*3.3.3 The Tab panel*

• The first Tab is shown in **Figure 5** is the "Acquisition" Tab. It is used for timescan, both with electro-optic detection, as in regular THz-TDS, or for scanning in time on a single wavelength in a THYR experiment, or for THz-TDS with alternative techniques such as the already mentioned ABCD technique. There are two graphs, one for the measured signal in time domain and the other which displays the Fast Fourier Transform (FFT) of the signal. In each graph two lines with different colors are displayed. The red one is the measurement on the current scan, and it is updated point-by-point, so that the user can immediately see if there are problems during the measurement without need to wait one full scan to be finished. The white curve is the average of all previous scans, and it is updated at the end of each scan. In the saved txt data file and jpeg report only the averaged curve is stored. In addition, the user has all the usual LabVIEW graph tools for changing the scale, labels, and more. Finally a vertical bar is tracing the progress of the measurement. This progress is not a theoretical estimate. It is calculated by measuring the real elapsed time since

#### **Figure 6.**

*The report jpeg file generated at the end of each experiment.*

the beginning of the measurement, dividing this time by the number of measured points so to obtain a realistic estimate of the time-per-point, and finally a multiplication for the number of total points and remaining points gives an estimate of the overall progress, in percent.

• The second Tab is the "Loop Scan" Tab. In this Tab the time scan is shown with no averaging. The delay stage is moved continuously in between the start and stop point, and the positions are not recorded. This speeds up a lot the acquisition, so that the waveform appears on the PC screen in real-time exactly as if it was displayed on an oscilloscope screen. In this Tab it is possible to set the velocity and acceleration of the Delay Stage, which will be then used also

**55**

*TeraVision: A LabVIEW Software for THz Hyper-Raman Spectroscopy*

elsewhere. The reason for this utility is to monitor the signal while adjusting the alignment of the setup. Who works with THz knows that the setup optimization can be quite difficult because if one looks at some signal on the oscilloscope, for instance the peak of the waveform, this peak position will change in time as the user is optimizing the alignment, because the relative path between THz and Gate pulses is changed. If one has the signal on screen in real-time, the displacement of the peak position will not be a problem, as long as the peak will fall between the start and stop points. The major drawback of this method is that the very poor signal-to-noise ratio makes difficult to distinguish the signal when it is very far from optimal intensity. It is very useful to quickly

reach the optimum once the signal is already roughly optimized.

• The third Tab is the "Move Stage" Tab. Here the user can set the Delay Stage parameters, and it displays a real-time chart which shows the stage position in time. This utility is used to manually move the stage step-by-step when searching for the signal after a major realignment, or to quickly move the stage in between two fixed positions. For instance, when a filter is placed in one of the two arms THz or Gate, it will change the relative optical path by a fixed known amount of time, so that it will be possible to quickly switch between the peak-

• The fourth Tab is the "Single Channels" Tab. In this Tab a graph in real time shows just the output of the DAQ for a given number of points. The DAQ receives two signals. One is the photodetector (PMT or balanced photodiode), and the second channel is linked to a photodiode which monitors the THz generation optical pulse right after the chopper. This monitor is needed for two reasons: the first is to keep trace of the laser intensity, and the second to know exactly which pulse has been chopped and which one has not. Even if in principle this signal could be used to normalize the measured signal on channel 1 in order to account for laser intensity fluctuations, our test measurements reveal that the amount of noise introduced by this procedure greatly overcomes the benefits of having a normalized signal, and therefore the channel 2 is used only to recognize the ON and OFF signals so to perform the differential measurement. In this Tab it is possible to set the DAQ parameters such as number of triggers to wait and average for each experimental point (delay stage position, monochromator wavelength). The utility is also capable of warning the user if the channel 2 photodiode signal difference between ON and OFF pulses is lower than 3 times the sum of the two standard deviations of each series of points (ON and OFF). When this condition is met, it means that the chopper phase has somehow changed and the chopping does not completely block every second pulses (this may happen for instance when the laser trigger has significant jitter or when the trigger parameters in the chopper motor are not correctly set). In this case, a red indicator blinks and the user is invited to

• The fifth Tab is the "PCM Spectrum" Tab. Here the signal is measured as a function of wavelength for a fixed Delay Line position (time position). In this Tab it is possible to set the monochromator parameters and to record a curve of the signal when THz is ON, OFF or the difference between the two. This is important in the case of THYR measurements, in order to directly compare the signal with no THz (standard SHG signal) and the signal with THz applied to the sample (THYR signal). It is also quite useful for quickly checking the pres-

ence of spurious effects, as for instance 2-photons luminescence.

*DOI: http://dx.doi.org/10.5772/intechopen.96663*

signals in the two different cases.

check the chopper phase.

the beginning of the measurement, dividing this time by the number of measured points so to obtain a realistic estimate of the time-per-point, and finally a multiplication for the number of total points and remaining points gives an

• The second Tab is the "Loop Scan" Tab. In this Tab the time scan is shown with no averaging. The delay stage is moved continuously in between the start and stop point, and the positions are not recorded. This speeds up a lot the acquisition, so that the waveform appears on the PC screen in real-time exactly as if it was displayed on an oscilloscope screen. In this Tab it is possible to set the velocity and acceleration of the Delay Stage, which will be then used also

estimate of the overall progress, in percent.

*The report jpeg file generated at the end of each experiment.*

**54**

**Figure 6.**

elsewhere. The reason for this utility is to monitor the signal while adjusting the alignment of the setup. Who works with THz knows that the setup optimization can be quite difficult because if one looks at some signal on the oscilloscope, for instance the peak of the waveform, this peak position will change in time as the user is optimizing the alignment, because the relative path between THz and Gate pulses is changed. If one has the signal on screen in real-time, the displacement of the peak position will not be a problem, as long as the peak will fall between the start and stop points. The major drawback of this method is that the very poor signal-to-noise ratio makes difficult to distinguish the signal when it is very far from optimal intensity. It is very useful to quickly reach the optimum once the signal is already roughly optimized.


**Figure 7.** *2D scan tab for THYR spectroscopy.*

