Abstract-Preference of subpicosecond laser pulses for terahertz wave generation from liquids

Qi Jin,  Yiwen E,  Shenghan Gao,  Xi-Cheng Zhang,

https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-2/issue-01/015001/Preference-of-subpicosecond-laser-pulses-for-terahertz-wave-generation-from/10.1117/1.AP.2.1.015001.full

Terahertz (THz) wave generation from laser-induced air plasma generally requires a short temporal laser pulse. In contrast, it was observed that THz radiation from ionized liquid water prefers a longer pulse, wherein the mechanism remains unclear. We attribute the preference for longer pulse duration to the process of ionization and plasma formation in water, which is supported by a numerical simulation result showing that the highest electron density is achieved with a subpicosecond pulse. The explanation is further verified by the coincidence of our experimental result and simulation when the thickness of the water is varied. Other liquids are also tested to assure the preference for such a pulse is not exclusive to water.

Abstract-Enhancement of terahertz emission by a preformed plasma in liquid water

Applied Physics Letters

Yiwen E, Qi Jin, X.-C. Zhang,

Experimental setup. (a) Two collinearly propagating main- and prepump beams are focused by a lens to ionize a water line (top view). The time delay (Δτ) between two optical pumps is controlled by a translation stage. The THz signal is detected through electro-optical sampling (EOS) in the forward direction. Additionally, the pulse duration is monitored by an optical autocorrelator. (b) A photo of a 210 μm water line (side view).

https://aip.scitation.org/doi/abs/10.1063/1.5119812

Terahertz (THz) wave generation from liquids under optical excitation has been experimentally confirmed. Here, we report the observation of THz emission enhancement from liquid water with a preformed plasma. Two collinear optical beams with a controlled time delay are focused into a liquid water line. With a plasma created by the first optical pump, the THz emission generated by the second pump is enhanced significantly. By using the same total incident energy compared to the commonly used single-pump excitation, an enhancement over 8 times is observed when the prepump is s-polarized. This observation provides an alternative strategy to boost THz generation from liquids and helps to further understand the laser-liquid interaction process.

The research at the University of Rochester was sponsored by the Army Research Office under Grant No. W911NF-17-1-0428, Air Force Office of Scientific Research under Grant No. FA9550-18-1-0357, and National Science Foundation under Grant No. ECCS-1916068.

Abstract-Spatial sampling of terahertz fields with sub-wavelength accuracy via probe-beam encoding

Jiapeng Zhao, Yiwen E, Kaia Williams, Xi-Cheng Zhang,  Robert W. Boyd, 

Fig. 1: Experimental Configuration

https://www.nature.com/articles/s41377-019-0166-6

Recently, computational sampling methods have been implemented to spatially characterize terahertz (THz) fields. Previous methods usually rely on either specialized THz devices such as THz spatial light modulators or complicated systems requiring assistance from photon-excited free carriers with high-speed synchronization among multiple optical beams. Here, by spatially encoding an 800-nm near-infrared (NIR) probe beam through the use of an optical SLM, we demonstrate a simple sampling approach that can probe THz fields with a single-pixel camera. This design does not require any dedicated THz devices, semiconductors or nanofilms to modulate THz fields. Using computational algorithms, we successfully measure 128 × 128 field distributions with a 62-μm transverse spatial resolution, which is 15 times smaller than the central wavelength of the THz signal (940 μm). Benefitting from the non-invasive nature of THz radiation and sub-wavelength resolution of our system, this simple approach can be used in applications such as biomedical sensing, inspection of flaws in industrial products, and so on.

Abstract-Flat liquid jet as a highly efficient source of terahertz radiation

Anton N. Tcypkin, Evgenia A. Ponomareva, Sergey E. Putilin, Semen V. Smirnov, Sviatoslav A. Shtumpf, Maksim V. Melnik, Yiwen E, Sergei A. Kozlov, and Xi-Cheng Zhang

Fig. 1 Experimental setup of terahertz generation in flat liquid jets. (a) Experimental layout for energy and spectral terahertz measurements (the inset shows an illustration of optical incident angle ϕ). Laser radiation is splat on pump and probe beams with beam-splitter (BS) with ratio of energy in the channels 1:49, for probe and pump, respectively. Parabolic mirror (PM1 with focal length equal 5 cm) focus the pump radiation on a liquid jet which leads to the generation of terahertz radiation asa result of filamentation inside ionizing liquid jet. The terahertz radiation is collected and collimated by TPX lens (TL) filtered by a teflon filter (F). For spectrum measurements we use conventional electro-optical system (EOS). Parabolic mirror (PM2 with focal length equal 12 cm) focus the terahertz radiation on the ZnTe crystal (EOC) with 1 mm thickness. (b) Photo of laser excitation of the liquid jet. Water moisture plum scatter the laser beam. Temporal terahertz signals (c) and spectrum (d) emitted from the jets of water and ethanol with a thickness of 150 μm at laser pulse duration of 400 fs and optical excitation energy of 600 μJ.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-11-15485

Polar liquids are strong absorbers of electromagnetic waves in the terahertz range, therefore, historically such liquids have not been considered as good candidates for terahertz sources. However, flowing liquid medium has explicit advantages, such as a higher damage threshold compared to solid-state sources and more efficient ionization process compared to gases. Here we report systematic study of efficient generation of terahertz radiation in flat liquid jets under sub-picosecond single-color optical excitation. We demonstrate how medium parameters such as molecular density, ionization energy and linear absorption contribute to the terahertz emission from the flat liquid jets. Our simulation and experimental measurements reveal that the terahertz energy has quasi-quadratic dependence on the optical excitation pulse energy. Moreover, the optimal pump pulse duration, which depends on the thickness of the jet is theoretically predicted and experimentally confirmed. The obtained optical-to-terahertz energy conversion efficiency is more than 0.05%. It is comparable to the commonly used optical rectification in most of electro-optical crystals and two-color air filamentation. These results, significantly advancing prior research, can be successfully applied to create a new alternative source of terahertz radiation.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Investigation of liquid lines as terahertz emitters under ultrashort optical excitation

Qi Jin, Yiwen E, Shenghan Gao, Xi-Cheng Zhang

https://arxiv.org/abs/1902.07150

Recently, there has been growing interest in terahertz (THz) wave generation from liquids under optical excitation. Here, we propose and demonstrate the use of liquid lines in place of liquid films as THz emitters to boost THz signals. The geometry of the emitter eliminates the total internal reflection at the flat liquid-air interface. In addition, we observe that the polarity of the liquid has a significant influence on the THz wave generation. Alpha-pinene, a nonpolar liquid, offers much stronger THz radiation than water does. Besides paving the way to develop intense liquid THz sources, our work indicates that THz waves could be a tool for the further study of laser-liquid interaction.

Abstract-Spatial Sampling of Terahertz Fields with Sub-wavelength Accuracy via Probe Beam Encoding

Jiapeng Zhao, Yiwen E, Kaia Williams, Xi-cheng Zhang, Robert Boyd

https://arxiv.org/abs/1901.05346

Recently, computational sampling methods have been implemented to spatially characterize terahertz (THz) fields. Previous methods usually rely on either specialized THz devices such as THz spatial light modulators, or complicated systems requiring assistance from photon-excited free-carriers with high-speed synchronization among multiple optical beams. Here, by spatially encoding an 800 nm near-infrared (NIR) probe beam through the use of an optical SLM, we demonstrate a simple sampling approach that can probe THz fields with a single-pixel camera. This design does not require any dedicated THz devices, semiconductors or nanofilms to modulate THz fields. Through the use of computational algorithms, we successfully measure 128×128 field distributions with a 62 μm transverse spatial resolution, more than 15 times smaller than the central wavelength of the THz signal (940 μm). Benefitting from the non-invasive nature of THz radiation and sub-wavelength resolution of our system, this simple approach can be used in applications such as biomedical sensing, inspection of flaws. in industrial products, and so on.

Abstract-Terahertz wave emission from a liquid water film under the excitation of asymmetric optical fields

Applied Physics Letters

Qi Jin, Jianming Dai, Yiwen E, Xi-Cheng Zhang,

Schematic diagram of the experimental setup. A phase compensator (PC) is applied to control the relative phase between ω and 2ω pulses. DWP, dual-wavelength wave plate. PM, parabolic mirror with an effective focal length of 1-in

https://aip.scitation.org/doi/10.1063/1.5064644

Liquid water excited by intense two-color laser pulses radiates electromagnetic waves at terahertz frequencies. Compared with one-color excitation, two-orders of magnitude enhanced terahertz energy are observed by using asymmetric optical excitation with the same total excitation pulse energy and focusing geometry. Modulation of the terahertz field is achieved via the coherent control approach. We find that modulated and unmodulated terahertz energies have, respectively, quadratic and linear dependence on the laser pulse energy. This work, as part of terahertz aqueous photonics, paves an alternative way of studying laser-liquid interactions and developing intense terahertz sources.

Abstract-Propagation of terahertz waves in a monoclinic crystal BaGa4Se7

Yiwen E, Jiyong Yao,  Li Wang, 

https://www.nature.com/articles/s41598-018-34552-y

The complex symmetric dielectric tensor of a monoclinic crystal cannot be diagonalized by a space rotation operation in general, which poses a serious difficulty in analyzing the propagation of electromagnetic fields in monoclinic crystals so far. This propagation issue is discussed in a general case without using the index ellipsoid scheme. It is found that, when incident waves travel along the mirror plane normal or 2-fold rotation axis of monoclinic crystals, two eigenmodes following specific dispersion relations are elliptically polarized with the same ellipticity and chirality but have spatially orthogonal elliptical principal axes. The frequency independent features are the unique manifestation of the crystal symmetry. Using polarization sensitive terahertz time-domain spectroscopy and our developed data analyzing and processing methods, three complex permittivity tensor elements for a monoclinic crystal BaGa4Se7 are straightforwardly extracted and the properties of the two eigenmodes are characterized in full. It is also interesting that the spectral components beyond 1.7 THz show a very high refraction index (>10) and low dissipation during propagation, which suggests that the bulk phonon-polariton waves may be excited and effectively propagate in the crystal, resulting from the coherent phonon excitations by the incident terahertz waves. Our results may promote to develop novel terahertz devices based on polariton excitation and propagation in monoclinic crystals.

Abstract-Coherent excitation of phonon polaritons in BaGa4Se7 by terahertz pulses

Bo Wang, Yiwen E, Jiyong Yao, and Li Wang

https://www.osapublishing.org/abstract.cfm?uri=fio-2018-JW4A.36&origin=search

Phonon polaritons are generated in BaGa4Se7 by linear excitation of terahertz pulses, and probed using 800 nm femtosecond laser pulses. The observed phonon polaritons can be perfectly reproduced by a damped harmonic oscillator model.

© 2018 The Author(s)

Generating terahertz radiation from water makes ‘the impossible, possible’

Researchers use lasers to generate terahertz pulses via interaction with a target. In this case, the target was an extremely thin water film—approximately 200 microns or about the thickness of two pieces of paper—created using water suspended between two aluminum wires. (University of Rochester photo / Kaia Williams)

http://www.rochester.edu/newscenter/generating-terahertz-radiation-from-water-makes-the-impossible-possible-271292/

Xi-Cheng Zhang has worked for nearly a decade to solve a scientific puzzle that many in the research community believed to be impossible: producing terahertz waves—a form of electromagnetic radiation in the far infrared frequency range—from liquid water.

Now, as reported in a paper published in Applied Physics Letters, researchers at the University of Rochester have “made the impossible, possible,” says Zhang, the M. Parker Givens Professor of Optics. “Figuring out how to generate terahertz waves from liquid water is a fundamental breakthrough because water is such an important element in the human body and on Earth.”

Terahertz waves have attracted increased attention recently because of their ability to nondestructively pass through solid objects, including those made of cloth, paper, wood, plastic, and ceramics, and produce images of the interiors of the objects. Additionally, the energy of a terahertz photon is weaker than an x-ray photon. Unlike x-rays, terahertz waves are non-ionizing—they do not have enough energy to remove an electron from an atom—so they do not have the same harmful effects on human tissue and DNA.

Because of these abilities, terahertz waves have unique applications in imaging and spectroscopy—everything from discovering bombs in suspicious packages, to identifying murals hidden beneath coats of paint, to detecting tooth decay.

“Terahertz waves have a capacity to see through clothing, which is why you have these sub-terahertz body scanners at airports,” Zhang says. “These waves can help to identify if an object is explosive, chemical, or biological, even if they can’t tell exactly what the object is.”

Zhang’s research group uses lasers to generate terahertz pulses via interaction with a target. In this case, the target is an extremely thin film of water—approximately 200 microns or about the thickness of two pieces of paper—created using water suspended by surface tension between two aluminum wires. Researchers focus a laser into the water film, which acts as an emitter for the terahertz radiation output.

The experimental set-up used to generate terahertz waves from liquid water. Researchers focus the optical pump beam into the water film and use a series of filters and off-axis parabolic mirrors (OAPMs) to detect the terahertz signal and block any other light waves simultaneously generated from the water film.

Previous researchers have generated terahertz waves from targets of solid crystals, metals, air plasma, and water vapor, but, until now, liquid water has proved elusive.

“Water was considered the enemy of terahertz waves because of water’s strong absorption,” Zhang says. “We always tried to avoid water, but it is a surprisingly efficient terahertz source.”

In fact, when researchers measured the terahertz waves generated by the water, they found they were 1.8 times stronger than the terahertz waves generated from air plasma under comparable experimental conditions.

Because water is such a strong absorber, however, many people in the research community believed it would be impossible to use water as a target. Zhang himself has spent years attempting a solution, and he found a likewise stalwart in Qi Jin, now a PhD candidate in optics at Rochester, and the lead author on the paper.

“Almost everybody thought we wouldn’t be able to get a signal from water,” Jin says. “At first, I didn’t believe it either.”

One of the challenges was creating a film of water thin enough that the terahertz photons generated by the laser beam would not be absorbed, but thick enough to withstand the laser’s energy.

Along with Yiwen E, a postdoctoral associate in Zhang’s research group, Jin spent months optimizing the thickness of the water film and the incident angle, intensity, and pulse duration of the laser beam.

“We increased the thickness of the water a little bit, and gradually increased the laser, and just kept trying until we could make it work,” Jin says.  “Water is one of the richest resources on Earth, so it was really important for us to be able to generate these waves from water. There were many times I wanted to give up on this, but people in the lab kept encouraging me.”

Zhang agrees: “I always tell my students and researchers here: if you try something, you might not get the result you wanted. But if you never try it, you definitely won’t get it.”

The research was sponsored by grants from the Army Research Office.

Abstract-Observation of broadband terahertz wave generation from liquid water

Qi Jin, Yiwen E, Kaia Williams, Jianming Dai,  X.-C. Zhang,

http://aip.scitation.org/doi/10.1063/1.4990824

Bulk liquid water is a strong absorber in the terahertz (THz) frequency range, due to which liquid water has historically been sworn off as a source for THz radiation. Here, we experimentally demonstrate the generation of broadband THz waves from liquid water excited by femtosecond laser pulses. Our measurements reveal the critical dependence of the THz field upon the relative position between the water film and the focal point of the laser beam. The THz radiation from liquid water shows distinct characteristics when compared with the THz radiation from air plasmas with single color optical excitation. First, the THz field is maximized with the laser beam of longer pulse durations. In addition, the p-polarized component of the emitted THz waves will be influenced by the polarization of the optical excitation beam. It is also shown that the energy of the THz radiation is linearly dependent on the excitation pulse energy.

With successive development, numerous research groups have demonstrated terahertz (THz) wave generation from solid crystals,2,3metals,4–6 gas plasmas7,8and water vapors.9 Nevertheless, THz wave generation from a liquid state has not been demonstrated. In fact, liquid water has been well studied as a source for various electromagnetic waves for over 10 years.10–12 For example, white-light generation10 and high-harmonic generation11,12 due to nonlinear processes from ultrashort-pulse lasers focused into water contained in cells, jets, or droplets were previously reported. Additionally, the dynamics of liquid water irradiated by laser pulses have been investigated to achieve a better understanding of laser-water interactions for over two decades.12–17 However, liquid water has not been demonstrated as a source for THz radiation. One possible reason leading to the impediment is that liquid water has strong absorption characteristics in the THz frequency regime. The power absorption coefficient of 220 cm−1 at 1 THz18,19 means that only one photon at 1 THz can go through 1 mm thick water with 3.6×109$3.6\times {10}^9$ THz photons entered. To mitigate the considerable loss of THz waves, the water with much less than 1 mm thickness is an intuitive choice to study THz wave generation. Recently, gravity-driven, free-flowing water films have been efficaciously used owing to their simple design and almost unmatched ability to generate a thin, continuous, and stable film of liquid water in free space,20,21 which offers us the liquid source for the THz radiation.

In this letter, we experimentally demonstrate broadband THz wave generation from liquid water and investigate the influence of optical pulse duration, polarization, and excitation energy on the THz radiation energy. The set-up is schematically shown in Fig. 1. An amplifier laser with a central wavelength of 800 nm and a repetition rate of 1 kHz is used. An optical polarizer is placed to verify the p-polarization of the optical beam. Two aluminum wires are used to form the water film. These wires have a diameter of 170 μm and are separated by about 4 mm. The incident angle of the laser beam on the water film is tilted to 25° from the normal to reduce the water sputtering onto the surface of optics. The thickness of the water film is 177±8 $177\pm 8\hspace{0.17em}$μm, which can be adjusted by throttling the water flow rate. The thickness is measured and calibrated using an optical second-harmonic intensity autocorrelator.21 The laser beam is focused into the water film using a 1 inch effective focal length parabolic mirror, forming a plasma inside the water film. Filters are placed to block the remaining pump laser light and any white light simultaneously generated from the water film in addition to the THz radiation. A tungsten wire-grid polarizer acts as the THz polarizer. Standard electro-optical sampling22 with a 3 mm thick ZnTe is used to detect the THz field. The flow rate of the water is about 1.3 m/s, meaning that the water film flows about 1.3 mm between two laser pulses. This distance is much greater than the diameter of the focal spot of the laser beam, which indicates that each THz pulse will not be affected by previous interactions between the water and laser pulses.

FIG. 1.Experimental setup. Broadband THz wave is generated by tightly focusing the optical laser beam into a gravity-driven wire-guided free-flowing water film. OAPM, off-axis parabolic mirror; HWP, half-wave plate.

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The waveform of the THz wave generated from liquid water is shown as curve B in Fig. 2(a). To confirm that the THz radiation is mainly emitted from the water film and has no contributions from the air plasma, the water film is translated along the direction of laser propagation. The pulse duration of the laser is about 550 fs during these measurements. The schema of relative positions between the water film and the plasma is shown on the left of Fig. 2(a). The corresponding THz waveforms are plotted on the converse side of Fig. 2(a). For curve A, the focal point of the laser is behind the film: the laser beam passes through the water and is focused to generate THz waves from an air plasma. For curve B, the focal point is near the center of the water film: a plasma is formed inside the water film, and the THz field emitted from liquid water is measured. For curve C, the laser beam is focused and forms an air plasma before the water film. Very weak THz radiation is detected due to the strong absorption of the water film. We note that the THz signals from air plasmas can be clearly observed if the thickness of the water film is reduced to 100 μm or less. Curve D is shown as a reference: no water film is present, and only THz generated from air plasmas is detected.

FIG. 2.Measurements of the THz fields when the water film is translated along the direction of laser propagation. (a) THz waveforms are plotted from curves A–C when the water film is before, near, and after the focus, respectively; Curve B shows the THz waveform generated from liquid water; Curve D is the reference with no water film. Yellow spark and bluish pane represent the plasma and the water film, respectively. The THz emission angle shown is not meant to be an indicative of the actual THz emission pattern. (b) THz waveforms when the water film is moved near the focal point. The 0 position is set to the place with the strongest THz field. Relative positions are listed with corresponding waveforms. The negative sign means that the water film is located after the focal point. The positive sign indicates the opposite case. (c) Comparison between the THz field from water and that from air plasma in the frequency domain. The dashed, solid, and dotted spectra correspond to curve A, curve B, and curve D in (a), respectively.

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By scanning the water film along the optical axis, THz radiation from different sources can be clearly differentiated. The timing distinctions in the waveforms in Fig. 2(a) are indicative of different generation sources. A time delay is observed from the THz waveform from liquid water compared with other generations. Figure 2(b) shows the measurements of THz waveforms as the water film is tracked along the direction of laser propagation marking a relative position across –60 μm to +60 μm. The measurement shows that the emitted THz waves are significantly sensitive to the relative position between the water film and the focus. The THz radiation can be detected only within a roughly 60 μm scanning range of the water film. It should be mentioned that no THz radiation is detectable when only part of the plasma is located outside the range of the water film. The plasma located at the interface between the surface of the water film and air does not give a spurious THz signal.

A comparison of the THz waveforms from liquid water and air plasma is shown in Fig. 2(a). In this measurement, the THz field from the water film is 1.8 times stronger than that from the air plasma. The corresponding comparison in the frequency domain is shown in Fig. 2(c). The measured bandwidth can be limited by the stretch of the probe laser pulses. The measured THz radiation from the water has more low-frequency and less high-frequency components. In addition, the bandwidth is narrower than the signal from the air plasma. We note that the air plasma generates a stronger THz field if a shorter pulsed laser is used.

To further study the dependence of the THz radiation on the optical pulse duration, the frequency chirp of the optical laser pulse is changed to achieve different pulse durations. Figure 3(a) shows normalized THz energy from water and air plasma versus various optical pulse durations. The THz energy from water and air plasma is normalized to their maxima, respectively. The optical pulse duration is at its minimum of 58 fs$58\hspace{0.17em}\mathrm{f}\mathrm{s}$ when no chirp is applied. The left of Fig. 3(a) shows the case of negative chirps, where the low-frequency component of the pulse lags the high-frequency component. Positive chirps indicate the opposite, and the corresponding measurements are shown on the right of Fig. 3(a). It can be observed that unlike the THz radiation from the air plasma, where the signal is maximized at a minimum pulse duration with no additional chirp, liquid water generates a maximum field at longer pulse durations. Furthermore, by comparing the left part and the right part of Fig. 3(a), it is shown that the frequency chirp of the optical beam is not a dominant factor compared with the contribution from the optical pulse duration. The above observation may result from the dependence of the ionization process in water upon the optical pulse duration. Multiphoton ionization and cascade ionization23,24 are the main processes of the free electron generation in distilled water with the laser pulses in our experiments. The contributions from the two ionization processes are varied with the optical pulse duration. With a longer pulse duration, cascade ionization dominates the process, leading to an exponential increase in the number of electrons. The THz radiation from liquid water may benefit from higher density of electrons in the water, which may cause the THz emission to be dependent on the optical pulse duration as shown in Fig. 3(a).

FIG. 3.(a) Normalized THz energy from liquid water and air plasma with different pulse durations of the laser beam. Black squares represent the THz energy from liquid water, and red dots represent the air plasma case. The optical pulse duration is at its minimum of 58 fs$58\hspace{0.17em}\mathrm{f}\mathrm{s}$ when no frequency chirp is applied. Negative chirps are applied to increase the optical pulse duration (left), while the case of positive chirps is shown (right). The energy of the laser pulse is 400 μJ for these measurements. (b) The energy of P-polarized THz field from liquid water with different linearly optical polarizations. 0°$0°$ refers to the p-polarized optical beam, and 90°$90°$ refers to the optical beam with s-polarization. (c) Normalized THz energy from liquid water as a function of incident optical pulse energy. The water film breaks when the energy of the excitation pulse is over 420 μJ.

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The relationship between the THz radiation and the optical polarization is exposed by measuring the THz field with different polarizations of the optical excitation beam. Considering the strong reflection of the s-polarized THz waves caused by the water-air interface, a wire-grid THz polarizer is applied to help to measure the p-polarized component of the THz field. The polarization of the optical beam is rotated by a half-wave plate (HWP) in the optical path (see Fig. 1). The corresponding result is shown in Fig. 3(b). 0°$0°$ refers to the p-polarized optical beam, and 90°$90°$ refers to the optical beam with s-polarization. It is shown that strong THz radiation is achieved with a p-polarized optical beam, while an s-polarized optical beam offers sparse contribution. One possible explanation is that the polarization of the THz field is dependent on the optical polarization, as the measured p-polarized component of the THz field has a nearly cosine squared relationship with the angle of the optical polarization. The result of the THz radiation from liquid water goes against the case of the single color air plasma THz generation, in which the ponderomotive force is dominantly involved. It is well known that the THz radiation from the air plasma with single color optical excitation does not depend upon the polarization of the optical beam,25,26 which means that the THz radiation energy will remain constant with various optical polarizations. Furthermore, a linear energy dependence observed in Fig. 3(c) is different from the quadratic relation of the single color air plasma THz generation.27 Figure 3(c) also shows a laser excitation pulse energy threshold at about 160 μJ$160\hspace{0.17em}\mu \mathrm{J}$ for the detectable THz field from liquid water. The 177 μm$177\hspace{0.17em}\mu \mathrm{m}$ thick water film will break when the energy of the excitation pulse is over 420 μJ$420\hspace{0.17em}\mu \mathrm{J}$. This rupture may be caused by shock waves and plasma expansion, and water ejection occurs when high-intensity laser pulses are focused into liquid water.13–17 These effects weaken the stability of the water film as the laser energy is increased.

In summary, we have observed broadband THz wave generation by focusing high-intensity laser pulses into a free-flowing water film. Compared with THz radiation generated from the air plasma, the THz radiation from liquid water has a distinct response to various optical pulse durations and shows linear energy dependence upon incident laser pulses. In addition, the optical polarization affects the THz radiation energy from liquid water. The reported results may not be fully interpreted through the extant understanding of the mechanism of THz wave generation. Our observations point towards potential research topics in the fields of THz and infrared radiation. Additionally, we predict that this work will contribute to the exploration of laser-liquid interactions and their future as THz sources.

Our research was sponsored by the Army Research Office and was accomplished under Grant Nos. W911NF-16-1-0381 and W911NF-16-1-0436. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

REFERENCES

1. 1.X. C. Zhang , A. Shkurinov , and Y. Zhang , Nat. Photonics 11(1), 16 (2017). https://doi.org/10.1038/nphoton.2016.249, Crossref, CAS
2. 2.F. Blanchard , G. Sharma , L. Razzari , X. Ropagnol , H.-C. Bandulet , F. Vidal , R. Morandotti , J.-C. Kieffer , T. Ozaki , and H. Tiedje , IEEE J. Sel. Top. Quantum Electron. 17(1), 5 (2011). https://doi.org/10.1109/JSTQE.2010.2047715, Crossref, CAS
3. 3.K.-L. Yeh , M. C. Hoffmann , J. Hebling , and K. A. Nelson , Appl. Phys. Lett. 90(17), 171121 (2007). https://doi.org/10.1063/1.2734374, Scitation
4. 4.F. Kadlec , P. Kužel , and J.-L. Coutaz , Opt. Lett. 29(22), 2674 (2004). https://doi.org/10.1364/OL.29.002674, Crossref
5. 5.G. H. Welsh , N. T. Hunt , and K. Wynne , Phys. Rev. Lett. 98(2), 026803 (2007). https://doi.org/10.1103/PhysRevLett.98.026803, Crossref
6. 6.J. Dai and X.-C. Zhang , Opt. Lett. 39(4), 777 (2014). https://doi.org/10.1364/OL.39.000777, Crossref, CAS
7. 7.T. Löffler , F. Jacob , and H. G. Roskos , Appl. Phys. Lett. 77(3), 453 (2000). https://doi.org/10.1063/1.127007, Scitation, CAS
8. 8.D. J. Cook and R. M. Hochstrasser , Opt. Lett. 25(16), 1210 (2000). https://doi.org/10.1364/OL.25.001210, Crossref, CAS
9. 9.K. Johnson , M. Price-Gallagher , O. Mamer , A. Lesimple , C. Fletcher , Y. Chen , X. Lu , M. Yamaguchi , and X. C. Zhang , Phys. Lett. A 372(38), 6037 (2008). https://doi.org/10.1016/j.physleta.2008.07.071, Crossref, CAS
10. 10.V. P. Kandidov , O. G. Kosareva , I. S. Golubtsov , W. Liu , A. Becker , N. Akozbek , C. M. Bowden , and S. L. Chin , Appl. Phys. B: Lasers Opt. 77(2), 149 (2003). https://doi.org/10.1007/s00340-003-1214-7, Crossref, CAS
11. 11.S. J. McNaught , J. Fan , E. Parra , and H. M. Milchberg , Appl. Phys. Lett. 79(25), 4100 (2001). https://doi.org/10.1063/1.1426266, Scitation, CAS
12. 12.A. Flettner , T. Pfeifer , D. Walter , C. Winterfeldt , C. Spielmann , and G. Gerber , Appl. Phys. B: Lasers Opt. 77(8), 747 (2003). https://doi.org/10.1007/s00340-003-1329-x, Crossref, CAS
13. 13.J.-Z. Zhang , J. K. Lam , C. F. Wood , B.-T. Chu , and R. K. Chang , Appl. Opt. 26(22), 4731 (1987). https://doi.org/10.1364/AO.26.004731, Crossref, CAS
14. 14.C. B. Schaffer , N. Nishimura , E. N. Glezer , A. M.-T. Kim , and E. Mazur , Opt. Express 10(3), 196 (2002). https://doi.org/10.1364/OE.10.000196, Crossref, CAS
15. 15.F. Courvoisier , V. Boutou , C. Favre , S. C. Hill , and J.-P. Wolf , Opt. Lett. 28(3), 206 (2003). https://doi.org/10.1364/OL.28.000206, Crossref
16. 16.A. Lindinger , J. Hagen , L. D. Socaciu , T. M. Bernhardt , L. Wöste , D. Duft , and T. Leisner , Appl. Opt. 43(27), 5263 (2004). https://doi.org/10.1364/AO.43.005263, Crossref, CAS
17. 17.C. A. Stan , D. Milathianaki , H. Laksmono , R. G. Sierra , T. A. McQueen , M. Messerschmidt , G. J. Williams , J. E. Koglin , T. J. Lane , and M. J. Hayes , Nat. Phys. 12(10), 966 (2016) https://doi.org/10.1038/nphys3779. Crossref, CAS
18. 18.L. Thrane , R. H. Jacobsen , P. Uhd Jepsen , and S. R. Keiding , Chem. Phys. Lett. 240(4), 330 (1995). https://doi.org/10.1016/0009-2614(95)00543-D, Crossref, CAS
19. 19.C. Ro/nne , L. Thrane , P.-O. Åstrand , A. Wallqvist , K. V. Mikkelsen , and S/r. R. Keiding , J. Chem. Phys. 107(14), 5319 (1997). https://doi.org/10.1063/1.474242, Scitation, CAS
20. 20.M. J. Tauber , R. A. Mathies , X. Chen , and S. E. Bradforth , Rev. Sci. Instrum. 74(11), 4958 (2003). https://doi.org/10.1063/1.1614874, Scitation, CAS
21. 21.T. Wang , P. Klarskov , and P. U. Jepsen , IEEE Trans. Terahertz Sci. Technol. 4(4), 425 (2014). https://doi.org/10.1109/TTHZ.2014.2322757, Crossref, CAS
22. 22.Q. Wu and X.-C. Zhang , Appl. Phys. Lett. 67(24), 3523 (1995). https://doi.org/10.1063/1.114909, Scitation, CAS
23. 23.P. K. Kennedy , IEEE J. Quantum Electron. 31(12), 2241 (1995). https://doi.org/10.1109/3.477753, Crossref, CAS
24. 24.J. Noack and A. Vogel , IEEE J. Quantum Electron. 35(8), 1156 (1999). https://doi.org/10.1109/3.777215, Crossref, CAS
25. 25.H. Hamster , A. Sullivan , S. Gordon , W. White , and R. W. Falcone , Phys. Rev. Lett. 71(17), 2725 (1993). https://doi.org/10.1103/PhysRevLett.71.2725, Crossref, CAS
26. 26.H. Hamster , A. Sullivan , S. Gordon , and R. W. Falcone , Phys. Rev. E 49(1), 671 (1994). https://doi.org/10.1103/PhysRevE.49.671, Crossref, CAS
27. 27.F. Buccheri and X.-C. Zhang , Optica 2(4), 366 (2015). https://doi.org/10.1364/OPTICA.2.000366, Crossref, CAS
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