Since the advent of the laser in 1960, laser technology has continued to develop and has received widespread attention^[1]. Mode-locking technology drives continuous progress in the development of ultrafast and intense lasers. Mode-locking technology can be used to shorten the pulse width of a laser from nanoseconds to femtoseconds and attoseconds, while greatly improving the peak power^[2,3]. Although visible (400–780 nm) and ultraviolet (UV) light (10–400 nm) only occupy a very narrow band in the electromagnetic spectrum, they have significant applications in various fields. Humans can directly observe the visible band, which allows them to obtain information through the perception of visible light. Blue and green light can be used as a light source for underwater communication, and green light can be used to treat retinal diseases. In addition, visible light has important applications in screen display, laser processing, quantum information, and other domains^[4–8]. The UV laser has the characteristics of a short wavelength and large single-photon energy, which allow many important applications, such as material processing, biomedicine, and spectral detection^[9–13].

At present, there are several ways to achieve visible and UV laser outputs. Visible output can be obtained when a Pr3+-doped crystal is pumped by a blue light laser diode (LD)^[14]. Gas excimer lasers mainly use the stimulated radiation of gas atoms to generate UV lasers, including XeF (354 nm), KrF (248 nm), ArF (193 nm), and other excimer lasers^[15–17]. Third-harmonic generation (THG), fourth-harmonic generation (FHG), and fifth-harmonic generation (FiHG) by a near-infrared solid-state laser can also generate UV lasers^[18–20]. Using a UV laser as the pump source, stimulated Raman scattering (SRS) can generate new UV spectral lines^[21]. However, the laser wavelengths obtained by these methods are not continuously adjustable. The cascaded sum frequency generation (SFG) and optical parametric methods have commonly been used to solve this problem. Starting from an infrared laser with a fixed wavelength of λp, tunable visible light (usually λ>400nm) can be obtained by the cascaded frequency conversions of the second-harmonic generation (SHG), THG/FHG, and optical parametric processes, which require at least three nonlinear optical (NLO) media. On that basis, tunable UV light can be obtained by further SHG or SFG, which require additional NLO media^[22,23]. In addition to the increase in production costs, multiple complex frequency conversion processes lead to large energy losses, and the overall conversion efficiency is reduced. Although tunable visible light can also be realized from a single NLO crystal based on cascaded frequency conversions using prior optical parametric oscillation (OPO) or optical parametric amplification (OPA) and posterior SFG, the tunable range of output wavelength λSFG is quite limited and cannot break though λp/2 in the short wavelength direction^[24–26] (e.g.,  when λp=827nm, and λSFG=484–512nm^[24]).

On the whole, no simple and efficient method is yet available to obtain wide waveband tunable visible to deep-UV lasers from a single wavelength near-infrared light source. Here, we present a new method that can realize this, which can be called the self-phase modulation and second-harmonic generation (SPM-SHG) method. When this method is applied to a β-BaB₂O₄ (BBO) crystal, efficient laser outputs with a wideband tunable characteristic from visible to deep-ultraviolet (DUV) have been obtained from femtosecond near-infrared laser sources.

The fundamental experimental setup of SPM-SHG is extremely simple. The laser source is a femtosecond laser with a fixed central wavelength of 800 nm (Legend Elite Ti:sapphire amplifier, Coherent Inc.). The repetition rate, nominal pulse width, beam diameter, and maximum output power were 2 kHz, 35 fs, 11 mm, and 0.6 W, respectively. The pump beam λp was focused into the NLO crystal with a quartz plano-convex lens (f=350mm). Correspondingly, the highest peak power of pump light in the NLO crystal is 1518GW/cm2. The NLO crystal was mounted on a three-dimensionally adjustable frame. After frequency conversion, the output power of the SPM-SHG beam λo was measured after the filter. The output spectrum was detected by two spectrometers with different work wavebands, one at 200–1100 nm (HR4000, Ocean Optics) and the other at 1000–2500 nm (AvaSpec-NIR256-2.5-HSC-EVO, Avantes).

A BBO crystal was used as the NLO medium. BBO crystals belong to the trigonal point group 3m and the space group R3c. A large-size BBO crystal with high optical quality can be obtained using the flux method. A BBO crystal has the advantages of A large nonlinear coefficient, short absorption cutoff edge, wide phase-matching (PM) range, high laser damage threshold, and stable physicochemical properties^[27,28]. Referring to the Sellmeier equations^[29], we chose (θ=29°, ϕ=30°) to process the experimental sample, which corresponded to the type-I SHG PM direction, when the pump wavelength was 800 nm and the SPM-SHG wavelength was 400 nm (the dimensions were 6mm×6mm×10mm). The transmission end-faces were polished and uncoated. During the experiments, the BBO sample was rotated in the ϕ=30° plane, and its PM angle θ varied. The rotation accuracy of the adjustment frame was 0.1°. According to the law of refraction, when the spatial rotation angle of the adjustment frame was α, the rotation angle α′ in the crystal was arcsin[sin(α)/n], where n was the refractive index of the BBO crystal at λp. The normally incident direction of the BBO sample was (29°, 30°). When the BBO sample rotated by α clockwise or counterclockwise in the ϕ=30° plane, the corresponding direction in the BBO sample was (29±α′°,30°), which was the new PM direction for λo.

The variation of the SPM-SHG spectrum λo with crystal rotation angle α of the (29°, 30°) sample is displayed in Figs. 1(a)–1(f). As shown in Figs. 1(a)–1(f), the output wavelength of 400 nm corresponds to the direction of α=0°. In other words, pump beam λp is normally incident on the BBO sample. As α is varied from −4.7° to +12.1°, λo changes from 444 to 329 nm. Using a (45°, 30°)-cut BBO crystal as the nonlinear optical, the shortest wavelength of the tunable SPM-SHG output reaches 225 nm at a rotation angle of α=31.4°, corresponding to a PM angle of θ=63.7°, as shown in Figs. 1(g) and 1(h).

A BFL-1030-10 H (Tianjin BWT Laser Ltd.) laser was used as the 1030 nm fundamental light source. The parameters included a repetition rate of 100 kHz, a pulse width of 200 fs, a beam diameter of 3 mm, and a maximum output power of 1.2 W. After the focusing of a f=300 mm convex lens, the peak power on the nonlinear optical crystal was calculated to be 299GW/cm2. Figures 2(a)–2(f) display the variation of λo with rotation angle α for the (29°, 30°) sample. The SPM-SHG wavelength red shifts as θ decreases and blue shifts as θ increases. As α varies from −9.1° to +18.2°, the tunable range of SPM-SHG wavelength is 515–302 nm. The output wavelength corresponding to the pump light normal incidence on the BBO sample is 402 nm. Similarly, with a (45°, 30°)-cut BBO sample as a nonlinear optical crystal, the shortest wavelength of the tunable SPM-SHG output can extend to 225 nm at a rotation angle of α=31.0°, i.e., the PM angle of θ=63.8°, as shown in Figs. 2(g) and 2(h).

Two kinds of filters were used to measure the conversion efficiency. When the SPM-SHG output wavelength was longer than 400 nm, a band-pass filter with high transmittance at 400–500 nm and high reflection at 500–1000 nm was used to filter the pump light. When the SPM-SHG output wavelength was shorter than 400 nm, a UV transmission filter with high transmittance at 260–390 nm and high reflection at 390–1000 nm was used to filter the pump light. The power of the SPM-SHG light λo was measured at different pump power values using an 800 nm laser source, and the results of λo=341, 361, and 383 nm are displayed in Fig. 3(a). When λo is 361 nm, the maximum output power is 108 mW at a pump power of 600 mW, and the corresponding conversion efficiency is 18.1%. The maximum conversion efficiencies for 341 and 383 nm are 15.1% and 14.3%, respectively. Figure 3(b) displays the conversion efficiencies for different SPM-SHG wavelengths λo pumped by an 800 nm laser source when the pump power is fixed at 600 mW. When λo varies from 307 to 361 nm, the conversion efficiency increases from 3.1% to 18.1%; when λo varies from 361 to 460 nm, the conversion efficiency decreases from 18.1% to 5.0%.

The power of the SPM-SHG light λo was measured under different pump power values by a 1030 nm laser source. Figure 3(c) displays the results of λo=487, 465, and 440 nm. When λo is 487 nm, the maximum output power is 55.8 mW at a pump power of 1200 mW, and the corresponding conversion efficiency is 4.7%. The maximum conversion efficiencies for 465 and 440 nm are 4.1% and 1.9%, respectively. Figure 3(d) displays the conversion efficiencies of different SPM-SHG wavelengths λo pumped by a 1030 nm laser source when the pump power is fixed at 1200 mW. The conversion efficiency increases from 0.2% to 4.7% as λo varies from 310 to 487 nm.

The pulse widths of the fundamental wave and SPM-SHG waves were measured by an autocorrelator (Pulsecheck, NX150, APE Inc.), as displayed in Fig. 4. Figure 4(a) displays the pulse width of the fundamental wave with a wavelength of 800 nm, which is 38 fs. Figures 4(b) and 4(c) display the pulse widths of SPM-SHG waves with wavelengths of 440 and 460 nm, which are 360 and 390 fs, respectively.

The SPM-SHG method combined two physical effects, self-phase modulation and second-harmonic generation. A typical SPM spectrum of the 800 nm light source generated by the BBO sample is shown in Fig. 1(h), which outputs a broadband supercontinuum spectrum from 450 to 1000 nm. The SPM spectrum of the 1030 nm light source is shown in Fig. 2(h), which means broadband supercontinuum generation ability from 400 to 1060 nm.

Defining the SPM frequency as ωSPM, when the BBO crystal is rotated to meet different SHG PM conditions (energy conservation condition: ωSPM+ωSPM=ωo, and momentum conservation condition: kSPM+kSPM=ko), the SPM-SHG light λo is continuously tunable. The most direct evidence is that the SPM photons at the corresponding fundamental wavelength λSPM are obviously consumed, thus forming a spectral valley, as illustrated in Fig. 1. For example, in Fig. 1(f) where λo=329nm, the corresponding fundamental frequency location at λSPM=658nm has a valley, which means the consumption of ω658nm photons and the occurrence of ω658nm+ω658nm=ω329nm.

To further verify the above mechanisms, the most direct and convincing method was to check whether the experimental values of the PM angle were consistent with the theoretical calculation results. Using the Sellmeier equations for BBO crystal^[29], the PM curves of SPM-SHG were calculated for the pump wavelengths of 800 and 1030 nm, as displayed in Fig. 5. As seen in Fig. 5(a), when λp is 800 nm, the experimental points are consistent with the PM curve for SPM-SHG, which proves that its mixing frequency mechanism is ωSPM+ωSPM=ωo. Figure 5(b) presents the situation for λp=1030nm. As PM angle θ increases, the output wavelength λo is blue-shifted from the SHG direction of θ=23.5°. The experimental points are consistent with the PM curve. As demonstrated in Fig. 5, the spectral tuning range is 460–225 nm for the 800 nm pump and 515–225 nm for the 1030 nm pump.

As shown in Figs. 1 and 2, from two commercial near-infrared ultrafast lasers of 800 and 1030 nm, the shortest wavelength presently obtained by the SPM-SHG method is 225 nm. To further extend the laser output to a shorter spectral region like the vacuum-ultraviolet (VUV) waveband (<200nm), additional frequency conversion steps are required, and here we present an experimental example. With the 1030 nm laser (BFL-1030-10 H) as the fundamental light source, a tunable laser output ranging from 193 to 207 nm has been achieved from three cascading frequency conversions, as demonstrated in Fig. 6. In this scheme, three BBO samples with different processing directions were used, which were (45°, 30°)-cut BBO1, (70°, 30°)-cut BBO2, and (57°, 30°)-cut BBO3. Firstly, the BBO1 sample produced a tunable laser output of 225–515 nm via the SPM-SHG effect. Secondly, the BBO2 sample performed SHG for the SPM-SHG output of the BBO1 sample, and realized a tunable laser output of 205–258 nm, where 205 nm is the shortest SHG PM wavelength of a BBO crystal. Finally, the BBO3 sample performed SFG for the SHG output of the BBO2 sample and the residential 1030 nm fundamental laser, and achieved tunable UV laser output of 193–207 nm.

In this way, by utilizing the SPM-SHG method, the VUV lithography laser wavelength of 193 nm is conveniently produced from a commercial 1030 nm laser source. To further reduce frequency conversion processes, a KBe2BO3F2 (KBBF) crystal can be used to replace BBO2 and BBO3, which realizes the SHG process from 386 to 193 nm. The other route is using a DUV half-waveplate to adjust the linear polarization of the SPM-SHG output of the BBO1 crystal to an approximately parallel direction with the linear polarization of the fundamental laser, and then using the BBO3 crystal to perform type-I SFG to achieve the 193 nm laser. In this way, the BBO2 crystal and corresponding SHG process can be saved. For the above two schemes, only two-step frequency conversions are required to realize tunable VUV laser output. Starting from a single-wavelength near-infrared laser, by utilizing the SPM-SHG method, they give out the simplest and most direct solutions to generate a tunable VUV solid-state laser, as far as we know.

SPM-SHG is an extremely simple, one-step frequency conversion scheme to obtain a tunable visible to DUV ultrafast laser. An ultrafast laser source and a nonlinear optical medium are the only optics required to achieve a short-wavelength, wideband tunable output, which simplifies the installation and significantly reduces the intermediate losses. This technology was successfully applied to two near-infrared laser sources with different wavelengths. For the 800 nm fundamental light source, the tunable range of SPM-SHG wavelength λo was 225–460 nm, and the highest optical conversion efficiency was 18.1%. For the 1030 nm fundamental light source, the tunable range of λo was 225–515 nm. By three-step cascading frequency conversions, i.e., SPM-SHG, SHG, and SFG, a tunable laser output ranging from 193 to 207 nm has been achieved.

Since the SPM-SHG effect is the synergistic effect of the third-order nonlinear effect SPM and the second-order nonlinear effect SHG, the peak power of the pump source has a significant impact on the output power. Compared to the 1030 nm, 200 fs pump pulse, the 800 nm, 35 fs pulse exhibits a higher peak power. To further enhance the overall performance of SPM-SHG, strategies such as increasing the pump energy and reducing the pulse duration can be considered. From the perspective of SHG, broadband and tunable PM characteristics are crucial for nonlinear optical materials. Other desirable properties include a large second-order nonlinear coefficient, a high laser damage threshold, and a broad transmission window.

Currently, common commercial femtosecond lasers primarily include Ti:sapphire lasers (800 nm), Yb3+ solid-state lasers (1030 nm), Nd3+ solid-state lasers (1064 nm), and Er3+ fiber lasers (1550 nm)^[30–33]. Using commercial lasers to pump nonlinear optical crystals typically requires four to six steps to achieve 193 nm laser output^[34–38]. The SPM-SHG technology simplifies this process to three steps or fewer. It brings many advantages, including small intermediate losses, high conversion efficiency, large laser output, simple optical arrangement, compact sizes, long-term reliability, as well as economic production cost. This new technology is hopeful to realize important applications in many fields, such as lithography, optical inspection, micromachining, biology, medicine, and scientific research.