Showing papers in "Optical Materials Express in 2015"

Time-varying metasurfaces and Lorentz non-reciprocity

Abstract: A cornerstone equation of optics – Snell's law – relates the angles of incidence and refraction for light passing through an interface between two media. It is built on two fundamental constraints: the conservation of tangential momentum and the conservation of energy. By relaxing the classical Snell’s law photon momentum conservation constrain when using space-gradient phase discontinuity, optical metasurfaces enabled an entirely new class of ultrathin optical devices. Here, we show that by eradicating the photon energy conservation constrain when introducing time-gradient phase discontinuity, we can further empower the area of flat photonics and obtain a new genus of optical devices. With this approach, classical Snell’s relations are developed into a more universal form not limited by Lorentz reciprocity, hence, meeting all the requirements for building magnetic-free optical isolators. Furthermore, photons experience inelastic interaction with time-gradient metasurfaces, which modifies photonic energy eigenstates and results in a Doppler-like wavelength shift. Consequently, metasurfaces with both space- and time-gradients can have a strong impact on a plethora of photonic applications and provide versatile control over the physical properties of light.

Microfiber-based WS 2 -film saturable absorber for ultra-fast photonics

Abstract: In this paper, we demonstrated a passively mode-locked erbium-doped fiber (EDF) laser by incorporating a tungsten disulfide (WS2) film SA fabricated by pulsed laser deposition (PLD) method. The WS2 film was thickness-dependent, which had two different states: the bulk WS2 [faced to plasma plume] and tiny WS2 flakes [in the shadow of plasma plume]. This SA device demonstrated low insertion loss (IL) and high power tolerance ability. Interestingly, the SA device possessed different nonlinear absorption regimes related with the film states. By employing this new type of SA, we obtained stable fundamental mode-locking (FML) at pump power of 54 mW, and the generated soliton pulse had pulse duration of 675 fs and signal-to-noise ratio (SNR) of 65 dB. At the maximum pump power of 395 mW, we also obtained up to 1 GHz repetition rate of harmonic mode-locking (HML) with pulse duration of 452 fs and SNR of 48 dB. The experimental results show that WS2-PLD film can serve as a promising SA for ultrafast laser systems.

Pulsed laser deposition of gallium oxide films for high performance solar-blind photodetectors

Abstract: Monoclinic gallium oxide thin films were grown on (0001) sapphire at various substrate temperatures ranging from 400 to 1000 °C by pulsed laser deposition using a KrF excimer laser. The structural, optical and compositional properties of the films were analyzed by using x-ray diffraction, transmission electron microscopy, optical transmittance, and Rutherford backscattering spectroscopy. As the substrate temperature was increased to 800 °C, the gallium oxide film possesses single crystalline phase with a preferred growth orientation of (−201) plane and higher crystal quality than those at the other temperatures. Optical transmittance measurements reveal the films grown at 600-1000 °C exhibit a clear absorption edge at the deep ultraviolet region around 250 nm wavelength. Based on the results of Rutherford backscattering spectroscopy, the O/Ga ratio of gallium oxide film increased gradually with increasing substrate temperature. When the substrate temperature was raised to 800-1000 °C, the film composition was close to the formation of Ga2O3, indicating the O vacancies and defects were reduced. Furthermore, the films grown at 600 and 800 °C were chosen to fabricate solar-blind metal-semiconductor-metal photodetectors. At an applied bias of 5 V, the photodetector prepared with 800 °C-grown film has a lower dark current of 1.2 × 10−11 A and a higher responsivity of 0.903 A/W (at a wavelength of 250 nm) than those with 600 °C-grown films. The better device performance is ascribed to the higher crystal quality and fewer O vacancies in the 800 °C-grown film. Moreover, the results indicate the gallium oxide films presented in this study have high potential for deep ultraviolet photodetector applications.

Hyperbolic metasurfaces: Surface plasmons, light-matter interactions, and physical implementation using graphene strips

Abstract: We investigate ultrathin metasurfaces defined by anisotropic conductivity tensors using a Green’s function approach, focusing on their exciting plasmonic interactions and dramatic enhancement of light-matter interactions for hyperbolic dispersion. We apply our analytical formulation to explore several practical implementations at THz and near infrared frequencies, including electrically and magnetically-biased graphene sheets – a natural isotropic elliptic metasurface – and densely-packed arrays of graphene ribbons modelled through an effective medium approach. This latter configuration allows the electrical control of their band diagram topology – from elliptic to hyperbolic, going through the extremely anisotropic σ-near-zero case – providing unprecedented control over the confinement and direction of plasmon propagation while simultaneously boosting the local density of states. Finally, we study the influence of the strip granularity to delimit the accuracy of effective medium theory to model the electromagnetic interactions with hyperbolic metasurfaces. Our findings may lead to the development of ultrathin reconfigurable plasmonic devices able to provide extreme confinement and dynamic guidance of light while strongly interacting with their surroundings, with direct application in sensing, imaging, hyperlensing, on-chip networks, and communications.

All-fiber Er-doped Q-Switched laser based on Tungsten Disulfide saturable absorber

Abstract: We demonstrated a Q-switched fiber laser based on Tungsten Disulfide (WS2) saturable absorber. The WS2 nano-sheets were prepared by liquid phase exfoliation method and the saturable absorber was fabricated by spin-coating of few-layer WS2 nano-sheets on a side-polished fiber for pulsed operation of a fiber laser. By inserting the absorber into an Erbium-doped fiber laser cavity pumped by a 980 nm laser diode, a stable Q-switched laser operation was achieved with a tunable repetition rates from 82 kHz to 134 kHz depending on the applied pump power. The properties of the deposited WS2 film was examined using scanning electron microscopic (SEM) and atomic force microscope (AFM). Detailed optical properties of the laser output are also discussed.

Wide tuning of the optical and structural properties of alternative plasmonic materials

Abstract: Alternative plasmonic materials have attracted considerable attention due to their advantages compared to conventional metals, including compatibility with Si processing, tunability of optical properties, and reduced losses. In this work, we demonstrate that post-deposition annealing of materials fabricated by magnetron sputtering allows large tuning of the structural and the optical dispersion properties of Indium Tin Oxide (ITO), Al-doped ZnO (AZO) and Titanium Nitride (TiN) nano-layers. By measuring their optical bandgaps, we show that thermal annealing treatments can dramatically modulate the carrier concentration in these materials, thus providing tunability of the optical losses and enabling the engineering of Epsilon-Near-Zero (ENZ) regime. Besides, we perform X-ray diffraction (XRD) measurements to show that thermal annealing can also effectively tune the materials grain sizes. Eventually, the effect of different annealing gases on the free carrier concentration has also been investigated. The wide tunability and control of the optical and structural properties that we demonstrated in this work is important to engineer resonant optical responses across a wide frequency spectrum for device applications to plasmonics, metamaterials and transformation-optics.

Refractive index and extinction coefficient of CH 3 NH 3 PbI 3 studied by spectroscopic ellipsometry

Abstract: Research concerning CH3NH3PbI3 solar cells (SCs) has attracted great attention. However, the CH3NH3PbI3 material’s critical dispersion relationships, i.e. the refractive index and the extinction coefficient, n(λ) and k(λ), as functions of λ, have been little studied. Without this knowledge, it will be difficult to quantitively investigate the optical properties of the CH3NH3PbI3 SCs. We studied n(λ) and k(λ) of CH3NH3PbI3 with spectroscopic ellipsometry. The CH3NH3PbI3 film was fabricated by dual-source evaporation, and the surface roughness was investigated to facilitate SE modeling. With the acquired n(λ) and k(λ), we applied the finite difference time domain method to calculate the ultimate efficiency, η(d), without considering carrier recombination, of the planar CH3NH3PbI3 SC as a function of the film thickness, d, from 31.25 nm to 2 μm, and compared with those of GaAs, c-Si, and a-Si:H(10%H) SCs. It is demonstrated that, η(d) for CH3NH3PbI3 SC is a little smaller than, but very close to that for the GaAs SC, however, much larger than that for the c-Si SC, for all d calculated; and much larger than that for the a-Si:H(10%H) SC when d > 100 nm. Apart from an appropriate band gap near 1.5 eV, the larger k(λ) and smaller n(λ) of CH3NH3PbI3 explain why the CH3NH3PbI3 SC has high efficiency.

Complex electrical permittivity of the monolayer molybdenum disulfide (MoS 2 ) in near UV and visible

Abstract: Temperature and Fermi energy dependent exciton eigenenergies of monolayer molybdenum disulfide (MoS2) are calculated using an atomistic model. These exciton eigen-energies are used as the resonance frequencies of a hybrid Lorentz-Drude-Gaussian model, in which oscillation strengths and damping coefficients are obtained from the experimental results for the differential transmission and reflection spectra of monolayer MoS2 coated quartz and silicon substrates, respectively. Numerical results compared to experimental results found in the literature reveal that the developed permittivity model can successfully represent the monolayer MoS2 under different biasing conditions at different temperatures for the design and simulation of MoS2 based opto-electronic devices.

Twenty years of Tm:Ho:YLF and LuLiF laser development for global wind and carbon dioxide active remote sensing

Abstract: NASA Langley Research Center (LaRC) has a long history of developing pulsed 2-μm lasers. From fundamental spectroscopy research, theoretical prediction of new materials, laser demonstration and engineering of lidar systems, it has been a very successful progress spanning around two decades. This article covers the 2-μm laser development from early research to current state-of-the-art instrumentation and projected future space missions. This applies to both global wind and carbon dioxide active remote sensing. A brief historical perspective of Tm:Ho work by early researchers is also given.

Optimization of sputtered titanium nitride as a tunable metal for plasmonic applications

Abstract: Alternative materials for plasmonic devices have garnered much recent interest. A promising candidate material is titanium nitride. Although there is a substantial body of work on the formation of this material, its use for plasmonic applications requires a more systematic and detailed optical analysis than has previously been carried out. This paper describes an initial optimization of sputtered TiN thin films for plasmonic performance from visible into near-IR wavelengths. The metallic behavior of TiN films exhibits a sensitive dependence on the substrate and deposition details. We explored reactive and non-reactive sputter deposition of TiN onto various substrates at both room temperature and 600°C. Metallic character was compared for films grown under different conditions via spectroscopic ellipsometry and correlated with compositional and structural measurements via x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and scanning transmission electron microscopy (STEM).

Spectroscopic properties of cerium-doped aluminosilicate glasses

Abstract: The spectroscopic properties of cerium ions in various aluminosilicate glasses modified by Mg2+, Ca2+, Ba2+ and Na+ were investigated in order to optimize these for the potential utilization as Ce3+/Yb3+ quantum cutting material. An increasing optical basicity of the glasses results in a shift in the peak position of the 5d-4f emission of Ce3+ to longer wavelengths and in a decrease in the Ce3+ fluorescence intensity due to decreasing Ce3+/Ce4+ ratios. Argon-bubbling of the melt and supplying argon as melting atmosphere and/or using small amounts of metallic aluminum powder as raw material led to an almost complete reduction of Ce4+ to Ce3+. This resulted in much higher intensities of the Ce3+ fluorescence emission which runs parallel to a decreasing charge transfer absorption of Ce4+. From the absorption spectra of these samples extinction coefficients for Ce4+ and Ce3+ were calculated. For this purpose, an additional sample was prepared by using oxygen bubbling of the melt. An increasing cerium concentration shifts the Ce3+ emission peak position to longer wavelengths, while up to 2·1020 ions per cm3 only a slight increase in the Ce3+ emission intensity was observed. At higher dopant concentrations, a drastic decrease in the Ce3+ fluorescence emission is observed which is most likely attributed to an increasing Ce4+ concentration. High intensity Ce3+ blue emission matching the spectroscopic requirements for potential quantum cutting in Ce3+/Yb3+ codoped glasses could be achieved with a barium aluminosilicate glass.

Low loss Ge-As-Se chalcogenide glass fiber, fabricated using extruded preform, for mid-infrared photonics

Abstract: Chalcogenide glass fibers have attractive properties (e.g. wide transparent window, high optical non-linearity) and numerous potential applications in the mid-infrared (MIR) region. Low optical loss is desired and important in the development of these fibers. Ge-As-Se glass has a large glass-forming range to provide versatility of choice from continuously varying physical properties. Recently, broadband MIR supercontinuum generation has been achieved in chalcogenide fibers by using Ge-As-Se glass in the core/clad. structure. In the shaping of chalcogenide glass optical fiber preforms, extrusion is a useful technique. This work reports glass properties (viscosity-temperature curve and glass transition) and optical losses of Ge-As-Se fiber fabricated from an extruded preform. A robust cut-back method of fiber loss measurement is developed and the corresponding error calculation discussed. MIR light is propagated through 52 meters of a fiber, which has the lowest loss yet reported for Ge-As-Se fiber of 83 ± 2 dB/km at 6.60 μm wavelength. The fiber baseline loss is 83-90 dB/km across 5.6-6.8 μm, a Se-H impurity absorption band of 1.4 dB/m at 4.5 μm wavelength is superposed and other impurity bands (e.g. O-H, As-O, Ge-O) are ≤ 20 dB/km. Optical losses of fiber fabricated from different positions of the extruded preform are investigated.

Ultra-low viscosity liquid crystal materials

Abstract: We report five ultra-low viscosity nematic liquid crystal mixtures with birefringence around 0.1, dielectric anisotropy in the range of 3 to 6, and clearing temperature about 80°C. A big advantage of these low viscosity mixtures is low activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Using our mixture M3 as an example, the response time of a 3-μm cell at −20°C is only 30 ms. Widespread application of these materials for display devices demanding a fast response time, especially at low temperatures, is foreseeable.

Multilayer graphene-based saturable absorbers with scalable modulation depth for mode-locked Er- and Tm-doped fiber lasers

Abstract: We demonstrate an experimental study on the influence of the parameters of a graphene-based saturable absorber (SA) on the performance of mode-locked Er- and Tm-doped fiber lasers. We have fabricated a set of saturable absorbers with different number of graphene layers: 9, 12, 24, 37 and 48. Each SA was characterized in terms of nonlinear optical parameters (modulation depth, saturation intensity, saturation fluence) and tested in two state-of-the-art, low-power Er- and Tm-doped fiber lasers. Our results show, that in the Er-laser the broadest output spectrum (11 nm) and shortest pulses (345 fs) are generated using 37 layers of graphene in the SA. In case of a Tm-laser, the best performance (737 fs pulses with 5.82 nm bandwidth) was achieved with 24 layers. Additionally, we show that the modulation depth of a 9-layer SA is insufficient to initiate mode-locking in both lasers. This is the first reported comprehensive study on controlling of the parameters of a SA by scaling the number of graphene layers.

Frequency tunable metamaterial absorber at deep-subwavelength scale

Abstract: Metamaterial-based absorbers utilize the intrinsic loss, with the aid of appropriate structure design, to achieve near unity absorption at a certain frequency. The frequency of the reported absorbers is usually fixed and operates over a limited bandwidth, which greatly hampers their practical applications. Active or dynamic control over their resonance frequency is urgently necessary. Herein, we theoretically present a novel frequency tunable terahertz metamaterial absorber formed by a square metallic patch and a ground plane separated by a strontium titanate dielectric layer. Up to 80.2% frequency tuning is obtained by changing the temperature of the absorber, and there is very little variation in the strength of the absorption. The frequency shift is attributed to the temperature-dependent refractive index of the dielectric layer. Furthermore, the ratio between the lattice period and the resonance wavelength is close to 1/36 at 0.111 THz, which is smaller than the previously reported results. The proposed absorber has potential applications in detection, sensors, and selective thermal emitters.

Dynamically tunable electromagnetically induced reflection in terahertz complementary graphene metamaterials

Abstract: We presented a dynamically tunable electromagnetically induced reflection (EIR) based on the complementary graphene metamaterials composed of the wire-slot and split-ring resonators slot (SRRs-slot) array structures for the terahertz region. In this structure, the dark mode excited by the near field coupling between wire-slot and SRRs-slot structures, induces a reflection window. Moreover, the reflection window can be actively controlled by varying the lateral displacement between two slot-type resonant structures or Fermi energy of graphene without reoptimizing and re-fabricating structure. In addition, the large positive group delay obtained within the reflection peak can be also tuned over a broad terahertz region by changing the Fermi energy of graphene. Therefore, the work opens up the possibility for the development of compact elements such as modulators, tunable sensor, switches and slow light devices.

Materials for hot carrier plasmonics [Invited]

Abstract: While the field of plasmonics has grown significantly in recent years, the relatively high losses and limited material choices have remained a challenge for the development of many device concepts. The decay of plasmons into hot carrier excitations is one of the main loss mechanisms; however, this process offers an opportunity for the direct utilization of loss if excited carriers can be collected prior to thermalization. From a materials point-of-view, noble metals (especially gold and silver) are almost exclusively employed in these hot carrier plasmonic devices; nevertheless, many other materials may offer advantages for collecting these hot carriers. In this manuscript, we present results for 16 materials ranging from pure metals and alloys to nanowires and graphene and show their potential applicability for hot carrier excitation and extraction. By considering the expected hot carrier distributions based on the electron density of states for the materials, we predict the preferred hot carrier type for collection and their expected performance under different illumination conditions. By considering materials not traditionally used in plasmonics, we find many promising alternative materials for the emerging field of hot carrier plasmonics.

Quasi-coherent thermal emitter based on refractory plasmonic materials

Abstract: The thermal emission of refractory plasmonic metamaterial – a titanium nitride 1D grating – is studied at high operating temperature (540 °C). By choosing a refractory material, we fabricate thermal gratings with high brightness that are emitting mid-infrared radiation centered around 3 µm. We demonstrate experimentally that the thermal excitation of plasmon-polariton on the surface of the grating produces a well-collimated beam with a spatial coherence length of 32λ (angular divergence of 1.8°) which is quasi-monochromatic with a full width at half maximum of 70 nm. These experimental results show good agreement with a numerical model based on a two-dimensional full-wave analysis in frequency domain.

Mid-infrared photoluminescence in small-core fiber of praseodymium-ion doped selenide-based chalcogenide glass

Abstract: Rare earth (RE)-ion doped chalcogenide glasses are attractive for mid-infrared (MIR) fiber lasers for operation >4 μm. Our prior modeling suggests that praseodymium (Pr) is a suitable RE-ion dopant for realizing a selenide-based, chalcogenide-glass, step index fiber (SIF) MIR fiber laser operating at 4-5 μm wavelength. Fabrication of RE-ion doped chalcogenide glass fiber, especially with a small core, is a demanding process because crystallization must be avoided during the heat treatments required to effect shaping. Here, a 500 ppmw (parts per million parts, by weight) Pr3+-doped Ge-As-Ga-Se glass SIF with a 10 μm or 15 μm diameter core is reported; the cladding glass is Ge-As-Ga-Se-S. The multistage process to produce the fiber is outlined. Thermal and optical properties of the core/clad. glass pair, and the crystalline/amorphous nature and optical behavior of the small core fiber are reported. MIR photoluminescence and lifetime of a RE-ion doped chalcogenide glass small core fiber are reported for the first time.

Large-area fabrication of TiN nanoantenna arrays for refractory plasmonics in the mid-infrared by femtosecond direct laser writing and interference lithography [Invited]

Abstract: Robust plasmonic nanoantennas at mid-infrared wavelengths are essential components for a variety of nanophotonic applications ranging from thermography to energy conversion. Titanium nitride (TiN) is a promising candidate for such cases due to its high thermal stability and metallic character. Here, we employ direct laser writing as well as interference lithography to fabricate large-area nanoantenna arrays of TiN on sapphire and silicon substrates. Our lithographic tools allow for fast and homogeneous preparation of nanoantenna geometries on a polymer layer, which is then selectively transferred to TiN by subsequent argon ion beam etching followed by a chemical wet etching process. The antennas are protected by an additional Al2O3 layer which allows for high-temperature annealing in argon flow without loss of the plasmonic properties. Tailoring of the TiN antenna geometry enables precise tuning of the plasmon resonances from the near to the mid-infrared spectral range. Due to the advantageous properties of TiN combined with our versatile large-area and low-cost fabrication process, such refractory nanoantennas will enable a multitude of high-temperature plasmonic applications such as thermophotovoltaics in the future.

Effective Third-Order Nonlinearities in Metallic Refractory Titanium Nitride Thin Films

Abstract: Nanophotonic devices offer an unprecedented ability to concentrate light into small volumes which can greatly increase nonlinear effects. However, traditional plasmonic materials suffer from low damage thresholds and are not compatible with standard semiconductor technology. Here we study the nonlinear optical properties in the novel refractory plasmonic material titanium nitride using the Z-scan method at 1550 nm and 780 nm. We compare the extracted nonlinear parameters for TiN with previous works on noble metals and note a similarly large nonlinear optical response. However, TiN films have been shown to exhibit a damage threshold up to an order of magnitude higher than gold films of a similar thickness, while also being robust, cost-efficient, bio- and CMOS-compatible. Together, these properties make TiN a promising material for metal-based nonlinear optics.

Dynamically controlled plasmonic nano-antenna phased array utilizing vanadium dioxide [Invited]

Abstract: We propose and analyze theoretically an approach for realizing a tunable optical phased-array antenna utilizing the properties of VO2 for electronic beam steering applications in the near-IR spectral range. The device is based on a 1D array of slot nano-antennas engraved in a thin Au film grown over VO2 layer. The tuning is obtained by inducing a temperature gradient over the device, which changes the refractive index of the VO2, and hence modifies the phase response of the elements comprising the array, by producing a thermal gradient within the underlying PCM layer. Using a 10-element array, we show that an incident beam can be steered up to ±22° with respect to the normal, by applying a gradient of less than 10°C.

Three-dimensional femtosecond laser fabrication of waveguide beam splitters in LiNbO 3 crystal

Abstract: We report on the fabrication of waveguide beam splitters in z-cut LiNbO3 crystal by direct femtosecond laser writing. The guidance is valid only for TM polarization due to the Type I modification of extraordinary refractive index (ne) induced by femtosecond laser pulses. With single scan of femtosecond laser beams over the crystal bulk, the structures with channel geometry have been produced. In this work, such waveguide configurations were created as one-dimensional (1D) straight waveguide, two-dimensional (2D) 1 × 2 and three-dimensional (3D) 1 × 4 waveguide beam splitters. The waveguide beam splitters are characterized at the wavelength of 632.8 nm and 1064 nm both experimentally and numerically. This work opens the way for laser-written 3D LiNbO3 waveguide beam splitters as novel 3D nonlinear photonic devices.

Fast-response liquid crystals for high image quality wearable displays

Abstract: We demonstrate two ultra-low viscosity liquid crystal mixtures to enable field-sequential-color wearable displays for low temperature operation, while keeping a wide color gamut. Our mixtures offer ~4X faster response time than a commercial material at 20°C and ~8X faster at −20°C. Other major attractive features include: (1) submillisecond response time at room temperature and vivid color even at −20°C without a heating device, (2) high brightness and excellent ambient contrast ratio, and (3) suppressed color breakup with 360Hz frame rate.

Fabrication of hierarchical photonic nanostructures inspired by Morpho butterflies utilizing laser interference lithography

Abstract: We introduce laser interference lithography (LIL) as a tool to fabricate hierarchical photonic nanostructures inspired by blue Morpho butterflies. For that, we utilize the interference pattern in vertical direction in addition to the conventional horizontal one. The vertical interference creates the lamellae by exploiting the back reflection from the substrate. The horizontal interference patterns the ridges of the hierarchical Christmas tree like structure. The artificial Morpho replica produced with this technique feature a brilliant blue iridescence up to an incident angle of 40°.

Fast-response infrared phase modulator based on polymer network liquid crystal

Abstract: We report a high birefringence terphenyl liquid crystal mixture, designated as M3, for infrared phase modulation with special emphasis on mid-wave infrared (MWIR). In addition to high birefringence, M3 exhibits excellent UV stability, modest dielectric anisotropy, and a very broad nematic range. The high birefringence enables a thin cell gap to be used for achieving a 2π phase change while maintaining a high transmittance (T>98%) in the MWIR region. To achieve fast response time, we employed a polymer network liquid crystal using M3 with 2π phase change at λ = 4μm and 3.6-ms response time. This response time is about 100X faster than that of a nematic LC phase modulator.

Versatile preparation of ultrathin MoS_2 nanosheets with reverse saturable absorption response

Abstract: High-yielded ultrathin MoS2 nanosheets (UMS) with thickness below 4 nm were successfully synthesized by a simple, cost-effective and reproducible solid-state reaction method. Significant reverse saturable absorption and nonlinear refraction responses of the UMS were measured by the z-scan experiment under femtosecond pulses at 800 nm. The figure of merit is calculated to be ~2.52 × 10−15 esu cm. Furthermore, optical limiting (OL) effects of the UMS were observed with low threshold FOL ~44 mJ/cm2. These results reveal that solid-state reaction is a feasible method for the fabrication of optical nanomaterials used in nanophotonic devices including optical limiter, which can be expanded to prepare other two-dimensional nanomaterials.

Low-temperature enhancement of plasmonic performance in silver films

Abstract: While plasmonic metals can manipulate optical energy at the nanoscale, they suffer from significant losses at visible wavelengths. We investigate the potential of low temperature to decrease such losses in optically thick Ag films. We extract the complex dielectric function (or relative permittivity) from spectroscopic ellipsometry measurements for smooth single-crystalline, smooth polycrystalline, and rough polycrystalline films down to liquid-helium temperatures and fit these data to a temperature-dependent Drude model. Smooth single-crystalline films exhibited the largest improvements relative to room temperature. Below 50 K, the surface plasmon polariton propagation lengths increased by ~50% at 650 nm. In rough polycrystalline films, improvements of 10% are expected.

Passively mode-locked fiber lasers at 1039 and 1560 nm based on a common gold nanorod saturable absorber

Abstract: We demonstrated passively mode-locked ytterbium and erbium doped fiber lasers operating at 1039 and 1560 nm by using a common gold nanorods (GNRs) saturable absorber (SA). The GNRs with designed aspect ratios were mixed with sodium carboxymethylcelluose to form the GNRs SA film. The film had broadband longitudinal SPR (surface plasmon resonance) absorption from 800 to 1800 nm. By inserting the same film into a ytterbium or erbium doped fiber laser cavity pumped by a 980 nm laser diode, stable passively mode-locked laser operation at 1039 or 1560 nm was achieved for a threshold pump power of ~100 or ~70 mW, respectively. The pulse width, the output power, and the repetition rate of the 1039 nm mode-locked laser were 460 ps, 1.47 mW, and 43.5 MHz for a pump power of ~110 mW, respectively. The corresponding output parameters of the 1560 nm mode-locked laser were 2.91 ps, 2 mW, and 35.6 MHz for a pump power of ~74 mW, respectively. Our results showed that one GNRs SA could be used for constructing broadband mode-locked lasers.

Design of deep-red persistent phosphors of Gd_3Al_5-xGa_xO_12:Cr^3+ transparent ceramics sensitized by Eu^3+ as an electron trap using conduction band engineering

Abstract: We developed bright deep-red persistent phosphors of Cr3+-Eu3+ co-doped Gd3Al5-xGaxO12 garnet (GAGG:Cr3+-Eu3+), in which only Cr3+ ion shows emission bands centered at 730 nm after ceasing UV illumination and Eu3+ ion acts as an excellent electron trap capturing one electron to be Eu2+ with tunable trap depth by varying conduction band with Ga3+ content, x. The persistent radiance of the GGG:Cr3+-Eu3+ (x = 5) sample at 1 h after ceasing UV light is approximately 25 times higher than that of the Cr3+ singly doped GGG sample, and is over 6 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor.