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Controlling terahertz radiation is essential in many areas of terahertz photonics. This is primarily achieved using parabolic mirrors and spherical lenses. However, the latter can be replaced with f-theta lenses for focusing radiation in terahertz imaging and scanning systems, as f-theta lenses provide a flat focal surface and nearly constant spot size across the entire scanning field.
Figure 1.THz Aspherical Flat-Field Focusing Lens
Such a lens is designed to provide a linear beam displacement dependent on the deflection angle, thereby generating a constant scanning speed on a plane. The F-theta lens also offers a constant broadband terahertz phase and high image resolution on the scanning plane. The image formed by the f-theta lens has barrel distortion, and the position of the light spot on the image plane is proportional to the scanning angle. This feature eliminates the need for complex electronic settings and allows for the assembly of fast, relatively inexpensive and compact scanning systems.
Below is the parameter of a non-spherical f- theta lens example.
| Material | COC TOPAS(Cyclic Olefin Co-polymer TOPAS) |
| Diameter,mm | 100 |
| Thickness,mm | 40.3 |
| Operating Wavelength Range,μm | 300-3000 |
| Focal Length,mm | 90 |
Figure 2. Spatial distribution of the electric field intensity of the focused terahertz beam by the f-θ lens
Figure 3. Transmission spectrum of a 40 millimeter thick f-θ lens along the axis
Figure 4 shows the relationship between the transverse coordinate of the maximum value of terahertz radiation at the focal point of the lens and the deflection angle (the incident angle of the terahertz beam on the f-θ lens). From the figure, it can be seen that within the range of -25 degrees to +25 degrees of the angle, this dependency is linear, which allows for a constant-speed scan of the plane.
Figure 4. Linear characteristics of the f-θ lens
The terahertz aspherical f-θ lens is manufactured according to the customer's specifications. For the calculation, the diameter, wavelength, focal length, scanning angle, scanning field and input beam size need to be specified.
Effective parameters:
| Material | COC TOPAS(Cyclic Olefin Co-polymer TOPAS) |
| Diameter,mm | 25.4-200 |
| Thickness,mm | Up to 200 |
| Operating wavelength range,μm | 300-3000 |
| Focal Length,mm | ≤200 |