**1.4 THz Hyper-Raman time domain spectroscopy**

The standard THz-TDS technique has already proved to be very effective. Nonetheless, it is important to highlight that, despite the use of nonlinear optics to generate and reveal THz radiation, this is a linear spectroscopic technique, i.e. it probes the linear dielectric function of the target material. Sometimes, it could be not so easy to access the target observables, such as the low-frequency excitations (phonons, magnons, excitons, etc.). For example, a large absorption can obscure a significant part of the spectral range, the index of refraction and/or the dielectric function could be difficult to interpret if the target materials have a complicated structure, the background from the bulk can overcome the weaker superficial contribution, which is perhaps the real target of the experiment, and finally some low-energy excitations are simply out-of-reach because of symmetry, i.e. they just do not show up in the linear parameters due to selection rules. Accessing the nonlinear optical parameters can in principle remove most of these hurdles. With this aim in mind, a new spectroscopic technique has been developed by our group a couple of years ago. Through the application of this new technology, femtosecond laser pulses and intense sub-picosecond broadband THz pulses produce THzoptical four-wave mixing in the material. The spectrum of the resulting signal is decomposed both in wavelengths and time and it appears as two distinct frequency side bands around the optical SHG central frequency 2ω*<sup>L</sup>* , whereω*<sup>L</sup>* is the optical central frequency of the fundamental fs-pulse. This effect resembles the wellknown Hyper-Raman (HYR) effect, and therefore it has been named THz Hyper-Raman - THYR.

In standard all-visible HYR, the coupling between optical frequencies happens through a high-order nonlinear susceptibility tensor. The model at the base of the THYR effect, instead, is a regular four-wave mixing effect, in which one of the optical photons is replaced by a THz photon. The mathematical formalism is therefore that of a nonlinear Raman effect, but because of the very small THz photon energy, the sidebands are appearing very close to the SHG central frequency

#### **Figure 3.**

*Level diagram of THYR effect. Labels are: 1) L = laser 2) T = terahertz 3) S = stokes 4) a = anti-stokes.*

instead of the fundamental central frequency, resembling what happens in HYR. Taking the same nomenclature routinely used for Raman and HYR effects, we define "Stokes" and "anti-Stokes" bands, developing around the SHG frequency 2ω*L* :ω ωω *sa L T* ; = 2 (whereω*<sup>T</sup>* is the THz central frequency. For all details about THYR theory and applications we invite the reader to the following publications: [33, 34]). The energy diagram of the effect is shown for Stokes (left) and anti-Stokes (right) bands in **Figure 3**. From an experimental point of view, the signal has to be measured in both frequency and time domain, so that the software TeraVision which has been developed in order to drive the experiment and record the data must be more sophisticated than the regular kind of software used for THz-TDS. Moreover, we intended to create a software capable of performing several different kinds of measurements, thought to be independent from the THYR spectrometry, and/or used as ancillary measurements in THYR spectrometry as well. In addition to this, we inserted some features in order to simplify the signal detection and optimization, which could be of use more generally for whoever needs to perform any kind of optical Pump/Probe experimental scheme.

### **2. Experimental set-up**

The experimental setup which was built in order to perform THYR experiments is shown in **Figure 4**. The femtosecond laser is a Ti:Sa mode-locked seed laser, amplified by a regenerative cavity. The fs-laser pulse, after passing through the beam splitter, is divided into a Pump (transmitted) pulse and a Probe pulse (about 10% of total power). The Pump is chopped by a mechanical chopper locked to half of the laser trigger frequency in order to block every second pulse. The pulse train chopping is very important because it allows measuring the difference between THz-ON and THz-OFF signals, as it will be further explained in the following. The pulse is then sent on a nonlinear optical crystal (Beta Barium Borate, β-BBO) where about 20% of the optical power is converted in SHG and then both fundamental and doubled pulses are focused in air by an achromatic doublet. In the plasma filament, usually about 1 cm long, the generated plasma is producing THz pulses via fourwaves mixing processes in which the maximum THz amplitude depends on relative phase between the fundamental and second harmonic light [35]. The Probe is sent to a delay stage in order to introduce a controllable delay between Pump and Probe pulses and then it is incident on the detection crystal (usually ZnTe, GaSe or GaP, or just air for ABCD technique) together with the THz pulse in a collinear geometry, through a hole in the last parabolic mirror. In the detection crystal the polarization

**47**

**Figure 4.**

*Experimental setup of THz-TDS and THYR.*

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

state of the optical pulse is altered by the presence of a transient birefringence created by the THz electric field, and this change can be detected by separating two orthogonal polarizations of the pulse with a Wollaston Prism and measuring their signal difference in a balanced photodiode. As the delay between the two pulses is adjusted, the amplitude of the THz pulse can be sampled in time, and the full THz waveform can be reconstructed. In a standard THz-TDS experiment, the sample will be placed in the first focus (f1) of the parabolic mirror and the detection crystal is placed in the second focus (f2), as we can see from **Figure 1**, to measure the THz waveform transmitted through the sample. The complex transmission coefficient is then obtained by measuring the signal without the sample in place, as an estimate of the incoming electromagnetic wave, and finally the complex ratio between the Fourier Transforms of those two measurements will deliver the complex transmission coefficient spectrum. In reflection geometry, the problem is more complicated by the difficulty to place sample and reference mirror with sufficient accuracy. We are not going to tackle the problem of THz reflectivity measurements here, and we refer to this publication and the references therein [36]. In the THYR experiment, the first focus (f1) is left empty, the detection crystal is removed, the sample is place in the second focus (f2) and the whole detection line after sample is replaced (by means of flippable mirrors) with the setup shown in the inset of **Figure 4**. The signal produced in the sample by the interaction between THz and optical pulses is filtered in order to remove the 800 nm light, then it is sent to a monochromator for spectral analysis and finally to a photon multiplier tube (PMT) for detection. The PMT is needed because of the very small amount of photons which are created by THYR effect, but its operation is usually in continuous mode rather than in photon counting mode. Anyway, if the signal from a given sample would be too small, it is straightforward to switch the detection to a photon counting regime, provided that the laser and setup stability is good enough to support a much longer data acquisition time. Finally, the ON and OFF trains of pulses are separated by the TeraVision

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

**Figure 4.**

instead of the fundamental central frequency, resembling what happens in HYR. Taking the same nomenclature routinely used for Raman and HYR effects, we define "Stokes" and "anti-Stokes" bands, developing around the SHG fre-

*Level diagram of THYR effect. Labels are: 1) L = laser 2) T = terahertz 3) S = stokes 4) a = anti-stokes.*

about THYR theory and applications we invite the reader to the following publications: [33, 34]). The energy diagram of the effect is shown for Stokes (left) and anti-Stokes (right) bands in **Figure 3**. From an experimental point of view, the signal has to be measured in both frequency and time domain, so that the software TeraVision which has been developed in order to drive the experiment and record the data must be more sophisticated than the regular kind of software used for THz-TDS. Moreover, we intended to create a software capable of performing several different kinds of measurements, thought to be independent from the THYR spectrometry, and/or used as ancillary measurements in THYR spectrometry as well. In addition to this, we inserted some features in order to simplify the signal detection and optimization, which could be of use more generally for whoever needs to perform any kind of optical Pump/Probe experimental scheme.

The experimental setup which was built in order to perform THYR experiments

is shown in **Figure 4**. The femtosecond laser is a Ti:Sa mode-locked seed laser, amplified by a regenerative cavity. The fs-laser pulse, after passing through the beam splitter, is divided into a Pump (transmitted) pulse and a Probe pulse (about 10% of total power). The Pump is chopped by a mechanical chopper locked to half of the laser trigger frequency in order to block every second pulse. The pulse train chopping is very important because it allows measuring the difference between THz-ON and THz-OFF signals, as it will be further explained in the following. The pulse is then sent on a nonlinear optical crystal (Beta Barium Borate, β-BBO) where about 20% of the optical power is converted in SHG and then both fundamental and doubled pulses are focused in air by an achromatic doublet. In the plasma filament, usually about 1 cm long, the generated plasma is producing THz pulses via fourwaves mixing processes in which the maximum THz amplitude depends on relative phase between the fundamental and second harmonic light [35]. The Probe is sent to a delay stage in order to introduce a controllable delay between Pump and Probe pulses and then it is incident on the detection crystal (usually ZnTe, GaSe or GaP, or just air for ABCD technique) together with the THz pulse in a collinear geometry, through a hole in the last parabolic mirror. In the detection crystal the polarization

*<sup>T</sup>* is the THz central frequency. For all details

ω

**46**

quency 2

**Figure 3.**

ω*L* :ω

**2. Experimental set-up**

 ωω*sa L T* ; = 2 (where

*Experimental setup of THz-TDS and THYR.*

state of the optical pulse is altered by the presence of a transient birefringence created by the THz electric field, and this change can be detected by separating two orthogonal polarizations of the pulse with a Wollaston Prism and measuring their signal difference in a balanced photodiode. As the delay between the two pulses is adjusted, the amplitude of the THz pulse can be sampled in time, and the full THz waveform can be reconstructed. In a standard THz-TDS experiment, the sample will be placed in the first focus (f1) of the parabolic mirror and the detection crystal is placed in the second focus (f2), as we can see from **Figure 1**, to measure the THz waveform transmitted through the sample. The complex transmission coefficient is then obtained by measuring the signal without the sample in place, as an estimate of the incoming electromagnetic wave, and finally the complex ratio between the Fourier Transforms of those two measurements will deliver the complex transmission coefficient spectrum. In reflection geometry, the problem is more complicated by the difficulty to place sample and reference mirror with sufficient accuracy. We are not going to tackle the problem of THz reflectivity measurements here, and we refer to this publication and the references therein [36]. In the THYR experiment, the first focus (f1) is left empty, the detection crystal is removed, the sample is place in the second focus (f2) and the whole detection line after sample is replaced (by means of flippable mirrors) with the setup shown in the inset of **Figure 4**. The signal produced in the sample by the interaction between THz and optical pulses is filtered in order to remove the 800 nm light, then it is sent to a monochromator for spectral analysis and finally to a photon multiplier tube (PMT) for detection. The PMT is needed because of the very small amount of photons which are created by THYR effect, but its operation is usually in continuous mode rather than in photon counting mode. Anyway, if the signal from a given sample would be too small, it is straightforward to switch the detection to a photon counting regime, provided that the laser and setup stability is good enough to support a much longer data acquisition time. Finally, the ON and OFF trains of pulses are separated by the TeraVision

software and each of them is subtracted with the following one. As the timedistance of two subsequent pulses is 1 ms, this differential measurement scheme will quench all noises below the 1 kHz cutoff frequency. Moreover, this procedure ensures that even in presence of a strong non-THz-related background signal (as for instance a static SHG signal, or 2-photons luminescence) the measured signal will not be too much affected.

Here and in the following, we will name the Pump pulse simply "THz pulse", as it is responsible for THz generation, and the Probe pulse will be named "Gate pulse", as it sets the time-gate at which the signal is sampled. This nomenclature is quite common in THz spectroscopy, and it is particularly important when performing a THz Pump/Probe measurement, because in that case the pulses will be three: an optical Pump pulse which is exciting the sample, the THz and the Gate pulses, which are both together acting as a Probe. Let us now briefly give some details about the main components of the setup.

### **2.1 Laser**

The laser system used to run the spectrometer is a Coherent Legend regenerative amplifier seeded by a Coherent Mantis fs-oscillator (800 nm central wavelength, 20 fs FWHM pulse duration, 80 MHz repetition rate, 500 mW output power) and pumped by Coherent Evolution (527 nm central wavelength, ~ 10 ns pulse width, 1 kHz repetition rate, 20 W output power). The Legend delivers ~4 W output power at 1 KHz repetition rate (~ 4 mJ energy per pulse) at 800 nm central wavelength (1.5 eV), ~ 80 nm bandwidth, and with about 35 fs FWHM time duration [37].

#### **2.2 Signal detection**

### *2.2.1 Monochromator*

It is an optical device which disperses the light spectrum by means of a suitable optical grating (1800 groves/mm, in this case) and filters out all wavelengths except those passing through a slit at the output of the device. We use a monochromator from the Optometrics Group which is a Ebert-Fastie type with focal length (path length) of 74 mm, model number SDMC1–02 (Scanning Digital MiniChrom). The wavelength range of this monochromator is 200-800 nm. The wavelength range is suitable for all our measurements (THYR, THz-TDS and SHG). The reciprocal linear dispersion factor is 3 nm/mm. We can operate our monochromator manually using a knob for selection and a digital counter for wavelength readout. However, the device is routinely controlled via TeraVision software which drives a stepping motor controller (PCM-01) connected to PC by RS-232 serial port. The monochromator is used only for SHG and THYR detection, in combination with a PMT. It is possible to use it also for THz-TDS detection in case of ABCD technique, but we will not discuss this method here.

#### *2.2.2 Photomultiplier tube and balanced photodiode*

A PMT is a device consisting of a photocathode, several dynodes and one anode, all placed in vacuum. Modern PMTs deliver low noise and low jitter detection over a wide dynamic range. We use a PMT by Hamamatsu (model number R928) having spectral response from 185 to 900 nm. The PMT is used only for SHG and THYR detection, and it can be used in combination with the monochromator for THz-TDS by means

**49**

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

of the ABCD technique. In case of standard electro-optic sampling (THz-TDS) we use a balanced amplified photodetector by Thorlabs (model number PDB210A) with broadband detection range (320-1060 nm). The detector has two distinct low-noise photodiodes, whose difference is strongly amplified by the device in order to achieve a low noise measurement of the differential signal. The main advantage of this device compared to performing the subtraction of independent photodiodes signals is that the amplification on the difference is affected by a much lower noise than the subtrac-

In our experiments we use a linear stage by Physik Instrumente (PI, M-5x1 series) which are low profile, high-accuracy linear translation stages for laboratory applications. The stage is controlled by a DC-Motor controller (Mercury) and the PC communication is ensured through a standard RS-232 port. The minimal step size corresponds to a minimum incremental motion of 0.1 μm and 1 μm full travel accuracy. Therefore, since the optical path is double (forward-and-backward) than the physical path, we can achieve a minimal time delay of about 6 fs. As the optical pulse duration is about 35 fs, this Delay Line allow us to reach the full time-resolution

To control the input and output optical polarizations of light for THYR and THz-TDS experiments, we used mechanical rotors controlled by DC servo controllers. We use Thorlabs rotors (model PRM1/MZ8) which are small, compact, DC servo motorized 360° rotation stages. The user can measure the angular displacement manually by using the Vernier dial and the marks that are marked on the rotating plate in 1° increments. In order to rotate the rotors precisely we are using DC servo controllers (model TDCOO1) or "T-cube DC", controlled in turn by our TeraVision software. The software allows the user both to set the in-out angles as fixed parameters in any kind of measurement, or to measure the signal as a function of in-and/or-out angles by fixing all other parameters, in the "Rotors" Tab.

The electrical signal is generated by the PMT or the balanced photodiodes and enters a Stanford Gated Integrator or Boxcar Averager (GI). Here, it is splitted in two signals (50%). One is sent to a Tektronix DPO 4054 oscilloscope for monitoring the signal and, more importantly, adjusting the synchronization with the electronic gate, while the second signal enters the GI for processing. In our lab we use SR250 GI that is a versatile, high speed, low cost module designed to recover fast input analog signals from noisy background. It consists of a gate generator, a fast gated integrator, and an exponential averaging circuitry. The gate generator can be triggered internally or externally, and it provides an adjustable delay from a few nanoseconds to 100 ms before it generates a continuously adjustable gate with a width between 2 ns and 15 μs. The delay can be set by a front panel potentiometer. The fast gated integrator integrates the input signal during the gating and the output signal is normalized by the gate width to provide a voltage which is proportional to the average of the input signal. This signal can be further amplified by using different settings on the front panel of the boxcar integrator. The final analog output signal is sent to a data acquisition system (DAQ ) based on aPCIe-6351 interface by National Instruments

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

tion of already amplified separate signals.

**2.3 Delay line**

possible for our setup.

**2.5 Data acquisition**

**2.4 Rotors**

*TeraVision: A LabVIEW Software for THz Hyper-Raman Spectroscopy DOI: http://dx.doi.org/10.5772/intechopen.96663*

of the ABCD technique. In case of standard electro-optic sampling (THz-TDS) we use a balanced amplified photodetector by Thorlabs (model number PDB210A) with broadband detection range (320-1060 nm). The detector has two distinct low-noise photodiodes, whose difference is strongly amplified by the device in order to achieve a low noise measurement of the differential signal. The main advantage of this device compared to performing the subtraction of independent photodiodes signals is that the amplification on the difference is affected by a much lower noise than the subtraction of already amplified separate signals.

## **2.3 Delay line**

*LabVIEW - A Flexible Environment for Modeling and Daily Laboratory Use*

not be too much affected.

**2.1 Laser**

time duration [37].

**2.2 Signal detection**

*2.2.1 Monochromator*

not discuss this method here.

*2.2.2 Photomultiplier tube and balanced photodiode*

the main components of the setup.

software and each of them is subtracted with the following one. As the timedistance of two subsequent pulses is 1 ms, this differential measurement scheme will quench all noises below the 1 kHz cutoff frequency. Moreover, this procedure ensures that even in presence of a strong non-THz-related background signal (as for instance a static SHG signal, or 2-photons luminescence) the measured signal will

Here and in the following, we will name the Pump pulse simply "THz pulse", as it is responsible for THz generation, and the Probe pulse will be named "Gate pulse", as it sets the time-gate at which the signal is sampled. This nomenclature is quite common in THz spectroscopy, and it is particularly important when performing a THz Pump/Probe measurement, because in that case the pulses will be three: an optical Pump pulse which is exciting the sample, the THz and the Gate pulses, which are both together acting as a Probe. Let us now briefly give some details about

The laser system used to run the spectrometer is a Coherent Legend regenerative amplifier seeded by a Coherent Mantis fs-oscillator (800 nm central wavelength, 20 fs FWHM pulse duration, 80 MHz repetition rate, 500 mW output power) and pumped by Coherent Evolution (527 nm central wavelength, ~ 10 ns pulse width, 1 kHz repetition rate, 20 W output power). The Legend delivers ~4 W output power at 1 KHz repetition rate (~ 4 mJ energy per pulse) at 800 nm central wavelength (1.5 eV), ~ 80 nm bandwidth, and with about 35 fs FWHM

It is an optical device which disperses the light spectrum by means of a suitable optical grating (1800 groves/mm, in this case) and filters out all wavelengths except those passing through a slit at the output of the device. We use a monochromator from the Optometrics Group which is a Ebert-Fastie type with focal length (path length) of 74 mm, model number SDMC1–02 (Scanning Digital MiniChrom). The wavelength range of this monochromator is 200-800 nm. The wavelength range is suitable for all our measurements (THYR, THz-TDS and SHG). The reciprocal linear dispersion factor is 3 nm/mm. We can operate our monochromator manually using a knob for selection and a digital counter for wavelength readout. However, the device is routinely controlled via TeraVision software which drives a stepping motor controller (PCM-01) connected to PC by RS-232 serial port. The monochromator is used only for SHG and THYR detection, in combination with a PMT. It is possible to use it also for THz-TDS detection in case of ABCD technique, but we will

A PMT is a device consisting of a photocathode, several dynodes and one anode, all placed in vacuum. Modern PMTs deliver low noise and low jitter detection over a wide dynamic range. We use a PMT by Hamamatsu (model number R928) having spectral response from 185 to 900 nm. The PMT is used only for SHG and THYR detection, and it can be used in combination with the monochromator for THz-TDS by means

**48**

In our experiments we use a linear stage by Physik Instrumente (PI, M-5x1 series) which are low profile, high-accuracy linear translation stages for laboratory applications. The stage is controlled by a DC-Motor controller (Mercury) and the PC communication is ensured through a standard RS-232 port. The minimal step size corresponds to a minimum incremental motion of 0.1 μm and 1 μm full travel accuracy. Therefore, since the optical path is double (forward-and-backward) than the physical path, we can achieve a minimal time delay of about 6 fs. As the optical pulse duration is about 35 fs, this Delay Line allow us to reach the full time-resolution possible for our setup.

### **2.4 Rotors**

To control the input and output optical polarizations of light for THYR and THz-TDS experiments, we used mechanical rotors controlled by DC servo controllers. We use Thorlabs rotors (model PRM1/MZ8) which are small, compact, DC servo motorized 360° rotation stages. The user can measure the angular displacement manually by using the Vernier dial and the marks that are marked on the rotating plate in 1° increments. In order to rotate the rotors precisely we are using DC servo controllers (model TDCOO1) or "T-cube DC", controlled in turn by our TeraVision software. The software allows the user both to set the in-out angles as fixed parameters in any kind of measurement, or to measure the signal as a function of in-and/or-out angles by fixing all other parameters, in the "Rotors" Tab.

#### **2.5 Data acquisition**

The electrical signal is generated by the PMT or the balanced photodiodes and enters a Stanford Gated Integrator or Boxcar Averager (GI). Here, it is splitted in two signals (50%). One is sent to a Tektronix DPO 4054 oscilloscope for monitoring the signal and, more importantly, adjusting the synchronization with the electronic gate, while the second signal enters the GI for processing. In our lab we use SR250 GI that is a versatile, high speed, low cost module designed to recover fast input analog signals from noisy background. It consists of a gate generator, a fast gated integrator, and an exponential averaging circuitry. The gate generator can be triggered internally or externally, and it provides an adjustable delay from a few nanoseconds to 100 ms before it generates a continuously adjustable gate with a width between 2 ns and 15 μs. The delay can be set by a front panel potentiometer. The fast gated integrator integrates the input signal during the gating and the output signal is normalized by the gate width to provide a voltage which is proportional to the average of the input signal. This signal can be further amplified by using different settings on the front panel of the boxcar integrator. The final analog output signal is sent to a data acquisition system (DAQ ) based on aPCIe-6351 interface by National Instruments

(NI). The latter is a X-series multifunction I/O Device, which offers analog (16 bit) and digital I/O, four 32-bit counter/timers for encoder, frequency, event counting etc. It relies on a high-speed PCI Express bus. In our TeraVision software we used NI-VISA which is an API (Application Programming Interface) that provides a programming interface to control GPIB, serial, USB, PXI, VXI etc. instruments in National Instruments application development environments like LabVIEW.
