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We report the generation of free space terahertz (THz) pulses with energy up to 8.3 ± 0.2 µJ from an encapsulated interdigitated ZnSe Large Aperture Photo-Conductive Antenna (LAPCA). An aperture of 12.2 cm2 is illuminated using a 400 nm pump laser with multi-mJ energies at 10 Hz repetition rate. The calculated THz peak electric field is 331 ± 4 kV/cm with a spectrum characterized by a median frequency of 0.28 THz. Given its relatively low frequency, this THz field will accelerate charged particles efficiently having very large ponderomotive energy of 15 ± 1 eV for electrons in vacuum. The scaling of the emission is studied with respect to the dimensions of the antenna, and it is observed that the capacitance of the LAPCA leads to a severe decrease in and distortion of the biasing voltage pulse, fundamentally limiting the maximum applied bias field and consequently the maximum energy of the radiated THz pulses. In order to demonstrate the advantages of this source in the strong field regime, an open-aperture Z-scan experiment was performed on n-doped InGaAs, which showed significant absorption bleaching.
© 2016 Optical Society of America
1. Introduction
The past decade has seen the active development of various laser-based, intense THz sources, which has allowed the generation of sub-picosecond THz pulses with µJ energies, resulting in MV/cm focused fields [1]. These advances have paved the way for studying new physical phenomena and novel applications in the THz frequency range, such as nonlinear THz optics and imaging [2,3]. For example, single-cycle high-field THz pulses have been used to study the anisotropic effective mass of hot electrons in the non-parabolic band of InGaAs [4], inter-band impact ionization in graphene [5], insulator-to-metal transitions in VO2 [6], and THz-induced changes in human gene expression [7]. In parallel with these fascinating studies, intense THz radiation is being used to engineer transient states of matter [8].
For example, THz pulses have been used to control molecular orientation in ionization experiments [9, 10] and to modify the magnetic structure of multiferroic compounds, such as TbMnO3, by modulating the amplitude of spin-cycloid plane rotation [11]. Moreover, these sources have enabled the observation of strong field phenomena in matter such as impact ionization [12–14], field ionization of excitons, impurities and excited atoms [15–17], strong field transport in crystals [18, 19] extreme high harmonic generation [20, 21] and field emission at THz frequencies [22]. Many of these phenomena capitalize on the unique characteristic of intense THz radiation to accelerate electrons to very large kinetic energy over half a cycle of the driving field. The energy given to an electron by the THz field over a half-cycle pulse may be estimated by its ponderomotive energy [8]:
Here e and m0 are the charge and effective mass of the carriers, and Epeak is the peak THz electric field at frequency ω. For example, THz pulses with 1 MV/cm field at 1 THz accelerate free electrons up to 11 eV, sufficient to create many of the non-linear effects described above [8].THz pulses with MV/cm peak electric fields are generated primarily using two methods: (i) optical rectification in bulk non-linear media such as LiNbO3 using the tilted wavefront technique [23,24] or in organic crystals such as 4-N,N-dimethylamino-4’-N’–4’-dimethylamino- N- methyl-stillbazolium tosylate (DAST) with an infrared optical pump pulse [25] and (ii) generation in photo-induced gas plasmas [26–28]. A key characteristic of the different intense THz sources is that they cover different parts of the THz spectrum. For example, the LiNbO3 source efficiently covers the range from 0.5 THz up to 2 THz while the two-color plasma source covers the range from 2 THz up to 15 THz. From this point of view, we can see that it is very important to develop an intense THz source that generates THz pulses that cover the range from 0.1 up to 1 THz. LAPCAs are well known to generate THz pulses with low THz frequencies. It can also be advantageous to work with THz sources having lower frequencies, if the goal is to observe non-resonant, non-linear effects in matter with intense THz pulses, since they can have a large ponderomotive potential associated with them.
Despite recent success with optical rectification and air plasma THz sources, the first intense, tabletop THz source was a GaAs LAPCA [29]. The reported LAPCA source generated half-cycle THz pulses with 0.8 µJ pulse energy, with a calculated peak THz electric field of 150 kV/cm. The spectrum of the radiated field showed a peak frequency of approximately 0.25 THz. From Eq. (1), even with such relatively modest THz fields, one can estimate an associated ponderomotive energy at this frequency of 4 eV.
In addition, the asymmetry of the radiated THz pulse can impart a non-zero transverse momentum to electrons because they are not decelerated by an equal, but opposite polarity swing in the field associated with a full-cycle of radiation. Despite this uniqueness, LAPCAs have not found widespread use as intense THz field sources since they have traditionally been limited by various technical factors, including thermal instability, lack of reliability in terms of premature failures, and saturation of the THz electric field at relatively low pump laser fluence, thus requiring excessively large apertures [30, 31].
Recent research has focused on enhancing THz fields radiated from LAPCAs. One approach consists of implementing an interdigitated geometry for the LAPCA. For example, a GaAs interdigitated LAPCA with a 5 µm gap size has generated THz pulses with 36 kV/cm focused electric fields and an optical-to-THz conversion efficiency of 210−3 [32]. Another approach consists of enhancing the optical-to-THz conversion efficiency by incorporating nanoantennas [33] or implementing plasmonic electrodes [34]. While the nanoantennas increase the light concentration near nanorod arrays localized at the plasmon resonance, the plasmonic electrodes reduce the average photo-carrier transport path, allowing the collection of a larger number of carriers on a sub-picosecond time scale, thus increasing quantum efficiency. For example, Yang et al. used a three dimensional plasmonic electrode structure on a logarithmic spiral antenna and reported an optical-to-THz conversion efficiency of 7.5%, which is, to date, the highest optical-to-THz conversion for tabletop THz sources [35]. Even though this process has been demonstrated for LAPCAs with dimensions of 1 mm2, enabling an efficiency of 1.6% [36], these nano- and micro-scale structures are extremely difficult to fabricate on very large areas (~cm dimensions), which would be necessary to generate intense THz pulses with MV/cm focused fields.
The THz pulses radiated from LAPCAs show fields that are linearly proportional to the applied bias field [37]. The traditional GaAs substrate has a moderate breakdown strength, which ultimately limits the conversion efficiency and radiated pulse energy. The use of wide bandgap semiconductor crystals can significantly improve the radiated pulse energies, since they have higher dielectric strength, thus allowing larger bias fields to be applied. In the past, diamond [38], ZnO [39] and GaN [40] substrates have been tested as substrates for THz generation with photoconductive antennas. Despite their very attractive electronic and thermal properties, these crystals have a bandgap larger than 3.1 eV. As a result, the driving laser needs to have a wavelength shorter than the second harmonic of a Ti:sapphire laser to excite the carriers above the bandgap, thus decreasing their attractiveness. To this end, ZnSe is another wide bandgap semiconductor crystal that has recently been studied for THz generation [41–43]. Although having less attractive electrical and thermal properties compared to diamond, the 2.7 eV bandgap of ZnSe allows such LAPCAs to be pumped above the bandgap with the second harmonic (400 nm) of the Ti:sapphire laser [42,43] and below the bandgap via two-photon absorption with an 800 nm pulse [41,42]. Recently, it has been demonstrated that a 5.5 cm2 area ZnSe interdigitated LAPCA covered with a binary phase mask (phase delay on every second gap) and excited above the bandgap radiated THz pulse energies of 3.6 µJ and an equivalent calculated THz peak electric field of 143 kV/cm. However, the radiated THz peak electric field was still significantly less than what can be obtained with other tabletop THz sources. Since the field of THz pulses generated by an interdigitated LAPCA is linearly proportional to the illuminated surface area [44], it is beneficial to increase the photo-illuminated area of the interdigitated LAPCA.
In this work, an interdigitated ZnSe LAPCA with an area of 12.2 cm2 was illuminated using 400 nm laser pulses. This source generated quasi-half-cycle THz pulses with an energy of 8.3 ± 0.2 µJ when the LAPCA was covered by a shadow mask (every second gap covered). The calculated THz field was estimated to be 331 ± 4kV/cm. Considering that the median frequency of these THz pulses was 0.28 THz, we estimated the ponderomotive energy of these pulses to be 15 ± 1 eV, which is higher than what can be obtained with a 1 MV/cm field at 1 THz. We find that increasing the number of interdigitated electrodes led to a significant increase in the capacitance of the LAPCA, which in turn led to distortion of the applied voltage. This distortion fundamentally limited the magnitude of our bias field and also limited the maximum radiated THz field. In order to demonstrate the large ponderomotive potential, we conducted a z-scan experiment showing the nonlinear absorption bleaching of n-doped InGaAs resulting from THz electric field driven inter-valley scattering of electrons in the conduction band.
2. Device architecture and experimental set up
For these experiments, we used an interdigitated ZnSe LAPCA with an active area that is larger than that has been previously used [43]. The polycrystalline ZnSe crystals (Crystran Ltd., UK) had dimensions of 5 cm square and 1 mm thick. Two different antennas were fabricated. Both antennas have the same interdigitated structure, which was composed of 35 identical electrodes with 0.7 mm gap spacing, chosen to ensure operation in the THz field-screening regime where the radiated THz field is not limited by space-charge screening [45]. The electrodes were 35 mm long and 300 µm wide. Two different metallization types were implemented using conventional optical lithography and e-beam evaporation. The contacts of the first antenna were made using 20 nm of Cr and 150 nm of Au, while the contacts of the second antenna were made using 20 nm of Cr and 150 nm of Pt. The entire interdigitated structure was covered with a dielectric layer of alumina (Al2O3) with a thickness of 300 nm, which was deposited by e-beam deposition at room temperature. The dielectric film was used to encapsulate the antenna to limit air breakdown at large applied bias fields and protect the electrodes and crystal against damage. Instead of making a second metallization in order to cover antennas with one bias field direction, we used a shadow mask composed of 19 opaque plates constructed from 3 mm thick RG-850 Schott glass filters that we aligned on the interdigitated structure.
Figure 1. shows the experimental setup used to generate and detect THz radiation. We used an amplified Ti:sapphire laser that generates 800 nm laser pulses with an energy up to 300 mJ, a pulse duration of 60 fs at 10 Hz repetition rate. This optical source was used so that the entire area of the LAPCAs could be illuminated with a 400 nm laser fluence that is large enough to observe saturation and radiate the highest field possible. The experimental setup is comprised of two main parts: (i) the optical pump beam that is used to illuminate the ZnSe LAPCAs and (ii) the probe beam used for Electro-Optic (EO) sampling.
Fig. 1 Schematic of the setup for generation of intense THz pulses with an interdigitated ZnSe LAPCA excited at 400 nm.
A KDP crystal was used to generate the second harmonic (400 nm). The maximum laser energy obtained at 400 nm was 20 mJ. To ensure the full illumination of the interdigitated ZnSe LAPCAs and limit saturation, the beam was expanded to a diameter of 4.7 cm FWHM with a telescope. The external bias field is provided using a pulsed high voltage-generator, which can generate 20 ns, 7 kV pulses. The electrodes were oriented vertically to produce horizontal polarization. Any residual 400 nm light was blocked using two layers of black polyethylene, which is higly transparent to THz radiation. The THz beam is collected and focused in air by a gold-coated 90° off-axis parabolic mirror (F/1, 2” diameter, 2” focal length) and then recollimated and refocused by a pair of two off axis mirrors with 4” diameters and 4” focal lengths.
Leakage light from a dielectric mirror placed before the KDP crystal was used as the optical probe beam. This beam was delayed relative to the pump using an optical delay line, and subsequently guided collinearly with the THz beam after passing through the third off-axis mirror. We used a (110) GaP crystal, 0.3 mm thick, for the EO detection. High-resistivity Si wafers were used (if necessary) to attenuate the THz field before the EO crystal to avoid over-rotation of the probe beam, and to ensure operation in the linear regime to avoid distortion in the detected THz waveform. For measuring the THz energy and imaging the focused THz beam, we placed a pyroelectric detector (Model Gentec THz 5I-BL-BNC) and a ferroelectric infrared camera (Electro Physics model PV320-L2Z) at the focus of the first parabolic mirror. The pyroelectric detector has a sensitivity of 78 kV/W at 0.6 µm wavelength as per the manufacturer’s specifications. However, calibration of the response in the THz frequency range was carried out by cross calibration with a Coherent-Molectron pyroelectric detector, previously calibrated in the THz frequency range [46]. This calibration gives a THz sensitivity of 0.87 V/µJ, or 87 kV/W at 10 Hz repetition rate.
3. Experimental results and discussion
3.1 THz waveform and spectrum
Figure 2 (a) shows the radiated THz field profile from the interdigitated ZnSe LAPCA with Ti/Au metallization, covered by a shadow mask, excited with 16 mJ, 400 nm optical pump and biased at 30 kV/cm. Figure 2 (b) shows the corresponding amplitude spectrum. The waveform is asymmetric and consists of a large positive peak followed by a long but weak negative tail. The maximum of the negative tail arrives 1.7 ps after the main peak and is only 14% of the main peak. This kind of asymmetry is characteristic of LAPCAs. The 1/e THz pulse duration is 0.84 ps. The oscillations observed in the negative tail result from water absorption, as this experiment was not conducted in a purged environment. In Fig. 2 (b), the THz spectrum extends up to about 2 THz. The main frequencies are below 1 THz, and the peak of the spectrum is located at 0.12 THzTHz.
Fig. 2 (a) THz waveform acquired by EO sampling of emission from ZnSe interdigitated LAPCA with Ti/Au contacts, using a shadow mask, excited with 16 mJ of 400 nm optical pump and biased with 30 kV/cm. (b) Respective amplitude THz spectrum.
3.2 Influence of the metallization
Figure 3 shows the scaling of the energy of THz pulses generated by a ZnSe interdigitated LAPCAs with Cr/Au and Cr/Pt metallization covered by the shadow mask (a) as a function of the bias field excited by a driving laser energy of 16 mJ and (b) as a function of the optical energy when biased at 20 kV/cm.
Fig. 3 Energy of the THz pulse generated by ZnSe LAPCAs with the Cr/Pt and Cr/Au metallization covered by a shadow mask (a) as function of the bias field when the LAPCAs are excited with 16 mJ and (b) as a function of the optical energy when the LAPCAs are biased at 20 kV/cm.
The THz energy follows a quadratic behavior as a function of the bias field for both metallizations and the solid curves represent quadratic fits to the experimental data in Fig. 3 (a). In Fig. 3 (b) the THz energy scales sub-linearly as a function of the optical energy when the energy is low. However, a saturation of the emission appears for optical energies that are larger than 15 mJ. Figure 3 shows that the Cr/Au metallization is slightly more efficient than the Cr/Pt metallization. However, the difference is not large. We attribute this difference to the higher conductivity of Au versus Pt. This experiment was performed without a 400 nm filter located before the telescope, so it is possible that 800 nm leakage was also illuminating the ZnSe LAPCAs, leading to the sub-linear scaling observed in Fig. 3(b).
3.3 Energy and efficiency
Figure 4 shows (a) the measured energy and (b) the optical to THz conversion efficiency as a function of the optical energy for THz pulses generated by the ZnSe LAPCA with the Cr/Au metallization covered by the shadow mask and biased at 42 kV/cm. This bias field was the maximum electric field we could apply to the antenna before the onset of Corona discharge. At low optical energy, the THz energy follows a sub-linear relationship and then saturates when the optical energy is higher than 15 mJ. The maximum energy is 8.3 ± 0.2 µJ, more than 2 times larger than the previous record [43], even while using a shadow mask. If, instead, a binary phase mask was used, the generation of a symmetric single cycle THz pulse would be expected and double the radiated energy in the THz pulses [47]. From Fig. 4 (b), we observe that the maximum optical to THz conversion efficiency is 0.106% when the LAPCA is excited at 2.1 mJ energy. By increasing the fluence, we observe a drastic decrease of the efficiency, which is a consequence of operating the LAPCA in the THz field screening regime. This THz efficiency was possible, in part, because of the presence of the 300 nm thick alumina (Al2O3) dielectric layer that acts as a partial antireflection coating [48]. This dielectric layer also protects the electrodes from damage by corona discharge. This considerably increases the lifetime of the LAPCA, which is required if these sources are to be used for nonlinear THz spectroscopy.
Fig. 4 (a): Energy and (b) optical-to-THz conversion efficiency as a function of optical energy for the radiation of THz pulses from interdigitated ZnSe LAPCA with Cr/Au contacts illuminated and biased at 42 kV/cm
3.4 Estimation of the THz field and the ponderomotive energy
Figure 5 (a) and (b) shows the real-time image and the transverse intensity profile of the THz pulse with an energy of 3.6 µJ measured at the focus position after the 2 inch focal length off-axis mirror. Assuming a Gaussian profile, we estimate the THz beam spot size at 1/e2 at the focus to be 2.34 mm on the horizontal axis and 2.74 mm on the vertical axis for a total spot area of 5.07 mm2. These dimensions are slightly smaller than what was previously obtained with a smaller interdigitated ZnSe LAPCA [43]. However, these dimensions are still much larger than can be obtained with a two-color plasma [28] source or LiNbO3 source [23], whose higher THz frequencies allow them to be focused more tightly [49]. The low frequencies of the LAPCA source fundamentally limit the focused beam diameter and consequently the magnitude of the focused field. However, these dimensions of the THz spot size are similar to what can be obtained with a LiNbO3 source when a high fraction of the low THz frequencies, present in the THz pulse, are collected by a first off axis-mirror with larger numerical aperture [24].
Fig. 5 (a) Real time image of the THz pulse with an energy of 3.6 µJ measured at the focus position (after the off axis mirror) and (b) cross sections intensity profiles of the THz image in (a).
The THz electric field is estimated here by using Eq. (2) which is derived from Maxwell equations [46]:
Here η0 is the impedance of free space, W is the THz energy, wx and wy are the intensity beam waist for the horizontal and vertical directions and τ is the 1/e THz pulse duration that can be retrieved from Fig. 2 (a). In order to have a better estimation of the field, we calculated by integrating the square modulus of the THz waveform in Fig. 2 (a) that 74% of the total energy of the THz pulses is located inside the main positive peak. Taking an energy of the measured values of W = 6.2 ± 0.2 µJ, wx = 1.17 mm and wy = 1.37 mm, and τ = 0.84 ps the THz electric field has been evaluated to be 331 ± 4 kV/cm, which is, to the best of our knowledge, the largest field observed from an LAPCA THz source.In the experiments that follow, we measured a maximum modulation of 31% in the electro-optic signal, with a 300 µm thick (110) GaP crystal and two Si wafer filters (to attenuate the THz beam) at the position of the 3rd off-axis mirror for a THz energy of 4.1 µJ. This modulation corresponds to a peak electric field of 202 kV/cm [23]. The energy of the THz pulses, and as a consequence the electric field, was somewhat smaller as a result of strong laser pre-pulse that was not present in the previous experiments.
In order to estimate the ponderomotive energy associated with the THz field, we adopt the same method reported in [17], where we calculated the median frequency of the power spectrum of the half-cycle pulse. By excluding the negative tail, we found that the median frequency of the half-cycle pulse is located at 0.28 THz. The ponderomotive energy associated to this THz pulse is evaluated to be 15 ± 1 eV. This ponderomotive energy is slightly higher than the ponderomotive energy associated to a THz pulse of a 1 MV/cm field at 1 THz as is typically generated by a LiNbO3 source.
3.5 Effects of the large capacitance of the LAPCA
We could apply a maximum bias field of 42 kV/cm before the appearance of corona discharge. In a previous study, we noticed corona discharge when a 47 kV/cm bias field was applied to an interdigitated ZnSe LAPCA with 0.6 mm gap size [43]. This limiting bias field determines the maximum radiated energy and field of the THz pulses. Figure 6 compares the shape of the high voltage pulse when the high-voltage source is connected to the interdigitated ZnSe LAPCA and when it is connected to (a) no capacitance, (b) a capacitance of 24 pF, (c) 36 pF and (d) 50 pF. When the antenna is connected, we observe strong distortions of the temporal shape of the voltage profile and a reduction in the amplitude of the voltage pulse (1.5 kV with no capacitance compared to 0.96 kV with the ZnSe LAPCA). More importantly, we notice that the voltage pulse duration almost doubles when the ZnSe antenna is connected.
Fig. 6 (a) Trace of the high voltage pulse when the high voltage source is connected to (a) no capacitance, (b) a 24 pF capacitance, (c) a 36 pF capacitance and (d) a 50 pF capacitance. All the traces are compared to the trace of the high voltage pulse when the source is connected to the interdigitated ZnSe LAPCA.
Increasing the capacitance leads to stretching, distortion and a reduction of the high voltage bias. The distortion and the stretching may be the cause of the reduced air-breakdown voltage. Martin et al. reported an empirical scaling relationship between the mean electric field and the breakdown time [50]. This empirical relationship is based on a collection of data for a variety of gases, electrode geometries, and pressures. The formula is given as:
Here ρ is the gas density in g/cm3, t is the breakdown time in seconds, and E is the mean electric field in kV/cm. This formula is valid for average electric fields between 5 and 100 kV/cm and for times between 500 ps and 10 µs. From this formula, one can see that a longer high-voltage pulse duration leads to air-breakdown at lower fields. We estimate the capacitance of the ZnSe interdigitated LAPCA to be between 36 and 50 pF.The relatively high capacitance of our interdigitated ZnSe LAPCA imposes other limitations on the generated THz radiation. It tends to limit the maximum radiated THz power, which is given by [51]:
Here, Sp(ω) is the photoconductor current responsivity, Popt(ω) is the pump power, ζ is the antenna efficiency, Gp and Cp are the conductance and capacitance of the ultrafast photoconductor connected to a THz antenna represented by a complex admittance YA = GA + jBA. From (4), we can see that high impedance limits the THz radiation power. Since an interdigitated structure is the sum of single antennas connected in parallel, the total capacitance of the antenna will be the sum of the capacitances of each single antenna, and consequently, increasing the number of single antennas leads to an enhancement of the capacitance, fundamentally limiting the maximum radiated THz power.3.6 Absorption bleaching in n-doped InGaAs
In order to demonstrate the very high ponderomotive energy of this source, a nonlinear THz experiment was conducted showing a strong modulation of the optical absorption in an n-doped semiconductor using a Z-scan technique. We use an n-doped InGaAs crystal, which consisted of a 500 nm thick n-type In53Ga47As epilayer with a doping concentration of 2.0x1018 cm−3 grown on a lattice-matched 350 µm thick semi-insulating InP wafer with a dark resistivity that is higher than 1x107 Ω-cm. This InGaAs sample is identical to the one that was used by Razzari et al. for the demonstration of the bleaching induced by a multi-cycle intense THz pulse [52]. Figure 7 shows the normalized time integral of the modulus square of the transmitted THz waveforms as a function of the z position of the sample for the asymmetric, quasi-half-cycle THz pulses generated in this report using the LAPCA. 3 different THz fields were applied by changing the bias voltage on the ZnSe interdigitated LAPCA. The peak electric field values at the InGaAs sample were 174, 144 and 115 kV/cm.
Fig. 7 Normalized time integral of the square modulus of the transmitted electric field as a function of the Z-scan position of the InGaAs sample for three different peak THz fields (174, 144 and 115 kV/cm) and for the bare InP substrate at a peak field of 174 kV/cm.
Strong absorption bleaching is observed, which results from electrons accelerated by an electric field, and gaining enough kinetic energy to scatter from the Γ valley to the L valley where the effective mass is significantly higher. This results in a lower conductivity and consequently, a higher transmission of the THz pulse. Electrons relax to the Γ valley with an approximate intervalley relaxation time of 3.1 ps [53]. From Fig. 7, it is clear that no obvious changes in transmission appear for the bare InP wafer along the Z-scan position. When we illuminated the InGaAs wafer with the asymmetric THz pulse and a peak electric field of 115 kV/cm, we observe a maximum transmission enhancement of 2.8. This transmission enhancement is exactly equal to what has been observed by Razzari et al. with a THz field of 200 kV/cm and a peak frequency located at 1 THz [52]. When we increase the peak electric field up to 144 kV/cm, the transmission increases up to 8.0. The maximum transmission enhancement is 12.7 and is reached with the highest electric field of 174 kV/cm. In fact, under the strongest focused field, the maximum transmission through the InGaAs wafer, relative to the transmission of the InP substrate, was 60% at the focus position. The minimum transmission was 15% when the sample was furthest from the focus, demonstrating a huge bleaching of the absorption. From this example, we clearly demonstrate the high ponderomotive potential of this THz source, which induces strong nonresonant nonlinear absorption bleaching in InGaAs.
4. Conclusion
In summary, the photo-illuminated surface of an encapsulated interdigitated ZnSe LAPCA with an aperture of 12.2 cm2 was studied with two different metallizations. The maximum energy of the quasi-half-cycle THz pulse are 8.3 ± 0.2 µJ when the bias is 42 kV/cm. The corresponding calculated peak electric field is 331 ± 4 kV/cm. Assuming a median frequency of 0.28 THz, we estimate that these THz pulses, for interaction with free electrons, have a ponderomotive energy of 15 ± 1 eV. This ponderomotive energy is slightly larger than can be achieved with a 1 MV/cm electric field at 1 THz. We observed large capacitance that fundamentally limits THz emission from this source. The two major consequences of the large capacitance are: (i) it introduces distortions and stretching of the high-voltage bias, leading to an early air-breakdown limiting the maximum radiated THz field and (ii) it limits the THz power radiated into free space. These limitations are the consequence of increasing the surface of the LAPCA which is the solution for increasing the maximum radiated THz field. Finally, in order to demonstrate the high ponderomotive potential of these pulses, we studied the nonlinear absorption bleaching in an n-doped semiconductor induced by the THz pulses in a Z-scan experiment. Even with a maximum peak electric field of only 174 kV/cm, a maximum transmission enhancement of 12.7 was observed, which is 4.5 times higher than previously reported using a peak electric field of 200 kV/cm at 1 THz.
Acknowledgements
We wish to acknowledge Axis-Photonique for their technical assistance with the pulsed high voltage source. We also thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support.
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