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Abstract
Optical rectification of near-infrared laser pulses generates broadband terahertz radiation in chalcopyrite crystals CdGeP2, ZnGeP2 and CdSiP2. The emission is characterized using linear-polarized excitation from 0.8 eV to 1.55 eV (1550 nm – 800 nm). All three crystals are (110)-cut and polished to 0.5 mm, thinner than the coherence length across most of the excitation photon energy range, such that they all produce a bandwidth ~2.5 THz when excited with ~100 fs pulses. It is found that CdGeP2 produced the strongest emission at telecoms wavelengths, while CdSiP2 is generally the strongest source. Pump-intensity dependence provides the nonlinear coefficients for each crystal.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solid-state nonlinear optical frequency conversion is central to photonics, allowing for the development of new sources for spectroscopy [1,2] and other optical devices [3]. In optical rectification (OR), the relevant nonlinear tensor is χ(2)(0; ω, –ω), so that short optical pulses are converted to pulses with Terahertz (THz) center frequencies and THz bandwidths that are proportional to the source bandwidth.
THz pulses are routinely generated by OR in (110)-cut zincblende crystals, such as GaAs [4–6], GaP [7–9] and ZnTe [10–12], in tilted pulse front LiNbO3 [13–15] and tilted GaSe [16]. Only in 2012, Rowley et al demonstrated that the chalcopyrite crystal ZnGeP2 (ZGP) also produces THz pulses by OR [17], determined the optimum phase-matching conditions considering the uniaxial birefringence [18] and that the it is suited to pulsed optical excitation at 1200 nm [17,19].
ZGP and other chalcopyrite crystals have 42m symmetry [17], have strong uniaxial birefringence [18] and non-zero χ(2) tensor elements d14 = d25 ≠ d36, although d36 is approximately equal in strength to the other tensor elements. Hence, ZGP is known for optical parametric generation to down convert near-infrared optical pulse into mid-infrared pulses [20,21]. Moreover, chalcopyrite crystals have been explored for photovoltaic [22–25] and spintronic [26–28] applications. Recently, chalcopyrite CdSiP2 (CSP) has also been shown to be as a source of THz in the low-power regime [29]. This paper complements the previous work by showing the first demonstrate of optical rectification in CdGeP2 (CGP), comparing it to the THz emission from ZGP and CSP. Measurements from the three crystals are presented over a broad excitation tuning range and into the high-power regime.
2. Sample details
Single crystal ingots of chalcopyrite ZGP, CGP and CSP are grown using a horizontal-gradient freeze method [30,31]. From these high-quality (110)-cut crystals, double-side polished chips with an area of ~1 cm2 are thickness L = 0.5 mm for nonlinear optical applications [32–35]; see the photograph in Fig. 1(a). Details of the growth process can be found in [30], Zawilski et al. Characterization of the quality is performed using an ellipsometry technique that exploits the strong birefringence of the material [36,37].
Fig. 1 (a) Photographs of the polished CdGeP2 (CGP), ZnGeP2 (ZGP) and CdSiP2 (CSP) crystals from top to bottom. Visible light is reflected from the CGP. (b) Tauc plot for absorption edges using direct-gap normalization.
Linear optical transmission measurements are performed with Fourier transform infrared and UV-VIS spectrometers, results of which are combined and converted into Tauc plots to identify the absorption edges for each crystal. Figure 1(b) shows the Tauc plot of linear spectra for ZGP (red), CGP (black) and CSP (blue). Absorption for all three crystals is extremely low at low photon energies and a sharp absorption edge is observed at 1.70 eV, 1.97 eV and 2.12 eV for CGP, ZGP and CSP. These three values are consistently lower than previous measurements of the respective band gaps (εg), due to the samples being a couple of orders of magnitude thicker than the optical penetration depth at εg. The lack of structure below εg indicates a negligible concentration of defects or dopants in the gap, although it is possible that there is slight disorder producing Urbach tails [38].
3. THz experimental
THz emission measurements are performed using ~100 fs pulses from a 1 kHz regenerative amplifier centered at 800 nm (1.55 eV) and an optical parametric amplifier (OPA) tunable from 1160 nm – 2600 nm (1.07 eV – 0.48 eV). Pulse from the OPA impinge the chalcopyrite crystals at normal incidence. For (110)-cut chalcopyrite crystals under linear excitation along the [001] crystal axis, this excitation produces the maximum THz by a type-I process resulting in maximum emission along the [110] axis. This azimuthal emission dependence for this configuration is two-fold, so the sample is azimuthally rotated about the normal to maximize the emission with respect to the input linear polarization. The pump pulses are loosely focused to a 1/e2 spot diameter of 1.35 mm, with a pump fluence in the range 0.5 GW/cm2 – 30 GW/cm2. The emitted THz is collected with a pair of off-axis parabolic mirrors and focused onto a 0.5 mm thick, (110)-cut ZnTe crystal. Electro-optic sampling is performed by a sampling pulse with intensity Ig = 0.05 GW/cm2, derived from the laser amplifier at λg = 800 nm, measuring the THz electric field amplitude as a function of delay time, ETHz(t). The signal is recorded by balanced detectors feeding a lock-in amplifier that is phase-locked to a mechanical chopper in the pump beam, chopping at 250 Hz and synchronized to the regenerative amplifier.
4. Results and discussion
Figure 2(a) shows typical THz transients for CGP, ZGP and CSP, measured with pump intensity Ip = 5 GW/cm2 and photon energy εp = 0.95 eV (1300 nm). The THz electric field is determined by ETHz(t) = λg is(t)/(2πLIgn03r41 responsivity ρdet), where the is is the measure photocurrent in the lock-in amplifier, the ZnTe refractive index is n0 = 2.85 [39], the emitted THz the electro-optic coefficient is r41 = 4 pm/V [40] and ρdet = 0.009 A/W at these power levels. The emission from the three crystals comprise of a fast, strong response with a duration of ~1 ps, which is followed by weaker oscillations arising from interaction in the crystal and systematic ambient absorption in the THz path [17]. Longer transients reveal a replica of the THz signal due to reflection of the optical pulse in the detection crystal (not shown). The three transients look very similar, with slight difference in the magnitude at the first shallow trough, the first tall maximum and the second deeper trough. This result essentially illustrates different phases of the emitted radiation due to linear and nonlinear dispersions at both the optical excitation photon energy and the THz emission frequency [19].
Fig. 2 (a) Transient emission from CdGeP2 (CGP), ZnGeP2 (ZGP) and CdSiP2 (CSP) at low intensity pump at 1300 nm. Also shown is the window function (not to scale). (b) Fourier transform of the transients to determine the emission amplitude spectra.
Figure 2(b) shows the amplitude of the numerical Fourier transform of the experimental transients, which has been treated with an arctan window function (dashed green line) to remove the first reflection in the EO crystal at ~12 ps and zero padded before the transform. All three THz amplitude spectra are similar, peaking near ~1 THz with bandwidths of ~2.5 THz and ambient absorption, seen as a dips at 1.2 THz, 1.4 THz and 1.7 THz [17]. For the current excitation conditions, the strongest peak and integrated signal is from the CSP crystal. Emission from the CGP and ZGP is weaker, with only slightly different spectral signatures. The details are expected to depend on the crystal and the excitation conditions [36].
Figure 3(a) shows the THz conversion efficiency, η = ITHz / Ip as a function of excitation photon-energy from 0.8 eV to 1.55 eV. The pump intensity is fixed at Ip ≈5 GW/cm2 and the THz intensity is ITHz = ξ |Epp|2, where Epp is the peak-to-peak value of the emitted THz [illustrated on Fig. 2(a)] and ξ ≈9.7 is a scaling factor to account for the 2.5 THz bandwidth and the finite-lens correction for the off-axis parabolic mirrors [41]. Guides to the eye, based on a 15-point spline average, reveal that the CGP, ZGP and CSP signal peaks at approximately 0.8 eV, 1.2 eV and 1.4 eV respectively. This spectral range covers the data/telecoms range accessible by fiber lasers, the photon energy of Nd:YAG (and similar) lasers, as well as the range of Ti:sapphire lasers, illustrating the usefulness of these crystals as THz sources applicable to broad excitation operation.
Fig. 3 (a) Pump photon energy dependence of the emitted THz for CdGeP2 (CGP), ZnGeP2 (ZGP) and CdSiP2 (CSP). (b) Calculation of the coherence length of the three crystals.
The spectral dependence of the optical rectification is governed by the linear and nonlinear dispersion of the crystal at optical and THz frequencies, the nonlinear generation parameters determined from χ(2) and the various absorption processes of the excitation and emission. Low-intensity investigations, such as that shown in Fig. 3(a), are only weakly dependent on nonlinear or free-carrier absorption processes and can be determined from linear and nonlinear refractive indices. The dispersion of the real part of χ(2) is small and even unknown for some chalcopyrites. Hence, the initial comparison η (or Epp) is only to the coherence length, defined as lc = c/(2νTHz|nTHz – ng|), and which quantifies the velocity matching of the excitation pulse and the generated THz pulse. Here νTHz and nTHz are the THz phase frequency and refractive index respectively, ng = n – λdn/dλ is the optical group velocity of the pump with n as its phase refractive index at the corresponding wavelength λ and c is the speed of light. Figure 3(b) shows the calculation of lc plotted as a function of photon energy for comparison to the above data. Values used to calculate the lc curves are shown in Table 1.
Table 1. Typical values for determining the coherence length
A horizontal line is drawn at 0.5 mm in Fig. 3(b) to indicate the measured thickness of the crystals. If the lc estimates fall below this line, then in those regions the emitted signals are expected to be reduced. The CGP and CSP follow the trend of the coherence length calculation, peaking close to the position of optimum lc in each case. In comparison, ZGP has a much weaker dependence on the excitation photon energy. Overall, these results indicate that CGP is best suited to excitation with fiber lasers, ZGP is best suited to excitation with fiber and Nd:YAG lasers and CSP is best suited to excitation with Nd:YAG and Ti:sapphire lasers.
The scalable performance of the three THz sources is measured through the excitation-intensity dependence at several pump excitation photon energies. Figure 4 shows the strength of the THz emission Epp over a range of excitation densities up to 4(a) 15 GW/cm2 at 0.805 eV, 4(b) 30 GW/cm2 at 0.953 eV and 4(c) 120 GW/cm2 at 1.55 eV. All three results are plotted over the same range and the extended range excited at 1.55 eV is shown in the inset of Fig. 4(c). For 0.805 eV excitation the CGP emission remains strongest throughout the excitation intensity range, whereas for the other excitation photon energies CSP emits the most.
Fig. 4 Excitation-intensity dependence of CGP, ZGP and CSP, excited at (a) 0.805 eV, (b) 0.953 eV and (c) 1.55 eV. The inset of (c) shows an extended range.
In each measurement, the intensity dependence of ETHz grows linearly because ∂ETHz/∂z ∝ d36I(z), where d36 is the nonlinear optical tensor for optical rectification and z is the direction of propagation through the THz source. Here, d36 and the effective tensor element deff are interchangeable because the crystals have the identical orientation and birefringence is not considered because the excitation field is purely o-wave. At higher excitation intensities the signals saturate primarily due to nonlinear absorption, which follows a generalized form of Beer’s law: ∂I(z)/∂z = -(α I(z) + β I2(z) + γ I3(z) + …), where α is the linear absorption (shown in Fig. 1), β and γ are the two- (2PA) and three-photon absorption (3PA) coefficients.
Table 2 shows the values used to fit the curves for the three crystals and excitation photon energies. Following the scaling rules for 2PA and 3PA [47], β and γ are set to zero when the excitation is below the half (εg/2) and one-third (εg/3) bandgap of the crystal being excited respectively. Moreover, values for deff are fixed based on literature values [48,49], along with literature values of deff for ZnTe and LiNbO3 for comparison [50,51].
Table 2. Fit values for absorption of integrated THz emission and nonlinear coefficients.
For 0.805 eV excitation, the linear dependence of the THz amplitude rolls over, saturating due to 3PA. The fit values for the 3PA process scale with the strength of deff, indicating a fairly consistent nonlinear figure of merit far below bandgap. At 0.953 eV, direct comparison of the nonlinear absorption parameters is less straightforward because CGP exhibits 2PA. Whereas at 1.55 eV, all three crystals exhibit 2PA, although its effect is weak compared to that for CGP at 0.953 eV excitation. This occurs because 2PA is reduced closer to the direct bandgap and absorption preferentially transfers to the single photon contribution. Hence, all fit values extracted at the respective excitation photon energies are consistent with the expectations for the linear and nonlinear response for these crystals.
Finally, even in the extended range of excitation density (shown in the inset of Fig. 4), the weakly focused laser is below the laser damage threshold, which has been previously measured for anti-reflection-coated ZGP [52] and CSP [53] to be >2 J·cm−2 at 2 μm excitation and somewhat lower at 1 μm. Such values have not been explicitly measured for CGP, but they are expected to be of the same order.
5. Conclusion
In conclusion, chalcopyrite crystals have promising application in nonlinear optics based on their large nonlinear coefficients, birefringence and the availability of large area growth. In this study, optical rectification has been explored in three chalcopyrite crystals showing results that complement emission of THz from semiconductors in the zincblende family. The comparative excitation photon energy dependence reveals that both CGP and CSP are better emitters in their respective wavelength ranges than ZGP, which is reflected in the known values of the nonlinear optical coefficients. CGP is well suited to excitation at optical wavelengths near the center of the data/telecoms bands, where as ZGP and CSP are well suited for shorter wavelengths in the near infrared. Consequently, chalcopyrite THz emitters are compatible with the rapidly growing market of commercially available fiber and solid-state laser pump sources. Moreover, further emission strength could be achieved by using (012)- or (114)-cut crystals [18] and larger excitation areas [12,54], possibly making these sources suitable for nonlinear THz spectroscopy.
Funding
National Institute of Standards and Technology award 70NANB18H238_1 CD-451.
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