*Article* **Broadband Anisotropic Optical Properties of the Terahertz Generator HMQ-TMS Organic Crystal**

**Annalisa D'Arco 1,\*, Luca Tomarchio 1,2,\*, Valerio Dolci 1,2, Paola Di Pietro 3, Andrea Perucchi 3, Sen Mou 1,2, Massimo Petrarca 1,4 and Stefano Lupi 2,5**


Received: 19 May 2020; Accepted: 11 July 2020; Published: 14 July 2020

**Abstract:** HMQ-TMS (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 2,4,6 trimethylbenzenesulfonate) is a recently discovered anisotropic organic crystal that can be exploited for the production of broadband high-intensity terahertz (THz) radiation through the optical rectification (OR) technique. HMQ-TMS plays a central role in THz technology due to its broad transparency range, large electro-optic coefficient and coherence length, and excellent crystal properties. However, its anisotropic optical properties have not been deeply researched yet. Here, from polarized reflectance and transmittance measurements along the *x*<sup>1</sup> and *x*<sup>3</sup> axes of a HMQ-TMS single-crystal, we extract both the refraction index *n* and the extinction coefficient *k* between 50 and 35,000 cm−1. We further measure the THz radiation generated by optical rectification at different infrared (IR) wavelengths and along the two *x*<sup>1</sup> and *x*<sup>3</sup> axes. These data highlight the remarkable anisotropic linear and nonlinear optical behavior of HMQ-TMS crystals, expanding the knowledge of its properties and applications from the THz to the UV region.

**Keywords:** terahertz; THz spectroscopy; optical indices; nonlinear effects; optical rectification; organic crystals; HMQ-TMS

#### **1. Introduction**

THz radiation (1 THz∼<sup>33</sup> cm−<sup>1</sup> or 4 meV photon energy) has gained over the years a considerable interest due to its broad variety of applications. Starting from fundamental scientific investigations, where THz can be used as a resonant probe for the plethora of excitations in condensed matter physics [1–3], its applications reach also to various industrial and biomedical activities [4–7], security applications [8–10], and particle-accelerator physics [11,12]. Following the growing interest, a rapid development of both THz generators and detectors has been made possible thanks to novel technologies that have become available in these last two decades, such as quantum cascade lasers, photoconductive antennas, Gunn lasers, and sources based on nonlinear optical (NLO) effects. The latter realm has been the starting point for the production of single cycle, high-intensity THz signals comparable to those obtained from free-electron facilities [13,14]. The process of difference frequency generation [15,16] or optical rectification (OR) [15,17–22] still holds the greatest interest due to its capabilities of reaching electric field magnitudes up to tens of MV/cm providing a broad THz spectral range going from nearly 0.1 THz up to 15 THz [1,23]. Due to these properties, novel NLO materials have been highly investigated in terms of THz transparency and linear and nonlinear

optical responses. As already highlighted in literature, the production of THz radiation through OR process is highly dependent on the material properties, like the microscopic optical response functions of the crystal [15,24]. The real and imaginary parts of the refractive index, both in the optical and THz emission regions, give information about the phase matching condition and the absorption effects inside the crystal. Therefore, the knowledge of those optical parameters is of great importance in order to optimize the OR process and the emitted THz spectrum. Moreover, many efforts are also required in order to optimize future growth processes of new THz crystals [25,26].

Among the many materials already discovered, like inorganic NLO crystals such as ZnTe and GaP [27], organic NLO crystals offer the best platform, mainly due to their strong nonlinear optical response arising from the molecules hyperpolarizability and orientation inside the crystal [15]. Organic crystals like DAST, DSTMS, OH1 [28–30], 2-(4-hydroxy-3-methoxystyryl)-1 methylquinolinium 2,4,6-trimethylbenzenesulfonate (HMQ-TMS) [31] and BNA [32], are already widely used for THz photonics. Here, HMQ-TMS is an organic molecular crystal built upon HMQ (2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium) cations and TMS (2,4,6 trimethylbenzenesulfonate) counter anions. HMQ-TMS shows a polar axis oriented along the *x*<sup>3</sup> direction, as shown in Figure 1. Although the electromagnetic properties of HMQ-TMS have been partially studied [31,33,34], a complete investigation of its anisotropic optical properties is still missing. In recent works, Brunner et al. [33] estimated the crystal optical group index, with light polarized along the polar axis, through retardation of laser pulses [35], covering a range from 600 to 2000 nm. In addition, the absorption coefficient *α*<sup>3</sup> were also extracted from transmission measurements in the same spectral range and between 0.3 and 1.5 THz through THz time-domain spectroscopy. The same optical parameters have also been estimated for a broader THz spectral range (1.2–12 THz) in Reference [34]. In this paper, we extract from polarized reflectance and transmittance measurements, from THz to ultraviolet (UV), both the real (refraction index *n*) and the imaginary part (extinction coefficient *k*) of the complex refractive index *n*˜ = *n* − *ik*, along the *x*<sup>1</sup> and *x*<sup>3</sup> (polar) axes of a HMQ-TMS single-crystal. We further measure the THz radiation generated by optical rectification at different infrared (IR) pumping wavelengths and along the two *x*<sup>1</sup> and *x*<sup>3</sup> axes. These data highlight the remarkable anisotropic linear and nonlinear optical behavior of HMQ-TMS crystal, as predicted from the crystallographic theory.

**Figure 1.** (**a**) Chemical structure of HMQ-TMS system. (**b**) Orientation inside the 2-(4-hydroxy-3 methoxystyryl)-1-methylquinolinium 2,4,6-trimethylbenzenesulfonate (HMQ-TMS) crystal of the HMQ and TMS molecular groups projected on the crystallographic *b*-axis. A massive hyperpolarizability is associated to the HMQ chromophores, which are aligned along the polar axis *x*<sup>3</sup> in such a way to define the maximum possible value of the order parameter cos3 *<sup>θ</sup>* [15].

#### **2. Experimental Methods**

#### *2.1. Linear Response Study*

The Reflectance (R) and transmittance (T) at room temperature of a HMQ-TMS single crystal have been measured, from THz to UV (50–35,000 cm<sup>−</sup>1), along the *x*<sup>3</sup> and *x*<sup>1</sup> axes. The crystal is characterised by a thickness of 190 μm (as measured by a micrometer) and lateral dimensions of 4 mm × 2.5 mm. The face of incidence coincides with the crystallographic *ac*-plane, parallel to the radiation polarization. THz and Mid-Infrared (MIR) regions have been investigated at the SISSI Infrared beamline in Elettra Synchrotron (Trieste) through a Bruker Vertex 70V Michelson interferometer [36–38]. The region going from NIR to UV has been studied at the Physics Department of the University of Rome "La Sapienza" through a JASCO V-770 spectrometer. A calibrated gold (aluminium) mirror in the THz/MIR (NIR/UV) has been used as a reference in reflectance experiments. The linear complex refractive index as a function of frequency, *n*(*ω*) − *ik*(*ω*) (here *ω* is a wavenumber), has been obtained from *T*(*ω*) and *R*(*ω*) data by deriving the exact analytical solution to the inverse problem for a slab under the approximation *<sup>k</sup>*<sup>2</sup> *<sup>n</sup>* (no absorption at interfaces). The two indices can be expressed as [39]:

$$n(R, T, \omega) = \frac{1 + R\_F(R, T)}{1 - R\_F(R, T)} + \left\{ \frac{4R\_F(R, T)}{[1 - R\_F(R, T)]^2} - \left(\frac{1}{2\omega d}\right)^2 \ln^2 \left[\frac{R\_F(R, T)T}{R - R\_F(R, T)}\right] \right\}^{1/2} \tag{1}$$

$$k(R, T, \omega) = \frac{1}{2\omega d} \ln \left[ \frac{R\_F(R, T)T}{R - R\_F(R, T)} \right],\tag{2}$$

where *d* is the slab thickness and the single interface reflectance *RF* takes the form

$$R\_F = \frac{2 + T^2 - (1 - R)^2 - \{ [2 + T^2 - (1 - R)^2]^2 - 4R(2 - R) \}^{1/2}}{2(2 - R)}$$

This method, based on both *R* and *T*, is independent from any major approximation. It is thus expected to be very precise in the determination of *n* and *k* values across the broad spectroscopic range.

#### *2.2. Ir Pumping Scheme*

In order to study the THz emission from the HMQ-TMS crystal, an optical apparatus has been developed based on a collinear optical parametric amplifier (OPA) from Light Conversion®, which permits the production of femtosecond pulses at tunable IR wavelengths, going from 1200 nm up to 1600 nm. The system is shown in Figure 2. A femtosecond high-intensity pulse at 780 nm pumps the OPA, while a minor intensity is used for detection of the THz signal through the electro-optic effect in a GaP 0.2 mm thick crystal. The signal emitted from the OPA is then used in order to pump the HMQ-TMS crystal at varying wavelengths. At constant fluence (4 mJ/cm2), the wavelength range spans from 1300 nm to 1600 nm, and four different values have been compared for the THz generation: 1300, 1400, 1500 and 1600 nm, as suggested by previous literature [34].

**Figure 2.** Infrared (IR) pumping scheme for THz generation in a HMQ-TMS crystal. An initial 40 fs pulse at 780 nm, generated from a Ti:Sapphire laser (COHERENT® Legend Elite), is injected into the optical parametric amplifier (OPA) for the generation of IR femtosecond and high-fluence pulses. The residual pump is sent to the electro-optical detection system after passing through a delay stage and a series of optical elements. A GaP crystal is used for the detection.

#### **3. Results**

#### *3.1. Linear Optical Parameters*

In Figure 3, *R*(*ω*) and *T*(*ω*) measurements for the HMQ-TMS crystal are reported between 50 and 35,000 cm<sup>−</sup>1. The blue (red) solid-lines concern R data, while blue (red) dashed-lines refer to T data with light polarization along the *x*<sup>1</sup> and *x*<sup>3</sup> axes, respectively. From the T measurements, one can notice a broad transparent spectral region extending from the mid-IR to the VIS region (5000–20,000 cm<sup>−</sup>1) for both axes, with a plateau at 83% along *x*<sup>1</sup> and up to 80% along *x*3, respectively. This sligthly higher absorption is attributed to the major alignment of both HMQ cations and TMS anions along the *x*<sup>3</sup> axis (see Figure 1) [31]. The first electronic transition is approximately located around 20,000 cm−<sup>1</sup> and corresponds to a strong reduction of transmittance along both axes, with a relatively low cut-off wavelength < 580 nm, in accordance with the estimation of Brunner et al. [33]. This electronic transition is related to the HMQ cations, which exhibit (in a methanol solution) an absorption maximum around 439 nm (nearly 22,000 cm<sup>−</sup>1) [31], mapping a smaller modulation of the reflectance (Figure 3) along the *x*<sup>1</sup> axis. In the inset of Figure 3, a magnified plot of T (R) curves in the THz/MIR region is shown. Here, minima (maxima) correspond to both intra- and intermolecular (phonon) absorptions extending to the MIR region (see Figure 3).

In order to extract the real and imaginary parts of the refraction index from R and T data, the partial transparency of the HMQ-TMS single crystal in the MIR and VIS spectral region (see Figure 3) should be taken into account. Indeed, this transparency does not allow the use of Kramers-Kronig transformations. However, the complementary T and R data allow the derivation of an analytical method (see Equations (1) and (2) which considers Fresnel losses due to multiple reflections at the crystal surfaces [39]. The extracted optical parameters have been double-checked by using the RefFit constrained fitting program for a thin slab [40]. In Figure 4, the real (*n*) part of refraction index along *x*<sup>1</sup> (blue curves) and *x*<sup>3</sup> (red curves) axes are shown. For both axes, *n* is nearly constant from MIR to red, showing an average value of 1.6 (2.0) for the *x*<sup>1</sup> (*x*3) axis. A strong modulation of *n* can be observed between 20,000 and 25,000 cm<sup>−</sup>1, in correspondence to the electronic insulating gap. In the spectral range (5000–16,000 cm<sup>−</sup>1), and along the *x*<sup>3</sup> axis, the refractive index behaves accordingly to already published data [33]. Moreover, the inset of the same figure shows *n* in the THz/MIR spectral region, which behavior is in accordance with previous works [34].

**Figure 3.** Polarized T and R data of HMQ-TMS single crystal at room temperature along the *x*<sup>1</sup> and *x*<sup>3</sup> axis, in the 50–35,000 cm−<sup>1</sup> spectral region. T (R) data with light polarization along *x*<sup>1</sup> and *x*<sup>3</sup> are represented by dashed (solid) blue and red lines, respectively. In the inset, R and T data are plotted in the THz/MIR region.

**Figure 4.** Real part of refraction index (*n*) of the HMQ-TMS crystal at room temperature. Solid-blue (red) line corresponds to the value along the *x*<sup>1</sup> (*x*3) axis. In the inset, *n* is plotted in the THz/MIR spectral range. The strong variation of *n* around 20,000 cm−<sup>1</sup> is generated by an electronic transition of the HMQ cations.

#### *3.2. Spectral Analysis*

The absorption coefficients, along both *x*<sup>1</sup> and *x*<sup>3</sup> axes, are calculated through the extinction coefficient *k* as *α* = 4*πωk* (*ω* is a wavenumber). They are reported in the spectral range 400–4000 cm<sup>−</sup>1, where most of the vibrational excitations of HMQ and TMS chemical groups fall (see Figure 5a,b). Differently to the electronic transitions that show a remarkable anisotropy (see Figure 4), the two vibrational spectra have several peaks in common. A small anisotropy can be observed only below 1000 cm<sup>−</sup>1, where ring-structure bending and lattice modes are located, and can be attributed to

molecules orientation. In order to assign those peaks, one can observe that aromatic rings in the HMQ-TMS structure (see Figure 1) show several C-H and C=C-C vibrational modes. Specifically, the bending modes of quinolinium ring are present below 650 cm<sup>−</sup>1. C-H out-of-plane and in-plane bending vibrations occur in the regions 670–900 cm−<sup>1</sup> and 950–1225 cm<sup>−</sup>1, respectively [41]. Along axis *x*1, the band at 528 cm−<sup>1</sup> is identified with the C-N-C and C-C-N in-plane bending modes. For axis *x*3, in the region 450–600 cm<sup>−</sup>1, two shoulders are distinguished at 470 and 609 cm<sup>−</sup>1, and assigned to the symmetric and asymmetric bending vibrations of the -SO3 group [42–44]. The peaks at about 1030 cm<sup>−</sup>1, 1140 cm−<sup>1</sup> and 1350 cm−<sup>1</sup> can be assigned to the symmetric and asymmetric SO− <sup>3</sup> stretching, respectively [41]. Between 1260–1340 cm<sup>−</sup>1, three weak shoulders can be associated to aromatic primary amine C-N stretching. The peaks at 1530 and 1590 cm−<sup>1</sup> can be attributed to the vibrations of aromatic rings, while the absorptions around 1390, 1430 and 1480 cm−<sup>1</sup> are due to trimethyl CH3. The shoulder at 2652 cm−<sup>1</sup> is related to the stretching vibration of C-CH3, located in the trimethylbenzenesulfonate. The methylamino N-CH3 vibrational band is located at 2760 cm<sup>−</sup>1. Above 2800 cm<sup>−</sup>1, the C-H bonds vibrate with the methyl C-H symmetric and asymmetric stretching at 2860 and 2960 cm<sup>−</sup>1, respectively, and the methyl ether O-CH3 and C-H stretching corresponding to the band at 2815 cm<sup>−</sup>1.

**Figure 5.** Absorption coefficients of HMQ-TMS crystal at room temperature in the vibrational spectral region 400–4000 cm−<sup>1</sup> along the polar *x*<sup>3</sup> (**a**) and *x*<sup>1</sup> (**b**) axes. The shaded area pictorially assigns the absorption peaks (or vibrational regions) to specific vibrational modes of HMQ and TMS chemical groups. (The labels are: *δ* → bending vibration, *ν* → stretching vibration, *as* → asymmetric, *s* → symmetric, *ring* → quinolinium ring).

The narrow peak at 3000 cm−<sup>1</sup> and the shoulder around 3010 cm−<sup>1</sup> are attributed to C-H bonds around the aromatic rings [41]. The region between 3020–3230 cm<sup>−</sup>1, related to aromatic C-H stretching and hydroxyl group vibrations, shows a very strong absorption that is nearly saturated. The small shoulder, located at 3250 cm<sup>−</sup>1, can be attributed to O-H vibrational bonds. These general assignments are reported in Figure 5a,b.

#### *3.3. Thz Generation*

For completeness, the nonlinear properties in terms of THz generation vs. different pumping wavelengths, along the *x*<sup>1</sup> and *x*<sup>3</sup> axes, have also been measured. Referring to the scheme of Figure 2, and varying the timing overlap between the THz pulse and the 780 nm probe in a GaP detection crystal, it is possible to scan the THz electric field magnitude in order to compute the spectral amplitude. The amplitudes along the *x*<sup>3</sup> axis (coinciding with the maximum generation efficiency) at a fluence of 4 mJ/cm2, and at different IR pumping wavelengths (1300, 1400, 1500 and 1600 nm), are shown in Figure 6a. The comparable magnitudes of the field vs. the pumping wavelength suggest a nearly flat THz efficiency of the HMQ-TMs crystal in the whole infrared range.

**Figure 6.** (**a**) Spectral amplitude of the THz field generated by a HMQ-TMS single crystal pumped by a fs optical pulse at different IR wavelengths (1300, 1400, 1500 and 1600 nm). A broad frequency THz generation is visible from 0.5 up to 6 THz, with a further contribution around 7 THz. Minima in the spectra are mainly due to the water vapour absorption. (**b**) Spectral amplitude of the THz field generated along the *x*<sup>1</sup> axis. The intensity and spectral bandwidth are strongly reduced in comparison to the *x*<sup>3</sup> axis. Red-dashed lines in (a) and (b) represent the noise level in our experiment. (**c**) Peak-to-peak THz field magnitude *vs* the angle between the *x*<sup>3</sup> axis and the pumping polarization. The THz field value strongly decreases for an increasing angle, indicating a strong reduction of the THz emission efficiency for a pump polarization along the *x*<sup>1</sup> axis.

The anisotropic THz emission properties of HMQ-TMS have been studied by varying the crystal orientation with respect to the linearly polarized pump. In particular, both the incidence OPA polarization and the GaP detection crystal orientation have been kept fixed while the crystal has been rotated. Although we cannot exclude that some THz intensity might come from orientation misalignment and polarization losses, a small THz emission can be observed along *x*<sup>1</sup> centered around 2 THz (Figure 6b) for a pumping wavelength at 1500 nm (similar data have been obtained at the other wavelengths). More specifically, the THz magnitude vs. the crystal orientation (Figure 6c) progressively decreases when the pumping polarization approaches *x*1.

#### **4. Conclusions**

In this paper, we have measured the complex refraction index of a HMQ-TMS single crystal from terahertz to ultraviolet, both along the polar *x*<sup>3</sup> axis and the orthogonal *x*<sup>1</sup> axis on the crystallographic *ac*-plane. In the visible-ultraviolet region, we observe a remarkable anisotropy which is strongly attenuated in the infrared and terahertz range. The precise extraction of both the refractive indices and the absorption coefficients proposes an inverse problem approach for the THz generation study. Therefore, we have also measured the terahertz emission spectra along the same axes when pumping

in the infrared through a fs-amplifier laser. As expected from theoretical grounds, the THz emission shows a huge intensity reduction when the pumping polarization is parallel to the *x*<sup>1</sup> axis. These data expand our knowledge of the HMQ-TMS optical properties across the broad spectral range from THz to UV, allowing a better understanding of its possible applications in THz pump-probe experiments of exotic electronic systems [45,46].

**Author Contributions:** Conceptualization, A.D., L.T., S.L. and M.P.; methodology, A.D. and L.T.; software, A.D. and L.T.; validation, S.L., A.D. and L.T.; formal analysis, A.D. and L.T.; investigation, V.D., L.T., S.M., P.D.P. and A.P.; resources, S.L. and M.P.; data curation, A.D. and L.T.; writing—original draft preparation, A.D., L.T. and S.L.; writing—review and editing, all authors; visualization, A.D. and L.T.; supervision, M.P. and S.L.; project administration, M.P. and S.L.; funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR) (Rita Levi Montalcini); Italian Ministry of Foreign Affairs and International Cooperation (PGR00806).

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **Abbreviations**

The following abbreviations are used in this manuscript:


#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Spatially Resolved Spectral Imaging by A THz-FEL**

**Akinori Irizawa 1,\*, Masaki Fujimoto 1,**†**, Keigo Kawase 1,**‡**, Ryukou Kato 1,§, Hidenori Fujiwara 2, Atsushi Higashiya 3, Salvatore Macis 4,5, Luca Tomarchio 4, Stefano Lupi 4,5, Augusto Marcelli 5,6 and Shigemasa Suga <sup>1</sup>**


Received: 28 April 2020; Accepted: 2 June 2020; Published: 4 June 2020

**Abstract:** Using the unique characteristics of the free-electron-laser (FEL), we successfully performed high-sensitivity spectral imaging of different materials in the terahertz (THz) and far-infrared (FIR) domain. THz imaging at various wavelengths was achieved using in situ spectroscopy by means of this wavelength tunable and monochromatic source. In particular, owing to its large intensity and directionality, we could collect high-sensitivity transmission imaging of extremely low-transparency materials and three-dimensional objects in the 3–6 THz range. By accurately identifying the intrinsic absorption wavelength of organic and inorganic materials, we succeeded in the mapping of spatial distribution of individual components. This simple imaging technique using a focusing optics and a raster scan modality has made it possible to set up and carry out fast spectral imaging experiments on different materials in this radiation facility.

**Keywords:** THz; far infrared; FEL; spectroscopy; imaging

#### **1. Introduction**

Terahertz (THz)-wave and/or far-infrared photons represent radiation located in the frequency region, which is well known as the "terahertz-gap". This region is a frequency domain that is still particularly interesting in the field of radiation generation and detection technology. In terms of applicability, conventional light sources based on the blackbody emission and well-established spectrometers or interferometers are rather common tools. On the other hand, far-infrared radiation emitted by synchrotron radiation (SR) and free-electron-laser (FEL) sources is powerful for spectroscopy and microspectroscopy. Thanks to the high brilliance and focusing properties, these light sources

important results have been obtained with experiments performed with these non-thermal radiation sources. Pump-probe experiments using intense and coherent photon sources to investigate non-linear materials properties can be realized only by using high-intensity FELs. In the recent years, we have performed various linear and non-linear experiments using the terahertz-free-electron-laser (THz-FEL) in the Institute of Scientific and Industrial Research (ISIR) of Osaka University [1–4].

Regarding the electromagnetic waves, THz and far-infrared (FIR) radiation falling in the "terahertz gap" share both the behavior of high-frequency radio waves and low-energy photons. For this reason, in different areas such as physics, chemistry, engineering, and bio-medicine, this radiation can be described by different units such as frequency, wavenumber, wavelength, and energy. Their mutual relationship is shown in Equation (1) with the reference to approximately 3 THz, where only the wavelength is inversely proportional to the others:

$$13\text{ THz} = 100\text{ cm}^{-1} = 100\text{ }\mu\text{m} = 12.4\text{ meV} \tag{1}$$

The terahertz band is conventionally understood as ranging from approximately 0.3 to 3 THz around 1 THz within the International Telecommunication Union (ITU) designated band of frequencies. In addition to the light sources having characteristics of intensity, stability, monochromaticity, or broadbandness, detectors with high sensitivity, high-speed response, and wide (linear) dynamic range are under development. Besides far-infrared photons emitted from a black body in accordance with Planck's law, various other light sources exist, e.g., synchrotron radiation light sources, vacuum tubes including gyrotrons, gas, and solid-state lasers, superconducting devices, non-linear optical devices, and FELs, which are mainly described in this contribution.

The ISIR THz-FEL is a monochromatic, wavelength-tunable and highly coherent pulsed light source with megawatt-class peak intensity. For the first time in the 1970s and in the early phase, FELs produced radiation in the mid-infrared region [5]. The emission wavelength was continuously reduced down to the X-ray range with the goal of studying nano- and sub-nano-size objects and reach the pico- and sub-picosecond time resolution in different research areas [6,7]. On the other hand, although infrared FELs are relatively compact, the operation and the maintenance of these linear accelerators requite important human and/or economic supports from universities and institutions. Mainly for this reason, some FEL facilities in the infrared region were discontinued in the past. However, since the last decade, long wavelengths FELs are again attracting attention as high-intensity coherent radiation sources. Indeed, the increasing number of studies using infrared FEL radiation from the operational facilities is triggering the constructions of new infrared FELs and the plan of new IR and THz beamlines worldwide [8–12].

#### **2. Characteristics of the ISIR THz-FEL**

The FEL installed at the quantum beam science research facility in ISIR of the Osaka University can produce high-intensity pulsed light in the THz and FIR domains. The accelerator facility was established in 1957, and after almost 40 years, in 1978, the L-band electron linear accelerator was installed. After the first successful FEL oscillation [13], several upgrades have been realized. The details of the continuous improvement of the electron gun and the accelerator are summarized in Refs. [14–18]. In this contribution, we describe the experimental layout designed and assembled in ISIR and some recent experimental results obtained using this layout. The ISIR THz-FEL can be also used as a pump source thanks to the high intensity and time characteristics or as a probe source if combined with the irradiation of an external laser. In the next section, we will focus on microscopy and spectral-imaging experiments performed with the ISIR THz-FEL as the probe source and will discuss the typical FEL parameters we used for the experiments discussed below.

Figure 1 shows the pulsed time structure of the ISIR THz-FEL radiation reflecting the structure of an electron-bunch train. The FEL pulse train has a two-level structure. The first level is the micro-pulse, and the set of micro-pulses forms the macro-pulse. The long-time structure contains several macro-pulses emitted in sequence every 200 ms (5 Hz repetition). Each macro-pulse contains approximately 100 micro-pulses. The interval among micro-pulses depends on the FEL oscillation mode, i.e., 9.2 ns or 37 ns (= 9.2 ns × 4). The emission is due to ISIR THz-FEL's pre-bunching system, which aims to increase the charge of the electron bunches. The time interval is 12 or 48 times longer than the length of 0.77 ns associated with the RF frequency of 1.3 GHz of the Klystron's. The 9.2 ns interval is associated with the 108 MHz operation of the sub-harmonic buncher, while the 37 ns interval is due to the 27 MHz grid pulser installed on the thermal cathode electron gun.

**Figure 1.** Time structure of the pulsed Institute of Scientific and Industrial Research (ISIR) terahertz-free-electron-laser (THz-FEL). The pulse structure is approximately 4 μs in the macro-pulse and approximately 20 ps in the micro-pulse, respectively.

In the spectroscopy experiments using the FEL as the radiation probe, a stable intensity mode with the micro-pulse interval of 9.2 ns (108 MHz mode) has been usually employed. For irradiation experiments on materials and for non-linear response experiments, the mode with the 37 ns interval (27 MHz mode) has been selected.

In this mode, the light intensity per micro-pulse was greatly increased and used as the pump source. In both modes, beam conditions are searched for the best monochromaticity and stability, and the irradiation energy is up to 10 mJ/macro-pulse in the 27 MHz mode and up to 1 mJ/macro-pulse in the 108 MHz mode. Figure 2 shows the typical wavelength dispersions of the THz-FEL in the (a) 108 MHz mode and the (b) 27 MHz mode. Although the monochromaticity differs much depending on the beam parameters and the FEL mode, the bandwidth is approximately 3% at the 108 MHz mode and approximately 10% at the 27 MHz mode under the best monochromaticity conditions. Actually, deep care must be taken when one adjusts the beam condition to make the energy maximized in the 27 MHz mode, because it often shows a wideband wavelength spectrum that can no longer be considered monochromatic (Appendix A).

**Figure 2.** FEL spectra for different undulator gap values and modes. (**a**) Spectrum at variable gaps in the range of 30–37 mm at 108 MHz (monochromatic condition); (**b**) Spectrum at variable gaps in the range of 32–37 mm at 27 MHz (monochromatic condition).

#### **3. Spectral Imaging Using THz-FEL**

So far, THz imaging using various light sources and detectors has been performed, and technologies have been developed as in other wavelength regions [19–24]. Among them, time domain spectroscopy (TDS), quantum cascade laser (QCL), and terahertz parametric generator (TPG) using LiNbO3 non-linear optical crystals have been employed for the imaging at laboratory level below the 3 THz frequency domain. Moreover, the near field method has enabled the possibility of reaching a spatial resolution below the diffraction limit. However, THz/FIR radiation may not compete with X-rays in terms of transmittance and spatial resolution. That is, the most important motivation for imaging at the THz/FIR domain should be the observation of materials that have a characteristic response in this wavelength range. From this point of view, only a few imaging experiments at arbitrary monochromatic wavelengths, which can be called spectral imaging, are so far reported in the region below 3 THz [25,26]. In other words, conventional THz imaging has been hardly performed by probing the precise wavelength dependence of different materials characterized by specific absorptions, and most researchers have performed imaging by selecting an example that matches the wavelength maximum of each light source.

In the following, we will describe the status of spectroscopy for imaging using long wavelength radiation sources. Such a radiation, when emitted from a storage ring (SR), is competitive because of the brilliance, which is much higher than any standard laboratory light source based on thermal radiation. In this respect, SR is highly effective for any spectroscopic measurement of small areas using condensing optics, e.g., for microspectroscopy [27–29]. However, even in the case of SR sources, the total photon number is not enough for many experiments, especially in the far-infrared region. In general, it is necessary to accumulate spectra for several tens of seconds to several minutes using a Fourier transform infrared (FT-IR) spectrometer even for a single acquisition. Furthermore, it is well known that diffraction grating type spectrometers are more demanding in terms of photon numbers. Therefore, an impractical long-time integration is frequently required in 2D scan spectral imaging. For example, a scan of 50 × 50 points would require approximately 7 h, even if a FT-IR instrument takes only 10 s to measure each point. In contrast, THz-FEL is a quasi-monochromatic light source, and it is several orders of magnitude more intense than SR infrared radiation for both total photon number and illuminance. Thus, the spectral imaging using FEL is certainly realistic even when monochromatized through a diffraction grating-type spectrometer. We will present and discuss high-sensitive spectral imaging experiments performed with arbitrary monochromatic wavelengths in the range of 50–100 μm, i.e., the frequency range of 3–6 THz.

#### *3.1. Optical Scheme and Beam Profile*

For spectral imaging, the quasi-monochromatic FEL emission, as shown in Figure 2a, must be further monochromatized. Figure 3 shows the layout of the monochromator, the focusing optics, and the sample stage system to be really employed. FEL radiation transferred through the vacuum tube is first monochromatized by the diffraction grating in the monochromator and propagates from vacuum to atmosphere, as a parallel beam through a diamond window. The monochromatic FEL radiation is focused at the sample position by the lens and refocused on the detector in the transmission arrangement, or in the alternative reflection layout. The lens used is an F = 1.97 (effective aperture Φ = 25.4 mm, focal length f = 50 mm) Tsurupica lens, and the detector used is a COHERENT Energy Max energy sensor calibrated in the THz region. The measurement sample stage is a SIGMAKOKI motorized x-y stage that allows a raster scan in the x-y plane simultaneously with the pulse timing of the FEL.

**Figure 3.** Schematic layout of the monochromator, the focusing optics, and the sample stage system installed downstream the FEL. The blue layout is for transmission imaging, while the red layout refers to reflection imaging.

Figure 4 shows (a) (left hand) the beam profiles of the THz-FEL taken by an uncooled THz imager (NEC Corporation, IR/V–T0831) [30] just after the vacuum window and (right hand) the focal point condensed by the Tsurupica lens and (b) the intensity profiles by a knife-edge scan near the focal point. The parallel beam of the THz-FEL obtained through a 1-inch diameter diamond vacuum window is focused to an area of approximately 350 μm in 2σ at the focal point. The beam profiles observed by the THz imager (Figure 4a), measured with a knife-edge scan (Figure 4b), show an almost Gaussian distribution in both parallel areas just after the vacuum window and the focal point. The beam profile distribution near the focal point was probed by a knife-edge scan at several points in the z-axis direction (optical axis direction). As a result, the Rayleigh length was estimated to be approximately 3.5 mm. As a consequence, we expect that a clear transmitting image can be obtained up to a thickness of the sample of approximately 7 mm or less.

**Figure 4.** The beam profiles of the ISIR FEL (**a**) taken with a THz camera for the parallel beam (left) and at the focal point (right), and (**b**) the spatial resolution measured with the knife-edge scan around the focal point.

#### *3.2. Spectral Imaging of Solid Samples*

In the spectral-imaging experiment, the sample is placed at the FEL focal position. A He-Ne laser, which go through the same FEL optics, is used as a guide to set the sample at the FEL focal spot. In our layout, the FEL radiation after monochromatization by the diffraction grating spectrometer is collected by the lens, and the raster scan of the sample is automated by the stage to obtain a 2D image. The spatial resolution is determined by the degree of FEL focusing and by the step of the translation stage that moves the sample during the scan. In order to ensure that the sample is irradiated with the FEL pulse at each observation point, the stage is moved stepwise at 5 Hz, which is synchronized with the timing system of the linear accelerator [31]. The intensity of the monochromatized FEL radiation is sufficient for transmission and reflection experiments using one macro-pulse per point. When a

raster scan of the sample is performed on an area of 2 cm × 2 cm with a scanning step Δ = 500 μm, i.e., 1600 points at 5 Hz, a 2D spectral image can be collected within approximately 6 min.

Figures 5–8 show the results of spectral images collected on pellets, 3D samples made by composite materials, opaque objects, and biological systems. Versatility and short time acquisitions represent the really unique capabilities of this layout. The time required for a single 2D image ranges from 5 to 60 min depending on the sample size and the raster-scan step. The relevance of this experiment resides in the observation of clear changes in the images using high-wavelength resolution. These were the experiments performed for the first time in the THz/FIR domain with such resolution and the first successful imaging of almost opaque objects at these wavelengths. We also successfully collected images showing non-linear behavior due to the high FEL intensity.

**Figure 5.** (**a**) Comparison of transmittance spectra of polytetrafluoroethylene (PTFE) and polypropylene (PP) by in situ observation; (**b**) 2D images of PTFE and PP pellets collected in transmission at λTHz = 50 μm.

**Figure 6.** (**a**) Comparison of transmittance spectra of Cu2O and CuO by in situ observation; (**b**) 2D images of Cu2O and CuO pellets collected in transmission at several THz wavelengths.

#### 3.2.1. Pellets

Figures 5 and 6 compare the spectral imaging of pellet samples collected in transmission. Figure 5a compares the infrared spectra of the Teflon (polytetrafluoroethylene, PTFE) and of the polypropylene (PP). PP has no intrinsic absorption in the THz/FIR region and is highly transparent. Therefore, as it can be seen in the figure, PP shows only a flat spectrum. On the other hand, PTFE shows a spectrum with an intense absorption peak around 50 μm. Both pellets are made from 100% pure powders of each material using a compression mold. Figure 5b shows the images of these pellets at the wavelength of the absorption peak in PTFE. The upper part is a visible image of the two pellets that can be hardly distinguishable, while collecting images at the absorption wavelength, a clear difference can be observed with a clear contrast. Next, imaging was performed on diluted pellets of CuO and Cu2O powder. With each powder, we made pellets at the dilution of 5% by weight together with PP powders.

Figure 6a shows the infrared spectra of CuO and Cu2O, respectively. CuO shows absorption peaks at the wavelengths of 61.5 μm and 67 μm while Cu2O shows an absorption peak at 68 μm. Figure 6b shows the images at the three absorption wavelengths and at 70 μm that are not absorbed by both CuO and Cu2O. It takes 6 min to collect each image. In panel 6(b), it can be seen that the image contrast changes with respect to the wavelength. In particular, the contrast between the images of CuO and Cu2O is clearly reversed with just a difference of 1 μm going from 67 μm up to 68 μm, clearly demonstrating the effectiveness of the monochromatic FEL for long-wavelength high-resolution spectral imaging.

#### 3.2.2. Composite Materials with A Fine Structure

In Figure 7a, transmission imaging of an oily marker having a pen tip of 0.5 mm was performed while keeping the lid so that the pen tip would not dry. The lid is almost transparent, and the metal part and the liquid reservoir inside can be observed as opaque parts. The outer diameter is approximately 1 cm at the maximum. A clear image is obtained with a large depth of focus, which is expected from the Rayleigh length estimated at the focal point.

**Figure 7.** Comparison of 2D images of (**a**) a marker; (**b**) a mahjong tile; (**c**) a peanut (two nuts in a shell); (**d**) water collected in transmission at λTHz approximately 90 μm.

#### 3.2.3. Opaque Systems

Particularly important applications of spectral images refer to hard and soft opaque objects or liquid substances. Mahjong tiles are made of lumps of urea resin and hardly transmit THz/FIR radiation. However, when transmission imaging was performed using FEL with a high-intensity monochromatic beam, we succeeded in observing the engraved character on the surface from behind, as shown in Figure 7b. Since the character is observed from behind, the left and right of the image is reversed. The contrast observed is due to the difference in the reflection and the scattering contributions of the

FEL radiation between the flat part of the tile surface and the regions of the engraved text. There are several successful examples of THz transmission images of nuts at 0.1 THz (λTHz = 3000 μm) [32]. However, as far as we know, Figure 7c is the first in situ observation of dried peanut without peeling at approximately 3 THz (λTHz approximately 100 μm). The outer shell is shown with the orange color, while the two nuts touching each other can be recognized as dark areas in Figure 7c. Between the two nuts inside the shell, the lower nut seems to be drier and the internal embryo is seen through. Since the image including the outer shell is collected at the maximum FEL intensity, the detector was saturated in vacant space outside of the shell, causing a noise pattern due to the incorrect acquisition of the signal.

THz can be used also to collect images of liquids such as water. As an example, an image shown in Figure 7d was taken of a container filled with approximately 1 cm thickness of water. FEL radiation penetrates where the thickness of water is reduced: the neck and edge of the container, and the edge of water surface raised by surface tension. At the neck position with a red cap, the water thickness is approximately 2–3 mm and, to the best of our knowledge, in Figure 7d, we show the first THz transmission image of pure water. Since the image was obtained with a step scan over a wide area with a space resolution of 0.3 mm, it took one hour to collect this high-spatial resolution transmission image. The noise pattern near the edge of container was caused by the same reason of Figure 7c.

#### 3.2.4. Leaves

THz images of leaves showing water distribution are already available in other studies. At ISIR, we tried to investigate their wavelength dependence and time dependence. The wavelength selected for imaging lies in a region where there is no absorption by atmospheric water vapor. At variegation spots highlighted by a dark color in the visible image shown in Figure 8a, the FEL radiation is well transmitted through leaf at any wavelength, even in fresh conditions. It is important to emphasize here, from these experimental results and those of another leaf in Figure 8b, that moisture and nutrients are not homogeneously distributed in leaves.

The image collected at 103 μm is the first image of the present measurement, but it is also measured with the highest transmission compared to the images collected at 89 μm and 83 μm. We would like to point out that the difference between these three images at different wavelengths is not merely due to the amount of the water content through the evaporation occurring with time, but it is due to the different absorbance versus wavelength. The reproducibility of the image at 103 μm was confirmed by measuring it twice, at the beginning and at the end of the experimental series after 30 min.

In order to evaluate how much water remains in a leaf after being detached, several images of a leaf hanged in air for 10 h were collected at several wavelengths, as shown in Figure 8b. Since this measurement is not time-sensitive, four different wavelengths were selected for imaging. Two images at 103 μm, taken twice with different FEL power, are also shown for reference.

Comparisons of the different images in Figure 8b pointed out that at the wavelength of 67 μm, there is still a large absorption in the whole leaf area, while at 103 μm, FEL radiation is almost completely transmitted through, making the pattern of the vein unrecognizable. The strong wavelength dependence in the dried leaf suggests that the decrease of the transmittance in these THz images of leaves is not only due to water, but also due to other substances present in the veins, such as nutrients. Therefore, we assume that water in leaves was almost evaporated after approximately 10 h, while nutrients still remained in the leaf veins, as we can understand from the observed large wavelength dependence. In the freshly detached leaf showed in Figure 8a, nutrients and moisture were homogeneously distributed throughout all veins of the leaf. However, in the leaf that was hanged and dried in air shown in Figure 8b, it is presumed that only the nutrients were concentrated in the lower part of the leaf during water evaporation. In addition, when the power of the FEL is changed as the intensity is halved at the wavelength of 103 μm, the vein pattern comes to be exposed again. One possibility resides in the nutrients in the veins remaining after water evaporation showing non-linear transmittance with respect to FEL intensity. The component analysis and the survey of the distribution of nutrients in

leaves are indispensable for future biological science researches. The observed non-linear response can be the first case in THz imaging and could provide very useful information for future studies.

**Figure 8.** (**a**) shows a comparison of 2D images of a fresh leaf collected in transmission at different THz wavelengths. (**b**). Comparison of 2D images of a dried leaf collected in transmission at different THz wavelengths and different FEL powers.

#### **4. Conclusions**

We performed unique spectral-imaging experiments in the THz/FIR range using a grating-type spectrometer, a focusing optics, and a sample stage synchronized with the time structure of the high power ISIR THz-FEL at Osaka University. Successful different images with high wavelength resolution of Δλ = 1 μm have been obtained by tuning the gap of the undulator and the spectrometer. Moreover, thanks to the FEL time structure, fast spectral imaging was also feasible on different samples in various forms. Taking advantage of the high power of the FEL source, for the first time in the THz/FIR domain, we succeeded in the acquisition of transmission images of "opaque" solid materials and liquids. The versatile image acquisition system with raster scanning allowed monitoring changes in the composition of leaves. We also showed for the first time that non-linear phenomenon can be observed in images on changing the FEL intensity.

These results clearly point out the huge opportunities of these coherent high-power, high brilliance sources and the potential of scientific and technological researches in the THz/FIR domain and its interdisciplinary nature. The introduction of new sources, new detectors, control systems, and more advanced analysis techniques will be in great demand in the coming years.

**Author Contributions:** Conceptualization, project administration, A.I.; data curation, A.I. and M.F.; funding acquisition, A.I., S.L., A.M. and S.S.; investigation, A.I., K.K., M.F., R.K., H.F., A.H., S.M. and L.T.; resources, A.I., M.F., K.K. and R.K.; software, A.I., M.F., K.K. and R.K.; Writing—original draft, A.I; Writing—review and editing, A.M. and S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by JSPS KAKENHI Grant No.JP17K18989, by the research program "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials," and by the Bilateral Cooperation Agreement between Italy and Japan of the *Italian Ministry of Foreign A*ff*airs and of the International Cooperation* (MAECI) in the framework of the project of major relevance N. PGR0072.

**Acknowledgments:** We acknowledge K. Furukawa, and Y. Okada for their invaluable support during the THz-FEL operation.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A**

The emission characteristics of the ISIR THz-FEL may change greatly depending on the beam setting. When measuring the wavelength spectrum, the bandwidth, the peak intensity, and the total energy have to be changed. As described in the text, when the total energy is set to the maximum at 27 MHz, the bandwidth significantly increases and shows a complex spectral shape (Figure A1).

**Figure A1.** Comparison of two FEL spectra in the 27 MHz mode for the 33 mm (red) and the 37 mm (black) gap (wideband condition).

In this condition, the area of the spectrum, that is proportional to the total energy, increases about three times (approximately 30 mJ/macro-pulse) [33] or more in this wideband scenario if compared with the monochromatic setup (approximately 10 mJ/macro-pulse). However, the maximum intensity at the central wavelength does not increase, as shown in Figure A2. Therefore, such beam conditioning has no advantage for spectral imaging when the FEL is monochromatized by a diffraction grating. At variance, it is detrimental, since it is characterized by higher beam instability. Meanwhile, the pulse width estimated from the autocorrelation of the pulsed FEL is shortened from approximately 20 ps to about several ps by adjusting to the wideband condition, as shown in Figure A3. It must be always clarified whether a short pulse or a monochromatic beam is required for the THz radiation in each experiment, and then it sets the FEL with the necessary parameters.

**Figure A2.** Comparison of two FEL spectra in the wideband (black) and monochromatic (purple) condition in the 27 MHz mode and the undulator gap sets to 37 mm.

**Figure A3.** The interference waveform of the pulsed FEL by autocorrelation.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
