CALOMEL (Hg2Cl2) single crystals

A short list of applications of CALOMEL crystals


The polarizers are most developed applications - we have tested the apertures from 5x5 up to 20x20 mm of the Glan-Taylor type. We are able to help in the design of new polarizers for particular applications using CALOMEL. We have also designed special single-piece (monolithic) polarizers, featuring relatively simple operation, high quality and low light losses due to reflection.

CALOMEL Glan-Taylor polarizer CALOMEL Glan-Taylor polarizer CALOMEL Glan-Taylor polarizer
CALOMEL Glan-Taylor polarizer

Used in: optics (e.g., spectroscopy, polarimetry, defectoscopy, astronomy, material research, laser applications, light modulation, agricultural production, electric power generation, environmental control devices, molecular biology, biotechnology), microwave applications and various industrial processes.

Polarized light is a carrier of valuable information. Magnetic fields, chemical interactions, molecular structures, quality variations, mechanical stress all impose changes on the polarized state of an optical beam. In order to assess the value of any one polarizer, it is necessary to assess each type of polarizer as follows:

   - optical transmission range
   - degree of light polarization
   - angular field of view
   - reflection and absorption (light losses)
   - stability with powerful laser beams
   - polarization of IR- and UV-light either separately or with visible light or
   other combinations
   - angle of splitting of the ordinary and extraordinary rays
   - size and cost

Until recently, calcite was the only suitable material for the construction of polarizers in the visible and near infra-red spectral regions. In order to operate in the infra-red and far-infrared region, grid polarizers and specially designed thin film plate polarizers were required. CALOMEL is the only known material which can polarize light continuously from the visible to far infra-red regions. Therefore, one polarizer of CALOMEL can replace minimally two or more polarizers depending on the wavelength of the light beam. In commercial applications CALOMEL crystals should enable manufacturers to produce more powerful, smaller and lighter devices at a reduced cost.

Performance comparison of CALOMEL polarizers for infrared grid polarizers

Substrate material CaF2 KRS-2 CALOMEL AR-coated Ge BaF2
Spectral range (µm) 1-9 2-35 0.38-20 8-14 1-12
Degree of polarization 93% >88% 99.998% >99% 93%
(1-5 µm) (3 µm) (1-5 µm)
98% >99% (10 µm) 98%
(3-9 µm) (10 µm) (3-9 µm)
Power Rating:
- CW-mode 101 101 103-105 101 101
- Pulsed-mode 104 104 >5.107 104 104

In addition, CALOMEL has an exceptionally high index of refraction and degree of birefringence making it possible to produce traditional and also new types of optical devices.

Some Characteristic Features of Calcite versus CALOMEL

calcite CALOMEL
Birefringence value 0.256 to -0.147 +1.068 to +0.546
(acconrding to  ) (0.172 for 589 nm) (+0.683 for 589 nm)
The largest angle of splitting of the
ordinary and extra-ordinary rays

(at 632.8 nm)
22.74° - 14.34°
(at 632.8 nm)
Range of optical transmission 215 nm - 2.3 µm 380 nm - 20 µm
Index of reaction:
- ordinary ray 1.802 - 1.622
1.658 (for 589 nm)
2.122 - 1.898
1.973 (for 589 nm)
- extraordinary ray 1.546 - 1.475
1.486 (for 589 nm)
3.190 - 2.444
2.656 (for 589 nm)

CALOMEL Glan-Taylor polarizers will offer the following advantages:

CALOMEL Glan-Taylor polarizer will be approximately one-half the thickness of a corresponding calcite Glan-Taylor polarizer.

CALOMEL Glan-Taylor polarizer will have a broader spectral region of optical transmission from 0.38 to 20 µm as compared to calcite 0.2 15 to 2.3 µm.

CALOMEL Glan-Taylor polarizer will have a larger angular aperture of 18.36° as compared to 8.5° with calcite, allowing for the polarization of more light as the viewing field is five times broader. In addition, this larger angular aperture eliminates unwanted radiation.

CALOMEL exhibits a positive birefringence value permitting the ordinary ray to be used as the functional ray. Its wave front is spherical, and thus the refractive index does not vary with direction and it is less sensitive to imperfections. This fact diminishes the quality demanded of the crystal while still maintaining the high functional quality of polarizer.

CALOMEL Glan-Taylor polarizers are air-spaced (without immersion) and exhibit high optical load resistance, enabling them to handle large amounts of laser power).\

The degree of polarization is comparable to calcite (99.998%) which is only usable in the visible and near IR spectral regions, however, CALOMEL, also, works in the infra-red region, maintaining this high degree of polarization exceeding the capabilities of grid polarizers which varies from 88% to 99% depending on the type of polarizer.
Therefore, CALOMEL provides greater flexibility over a broader spectral range with a maximal degree of polarization.

Comparison of Glan-Taylor polarization prisms made of calcite and calomel

calcite CALOMEL
Lengh / aperture ratio 0.85 0.488
Full angular field of view 8.5° 18.36°
Wavelength range 230 nm to 2.3 µm 380 nm to 20 µm
Degree of polarisation 99.998% 99.998%

Comparation of basic acousto-optical properties of CALOMEL and tellurium dioxide

Materials TeO2 Hg2Cl2
Number of optical axes 1 1
Refractive indices 2.26(o), 2.41(e) 1.9(o), 2.6(e)
Birefrigence 0.15 0.7
Sound velocity in the [110] direction [m/s 620 347
Possible frequency bandwidth at 100 MHz [MHz] 45-47 20-25
Achievable resolution with 5 mm aperture [points] 370 330
Figure of merit in the [110] direction M2 [x10-18 s3/g] 1 200 800
Damage threshold:
- with single 30ns pulse [MW/cm2] >30 >150
- with 1kHz 100ns pulses [MW/cm2] >4 >4.6
- at high average power [kW/cm2] >1 >1


Due to the relatively low optical damage (occurring at powers higher than 50 MW/cm2) CALOMEL polarizers can be used in combination with high power lasers for laser processing (drilling, cutting, annealing, etc.). Unlike calcite polarizers which operate at low to medium power levels and film plate polarizers which have to be specially designed for each high power laser at every wavelength, CALOMEL has the flexibility to work across-the-board from low to high power. Generally, CALOMEL can be used in infra-red polarization optics i.e., photography, spectroscopy, environmental control devices and other applications.


Microwave, radar, television, signal processing, spectral analysis, robotics, Bragg Deflectors, birefringent filters of Lyot-Type, filters for environmental control, beam splitters, etc.

So far we have tested a wide field of technical applications of calomel units as listed below. The extended spectral range 0.38 to 20 µm gives a very large number of potential highly-promising implementations. We have developed the crystal growing technology and we are looking for partners in order to develop the industrial-scale crystal processing (i.e. cutting and polishing, AR layers, mounting) of this extraordinary matarial.


Due to extreme birefringence in the microwave region, quarter wave plates can be produced which convert the linearly polarized microwave signal into a signal polarized circularly. The advantage of using CALOMEL in microwave communication systems, for example, direct broadcasting from satellites, is that CALOMEL eliminates the requirement of a polarization switch and thereby reduces the number of moving parts.


CALOMEL, as a medium for delay of acoustic waves offers the longest possible time window (up to l50 µsec) of any known material. Such a slow shear acoustic wave can significantly reduce the size of generally used delay lines. Such a property makes CALOMEL ideal for signal processing - correlation, convolution and filtering, noise reduction, etc.


CALOMEL is extremely suitable for the construction of acousto-optic devices. In comparison, CALOMEL's figure of merit is comparable with tellurium dioxide TeO2 and more than one order better than lead molybdenate PbMoO4. The simplest application for CALOMEL would be as an integral light beam modulator. Because of the high value of the figure of merit, M2, the required power consumption is low and diffraction efficiency is high. It follows that due to the low value of shear wave velocity, dimensions can be reduced and the modulation bandwidth is slightly reduced.The transparency of CALOMEL (0.38 to 20 µm) enables the use of these modulators in a broad spectral range (compared with TeO2). The high value of optical damage in the infrared region could be efficiently exploited in industrial laser beam manufacturing (drilling, annealing, etc.). In addition, these properties are valuable in laser surgery, because CALOMEL has a broad band of transparency and is thereby suitable as a visible light beam for infra-red beam pointing. A more sophisticated acousto-optic application would be a spatial light beam modulator - optical processor. This acousto-optic unit is used to convert an electric signal into an optical image which can be processed by optical data processing techniques.

Due to a very slow shear acoustic wave, high resolution (about 5000 pts) and a large time window (about 150 µs) can be achieved. These parameters are excellent comparable to TeO2 and the time window is superior. This type of acousto-optic unit is used for optical spectrum analyzers, Kalman filters, convolvers, correlators, matrix multipliers, etc. Because of high speed parallel optical processing these systems are used in aircraft, space and military applications where real time processing of large amounts of data is fundamental.

The applications listed above are not a complete list. They are only examples. Each application can have many modifications (for example a broad variety of polarizers). Quite new possibilities can be found in CALOMEL homologues (Hg2Br2, Hg2I2) and mixed crystals (Hg2ClBr, Hg2BrI etc.). These single crystals are now in development. The production of CALOMEL on an industrial scale makes it possible to take advantage of CALOMEL's excellent properties and those of its homologues.

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