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High-reflection coatings with broad bandwidth can be achieved by pairing a low refractive index material, such as SiO2, with a high refractive index material, such as TiO2. To achieve high refractive index, low absorption TiO2 films, we optimized the reactive, ion-assisted deposition process (O2 levels, deposition rate, and ion beam settings) using e-beam evaporated Ti. TiO2 high-index layers were then paired with SiO2 low-index layers in a quarter-wave-type coating to achieve a broader high-reflection bandwidth compared to the same coating composed of HfO2/SiO2 layer pairs. However, the improved bandwidth exhibited by the TiO2/SiO2 coating is associated with lower laser damage threshold. To improve the laser damage resistance of the TiO2/SiO2 coating, we also created four coatings where HfO2 replaced some of the outer TiO2 layers. We present the laser damage results of these coatings to understand the trade-offs between good laser damage resistance and high-reflection bandwidth using TiO2 and HfO2.

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Abstract:We describe an optical coating design suitable for broad bandwidth high reflection (BBHR) at 45˝angle of incidence (AOI), P polarization (Ppol) of femtosecond (fs) laser pulses whose wavelengths range from 800 to 1000 nm. Our design process is guided by quarter-wave HR coating properties. The design must afford low group delay dispersion (GDD) for reflected light over the broad, 200 nm bandwidth in order to minimize temporal broadening of the fs pulses due to dispersive alteration of relative phases between their frequency components. The design should also be favorable to high laser-induced damage threshold (LIDT). We base the coating on TiO 2 /SiO 2 layer pairs produced by means of e-beam evaporation with ion-assisted deposition, and use OptiLayer Thin Film Software to explore designs starting with TiO 2 /SiO 2 layers having thicknesses in a reverse chirped arrangement. This approach led to a design with R > 99% from 800 to 1000 nm and GDD < 20 fs 2 from 843 to 949 nm (45˝AOI, Ppol). The design's GDD behaves in a smooth way, suitable for GDD compensation techniques, and its electric field intensities show promise for high LIDTs. Reflectivity and GDD measurements for the initial test coating indicate good performance of the BBHR design. Subsequent coating runs with improved process calibration produced two coatings whose HR bands satisfactorily meet the design goals. For the sake of completeness, we summarize our previously reported transmission spectra and LIDT test results with 800 ps, 8 ps and 675 fs pulses for these two coatings, and present a table of the LIDT results we have for all of our TiO 2 /SiO 2 BBHR coatings, showing the trends with test laser pulse duration from the ns to sub-ps regimes.

We compare designs and laser-induced damage thresholds (LIDTs) of hafnia/silica antireflection (AR) coatings for 1054 nm or dual 527 nm/1054 nm wavelengths and 0° to 45° angles of incidence (AOIs). For a 527 nm/1054 nm, 0° AOI AR coating, LIDTs from three runs arbitrarily selected over three years are ∼20 J/cm2 or higher at 1054 nm and <10 J/cm2 at 527 nm. Calculated optical electric field intensities within the coating show two intensity peaks for 527 nm but not for 1054 nm, correlating with the lower (higher) LIDTs at 527 nm (1054 nm). For 1054 nm AR coatings at 45° and 32° AOIs and S and P polarizations (Spol and Ppol), LIDTs are high for Spol (>35 J/cm2) but not as high for Ppol (>30 J/cm2 at 32° AOI; ∼15 J/cm2 at 45° AOI). Field intensities show that Ppol discontinuities at media interfaces correlate with the lower Ppol LIDTs at these AOIs. For Side 1 and Side 2 dual 527 nm/1054 nm AR coatings of a diagnostic beam splitter at 22.5° AOI, Spol and Ppol LIDTs (>10 J/cm2 at 527 nm; >35 J/cm2 at 1054 nm) are consistent with Spol and Ppol intensity behaviors.