Development of a Table-Top High-Power, High-Stability, High-Harmonic-Generation Extreme-Ultraviolet Laser Source

Abstract

In this study, we present the development of a high-average-power, exceptionally stable extreme-ultraviolet (EUV) laser source based on a high-order harmonic generation (HHG) technique. The spectrum of an ytterbium-doped laser is broadened through self-phase modulation (SPM) in a gas-filled hollow fiber and compressed down to 25.3 fs for efficient harmonic generation. The high harmonics are generated in a krypton (Kr) gas cell, delivering a total power of 241 μW within the 30–60 nm spectral range, corresponding to a single harmonic output of 166 μW at a central wavelength of 46.8 nm. Notably, the system demonstrates good power stability with a root-mean-square (RMS) deviation of only 1.95% over 12 h of continuous operation. This advanced light source holds great potential for applications in nano- and quantum-material development and in semiconductor wafer defect detection. Future work aims to further enhance the output power in the 30–60 nm band to the milliwatt level, which would significantly bolster scientific research and technological development in related fields.

1. Introduction

Since the first observation of high-order harmonic generation in 1987 [1], this field has witnessed over three decades of innovative development, with theoretical models evolving from semiclassical systems to sophisticated strong-field approximation numerical frameworks [2]. This progress has significantly deepened our understanding of the high-order harmonic generation process. In the 21st century, HHG technology has driven the birth of attosecond science [3,4], opening new frontiers for ultrafast phenomenon exploration [5,6,7,8]. Nevertheless, the intrinsic limitations of HHG, particularly its $~{10}^{-6}$ conversion efficiency stemming from nonlinear frequency conversion mechanisms, continue to challenge practical implementations. This bottleneck has spurred the research community to continuously pursue optimization strategies for enhancing harmonic output and improving HHG conversion efficiency, with the ultimate goal of increasing EUV power output to accelerate the technology’s practical applications.

The enhancement of the EUV output power of high-order harmonic lasers can be achieved through two primary approaches (the major milestones of EUV generation over the years are reviewed in Ref. [9]). One is to enhance the power parameters of the driving laser, and the other is to boost the conversion efficiency of HHG itself. To obtain harmonic output with high photon flux, researchers have made significant progress in improving the average power of the driving laser [10], compressing the pulse width [11], and increasing the energy of a single pulse through innovative driving light source design [12]. It is worth noting that the use of a central wavelength of 1030 nm has been proven to be beneficial for EUV light sources with high average power [13]. With the integration of innovative technologies such as pulse compression [14], multi-frequency regulation [15,16], and multi-array nozzle phase modulation [17], the conversion efficiency of HHG has achieved a breakthrough improvement [18]. The combination of high average power and ultrashort pulse duration significantly increases the average power of EUV, further promoting the generation of higher cutoff energy and shorter attosecond pulses [19,20]. When enhancing the power of EUV light sources, maintaining the stability of lasers is equally important.

The improvement of the power stability of EUV sources holds substantial significance. In scientific research domains, such as nanoimaging and spectroscopy [21], highly stable EUV light sources can provide more precise experimental data, underpinning high-precision scientific endeavors. In imaging applications, unstable EUV power can lead to image distortion or resolution degradation. In terms of instrument usage, higher stability can reduce maintenance frequency and costs, while enhancing the reliability of the equipment. As technology evolves, the demands for stable EUV source continue to rise [22]. Therefore, high-stability EUV light sources are essential for promoting the development of technology and its applications.

To address existing technical bottlenecks, our research team has successfully developed a high harmonic EUV source based on an ytterbium fiber laser. The conversion efficiency of the harmonics yield reaches ${1\times 10}^{-5}$ when an ultrashort 25 fs pulse at 515 nm is used as the HHG driver. At 46.8 nm central wavelength, the HHG achieves an average power of 0.166 mW (single-pulse energy of 1.66 nJ). By using active beam pointing, the EUV source demonstrates good power stability of 1.95% RMS deviation during 12 h of continuous operation. This represents an innovative solution for the engineering application of high-stability EUV sources, marking a significant step forward in the field.

2. The 515 nm Driving Laser for HHG

2.1. The Driving Laser and Frequency Doubling

In the experiment, we chose an ytterbium-doped fiber laser with a central wavelength of 1030 nm as the driving light source. This laser runs at 100 kHz, with an output power of 14.5 W, a pulse width of 220 fs, and a beam quality factor of $M^2<1.11$. This provides a robust foundation for generating high-power EUV radiation.

For high harmonic generation, driving pulses with a shorter wavelength can significantly boost EUV generation efficiency ($\eta \left(\lambda \right)\propto {\lambda }^{-\left(5~6\right)}$) since the HHG atomic dipole scales with the wavelength [9,23,24]. We first set up a frequency doubling module (see Figure 1) to convert the 1030 nm driving light into 515 nm green light, which served as the driving source for HHG.

The 1030 nm laser was frequency doubled using a 12 × 12 × 0.5 mm³ BBO crystal. In order to enhance the frequency doubling efficiency, we installed a beam shrinkage system (consisting of lenses with respective focal lengths of 500 mm and −250 mm) to increase the laser peak intensity to $9.23\times {10}^9 \mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ on the BBO. Precise adjustment of the BBO crystal’s angle ensured phase matching, achieving efficient second harmonic generation with 44.14% conversion efficiency. This process yielded a 515 nm laser with an output power of 6.4 W. The frequency doubling was optimized to maximize the power conversion while maintaining good beam quality, which was critical for subsequent SPM.

2.2. The Spectrum Broadening and Pulse Compression

A driving laser with a short pulse duration and higher peak power can generate higher-order harmonics with better phase matching conditions [25]. To keep the device relatively simple and keep the loss within an acceptable range, we chose a hollow capillary fiber filled with Kr gas to broaden the spectrum and then compress the pulse using chirped mirrors [26].

As shown in Figure 1, spectral broadening was achieved by filling 2.5 bar of Kr into the capillary. The fiber length was 1.1 m with a core diameter of 250 μm. The measured transmittance was 78% and was identical for evacuated and gas-filled capillaries, indicating that there was no obvious ionization loss during the SPM process [27]. A chirped mirror compressor was then employed to compress the broadened spectrum. The total transmittance for chirp mirror reflection was 90%, offering −2300 fs² total dispersion (−100 fs² per reflection and 23 reflections on the CMs), which also compensated for the material dispersion of the lens and vacuum windows. Finally, we measured the compressed pulse with an autocorrelation; the retrieved pulse duration was 25.3 fs (see Figure 2).

2.3. The Stability Control by the Point Lock System

For the above-mentioned capillary spectrum broadener, good coupling of the incident laser with the fiber entrance needs to be ensured. This is because it not only is related to the overall transmission efficiency of the fiber but also affects the stability of the output laser. When the coupling is poor, it may even cause degradation of the output beam profile and irreversible damage to the fiber entrance.

During the subsequent HHG process, the position where the driving laser is focused also significantly affects the stability of the output EUV. These issues require us to have precise control over the direction of the laser pointing.

We then employed two sets of stability control systems to record and feed back the beam’s direction; these are shown in the two blue dotted boxes in Figure 1.

We utilized two piezoelectric-controlled mirror mounts to fine tune the beam orientation at the laser focusing point. The dielectric mirror had a reflectivity over 99.5% for the 515 nm laser, and a very small amount of input laser leaked through the mirror. This part of the light was used for the beam position record. CCD1 and CCD2 (MV-CS060-10UM-PRO, produced by Hikvision in Hangzhou, China) were used to continuously monitor the near-field and far-field beam position.

With the help of the beam pointing control system, the stability of the beam’s direction reached 6.90 μrad STD (standard deviation) in the horizontal direction and 7.26 μrad STD in the vertical direction (see Figure 3). External disturbances such as air flow, thermal deformation, and vibration which lead to beam pointing drift can be effectively compensated. This ensures pointing stability and good beam coupling at the fiber entrance and HHG focusing point, which reduces the risk of laser damage and increases the power stability.

3. HHG-EUV Source

3.1. The Generation and Spectrum Measurement of the EUV Light Source

The 515 nm driving laser, after passing through the stabilization system, is focused into a Kr gas cell with an inner diameter of 1 mm. Compared with a gas jet, the gas cell can keep a uniform gas density in the interaction area and leak less gas medium into the vacuum chamber. A multi-stage vacuum pump maintains the gas pressure of the generation chamber below 0.01 torr, reducing the gas leakage of the subsequent chambers. We enhanced the mechanical stability of the gas cell to isolate vibrations from the pump device.

The estimated beam focus size at the gas cell is 30.4 μm when using an $f=175\mathrm{m}\mathrm{m}$ lens, resulting in a peak intensity of $4.29\times {10}^{14} \mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ and an HHG cutoff energy of 47.68 eV (or 26 nm). In experiments, we optimized the Kr gas pressure for maximum EUV flux centered at 46.8 nm.

Subsequently, a 200 nm thin aluminum (Al) film was used to completely block the 515 nm driving laser, while the EUV light could pass through with 21.8% transmittivity. An EUV spectrometer, composed of a diffraction grating, a gold-coated toroidal mirror, and a CMOS camera, was used for spectral analysis (in Section 3.3) of the EUV beam [28].

3.2. Harmonic Beam Profile and Power Stability

In the experiment, the average power of the multi-harmonic EUV beam was measured using a photodiode (AXUV100G, produced by Opto Diode Corporation in Camarillo, CA, USA), while the EUV beam profile was recorded directly by a CMOS camera. When Kr gas was used as the generating gas, the EUV spectrum transmitted through the Al film was centered at 46.8 nm. The EUV power was measured using a silicon photodiode. After passing through two Al films with different thickness parameters, the measured electrical signal of the EUV was stable at 13.5 nA. The photoelectric conversion efficiency of the photodiode was 0.26 A/W; therefore, the calculated average power was 51.9 nW. The measured transmittance values of the two Al films near 46.8 nm were 0.33% and 21.8%. Additionally, accounting for a gas absorption loss of 30% during EUV propagation from the generation point to the photodiode, the total EUV power at the HHG generation point was calculated to be 241 μW.

In order to obtain the divergence angle of high-order harmonics, we used a CMOS to collect the shape of the beam profile at a distance of 1 m from the light source. Figure 4 shows the harmonic beam profile. The size of the $1/e^2$ beam profile was 3.35 mm × 3.04 mm. The calculated divergence angle in the horizontal-direction of the beam profile was 3.35 mrad, and the divergence angle in the vertical-direction was 3.04 mrad.

To further test the power stability of our EUV source, we recorded the total intensity of the beam profile with the beam pointing stability continuously working over a 12 h period. The result is shown in Figure 5. It can be seen that although the average power decreased slightly within the 12 h period, the overall situation remained within the normal range. The measurement of its long-term stability showed that the root-mean-square deviation was 1.95%.

3.3. Spectral Analysis

The EUV spectrum generated by HHG typically includes multiple peaks. We use a grating-based spectrometer to record and calibrate the harmonic peaks. The spectrometer mainly consists of a toroidal mirror and a grating. The toroidal mirror serves as the focusing component which re-focuses the divergent light of higher harmonics and projects the resulting point images onto the detector plane. The grating separates different harmonics [28].

When we used the Al film, we detected four high-order harmonic peaks. To determine the specific wavelength of each peak, we used zirconium (Zr) film to replace the Al film. Because, Zr film has a steep absorption edge near 47 nm (see Figure 6), only the 46.8 nm component in our EUV can pass through it. Therefore, with the addition of the Zr film, the position of 46.8 nm on the spectrometer’s detection plane could be determined. Subsequently, the positions of the other harmonics could also be determined.

After the EUV, which contained harmonics of different orders, passed through the spectrometer, we could finally obtain the data photo on a CMOS camera (Dhyana XF95, produced by Tucsen in Fuzhou, China) which was placed on the focusing plane. The spectrum shown in Figure 6 was obtained by appropriately processing the data from the recorded image. Among the harmonics, the target harmonic, that is, the component with a central wavelength of 46.8 nm, accounts for 68.89% of all HHG energy. Given that the total EUV power measured in Section 3.2 was 241 μW, the harmonic power of 46.8 nm output was 166 μW. Using the 515 nm driving light source, the efficiency of HHG into the single harmonic line at 26.5 eV was $4\times {10}^{-5}$.

4. Conclusions

In this study, we successfully developed a high-power, high-stability HHG-EUV light source with a wavelength centered at 46.8 nm using both an ytterbium-doped fiber laser as the driving source and a gas-filled hollow capillary technology for pulse compression. In a Kr gas environment, the light source achieved a total average power of 241 μW within the 30–60 nm spectral range with an average power of 166 μW at a 46.8 nm central wavelength. With the help of active point stabilization, our HHG system achieved a remarkable power stability of 1.95% RMS deviation over 12 h of continuous operation. This breakthrough lays a solid foundation for the industrial application and promotion of this type of light source.

Looking ahead, further optimization of the HHG scheme is expected to boost the output power of the source in the 30–60 nm band to the milliwatt level. This would provide an even more capable tool for scientific research and technological development in related fields, driving innovation and progress in areas such as coherent diffraction imaging (CDI), nanotechnology, and materials science.