Ultra-widely tunable superradiant terahertz free electron laser from electron microbunch trains

doi:10.21203/rs.3.rs-4949302/v1

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Ultra-widely tunable superradiant terahertz free electron laser from electron microbunch trains

https://doi.org/10.21203/rs.3.rs-4949302/v1

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High-intensity and widely tunable sources in the terahertz (THz) frequency range are highly desirable for both fundamental and applied researches. Free electron lasers (FELs) coupling between a relativistic electron beam and a copropagating electromagnetic wave in an undulator can generate intense THz light. However, most THz FEL facilities are still working with a relatively long and low-peak-current electron beam, limiting the emission efficiency and the THz pulse energy. Here, we demonstrate a superradiant THz FEL by pre-modulating the electron beam into microbunches which emit in phase and interact strongly with the generated THz waves in a one-meter-long undulator. The measurements show that the narrow-band radiation frequency can be tuned throughout the 1 THz to 15 THz range at the fundamental harmonic of electron microbunches and up to 20 THz at the second harmonic. The detected pulse energy reaches 150 microjoules at 10 THz without consideration of the transportation loss. Such kind of THz source is stable in terms of shot-to-shot pulse intensity and frequency, and is expected to enable many novel researches.

Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Free-electron lasers

Physical sciences/Optics and photonics/Optical physics/Terahertz optics

THz radiation is a fascinating tool that plays a pivotal role in a broad variety of scientific researches. For instance, THz radiation allows direct access to numerous low energy excitations, including phonons^1,2, magnons^3,4 and quasiparticle collisions^5,6, providing new insights in light-matter interactions. Intense THz waves can drive the non-perturbative nonlinearities, such as THz-field-induced phase transition⁷, coherent interband polarization⁸ and large-amplitude spin oscillation⁹. Although various sources in the THz range have been developed in recent decades, an intense source that can cover the whole THz band to fulfill the requirements of versatile applications is still scarce.

Free electron lasers are powerful THz sources as they can generate intense narrow-band THz radiation with wide tunability. Present THz FELs^10-15 are typically based on an oscillator configuration to accumulate power in an optical cavity, amplifying FEL pulses by successive electron bunches. In this oscillator mode, a relatively long and low-peak-current electron beam with a high repetition rate is always utilized to drive the FEL process. The energy extraction efficiency and the peak power is relatively low and the micropulse energy mainly depends on the electron bunch charge which is hard to be further increased under current accelerator technology. Modern accelerator technology can produce high-brightness electron beams with low emittance and low energy spread, making the single-pass high-efficiency THz FEL possible. One commonly used single-pass FEL mechanism is self-amplified spontaneous emission (SASE), which starts from the incoherent spontaneous radiation and amplifies this noise signal through long interaction length in the undulator¹⁶. Since the electron beam velocity is slower than the electromagnetic wave velocity in the undulator (slippage), the electron bunches must be many wavelengths long for the SASE process and consequently have low peak current, resulting in a long undulator length for saturation and limiting the energy extraction efficiency. A novel scheme in THz FEL uses a waveguide to slow the THz wave and match the wave group velocity with the electron velocity, realizing zero slippage in the undulator and strongly enhancing the beam-wave interaction¹⁷. However, this scheme has only been demonstrated at 0.16 THz, and the tunability is quite limited.

When the electron bunch is much shorter than the radiation wavelength, the fields emitted by different electrons will add coherently in phase and the resultant intensity is proportional to the square of the electron number, which is called superradiance¹⁸. For the beam consisting of a train of ultrashort electron microbunches, superradiant FEL process takes place when the spacing between microbunches is equal to the resonant wavelength of the radiator and the microbunches interact strongly with the generated electromagnetic wave to amplify the emission intensity. In the optical spectrum, electron bunches are usually density modulated with a seeding laser in a modulator. However, there are few suitable THz sources that can act as the seed to modulate the electron beam into microbunches with sub-picosecond spacing. Some experimental researches have studied the generation of THz electron microbunch trains by employing novel techniques^19-29, but the frequency tunability and the bunching factor are both limited. Until the work published in 2023, THz microbunch trains with high bunching factors and a wide frequency tuning range can be experimentally produced³⁰. The scheme utilizes the nonlinear longitudinal space charge (NLSC) force to produce a sharp energy modulation that is subsequently transformed into deep density modulation.

In this article, we experimentally demonstrate a single-pass superradiant THz FEL driven by THz electron microbunch trains and show an ultrawide tunability from 1 THz to 20 THz in a 1-meter-long undulator. The electron microbunch trains were generated by converting the energy modulation induced by the NLSC force into density modulation. An energy chirp, linearized by a harmonic cavity, was imposed on the beam to tune the spacing between microbunches. The linearization of the energy chirp makes it possible to maintain the bunching while increasing the beam compression rate. The intense THz superradiant emission from the ultrashort electron microbunches after entering the undulator served as the seed for the FEL amplification and further enhance the bunching. We demonstrated the superradiant FEL emission up to 15 THz at the fundamental harmonic of electron microbunch trains and up to 20 THz at the second harmonic. The detected THz pulse energy was around tens of microjoules to 150 microjoules with 700-pC beam charge.

The experiment was conducted at Dalian coherent light source (DCLS)³¹. The schematic layout is shown in Fig. 1. To enable superradiant THz FEL, we first modulated the beam into microbunches with subpicosecond spacing by utilizing the NLSC force³⁰, and measured the spectrum of the coherent transition radiation emitted by the electron microbunch trains which passed through an aluminum-coated mirror. A typical autocorrelation function measured by a Michelson interferometer and its Fourier transform are shown in Fig. 2a, indicating that the central frequency of this microbunch train is about 2.85 THz. We then transported the beam to the end of the beamline to diagnose the longitudinal distribution with a deflector. The measured beam longitudinal distribution and longitudinal phase space are shown in Fig. 2b and Fig. 2c. In the DCLS beamline, a one-meter-long undulator was placed right after its compressing chicane and there were still 70 meters from the chicane to the beamline end. The beam was accelerated to about 255 MeV and the energy chirp was compensated by four linacs after the chicane. During the transportation from the chicane exit to the beamline end, the spacing between the microbunches did not change as the energy chirp had been compensated, but the electron microbunch trains would experience non-negligible longitudinal space charge force that would first smear out the energy modulation and then form the energy modulation at an opposite direction, resulting in that the microbunch heads had higher energies than the microbunch tails as shown in Fig. 2c (the beam head is to the left). The bunching frequencies were calibrated to be \(\:2.82\pm\:0.07\) THz in Fig. 2b and \(\:2.88\pm\:0.06\) THz in Fig. 2c respectively, which agree well with the autocorrelation measurements.

The undulator gap was then closed to enable superradiant THz FEL emission. By tuning the whole-bunch energy chirp and the magnetic chicane strength, the beam was stretched or compressed and the bunching frequency was tuned continuously from 1 THz to 15 THz. Meanwhile, the beam energy at the undulator was adjusted from 16 MeV to 47 MeV to ensure that the undulator resonant frequency can match the microbunching frequency according to the FEL resonant condition \(f=\frac{{2c{\gamma ^2}}}{{{\lambda _U}\left( {1+{K^2}/2+{\gamma ^2}{\theta ^2}} \right)}}\), where is the resonant frequency, c is the speed of light, \(\gamma\)is the beam relativistic factor, \({\lambda _U}\) is the undulator period length, is the undulator parameter and \(\theta\) is the observation angle relative to the beam axis. The autocorrelation functions of the THz FEL pulses were measured and the spectrums were obtained by Fourier transformation. The spectrums of THz FEL pulses are shown in Fig. 3a, where frequencies below 15 THz are the fundamental harmonics of the microbunch trains and 20 THz is the second harmonic of a beam modulated at 10 THz.

For each FEL pulse, we measured the pulse energy by inserting a THz attenuator with transmission efficiency of 1% and a band-pass filter (BPF) or a low-pass filter (LPF) before the detector (see Methods). Figure 3b shows the measured FEL pulse energy at the corresponding frequencies. The data shown here have considered the transmission efficiency of the attenuator and the filter, but are not corrected with the transportation loss. The THz pulse energy measurements are averaged by 100 shots and the error bars represent the root-mean-square (RMS) intensity fluctuations. The low shot-to-shot intensity fluctuations indicate that this kind of THz source is quite stable. The start-to-end simulation shows a good prediction of the radiated pulse energy.

As the length of the generated microbunches are much shorter than the spacing between them, the electron microbunch trains could emit at its second harmonic when the beam resonant frequency in the undulator was tuned to the second harmonic of the bunching frequency. Take the beam with 10 THz modulation as an example. We scanned the undulator parameter and measured the total pulse energy as well as the pulse energy filtered by a 10 THz BPF. The results are shown in Fig. 4a. A main energy peak was observed when the resonant frequency was around 10 THz and a small energy hump appeared when the resonant frequency was around 20 THz. We measured the autocorrelation function without any filter and obtained the spectrum when the undulator parameter was 4.3 (resonant at 10 THz) and 2.9 (resonant at 20 THz) respectively. The results are shown in Fig. 4b and 4c. Only one frequency peak at 10 THz could be observed when the resonant frequency coincided with the microbunching frequency. The 20 THz component rose as the resonant frequency was tuned to the second harmonic of the microbunching. According to the resonant condition, the 10 THz component was located around 25 mrad off axis when the resonant frequency on the axis was 20 THz and it was in the collection angle of the THz optics, which makes the 10 THz frequency peak noticeable in the spectrum shown in Fig. 4c.

When the beam was compressed with a high compression rate, the nonlinearity of the beam energy chirp would deteriorate the spacing uniformity between microbunches, which would broaden the bunching spectrum and decrease the bunching factor. The X-band harmonic cavity was employed to linearize the energy chirp and maintain the bunching uniformity during compression, which guaranteed a high bunching factor up to 10 THz. The undulator radiation energy is proportional to the photon energy and the square of the bunching factor. If the bunching factor remained at almost the same level when tuning from 1 THz to 10 THz, the radiation energy would increase because the photon energy increased. This was clearly observed in Fig. 3b. The resonant bandwidth of the 20-period undulator was as wide as 5%. However, the energy spread of 12% needed to achieve the bunching frequency of 10 THz was much larger than the resonant bandwidth. When tuning from 10 THz to 15 THz, we did not increase the beam energy chirp anymore and just increased the chicane strength to further compress the beam, which maintained the beam energy spread but resulted in beam overbunching. A decrease of pulse energy was observed when tuning to the 15 THz.

The undulator employed in this experiment can provide a maximum undulator parameter of 5.2, restricting the resonant beam energy to below 17 MeV at 1 THz and 56 MeV at 10 THz. The electron beam with density spikes and energy of tens of MeV drifted for a long distance (about 14 meters) from the linac entrance to the chicane entrance, during which the NLSC force induced a large energy modulation amplitude. The large energy modulation amplitude is beneficial for obtaining a high bunching factor after passing through the chicane but detrimental to frequency tuning because under this condition only a small dispersion is required to achieve best bunching and the energy chirp should be rather large if a higher compression rate or a higher bunching frequency is needed. To increase the compression rate and reach the higher bunching frequencies, one possible solution is decreasing the energy modulation amplitude by increasing the beam energy and thus decreasing the longitudinal space charge force during the beam transportation. The energy modulation amplitude in this experiment was estimated to be approximately 0.4% relative to the beam energy according to the dispersion needed for the best bunching. The simulation predicts that if the energy modulation was decreased to 0.1%, the bunching frequency could be tuned to 30 THz with the help of a harmonic cavity to linearize the compression.

It should be pointed out that the DCLS beamline was not initially designed for this mechanism. The microbunching and radiation performance can be much improved after optimization. There are three linacs before the compressing chicane in the DCLS beamline. In this experiment, we just used the first one to accelerate the beam, and used the other two linacs to impose an energy chirp on the beam and decelerate the beam to the resonant energy. If we utilize a new undulator with a period length of 10 cm, the increased resonant beam energy would decrease the energy modulation amplitude, which is beneficial for increasing the compression rate. Moreover, the copper photocathode used in this experiment had been working for over 3 years and the quantum efficiency was only half of the initial value, which restricted the bunch charge to a maximum of 700 pC. The beam charge can easily go beyond 1 nC with a new photocathode. The simulation using a 1-nC electron beam accelerated by an optimized combination of the acceleration gradients and phases shows that the central frequency of the electron microbunch trains can be tuned between 1 THz and 30 THz with bunching factor around 0.3, as illustrated in Fig. 5a. The radiation results utilizing such electron microbunch trains indicate that mJ-level narrow-band THz radiation tunable between 1 THz to 30 THz can be generated. A typical longitudinal phase space and the current distribution of a beam modulated at 20 THz are shown in Fig. 5b. The RMS microbunch length is about 1.5 fs. The parameters in this experiment and the optimized parameters are shown in Table 1. In addition, the THz radiation energy can be further increased by changing the plane undulator to a tapered helical one³².

In summary, we demonstrated a compact superradiant THz FEL with ultra-wide tunability (1–20 THz) and high pulse energy (up to 150 µJ measured with a 700-pC beam). In this work, we pre-modulated the electron beam into microbunch trains using the NLSC force and linearized the beam compression with a harmonic cavity. By pre-modulating the beam, the spontaneous collective instability is bypassed which preserves the beam quality and leads to higher energy extraction efficiency compared to the normal SASE regime. FEL simulations further show that an optimized setup of beamline will extend the tuning range to 1–30 THz and pulse energy up to 2 mJ with 1 nC beam charge. This proposed scheme exhibits no inherent limitations on repetition rate and beam charge, allowing it to deliver THz radiation with unparalleled pulse energy and average power. Moreover, the experimental results verify the remarkable stability in shot-to-shot pulse intensity and central frequency, positioning this scheme as a strong contender in stability-critical applications. This mechanism can be applied in most of the modern accelerator facilities equipped with a photocathode gun, making the intense THz source covering the whole THz band available to a much wider scientific society.

DCLS beamline

A 700-pC electron beam was generated by a radiofrequency photocathode gun and accelerated to 5 MeV before injection to three S-band linacs. Two solenoids were installed to focus the beam, with one at the gun exit and the other along the linac. The acceleration phase of the first linac was set on the crest and accelerated the beam to about 48 MeV. The consecutive two linac phases were set off crest to impose a negative or positive energy chirp on the electron beam and decelerate the electron beam to the resonant energy. The peak field in these two linacs were tuned to control the energy chirp amplitude and the compression rate. A fourth-harmonic cavity was used to linearize the energy chirp. The beam then passed through a magnetic chicane and was stretched or compressed depending on the energy chirp rate. One plane undulator with 20 periods and period length of 5 cm was installed after the chicane. Four linacs after the chicane accelerated the beam to 255 MeV and compensated the energy chirp. After drifting for another 50 meters, the beam was diagnosed by a deflecting cavity and a dipole. The DCLS beamline is illustrated in extended data Fig. 1.

Photoinjector drive laser generation and measurements

The drive laser for the photoinjector was generated by a frequency-tripled Ti:sapphire laser system as shown in extended data Fig. 2. The 260-nm ultraviolet (UV) pulse was directed into four alpha-barium borates (α-BBO) crystals to produce a laser pulse train consisting of 16 pulses equally spaced at intervals of 0.5 ps. To equivalently measure the longitudinal profile on the photocathode, a movable mirror was utilized to switch the UV pulse train onto the measurement path which replicated the actual transportation path to the photocathode with the same optics and the same distance. An Infrared (IR) pulse with a duration of about 50 fs temporarily swept over the UV train inside a 0.1-mm-thick β-BBO crystal, producing a difference-frequency signal which then got detected by a photomultiplier tube (PMT). The full width at half maxima (FWHM) of the single UV pulse was measured to be about 140 fs.

THz measurements

The THz measurement platform was mounted right after the undulator exit. As illustrated in the extended data Fig. 3, the THz radiation reflected out of the vacuum chamber through a diamond window was collected by two gold-coated off-axis-mirrors (OAPs). A flipping mirror switched the THz radiation directly to a Golay cell (TYDEX, model GC-1D), or to a homemade Michelson interferometer. The Golay cell detector was calibrated at 2.5 THz to 12 μJ/V. A THz attenuator with transmission of 1% and a BPF or a LPF were placed before the detector. We first measured the THz pulse spectrum by the Michelson interferometer to obtain the radiation frequency, and then measured the pulse energy. At frequencies 1.5 THz, 3 THz, 5 THz and 10 THz, we set BPFs at the respective frequencies before the Golay cell to measure the energy. At frequencies 8 THz, 11 THz, 13 THz and 15 THz, we set a LPF with cutting frequency at 4.3 THz to obtain the energy of the frequencies lower than 4.3 THz and then calculated the radiation energy around frequency peaks by using the total energy subtracting the energy lower than 4.3 THz. The pulse energy at 20 THz was calculated by using the total energy subtracting the energy around 10 THz and the energy lower than 4.3 THz. All the measured energy data shown here have considered the transmission efficiency of the attenuator and the filters, but does not consider the transportation loss mainly due to the transverse profile truncation and the transmittance efficiency of the vacuum window which was only 20 mm in diameter.

Simulation method

The particle-tracking code ASTRA³³ and ELEGANT³⁴ were used to simulate the beam dynamics from the photocathode to the end of the beamline with the space charge effects considered. The time-dependent FEL code Genesis³⁵ was utilized to simulate the FEL from THz electron microbunch trains. The 6D beam distribution from the ELEGANT simulations was directly imported into Genesis and used to load the phase space.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

Acknowledgement:

We thank the Dalian Coherent Light Source for excellent support during the experiments. We would also like to thank Wei Wang, Hongfei Wang, Wei Wei, Likai Wang, Ming Liu, Liuping Chen, Guoqing Zhang, Lei He, Chunguang Wang, Feng Zhao, Jianping Wei, Zhijun Qi and Wenjie Cheng for technical supports. This work was supported by the National Natural Science Foundation of China (Grant No. 12305153, 11835004 and 22288201), Beijing College Outstanding Young Scientist Program (No. JWZQ20240101006) and the Scientific Instrument Developing Project of Chinese Academy of Sciences (Grant No. GJJSTD20220001).

Contributions

L.Y. and Y.L. conceived and designed the experiment. Y.L., T.L., J.S., Z.L., X.W., Y.Y., Q.T., Z.H., L.Z., and H.Y. conducted the experiment. Q.T., Z.H., T.L., Y.L., L.Y. and G.W. contributed to the generation and measurements of laser pulse trains. J.Y. contributed to the modification of the beamline. Y.L. and T.L. prepared the manuscript with contributions from all co-authors. L.Y., G.W. and W.Z. provided management and oversight to the project.

These authors contributed equally: Yifan Liang, Tong Li.

Corresponding authors: Lixin Yan, Weiqing Zhang, Guorong Wu.

Competing interests

The authors declare no competing interests.

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Table 1

Parameters in this experiment and the optimized values
Parameter	This experiment	Optimized
Beam energy	16–47 MeV	25–100 MeV
Beam charge	700 pC	1 nC
X-band voltage	0–2 MV	0–15 MV
Undulator period length	5 cm	10 cm
Undulator period number	20	30
Undulator parameter	2.8–5.2	3.5–6.5
THz frequency tuning range	1–20 THz	1–30 THz
THz radiation energy	0.1–0.2 mJ	0.1–2 mJ

There is NO Competing Interest.

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Ultra-widely tunable superradiant terahertz free electron laser from electron microbunch trains

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