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Measuring the refractive index of thin transparent films using an extended cavity diode laser
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Image of Fig. 1.
Fig. 1.

Simplified experimental setup. Shown is the diode laser, which, together with the diffraction grating, forms the extended cavity diode laser (ECDL). The beam propagates through a vapor cell in pump-probe configuration and is recorded on a photodiode. The slide or liquid cell is inserted in the extended cavity. The angle that the text refers to cannot be seen in this top view because the angle is measured with respect to the vertical.

Image of Fig. 2.
Fig. 2.

Absorption spectrum of rubidium. Shown is the absorption as a function of frequency for the 85Rb isotope. For comparison, we show the Doppler-broadened absorption profile (gray) and the saturated absorption spectrum (black). During the experiment, we use the saturated absorption spectrum and track the position of one of the peaks.

Image of Fig. 3.
Fig. 3.

Refraction inside the glass slide. The angles and are related via Snell's Law , where is the index of refraction of the slide. The distance Δ is the vertical displacement of the beam compared to the beam without the slide (dashed line), and .

Image of Fig. 4.
Fig. 4.

The assembled liquid cell. The liquid has been inserted between two cover slips (note the bubbles in the figure). Most of the epoxy is around the edges to ensure that the thickness of the liquid layer is as uniform as possible. Completely sealing the cell also ensures that liquid cannot evaporate during the measurements.

Image of Fig. 5.
Fig. 5.

Repeated refraction in the liquid cell; () is the thickness of the glass (liquid) layer. The angles follow Snell's Law and are described in more detail in the text.

Image of Fig. 6.
Fig. 6.

Raw data displaying the angles for which a half wavelength has been added to the optical path length within the extended laser cavity. Shown are the data for the air-filled liquid cell (squares), the water-filled liquid cell (triangles), and the vegetable-oil-filled liquid cell (circles). The solid lines are a fit to the data using Eq. (3) (see the text for more detail about this fit).

Image of Fig. 7.
Fig. 7.

Refraction inside the glass slide for two different angles of incidence θ and θ + Δ. Shown are the lengths and within the cell as well as the displacements , , and that are described in the text. The circled part of the figure is shown in more detail in Fig. 8 .

Image of Fig. 8.
Fig. 8.

Magnified inset of Fig. 7 showing the exiting beams; this figure is used to derive an expression for . The distances Δ and are the beam displacements.


Generic image for table
Table I.

Measured refractive indices for three different transparent substances. Values with uncertainties are our measurements. Also listed in square brackets are the accepted values for 780 nm light. 24


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Measuring the refractive index of thin transparent films using an extended cavity diode laser