Low Optical Absorption Research Articles

Damage on fused silica optics caused by laser ablation of surface-bound microparticles.

High peak power laser systems are vulnerable to performance degradation due to particulate contamination on optical surfaces. In this work, we show using model contaminant particles that their optical properties decisively determine the nature of the optical damage. Borosilicate particles with low intrinsic optical absorption undergo ablation initiating in their sub-surface, leading to brittle fragmentation, distributed plasma formation, material dispersal and ultimately can lead to micro-fractures in the substrate optical surface. In contrast, energy coupling into metallic particles is highly localized near the particle-substrate interface leading to the formation of a confined plasma and subsequent etching of the substrate surface, accompanied by particle ejection driven by the recoil momentum of the ablation plume. While the tendency to create fractured surface pitting from borosilicate is stochastic, the smooth ablation pits created by metal particles is deterministic, with pit depths scaling linearly with laser fluence. A simple model is employed which predicts ~3x electric field intensity enhancement from surface-bound fragments. In addition, our results suggest that the amount of energy deposited in metal particles is at least twice that in transparent particles.

Ohmic Contacts With Ultra-Low Optical Loss on Heavily Doped n-Type InGaAs and InGaAsP for InP-Based Photonic Membranes

In this paper, we present significant reductions of optical losses and contact resistances in AgGe-based ohmic contacts to InP membranes. Due to the high solubility of Si in InGaAs and InGaAsP, heavily doped n-type contact layers are grown on InP wafers. This high doping concentration gives rise to annealing-free ohmic contacts and low contact resistances at the level of $10^{-7} \Omega \textrm{cm}^{2}$ . It also leads to strong band-filling effects in InGaAs and InGaAsP, which result in low optical absorption losses in the contact layer. Combined with the low optical loss of AgGe, a massive reduction of the propagation loss in membrane waveguides is observed compared with other existing solutions. An additional advantage is the minimal influence of thermal treatments during the processing, leading to very stable high-performing contacts.

Optimization of the post-deposition annealing process of high-mobility In2O3:H for photovoltaic applications

High-mobility hydrogen-doped indium oxide In2O3:H (IOH) were deposited by magnetron sputtering (radio frequency 13.56MHz) from an In2O3 target in Ar/O2/H2O gas mixtures onto unheated substrates. The as-deposited films were amorphous as shown by X-ray measurements and were crystallized in a post-annealing step in vacuum and air at 180°C for 15 min. The optical, electrical, and morphological properties as well as the crystallization behavior of these transparent conductive oxides films were investigated in detail. A dependence of the annealing behavior in air on the total pressure during deposition could be shown. High carrier mobilities >100cm2/Vs allow for very low optical absorption and a low resistivity around 350μΩcm. Amorphous IOH films were crystallized by short-term flash lamp annealing (FLA) in argon atmosphere for around 2.7ms. Spatial temperature distributions in a typical layer stack (crystalline silicon, amorphous silicon, silicon oxide, and IOH film) were calculated within milliseconds after the FLA-treatment. Electron backscattering diffraction measurements of IOH films crystallized by FLA reveal a polycrystalline microstructure with an average lateral crystallite size of 333nm. The crystallization process of these IOH films was studied by XRD and Hall measurements.

Reactive magnetron sputtering of Nb-doped TiO2 films: Relationships between structure, composition and electrical properties

Niobium-doped TiO2 films as highly transparent conducting oxides for electrical contacts were investigated. As-deposited films were amorphous and exhibited high resistivities ranging from 10 to 105Ωcm. A slight oxygen deficiency in as-deposited films was essential to gain low resistivities (10−3Ωcm) and low optical absorption coefficients (α550nm<2×103cm−1) in the annealed films. Therefore, we controlled the oxygen stoichiometry during the film deposition by adjusting the magnetron discharge voltage, while the oxygen gas flow was kept constant. The Hall mobility of degenerately doped films (electron concentration>1020cm−3) increased with decreasing substrate temperature owing to metal-like phonon scattering in these samples.

Optical properties of polydimethylsiloxane (PDMS) during nanosecond laser processing

This article presents experimental investigations of effects of the process parameters on the medical grade polydimethylsiloxane (PDMS) elastomer processed by laser source with irradiation at UV (266 and 355nm), VIS (532nm) and NIR (1064nm).Systematic experiments are done to characterize how the laser beam parameters (wavelength, fluence, and number of pulses) affect the optical properties and the chemical composition in the laser treated areas. Remarkable changes of the optical properties and the chemical composition are observed. Despite the low optical absorption of the native PDMS for UV, VIS and NIR wavelengths, successful laser treatment is accomplished due to the incubation process occurring below the polymer surface.With increasing of the fluence and the number of the pulses chemical transformations are revealed in the entire laser treated area and hence decreasing of the optical transmittance is observed. The incubation gets saturation after a certain number of pulses and the laser ablation of the material begins efficiently. At the UV and VIS wavelengths the number of the initial pulses, at which the optical transmittance begins to reduce, decreases from 16 up to 8 with increasing of the laser fluence up to 1.0, 2.5 and 10Jcm−2 for 266, 355 and 532nm, respectively. In the case of 1064nm the optical transmittance begins to reduce at 11th pulse incident at a fluence of 13Jcm−2 and the number of the pulses decreases to 8 when the fluence reaches value of 16Jcm−2. The threshold laser fluence needed to induce incubation process after certain number of pulses of 8 is different for every wavelength irradiation as the values increase from 1.0 for 266nm up to 16Jcm−2 for 1064nm. The incubation and the ablation processes occur in the PDMS elastomer material during its pulsed laser treatment are a complex function of the wavelength, fluence, number of pulses and the material properties as well.

Plasmonic Optical Interferences for Phase-Monitored Nanoscale Sensing in Low-Loss Three-Dimensional Metamaterials

We report a powerful and general concept for the design of three-dimensional metamaterials suitable for optically probed nanoscale sensing, combining the environmental sensitivity of localized surface plasmon resonances (LSPRs) and a low absorption of the probe light, even when the LSPRs are supported by lossy plasmonic elements. This concept is based on the tuning of the plasmon-induced Brewster conditions (angle of incidence and photon energy) of the material by suitable tailoring of its structure. This tuning is made possible by the interplay between LSPRs and optical interferences, i.e., the so-called plasmonic optical interferences. The plasmon-induced Brewster photon energy can be set outside the spectral range of the LSPR, thus in a spectral region of low optical absorption. Such “off-resonant” plasmon-induced Brewster conditions are examplified in a self-assembled lamellar three-dimensional metamaterial consisting of stacked polymer layers doped with gold nanoparticles alternating with undoped pol...

Second-order nonlinear optical metamaterials: ABC-type nanolaminates

We demonstrate a concept for second-order nonlinear metamaterials that can be obtained from non-metallic centrosymmetric constituents with inherently low optical absorption. The concept is based on iterative atomic-layer deposition of three different materials, A = Al2O3, B = TiO2, and C = HfO2. The centrosymmetry of the resulting ABC stack is broken since the ABC and the inverted CBA sequences are not equivalent—a necessary condition for non-zero second-order nonlinearity. In our experiments, we find that the bulk second-order nonlinear susceptibility depends on the density of interfaces, leading to a nonlinear susceptibility of 0.26 pm/V at a wavelength of 800 nm. ABC-type nanolaminates can be deposited on virtually any substrate and offer a promising route towards engineering of second-order optical nonlinearities at both infrared and visible wavelengths.

Faraday effect of bismuth iron garnet thin film prepared by mist CVD method

Metastable bismuth iron garnet (BIG, an abbreviation of Bi3Fe5O12), one kind of garnet-type ferrites, is known to manifest very large Faraday rotation as well as low optical absorption in the visible to infrared region. We report on successful synthesis of thin film composed of single-phase BIG epitaxially grown on single-crystalline gadolinium gallium garnet (Gd3Ga5O12, GGG) substrate by using mist chemical vapor deposition (CVD) method, which is an emerging technique for preparation of thin films. The crystal structure, surface morphology, and magnetic, optical and magneto-optical properties of the resultant thin films have been explored. The BIG thin film has a relatively flat surface free from roughness compared to those prepared by other vapor deposition methods. Saturation magnetization is about 1620 G at room temperature, which is close to that expected from the ideal magnetic structure of BIG. The maximum value of Faraday rotation angle reaches 54.3 deg/µm at a wavelength of 424 nm. This value is rather large when compared with those reported for BIG thin films prepared by other techniques. The wavelength dependence of Faraday rotation angle is analyzed well in terms of the crystal electric field (CEF) level schema. Our result suggests that the mist CVD method is a simple and effective technique to synthesize BIG thin film with excellent magneto-optical properties.

Preparation and measurement of highly efficient a‐Si:H single junction solar cells and the advantages of μc‐SiOx:H n‐layers

AbstractReducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiOx:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiOx:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiOx:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiOx:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiOx:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiOx:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Enhanced absorption in two-dimensional materials via Fano-resonant photonic crystals

The use of two-dimensional (2D) materials in optoelectronics has attracted much attention due to their fascinating optical and electrical properties. However, the low optical absorption of 2D materials arising from their atomic thickness limits the maximum attainable external quantum efficiency. For example, in the visible and near-infrared regimes monolayer MoS2 and graphene absorb only ∼10% and 2.3% of incoming light, respectively. Here, we experimentally demonstrate the use of Fano-resonant photonic crystals to significantly boost absorption in atomically thin materials. Using graphene as a test bed, we demonstrate that absorption in the monolayer thick material can be enhanced to 77% within the telecommunications band, the highest value reported to date. We also show that the absorption in the Fano-resonant structure is non-local, with light propagating up to 16 μm within the structure. This property is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors in which separation of photo-generated carriers only occurs ∼0.2 μm adjacent to the graphene/electrode interface.

An Atomically Layered InSe Avalanche Photodetector.

Atomically thin photodetectors based on 2D materials have attracted great interest due to their potential as highly energy-efficient integrated devices. However, photoinduced carrier generation in these media is relatively poor due to low optical absorption, limiting device performance. Current methods for overcoming this problem, such as reducing contact resistances or back gating, tend to increase dark current and suffer slow response times. Here, we realize the avalanche effect in a 2D material-based photodetector and show that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector. This is achieved by exploiting the large Schottky barrier formed between InSe and Al electrodes, enabling the application of a large bias voltage. Plasmonic enhancement of the photosensitivity, achieved by patterning arrays of Al nanodisks onto the InSe layer, further improves device efficiency. With an external quantum efficiency approaching 866%, a dark current in the picoamp range, and a fast response time of 87 μs, this atomic layer device exhibits multiple significant advances in overall performance for this class of devices.

Optical properties of pure and Ce3+ doped gadolinium gallium garnet crystals and epitaxial layers

Results of X-ray diffraction and low temperature optical absorption measurements of cerium doped gadolinium gallium garnet single crystals and epitaxial layers are reported. In the region of intra-configurational 4f–4f transitions the spectra of the bulk crystals exhibit the signatures of several different Ce3+ related centers. Apart from the dominant center, associated with Ce substituting gadolinium, at least three other centers are found, some of them attributed to the so-called antisite locations of rare-earth ions in the garnet host, i.e., in the Ga positions. X-ray diffraction data prove lattice expansion of bulk GGG crystals due to the presence of rare-earth antisites. The concentration of the additional Ce-related centers in epitaxial layers is much lower than in the bulk crystals. However, the Ce-doped layers incorporate a large amount of Pb from flux, which is the most probable source of nonradiative quenching of Ce luminescence, not observed in crystals grown by the Czochralski method.

Microstructural and band gap exploration on Ni-doped SnO2 nanoparticles co-doped with Cu

Sn0.97−xNi0.03CuxO2 (x = 0, 0.01, 0.02) nanoparticles have been successfully synthesized by employing a simple co-precipitation method. Tetragonal rutile structure of the samples was confirmed by X-ray diffraction technique. The change in crystallite size and optical properties such as absorption, transmittance and band gap were discussed based on Cu concentration and density of defect states. Morphologies and composition of products were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Lower optical absorption and higher transmittance of Sn0.96Ni0.03Cu0.01O2 nanoparticles described that it is one of the suitable materials for opto-electronic device applications. The shift of band position towards lower wave number side and the decrease in the intensity of Fourier transform infrared spectra peaks confirmed the presence of Cu in Sn–Ni–O lattice.

Optical properties of transition metal atom adsorbed graphene: A density functional theoretical calculation

Electronic and optical properties of 3d-transition metal adsorbed graphene system, theoretically studied in the framework of density functional theory, reveals significant modification compared to the pristine system. Due to adsorption of transition metal, the emergence of closely separated electronic bands leads to substantial amount of low energy optical absorption below 2.0eV photon energy. Very significant enhancement of static dielectric constant and large value of reflectivity in the low optical energy regime has been identified for different adsorbed systems. In the different 3d-transition metal adsorbed systems, particularly up to the half filled d-shell transition metal atom, pronounced emergence of optical absorption line in the deep ultraviolet regime beyond 30.0eV photon energy is observed.

Thermal noise reduction and absorption optimization via multimaterial coatings

Future gravitational wave detectors (GWDs) such as Advanced LIGO upgrades and the Einstein Telescope are planned to operate at cryogenic temperatures using crystalline silicon (cSi) test-mass mirrors at an operation wavelength of 1550 nm. The reduction in temperature in principle provides a direct reduction in coating thermal noise, but the presently used coating stacks which are composed of silica (${\mathrm{SiO}}_{2}$) and tantala (${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$) show cryogenic loss peaks which results in less thermal noise improvement than might be expected. Due to low mechanical loss at low temperature amorphous silicon (aSi) is a very promising candidate material for dielectric mirror coatings and could replace ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$. Unfortunately, such an $\mathrm{aSi}/{\mathrm{SiO}}_{2}$ coating is not suitable for use in GWDs due to high optical absorption in aSi coatings. We explore the use of a three material based coating stack. In this multimaterial design the low absorbing ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ in the outermost coating layers significantly reduces the incident light power, while aSi is used only in the lower bilayers to maintain low optical absorption. Such a coating design would enable a reduction of Brownian thermal noise by 25%. We show experimentally that an optical absorption of only $(5.3\ifmmode\pm\else\textpm\fi{}0.4)\text{ }\text{ }\mathrm{ppm}$ at 1550 nm should be achievable.

Industrial-scale deposition of highly uniform and precise optical interference filters by the use of an improved cylindrical magnetron sputtering system

The manufacture of precision optical interference coatings requires a process with sufficient accuracy, precision, uniformity, and stability to produce economic quantities of coatings with acceptable yield. This paper compares the capabilities of the plasma assisted reactive magnetron sputtering (PARMS) process with the compound-assisted reactive sputtering process (CARS). Compared with PARMS, CARS exhibited superior process accuracy, stability and uniformity by being less sensitive to equipment and process temperatures, and to cathode and machine conditioning. A uniformity deviation of less than ±0.15% is demonstrated for different optical multilayer filters over substrates of 200mm diameter. The CARS process stability and precision are also demonstrated using many coating runs of a sensitive, broadband antireflective coating without the need for optical monitoring. Thus, it is shown that CARS combines the advantages of the Metamode (high stability) and the PARMS process (low optical absorption).

Low-optical-loss, low-resistance Ag/Ge based ohmic contacts to n-type InP for membrane based waveguide devices

We present the development of Ag/Ge based ohmic contacts to n-type InP with both low contact resistances and relatively low optical losses. A specific contact resistance as low as 1.5×10−6 Ω cm2 is achieved by optimizing the Ge layer thickness and annealing conditions. The use of Ge instead of metal as the first deposited layer results in a low optical absorption loss in the telecommunication wavelength range. Compared to Au based contacts, the Ag based metallization also shows considerably reduced spiking effects after annealing. Contacts with different lengths are deposited on top of InP membrane waveguides to characterize the optical loss before and after annealing. A factor of 5 reduction of the propagation loss compared to the conventional Au/Ge/Ni contact is demonstrated. This allows for much more optimized designs for membrane photonic devices.

Spin-orbit coupling and optical detection of spin polarisation in triangular graphene quantum dots

We present a theory of the effect of spin–orbit coupling on optical properties of triangular graphene quantum dots (TGQD). TGQDs with zigzag edges exhibit a degenerate band of states at the Fermi level. For the charge neutral TGQD, the shell is expected to be half–filled by spin polarised electrons leading to finite magnetisation. Using four–band tight–binding and effective Kane–Mele models, we show that, if the TGQD is spin polarised, the low energy optical absorption spectrum reveals two distinct peaks corresponding to left and right circularly polarised light while the unpolarised TGQD shows only one peak. This allows optical detection of spin polarisation, its direction and the strength of spin–orbit coupling in TGQDs.

Removal of Verrucaria nigrescens from Carrara marble artefacts using Nd:YAG lasers: comparison among different pulse durations and wavelengths

The periodical removal of biodeteriogens is a fundamental need for the preservation of outdoor stone cultural heritage, which is stimulating significant efforts toward the development of low-impact conservation strategies. In the present work, the potential of laser removal of Verrucaria nigrescens Pers. from Carrara marble and the evaluation of the associated biocide effect on the organism residues embedded in the surface texture and through the outer porosities of the stone substrate were investigated. The fundamental wavelength of Nd:YAG laser (1,064 nm), commonly used in stone cleaning, and its second harmonic (532 nm) were comparatively tested. The phenomenology of laser treatments carried out in different irradiation conditions was characterized using optical, epifluorescence, and electron microscopes along with chlorophyll fluorescence with pulsed amplitude-modulated imaging. The results achieved show that 532 nm can provide significant advantages with respect to 1,064 nm. The potential of the latter against the biodeteriogens appears rather limited because of the low optical absorption, whereas the former can allow effective and practicable laser treatments, which disclose a significant application perspective.

Improved photovoltaic properties of amorphous silicon thin-film solar cells with an un-doped silicon oxide layer

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) deposited on the n-μc-Si:H layer of amorphous silicon single-junction superstrate configuration thin-film solar cells produced through 40MHz very high frequency plasma-enhanced chemical vapor deposition. Raman spectroscopy and optoelectronic analyses of the undoped μc-SiOx:H thin film revealed that adding a small amount of oxygen into a μc-network results in a low optical absorption, wide band gap, high optical band gap E04, high refractive index, reasonable conductivity, and crystalline volume fraction, which are advantageous properties in solar cells. Compared with a standard cell, the current density–voltage (J–V) characteristics of the cell with an undoped μc-SiOx:H/n-μc-Si:H structure showed an enhancement in short-circuit current density Jsc from 13.32 to 13.60mA/cm2, and in conversion efficiency from 8.53% to 8.61%. The increased Jsc mechanism can be attributed to an improved light-trapping capability in the long wavelength range between 510 and 660nm, as demonstrated by the external quantum efficiency.