We have developed and tested a novel application of interferometry to determine the absolute refractive index of individual infrared materials having flat and parallel surfaces without alteration of the sample in any way, and measured no for ZnGeP2

Gillen, Glen

Guha, Shekhar

English

DigitalCommons@CalPoly

                                                                            © 2003 OSA/CLEO 2003 Measurement of the index of refraction of a Flat zinc germanium phosphide wafer in the infrared Glen D. Gillen Air Force Research Laboratory, Materials and Manufacturing Directorate/MLPJ, Anteon Corporation,Building 71-A, Area B,Wright-Patterson Air Force Base, Ohio 45433 Voice: 937 252-3132 ext. 3039, Fax: 937 252-0418, Email: glen.gillen@wpafb.af.mil Shekhar GuhaAir Force Research Laboratory, Materials and Manufacturing Directorate,Wright-Patterson Air Force Base, Ohio 4543 Voice:  937 252-3132 ext. 3022, Fax: 937 252 4018, Email:  shekhar.guha@wpafb.af.mil Abstract: We have developed and tested a novel application of interferometry to determine the absolute refractive index of individual infrared materials having flat and parallel surfaces withoutalteration of the sample in any way, and measured no for ZnGeP2.2000 Optical Society of AmericaOCIS codes: (160.4760) optical properties, (190.4400) nonlinear optics, materials 1. IntroductionIt is important to know the absolute refractive indices of materials for most optical applications, especially for nonlinear light-matter interactions. Currently, the standard method for refractive index measurements of materials isthe minimum deviation method as light propagates through a wedge or a prism. The use of this method presents aunique challenge to the majority of infrared materials, which are manufactured in flat layers. Although refractive index measurements using the wedge/deviation method are highly accurate, they do require the sacrifice of some portion of the material to be precisely, and destructively, ground into that shape. In this talk we present analternative, non-destructive, sample-specific method for determining the refractive index of flat infrared materials using interferometry. 2. MethodThe experimental setup for these refractive index measurements is rather straightforward: the flat sample is placed in the beam path of one arm of a Michelson interferometer on a computer controlled rotation stage. As the sample is rotated, the optical path of that arm of the interferometer will change. If the refractive index of the material is greater than 2, the change in optical distance as a function of the rotation angle is ∆OD(θ) = 2d(1 + (n2 – sin2θ)1/2 – cosθ – n), (1) where n is the refractive index, d is the sample thickness, and θ is the angle of the surface normal with respect to the incident beam path. A small area of the fringe pattern is recorded as a function of θ. The locations of the maximaand minima are extracted from the raw data and fit to∆OD(θ) = m(θ)λ + moλ,  (2) where m is the relative fringe value, and moλ is the initial fringe value for θ=0. 3. Results and DiscussionThe raw data of intensity versus sample angle, measured using a wavelength of 10.611µm obtained from a low power (700mW) CO2 laser, is displayed in Figure 1, for an 8×8×2.94 mm sample of ZnGeP2. Figure 2 shows thelocations of the maxima and minima as a function of the sample angle and fit to equation (2), resulting in a refractive index of the ordinary axis of no = 3.03 ± 0.01, comparable to that predicted by Zelmon et al. of 3.07 [1].                             -1'l1tbl11-4.S-,«I.. ..,j ..o!l ..]~ 1lJ_111-6 Go!i: 10Sample Anllle (degreB9)..2tJ -l(l (I 101 :IllSample AngiE> (di>gree.)"© 2003 OSA/CLEO 2003 Figure 1.  Observed data of the interferometric fringe pattern intensity as a function of the sample angle, for the ZnGeP2 sample. Figure 2. Measured optical distances, and their fit to equation (2).A calibration check was performed with germanium, yielding n = 4.00 ± 0.01, agreeing well with the widely accepted value of 4.003. The major contributor to the error is the uncertainty in measuring the sample thickness of ±10 microns. A more accurate measurement of the sample thickness will improve the resolution of this method to 3 to 4 digits of precision. The advantages of this method are:  (i) the ability to measure thin (∼ 1 mm) samples, (ii) localized refractive index measurements, and (iii) ease of adaptation of the setup for samples at cryogenic temperatures, which is important for many infrared applications. We will present the results of refractive indexmeasurements for a variety of infrared samples for temperatures of 77K and 300K. To the authors’ knowledge this is the first application of interferometry to find the absolute refractive index for LWIR materials. 1.	 Zelmon, D. E., Hanning E. A and P. G. Schunemann, “Refractive-index measurements and Sellmier coefficients for zinc germaniumphosphide from 2 to 9 µm with implications for phase matching in optical frequency-conversion devices”, J. Opt. Soc. Am. B 18, 1307-1310 (2001).

Measurement of the Index of Refraction of a Flat Zinc Germanium Phosphide Wafer in the Infrared

A method to determine the absolute refractive index of materials available in the shape of flat wafers with parallel sides by using interferometric techniques is presented. With this method, nondestructive, sample-specific measurements can be made. The method is tested by using silicon, germanium and zinc selenide, and measurements for both the ordinary and extraordinary axes of ZnGeP2 for temperatures of 300 and 77 K are reported

Gillen, Glen D

Refractive-Index Measurements of Zinc Germanium Diphosphide at 300 and 77 K by Use of a Modified Michelson Interferometer

https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1275&amp;context=phy_fac

Measurement of the Index of Refraction of a Flat Zinc Germanium Phosphide Wafer in the Infrared

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