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Abstract
The organic terahertz (THz) generation crystal BNA has recently gained traction as a source for producing broadband THz pulses. When pumped with 100 fs pulses, the thin BNA crystals can produce relatively high electric fields with frequency components out to 5 THz. However, the THz output with 800-nm pump wavelength is limited by the damage threshold of the material, particularly when using a 1 kHz or higher repetition rate laser. Here, we report that the damage threshold of BNA THz generation crystals can be significantly improved by bonding BNA to a high-thermal conductivity sapphire window. When pumped with 800-nm light from an amplified Ti:sapphire laser system, this higher damage threshold enables generation of 2.5× higher electric field strengths compared to bare BNA crystals. We characterize the average damage threshold for bare BNA and BNA-sapphire, measure peak-to-peak electric field strengths and THz waveforms, and determine the nonlinear transmission in BNA. Pumping BNA bonded to sapphire with 3 mJ 800-nm pulses results in peak-to-peak electric fields exceeding 1 MV/cm, with broadband frequency components >3 THz. This high-field, broadband THz source is a promising alternative to tilted pulse front LiNbO3 THz sources, enabling many research groups without optical parametric amplifiers to perform high-field, broadband THz spectroscopy.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-field terahertz (THz) science is enabling fascinating studies of condensed matter systems, including recent advances in powerful two-dimensional THz spectroscopy [1–7]. The most common sources of amplified ultrafast laser pulses are Ti:sapphire laser systems providing 800-nm pump wavelengths. To date, nonlinear THz studies with strong peak electric fields (>100 kV/cm) have either been carried out by a) directly pumping inorganic LiNbO3 using 800-nm in a tilted-pulse front configuration to produce relatively low frequencies (<3 THz) [8,9], or b) using organic THz generation crystals like DAST [10], DSTMS [11], OH1 [12], and EHPSI-4NBS [13] that produce broader generation bandwidths (1-6 THz). However, the organic crystals are most efficiently pumped at longer wavelengths, often necessitating an optical parametric amplifier (OPA) to produce pump wavelengths in the 1200-1600 nm range [10–14]. Other THz generation schemes exist using inorganic crystals like GaP [15] or ZnTe [16], or plasma THz generation techniques [17–20], but none of these can currently match the peak field strengths with the spectral amplitude in the 1-6 THz range associated with optical rectification in LiNbO3 or organic crystals (when standard Ti:sapphire laser systems are used). For example, we showed that with 100-fs 800-nm pump pulses, two-color THz generation saturates at ∼150 kV/cm [18]. Others have shown larger peak field strengths with plasma THz generation with kHz laser systems, but with extremely broad bandwidth THz pulses that have significant spectral amplitude extending beyond 10 THz [21]. However, when peak-to-peak electric field strengths exceeding 1 MV/cm with large spectral amplitudes in the 1-6 THz range are needed, pumping LiNbO3 with 800 nm or organic crystals with longer wavelengths are the preferred methods that have enabled a host of powerful measurements [1–7,22–25].
Currently, many ultrafast laser research groups that lack an optical parametric amplifier or a specialized laser system [26] are limited to lower frequencies accessible with inorganic nonlinear optical crystals. When an OPA is available with substantial output pulse energies, organic crystals have demonstrably improved THz generation efficiencies, which can make up for conversion losses in the OPA generation of 1200-1600 nm light. Furthermore, the good phase-matching in organic crystals enables a simple collinear THz generation geometry. This also allows for easily changing the polarization of generated THz by simply rotating the THz generation crystal along with the polarization of the pump light. In short, LiNbO3 currently has best utilized the Ti:sapphire output of 800-nm, but organic crystals can offer advantages of broader generation bandwidths, easy polarization control, and simple THz generation geometries when an OPA or longer wavelength amplified laser are available. Therefore, finding a nonlinear optical crystal that offers the advantages organic crystals offer without the need of an OPA or specialized laser system to produce pump light would be a boon to many research groups interested in improving their THz sources or entering the exciting area of high-field THz science.
We recently reported that thin crystals of the organic crystal BNA can generate broad bandwidth THz pulses when pumped with 800-nm light [27]. However, the low melting point of BNA (103° C) leads to a relatively low laser induced damage threshold (LIDT) of 4 mJ/cm2 when pumped with ∼100 fs ultrafast pulses of 800-nm light at a 500 Hz repetition rate; the damage threshold drops to 2 mJ/cm2 when pumped at 1 kHz repetition rate, suggesting that BNA heats to its relatively low melting point and melts. Other papers report slightly higher LIDT fluence values when using lower repetition rate laser systems, and therefore lower average powers [28,29]. The low damage threshold prohibits many with Ti:sapphire laser systems, particularly those that operate at 1 kHz or higher repetition rates, from utilizing their full power to pump BNA crystals for THz generation. Here we show that by bonding BNA to high-thermal conductivity sapphire plates, the damage threshold is increased considerably, boosting the THz output significantly. Thermal management techniques have been utilized in laser science for years, but to our knowledge, this is the first comprehensive test showing the benefits for high-field THz generation with an efficient organic THz emitter.
The increased heat dissipation increases the damage threshold of the BNA-sapphire structure almost threefold compared to bare BNA, which also increases the attainable electric field by a factor of ∼2.4 (leading to a ∼6${\times} $ increase in intensity). The use of BNA-sapphire therefore can enable high-field, broadband THz spectroscopy in any research lab with an amplified, ultrafast Ti:sapphire laser system, using a simpler experimental setup, and producing a broader THz spectrum than is possible with LiNbO3.
2. Methods
Early studies showed a dramatic improvement in THz output from BNA-sapphire structures, and we set out to determine if both thin and thick BNA crystals would benefit equally. Bare BNA crystals were synthesized, grown, and cleaved to the appropriate thickness according to our previously reported methods [26]. In brief, to synthesize N-benzyl-2-methyl-4-nitroaniline (BNA), 50 g (0.328 mol) of 2-methyl-4-nitroaniline was placed in a 1 L round bottom flask with 500 mL of dry dimethylformamide (DMF). 27.5 g (0.328 mol, 1 equiv) of sodium bicarbonate was added. The solution was then heated to 80 °C and stirred vigorously under an argon atmosphere. 45 mL of benzyl bromide (0.378 mol, 1.15 equiv) is added dropwise and the solution was left to react over 48 hours. 500 mL of deionized water was then added, and the reaction left to cool to room temperature. The mixture was extracted with 3 ${\times} $ 500 ml diethyl ether and the combined organic layers were washed with brine and the solvent was removed under vacuum. The resulting solid was recrystallized from ethanol. High quality BNA crystal plates were grown from the melt by suspending a seed crystal in slightly supercooled liquid BNA (101-102 °C) and allowing the seed crystal to slowly grow while maintaining the temperature of the melt. Crystal plates of the purified BNA were then cleaved to the appropriate thickness before polishing.
BNA crystals with thicknesses less than ∼300 μm were classified as “thin” (most were approximately 200 μm thick), and BNA crystals with thicknesses greater than ∼300 μm were classified as “thick” (most were approximately 400-500 μm thick). Thin and thick BNA crystals were used bare (mounted to a 1-inch metal disk with an appropriate aperture) or fused to 0.5-mm thick, 1-inch diameter sapphire plates to test the impact of the sapphire layer on the THz generation output of BNA. BNA crystals were fused to sapphire using a method similar to annealing. Heating the sapphire plate up to within 1 °C of the melting point of BNA (103.4 °C) with the BNA crystal on top of the plate allows for the interface of the crystal to become liquid without melting the entire crystal. This allows for the crystal to maintain its structure and quality, while also inducing intermolecular interactions between the BNA and sapphire, effectively fusing the two together. After ∼24 hours of annealing, the substrate is then allowed to cool slowly until it reaches room temperature. We emphasize that no adhesives are used; the BNA is in direct contact with the sapphire without any intermediate layer or surrounding material. These BNA crystals and BNA-sapphire structures were invented in collaboration with Terahertz Innovations, LLC.
We classified the four types of samples tested as thin bare BNA, thick bare BNA, thin BNA-sapphire, and thick BNA-sapphire. Larger BNA crystals (14 mm ${\times} $ 25 mm) were irradiated with fluences up to the damage threshold multiple times so that damage spots did not overlap. The LIDT of each sample type was measured at least five times.
To perform the THz generation experiments, two optical rectification setups were utilized. In each, ultrafast laser pulses were generated by a Ti:sapphire system with a central wavelength at 800 nm. The pulse duration was ∼100 fs with a 500 Hz repetition rate (mechanically chopped from 1 kHz). Even though the signal-to-noise in these measurements is adequate to not necessitate chopping, it is beneficial to test in the same type of experimental setup and conditions where real experiments are made and chopping provides benefits. In the first setup, the pump beam was reduced to a 2.6 mm 1/e2 radius. 800-nm pulses were directed to the BNA or BNA-sapphire structures, a Teflon filter removed the remaining pump light, and the generated THz naturally diverged to nearly fill and was focused by a 2-inch diameter off-axis parabolic mirror. The focused THz (∼500 μm radius) waveform was detected via electro-optic (EO) sampling. We used 100-fs 800-nm probe pulses in 100 μm (110) GaP layer bonded to a 1 mm (100) GaP crystal to minimize the effect of signal echoes. The probe light was passed through a λ/4 waveplate and Wollaston prism, and photodiode intensities were recorded using gated integrators and a computer-based data acquisition system [30]. The electric field strength was determined using standard EO-sampling equations and electro-optic coefficients for GaP [8,31,32].
In the second upgraded setup with tighter THz focus, the pump beam was larger with 4.5 mm 1/e2 radius, and a series of 3 off-axis parabolic mirrors (1-inch diameter, 1-inch effective focal length (EFL); 3-inch diameter, 5-inch EFL; 3-inch diameter, 2-inch EFL) were used to focus the THz beam to approximately 300 μm radius with the same EO-detection scheme. In both setups, Gaussian beam propagation was used to model the beam divergence and size at each point in the setup, and the predicted beam size at the focus was checked with knife-edge scans. We note that in the upgraded, smaller focus THz setup, the electric field strength was strong enough that the GaP signal saturated. By rotating the first of a pair of wire-grid polarizers, we could attenuate the THz, measured at several angles, and then extrapolate to the maximum transmission angle, giving a more accurate evaluation of the maximum THz field strength (this correction wasn’t needed in the first setup). In both setups, the pump fluence and spot sizes were measured just before the BNA sample.
Typical THz waveforms with accompanying spectra measured in the first setup, with pump fluence just below the respective damage thresholds, are displayed in Fig. 1. In Figs. 1(a) and 1(b), the lighter lines were produced with bare BNA crystals, and the darker lines were generated with BNA bonded to sapphire (thin crystals for Fig. 1(a) and thicker crystals for Fig. 1(b)). Thicker crystals exhibit a generation spectrum similar to LiNbO3 (up to about 3 THz), and thinner crystals contain significant frequency content out to 5 THz. Both thin and thick BNA exhibit a main dip in the generated spectrum at 2 THz, and small dips at 1.8 THz and 3.2 THz corresponding to phonon absorptions and worse phase-matching at frequencies larger than 2 THz [27]. These traces are represented by the highest field strength points in Fig. 3 below, for bare and sapphire-bonded BNA THz generators. The THz generation spectral data for bare crystals and crystal-sapphire structures are shown in the insets of Fig. 1, showing no large change in the frequency content of the generation spectra.
Fig. 1. Generated THz electric fields for bare crystals and BNA-sapphire structures pumped just below their respective damage thresholds. a) thin bare BNA crystal pumped with 4 mJ/cm2 fluence (light blue) and thin BNA-sapphire structure pumped with 14 mJ/cm2 (navy blue). b) thick bare BNA crystal pumped with 3.9 mJ/cm2 (light purple) and thick BNA-sapphire structure pumped with 13 mJ/cm2 (purple).
3. Results and discussion
Table 1 shows representative photos and diagrams of the tested architectures, with the corresponding LIDT values and maximum generated electric field results from the average maximum fluence (just before damage). To test for the damage threshold, the sample was exposed to the laser for at least 15 seconds at each fluence. Thermal imaging measurements indicated this was enough exposure time for the sample to reach a steady state temperature. Additionally, we irradiated several samples for much longer than 15 seconds and the damage thresholds were similar. After increasing the fluence, a drop in the output THz electric field was an indication that the sample had been damaged. We report average damage threshold values of 4.84 ± 0.95 and 4.03 ± 0.17 mJ/cm2 for thin and thick bare crystals, respectively. At these fluence values, bare BNA crystals heat and melt, leaving a hole in the BNA crystals that results in a significant decrease of THz signal (see Fig. 2(a)). The obtained LIDT results are in agreement with previously reported damage threshold values of 4 and 6 mJ/cm2 [27,28]. The BNA-sapphire structures, however, maintain their integrity up to an average fluence of 13 mJ/cm2. Even at 16 mJ/cm2, BNA-sapphire structures show damage as a browning of the crystal with no melted holes, as illustrated in Fig. 2(b). After damage and lowering the fluence below the damage threshold, the BNA-sapphire structures still produced strong THz, but the electric-field output was reduced ∼10%. This is similar to crystal degradation seen in other organic THz generation crystals like OH1, DSTMS, and DAST over prolonged exposure (1-2 years of regular use), which degradation can be additionally increased at higher pump fluences. We note that the damage thresholds for our BNA-sapphire structures are similar to damage thresholds reported for bare BNA when using a laser system with <25 Hz repetition rate (when average heating does not play an important role) [29].
Fig. 2. Damaged BNA crystals, a) Bare BNA, and b) BNA-sapphire structure. Damaged spots are marked with red circles.
Table 1. Representative photos and diagrams of thin and thick bare BNA and fused to sapphire crystals. Average and maximum laser-induced damage threshold (LIDT) values were determined for a ∼2.6 mm pump radius. Electric field values were obtained for just below average LIDT fluence.
The marked increase in the LIDT of the BNA-sapphire structures corresponds on average to 2.8 times higher stability compared to bare BNA crystals for both thin and thick samples. To demonstrate the reproducibility of a) our THz generation measurements and b) the BNA-sapphire structures, Table 1 reports the average damage threshold for bare BNA and BNA-sapphire samples. The bare thin crystals show a larger standard error compared with the thick samples and similar uncertainties are observed for crystal-sapphire measurements.
To demonstrate the impact of the featured BNA-sapphire structures on the generated THz fields, we show in Fig. 3 the evolution of the peak electric fields with increasing pump fluences. Importantly, the bare crystals reach their damage threshold at ∼4 mJ/cm2, when the crystals begin to melt and the THz signal significantly decreases. For the fused structures, the magnitude of the THz electric field continues to increase up to 12 mJ/cm2, on average. While there is not a linear relationship between pump fluence and output THz electric field due to nonlinear absorption, an increase in pump fluence nonetheless results in an increase in generated electric field until the BNA sample is damaged. As seen from Fig. 3, an increase of ∼3 times in the fluence with the crystal-sapphire ensemble results in around 2.4-fold improvement in the generated THz fields. We also measured the THz pulse energy with pyroelectric detector (Gentec-EO QSIL-3). After accounting for all filters in the path from the THz generator to the detector, we report the efficiency in the inset to Fig. 3. The triangles are data recorded with BNA-sapphire samples, and the solid line (with arbitrary units) is proportional to the electric field squared divided by the pump fluence (extracted from the thin-BNA electric field data in Fig. 3). The vertical dashed grey lines in the inset indicate LIDT values. This shows a ∼2${\times} $ increase in efficiency for BNA-sapphire samples at higher fluences, compared to bare BNA crystals just below their damage threshold.
Fig. 3. Electric field values at different fluences for representative crystal spots. Bare and crystal-sapphire samples are represented with filled and open shapes, respectively. The dashed and dot-dashed lines represent the LIDT, while the shaded portions represent standard deviations of the LIDT. Inset: Triangles are measured efficiency for two different BNA-sapphire samples. The solid line is a relative efficiency calculated from electric field data in the main figure as described in the text.
Figure 1 above shows THz waveforms that were recorded using thin and thick crystal structures at fluences just below the laser-induced damage threshold. As seen in Fig. 1(a) and Fig. 3, a thin BNA-sapphire structure pumped at 13.2 mJ/cm2 generates a peak-to-peak THz field of 551 kV/cm, and as observed in Fig. 1(b) and Fig. 3, the thick BNA-sapphire generates 636 kV/cm. These peak fields more than double the maximum THz fields of 238 and 266 kV/cm obtained for bare thin and thick crystals, respectively (also in Figs. 1(a) and 1(b)). For all crystals, the amplification caused by the increased pump fluence reaches a maximum at 1.4 THz, where phase-matching is optimal [27]. The broadband generation for the thin BNA-sapphire architecture extends up to 5 THz and is similar to our previous study with thin BNA crystals [27], while the thick BNA-sapphire crystals produce very strong fields below 2 THz, with smaller amounts of light produced up to 3 THz.
As a further test of the capabilities enabled by the sapphire structures, we upgraded our experimental setup to achieve better focusing using three parabolic mirrors instead of one [8]. We pumped the BNA-sapphire structures with the natural laser beam radius of 4.5 mm, with a pulse energy of 3.0 mJ (corresponding to a fluence of 4.7 mJ/cm2). As shown in Fig. 4, we achieved a peak-to-peak electric field of ∼1.5 MV/cm with a thin BNA-sapphire sample. The inset to Fig. 4 shows the spectrum with frequency components extending out to 5 THz. We also measured a THz pulse energy of 15 μJ, corresponding to a 0.5% conversion efficiency. These field strengths and pulse energies now rival those produced with tilted-pulse front THz generation in LiNbO3 [8,9], but with a simple collinear pump and a broader THz bandwidth.
Fig. 4. THz waveforms collected with BNA-sapphire pumped with near full laser pulse energy (3 mJ 800 nm), compared to THz generated with maximum output from the OPA used to pump DAST or OH1 crystals (0.84 mJ 1450 nm). Inset: Corresponding spectra.
Furthermore, the broadband THz pulses generated with BNA-sapphire now rival the strength of other organic THz generators pumped at longer wavelengths. BNA is not as efficient at THz generation as DAST or OH1, but with the ability to use more power directly from a Ti:sapphire laser, the overall BNA THz output is now similar to more efficient THz generators. In Fig. 4, we also show THz pulses generated using DAST or OH1 pumped at 1450 nm. These THz traces were measured using exactly the same 3-parabolic mirror experimental setup for all crystals. ∼300 μm DAST and OH1 crystals were pumped by the maximum output of our OPA with 0.84 mJ per pulse (corresponding to 4 mJ input to the OPA, giving a 1450-nm pump fluence of ∼2 mJ/cm2), while BNA is pumped with 3.0 mJ of 800-nm light. The lower THz generation efficiency of BNA-sapphire (∼27% less efficient than DAST and OH1) is similar to the power conversion losses in the OPA needed to efficiently pump DAST and OH1.
To further investigate this increase in damage threshold, we analyzed the transmission of 800-nm light through bare BNA and BNA-sapphire as a function of fluence. A large 10 mm ${\times} $ 20 mm, ∼650 μm thick BNA crystal was fused to a sapphire plate, half on and half off so we could directly compare transmission through the bare crystal to transmission through the BNA-sapphire. UV-vis-IR transmission measurements show a relatively low linear absorption coefficient of ∼5 cm-1 at 800 nm [27]. Figure 5 shows that the transmission nonlinearly drops as the fluence is increased for both the bare BNA and BNA-sapphire. The nonlinear transmission is explained well with a multiphoton absorption fit (dashed line in Fig. 5). At the lowest fluences, linear absorption does not lead to much absorbed light. At the highest fluences, nearly half of the pump light is absorbed by the sample through multi-photon absorption.
Fig. 5. Transmittance of the 800-nm pump through bare BNA and BNA-sapphire as a function of pump fluence. The dashed line shows a fit to a multiphoton absorption model, including two- and three-photon absorption.
As the structure absorbs more and more pump light, the BNA heats up more and more. Using the nonlinear absorption (adjusted for reflective losses using a refractive index of 1.76 for sapphire, 1.836 for BNA, and 1 for air), and estimating the thermal conductivity of BNA as 0.18 W/mK (which gives us temperature rises in line with thermal imaging measurements, and is similar to many organic crystals [33]), we can approximate the steady-state temperature rise of the BNA and compare this to the temperature rise of the BNA-sapphire structure (see Eqs. (6) and (8) from [34]). For bare BNA, the calculations predict a temperature increase of over 80 K, which, when starting at room temperature, brings BNA above the melting point at the center of the pump beam, resulting in a melted hole in the BNA (see Fig. 2(a) above). We can also calculate the temperature change of BNA fused to sapphire. When fused to high-thermal conductivity sapphire, we estimate BNA only heats up ∼5-10 K, even at the highest fluences reached. We note that these equations neglect an interface thermal conductance, heat transfer into the optical mount or surrounding air, as well as any time dependence to the temperature rise. The calculations also neglect a potential thermal gradient due to absorption across the BNA crystals. Our main reason for measuring both thick and thin BNA-sapphire was to investigate how these thermal gradients may alter damage thresholds, particularly in the thicker crystals. Similar damage thresholds in all samples indicate that this was not an issue. Additionally, imaging the BNA crystals with a thermal imaging camera confirms the temperature rise of only ∼10 degrees for the BNA-sapphire and ∼70-degree rise for the bare BNA pumped below the damage threshold (pumping above the damage threshold quickly melts the bare BNA making it not feasible to measure the temperature rise).
If we consider that the steady-state temperatures of the BNA-sapphire structures are well below the BNA melting point, we conclude that a different damage mechanism is occurring when the BNA browns, likely due to multi-photon absorption that degrades the sample (see Fig. 2(b) above). Fortunately, this browning-damage is not catastrophic, and the BNA can still be used, albeit with less THz output. In a recent study using a 10 Hz repetition rate Ti:sapphire laser, this drop in THz output also occurred, indicating that at lower repetition rates, and therefore lower average powers, a similar damage mechanism can occur [29]. Additionally, our thermal modeling suggests that similar fluence damage thresholds would be maintained with higher repetition rates, potentially up to 2-3 kHz. Variations in laser spot size, pulse duration, and pump wavelength may lead to differences in damage threshold compared to what we report here.
4. Conclusions
In summary, we have shown the utility of fusing BNA crystals to high-thermal conductivity sapphire windows to improve thermal stability, significantly increase the damage threshold, and enable large increases in THz output via pumping with Ti:sapphire laser sources. The 3-fold increase in damage threshold for BNA-sapphire ensembles compared to bare crystals is due to improved heat dissipation from the BNA into the sapphire layer. This increased damage threshold enables significantly increased THz generation. With a 3 mJ pump pulse energy, we generated THz pulses with peak-to-peak electric fields exceeding 1 MV/cm. The BNA-sapphire generated THz fields and broad bandwidths are comparable in magnitude to those produced by the organic DAST and OH1, which are optimally pumped using the longer wavelength output of an optical parametric amplifier. BNA-sapphire enables any lab with an amplified Ti:sapphire laser system to now perform high-field THz spectroscopy with broader bandwidth THz pulses than is possible with tilted-pulse front THz generation with LiNbO3.
We additionally note that BNA is optimally phase matched close to 1 μm pump wavelength. With relatively new and increasingly common high-average power Ytterbium based amplified laser systems used for THz generation [35–37], BNA-sapphire should prove an excellent THz source. However, with the high average powers, we anticipate that the BNA melting point may still be reached, and we are currently investigating active cooling schemes which were not needed in the current study.
Acknowledgements
We thank the Department of Chemistry and Biochemistry at Brigham Young University for funding.
Disclosures
D. J. Michaelis and J. A. Johnson are co-founders of Terahertz Innovations, LLC.
Data Availability
The data that support the findings of this study are available within the article and from the corresponding author upon reasonable request.
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