PEM APPLICATIONS

he photoelastic modulators (PEMs) are polarization modulation devices. The PEM is typically used as the key component for generating modulated polarization states of light in an integrated instrument. PEMs have several unique features such as wide spectral range, large aperture, wide acceptance angle, and high precision phase modulation. The most important reason for using the PEM is to improve the sensitivity of a measurement. Here is a summary of some basic experimental set-ups for selected applications. BY ABSORPTION Circular Dichroism (CD)
Linear Dichroism (LD)
Magnetic Circular Dichroism (MCD)
Vibrational Circular Dichroism (VCD)
Vibrational Linear Dichroism (VLD)
Ellipsometry Polarization Modulation IR Reflection-Absorption Spectroscopy (PM-IRRAS) Faraday Rotation
Intensity Modulation/Chopping
Low-level Linear Birefringence
Optical Rotation
Stokes Polarimetry/Polarization Analyzer
Wave-plate Measurement

BY REFLECTION

BY TRANSMISSION

APPLICATIONS

OTHERS

There are many other PEM applications that are not described here, such as: Polarization modulation in scattering media Polarization modulation with florescence, emission and luminescence Reflection Difference Spectroscopy Magneto-optical Kerr Effect Mueller Polarimetry Rheology Radiometry Interferometry Please feel free to contact our applications scientists for a discussion of your particular PEM applications. You may also request technical literature regarding the PEM and its applications. We at Hinds are eager to learn from you and to help you with your applications.

FARADAY ROTATION
Faraday rotation is the rotation of the polarization plane when light passes through a sample with the existence of an external longitudinal magnetic field. The rotation of the polarization plane is caused by the magnetic field induced discrepancy between the refractive indices for right and left circularly polarized light inside the sample. Therefore, Faraday effect is universal and it exists in all substances.

TRANSMISSION

The Faraday rotation can be measured using the same

principle and set-ups for optical rotation. Both Faraday rotation and

optical rotation is a measurement of rotated degree of polarization plane due to the discrepancy between the refractive indices for right and left circularly polarized light. Natural optical rotation arises from the inherent chirality of the molecules of a sample, while Faraday rotation is caused by a longitudinal external magnetic field.

INTENSITY MODULATION/ CHOPPING

The PEM modulates light polarization. When a PEM is used between two crossed polarizers, the set-up can modulate the intensity of a light beam (chopping). This intensity modulation is at a fixed frequency between 20 and 200 kHz. SETUP

Scope

TRANSMISSION

Light Source

Detector

45 Polarizer

0 PEM

- 45 Polarizer Detector

OBTAINED WAVEFORMS

Chopping frequency: PEMs 2nd harmonic (2f)

1 /2

0.5

/4 0 0 0.5 PEM cycles 1 1.5

APPLICATIONS

chopping at tens of kHz without moving mechanical parts, vacuum UV to mid-IR I/FS50 for UV-Vis and near IR; II/ZS50 for mid-IR

TYPICAL PEMs FURTHER READING

T. C. Oakberg and B. Wang, Application noteLight Intensity Modulation Using A PEM, Hinds Instruments, Inc. (1995).

LOW-LEVEL LINEAR BIREFRINGENCE

The residual linear birefringence in an optical component affects its quality, especially when used in polarization related instruments. The PEM can be applied to the measurement of linear birefringence Light of transparent optical materials Source in several different ways. ONE EXAMPLE OF THE COMMON SETUPS
45 Polarizer 0 PEM

TRANSMISSION

EQUATION

B=sin-1

1 -J0(A0) 2 J1(A0)

I1f IDC

Sample

linear birefringence of
a sample
1f signal
I1f: DC signal
IDC: PEMs retardation setting
A0: J0(A0): the 0th order Bessel function
J1(A0): 1st order Bessel function
B:

V PEM If reference

- 45 Polarizer

Computer Detector If signal Lock-in Amp DC

In reality, due to different amplification gains

Low-pass filter being used for the AC and DC signal channels,
a calibration with a known birefringence is normally used.
in optical materials; laser optics; quality control of optics and glasses I/FS50 with NIO-1

APPLICATIONS

TYPICAL PEM FURTHER READING

J. C. Kemp, Basic Laboratory set-up for various measurements possible with the photoelastic modulator, Application note, Hinds Instruments, Inc. (1975) J. Schellman and H. P. Jensen, Optical Spectroscopy of Oriented Molecules, Chem. Rev. 87, 1359-1399 (1987). S. J. Johnson, Simultaneous dichroism and birefringence measurements of sheared colloidal suspension in polymeric liquids," Ph. D thesis, Stanford Univ. (1985). T. C. Oakberg, Measurement of Low-level Strain Birefringence in Optical Elements Using a Photoelastic Modulator, SPIE, 2873, 17-20 (1996).

OPTICAL ROTATION/ CIRCULAR BIREFRINGENCE

If a linearly polarized light passes through a chiral media, for example, a solution of chiral molecules, the polarization plane of the incident light will be rotated. This is called optical rotation (or natural optical rotation in order to distinguish with Faraday rotation in a magnetic field). Optical rotation is also called circular birefringence. A linearly polarized light can be decomposed into two orthogonal components for right and left circularly polarized light. Any phase difference (retardance) between the two circular components produces a rotation of the polarization plane. A COMMON SETUP

TRANSMISSION

PEM If reference V Computer

Lock-in

Light Source 0, /4 Retarder Sample

Detector

45 Polarizer

0 PEM

0 Analyzer Detector

A slight modification to this set-up is to remove the quarter-wave plate and to detect the 2f signal. Optical rotation has been measured using these two setups at Hinds. High sensitivity for optical rotation (0.001 degree) has been achieved.

APPLICATIONS

chiral analysis in chemistry, pharmaceutical and biological industries I/FS50; II/FS42 J. C. Kemp, Basic Laboratory setup for various measurements possible with the photoelastic modulator, Application note, Hinds Instruments, Inc. (1975) T. C. Oakberg, Linear Birefringence and Optical Rotation, Application note, Hinds Instruments, Inc. (1992).

TYPICAL PEMs FURTHER READING

STOKES POLARIMETRY/ POLARIZATION ANALYZER

One method to represent light polarization is to use the Stokes vectors (I, Q, U, and V). A dual PEM system can be used to measure all four Stokes parameters simultaneously, thus to completely characterize light polarization state. SETUP
Light Source PEM 1 control PEM 1, 45

TRANSMISSION

PEM 2 control PEM 2, 0 Analyzer, 22.5 Lock-in II PEM 2 (2f) Detector Lock-in III PEM 1 (1f) Lock-in I PEM 1 (2f) PC

PC

PC

HOW STOKES PARAMETERS ARE MEASURED

I Q U V

total intensity; proportional to the DC signal linear polarization component at 0o or 90o; PEM2s 2f frequency linear polarization component at 45o; PEM1's 2f frequency circular polarization component; PEM1's 1f frequency

The Q, U, and V can be determined simultaneously at a fast data acquisition speed from the outputs of lockins I, II and III, respectively. astronomy; light source characterization; current diagnostic in Tokamak II/FS20&23; II/FS42&47; I/FS50&55 for UV-vis II/ZS37&50 for mid-IR

APPLICATIONS

DUAL PEM SYSTEMS

FURTHER READING

T. C. Oakberg, Application noteStokes Polarimetry, Hinds Instruments, Inc. (1991). J. C. Kemp, G. D. Henson, C. T. Steiner and E. R. Powell, Nature, 326, 270-273 (1987). H. Povel, et al. Applied Optics 33, 4254 (1994). D. Wroblewski and L. L. Lao, Rev. Sci. Instrum. 63, 5140 (1992).

WAVE-PLATE RETARDATION
Wave-plates are important optical component for light polarization
related measurements. The PEM can be employed to determine both
the amplitude and orientation
of the retardation of a wave-plate.
SETUP
Light Source

45 Polarizer

TRANSMISSION

0 PEM

Sample 0 Rotating Platform - 45 Polarizer

V PEM If reference

EQUATION

B=

J1(A0) -tan-1 J2(A0) 2

I2f |I1f|

[
Lock-in Amp 1 PEM 2f reference Lock-in Amp 2

Computer If signal 2f signal

Detector I1f: 1f signal I2f: 2f signal A0: PEMs retardation setting J2(A0): 2nd order Bessel function J1(A0): 1st order Bessel function

In this set-up, either the fast axis or the slow axis of the quarter-wave plate is required to be parallel to the PEMs optical axis. The positive and negative signs of I2f, after calibration, indicate which axis (slow or fast) it is. wave-plate inspection and characterization I/FS50 with NIO-1

APPLICATIONS

TYPICAL PEM FURTHER READING

J. C. Kemp, Basic Laboratory set-up for various measurements possible with the photoelastic modulator, Application note, Hinds Instruments, Inc. (1975) J. Schellman and H. P. Jensen, Optical Spectroscopy of Oriented Molecules, Chem. Rev. 87, 1359-1399 (1987). T. C. Oakberg, Measurement of Wave-plate Retardation Using a Photoelastic Modulator, SPIE, 3121, 19-22 (1997).