Burkhardt, Matthias

Glaser, Tilman

Koch, Felix

Lehr, Dennis

Schönbrodt, Oliver

Sinzinger, Stefan

Digitale Bibliothek Thüringen

   TU Ilmenau | Universitätsbibliothek | ilmedia, 2019 http://www.tu-ilmenau.de/ilmedia Koch, Felix; Schönbrodt, Oliver; Burkhardt, Matthias; Lehr, Dennis; Glaser, Tilman; Sinzinger, Stefan: Optimization and manufacturing of gold gratings for pulse compression  Zuerst erschienen in: DGaO-Proceedings. - Erlangen-Nürnberg : Dt. Gesellschaft für angewandte Optik. -119 (2018), art. A27, 2 S. Erstveröffentlichung: 20.07.2018 ISSN: 1614-8436 URN: urn:nbn:de:0287-2018-A027-3 [Gesehen: 30.08.2019]  DGaO Proceedings 2018 http://www.dgao-proceedings.de ISSN: 1614-8436 urn:nbn:de:0287-2018-XXXX-Ysubmitted: XX.XXX.2018       published: XX.XXX.2018Optimization and manufacturing of gold gratings for pulse compressionFelix Koch*, Oliver Schönbrodt*, Matthias Burkhardt*, Dennis Lehr*, Tilman Glaser*, Stefan Sinzinger***Carl Zeiss Jena GmbH, Jena**Technische Universität Ilmenau, Ilmenaumailto:felix.koch@zeiss.comTo achieve decent diffraction efficiencies with gold-coated gratings, e.g. for pulse compression, the profile shape has to be optimized based on the manufacturing process. In our contribution, we report on the design and holographic manufacturing of Au-gratings with up to 94 % diffraction efficiency at 1050 nm central wavelength.1 IntroductionThere are several applications for diffraction grat-ings in laser physics and -engineering, e.g. as filters for bandwidth-narrowing and wavelength-tuning or for spectral beam combining and temporal pulse shaping in ultrafast lasers. Gold-coated diffraction gratings are still popular for laser pulse compressiondue to their rather simple and low-cost fabrication, especially compared to more sophisticated struc-tured dielectric multilayers or dielectric transmission gratings [1]. To achieve high diffraction efficiencies(>90 %) with a practically feasible design, it is ben-eficial to consider the manufacturing process al-ready in the design stage. In our contribution, we demonstrate a holistic design approach for holo-graphically recorded gratings optimized for laser pulse compression in the NIR range as well as firstexperimental results from the manufacturing of these gratings yielding close to theoretical perfor-mance.2 Design approachThe direction of the diffraction orders for one-dimen-sional gratings with in-plane incidence is completely described by the grating equation that arises from the interference of the light periodically scattered from the grating grooves:, (1)where and are the incidence and diffracted angle for order m respectively, is the wavelength and the period. To achieve the desired high angu-lar dispersion, the gratings utilized for laser pulse compression typically work in the resonance do-main with only one propagating diffraction order be-sides the 0th order. Hence, neglecting scattering and absorption, also the energy can only be distributed between these two orders and high efficiencies can be achieved by controlling the profile shape [2]. In this highly dispersive regime, the polarization and the field propagation inside the grooves cannot be neglected, and hence, to correctly model the grat-ing, rigorous numerical methods have to be applied, ther physical approximations. For our contribution, we applied the rigorous boundary integral equation solver PCGrate, that has particularly proven well-suited for metal-coated gratings with high refractive index contrasts and continuous profiles. The math-ematical details of this method are beyond the scope of this paper, the reader is referred to [3]. To incorporate the manufacturing process, the Maxwell solver was combined with a self-written semi-empir-ical simulation of the photoresist development based on the string method [4]. In Fig. 1, the for-mation of a symmetric grating profile inside the pho-toresist during the development process is demon-strated. The false colors in (a) correspond to the lo-cal development rate [nm/s]. Exposure of AZ1505positive resist with 250 mJ/cm² dose from each arm and a contrast of 0.8 was assumed. The model is based on the experimentally determined resist char-acteristic depicted in (b).Fig. 1 Semi-empirical resist simulation. (a) Intermediate steps for symmetrical grating, (b) experimental resist re-sponse for 460 nm and diluted AZ303 developer.Although, the complex resist chemistry is neglected, our semi-empirical model was found to exhibit good correspondence to experimental profiles.Furthermore, to efficiently address the parameter space of the resist model, a multi-parameter optimi-zation based on the Interior Point method was incor-porated into the design routine. The former is a de-terministic local gradient-based optimizer. A merit function based on the RMS-deviation from 100 % -1st reflected order efficiency for TM polarization was minimized for 1740 L/mm, 72° angle of incidence(10° deviation from the diffracted order at 1054 nm)and a wavelength range from 1040 to 1070 nm in DGaO-Proc edings 2018 – http:// w.dgao-proc edings.de – I SN: 1614-8436 – urn:nbn:de:0287-2018-A027-3submitted: 19.Jul.2018       published: 20.Jul.2018DGaO Proceedings 2018 http://www.dgao-proceedings.de ISSN: 1614-8436 urn:nbn:de:0287-2018-XXXX-Ysubmitted: XX.XXX.2018       published: XX.XXX.2018steps of 5 nm. The parameters were modulationdose, offset dose, development time and resist thickness and have been restricted to reasonable bounds. A statistic multi-start approach with 100 start points was used to avoid poor local minima. More details on the optimization method and merit function are given in [5] and the references therein, where a similar optimization routine is described for dielectric gratings.3 Results and manufacturingThe resulting target profiles were expectably sine-like with a depth of about 140 nm and a duty cycle of 0.46. For manufacturing, 90x90 mm² fused silica substrates were spincoated with 140 nm of AZ1505 positive resist and exposed to the interference pat-tern of two filtered and collimated waves [6]. The pa-rameters from the resist optimization were adapted and slightly refined based on AFM measurementsto resemble the target profiles as closely as possi-ble. After mastering, the grating was, without etch-ing, directly replicated to an even generation (for best profile conservation) using an epoxy-resin-based in-house process and coated with approx. 100 nm gold using thermal evaporation. In Fig. 2, the elementary cells of one master (S1) and one replica grating (S1-rep.) are compared to the target profile from the optimization and show good corre-spondence.Fig. 2 Target profile from resist optimization compared to experimental AFM elementary cells from one master and one replica grating.Spatially resolved diffraction efficiency measure-ments were carried out using a tunable diode laser and an automated x-y-stage. Due to the measure-ment setup, the incidence angle was 60.3°. Figure 3 shows the spectral efficiency compared to simula-tions based on the target and measured profiles as well as a homogeneity measurement at 1045 nm. Amaximum TM efficiency of 93.3 % was attained, which is close to the optimum curve. For smaller de-viations from Littrow incidence, simulations with the real profile yield even higher values of up to 96 %.4 ConclusionWe described an integrated design approach for holographic diffraction gratings recorded in positive photoresist. A semi-empirical photoresist model was combined with a rigorous boundary integral equation solver and a multi-start Interior Point opti-mizer to find both, performant and experimentallyFig. 3 Diffraction efficiency measurements vs. simulation. (a) Spectral dependence for center position compared to simulations based on the corresponding AFM profile and the target profile. (b) Spatially resolved efficiency meas-urement.feasible solutions for a given diffraction efficiency requirement. Reflective gold-coated gratings for la-ser pulse compression were manufactured using in-terference lithography and subsequent replication. Diffraction efficiencies of >93 % at 1045 nm were achieved and agree well with the theoretical design. The described approach can help tailoring diffrac-tion efficiencies for various applications.References[1] R. D Boyd, J. A Britten, D. E Decker, B. W. Shore, B. C. Stuart, M. D Perry, L. Li High-efficiency metallic diffraction gratings for laser applications Appl. Opt. 34(10):1697-1706 (1995).[2] E. Popov: Light Diffraction by Relief Gratings: A Ma in Prog. in Opt.31: 139-187 (1993).[3] B. Kleemann: Fast integral methods for integrated optica Proc. SPIE Vol. 9630 (2015).[4] B. D. A. Mello, I. F. da Costa, C. R. Lima, L. Cescato: Developed profile of holographically exposed photo-Appl. Opt. 34(4): 597-603 (1995).[5] F. Koch, D. Lehr, T. Glaser: Optimization and toler-ancing of highly dispersive multilayer gratings for la-ser applications Proc. SPIE Vol. 10694 (2018).[6] F. Koch, D. Lehr, T. Glaser: Stability requirements for two-beam interference lithography diffraction grat-ing manufacturing Proc. SPIE Vol. 10448 (2017).808284868890929496981001000 1020 1040 1060 1080/nmMeasurement 60.3°Sim. AFM 60.3°Sim. AFM 72°Sim. Optim. 1 60.3°Sim. Optim. 1 72°a -Proceedings 2018 – http:// .dgao-proceedings.de – ISS : 1614-8436 – urn:nbn:de:0287-2018-A027-3submitted: 19.Jul.2018       published: 20.Jul.2018

Optimization and manufacturing of gold gratings for pulse compression

https://www.db-thueringen.de/servlets/MCRFileNodeServlet/dbt_derivate_00045182/1614-8436_119_2018_A27.pdf

Optimization and manufacturing of gold gratings for pulse compression

Abstract

Similar works

Full text

Available Versions

Digitale Bibliothek Thüringen