Enhanced Circular Dichroism for Achiral Sensing Based on a DNA-Origami-Empowered Anapole Metasurface.

Circular dichroism (CD) spectroscopy has been extensively utilized for detecting and distinguishing the chirality of diverse substances and structures. However, CD spectroscopy is inherently weak and conventionally associated with chiral sensing, thus constraining its range of applications. Here, we report a DNA-origami-empowered metasurface sensing platform through the collaborative effect of metasurfaces and DNA origami, enabling achiral/slightly chiral sensing with high sensitivity via the enhanced ΔCD. An anapole metasurface, boasting over 60 times the average optical chirality enhancement, was elaborately designed to synergize with reconfigurable DNA origami. We experimentally demonstrated the detection of achiral/slightly chiral DNA linker strands via the enhanced ΔCD of the proposed platform, whose sensitivity was a 10-fold enhancement compared with the platform without metasurfaces. Our work presents a high-sensitivity platform for achiral/slightly chiral sensing through chiral spectroscopy, expanding the capabilities of chiral spectroscopy and inspiring the integration of multifunctional artificial nanostructures across diverse domains.

Real-time synchronized monitoring for the overlap accuracy of the combining beam spot in a wavelength beam combination based on confocal planes.

Beam overlap accuracy in a wavelength beam combination system determines the beam quality and efficiency, so systematic monitoring of overlap accuracy is essential. In this work, a method of performing real-time synchronized monitoring and recording overlap accuracy for a combining beam spot is proposed. Firstly, theoretical calculations for monitoring different wavelength sub-beam positions and angular errors are established. Then, an optical design and grayscale centroid algorithm are developed to analyze and simulate the combination spots. A monitoring device was designed and constructed to meet the requirements of combining system applications, which achieved an accuracy of 8.86 µrad. Finally, the method successfully monitored the system spot fluctuation range within ±22 µrad. This study resolves the issue of distinguishing the different wavelength sub-beams and their response delays in traditional combining beams. It offers precise error data for real-time synchronized calibration of the overlap accuracy in laser beam combining technology.

Group delay dispersion monitoring for computational manufacturing of dispersive mirrors.

We present a computational manufacturing program for monitoring group delay dispersion (GDD). Two kinds of dispersive mirrors computational manufactured by GDD, broadband, and time monitoring simulator are compared. The results revealed the particular advantages of GDD monitoring in dispersive mirror deposition simulations. The self-compensation effect of GDD monitoring is discussed. GDD monitoring can improve the precision of layer termination techniques, it may become a possible approach to manufacture other optical coatings.

Production of ultra-steep dichroic filters with broad band optical monitoring.

The feasibility of using direct broad band optical monitoring control in the fabrication of the ultra-steep dichroic filters based on resonant structures is investigated. Using computational manufacturing and deposition experiments, the role of the errors self-compensation effect is clarified by comparing the results of direct broad band optical monitoring and time monitoring. The errors correlation strength of ultra-steep dichroic filter is analyzed and it shows that the correlation calculated by the current model is not strong. The relationship between errors correlation and errors self-compensation effect for the ultra-steep dichroic filter is discussed.

High performance ZnS antireflection sub-wavelength structures with HfO2 protective film for infrared optical windows.

Antireflection sub-wavelength structures (SWSs) on ZnS were designed and ZnS SWSs with HfO2 protective film were prepared, and their properties in long-wave infrared applications were examined and compared to AR coatings. The SWS has good antireflection performance and exhibits less polarization sensitivity than the AR coating. At temperatures above 500 °C, the SWS with HfO2 protective film has a better thermal endurance property than the multilayer AR coating. Moreover, the HfO2 protective film significantly improved the mechanical properties of the ZnS SWS and was similar to HfO2 covered AR coating when the HfO2 film was not broken. This study shows that the ZnS SWS with HfO2 protective film has promising application prospects in infrared optical windows.

Near-infrared broadband Si:H/SiO2 multilayer gratings with high tolerance to fabrication errors.

Si:H and TiO2 multilayer dielectric gratings (MDGs) were studied comparatively to highlight the influence of refractive indices on fabrication tolerances, including tolerances for grating width errors and cross-sectional shape errors. The fabrication tolerance of Si:H MDGs is two to four times greater than that of TiO2 MDGs, because the higher refractive index of Si:H has a stronger ability to restrain electric fields. It was further revealed in these studies that a Si:H MDG with positive trapezoidal errors has minor influence on high diffraction efficiency bandwidth. Finally, a Si:H MDG was prepared without iterative corrections. Although large fabrication errors of grating width and cross-sectional shape existed, the MDG still had a 146 nm bandwidth with diffraction efficiency over 97%, which verifies the robustness of the proposed Si:H MDG.

Design and fabrication of a superior nonpolarizing long-wavelength pass edge filter applied in laser beam combining technology.

The wavelength combining technique has been widely used to increase the power scaling of lasers. The edge filter, which is nonpolarizing for wavelength, has proven to be a critical component for the combining process, and should have high spectral efficiency, high laser induced damage threshold, and excellent thermal stability. Here, we design a superior nonpolarizing long-wavelength pass edge filter using a novel depolarized initial film structure based on a tuned Fabry-Perot interference filter. The coating fabricated by electron beam evaporation can withstand 130kW/cm2 of continuous laser energy with high spectral efficiency at 45° and a small temperature rise.

Influence of dry etching on the properties of SiO2 and HfO2 single layers.

A comparative study was performed to investigate how etching methods and parameters affect the properties of SiO2 and HfO2 coatings. SiO2 and HfO2 single layers were prepared by electron-beam evaporation (EBE), ion-beam assisted deposition (IAD), and ion-beam sputtering (IBS). Then, ion-beam etching (IBE), reactive ion etching (RIE), and inductively coupled plasma etching (ICPE) were used to study the influence of ion bombardment energy and chemical reaction on the etching rates and properties of the prepared SiO2 and HfO2 single layers. For SiO2 coatings, chemical reaction plays a dominant role in determining the etching rates, so ICPE that has the strongest CHF3 plasma shows the highest etching rate. Moreover, all three etching methods have barely any influence on the properties of SiO2 coatings. For HfO2 coatings, the etching rates are more dependent on the ion bombardment energy, although the chemical reaction using CHF3 plasma also helps to increase the etching rates to some extent. To our surprise, the ion bombardment with energy as high as 900 V does not change the amorphous microstructure or crystalline states of prepared HfO2 coatings. However, the high-energy ion bombardment in IBE significantly increases the absorption of the HfO2 coatings prepared by all deposition techniques and decreases their laser damage resistance to different extents.

Demonstration of a dual-channel two-dimensional reflection grating filter.

A dual-channel two-dimensional (2D) reflection grating filter operating around the 1.55 µm wavelength region is demonstrated, exhibiting dual-channel reflection peaks at 1.492 µm and 1.647 µm. The sidebands intrinsic to this kind of grating are suppressed by appropriately designed antireflective thin films, and this can be proved by equivalent medium theory. Using the modal analysis method, the excitation modes of the dual-channel reflection peaks are determined to be the TM0 (1.490 µm) and TE0 (1.638 µm) modes. The estimated relative errors in the wavelength determination of these modes are less than 1%. This is found to be in accord with analyses of the reflectivity spectra and electromagnetic fields. The dual-channel reflection peaks are sensitive to the background refractive index and may be useful in biosensing applications.

Morphological and damage growth characteristics of shell-type damage of fused silica optics induced by ultraviolet laser pulses.

Under the radiation of a nanosecond pulse laser, the types of damage and damage profile of fused silica optics are closely related to the surface and subsurface defects of the component. Using the raster-scanning mode to measure the laser-induced damage threshold of fused silica optics, three different types and sizes of damage patterns are found, of which shell damage is the intermediate state and is a bridge connecting submicrometer-size damage and catastrophic damage. This paper mainly studies the mechanism and damage growth law of shell damage, analyzes the model of laser-induced shell damage, and discusses the probability and cause of shell-damage growth.

Morphology and growth properties of nano- and submicrometer-scale initial damage sites under 355 nm wavelength pulsed laser irradiation.

The initial damage of optical elements under irradiation of a 355 nm wavelength nano-second pulsed laser below the laser-induced damage threshold mainly includes nano- and submicrometer-scale craters. The morphology and growth characteristics of such small-scale damage, which are different from those of larger-scale damage, have not been fully clarified. In this study, laser-induced nano- and submicrometer-scale initial damage of fused silica was investigated by atomic force microscope. The morphology of craters and ejecta was analyzed, and the relationship between the structure characteristics and the irradiation parameters was obtained. In addition, the growth and evolution features under multipulsed laser irradiation were classified.

Laser damage properties of broadband low-dispersion mirrors in sub-nanosecond laser pulse.

Broadband low dispersion (BBLD) mirrors are an essential component in femto-second (fs) pulse laser systems. We designed and produced Ta2O5-HfO2/SiO2 composite quarter-wave and non-quarter-wave HfO2/SiO2 BBLD mirrors for the 30fs petawatt laser system. The laser damage properties of the BBLD mirrors were investigated in an uncompressed sub-nanosecond laser pulse. It showed that the Ta2O5-HfO2/SiO2 composite BBLD mirror possessed higher LIDT due to the low electric-field intensity (EFI) in the case of the coating without artificial nodules. Nevertheless, the LIDT of the composite mirror was significantly lower than the non-quarter-wave HfO2/SiO2 mirror when the nodules exist. The EFI simulation and damage morphology of the nodules analysis demonstrated that the nodule leading to the light intensification in the middle of the boundary between the nodular and the surrounding coating, thus the outermost HfO2/SiO2 layers cannot protect the Ta2O5/SiO2 layers, and resulting to the significantly low LIDT. This study shed some light on the development of high-laser-damage BBLD mirrors for pulse compression laser systems.

Contribution of angle-dependent light penetration to electric-field enhancement at nodules in optical coatings.

The influence of angle-dependent light penetration on electric-field intensity (EFI) enhancement at nodules was investigated in this Letter. An experiment consisting of 3D finite-difference time-domain simulations was conducted on two types of polarizers that prevent light penetration at a low and a high incident angular range (IAR). The EFI at the focal point region is six times lower, and the laser damage resistance is three times higher in the polarizer blocking light penetration in a high IAR. These results reveal for the first time, to the best of our knowledge, that light penetration at a high IAR, rather than at a low IAR, contributes to EFI enhancement at the focal region of the nodules. Our findings may provide useful guidance in selecting optimal designs to suppress EFI enhancement at nodules in multilayer coatings.

Reducing light scattering in high-reflection coatings through destructive interference at fully correlated interfaces.

Light scattering in quarter-wave high-reflection (QWHR) coatings with fully correlated interfaces was reduced by adding Fabry-Perot (FP) cavity structures on top of the multilayer. The properly designed FP cavity can induce destructive interference for fully correlated interfaces and reduce the scattering loss. Compared to QWHR coatings, adding one FP cavity could decrease the scattering at the near specular angles, and two FP cavities have the potential to reduce light scattering in a broad angular range. A low-scattering HR (LSHR) coating using two FP cavities has been realized to suppress light scattering. The numerical scattering calculation illustrated that the total scattering loss of the LSHR was about 30% less than that of the QWHR coatings. The measured angle-resolved scattering of the LSHR coating showed a good correspondence to the numerical calculation, although a small deviation exists in a limited angular range.

Effect of boundary continuity on nanosecond laser damage of nodular defects in high-reflection coatings.

Two types of engineered nodules in Ta2O5/SiO2 high-reflection coatings were prepared using electron beam evaporation and an ion-assisted deposition processes to facilitate poor and good boundary continuity, respectively. The influence of nodular boundary continuity on their nanosecond laser damage characteristics was investigated through experimental studies, combined with 3D finite-difference time domain simulations and photo-thermal micro-characterizations. Better boundary continuity led to improved mechanical stability and higher ejection fluence of nodules, in accordance with the thermomechanical damage model. In contrast, the ejected nodules that initially had better boundary continuity exhibited higher localized absorption and lower damage growth fluence, which is attributed to the creation of mechanically induced electronic defects or stronger electric field intensity enhancement at the ejected nodules.

Characterization of grain sizes and roughness of HfO2 single layers.

The grain sizes and their influence on the roughness of an HfO2 single layer prepared with ion-assisted deposition were investigated. Three methods, x ray diffractometry, atomic force microscopy, and the k-correlated power spectral density function model, were used to obtain the grain sizes in two HfO2 single layers with 16 and 20 nm thicknesses. X ray diffractometry showed that the grain sizes were about 7 and 9 nm, respectively, whereas the other two methods demonstrated that the grain sizes were about 14 and 16 nm. It was thought that x ray diffractometry underestimated the grain size due to micro strain or a shallow penetration depth. The grains in an HfO2 single layer lead to a rough surface, which had a significant bulge at the middle-high frequency range in a power spectral density function curve. The coating intrinsic roughness of the HfO2 single layer was separated from the substrate roughness.

Fabrication of broadband absorbing coatings for amplified spontaneous emission suppression.

Amplified spontaneous emission (ASE) is a critical factor that limits the output power of slab lasers. To suppress the ASE effect in slab lasers, this paper proposes a metal/dielectric broadband absorbing coating between the laser gain medium and the metal heat sink to suppress ASE from zero to total internal reflection angle on the facet from inside the laser medium. Based on accurate characterization of Cr metal thin films, Cr/SiO2 broadband absorbing films for 1064 nm YAG slab lasers are designed and fabricated to suppress ASE. The good agreement between experiment and design shows the reliability to suppress ASE in slab lasers.

Research on the mechanical stability of high laser resistant coatings on lithium triborate crystal.

The thermomechanical property of the hafnium/silica antireflection (AR) coatings on lithium triborate (LBO) crystal was investigated by simulation and experiment. From the analysis of the stress and fracture toughness, it was found that the crack originated due to the high tensile stress in hafnium coating. Then we proposed the approaches of decreasing the deposition temperature and substituting the hafnium layers with alumina to improve the mechanical stability of AR coatings on LBO crystals, and cracks were effectively suppressed. The laser damage threshold of different coatings on LBO crystal was tested, and it illustrated that the alumina/silica coatings possess better laser resistance than hafnium/silica AR coatings deposited in low deposition temperature.

Influence of coating thickness on laser-induced damage characteristics of anti-reflection coatings irradiated by 1064 nm nanosecond laser pulses.

The influence of coating thickness on laser-induced damage (LID) characteristics of anti-reflection (AR) coatings irradiated by 1064 nm nanosecond laser pulses was investigated. Two HfO2/SiO2 AR coatings with different physical thicknesses, 0.7 and 2.7 µm, were prepared and tested. To study the effect of coating thickness on a laser-induced damage threshold (LIDT) in isolation, electric field intensities (EFIs) at the substrate-coating interface were kept the same by using proper AR designs. Moreover, 2 nm artificial gold particles with a density of 10 mm-2 were implanted into the substrate-coating interface to achieve reliable experimental results. An optical microscope (OM) and a scanning electron microscope (SEM) were used for an online LIDT test and offline LID morphology observation, respectively. The typical LID morphology of thicker AR coatings was flat bottom craters with diameters of 20-50 µm, which can be easily observed by an online OM. For thinner AR coatings, hemispherical craters with diameters down to 1 µm were found as typical LID morphology by a SEM. However, these tiny craters could not be observed by an online OM. Moreover, such tiny craters did not grow with subsequent pulses, so they did not degrade the functional laser damage resistance of the thin AR coatings. When identified with an online OM, the LIDT of thinner AR coatings is found to be about two times higher than the thicker ones, and large delamination was mainly found as the LID morphology of AR coatings with high fluence. When observed with a SEM, the LIDT of thin AR coatings with tiny craters was over 60% lower than the LIDT of thick AR coatings, which agrees with the model that less energy is required to form smaller LID craters of thinner coatings.

Optimal coating solution for a compact resonating cavity working at Brewster angle.

In a compact Nd:Glass resonator, the laser that enters the gain medium at Brewster angle can work either for P-polarization or S-polarization, in which polarization the optical coatings possess higher laser-induced damage threshold (LIDT) was investigated. For the P-polarized configuration, only one high reflection (HR) coating on the rear surface of the Nd:Glass substrate is needed, and the laser-induced damage occurred near the substrate-coating interface at a fluence of 10 ± 2 J/cm2 (1064nm 10ns). Although S-polarized configuration needs two coatings, one HR coating and one anti-reflection (AR) coating on the rear and front surface of the Nd:Glass substrate respectively, its overall LIDT was about 1.8 times higher than that of the P-polarized configuration. The laser-induced damage occurred at the interface between the S-polarized AR coating and the Nd:Glass substrate. The observed interfacial damage behaviors were interpreted using a phenomenological model that took the nano-sized absorbers, electric-field intensity (EFI) distribution and coating thickness into consideration.