In this paper, a concave holographic grating aberration optimization method is designed, which performs well in the aberration optimization of multi-channel narrow-band gratings. Taking the grating in the the project of CAFE (the Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer) proposed by the Purple Mountain Observatory, Chinese Academy of Sciences, as the optimization target, this paper compares the method proposed with the traditional aberration function method and the spot diagram method commonly used currently in terms of the evaluation perspectives such as the line density deviation, the focusing curve offset, and the exit slit luminous flux, etc., and the optimization time is one thirtieth and one sixtieth of the two methods, respectively.
Multilayer Dielectric Gratings (MLDGs) are the core optics of the picosecond-petawatt laser systems based on Chirped Pulse Amplification (CPA). The MLDGs encounter nanosecond- and picosecond-laser irradiation during the broadening and recompression of the laser pulse in CPA. Therefore, a comprehensive evaluation of the laser-induced damage performance of MLDG in the nanosecond (ns) and picosecond (ps) regimes is required. Herein, we examined the laser damage characteristics of MLDGs induced by the 8-ns and 8.6-ps laser pulses. In the two tested laser pulse widths, the damage of the MLDG was dominated by the nano absorbing defects and nodular defects, but the damage mechanism has changed. When the laser pulse width is shortened from ns to ps, the damage sites caused by the absorbing defects transfer from the interface to the grating pillars, where there is the maximum electric field. And the nodular ejection pit changes from a complete eruption to a local damage corresponding to the electric field enhancement region. For the nodular defects, ns-laser conditioning was introduced for removing them and achieved a maximum enhancement of 40% in the ps laser-induced damage threshold.
Gold-coated photoresist grating (GCPG) is commonly employed in high-power femtosecond laser systems for its broadband and simple structure. The pyrolysis and low heat conductivity of photoresist substrates are well known to result in a low GCPG laser damage threshold, which restricts the power increase of laser systems. Gold-coated fused silica grating (GCFSG) has a high threshold potential since the grating pattern is transferred from the photoresist to the fused silica substrate by etching. In this paper, a rectangular GCFSG with a period of 1740 l/mm was designed and fabricated. Using rigorous coupled-wave analysis (RCWA), the ideal slot form for GCFSG with high diffraction efficiency was designed. After a comprehensive analysis of the impact of gold plating coating flaws on efficiency, iterations of the coating process were carried out to optimize the slot shape. For these GCFSG samples, magnetron sputtering was used as the gold deposition process and the samples had a bandwidth of at least 150 nm with the -1st-order diffraction efficiency of 93% around the central wavelength of 800 nm. The measured efficiency results were compared with our simulation calculations and good agreement was achieved. After being damaged by lasers, GCFSG can be reused with good economics by being cleaned and then gold-coated.
According to the request for large-scale and high-efficiency transmission grating for astronomical ground-based telescope spectrographs, we report a method to control the duty cycle during the fabrication of deep-etched grating on fused silica substrate by holograph and ion-beam etching simply and conveniently. Simulation shows that 98% diffraction efficiency is achieved under Littrow conditions when groove depth is about 830nm, ridge width is about 240nm (duty cycle is about 0.60) at a wavelength of 400nm. In this work, we successfully manufactured serval gratings with different duty cycle controlled by exposure dose and thickness of chromium layer on the photoresist mask and achieved a high depth-to-width ratio deep-etched grating with the Cr-photoresist double mask at optimal grating period and groove depth for efficiency based on simulation results. It lays an essential foundation for the realization of large-area deep etching grating, which can be used in huge telescopes in the future.
With the widespread application of polarizers in infrared detection, military, medical and other fields, infrared polarizers have developed rapidly in the last decade, and it is becoming increasingly crucial to fabricate high-performance and low-cost polarizers. The paper presents a design for fabricating a linear polarizer operating in the 7-14 um by metal-assisted chemical etching. A subwavelength gold grating is fabricated on a silicon substrate by the lift-off process, then a subwavelength silicon grating structure is fabricated by the metal-assisted chemical etching process of silicon, an aluminum film is coated on the bottom of the groove and the ridge of the fabricated subwavelength silicon grating by electron beam evaporation coating. The structure is optimized by the rigorous coupled-wave analysis, we assess the effect of the grating parameters on the extinction ratio and transverse magnetic wave transmittance and optimize the parameters, and finally determined that the grating period is 400 nm, the duty cycle is 0.5, and the groove depth is 1200nm. The numerical simulation results demonstrate that the polarizer has the advantages of high extinction ratio and wide manufacturing tolerance in long wave infrared region (7-14um). Grating structures with high aspect ratio can be fabricated by metal-assisted chemical etching, which makes it possible to enhance the transmission of the silicon substrate. Therefore, anti-reflection structures can be designed to enhance the transmission of the silicon substrate in the long-wave infrared based on the effective medium theory.
We present the current design of WFOS, a wide-field UV/optical (0.31-1.0 µm) imaging spectrograph planned for first-light on the TMT International Observatory 30 m telescope. WFOS is optimized for high sensitivity across the entire optical waveband for low-to-moderate resolution (R ∼ 1500-5000) long-slit and multi-slit spectroscopy of very faint targets over a contiguous field of view of 8′ .3×3 ′ .0 at the f/15 Nasmyth focus of TMT. A key design goal for WFOS is stability and repeatability in all observing modes, made possible by its gravity-invariant opto-mechanical structure, with a vertical rotation axis and all reconfigurable components moving only in planes defined by tiered optical benches parallel to the Nasmyth platform. WFOS’s optics include a linear ADC correcting a 9′ diameter field, including both the science FoV and 4 patrolling acquisition, guiding, and wavefront sensing camera systems; a novel 2-mirror reflective collimator allowing the science FoV to be centered on the telescope optical axis; a dichroic beamsplitter dividing the collimated beam into 2 wavelength-optimized spectrometer channels (blue: 0.31-0.56 µm; red: 0.54-1.04 µm); selectable transmissive dispersers (VPH and/or VBG) with remotely configurable grating tilt (angle of incidence) and camera articulation that enable optimization of diffraction efficiency and wavelength coverage in each channel; all-refractive, wavelength-optimized f/2 spectrograph cameras, and UV/blue and red-optimized detector systems. The predicted instrumental through put of WFOS for spectroscopy averages > 56% over the full 0.31-1 µm range, from the ADC to the detector. When combined with the 30 m TMT aperture, WFOS will realize a factor of ∼20 gain in sensitivity compared to the current state of the art on 8-10 m-class telescopes.
We report on high-efficiency visible and near-infrared transmission gratings in fused silica generated by holographic recording and reactive ion beam etching technology. At a wavelength of 740 nm, near 100% diffraction efficiency is achieved under Littrow conditions. The design is based on the phenomenon of the high aspect ratio gratings by using the rigorous coupled wave analysis. A binary grating with the optimum grating period of 740 nm and groove depth of 1.55 had been fabricated in the paper. The grating wavelength bandwidth and angular bandwidth are extremely enhanced compared with conventional volume phase holographic gratings, making these gratings the key elements in high-resolution astronomical ground-based telescope spectrographs.
As an essential component of the slitless spectrometer, the UV transmission blazed grating has the capability of high dispersion and high resolution. In this paper, a method for fabricating UV transmission blazed gratings by holography-ion beam etching is proposed. Holographic interference lithography is used to generate photoresist grating masks. The ion beam vertical etching transfers the photoresist mask pattern to the substrate to form a SiO2 grating mask. When the ion beam incident direction is at a certain angle to the normal direction of the substrate, the SiO2 mask is used to block the inclined ion beam, so that different parts of the mask bottom are bombarded by the ion beam with different fluxes, forming a blazing facet. When the mask is etched completely, the blazed grating is formed. Based on the idea of the line motion algorithm, the article establishes the geometric model of blazed grating etching, which provides the parameter guidance for precise control of the groove structure. Combined with the theoretical model, a UV transmission blazed grating with a line density of 333 lines/mm and a blazing angle of 13.2° is successfully fabricated.
In this article, we designed and fabricated a high linear density concave variable line space holographic grating for the Lyman spectrometer, with level 3 working and the center line density is 3300line/mm. The focus curve is a circle with the grating vertex O as the center and a radius of 900 mm. We analyze the influence of the exposure error on the line density distribution and reduce the harm of errors through the method of multi-error compensation. To improve the diffraction efficiency, we use Finite Element Method software to get the best grating groove parameters. Finally, we initially prepared a holographic grating with a symmetrical arch groove with a groove depth of 175nm and a bottom duty cycle of 0.3.
Multilayer dielectric gratings(MLDGs)have been widely used in chirped pulse amplification due to their high laser induced damage thresholds(LIDTs). The quest for MLDG LIDT improvement is endless. For MLDGs applied in picosecond(ps) lasers, damage shows the characteristics of both thermal effect and nonlinear effect. The thermal damage of multilayer dielectric films (MLDFs) and MLDGs were investigated using a 1064 nm laser with a duration of 8 ns in our study. Differently from previous 1-on-1 studies, Raster Scan method is adopted to investigate the effect of low-density defects on the laser damage resistance of MLDFs with different top layers and MLDGs. The results show that the LIDTs of MLDGs are half of those MLDFs. For MLDFs with the top layer of HfO2, the damage behaves the ablation of the top layer material due to the surface strong electrical field. For MLDFs with top layer Ta2O5 and SiO2, the typical morphologies are nodule ejections. The initial damage of MLDGs fabricated by etching these three kinds of grating films are similar, and all behave nodule ejections. This indicates that reducing nodule defects can help the MLDGs LIDT improvement in ps pules. These results provide guidance for process optimizations of MLDG fabrication.
Multilayer dielectric gratings (MLDGs) have been widely used as pulse compression grating (PCG) in chirped pulse amplification (CPA) technology due to their high laser induced damage thresholds (LIDTs). The quest for MLDGs LIDTs improvement is endless. As one of the core components of CPA process, MLDGs will encounter laser irradiation of nanosecond, picosecond and femtosecond. Therefore, the damage characteristics of MLDGs should be studied at various pulse widths. We performed the LIDTs test on a Nd:YAG laser system with a wavelength of 1064 nm and a pulse width of 8 ns. Damage characteristics of both MLDFs and MLDGs were investigated. MLDFs were deposited on the substrates cleaned by hand wipe or ultrasonic cleaning. The results show that the LIDTs of MLDGs are approximately 60% of MLDFs. Besides, LIDTs of MLDFs with HfO2 top layer will not be affected by the methods of substrates cleaning due to its surface damage characteristic related to the non-zero EFI on the surface material. However, for the MLDFs with top layer of Ta2O5, LIDTs of MLDFs deposited on substrates cleaned by hand wipe are higher than those deposited on the ultrasonically cleaned substrates.
The CAFE (Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer) and LyRIC (Lyman UV Radiation from Interstellar medium and Circum-galactic medium) have been proposed to the space agencies in China respectively. CAFE was first proposed as a joint scientific CAS-ESA small space mission in 2015. LyRIC was proposed as the independent external payload operating on the Chinese Space Station (CSS) in 2019. Both missions are dedicated to mapping the Lyman UV emissions ( ionized oxygen (OVI) resonance lines at 103.2 and 103.8 nm, and Lyman series) for the diffuse sources either in our Galaxy or the circum-galactic mediums of the nearby galaxies. We present the primary science objectives, mission concepts, the enabling technologies, as well as the current status.
The morphology evolution and optical characteristic within longitudinal direction of self-organized nanopatterns induced during Ar+ ion bombardment (IB) with Fe co-deposition on fused silica (SiO2) surfaces are investigated in this work. At the same ion fluence and angle of incidence, simultaneous Fe co-deposition can result in significant terrace morphology effect with increased roughness and horizontal feature size compared to conventional pure IB. Due to the interaction between the pure IB and Fe co-deposition, well-ordered corrugation patterns can be produced on the surface of the fused silica. Such structure can be used to synthesize ordered self-organized metal nanowire arrays on dielectric materials. This method combines nano-patterning of ion bombardment, impurity co-deposition and metal shadow deposition effect, which provides a feasible way to replace traditional continuous nano-processing methods. Due to the excitation of metal plasmon resonance, nanowire arrays exhibit different optical transmission spectral characteristics at different position in the structure region and show optical anisotropy in the longitudinal direction due to linear changes of structure horizontal feature size.
In this note, we present our results about the process design and characterization of deep reactive ion beam etching of high line density fused silica transmission gratings. Up to 1 μm deep and 1880 lines/mm microstructures have been successfully fabricated for 532nm ultrashort pulse laser pulse compression using the developed processes. Considering the requirements of the manufacturing process, a certain range of parameters needs to be determined to reduce the need for etching precision. Therefore, in this paper, the strict coupling wave method is used to determine the parameter range of the grating in the range of the calculated grating parameters, and the diffraction efficiency of the simplified mode method can be calculated. To our knowledge, this is the first note on the design parameters of transmission pulse compression grating made on fused quartz substrate are given using the simplified mode method to design and a non-ICP-based reactive ion etcher.
Near infrared filters for thermophotovoltaic(TPV) generators have been invested widely and different programs are proposed in terms of design and fabrication. However, system conversion efficiency is limited by their own drawbacks, although some of them are combined to solve these problems. Resonance Antenna Array Filters(RAA) is candidate for TPV system with high conversion efficiency, which is metal pattern with less than 150nm linewidth. Under the existing technological conditions, there are large fabrication deviation, which will make the expected filtering characteristics offset and hardly meet the requirements. In this paper, a finite element method(FEM) combined with TPV model in practice is built to design the geometric characteristics of RAA filters in order to achieve high system efficiency, including square-loop, circular rings and cross slots. The results show that slot aspect is the main parameters affecting the filtering characteristics, and the fabrication with 10% deviation can meet the requirements of the design value. In addition, high-order harmonics exist in all three groove structures near 700nm wavelength. This paper proposes a circle ring with specific structure to effectively suppress the harmonics, which is helpful to improve the conversion efficiency of thermophotovoltaic system.
At present, scanning electron microscopy (SEM) and atomic force microscopy (AFM) are the main means of measuring grating parameters. When SEM is used, irreversible damage will be caused to the sample. AFM can only detect small areas of samples. AFM is too inefficient for large area diffraction gratings. Therefore, non-destructive testing of grating groove shapes is very necessary. In this paper, the light intensity 、diffraction angle of first-order and diffracted light interferometric fringes are studied and a new method for non-destructive measuring of grating parameter is proposed. We will study the grating parameters based on the rigorous coupled wave theory (RCWA).
Soft x-ray varied line spacing grating (VLSG), which is a vital optical element for laser plasma diagnosis and spectrometry analysis, is conventionally fabricated by holographic lithography or mechanical ruling. In order to overcome the issues of the above fabrication methods, a method based on electron beam lithography-near field lithography (EBL-NFH) is proposed to make good use of the flexibility of EBL and the high throughput of NFH. In this paper, we showed a newly designed soft x-ray VLSG with a central groove density of 3600 lines/mm, which is to be realized based on EBL-NFH. First, the optimization of the spatial distribution of line density and groove profile of the VLSG was shown. As an important element in NFH, a fused silica mask plays a key role during NFH in order to obtain a required line density of VLSG. Therefore, second, the transfer relationship of spatial distribution of line densities between fused silica mask and resist grating was investigated in different exposure modes during NFH. We proposed a formulation about the transfer of line density to design of the groove density distribution of a fused silica grating mask. Finally, the spatial distribution of line densities between the fused silica mask, which is to be fabrication by using EBL, was demonstrated.
The chirped-pulse amplification system plays a critical role in the process of achieving high-peak ultrashort pulses. Chirped-pulse amplification technology performance mainly depends on the pulse compression gratings. Diffraction efficiency is the critical parameter of the pulse compression gratings, and optimization of grating shape can achieve higher diffraction efficiency. If the photoresist grating mask bottom is not clean, the side walls would not be steep and duty cycle would be too big or too small, thus pulse compression grating diffraction efficiency would decrease. Solving these problems is the key to improve the diffraction efficiency. In this paper, oxygen etching methods are used to reduce the photoresist mask duty cycle, and PDMS pressing method is used to increase the duty cycle of photoresist mask, and aperture up to 100 mm×100 mm. Best photoresist grating mask parameter could be obtained by effectively combining the above two methods. Based on above techniques, a number of pulse compression gratings with line densities of 1740 lines∕mm was achieved. The diffraction efficiency at the-1st order was greater than 99% for TE polarized light. A qualitative judgments for graphic transfer of ion beam etching is received through the picture before and after etching which is get from the SEM. These experimental results proved the accuracy, stability, and success rates of the technique.
With the development of micro- & nanofabrication technology, micro- & nanostructures have been widely used in many fields, including spectroscopy, coding, sensor, subwavelength element, etc. With phase masks realized by a combination of electron beam lithography (EBL), near field lithography (NFH) has great potential to fabricate versatile nanostructures, because it combines the advantages of both lithographic methods. Currently, subwavelength structures attract much attention due to their various functions, such as antireflection, polarization beam splitter and filter. In this presentation, aiming at reducing the interface reflection of a fused silica mask of NFH at a wavelength of 441.6 nm and incidence angles of either 0° or 32°. First, we will compare the difference of antireflection property of one-dimensional (1D) and two-dimensional (2D) subwavelength structures with line density of 3600 lines/mm by simulation. Then, the optimized 1D and 2D subwavelength structures with 3600 lines/mm will be fabricated by using EBL-NFH method. Finally, the antireflection property of these 1D and 2D subwavelength structures will be characterized at the wavelength of 441.6 nm.
KEYWORDS: Etching, Ion beams, Photoresist materials, Diffraction gratings, Monte Carlo methods, Sputter deposition, Lithium, Quartz, Ions, Chemical species
A new method for controlling the groove profiles of diffraction gratings which changes the etching angle and etching time, meanwhile divides the etching area in the substrate into multi-layers to have a good approximation for the theory is introduced. We put forward a multi-layers etching model on the base of the ion bean sputtering (IBS) which can calculate the etching time and etching angle. We test the curved grooves profiles and get the optimizations for the number of the multi-layers, etching time and etching time in this model. Also a photoresist grating is applied for the etching experiment. The results indicate that the optimized parameters such as the number of the multi-layers result in a smaller root mean square deviation (RMSD) between the theory and the real etching result which show good agreement with the theoretical groove within the variation of ±6% of the etching rate. The simulation predictions and experimental results show that the multi-layers etching model to control the groove profiles of diffraction gratings is available.
Fabrication and testing results of sine-top, high-efficiency, broadband gold-coated gratings (BGCG) for high-power laser pulse compression applications are reported. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by directly etching the quartz substrate. The groove depth and duty cycle of the photoresist mask was controlled by changing photoresist thickness and adjusting exposure and development times, respectively. The duty cycle of the photoresist mask was further corrected by oxygen plasma etching. Using this method, high efficiency, sine-top, BGCG with line densities of 1740 lines/mm was achieved. The average diffraction efficiency at the-1st order was 89.2% and the peak value was 90% for TM polarized light as the wavelength increases from 750 to 850 nm.
Broadband gold-coated grating (BGCG) is one of the key elements of large pulse compression systems. Compared with
other pulse compression grating (PCG), BGCG have the advantages of simple structure and low cost etc. More
importantly, this kind of grating can get high diffraction efficiency within a broadband range (usually 200 nm or more).
In this paper the authors report a process for fabrication of sine-top BGCG. When gratings are intended for use with
high-power lasers, their laser-damage threshold has an importance equal to that of the diffraction efficiency. These
gratings fabricated by this method differ from conventional metal-on-photoresist PCGs in that the gratings patterns are
generated by etching the fused silica substrate directly. This can improve the laser damage threshold. The groove depth
and duty cycle of the photoresist mask were controlled by changing photoresist thickness and adjusting exposure and
development time. The duty cycle of the fused silica grating was further corrected by oxygen plasma etching. Using this
method, high efficiency sine-top BGCGs with line densities of 1740 lines /mm have been achieved, this paper has a good
reference value to the further fabrication of larger aperture gold-coated PCG.
We proposed a technique for conducting on-the-fly fine adjustment of etch depths with sub-nanometer precision during the course of ion beam etching (IBE). Simulations were performed to evaluate the etch-depth control precision. The simulation prediction shows that the precision of fine control of etch depths is at the level of 0.1nm. The preliminary experiment was conducted. The early result and the simulation prediction are in agreement with each other, which indicates that this approach is feasible for finely controlling groove-depth variations of large-area diffraction gratings.
The authors report a new process combining interference lithography with potassium hydroxide (KOH) anisotropic etch
technique for fabrication of high aspect ratio silicon gratings on (110) oriented silicon wafers. This new process has the
ability in fabricating high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. An
alignment method was developed to align interference fringes to the vertical (111) planes of (110) oriented wafers. In
addition, a room temperature etch process with 50 wt % KOH solution was chosen to finally get an etch anisotropy of 188.
Better etch uniformity was achieved by adding a surfactant to the aqueous KOH to promote the release of hydrogen bubbles.
To increase latitude in KOH etching process, deposition of aluminum under a sloped angle with respect to the grating
structures was utilized to obtain a high duty cycle nitride mask. To prevent the collapse of high aspect ratio grating
structures caused by surface tension, a liquid carbon dioxide supercritical point dryer was used in the drying process. The
authors successfully fabricated 320nm period gratings with aspect ratio up to 100 on 5-μm-thick silicon membranes on
(110) oriented silicon-on-insulator wafers. The sample area is about 50 mm × 60 mm. The roughness (root mean square)
of the sidewall is about 0.267 nm.
As a new type of grating, the freestanding blazed transmission grating combines the advantages of traditional transmission gratings (low mass, relaxed alignment and flatness figure insensitivity) with those of reflection gratings (high broadband diffraction efficiency, high spectral resolution). A freestanding blazed transmission grating with period of 1μm was successfully fabricated by holographic lithography and anisotropic wet etching of silicon. The duty cycle is about 0.13. The aspect ratio of a single grating bar is about 77. The thickness is 10μm and the open area fraction is about 58.8%. The size of a single die is 15mm×15mm divided into four 5mm ×5mm windows. The diffraction efficiency of the grating was measured at the National Synchrotron Radiation Laboratory in the wavelength region of 5-50nm. The results show a strong blazing effect in the direction of specular reflection from mirror-like grating sidewalls, as expected.
It is known that rectangular grooves profile and various duty cycles (line to pitch) are required to attain maximum
efficiency. The gold transmission phase gratings have been fabricated using holography and ion-beam etching
techniques. The latent image monitoring technique and real time end-point detection technique were utilized to improve
control of the shape of grooves and duty cycle during exposure and development, respectively. It is difficult to produce
gratings with larger duty cycle in photoresist on gold for standing wave, nevertheless it is revealed in our work that the
duty cycle can be more than 0.4 for the case for photoresist on gold substrates if development is stopped where the slope
of the monitoring curve begins to drop off, not at the peak of the diffracted signal. In ion-beam milling, the influence of
redeposition on the shape of grooves and duty cycle has been overcome by using a thin mask of Chromium with a
comparatively low ion-etch rate and tilting and rotating substrates beneath the ion beam. Finally, some gold phase
gratings with the duty cycles in the range of 0.25-0.45 have been obtained, whose duty cycle value is about 0.45, and
their grooves profile is trapezoidal profile whose left and right facets are both more than 83 degrees approximately.
A binary transmission phase grating is designed to operate as a dispersive element used in soft x-ray plasma diagnostic in ICF (inertial confinement fusion). The first and zero orders diffraction efficiency is simulated as a function of wave length and grating thickness. It is shown that the first order diffraction efficiency of a transmission phase grating can be up to 23.3% of the incident light, whereas that is not more than 10% for a conventional amplitude transmission grating. The transmission phase grating is fabricated in gold film supported by polyimide membranes using holography and ion beam etching method. The primary experimental results are presented.
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