THz Photoconductive Antenna for 800 nm Femtosecond Laser Input

Time-domain spectroscopy (TDS) using THz radiation allows for measurements of both the amplitude and the phase of the interrogating radiation, unlike spectroscopy with optical fields, where only the intensity of the field can be directly measured. A wide range of materials, including metals and gases, can be measured using this technique. The THz radiation used in these systems can be both generated and detected by a pair of PCA800 Photoconductive Antennas, allowing for spectroscopic measurements in the range of 0.1 - 3 THz. The system above gives an example of the types of components used and the basic optical layout of a THz TDS system that could be built using a pair of PCA800 THz antennas. For a detailed tutorial on THz TDS, please see Neu and Schmuttenmaer's "Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS)."¹

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The frequency-doubled output of the FSL1550 Femtosecond Fiber Laser can be used as the pulse source for the PCA800 antenna.

THz Radiation Generation
The main input to the THz TDS system, shown at the bottom left of the diagram above, is a femtosecond laser pulse centered around 800 nm. This pulse could come directly from a source, or by frequency-doubling the output of the FSL1550 Femtosecond Fiber Laser using an NLC07 β-BBO crystal. Both lasers have been used in a system like this to provide performance data for a pair of PCA800 antennas.

In the system shown above, the optical input pulse is split into two beams, with the majority (90%) of the light going into generating the THz radiation (labeled the Pump beam in the image above). Because the signal at the receiver antenna will be small, and a lock-in amplifier is recommended, the THz emission signal should be modulated. This can be done either through the use of an optical chopper in the Pump beam, or through modulation of the of the bias voltage on the antenna. The system above uses an optical chopper to modulate the input optical pulse, which will in turn result in a modulated THz emission from the antenna.

The Pump beam is then directed normal to the optical input surface of the antenna and focused down to the active area of the antenna for efficient THz generation. Note that the laser polarization needs to be aligned parallel to the Pol. Axis marking on the input side of the PCA800 antenna. Be careful not to exceed recommended fluence levels (J/cm²) on the antenna.

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This graph shows the THz electric field measured using a frequency-doubled FSL1550 Femtosecond Fiber Laser input and two
PCA800 antennas.

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This graph shows the Fourier Transform of the THz electric field. The blue shaded region of the THz Spectrum graph indicates the range of frequencies over which the PCA800 antenna is recommended for use.

Up to the saturation level of the antenna, more optical power into the antenna will result in more THz power out. The THz output spectrum also depends on the optical input characteristics. For example, the dispersion compensation of the input pulse has been shown to affect both the efficiency of THz generation and the spectral profile of the THz output.² A shorter, transform-limited input pulse will result in THz output radiation with broader spectral content than that resulting from a longer, chirped input pulse, because the temporally delayed frequency components of the chirped pulse lead to destructive interference of the THz radiation and affect its spectral profile.

In addition to the input optical pulse, a voltage is applied across the PCA800 antenna through the integrated coaxial cable. For the system above, a DC voltage is applied to the antenna. A larger applied voltage, within the recommended limits, will result in more THz power emitted from the antenna. Alternatively, if an optical chopper is not used, a modulated voltage can be applied to the antenna to modulate the THz output.

The THz output from the emitter antenna has a divergence angle of 15° relative to the optical axis, which can be collected and collimated or focused using either THz lenses or off-axis parabolic mirrors. Once the radiation has traversed the sample under test, the same types of lenses or mirrors can be used to send the light into the THz receiver antenna. 1:1 imaging from the emitter antenna output to the receiver antenna input is recommended. The THz input beam will be collected by the hyper-hemispherical Si lens on the THz input side of the receiver antenna.

Optical Delay Line and Detection of THz Signal
Both the THz input and the femtosecond laser input pulse are required for detection at the receiver antenna. As shown in the figure above, 10% of the pulse used to generate the THz radiation at the emitter antenna is split off for use with the receiver antenna. This is labeled the Delay beam in the figure above.

For a given optical pulse, the paths are set so the Delay portion of that pulse arrives at the receiver antenna simultaneously with the THz pulse generated by the Pump portion of the optical pulse. In setting this Delay path, be sure to consider the full optical path from the 90/10 beamsplitter to the PCA800 receiver antenna. This includes the HRFZ-Si lenses integrated into the PCA800 antennas, which are 7.1 mm thick with an index of refraction of ~3.41, as well as any additional optical components in the THz portion of the optical train. At the receiver antenna, align the input laser beam onto the active area of the detector antenna using the same conditions as the emitter arm, except the intensity can be much lower, ~10% of that used at the emitter antenna.

The femtosecond optical pulse is significantly shorter in duration than the THz pulse. Therefore, they only overlap in time at the antenna for a short time, given by the optical pulse width. In order to sample the entire THz pulse, the optical pulse delay is scanned, for example through the use of retroreflector mirror mounted on a motorized stage (such as one driven by a stepper, DC servo, or voice coil). Thorlabs also offers integrated solutions for a variable delay of up to 4000 ps through the optical delay line systems.

The signal level on the receiver antenna output BNC cable will be low, so a lock-in amplifier and averaging are recommended. The signal to the optical chopper, or the modulation signal to the emitter antenna, is used for triggering the lock-in amplifier.

An example of THz field data is presented in the graph on the top right. For this experiment, the input to the PCA800 Antenna was the second harmonic generation of an FSL1550 Femtosecond Fiber Laser. The optical input spectrum was centered at 775 nm and dispersion compensated, resulting in a 24 fs pulse duration. The antenna had a 15 V DC bias applied through the BNC connector, and the signal modulation frequency was 4 kHz, driving an optical chopper in the input laser beam. A pair of MPD229-M03 off-axis parabolic mirrors were used to collimate the THz radiation from the PCA800 antenna remitter and refocus it onto the PCA800 antenna receiver. By taking the Fourier Transform of the electric field, the spectrum of the THz radiation can be calculated, as shown in the graph to the right.

This system can be used for time-domain spectroscopy experiments. The optical path length of a sample can be measured by measuring its effect on the electric field signal. Any additional path length inserted into the THz radiation beam path would result in a shift of the electric field signal in time. The spectral properties in the THz regime may also be measured by comparing the THz spectrum with and without a sample inserted into the beam path.

References

1. J. Neu and C. A. Schmuttenmaer, "Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS)," Journal of Applied Physics 124.23 (2018) p. 231101.
2. J. Hamazaki, K. Furusawa, N. Sekine, A. Kasamatsu, and I. Hosako, "Effects of chirp of pump pulses on broadband terahertz pulse spectra generated by optical rectification," Japanese Journal of Applied Physics 55.11 (2016) p. 110305.