High-reflectivity Mirrors Research Articles

WGM lasing in irregular cavities with arbitrary boundaries.

Because of its limited light field mode and high Q value, the whispering-gallery-mode (WGM) cavity has been widely studied. In this study, we propose a simple, rapid, low-cost and no-manufacturing technology method that we call the drip-coating method to obtain an irregular cavity with arbitrary boundaries. By using polyvinyl alcohol (PVA) solution doped with rhodamine 6G, the irregular cavity with arbitrary boundaries was drip-coated on a high-reflection mirror, forming a WGM laser. The sample was pumped with a 532 nm pulsed laser, and the single-mode WGM and multi-WGM lasing were obtained. All WGMs are the vertical oscillation modes, which originate from both the total internal reflection of the PVA/air interface and vertical reflection of the PVA/mirror interface. The other boundaries of the cavity were not involved in the reflection and could have any shape. The mechanism of producing single-mode lasing is due to the action of the loss-gain cavity. Multi-WGM lasing is attributed to at least two groups of different WGMs existing in an irregular cavity. This can be confirmed by using a microsphere model and intensity correlation method. These results may provide an alternative for the design of WGM lasers.

High Optical Reflection in Two-dimensional ZnO-Si Photonic Crystals Induced by Coupled Optical Micro-cavities

Photonic crystals can exhibit relevant optical properties when transmitting or reflecting a light beam. In particular in a two-dimensional photonic crystal the reflective properties can be of interest and consequently optimized for different technological applications such as tunable laser cavities, photovoltaic solar systems, and selective high reflection mirrors among many others. Taking this motivation into account, a study of the reflective optical properties of two-dimensional photonic crystals built on a hybrid substrate of ZnO:Si has been developed. The aim of the present research is to demonstrate the feasibility to control the optical reflectance spectra of a two-dimensional photonic crystal by the inclusion of an array of optical micro-cavities in a regular photonic structure. Moreover, in this research an explanation of the origin of the high optical reflectance predicted by numerical calculations and confirmed by experimental measurements in a photonic crystal that contains an array of micro-optical cavities is also given. The results of numerical calculations of the optical properties of one of the photonic crystals studied determined that the origin of the increase in its optical reflectance is the light emission from the silicon present in the ZnO-Si substrate where the photonic structure was built. Strong resonant modes of the optical electric field stablished mainly in silicon present in two types of resonant cavities recognized in the photonic crystal favor the stimulated emission of light that gives rise to the high optical reflectivity.

Quantification of iodine monoxide based on incoherent broadband cavity enhanced absorption spectroscopy

The quantitative method of iodine monoxide radical (IO) using incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) in the 435–465 nm band is described in this paper. In order to obtain the concentration of IO accurately, the parameters such as the mirror reflectivity, effective cavity length and sample loss of the IBBCEAS system are evaluated. Using the difference of Rayleigh scattering between nitrogen and helium, the reflectivity curve of the high-reflection mirror is obtained. The reflectivity <i>R</i> of the mirror at 436.1 nm of the IO absorption peak is about 0.99982, and the effective absorption optical path reaches 3.83 km under vacuum condition. According to the absorption of O<sub>4</sub>, the effective cavity length of the modified system is 60.7 cm. The Allan deviation is used to evaluate the performance of the system, and the standard deviation is used to analyze the detection sensitivity of the system. When the time resolution is 60 s, the detection sensitivity (2<i>σ</i>) of the system for IO and NO<sub>2</sub> are 1.9 pptv and 20 pptv (part per trillion by volume), respectively. The iodine dissolved in potassium iodide (KI) solution is taken out by the bubbling method and react with ozone after photolysis to produce a stable concentration of IO sample gas. The IO loss in the sampling tube is calibrated, and the results show that the sampling tube has no significant effect on the IO loss. The IBBCEAS system is used to determine the linearity of IO, and the correlation coefficient <i>R</i><sup>2</sup> between the measured concentration of IO and the proportioned concentration in a concentration range from 39 to 530 pptv is 0.99. The IO produced by the reaction of iodine released from kelp with ozone is measured.

Development and performance testing of reflector materials for concentrated solar power: A review

Concentrated solar power is a competitive renewable energy technology that offers many advantages. Development in the parabolic shape concentrator demands the curved mirrors to harness the maximum solar energy received through the aperture opening. The performance of the concentrated solar collector mainly depends on the characteristic of the reflecting material. High reflective and durable mirrors are required for the viability of a concentrated solar collector. This paper is aimed to present the up to date progress in the solar reflector material and their performance testing. Thick glass mirrors with a protective coating against the weathering have made the place in the solar thermal power plant. However, the use of the glass mirror is limited to only the flat surface reflector. On the other hand, polymer mirrors are flexible but found less durable in outdoor weather conditions and susceptible to elevated temperatures. The reflectivity of stainless steel is lower; it limits the stainless steel used as a solar reflector. The glass mirror and aluminium are the main candidate material for the solar reflector. Reflectivity, durability and cost are the major parameters considered during the performance testing of the reflector material. In this article, studies on reflective surface preparation techniques and their durability analysis are also discussed.

X-ray photoelectron studies of near surface oxidation and plasmon excitation in spatially confined bi- and tri- layers periodic multilayer mirrors

The alternate bi- and tri- layers nanoscale periodic multilayers of Mo/Si, Mo/Be and Mo/Be/Si are useful for the high reflectivity mirrors, operating effectively around 13.5 nm of wavelength. In X-ray photoelectron spectroscopy (XPS), binding energy shift of BeO was observed in the Mo/Be structures. However, the chemical shifts of BeO and SiO2 was observed in the Mo/Be/Si multilayer structures. These evidences have indicated the near surface oxidation of multilayers. In the spatially confined layers, the plasmon was excited by the inelastic scattering of Mo4p photoelectrons, generated by the AlKα X-ray radiation of XPS. In the periodic multilayers, the plasmon was excited at higher energy as compared to the bulk counterpart, which was associated to quantum confinement of the condensed Si and Be nanolayers. In the bi- and tri- layers, the quantum confinement of plasmon in the Si nanolayer was fundamentally described by the density of valence electron and energy width of the band gap. However, in the bi- layer of Mo/Be, the plasmon shift of metallic Be layer was associated to confinement of the conduction electron density.

Monitored wet-etch removal of individual dielectric layers from high-finesse Bragg mirrors.

It is prohibitively expensive to deposit customized dielectric coatings on individual optics. One solution is to batch-coat many optics with extra dielectric layers, then remove layers from individual optics as needed. Here we present a low-cost, single-step, monitored wet etch technique for reliably removing individual SiO2 and Ta2O5 dielectric layers, in this case from a high-reflectivity fiber mirror. By immersing in acid and monitoring off-band reflected light, we show it is straightforward to iteratively (or continuously) remove six bilayers. At each stage, we characterize the coating performance with a Fabry-Pérot cavity, observing the expected stepwise decrease in finesse from 92,000 ± 3,000 to 3, 950 ± 50, finding no evidence of added optical losses. The etch also removes the fiber's sidewall coating after a single bilayer, and, after six bilayers, confines the remaining coating to a 60-µm-diameter pedestal at the center of the fiber tip. Vapor etching above the solution produces a tapered "pool cue" cladding profile, reducing the fiber diameter (nominally 125 µm) to 95 µm at an angle of ∼0.3° near the tip. Finally, we note that the data generated by this technique provides a sensitive estimate of the layers' optical depths. This technique could be readily adapted to free-space optics and other coatings.

Lightweight metasurface mirror of silicon nanospheres [Invited

Many experiments in modern quantum optics require the implementation of lightweight and near-perfect reflectors for noise reduction and high sensitivity. Another important application of low mass and high reflectivity mirrors is related to the development of solar or laser-driven light sails for acceleration of ultra-light spacecrafts to relativistic velocities. Here, we present numerical results and theoretical analysis of a metasurface mirror consisting of periodically arranged silicon nanospheres embedded in a polymer. In the absence of material losses or disorder, this mirror demonstrates absolute 100% reflection at a single wavelength, which can be tuned by changing nanosphere dimensions or periodicity (for example, by mechanical stretching). We show that high reflectivity can be reached due to electric or magnetic dipole resonant responses of Si nanoparticles in the metasurface. Dependence of mirror reflectivity on surrounding conditions, nanoparticle sizes, and the disorder in the array is studied and discussed. The optimization and simulation procedures presented in this work can be used for the development of other optical devices with functional characteristics determined by the resonant interaction of light with metasurfaces made of nanospheres.

Phase-microstructure of Mo/Si nanoscale multilayer and intermetallic compound formation in interfaces

Microstructural investigation of the Mo/Si nanoscale multilayers (MLs) is essential to design high-reflective nanomirrors. Raman spectroscopy showed existence of silicon layer in the amorphous structure. Small fraction of the nanocrystalline silicon (nc-Si) phase was found to be embedded in the amorphous silicon (a-Si) matrix. The disorder in a-Si is increased with the decrease in the thickness of silicon layer in the Mo/Si MLs, determined by the increased bond-angle deviation (ΔΘ) of the Si–Si network. The MLs exposed to high-temperature (HT) showed a decrease in the ΔΘ value, which signifies the relaxation of both molecular disorder and residual stress. X-ray diffraction analysis confirmed the polycrystalline bcc phase of Mo and the formation of intermetallic t-Mo5Si3 phase in the interfaces. This phase is more prominent for higher value of β (thickness ratio of high atomic weight (z) layer to ML period d, i.e. β = dMo/d). Intensity of the Mo (110) plane is decreased for the sample annealed at HT, which is directly related to the degree of amorphization and the formation of intermetallic phases. However, HT annealed Mo/Si showed the growth of Mo (220) planes for higher β value of MLs. Current findings revealed that the disorder in the a-Si and microstructure of the Mo and intermetallic phase depend on the annealing temperature and the β value. The analysis of the intermetallic phase is critical for the development of high reflective mirrors.

Hollow Plasma Acceleration Driven by a Relativistic Reflected Hollow Laser.

In order to address the present difficulty in experimentally generating the relativistic Laguerre-Gaussian laser, primarily due to damage caused to optical modulators, a high-reflectivity phase mirror is applied in the femtosecond petawatt laser system to generate a relativistic hollow laser at the highest intensity of 6.3×10^{19} W/cm^{2} for the first time. A simple optical model is used to verify that the vortex laser may be generated in this new scheme; using such a relativistic vortex laser, the hollow plasma drill and acceleration are achieved experimentally and proven by particle-in-cell simulations. With the development of the petawatt laser, this scheme opens up possibilities for the convenient production of the relativistic hollow laser at high repetition and possible hollow plasma acceleration, which is important for a wide range of applications such as the generation of radiation sources with orbital angular momentum, fast ignition for inertial confinement fusion, and jet research in the astrophysical environment.

Sensitivity Enhancement Factor for Gain-Assisted Cavity Enhanced Spectroscopy

Fiber-based gain-assisted incoherent broadband cavity enhanced spectroscopy is a novel technique used to enhance spectroscopic measurement sensitivity. This technique overcome the drawbacks of the conventional cavity enhanced techniques, in the need for optical alignment and high reflectivity mirror as they are based on free-space Fabry-Perot cavity. However, to assess the value of this technique, the sensitivity enhancement factor over the direct path absorption spectroscopy should be calculated. In this work, an analytical, and experimental study for the sensitivity of the fiber-based incoherent gain-assisted ring enhanced spectroscopy techniques are demonstrated. Two fiber-based configurations are studied for different system parameters. The first configuration is a dual-coupler ring with the source outside the resonator and the gain medium inside the resonator. The broadband source is the amplified spontaneous emission (ASE) of a semiconductor optical amplifier (SOA). While the second configuration is single-coupler resonator with the gain medium acting as the broadband source. The optical absorption of acetylene (C 2 H 2 ) gas around 1540 nm wavelength has been measured. The output spectrum has been measured using an optical spectrum analyzer (OSA) of 70 pm resolution. The sensitivity is enhanced by a factor of 942 in the case of the single-coupler configuration, compared to the direct absorption of the gas cell. The study elaborates that intracavity (single-coupler) configuration provides SNR enhancement in thermal noise limited system. In contrast to conventional incoherent broadband techniques that don't provide any enhancement in case of thermal noise limited system.

Optimum Design of high reflection Mirror with ZEMAX

With the aid of ZEMAX-EE software, optimum design of multilayer dielectric mirrors were proposed. Homogeneous dielectric materials TiO2 and SiO2 (as couple) were used to construct the assembly of high reflecting mirror to be operate in the visible and near-IR spectral region (400-800nm) and over angular range (0-65°) of incident of light. Result demonstrate that the omnidirectional reflector was achieved over a wide wavelength band (470-700nm).

Controlling the beam quality in DPALs by changing the resonator parameters

The beam quality of diode-pumped alkali lasers (DPALs) is of major concern in the studies of these lasers. We report on experimental studies and modeling of the beam propagation factor M2 in flowing-gas Cs DPALs with stable optical resonators and of its dependence on the resonator geometry. The measured values of M2 are in agreement with those calculated using the optical model of multi-transverse mode operation (Auslender et al. in Opt Express 25:19767, 2017). Conditions for substantial improvement of the output laser beam quality, reducing M2 to close to unity, are found. Changing the length of the resonator, and/or the radius of curvature of the high reflection mirror, leaving all other parameters of the laser unchanged, makes it possible to control the beam quality. Techniques for controlling M2 in DPALs with wide-aperture beams have not been studied elsewhere and are very important for the applications of this laser. In addition to the optical model, a simple analytical method based on the model suggested by Siegman (IEEE J Quantum Electron 29:1212, 1993) is applied to the experimental results and predicts a simple scaling law for M2 as a function of the size of the resonator fundamental mode.

Enhancement of high reflectivity mirrors using the combination of standard and sculptured thin films

The total generated output power in high-power laser systems is mainly limited by the laser-induced damage phenomena in coated optics. A significant part of optics include high reflectivity coatings. A research was conducted for the development of advanced high reflectivity mirrors, capable to withstand extreme laser fluence and featuring spectral stability at different ambient conditions. A concept of combination of IBS and GLAD technologies was proposed and implemented to form a hybrid multilayer mirror for the 355 nm wavelength. An optical and structural properties were investigated to evaluate the advantages of both technologies. The relation between the number of sculptured silica layers and standard mirrors performance under the high power of laser radiation was determined. The results of analysis show an improved reflectivity and optical resistance of consolidated coatings. LIDT measurements indicate the potential of reaching 90 J/cm2 for 355 nm wavelength (pulse duration 2.5 ns) while maintaining the reflectivity value higher than 99.5%. New findings indicate the superiority of hybrid coatings if compared to both classical and all-silica coatings. This knowledge opens up the new possibilities in further development of extreme light sources.

Miniature force sensor for absolute laser power measurements via radiation pressure at hundreds of watts.

We present a small power meter that detects the radiation pressure of an incident high-power laser. Given its small package and non-destructive interaction with the laser, this power meter is well suited to realizing a robust real-time, high-accuracy power measurement in laser-based manufacturing environments. The incident laser power is determined through interferometric measurement of displacement of a 20 mm diameter high reflectivity mirror, mounted at the center of a dual element spiral flexure. This device can measure laser power from 25 W to 400 W with a 260 m W/H z noise floor and ≤ 3.2% expanded uncertainty. We validate our device against a calibrated thermopile with simultaneous measurements of an unpolarized 1070 nm laser and report good agreement between the two systems. Finally, by referencing to an identical mechanical spring that does not see the incident laser, we suppress vibration noise in the power measurement by 14.8 dB over a 600 Hz measured bandwidth. This is an improvement over other radiation pressure based power meters that have previously been demonstrated.

A special intracavity power-modulator using the TGG magneto-optical effect

A special intracavity power-modulator using the TGG magneto-optical effect is proposed. The cavity based on the magneto-optical effect includes two high-reflection mirrors, a polarized beam-splitter device, a magnetic field generator and a TGG crystal. The transmittance of the cavity can be changed periodically by means of the changing magnetic filed, and the output power could be also modulated. When the magnetic intensities change from 0 to 2805 Gs, the output power of a Nd:YVO4 laser can be changed between 5 and 598 mW periodically. This special intracavity power-modulator based on the TGG magneto-optical effect can also be used in the visible light efficiently.

Study on the Photoacoustic Technology to Simultaneous In-Situ Detection of the Cavity Ring-Down Spectrum for Multi-Optical Parameters

A simultaneous in-situ detection technique to obtain multi-optical parameters was proposed. The same 403.56 nm, low power, blue diode laser was used to conduct the cavity ring-down spectroscopy and photoacoustic spectroscopy, which were coupled. Four photoacoustic spectra were applied to the simultaneous in-situ detection of the cavity ring-down spectrum, which were systematically compared and analyzed for the first time. And the main performance of the systems varied with the arrangement of the photoacoustic pool. The results reveal that the photoacoustic response was optimized by the high reflectivity mirror with the pool constant of 1450.64 (Pα·cm)/W and the signal sensitivity of 0.2562 μv/ppb. The intensity value of the photoacoustic pool inside the cavity ring down was superimposed to 48.45% of the initial intensity. The corresponding experimental results were consistent with the theoretical analysis and shown the coupled cavity could suitable for simultaneous in-situ measurement of high concentration. The coupled two detection techniques detected different optical parameters and thus provided a new method for the simultaneous in-situ detection of the multi-optical parameters of specific atmospheric components.

Design of NO2 photoacoustic sensor with high reflective mirror based on low power blue diode laser**Project supported by the National Natural Science Foundation of China (Grant Nos. 91644107, 61575206, 51904009, and 41905130), the National Key Research and Development Program of China (Grant Nos. 2017YFC0209401, 2017YFC0209403, and 2017YFC0209902), and the Outstanding Young Talents Program of Anhui University, China (Grant No. gxyq2019022).

An NO2 photoacoustic sensor system with a high reflective mirror based on a low power blue diode laser is developed in this work. The excitation power is enhanced by increasing the number of reflections. Comparing with a traditional photoacoustic system, the pool constant is improved from 300.24 (Pa⋅cm)/W to 1450.64 (Pa⋅cm)/W, and the signal sensitivity of the photoacoustic sensor is increased from 0.016 μV/ppb to 0.2562 μV/ppb. The characteristics of temperature and humidity of the new photoacoustic sensor are also obtained, and the algorithm is adjusted to provide a quantitative response and drift of the resonance frequency. The results of this research provide a new method and concept for further developing the NO2 photoacoustic sensors.

Photonic Crystal Stimuli-Responsive Chromatic Sensors: A Short Review.

Photonic crystals (PhC) are spatially ordered structures with lattice parameters comparable to the wavelength of propagating light. Their geometrical and refractive index features lead to an energy band structure for photons, which may allow or forbid the propagation of electromagnetic waves in a limited frequency range. These unique properties have attracted much attention for both theoretical and applied research. Devices such as high-reflection omnidirectional mirrors, low-loss waveguides, and high- and low-reflection coatings have been demonstrated, and several application areas have been explored, from optical communications and color displays to energy harvest and sensors. In this latter area, photonic crystal fibers (PCF) have proven to be very suitable for the development of highly performing sensors, but one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) PhCs have been successfully employed, too. The working principle of most PhC sensors is based on the fact that any physical phenomenon which affects the periodicity and the refractive index of the PhC structure induces changes in the intensity and spectral characteristics of the reflected, transmitted or diffracted light; thus, optical measurements allow one to sense, for instance, temperature, pressure, strain, chemical parameters, like pH and ionic strength, and the presence of chemical or biological elements. In the present article, after a brief general introduction, we present a review of the state of the art of PhC sensors, with particular reference to our own results in the field of mechanochromic sensors. We believe that PhC sensors based on changes of structural color and mechanochromic effect are able to provide a promising, technologically simple, low-cost platform for further developing devices and functionalities.

Multiple reflection assisted Laser Doppler Vibrometer setup for high resolution displacement measurement

An experimental design of high resolution non-intrusive displacement sensor working on the principle of basic LDV is being presented. Resolution enhancement has been achieved through phase multiplication using multiple reflections within a high-Q cavity created by a vibrating surface and a high reflection mirror placed parallel to each other. Displacement of 72 nm has been measured with an error range of ±14 nm through direct counting of characteristic peaks in “half cycle” of a interferogram. A linear relationship between displacement and number of reflections has been obtained which offers a straightforward route to measure displacements up to sub-nanometer scale and the resolution is essentially dictated by the reflectivity and physical dimension of moving source.

Long-Length and Thermally Stable High-Finesse Fabry-Perot Interferometers Made of Hollow Core Optical Fiber

We made and characterized two Fabry-Perot interferometer samples made of the latest-generation hollow core fiber with sub-1-dB/km loss. Thanks to this low transmission loss, we achieved a finesse of over 140 and 120, for interferometer lengths of 5 and 23 m, respectively. This resulted in transmission peaks as narrow as 47 kHz. Our all-fiber Fabry-Perot interferometers have standard single-mode fiber pigtails (for easy integration in conventional fiber optic systems) and employ fiber mode field adapters to enable low-loss coupling between the pigtails and the low-loss hollow core fiber. The high-reflectivity mirrors (>98%) were deposited directly on the fiber mode field adapters, which were glued to the hollow core fiber, resulting in permanently-aligned Fabry-Perot interferometers. We also measured how the position of the transmission peaks change with temperature (an important performance metrics for most applications, e.g., when used as a narrow-band band-pass filter) and found that it changed 14.5 times less in our Fabry-Perot interferometer relative to a similar device made of standard single mode fiber.