Lasing Technology Research Articles

Color tuning halide perovskites: Optical amplification and lasing

Light emission with delicate wavelength control generates fundamental colors that are essential for vision-based applications such as displays, information communication, visual and augmented reality. Yet these light emitting technologies critically rely on the development of photonic sources for proper lighting and color matching. Halide perovskites have recently emerged as excellent and efficient photonic sources due to their outstanding photophysical properties, such as low defect trap densities, long carrier lifetime, large absorption coefficient, high quantum yield and optical gain. On a par with the rapid advances of light-emitting diodes (LEDs), perovskite-based optical amplifiers and lasers have made great strides in the past few years, which have much more accurate control of color-matched lighting, high power, spatial and wavevector control of color emission. In this review, we aim to review the recent progress of perovskite lasers at micro-, nano- and subwavelength scales, discuss the properties of halide perovskites and optical cavity structures that benefit color light emission, and examine the remaining challenges in the field for the future development of perovskite-based lasing technologies.

Lasing from Narrow Bandwidth Light-Emitting One-Dimensional Nanoporous Photonic Crystals

Nanoporous anodic alumina (NAA) is an emerging platform material for photonics and light-based applications. However, demonstrations of narrow bandwidth lasing emissions from this optical material remain limited. Here, we demonstrate that narrow bandwidth NAA-based gradient-index filters (NAA-GIFs) can be optically engineered to achieve high-quality visible lasing. NAA-GIFs fabricated by a modified sinusoidal pulse anodization approach feature a well-resolved, intense, high-quality photonic stopband (PSB). The inner surface of NAA-GIFs is functionalized with rhodamine B (RhoB) fluorophore molecules through micellar solubilization of sodium dodecyl sulfate (SDS) surfactant. Systematic variation of the ratio of SDS and RhoB enables the precise engineering of the light-emitting functional layer to maximize light-driven lasing associated with the slow photon effect at the red edge of NAA-GIFs’ PSB. It is found that the optimal surfactant-to-fluorophore ratio, namely, 20 mM SDS to 0.81 mM RhoB, results in a strong, polarized lasing at ∼612 nm. This lasing was characterized by a remarkably high-quality–gain product of ∼536, a Purcell factor of 2.2, a lasing threshold of ∼0.15 mJ per pulse, and a high-quality polarization ratio of ∼0.7. Our results benefit the advancement of the NAA-based lasing technology for a variety of photonic disciplines such as sensing, tweezing, light harvesting, and photodetection.

Precise Chemical Sensing via External Cavity Lasing Technology

We report a method of precise chemical sensing by using on-chip external cavity lasing technology, in which a photonic micro-disk resonator acting as a sensing element is embedded into a laser's external cavity. The bandwidth of resonance peak of the passive resonator is greatly reduced. The lasing technology improves the quality factor of the system and the detection limitation in chemical sensing by 1-order of magnitude comparing with conventional passive devices.

Real-time, quantitative and sensitive detection of urea by whispering gallery mode lasing in liquid crystal microdroplet

In this study, we demonstrate a novel nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) microdroplet-based biosensor for real-time, quantitative and sensitive detection of urea by whispering gallery mode (WGM) lasing technology. The single stearic acid-doped 5CB microdroplet is taken as both optical resonator and sensing reactor. To the best of our knowledge, this work is the first report of urea detection with WGM lasing. The enzymatic reaction between urea and urease produces hydroxide ions. Deprotonation and self-assembly of stearic acid occur at the aqueous/LC interface with increased pH value, promoting the reorientation of LC molecules. The detectable shift of WGM lasing spectra related to the configuration transition of LC microdroplets can be used as an indicator of enzymatic reaction, and this allows detection of urea molecules by the sensor at a concentration of only 0.1 mM. The proposed sensor also demonstrated the ability to detect urea in real urine samples. Compared with conventional POM observations, monitoring and identification of lasing spectra from LC microdroplets as a versatile method allow for more quantitative and sensitive detection of analyte reactions and can be expected to replace POM imaging technology for current LC-based biosensor systems.

Single-fiber tweezers applied for dye lasing in a fluid droplet.

We report on the first demonstration of a single-fiber optical tweezer that is utilized to stabilize and control the liquid droplet for dye lasing. In order to trap a liquid droplet with a diameter of 15-30 μm, an annular core micro-structured optical fiber is adopted. By using wavelength division multiplexing technology, we couple a trapping light source (980 nm) and a pumping light source (532 nm) into the annular core of the fiber to realize the trapping, controlling, and pumping of the oil droplet. We show that the laser emission spectrum tunes along the same size as the oil droplet. The lasing threshold of the oil droplet with the diameter of 24 μm is 0.7 μJ. The presented fiber-based optical manipulation of liquid droplet micro-lasers can be easily combined with the micro-fluidic chip technology and also may extend the application of optical fiber tweezers for micro-droplet lasing technology in the biological field.

Effect of Auger Recombination on Lasing in Heterostructured Quantum Dots with Engineered Core/Shell Interfaces.

Nanocrystal quantum dots (QDs) are attractive materials for applications as laser media because of their bright, size-tunable emission and the flexibility afforded by colloidal synthesis. Nonradiative Auger recombination, however, hampers optical amplification in QDs by rapidly depleting the population of gain-active multiexciton states. In order to elucidate the role of Auger recombination in QD lasing and isolate its influence from other factors that might affect optical gain, we study two types of CdSe/CdS core/shell QDs with the same core radii and the same total sizes but different properties of the core/shell interface ("sharp" vs "smooth"). These samples exhibit distinctly different biexciton Auger lifetimes but are otherwise virtually identical. The suppression of Auger recombination in the sample with a smooth (alloyed) interface results in a notable improvement in the optical gain performance manifested in the reduction of the threshold for amplified spontaneous emission and the ability to produce dual-color lasing involving both the band-edge (1S) and the higher-energy (1P) electronic states. We develop a model, which explicitly accounts for the multiexciton nature of optical gain in QDs, and use it to analyze the competition between stimulated emission from multiexcitons and their decay via Auger recombination. These studies re-emphasize the importance of Auger recombination control for the realization of real-life QD-based lasing technologies and offer practical strategies for suppression of Auger recombination via "interface engineering" in core/shell structures.

Ultra-Wideband Tunable Dual-Mode Laser for Continuous Wave Terahertz Generation

An innovative external cavity dual-mode laser has been developed for continuous-wave terahertz generation via photomixing. Photomixing is a method of generating terahertz radiation at the beat frequency developed by two input optical light modes. Dual-mode lasers are an intriguing option for photomixing sources because the differential frequency produced is stabilized by the common mode rejection ratio effect. The laser presented here utilizes an acousto-optic tunable filter for frequency selection, which creates an ultra-wideband tunable range and provides the potential for tuning speed on the order of microseconds. The unique design of this laser enables 100% solid state operation resulting in a robust and compact system for applications in terahertz imaging or spectroscopy. Operating at around 1550 nm, this laser is capable of producing differential frequencies between 300 GHz and 18 THz with a side-mode-suppression-ratio of 50 dB and peak amplitudes of over 0 dBm. The device presented here represents a significant achievement in the advancement of dual mode lasing technology for tunable terahertz generation with high phase and amplitude stability.

Copper-Doped Inverted Core/Shell Nanocrystals with “Permanent” Optically Active Holes

We have developed a new class of colloidal nanocrystals composed of Cu-doped ZnSe cores overcoated with CdSe shells. Via spectroscopic and magneto-optical studies, we conclusively demonstrate that Cu impurities represent paramagnetic +2 species and serve as a source of permanent optically active holes. This implies that the Fermi level is located below the Cu(2+)/Cu(1+) state, that is, in the lower half of the forbidden gap, which is a signature of a p-doped material. It further suggests that the activation of optical emission due to the Cu level requires injection of only an electron without a need for a valence-band hole. This peculiar electron-only emission mechanism is confirmed by experiments in which the titration of the nanocrystals with hole-withdrawing molecules leads to enhancement of Cu-related photoluminescence while simultaneously suppressing the intrinsic, band-edge exciton emission. In addition to containing permanent optically active holes, these newly developed materials show unprecedented emission tunability from near-infrared (1.2 eV) to the blue (3.1 eV) and reduced losses from reabsorption due to a large Stokes shift (up to 0.7 eV). These properties make them very attractive for applications in light-emission and lasing technologies and especially for the realization of novel device concepts such as "zero-threshold" optical gain.

When group-III nitrides go infrared: New properties and perspectives

Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN) or 0.2 μm (6.2 eV for AlN). The continuous range of bandgap energies now spans the near infrared, raising the possibility of new applications for group-III nitrides. In this article we present a detailed review of the physical properties of InN and related group III-nitride semiconductors. The electronic structure, carrier dynamics, optical transitions, defect physics, doping disparity, surface effects, and phonon structure will be discussed in the context of the InN bandgap re-evaluation. We will then describe the progress, perspectives, and challenges in the developments of new electronic and optoelectronic devices based on InGaN alloys. Advances in characterization and understanding of InN and InGaN nanostructures will also be reviewed in comparison to their thin film counterparts.

Suppressed Auger Recombination in “Giant” Nanocrystals Boosts Optical Gain Performance

Many potential applications of semiconductor nanocrystals are hindered by nonradiative Auger recombination wherein the electron-hole (exciton) recombination energy is transferred to a third charge carrier. This process severely limits the lifetime and bandwidth of optical gain, leads to large nonradiative losses in light-emitting diodes and photovoltaic cells, and is believed to be responsible for intermittency ("blinking") of emission from single nanocrystals. The development of nanostructures in which Auger recombination is suppressed has recently been the subject of much research in the colloidal nanocrystal field. Here, we provide direct experimental evidence that so-called "giant" nanocrystals consisting of a small CdSe core and a thick CdS shell exhibit a significant (orders of magnitude) suppression of Auger decay rates. As a consequence, even multiexcitons of a very high order exhibit significant emission efficiencies, which allows us to demonstrate optical amplification with an extraordinarily large bandwidth (>500 meV) and record low excitation thresholds. This demonstration represents an important milestone toward practical lasing technologies utilizing solution-processable colloidal nanoparticles.

Excimer laser coronary angioplasty (ELCA) for diffuse in-stent restenosis: beneficial long-term results after sufficient debulking with a lesion-specific approach using various laser catheters.

Although brachytherapy became frequently used technique for treating in-stent restenosis (ISR) in 2000 (stenosis recurrence: 14-25%), removing the tissue (i.e., debulking) is still under evaluation. Regarding XeCl Excimer laser no randomised trials have been completed so far. The entirely attainable and published data originate from five one-centre and one multicentre registries which showed a high procedural success-(98%) and a very low complication rate (<2%), but heterogeneous results based on (I) manifold laser- and lasing technology due to evolving catheter and lasing technology during the studies and (II) inhomogeneous patient-populations, which had been documented only occasionally, although we now know that ISR-recurrence rates are significantly higher in patients with (1) high grade stenosis (>70%), (2) long lesions (>15mm in length), (3) recurrent ISR. (4) ISR in saphenous vein grafts, (5) early-ISR (<3 months after stent implantation), (6) multi-vessel disease and (7) low ejection fraction. Because all athero-ablative procedures are typically finalised by adjunctive balloon PCI, diameter stenosis before adjunctive PTCA was between 23% and 64%, reflecting the different lasing (and therefor debulking) intensity between the studies. The mean lesion-length, vessel diameter, stent-type and coronary differed significantly between the studies (i.e. mean lesion length between 14 and 32 mm) and although of major influence, the speed of lasing (=ablation) was not recorded in the various studies. Concerning eccentric ELCA the invention of the area-ablation technique provide a significant greater ablation effect and the new OS-technology provide 60% more ablating area. In summary, the feasibility, effectiveness, procedural success and presumably long-term outcome of the Excimer laser for treating ISR depend on multiple outcome-influencing aspects. The lack of recording variables make analysis of various laser-for-ISR trials difficult or even impossible. ELCA seems to have advantageous procedural and long term results in lesions, where aggressive debulking effect using eccentric ELCA catheters can be achieved. The OS-catheter can provide a maximum debulking in distal lesions and tortuous vessels, where the debulking abilities of the eccentric catheter are limited. If laser debulking is inadequate (DS 30%), the incidence of recurrent restenosis appear to be higher and brachytherapy is probably more suitable. Sufficient laser debulking can only be achieved through use of precise technique respecting the laser-specific abilities of plaque dissolution, appropriate energy levels and quantities, and possibly recovery time.