to develop and improve the TPS process, focusing on creating greater efficiency, higher resolution, and greater productivity. For the first time, an artistic microstructure has recently been produced with an ultraprecise spatial resolution, sub-30 nm nanofibers, 3D multilayer imprint stamps for mass production, and ceramic 3D microstructures. In this review, we report the progress of two-photon polymerization based on 3D microfabrication.

Metamaterial transmission lines are artificial lines consisting on a host conventional line loaded with reactive elements. The main relevant characteristic of these propagating structures is the fact that, due to the greater number of parameters as compared to conventional lines, it is possible to tailor their dispersion characteristics to achieve certain functionalities not achievable though conventional lines. Enhanced bandwidth components and multiband components are the main benefits of dispersion engineering with metamaterial transmission lines. In this paper, dispersion engineering will be carefully reviewed, and several examples illustrative of the application of this technique to microwave circuit design will be given.

We give an overview of slow- and fast-light effects in semiconductor active waveguides. Experimental and theoretical results are presented, emphasizing the physics of these phenomena and the limitations imposed by the carrier dynamical processes.

In this paper we review recent progress in obtaining laser action from semiconductor quantum cascade structures covering the low Thz region of the electromagnetic spectrum, from 2 THz (λ ≃ 155 μm) down to the sub-THz region (λ > 300 μm). Particularly, laser active region designs based on bound-to-continuum transition and magnetically assisted intra-well transition are presented. The wide scalability of active region designs is discussed and illustrated with experimental data. Latest results including the demonstration of laser action from quantum heterostructure at 950 GHz are presented.

We present an overview of solid-state optical refrigeration also known as laser cooling in solids by fluorescence upconversion. The idea of cooling a solid-state optical material by simply shining a laser beam onto it may sound counter intuitive but is rapidly becoming a promising technology for future cryocoolers. We chart the evolution of this science in rare-earth doped solids and~semiconductors.

We review recent work studying the dynamics in zinc oxide quantum wells (QWs) using ultrafast optical spectroscopy. These materials present exciting possibilities for optoelectronic device applications. In order to develop these applications, it is important to understand the mechanisms of the electronic processes occurring in ZnO/ZnMgO QWs. In this review, we discuss the excitonic lifetime and the impact of the internal electric field, the potential profile, phonons, and defects. We also discuss coherence dynamics and the observation of biexcitons at room temperature.

The advanced particle acceleration technique PASER enables bundles of electrons to be accelerated using the same principle as in laser. A proof-of-principle experiment has been conducted at the Brookhaven National Laboratory. A 45 MeV electron macrobunch was modulated by a high-power CO₂ laser in a wiggler, and then injected into an excited CO₂ gas mixture. The emerging microbunches reveal a 0.15 % relative change in the kinetic energy in less than 40 cm long interaction region. Advanced PASER concepts such as non-linear interaction of electrons and waves in an active medium, the occurrence of resonant absorption instability, and PASER-based optical Bragg accelerators are considered.

There has been an increasing interest in photonic generation of RF signals in the millimeter-wave (30 Ghz∼300 Ghz and/or terahertz-wave (0.1 Thz∼10 THz) regions, and photodiodes play a key role in it. This paper reviews recent progress in the high-power RF photodiodes such as Uni-Traveling-Carrier-Photodiodes (UTC-PDs), which operate at these frequencies. Several approaches to increasing both the bandwidth and output power of photodiodes are discussed, and promising applications to broadband wireless communications and spectroscopic sensing are described.

The photoluminescence properties of silicon quantum dots may be tailored by surface states via efficient coupling to resonant bulk states. Therefore various wet-chemistry procedures were developed to fabricate silicon quantum dots with adjustable sizes and surface properties. While the energy gap of the Si core is tuned by the size, resonant electronic surface states may be attained by varying the structure and chemical composition of the grafting. The origin of photoluminescence and therewith the quantum dot size distribution may be elucidated by stationary luminescence spectroscopy, ultrafast optical spectroscopy techniques. For instance the femtosecond transient absorption spectroscopy allows to monitor photo-induced electron transfer between surface and bulk states and carrier trapping on the subpicosecond timescale.

Noble metal nanoparticles exhibit a plasmonic resonance that provides them with unique optical properties. The sensitivity of the plasmonic resonance to the surrounding dielectric environment has lead to the development of metal nanoparticles as the basis of biosensing schemes. The sharp enhancement in scattering and absorption at the plasmonic resonance frequency has been exploited to develop metal nanoparticles as imaging contrast agents. In this review article, we recap recent efforts that combine both of these features of metal nanoparticles to enable simultaneous molecular imaging and environmental sensing through the use of darkfield microspectroscopy schemes.

In-vivo imaging of optical contrast in living tissues can allow measurement of functional parameters such as blood oxygenation and detection of targeted and active fluorescent contrast agents. However, optical imaging must overcome the effects of light scattering, which limit the penetration depth and can affect quantitation and sensitivity. This article focuses on a technique for high-resolution, high-speed depth-resolved optical imaging of superficial living tissues called laminar optical tomography (LOT), which is capable of imaging absorbing and fluorescent contrast in living tissues to depths of 2–3 mm with 100–200 micron~resolution.

Today, fluorescence microscopes are essential in biological and biomedical sciences for 3D noninvasive imaging of the interior of cells. Optical microscopes are subject to the diffraction barrier of light. In the recent past new techniques emerged that break the diffraction barrier and enable structural investigations with so far unmatched resolution. They are all based on the selective switching of fluorophores between a fluorescent and a nonfluorescent state and can be generalized under the denotation “Photoswitching Microscopy”. Recent progress in subdiffraction-resolution fluorescence imaging microscopy using various photoswitchable fluorophores and strategies is presented. Special emphasis is placed on the design and development of photoswitches and the requirements they have to fulfill for successful use in this kind of microscopy.

Optical trapping experiments of different complexities are making a significant impact in biology. This review seeks to highlight design choices for scientists entering the field or building new instruments and to discuss making calibrated measurements with optical traps. For specificity, this review focuses on nucleic acid-based assays, but the discussion reflects the general experimental design considerations of developing a biological assay and an optical trap to measure it.

In recent years there has been a growing interest in the interactions of fluorophores with metallic nanostructures or nanoparticles. The spectra properties of fluorophores can be dramatically modified by near-field interactions with the electron clouds present in metals. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. These interactions modify the emission in ways not seen in ensemble fluorescence experiments. In this review we provide an insightful description of the photophysics of metal plasmons and near-field interactions. Additionally, we summarize recent works on single-molecule studies on metal-fluorophore interactions and suggest how these effects will result in new classes of experimental procedures, novel probes, bioassays and devices.