Broad Tunability Research Articles

A compact laser platform for nonlinear Raman microspectroscopy: multimodality through broad chirp tunability

We demonstrate a compact laser platform that integrates stimulated and coherent Raman microscopy with high‐resolution nonlinear Raman spectroscopy. Ultrashort laser pulses with a carefully managed, broadly tunable chirp are shown to enable a comfortable switching between the high‐contrast microscopy and high‐resolution spectroscopy modalities in both stimulated and coherent versions of nonlinear Raman scattering. Sub‐10‐cm−1 spectral resolution in stimulated Raman spectroscopy is demonstrated through a careful compensation of nonlinear phase distortions of ultrashort laser pulses. This offers a powerful tool for a reliable identification of molecular vibrations with close frequencies, enhancing the chemical selectivity of spectroscopic analysis of complex multicomponent systems. Copyright © 2016 John Wiley & Sons, Ltd.

Light Emission near a Gradient Metasurface

Gradient metasurfaces have recently emerged as a powerful tool for the control of externally incident optical wavefronts. Here we present a theoretical study showing how they can also dramatically modify the radiation properties of light emitters located in their near-field zone. First, the spontaneous emission decay of a dipole source can be greatly accelerated through near-field interactions involving evanescent waves at the metasurface, by an amount that depends on the dipole lateral position in an oscillatory fashion. Second, as a result of these interactions, highly asymmetric directional radiation patterns can be produced in the far field, with broad geometrical tunability of the angle of peak emission. Furthermore, the total output radiation power can be increased in the case of low-efficiency emitters or quenched in favor of directional excitation of surface waves depending on the metasurface phase gradient. These phenomena could not be explained simply in terms of anomalous reflection of external...

An organic dye with very large Stokes-shift and broad tunability of fluorescence: Potential two-photon probe for bioimaging and ultra-sensitive solid-state gas sensor

Light-emitting nonlinear optical molecules, especially those with large Stokes shifts and broad tunability of their emission wavelength, have attracted considerable attention for various applications including biomedical imaging and fluorescent sensors. However, most fluorescent chromophores have only limited potential for such applications due to small Stokes shifts, narrow tunability of fluorescence emissions, and small optical nonlinearity in highly polar solvents. In this work, we demonstrate that a two-photon absorbing stilbene chromophore exhibits a large two-photon absorption action cross-section (ηδ = 320 GM) in dimethylsulfoxide (DMSO) and shows broad fluorescence tunability (125 nm) by manipulating the polarity of the surrounding medium. Importantly, a very large Stokes shift of up to 227 nm is achieved in DMSO. Thanks to these features, this chromophore can be utilized as a two-photon probe for bioimaging applications and in an ultrasensitive solid-state gas detector.

A SERS and electrical sensor from gas-phase generated Ag nanoparticles self-assembled on planar substrates.

Optical excitation of coupled plasmonic nanoparticles supports intense localized electromagnetic "hot-spots" which enable a variety of surface enhanced spectroscopies with the best known example being surface enhanced Raman scattering (SERS), currently of great interest for sensing applications. In this study, we present a novel SERS and electrical dual transduction chemical sensor based on gas-phase generated, negatively charged, silver nanoparticles self-assembled on glass slide forming a close-packed plasmonic monolayer thin-film that supports both SERS and electrical sensing. We demonstrate broad tunability of the localized surface plasmon resonance (LSPR) of the close-packed plasmonic nanoparticle monolayer thin-film sensors through control of the nanoparticle (NP) deposition time which directly influences the plasmonic coupling between neghibouring NPs. This broad tunability supports strong SERS activity from visible to near infrared (NIR) excitation wavelengths. We performed SERS and electrical measurements of a non-resonant molecule 4-mercaptobenzonitrile (4-MBN) as a sample Raman reporter molecule to determine the SERS enhancement factor of our SERS substrate. We measured an average SERS enhancement factor of 10(7) from our close-packed plasmonic nanoparticle monolayer thin-film sensor. Films which were grown below or above one nanoparticle monolayer both exhibited significantly lower SERS performance in one or more of SERS enhancement factor (EF), uniformity or repeatability. Our close-packed plasmonic nanoparticle monolayer thin-film sensors are highly uniform from point-to-point across the entire substrate and showed good reproducibility from batch-to-batch. These qualities are highly desirable for quantifiable detection of chemical and biological molecules. As an example application, this type of substrates provides an affordable and reliable sensing and identification capability for combatting new and emerging chemical and biological threats in support of security applications.

203 W all-polarization-maintaining picosecond thulium-doped all-fiber laser

High-power ultrafast fiber lasers are important sources for a number of applications including material processing, pump source for optical parametric oscillator, and supercontinuum generation. Ultrafast thulium-doped fiber lasers, which extend the wavelength range of fiber lasers from 1.8 to 2.1 m, have rapidly developed in the last several years and the average output power of the ultrafast thulium-doped fiber amplifiers has reached a hundredwatt level. The broad and smooth gain spectrum of thulium-doped fiber makes it a well-suited gain medium for generating the ultrashort laser pulses and broad wavelength tunability. However, previous reports on ultrafast thulium-doped fiber lasers and amplifiers were related to non-PM fiber configuration. These ultrafast thulium-doped fiber lasers and amplifiers may suffer the environmental instability, which means that these fiber sources are sensitive to externally-induced changes, like significant temperature variations and mechanical perturbations which will influence the fiber birefringence property. An effective method to eliminate this environmental instability is to build an all-PM, thulium-doped all-fiber MOPA configuration where the light polarizes only along the slow or fast axis in the PM fiber and PM-fiber components. Here, we demonstrate a high-power all-polarization-maintaining picosecond thulium-doped all-fiber master-oscillator power-amplifier (MOPA) system. The linearly-polarized thulium-doped all-fiber MOPA yields 203 W of average output power at central wavelength of 1985 nm with a polarization extinction ratio of 15 dB. The pulse duration of 15 ps at 611.5 MHz repetition-rate results in a peak-power of 22 kW in the final thulium-doped fiber power amplifier. To the best of our knowledge, this is the highest average output power ever reported for a picosecond-pulsed thulium-doped all-fiber laser at 2 m wavelength. Furthermore, high-power linearly-polarized thulium-doped fiber laser with compact and simple design is greatly demanded for a variety of applications, such as coherent polarization beam combination, and frequency conversion in nonlinear crystals.

Wavelength-tunable waveguides based on polycrystalline organic–inorganic perovskite microwires

Hybrid organic-inorganic perovskites have emerged as new photovoltaic materials with impressively high power conversion efficiency due to their high optical absorption coefficient and long charge carrier diffusion length. In addition to high photoluminescence quantum efficiency and chemical tunability, hybrid organic-inorganic perovskites also show intriguing potential for diverse photonic applications. In this work, we demonstrate that polycrystalline organic-inorganic perovskite microwires can function as active optical waveguides with small propagation loss. The successful production of high quality perovskite microwires with different halogen elements enables the guiding of light with different colours. Furthermore, it is interesting to find that out-coupled light intensity from the microwire can be effectively modulated by an external electric field, which behaves as an electro-optical modulator. This finding suggests the promising applications of perovskite microwires as effective building blocks in micro/nano scale photonic circuits.

Generalized colloidal synthesis of high-quality, two-dimensional cesium lead halide perovskite nanosheets and their applications in photodetectors.

All-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) nanocrystals (NCs) are emerging as an important class of semiconductor materials with superior photophysical properties and wide potential applications in optoelectronic devices. So far, only a few studies have been conducted to control the shape and geometry of CsPbX3 NCs. Here we report a general approach to directly synthesize two-dimensional (2D) CsPbX3 perovskite and mixed perovskite nanosheets with uniform and ultrathin thicknesses down to a few monolayers. The key to the high-yield synthesis of perovskite nanosheets is the development of a new Cs-oleate precursor. The as-synthesized CsPbX3 nanosheets exhibit bright photoluminescence with broad wavelength tunability by composition modulation. The excellent optoelectronic properties of CsPbX3 nanosheets combined with their unique 2D geometry and large lateral dimensions make them ideal building blocks for building functional devices. To demonstrate their potential applications in optoelectronics, photodetectors based on CsPbBr3 nanosheets are fabricated, which exhibit high on/off ratios with a fast response time.

Photothermal effects from Au–Cu2O core–shell nanocubes, octahedra, and nanobars with broad near-infrared absorption tunability

Other than the display of purely optical phenomenon, the recently-discovered facet-dependent optical properties of metal-Cu2O nanocrystals have become useful by illuminating Au-Cu2O nanocubes and octahedra having a surface plasmon resonance (SPR) absorption band in the near-infrared (NIR) region from octahedral Au cores with 808 nm light for heat generation. After 5 min of light irradiation, a solution of Au-Cu2O nanocubes can reach 65 °C with their Au SPR band matching the illuminating light wavelength. Photothermal efficiency has been found to be facet-dependent. In addition, short gold nanorods were employed to synthesize {100}-bound rectangular Au-Cu2O nanobars with a tunable longitudinal Au SPR absorption band covering a broad NIR range from ∼1050 to 1400 nm. Because the Au SPR bands can become fixed with relatively thin Cu2O shells of less than 15 nm, ultrasmall nanobars having a size of 61 nm directly red-shift the Au SPR band to 1047 nm. And 73 nm nanobars can give a Au SPR band at 1390 nm. Truncated nanobars exposing {100}, {110}, and {111} facets give a very blue-shifted Au SPR band. The nanobars also exhibit photothermal activity when illuminated by 1064 nm light. These small Au-Cu2O nanocrystals represent the simplest nanostructure design to absorb light covering the entire NIR wavelengths.

Investigation of the dynamic nature of 1,2-oxazines derived from peralkylcyclopentadiene and nitrosocarbonyl species.

We have investigated the reversible hetero-Diels-Alder reaction of 1,2-oxazines derived from a peralkylcyclopentadiene and a series of nitrosocarbonyl dienophiles. The nature of the dienophile was found to impart broad tunability to the dynamic character of the oxazine adducts. The reversibility was also observed in polymeric systems. The fidelity of the reaction and tunable sensitivity toward elevated temperature and water signify potential applications in the development of dynamic covalent materials or delivery systems for small molecule payloads.

Tuning a microcavity-coupled terahertz laser

Tunable oscillators are a key component of almost all electronic and photonic systems. Yet, a technology capable of operating in the terahertz (THz)-frequency range and fully suitable for widescale implementation is still lacking. This issue is significantly limiting potential THz applications in gas sensing, high-resolution spectroscopy, hyper-spectral imaging, and optical communications. The THz quantum cascade laser is arguably the most promising solution in terms of output power and spectral purity. In order to achieve reliable, repeatable, and broad tunability, here we exploit the strong coupling between two different cavity mode concepts: a distributed feedback one-dimensional photonic resonator (providing gain) and a mechanically actuated wavelength-size microcavity (providing tuning). The result is a continuously tunable, single-mode emitter covering a 162 GHz spectral range, centered on 3.2 THz. Our source has a few tens of MHz resolution, extremely high differential efficiency, and unprecedented compact and simple design architecture. By unveiling the large potential that lies in this technique, our results provide a robust platform for radically different THz systems exploiting broadly tunable semiconductor lasers.

Holmium-doped 21 μm waveguide chip laser with an output power > 1 W

We demonstrate the increasing applicability of compact ultra-fast laser inscribed glass guided-wave lasers and report the highest-power glass waveguide laser with over 1.1 W of output power in monolithic operation in the short-infrared near 2070 nm achieved (51% incident slope efficiency). The holmium doped ZBLAN chip laser is in-band pumped by a 1945 nm thulium fiber laser. When operated in an extended-cavity configuration, over 1 W of output power is realized in a linearly polarized beam. Broad and continuous tunability of the extended-cavity laser is demonstrated from 2004 nm to 2099 nm. Considering its excellent beam quality of M² = 1.08, this laser shows potential as a flexible master oscillator for single frequency and mode-locking applications.

Sample Grating Distributed Feedback Quantum Cascade Laser Array.

A sample grating distributed feedback quantum cascade laser array aim at broad tunability and enhanced side mode suppression ratios is presented. Utilizing a sample grating dependence on emission wavelength and epitaxial side down bonding technique, the array of laser ridges exhibited three separated single mode emissions centered at 4.760, 4.721, and 4.711 μm respectively, in continuous wave at room temperature. Side mode suppression ratios of >35 dB and continuous wave output powers of >10 mW per laser ridge were obtained.

Oxide-Free Gadolinium Nanocrystals with Large Magnetic Moments

Among the elements in the periodic table, gadolinium (Gd) has the highest number of unpaired electrons. However, the potential of this rare-earth metal has not yet been fully realized due to challenges in its chemical synthesis, namely, its high reduction potential, leading to the formation of oxides with suboptimal properties. This problem is also prevalent with other lanthanides, severely limiting their uses in industry. Herein, a fabrication approach along with a reduction process and appropriate capping have been developed to produce oxide-free, stable gadolinium nanoparticles. We demonstrate broad tunability of the particle size while maintaining remarkably narrow size distributions (<5%). The nanoconstructs displayed the highest magnetization measured to date for Gd, 206 emu/g Gd at 2 K along with a record high per particle nuclear magnetic resonance (NMR) transverse relaxivity (r2) of 2.7 × 108 mM–1 s–1, which corresponds to the highest per-particle r2 relaxivity reported for any T2 contrast agents...

(Invited) Lead Halide Perovskite Nanowire and Nanoplate Lasers with Low Lasing Thresholds and High Quality Factors

The remarkable performance of lead halide perovskites in solar cells can be attributed to the long carrier lifetimes and low non-radiative recombination rates, the same physical properties that are ideal for semiconductor lasers. Here we first report new insights on the crystal growth of the perovskite materials and the solution growth of single crystal nanowires, nanorods, and nanoplates of methylammonium lead halide and other perovskites via a dissolution-recrystallization pathway from lead iodide or lead acetate films coated on substrates. We further show room temperature and wavelength tunable lasing from single crystal lead halide perovskite nanowires with the lowest lasing thresholds (220 nJ/cm2) and highest quality factors (Q ~ 3600) reported to date for semiconductor nanowire lasers. The lasing threshold corresponds to a charge carrier density as low as 1.5×1016/cm3. Kinetic analysis based on time-resolved fluorescence reveals little charge carrier trapping in these single crystal nanowires and gives estimated lasing quantum yields approaching 100%. Such lasing performance, coupled with facile solution phase growth of single crystal NWs and broad tunability of emission color from stoichiometry, makes lead halide perovskites ideal materials for the development of nano-photonics, in parallel with the rapid development in photovoltaic technology from the same materials.

Unlocking Spectral Versatility from Broadly−Tunable Quantum−Dot Lasers

Wavelength−tunable semiconductor quantum−dot lasers have achieved impressive performance in terms of high−power, broad tunability, low threshold current, as well as broadly tunable generation of ultrashort pulses. InAs/GaAs quantum−dot−based lasers in particular have demonstrated significant versatility and promise for a range of applications in many areas such as biological imaging, optical fiber communications, spectroscopy, THz radiation generation and frequency doubling into the visible region. In this review, we cover the progress made towards the development of broadly−tunable quantum−dot edge−emitting lasers, particularly in the spectral region between 1.0–1.3 µm. This review discusses the strategies developed towards achieving lower threshold current, extending the tunability range and scaling the output power, covering achievements in both continuous wave and mode−locked InAs/GaAs quantum−dot lasers. We also highlight a number of applications which have benefitted from these advances, as well as emerging new directions for further development of broadly−tunable quantum−dot lasers.

Metal-halide perovskites for photovoltaic and light-emitting devices.

Metal-halide perovskites are crystalline materials originally developed out of scientific curiosity. Unexpectedly, solar cells incorporating these perovskites are rapidly emerging as serious contenders to rival the leading photovoltaic technologies. Power conversion efficiencies have jumped from 3% to over 20% in just four years of academic research. Here, we review the rapid progress in perovskite solar cells, as well as their promising use in light-emitting devices. In particular, we describe the broad tunability and fabrication methods of these materials, the current understanding of the operation of state-of-the-art solar cells and we highlight the properties that have delivered light-emitting diodes and lasers. We discuss key thermal and operational stability challenges facing perovskites, and give an outlook of future research avenues that might bring perovskite technology to commercialization.

Size-, Shape-, and Composition-Controlled Synthesis and Localized Surface Plasmon Resonance of Copper Tin Selenide Nanocrystals

We report a robust methodology for synthesizing monodisperse copper-tin-selenide (CTSe) nanocrystals (NCs) of tunable size, shape, and composition including single-crystalline nanosheets, nanoplates, spheres, and tetrahedra. Both the identity and concentration of the selenium precursor play important roles in determining the morphology and crystal structure of the CTSe NCs. In contrast to previous studies, we demonstrated broad tunability of the Cu to Sn ratio. The size of CTSe NCs continuously decreased with increasing Sn incorporation. Moreover, the near-infrared (NIR) localized surface plasmon resonance (LSPR) in CTSe alloy NCs was tuned over a broad range by varying the Cu:Sn ratio. The LSPR red-shifted and decreased in intensity with increasing Sn content. This indicates that the free charge carrier concentration can be manipulated by varying the cation ratio. The cation deficiency responsible for self-doping in these NCs decreases with increasing Sn content. The resulting CTSe NCs and related materi...

Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors.

The remarkable performance of lead halide perovskites in solar cells can be attributed to the long carrier lifetimes and low non-radiative recombination rates, the same physical properties that are ideal for semiconductor lasers. Here, we show room-temperature and wavelength-tunable lasing from single-crystal lead halide perovskite nanowires with very low lasing thresholds (220 nJ cm(-2)) and high quality factors (Q ∼ 3,600). The lasing threshold corresponds to a charge carrier density as low as 1.5 × 10(16) cm(-3). Kinetic analysis based on time-resolved fluorescence reveals little charge carrier trapping in these single-crystal nanowires and gives estimated lasing quantum yields approaching 100%. Such lasing performance, coupled with the facile solution growth of single-crystal nanowires and the broad stoichiometry-dependent tunability of emission colour, makes lead halide perovskites ideal materials for the development of nanophotonics, in parallel with the rapid development in photovoltaics from the same materials.

Superior Conductive Solid-like Electrolytes: Nanoconfining Liquids within the Hollow Structures.

The growth and proliferation of lithium (Li) dendrites during cell recharge are currently unavoidable, which seriously hinders the development and application of rechargeable Li metal batteries. Solid electrolytes with robust mechanical modulus are regarded as a promising approach to overcome the dendrite problems. However, their room-temperature ionic conductivities are usually too low to reach the level required for normal battery operation. Here, a class of novel solid electrolytes with liquid-like room-temperature ionic conductivities (>1 mS cm(-1)) has been successfully synthesized by taking advantage of the unique nanoarchitectures of hollow silica (HS) spheres to confine liquid electrolytes in hollow space to afford high conductivities (2.5 mS cm(-1)). In a symmetric lithium/lithium cell, the solid-like electrolytes demonstrate a robust performance against the Li dendrite problem, preventing the cell from short circuiting at current densities ranging from 0.16 to 0.32 mA cm(-2) over an extended period of time. Moreover, the high flexibility and compatibility of HS nanoarchitectures, in principle, enables broad tunability to choose desired liquids for the fabrication of other kinds of solid-like electrolytes, such as those containing Na(+), Mg(2+), or Al(3+) as conductive media, providing a useful alternative strategy for the development of next generation rechargeable batteries.

Sub-50-fs widely tunable Yb:CaYAlO(4) laser pumped by 400-mW single-mode fiber-coupled laser diode.

Yb:CaYAlO(4) has been investigated spectroscopically and compared to better known Yb:CaGdAlO(4). It turns out that both materials show very similar spectroscopic parameters relevant to ultrafast lasers design. Employing single-mode fiber-coupled 400-mW laser diode at 976 nm we measured pulses as short as 43 fs, and broad tunability of 40 nm with a simple single-prism setup.