Basic THz Spectroscopy
Basics of THz Time Domain Spectroscopy
In THz-Time Domain Spectroscopy, subpicosecond pulses of THz radiation are measured after propagation through a sample and an identical length of free space. A comparison of the Fourier transforms of these pulse shapes gives the absorption and dispersion of the sample. The THz-TDS setup shown in the figure below.
In brief, an ultrashort laser pulse from a mode-locked laser generates a freely propagating THz pulse by photoexciting the gap between biased coplanar transmission lines on GaAs. The generated THz radiation is collimated by a silicon lens and an off-axis paraboloidal mirror. An identical set of collection optics focuses the THz radiation on a micron scale dipole antenna fabricated on a radiation damaged silicon-on-sapphire chip, inducing a transient bias of the receiver. By photoexcitation of the receiver with a short laser pulse, the electric field of the incident THz pulse may be sampled with subpicosecond resolution.
The high-performance optoelectronic source chip is shown in the figure below. The simple coplanar transmission line structure consists of two 10 μm-wide metal lines separated by 80 μm, fabricated on high-resistivity GaAs, and DC biased at 80V. Irradiating the metal-semiconductor interface (edge) of the positively biased line with focused ultrafast laser pulses produces synchronous bursts of THz radiation. This occurs because each laser pulse creates a large number of photocarriers in a region of extremely high electric field. The consequent acceleration of the carriers generates the burst of radiation. The major fraction of the laser generated burst of THz radiation is emitted into the GaAs substrate in a cone normal to the interface. The THz radiation is then collected and collimated by a crystalline silicon lens attached to the back side of the chip.
The scattered THz beam is detected by a similar combination. The THz receiver consists of a paraboloidal mirror which focuses the beam onto a silicon lens, which in turn focuses it onto the THz receiver chip having the antenna geometry shown in the figure below. The antenna is fabricated on an ion-implanted silicon-on-sapphire (SOS) wafer. The 20 μm-wide antenna structure is located in the middle of a 20 mm-long coplanar transmission line consisting of two parallel 10 μm-wide, 0.5 mm-thick, 5 V/μm, aluminum lines separated from each other by distance of 5 to 200 μm. The electric field of the focused incoming THz radiation induces a transient bias voltage across the 5 μm gap between the two arms of this receiving antenna, which are directly connected to a low-noise current amplifier. The amplitude and time dependence of this transient voltage is obtained by measuring the collected charge (average current) versus the time delay (determined by a computer controlled stepping motor) between the THz pulses and the delayed laser pulses in the detection beam. These laser pulses synchronously gate the receiver, by driving the photoconductive switch defined by the 5 μm antenna gap. The detection process with gated integration can be considered as a sub-picosecond boxcar integrator. The response of the system is determined to a large extent by the geometry of the detector dipole antenna, with the 5 μm dipole having a response to beyond 5 THz while the 200 μm dipole allows one to measure frequencies below 50 GHz. The source-detector combination has polarization sensitivity of greater than 25:1 along the orientation of the dipole.
The THz system generates a highly directional beam of THz radiation with an average power of 10 nW with signal-to-noise ratios of approximately 10,000:1. The demonstrated detection limit is 10-16 W. Because the generation and detection of the THz (far-infrared) radiation is coherent, the THz receiver is intrinsically 1000 times more sensitive than an incoherent, liquid helium cooled bolometer. Thus this system has an extremely high dynamic range of over 10,000 in amplitude or 100,000,000 in power. This is shown in the figure below where a single data scan of a THz pulse is shown in the upper figure. The same pulse with the vertical scale expanded 1000 times is shown in the figure below. The RMS noise level for this signal is on the order of 0.1 pA.
To determine the characteristics of a sample in the THz frequency region we measure two data scans- one with the sample in place and a second reference scan with no sample. The time domain data is converted to the frequency domain by a numerical Fourier transform on a computer. By dividing the sample spectrum by the reference spectrum we are able to remove the system response of the experiment. This ratio gives the change in amplitude and phase of the THz beam caused by the sample from which we can determine the absorption coefficient and index of refraction. This process is shown in "flowchart" form in the figure below.
The most serious limitation to the accuracy of our measurements involves the relatively long term changes in the laser pulses and the consequent changes in the input THz pulses. Due to slow fluctuations during the time it takes to acquire the sample and reference data the amplitude spectral ratio varies by ±5% over the THz frequency spectrum. In the same manner, the relative phase of subsequent reference pulses varies by ±0.05 radians over the same spectrum.
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