UV/VIS AND UV/VIS/NIR SPECTROSCOPY APPLICATION NOTE

Polarizer/Depolarizer Options for the LAMBDA 650 and 850 UV/Vis Spectrophotometers and LAMBDA 950 UV/Vis/NIR Spectrophotometer

Nature of polarized light perpendicular to the wave’s Key Benefits direction of travel (k). In un- Natural light is a transverse Common beam depolarizer corrects electromagnetic wave with polarized light the electric field for instrument bias vectors vibrate in all directions of electric (E) and magnetic (H) -depolarizer drive allows the EH plane equally (Fig.2). Note fields perpendicular to each other analysis of oriented samples using (Fig. 1). The two fields are also that all the electric field vectors polarized light are of the same length. When the Reflectance and transmittance studies cater for a wide range E of materials Large sample compartment accommodates polarizer-depolarizer k and reflectance accessories

H

Figure 1. The field vectors of light. Figure 2. Electric field vectors of un-polarized light. www.perkinelmer.com beam axis. Typical measurements which may be affected include the measurement of single crystals, liquid crystals or stretched poly- mers. Use of a depolarizer is recommended for all types of transmission and reflectance meas- urements involving an angle of incidence which is greater than Figure 3. Electric field vectors in partially Figure 4. Electric field vectors in totally around 100 from the normal. polarized light. polarized light. distribution of the electric field Polarized light within Depolarization of radiation strength becomes non-uniform, a spectrophotometer The most practical way to control the light is defined as polarized. Radiation emitted by a spectro- the effects of instrument polariza- A partially polarized state, with photometer’s light source is by tion is to depolarize the instrument asymmetric electric vectors, is shown nature un-polarized. However, this beam. Here an optical device, called in Fig.3. The most extreme case light will become partially polarized a depolarizer is mounted in the occurs if the light consists of only by the spectrophotometer’s optical spectrophotometer’s light beam and one direction of polarization (Fig. 4). components before it interacts generates depolarized radiation by with the sample. The primary scrambling. The depolarizers used Polarizing materials causes of the polarization are: in the LAMBDA™ 650/850/950 range are of the Hanle type which Many polarizing materials produce • The consists of two wedges of different their polarized light by a process • The polarization contribution materials fastened together. The first of double refraction called birefrin- of mirror reflections wedge is made of double refracting gence. A birefringent material • The diffraction grating slits natural quartz and the second, which divides an entering beam of mono- corrects the direction of the beam, is chromatic radiation into two beams In opposition to these factors, if the manufactured from Suprasil quartz. having opposite and orthogonal light is scattered by dust particles polarizations. Polaroid film sheets or irregularities on optical surfaces produce polarized light from a it will become randomly polarized Depolarization options (which means a depolarization of specialized form of Two options are available: called dichroism, which is the the already polarized light). selective absorption of one plane • Common beam depolarization of polarization in preference to the Polarization effects other orthogonal polarization as on samples Here the depolarizer is located light is transmitted through them. inside the spectrophotometer just When samples such as liquids or Dichroic are made of before the chopper assembly in amorphous solids do not them- birefringent materials, often orient- which the beam is split into sample selves polarize light, the inherent ed plastics, in which one of the two and reference beam. The major polarization bias of the spectro- orthogonal polarizations is subject advantage of this approach is that photometer does not affect the data. to strong absorption, while the other only a single depolarizer is needed. However, if the sample polarizes is not. The preferential absorption A small amount of polarization light, numerous errors can occur. of one polarization is achieved by may be added after the depolarizer The nature and magnitude of the incorporating pigment molecules by the mirror reflections associated errors will be unique to the optical into the polymeric substrate of the with chopper and transfer optics design of the spectrophotometer polarizer material. These molecules which direct the beam back into the and the polarization characteristics selectively attach themselves to the sample compartments. The efficiency of the sample. An example of a aligned polymer molecules, result- of the depolarization is greater than common error is the variation of ing in very high absorption in one 92%, which is perfectly acceptable transmission values when a polariz- plane and very weak absorption in for most analyses. ing sample is rotated about the the other.

2 • Polarizer/depolarizer drive accessory Anti-reflectance coating on glass For more demanding analyses, 45 degree angle of incidence relative specular a depolarization efficiency of P/Random/S Polarization greater than 98% can be achieved by polarizing the beam just before the sample. Here the polarizer/ depolarizer drive accessory is used (Fig. 5). This mounts directly in the standard sample compartment of the LAMBDA™ 650/850/950 and can be configured with separate polarizers or depolarizers for sample and reference beams. The accessory is fully automated and is recognized by the software when installed.

Figure 6. Polarization spectra of an anti-reflection coating on glass. Polarization measurements Another important application of the polarizer/depolarizer drive is to the E field vector is parallel to the mission and reflectance measure- analyze samples using a specific plane of incidence of the sample, ments can be made. For materials polarization of incident radiation. the P vibrations are measured. like stretched polymers and material This is achieved by inserting a Rotation of the polarizer through 900 under magnetic or electrical fields, polarizer into the polarizer/ then allows the S vibrations perpen- the orientation of chromophores or depolarizer drive in the sample dicular to the plane of incidence to the conformation of molecules can compartment directly before the be measured. be deduced. For single crystals, sample, in combination with information about crystal symmetry As the sample compartment is large common beam depolarization. can be collected. enough to accommodate the polariz- For reflectance measurements, er drive in combination with a if the polarizer is orientated so that reflectance accessory, both trans- Applications

• Characterization of an anti- reflection coating on glass An anti-reflection coating on glass was measured using an absolute reflectance accessory at 450. Optical interference coatings respond differently to S and P polarized light and must be analyzed under different polarization conditions to fully characterize performance. Fig. 6 shows spectra from P (red), random (blue) and S (green) polar- izations. In each case the common beam depolarizer was used to remove the instrument polarization and a film polarizer and a depolar- izer were used in the drive unit.

Figure 5. LAMBDA 800/900 Polarizer/Depolarizer Drive Accessory (B050-5530).

www.perkinelmer.com 3 • Characterization of an anti-reflection coating on a laser window

An anti-reflection coating applied to Anti-reflectance coating on glass a window used in a ND-YAG laser P/S Polarization was characterized using a LAMBDA™ 950 and 450 relative reflectance accessory with a NIST SRM 2003 reference mirror. The performance depends on the polarization of the incident light. Measurements were made using a common beam depolarizer in combination with a Glan-Thompson polarizer in the polarizer drive. Spectra are shown from P (pink) and S (red) polariza- tions (Fig. 7). The specification of Figure 7. Polarization spectra of an anti-reflection coating on glass. the mirror is <0.5% reflectance at 1064 nm, the laser wavelength, using P-polarized radiation.

Specifications for the LAMBDA 650/850/950 depolarizer and polarizer accessories

Depolarizer/Polarizer Film Polarizer Glan-Thompson Glan-Taylor Depolarizer CB-Depolarizer

Wavelength Range 400 to 700 nm 300 to 2600 nm 210 to 1100 nm 185 to 2600 nm 185 to 2600 nm

Efficiency –– –> 98% > 92%

Part Number B220-5022 B050-5284 N101-0520 B220-5021 B050-1282

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