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Fused silica and natural z-cut Quartz optics

Fused silica and natural z-cut Quartz optics

Natural z-cut quartz and fused silica are optical materials that are colourless and transparent to visible light. Both are nominally chemically identical, both being silicon dioxde (SiO2), also known as silica. They are both reasonably tough materials and have good chemical resistance, with a very important exception. The notorious hydrofluoric acid will react with and possibly destroy both materials. You may have been told that hydrofluoric acid is such a strong acid that if you store it in a glass bottle, it will eat its way through. Firstly, hydrofluoric acid is a weak acid in that it is not fully dissociated in solution. Secondly, the reaction with the glass is not the reaction of an acid, it is the fluoride ion reacting. The glass might well be destroyed but not because of the acid properties of hydrofluoric acid.

 

Fused silica is also known as fused quartz, silica glass or quartz glass. Quartz might be called crystal quartz or just quartz, but be wary: it is not unusual for people to rather unhelpfully refer to fused silica as quartz. So what difference does it make if they are so similar, I hear you ask? Well, they are not all that similar.

The big difference is that fused silica is a glass, whereas quartz is crystalline. A glass has a disorderly structure at the molecular level, a crystal has an orderly, repeating structure at the molecular level. Imagine you had a bucket of loose Lego® bricks (other bricks are available). The loose bricks are disordered but free to move and take the shape of the bucket they are in: this is analogous to a liquid state. Let’s say I sit and assemble the bricks neatly together in an orderly, repeating pattern. This is analogous to a solid single crystal, like natural z-cut quartz. Let’s say instead of assembling them, I just stick the bucket of loose bricks in a hot oven for 10 minutes (or until golden brown) so that the bricks fuse together, as they lay, into a solid lump. This is now analogous to a glass material, like fused silica. At the risk of overstretching the analogy, I could assemble the bricks into smaller, but loose, orderly lumps and chuck these lumps into the bucket then into the oven. When the lumps are fused together (but still lumps) we have a solid polycrystal.

Glassy materials have a different temperature response than crystalline materials. Quartz has a distinct melting point (around 1710 °C, depending on where you look). Fused silica, on the other hand, does not have a distinct melting point. It gets progressively softer with temperature. Its official softening point is something like 1600 °C, but this varies with the grade of material. Softening point is defined by when the material achieves a defined viscosity which, in practice, would manifest itself as a certain degree of rubberiness. Fused silica has a very low coefficient of thermal expansion, so is fairly robust to changes in temperature that would put other materials at risk of shattering. It can see working temperatures up to around 1000 °C. In practice, a viewport with a fused silica window could not operate at this temperature due to some of the other materials (e.g. braze). Quartz is different. Despite having a melting point well above 1000 °C, it is best kept below 500 °C because somewhere around 573 °C it undergoes a phase change in which the crystal structure rearranges itself, causing a slight change in physical size. This change can lead to catastrophic failure from the internal stresses induced.

Synthetic quartz is usually manufactured by a hydrothermal process in which the quartz is precipitated from a circulating supersaturated solution. Before this process was fully developed, the main source of quartz was naturally occurring, mined material. This was probably the source of the description ‘natural’. Mined quartz is still available, but its quality and composition are less controlled than synthetic material. In the modern context, ‘natural’ does not necessarily mean the material was mined, just that it is single crystal quartz, like naturally occurring material, and that it is pure and undoped. Fused silica can be grown from gas precursors in a CVD (chemical vapour deposition) or related process. It can also be produced by melting high purity quartz, with a controlled cooling regime that maintains the glassy structure by not allowing the quartz to recrystallize.

Figure 1: Transmittance curves for z-cut crystal quartz and three different grades of fused silica

Both fused silica and quartz transmit visible light well from the UV through visible and up to near-IR. Fused silica, particularly standard and UV grades, tends to retain some impurities from the manufacturing process. The presence of -OH (hydroxyl) groups can lead to broad absorption peaks between wavelengths 2000 nm and 3000 nm (Figure 1). These are commonly referred to as water peaks. Some IR grades of fused silica are relatively free of these impurities and the absorption peaks are all but eliminated. In both fused silica and quartz, there is little transparency beyond about 5 µm, but quartz regains its transparency at around 50 µm and so is often used for infrared spectroscopy and THz (terahertz, with wavelengths up to 1 mm) applications. The latter includes scanners at airports that can detect concealed weapons under clothes (Figure 2). Fused silica does not become transparent again until around 100 µm. Fused silica finds widespread use in general purpose, high-quality optical components for UV through visible and near- to mid-IR. Quartz has good resistance to laser damage and so is useful for high-power, short wavelength lasers.

Quartz exhibits birefringence: its optical properties vary depending on the polarization direction of the light that enters the material. This is because the crystal structure lacks certain symmetries. Unpolarized light incident on quartz may be split into two beams on entering the bulk, transmitting a double image. In some optical applications, birefringence is useful. Waveplates are an example: they are used to alter the polarization state of transmitted light by retarding one component of polarization relative to the other. In many applications, birefringence is not welcome. A birefringent crystal always has one particular direction, known as the optical axis. If an optic is cut so that the faces are perpendicular to this axis (and to an arbitrary z-axis), light travelling in the same direction as the axis is not affected by birefringence. Such optics are said to be z-cut, as in z-cut natural quartz. Light entering z-cut natural quartz at normal incidence will not be split into two beams and no light of any polarization is retarded any more or less than, so there is no double image. Fused silica is not birefringent, as it has no regular crystal structure.

TSL manufactures viewports in clean facilities at its factory in Bexhill, near Hastings, East Sussex. The viewports are clean to UHV standards when shipping. We have seen that fused silica windows are rugged and ideal for precision optics for UV, visible and near-IR applications. They are tolerant to temperature changes and, being glassy, are not intrinsically birefringent. Fused silica viewports make excellent general-purpose windows. Different grades of fused silica can be supplied to enhance transmittance in the UV or near IR or to remove the broad ‘water absorbance peaks’ between 2000 nm and 3000 nm. Natural z‑cut quartz is crystalline and does have intrinsic birefringence, but being cut on the z-plane means that light at normal incidence is unaffected. Z-cut quartz is resistant to high-power laser damage. It has transparency between 50 µm and 100 µm where fused silica does not. Both materials have good chemical resistance except to hydrofluoric acid.