High-speed THz spectroscopic imaging at ten kilohertz pixel rate with amplitude and phase contrast

By combining the advantages of the high-speed ASOPS technology and efficient THz generation, we have realized a high-speed laser-based spectroscopic THz imaging system with more than 10,000 pixels per second acquisition speed and an excellent signal-tonoise ratio of more than 100. Unlike THz line cameras or mm-wave intensity detectors, the present device allows for a much higher spatial resolution and attributes each imaging pixel with phase and amplitude information up to several THz while simultaneously maintaining a very high scanning speed unmatched by any other technique presented so far. The high-speed acquisition allows for samples to be scanned even at sample velocities of 5 m/s or higher while preserving the fundamental resolution limit of the THz radiation, which is on the order of 500 μm in the present case. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
A multitude of different Terahertz spectroscopy techniques have been well established in the past two decades due to inherently interesting properties of the low THz frequency range for applications in fundamental sciences, biological and pharmaceutical industry or security [1][2][3][4][5].There are numerous advantages when employing THz radiation such as a strong dependence on water content, and the possibility for substance identification since many gases, molecules as well as solid state samples exhibit a unique fingerprint in this frequency range.Particularly interesting are applications where THz radiation is used for imaging since many materials are transparent for THz frequencies such as plastics or paper, in contrast to near-infrared or visible light [4].Also, THz radiation gives much better spatial resolution compared to microwave-based imaging systems due to the shorter wavelengths employed.
When designing THz imaging devices, the most crucial points are imaging speed, signalto-noise ratio as well as the type of information that can be interpreted at each pixel such as intensity or spectral amplitude and phase information.The fastest THz devices nowadays are built in the 100 GHz range since the rapid development in thermoelectric cameras enables direct bolometric measurements of CW sources [6] where an image can be formed through beam bending [7] or even 2D cameras for real time imaging [8] so that fast imaging devices can be built.Next generation THz imaging devices are suggested at even higher THz frequencies at specific wavelengths [3].However, in these measurements light is captured using thermoelectric cameras or an array of microwave antennas so that only the intensity contrast information can be gathered.In addition, the resolution is limited by the limited pixel matrix size of the detection camera and the limited number of antennas in an array or ultimately by the large wavelength of the radiation employed.Two-dimensional spectroscopic imaging has been demonstrated using electro-optic sampling and a CCD camera, however, these systems typically require amplified lasers with pulse energies in excess of 1µJ [9].
A common method for THz imaging is based on terahertz time domain spectroscopy.Here THz radiation is generated and detected using ultrafast lasers [2,4,5], permitting a powerful tool that exhibits typically a broad bandwidth up to several THz such that spectroscopic imaging is feasible, as well as amplitude and phase information.At each pixel these devices may gather the full spectral information of a sample with excellent signal-tonoise and high spectral resolution.However, due to the pixel-by-pixel scan technique employed and a mechanical delay line, imaging speed is usually very poor, of the order of 10 pixels per second or less, such that a final image can be acquired only after minutes or even many hours [1,2,5].THz spectroscopy based on asynchronous optical sampling (ASOPS) overcomes the abovementioned obstacles and provides a unique combination of extreme high data acquisition rates as well as allowing access to amplitude and phase information at the same time for low noise, high precision THz spectroscopy with GHz frequency resolution [10].Here, the very fast data acquisition rates can be achieved since there is no need for a mechanical delay between the two lasers that is normally required to sample the THz radiation.As is shown here, the presented THz imaging device based on ASOPS is orders of magnitudes faster and more powerful in terms of bandwidth and SNR compared to existing laser-based THz imaging devices [7,8] without the use of THz gratings and calibration of cameras.

High-speed ASOPS THz spectrometer
The ASOPS based THz spectroscopy system employed here is based on two high repetition rate lasers running at f R = 1 GHz repetition rate (models taccor power from Laser Quantum).The lasers are locked together in a master-slave configuration with a slight Δf R offset between them that can be as high as 20 kHz while maintaining an excellent time resolution [10,11].The combination of high scan rates and good time resolution can only be achieved with such high repetition rates and is thus a unique feature of the 1 GHz lasers compared to 80 MHz systems where the same physical performance is achieved with scan rates as low as 100 Hz.When Δf R = 10 kHz as in the present case, a 1 ns THz waveform can be acquired within only 100 µs of integration time.Figure 1 shows an example measurement of a THz transient and the corresponding Fourier transform.Even without averaging and at these short acquisition times, an excellent signal to noise ratio of more than 100 in the time domain and more than 30 dB in the frequency domain is achieved.Even larger signal to noise (SNR) values can be achieved when using thicker electro-optic detection crystals and when the setup is enclosed and purged with e.g.nitrogen.Note, that the SNR in the frequency domain is usually expressed in terms of spectral power while the time domain is typically related to amplitude information.shown represe e contrast of th asurement bu the data, as alculations w intensity cont a sample sh although the The spectrosc e detail in [14].

Summary
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