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Abstract
We demonstrate non-collinear optical parametric amplification (NOPA) in LiInS2 to generate ultrashort near-infrared pulses. White light pulses around 1400 nm generated in yttrium aluminum garnet are amplified by five orders of magnitude up to 1.0 µJ by three-stage NOPA in LiInS2. The dispersion of the amplified pulses is compensated by an acousto-optic programmable dispersive filter, resulting in the pulse compression down to 40 fs, which is 1.1 times the pulse width of the Fourier-transform limited pulse. The successful demonstration of NOPA in LiInS2 indicates the possibility as a new light source to obtain high peak intensity which enables us to access the regime of non-perturbative physics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Femtosecond (fs) pulses in the infrared region of the spectrum are widely used for ultrafast spectroscopy [1], laser processing [2,3], high-order harmonic generation (HHG) [4,5], etc. Optical pulses with a tunable wavelength are mainly generated by optical parametric amplification (OPA) [6], where signal light is amplified by pump light in a variety of nonlinear optical (NLO) crystals such as β-BaB2O4, LiNbO3, AgGaS2 due to their high performances [7]. The performance of OPA depends on the characteristics of a NLO crystal such as transparent and phase-matched wavelength ranges, nonlinear optical coefficient, and damage tolerance. Therefore, a wide variety of NLO crystals have been developed for higher performance of OPA. One of the most important conditions is the pump laser. Recently, high-power 1-µm lasers using ytterbium (Yb)-doped materials have been actively developed and applied to pump lasers for OPA [8–11].
Lithium thioindate (LiInS2), one of chalcogenide crystals, is a promising candidate for a NLO crystal in OPA in the infrared region [12,13], because LiInS2 has advantageous characteristics such as high birefringence, broadband transparency range (340–13200 nm), high nonlinear optical coefficient (d31(2300 nm) = 7.2 ± 0.4 pm/V), high thermal conductivity (7.6 Wm/K along z-axis), and high damage threshold (>140 GW/cm2 for fs pulse) [7]. These characteristics show promising potentials toward the generation of high-power broadband light pulses from the near-infrared to mid-infrared range. In these years, NLO phenomena in LiInS2 have been reported such as optical parametric oscillation (OPO) [14], second harmonic generation [15,16], OPA [17], etc [15,18–20].
Non-collinear OPA (NOPA) is one of methods to extend the wavelength range where the phase-matching condition is fulfilled. A degree of freedom of an angle between signal and pump lights (non-collinear angle) enables various combinations of the wave vectors, some of which satisfy the phase-matching condition over a broad bandwidth. Within only a few years from the first report applying non-collinear phase-matching to OPO [21], broadband amplification with sub-20 fs pulse width had been achieved by NOPA from the visible range to the infrared range [22–24]. Due to the broadband transparent range, LiInS2 is one of suitable candidates as a NLO crystal in NOPA to generate few-cycle light pulses, which enable us to explore new research fields such as carrier-envelope-phase controlled phenomena [25,26], attosecond science [27–30], soft X-ray physics [28,31–33], and so on.
In this study, we apply LiInS2 to NOPA to generate ultrashort near-infrared pulses. White light pulses around 1400 nm generated in yttrium aluminum garnet (YAG) are amplified by three-stage NOPA in LiInS2 with high-power 1-µm pump light from an Yb-doped YAG thin disk laser [34,35]. The amplified pulses are compressed by an acousto-optic programmable dispersive filter (AOPDF) [36]. We show that the white light is amplified by five orders of magnitude up to 1.0 µJ and compressed down to a pulse width of 40 fs, which is much shorter than that generated in LiInS2 in the previous study (sub-picosecond) [18].
2. Optical design
A phase-matching condition for broadband NOPA is calculated, as recently studied by L. Zhou et al [37]. We use a software (SNLO, AS-Photonics) for the calculation. Figure 1(a) shows optical arrangement used in the calculation. Similar crystals LiGaS2 and LiGaSe2 are also good candidates because their phase-matching bandwidths are as wide as that of LiInS2 [37]. However, we adopt LiInS2, which has the highest damage threshold of the three crystals under comparable conditions in the past studies [7,38–40]. The signal light is incident perpendicularly to the crystal surface with linear polarization along the y-axis of LiInS2. The pump light is introduced at a different incident angle with p-polarization. Here, we define the internal signal angle as the angle from the z-axis of LiInS2 to the wavevector of the signal light, and the non-collinear angle as that from the wavevector of the signal light to the pump light. Type-I configuration and room temperature (300 K) are adopted. The phase matching internal signal angle is calculated as a function of the signal wavelength at a non-collinear angle inside LiInS2 of 2.77° and at a pump wavelength of 1030 nm with φ = 0°-cut LiInS2, as shown in Fig. 1(b). The result indicates a broadband phase-matching around 1400 nm in wavelength at an internal signal angle of 19°. The nonlinear optical coefficient deff is calculated to be -2.45 pm/V. Therefore, we choose θ = 19°- and φ = 0°-cut LiInS2 crystals. Broadband idler light around 4 µm in wavelength is also expected to be generated with angular dispersion. In this study, we focus on signal light, which does not need complex optical setup to compensate angular dispersion [41]. The idler light will be used for ultrashort mid-infrared pulse generation in the future.
Fig. 1. (a) Optical arrangement of signal light, pump light, and a LiInS2 crystal. (b) The phase-matching internal signal angle as a function of the signal-wavelength when the non-collinear angle inside LiInS2 is set to be 2.77° for 1030 nm in pump wavelength with type-I configuration and φ = 0°-cut LiInS2 at 300 K. (c) A schematic of the experimental setup for NOPA in LiInS2. The incident angle of the pump light outside LiInS2 is 5.93°. White light generated in YAG is amplified by three-stage NOPA in LiInS2. The dispersion of the light pulses is compensated with AOPDF.
Our experimental setup is shown in Fig. 1(c). We use an Yb-doped YAG thin disk laser as a pump light source (center wavelength: 1030 nm, pulse energy: 2.1 mJ, pulse width: 1.0 ps, repetition rate: 3 kHz), which is similar to the light sources used in Refs. [34,35]. A partial output (0.085 mJ) from the thin disk laser is used for white light generation. The light pulses are adjusted by an iris to a pulse energy of 0.020 mJ and a beam diameter of 2.9 mm, and focused onto a 10-mm-long YAG crystal with a numerical aperture of 0.0095. The white light generated in YAG at a wavelength of around 1400 nm is amplified by three-stage NOPA in uncoated LiInS2 crystals with the condition derived in the last paragraph. The non-collinear angle inside (outside) LiInS2 is set to be 2.77° (5.93°). Other parameters at each NOPA stage are listed in Table 1. The dispersion of the light pulses is compensated with an AOPDF (Dazzler HR-1150-1600, FASTLITE) just after the second LiInS2 to compress the final output pulses as will be described in section 4.
Table 1. Experimental conditions for each NOPA stagea
3. Non-collinear optical parametric amplification
The result of the amplification is shown in Fig. 2(a). The spectrum of the input white light is plotted as a black curve. The spectra of the amplified light are plotted as red, blue, and green curves when NOPA is operated until the first, second and third stages, respectively. Note that all spectra are measured at a fixed position after the third LiInS2 for comparison. The white light pulses are amplified around 1400 nm to have a full width at half maximum of 108 nm after the third NOPA stage. The amplification factors of the spectral intensities are about 300–400, 5–25, 40–100 at the first, second and third NOPA stages, respectively. The result shows that the amplification factor per 5-mm-long LiInS2 is about one order of magnitude under our experimental condition. The spatial profile shown in Fig. 2(b) is almost Gaussian. The pulse energy is amplified up to 1.0 µJ. We should mention that there are some drawbacks: the bandwidth is narrower than expected by the calculation and the OPA efficiency is as low as 5 × 10−4, both of which should be improved in the future. However, a pulse energy of 1.0 µJ can induce non-perturbative phenomena as reported in Ref. [42]: high-order harmonic generation in a solid-state medium with a plasmon-assisted enhanced near-field.
Fig. 2. (a) Input spectrum and output spectra after NOPA in a logarithmic scale. (black curve) White light generated in YAG. (red, blue, and green curves) Amplified light when NOPA is operated until the first, second and third stages, respectively. Note that all spectra are measured at a fixed position after the third LiInS2. (b) Near-field beam profile after the third NOPA stage.
4. Pulse compression and measurement
For pulse compression, the negative dispersion is introduced by the AOPDF to cancel the positive dispersions of the three LiInS2 crystals (+6400 fs2/25 mm) and the AOPDF itself (+6100 fs2). Figure 3 shows two methods for pulse stretching and compression with the AOPDF inserted in different positions. Here we assume that the dispersion of the white light is almost zero [43]. When the AOPDF is placed before the first NOPA stage (Fig. 3(a)), a large negative dispersion introduced by the AOPDF stretches the signal pulses more than 1 ps in time. This leads to mismatch of the temporal overlap between the signal and pump pulses in the first NOPA stage, inducing gain narrowing. Therefore, the AOPDF is placed after the second NOPA stage as shown in Fig. 3(b). In this case, the signal light is not stretched so much in all stages to minimize the gain narrowing. The temporal profile of the compressed pulse is characterized by a second harmonic generation frequency-resolved optical gating (SHG-FROG) method using a 100-µm-thick β-BaB2O4 (type I phase-matching, θ = 21°, protection-coating from 1000 to 2000 nm, Crylight) in the same manner as Ref. [35] and retrieved with a home-made program.
Fig. 3. Two methods for pulse stretching and compression with the AOPDF inserted in different positions (a) when placed before the first NOPA stage and (b) used in this research. τsig and τpu denote pulse widths of signal and pump light, respectively. The designed dispersion at each position is listed.
The measured and reconstructed SHG-FROG traces are shown in Figs. 4(a) and 4(b), respectively. The similarity of the two traces and a FROG error of 0.91% indicate the validity of the measurement and the retrieval. The retrieved spectrum is centered around 1400 nm, similarly to the spectrum measured by the spectrometer (Fig. 2(a)). Variation in the group delay is also well suppressed by the AOPDF as shown in Fig. 4(c). The pulse width of the compressed pulse is derived to be 39.99 fs, which corresponds to 8.6 cycle and only 1.1 times the pulse width (35.9 fs) of the Fourier transform-limited pulse calculated from the retrieved spectrum shown in Fig. 4(c). This is the shortest pulse in the near-infrared region obtained by LiInS2. A pulse energy of 1.0 µJ is measured after the compression.
Fig. 4. (a) Measured and (b) reconstructed SHG-FROG traces (log scale). (c) Retrieved spectral intensity (black curve) and group delay (blue dashed curve). (d) Retrieved temporal profile. The FROG error is 0.91%.
5. Conclusion
We have demonstrated NOPA in LiInS2 to generate ultrashort near-infrared pulses around 1400 nm. White light pulses around 1400 nm generated in YAG are successfully amplified by five orders of magnitude up to 1.0 µJ by three-stage NOPA in LiInS2 with high-power 1-µm pump light from an Yb-doped YAG thin disk laser. The unsaturated amplification at the third NOPA stage indicates the possibility of further amplification with additional NOPA stages. The amplified pulses are compressed using an AOPDF down to 40 fs, which is 1.1 times the pulse width of the Fourier-transform limited pulse and much shorter than the pulse width generated in LiInS2 in the previous study. This successful demonstration of NOPA in LiInS2 indicates the possibility to obtain high peak intensity (typically 10−2–102 TW/cm2) which enables us to access non-perturbative phenomena [5,26–28,30–32,42].
Funding
Ministry of Education, Culture, Sports, Science and Technology (JPMXS0118068681); Precursory Research for Embryonic Science and Technology (JPMJPR2002); Japan Society for the Promotion of Science (JP19H02623).
Acknowledgments
This research is supported by QST President’s Strategic Grant (Creative research). The authors thank Prof. T. Momose, Prof. H. Katsuki, and Dr. M. Tsubouchi for support in the installation of the AOPDF.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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