cPCA-1000-05-05-800-x
Photoconductive THz antenna for laser excitation wavelength λ ~ 800 nm
cPCA – cross dipole Photo Conductive Antenna
For polarization sensitive THz measurements
Antenna performance
Antenna design
Antenna parameters
Parameter
minimum ratings
standard
maximum ratings
Dark resistance
1.5 MΩ
2.5 MΩ
3.5 MΩ
Voltage (2xVX, 2xVY),symmetrically
20 V
25 V
Optical mean power @ 50 – 100 MHz repetition rate
10 mW
15 mW
Optical pulse fluence
250 µJ/cm2
500 µJ/cm2
Attention: The F-number of the optical lens focusing the laser beam onto the antenna gap must be larger then a certain value to avoid too high pulse fluency. This means, that the minimum diameter of the focused beam waist must be about 120 % of the gap distance g. For a Gaussian beam the minimum focus length fmin of the optical lens can be estimated as
with g – gap distance of the antenna
λ - laser wavelength
D – diameter of the laser beam hitting the focusing lens.
For λ= 0.8 µm and g = 5 µm the minimum possible F-number of the lens is fmin/D = 1.9π λ 6.
Antenna applications
Possible applications of crossed dipole antennas are:
- Emission of THz waves with electronically controlled polarization direction
By using the crossed dipole antenna as emitter the polarization of the emitted THz waves is determined by the bias voltage ratio on the crossed dipoles. This allows polarization dependent reflection or transmission measurements on a sample without mechanical rotation of the antennas or the sample.
- Detection of the polarization direction of incoming THz waves
The induced signal voltage ratio on both arms of the crossed dipole receiver antenna can be used to determine the polarization direction of incoming THz waves.
- Functional principle of the crossed dipole antenna
By using the antenna as emitter the two dipoles x and y are driven by separate supply voltages VX and VY. The emitted electric field amplitudes EX and EY add up to the resulting THz electric field amplitude ETHz according to the superposition principle. The tilt angle between the THz field direction and the x-dipole is determined by the supply voltage ratio VY/VX according to
Crossed dipole antenna with separate supply voltages VX and VY on the single dipoles x and y. The resulting THz electric field ETHz is the superposition of the filed components EX and EY from both dipoles. The tilt angle φ between the THz field and the x-dipole direction is determined by the voltage ratio VY/VX according to equation (1).
Important: The supply voltages are symmetrical with respect to the ground potential located at the illuminated crossed dipole center.
The graph below shows the connection between the voltage ratio VY/VX and the tilt angle φ according to equation (1). If the same voltage is applied on both dipoles then the tilt angle is φ = 45 °.
Receiver antenna
If the antenna is used as polarization sensitive detector the incoming THz field ETHz induces the detector voltages VY and VX in the two dipoles. These voltages are also symmetrically with respect to the illuminated antenna gap ground point. Therefore two amplifier circuits with a symmetric input must be used for signal voltage detection. The tilt angle φ of the THz field against the x-dipole direction can be calculated also with equation (1).
Order information
cPCA-1000-5-15-800-x Photoconductive antenna
length l = 1000 µm
gap g = 5 µm
width w = 5 µm
laser wavelength l = 800 nm
x denotes the type of mounting as follows:
x = 0 unmounted chip 4 mm x 4 mm with 4 bond contact pads
x = h mounted on an Al disc with 25.4 mm Æ and hyperhemispherical siliconsubstrate lens, 1m four wire cable
x = a mounted on an Al disc with 25.4 mm Æ and aspheric focusing silicon substratelens, 1m four wire cable
x = c mounted on an Al disc with 25.4 mm Æ and aspheric collimating siliconsubstrate lens CL-20 for 20 mm THz beam diameter, 1m four wire cable
x = c-f fiber coupled antenna with collimating silicon substrate lens
x = l with aspheric focusing optical lens for free space laser excitation
描述 [描述] • Antenna length: 2 x 1000 µm<br />• Gap distance g: 5 µm<br />• Dipole width: 5 µm<br />• Laser wavelength λ: ≤ 800 nm