Optical transmission of polaroid sheet polarizers and neutral-density filters

Polarizing film sheets¹ and neutral-density (ND) filters² are two of the most commonly used optical components for applications in spectroscopy, imaging, photonics, and other fields of optics. Polarizing film sheets are used to produce polarized light as well as to create a visible light gate using parallel and perpendicular polarizing orientations, while ND filters are applied to reduce the intensity of visible light (400–700 nm). These optical components are commercially available for applications within the visible range (380–750 nm) and near-infrared (NIR) range (800–2500 nm).

The light transmission spectra of the polarizing film sheets are only active as cross polarizers within the visible range, but don’t have much effect on transmission within the NIR regime. This makes the plastic polarizing sheets useful for creating a collinear optical Kerr gate (see Fig. 1).

Optical Kerr effect

An optical Kerr effect (OKE) is a nonlinear optical phenomenon, in which an intense electric field induces a birefringence effect on an isotropic material and causes the total index of refraction to change.^3-5 A Kerr gate is based on two cross polarizers—±45 degrees (see magenta dotted boxes in Fig. 1)—with a sample in the middle (carbon disulfide), which opens the gate due to phase retardation.

The Kerr effect of a material is studied by a pump-probe technique, where the changes on the probe due to the pump are measured as a time delay that determines the response-time mechanisms of a material. The pump at 1034 nm (see Fig. 1, red arrow) and the probe at 517 nm (green arrow)³ eventually travel collinearly through the Kerr gate.

The collinear system is possible because the cross polarizers don’t affect the transmission of the IR regime (1034 nm), but do affect transmission of the 517 nm (green) light. A green light signal is observed across the crossed polarizers only when a phase change is produced (when the light changes from linear to elliptical polarization).

In the past, building a Kerr gate was challenging because the pump needed to pass through the sample at a specific angle. Circumventing this problem enables the assembly of a collinear gate.⁴ Plastic polarizer sheets can also be used for several key optical regimes within the shortwave-infrared (SWIR) range (1000–2500 nm) to improve deep imaging for the application of water-like media.^{5, 6}

ND filters

The light transmission spectra of different ND filters such as ND 70, ND 50, and ND 13 were measured from 200 to 2500 nm. Figure 1 shows the spectra of the measurements obtained from the ND filters (blue dotted box) and highlights the usefulness of optical ND filters within the NIR range. It also shows ND filters in the OKE setup on the pump arm (1034 nm). Different ND filters on the pump can influence the change of the total index of refraction because it’s proportional to the intensity of the external field multiplied by the nonlinear index of the studied material.⁷

The transmission of the polarizing sheets and ND filters is important for their use within certain experimental setups. One great example is using these optical components for a collinear optical Kerr gate. In general, a single ND filter for both the visible and NIR range will be cost-efficient and more versatile for applications and experiments. Studying the light transmission of ND filters and polarizing film sheets is helpful to determine their applications within different electromagnetic spectrum ranges.

Setup and results

For our work, we use a Cary 5000 spectrophotometer to measure the transmission of the ND filters and polarizing film sheets in parallel and perpendicular orientations within the ultraviolet (UV) range (100–400 nm), the visible range (400–700 nm), the NIR range (750–1000 nm), and the SWIR range (1000–2500 nm). And we calibrate the spectrophotometer by measuring the percent transmittance (%T) from 200 to 2500 nm without a sample, and set the measured spectrum as our baseline. The respective ND filter or polarizer are carefully placed and analyzed by the spectrophotometer.

The transmission of polarizing sheet and ND filters from 200 to 2500 nm are shown in Figures 2 and 3. Photographs of polarizing film sheets in both parallel and perpendicular are shown with background letters to highlight the light transmission, and photographs of common ND filters (ND 70, ND 50, and ND 13) are also shown in Figure 3.

FIGURE 2. Transmittance of ND filters from 200 to 2500 nm.

FIGURE 3. Transmission of crossed, parallel, and single polaroid polarizers from 200 to 2500 nm.

In Figure 2, we show the %T of a single polarizing film sheet, a pair of film sheets in parallel orientation, and a pair of film sheets in perpendicular orientation from the UV range to the NIR range. The salient feature is the ineffectiveness of the film sheets in perpendicular orientation (crossed polarizers) to block light transmission beyond 750 nm.

Using a 1034-nm laser pulse, the polarizer acts as a piece of glass that does not affect the degree of polarization. The polarization of the 1034-nm beam won’t change, but the polarization of a laser pulse within the visible range will. This property is ideal for a collinear optical Kerr gate using a wavelength from the NIR range as the pump beam and a wavelength from the visible range as the probe beam. A diagram of a collinear Kerr gate is shown in Figure 1.^3-5

Within the NIR range, the ND filters decrease in transmission. Their transmission is lower, but can still be used at about half their ND value. The %T of all ND filters changes in a similar way with respect to different optical ranges. Within the UV range, there is no transmittance for any ND filters. Within the visible range, they each seem to be very close to their designated 70%, 50%, and 13% transmittance for ND 70, ND 50, and ND 13, respectively. This means that they operate properly within the visible range.

The %T drops significantly and similarly for all ND filters within the NIR range. The ND 13 filter drops to a minimum of about 7% transmittance, while the ND 50 drops to a minimum of approximately 30% transmittance, and the ND 70 drops to a minimum of approximately 50% transmittance. For the SWIR range, the ND filter’s transmittance begins to rise until the very end of the range. The ND 13 rises the most to a maximum of approximately 40% transmittance at 2500 nm. The ND 50 rises to a maximum of 60% transmittance at 2500 nm, which is only an increase of about 10%; and the ND 70 rises to mostly maintain a maximum of approximately 70% transmittance within the SWIR range.

Useful spectral ranges

We measured the effectiveness of the transmission of polaroid sheet polarizers and neutral densities from 200 to 2500 nm to determine their useful spectral range. One of the applications of these optical components is the optical Kerr gate (OKG) system, in which a polarizing sheet within the visible and NIR ranges plays a key role in creating a collinear OKG experimental setup. The ND filters are also important when studying an intensity-dependent optical process, as is the case of the optical Kerr effect.

Polaroid polarizers only operate within the visible region from 400 to 750 nm and don’t alter the polarization of the light beyond 750 nm (NIR range). Quartz-crystal polarizers perform better within the UV, visible, and NIR ranges. Within the NIR range, the optical Kerr gate uses crystal polarizers in which the optical pump and probe beams are noncollinear. The overlap of the pump and probe beams is critical.^3,4,7 Neutral-density filters can be used within the visible and the SWIR range. There is no need to obtain a ND filter solely for the SWIR for the second (1100–1350 nm), third (1600–1870 nm), and fourth (2100–2300 nm) optical windows for water-like media such as fog, clouds, and tissue.^5-6

ACKNOWLEDGEMENT

We thank the EE department, Professor Edward Baurin, and Professor Roger Dorsinville for their support and for having a senior research project course to provide hands-on experience to the undergraduate students at The Grove School of Engineering at CCNY.

REFERENCES

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