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# Enhancement of terahertz fields in LiTaO3 waveguides using a conical pulse front

Eric Sung, Yun Kai, Thomas Pezeril, Keith Nelson

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Eric Sung, Yun Kai, Thomas Pezeril, Keith Nelson. Enhancement of terahertz fields in LiTaO3 waveguides using a conical pulse front. Optics Express, 2024, 32 (15), pp.26913. 10.1364/OE.519603 . hal-04650337

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# Enhancement of terahertz fields in LiTaO₃ waveguides using a conical pulse front

ERIC R. SUNG,<fnref n="1" /> ID YUN KAI,<fnref n="1" /> ID THOMAS PEZERIL,<fnref n="1" />,<fnref n="2" /> ID AND KEITH A. NELSON<fnref n="1" />,*, ID

**Abstract:** The development of methods for the generation of strong ultrafast electromagnetic pulses in the terahertz (THz) spectral range has led to a surge of progress in nonlinear THz spectroscopy and THz control of molecular and collective responses. For spectroscopy in the 1-THz range, the submillimeter wavelengths and associated large spot sizes, large optical elements, and short distances between final focusing elements and samples can lead to cumbersome experimental setups that are incompatible with some sample environments. Here, we introduce a novel terahertz ring excitation (TREx) optical pumping geometry to generate superposing, focusing fields in planar THz waveguides made out of the electro-optic material lithium tantalate. High THz fields, >175 kV/cm, are generated and measured optically with no free-space THz propagation. The field level achieved by pumping with a sequence of concentric rings of excitation light exceeds by about 20× the result of a single cylindrically focused line of pump light that has been used routinely in previous work. The technique opens new prospects for compact waveguide-based linear and nonlinear THz spectroscopy and signal processing.

© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

## 1. Introduction

The THz region of the electromagnetic spectrum has gained much interest due to the access it provides to a wide range of physical processes such as molecular rotations and spin precessions as well as responses of collective modes in solid-state materials [1,2]. Nonlinear spectroscopic studies in gases [3–5], liquids [6,7], and solids [8–10] have been conducted. THz fields have been used for control over the states of condensed matter including recent examples of ferroelectric phase transitions [11,12], magnetic domain reorientations [13,14], and electronic/structural phase transitions [15].

The development of tabletop THz sources has been instrumental in the study and control of these phenomena. In the 0.1-1 THz range, the robust electro-optic material lithium niobate (LiNbO₃, LN) has been used for generation of intense free-space THz fields at kHz repetition rates which, when focused, reach or approach amplitudes of 1 MV/cm. High field strengths are reached using pump laser light that is spatiotemporally shaped to form a tilted pulse front [16–19] with the tilt angle adjusted to match the THz phase velocity in order to optimally drive the THz field.

To make THz technology more accessible, THz waveguides have been used to provide a compact means for generating, manipulating, and measuring THz fields as well as having them interact with a sample of interest. Thin (10-100 µm) ferroelectric LN and lithium tantalate (LiTaO₃, LT) slab waveguides have been used as the basis for the “THz polaritonics” platform [20], named in reference to the phonon-polaritons formed as a result of the strong coupling between THz electromagnetic fields and polar lattice vibrations in LN and LT. Femtosecond optical pulses are used for the generation and direct visualization of THz fields within a slab with subwavelength spatial and subcycle temporal resolution [21]. Patterned structures can be fabricated in the slabs through femtosecond laser machining [22,23] or deposition [24–26]. This

<footnote n="1">¹Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA</footnote>

<footnote n="2">²Institut de Physique de Rennes, UMR CNRS 6251, Université Rennes 1, Rennes, 35042 Bretagne, France</footnote>

<footnote>*kanelson@mit.edu</footnote>
allows studies of THz wave interactions with photonic crystals [27–29] and other integrated photonics structures [24,26,30,31]. THz fields in the slabs can interact with integrated samples of interest in various compact geometries [32,33].

The utility of the THz polaritonics platform for applications in spectroscopy has been limited by the field strengths (roughly 5-10 kV/cm [20]) that can be routinely generated in the waveguide. Several methods have been developed to generate larger field strengths in waveguides. In one study, a tilted pulse front was used, resulting in a 3-fold increase in multi-cycle field strengths [34]. In another study, a conical prism (axicon) was used to shape the pump light into a “circular ring” pattern to generate THz fields that focused to give 6-fold increased single-cycle field strengths in LT [35]. Recently, pump light was inserted into a LN slab through a beveled edge and confined by total internal reflection, generating a multi-cycle THz field with 50% larger peak field amplitudes [36]. Here, we combine two of the previous approaches, pumping a LT waveguide with a timed sequence of concentric optical rings to generate superposed, converging THz fields. The Terahertz Ring Excitation (TREx) scheme yields strong fields in a configuration that is compatible with most environments that permit optical access to the waveguide. The method also permits selection of either a strong THz electric or magnetic field component.

## 2. THz ring excitation (TREx) scheme

The TREx scheme is illustrated in Fig. 1. A concave circular stair-step “echelon” mirror is used to transform an incident femtosecond pump pulse with a Gaussian spatial profile into a conical discrete tilted pulse front consisting of concentric rings of light which propagate in timed sequence, with the largest-diameter ring first and the smallest-diameter ring last. (See Supplement 1 for details on the discrete tilted pulse front.) The time between successive rings is determined by the echelon step height. The series of pump rings is imaged onto a 50-µm thick LT slab. There is a circular hole at the center of the echelon mirror, which results in a ~15 µm diameter circular region at the center of the rings where there is no pump light. Each individual pump ring generates converging and diverging single-cycle THz fields. The initial THz field is launched by the largest ring, which arrives at the LT slab first, and the later pump rings repeatedly drive the converging THz field as it focuses. The driving force is proportional to the pump light intensity [20] (independent of the optical phase), so the speed at which pump light in the conical pulse front moves across the sample, i.e., the lateral scan speed $v_{scan}$, must match the THz phase velocity $v_{ph}$ to achieve the maximum THz field amplitude. The velocity-matching condition is

$$v_{scan} = \frac{\Delta x}{\Delta t} = v_{ph} \qquad (1)$$

where $\Delta x$ is the spatial shift between successive rings, i.e., the difference between successive ring radii, at the LT slab and $\Delta t$ is the difference in arrival time between successive rings. As in the usual tilted pulse front geometry, the same lobe of the THz field is driven by each pump ring. Constructive interference along with the enhancement from focusing leads to a dramatic buildup of the THz electric field strength. We have neglected anisotropy and dispersion in the THz phase velocity, which are discussed further below and in Supplement 1.

The free-space delay between successive pump rings is $\Delta t = 2H/c$ where $H$ is the echelon step height and $c$ is the vacuum speed of light. The spatial shift between successive rings inside the LT slab is $\Delta x = W/M$ where $W$ is the echelon step width and $M$ is the demagnification of the imaging system which is adjusted using a zoom lens. (See Supplement 1 for details of the optical system.) Thus, the velocity-matching condition can be rewritten in terms of the echelon parameters and the demagnification as

$$v_{ph} = \frac{W_c}{2HM} \qquad (2)$$
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Illustration of the working principles of the TREx scheme. (a) The echelon mirror transforms a femtosecond Gaussian pulse into a sequence of concentric rings that form a conical discrete tilted pulse front. (b) Each ring excites a converging and diverging THz field in the LT slab. The converging field is superposed constructively with those generated in succession by the conical pulse front. At the THz focus, the contributions from all the pump rings interfere constructively to give a dramatically enhanced THz electric field amplitude.</figcaption></figure>

Using the echelon parameters *W* = 150 µm and *H* = 37.5 µm and the THz extraordinary refractive index in LT *n*<sub>e</sub> ≈ 6.2 [20], the demagnification required for effective buildup of the THz fields is approximately 12.4.

The output from a Ti:sapphire amplifier system (800 nm center wavelength, 100 fs pulse duration, 4 mJ pulse energy, 1 kHz repetition rate) was used to illuminate the circular “echelon” mirror (Sodick) made of Stavax nickel-phosphorus, and the resulting series of pump rings was imaged onto the *a*-cut 50-µm thick LT slab (NanoLN) to optically excite THz fields. Approximately 10% of the pump energy was diverted to a noncollinear optical parametric amplifier tuned to generate 532 nm probe pulses, which were used to image the THz fields in the LT slab via polarization gating imaging [37]. The 532 nm probe wavelength was chosen so any signal from the 800 nm pump light and 400 nm second harmonic signal generated inside LT could be easily removed using a spectral filter. The timing between the pump and probe beams was controlled using a variable delay stage. The THz electric field strength was determined via electro-optic sampling directly inside the LT slab using a method similar to that detailed in Ref. [21]. (The relevant equations are summarized in Supplement 1.) The pump polarization was set along the *c*-axis of the LT slab. The probe was polarized at 45 degrees relative to the pump, and the THz field-induced depolarization was measured as described in Supplement 1. Simulations of the THz fields were performed with the finite-difference time-domain (FDTD) method using the open-source software package MEEP [38]. See Supplement 1 for details.
3. Experimental results

The performance of the TREx method is characterized by imaging the THz fields as they converge. Fig. 2 shows the THz electric field at various times during focusing. At early times, the outer rings launch single-cycle THz fields which increase in amplitude while traveling toward the focus. When the pump light and optical probe overlap in the LT slab, a hyperpolarizability signal appears in the images. This in addition to measurement noise obscures the THz waveform at early times while the peak amplitude is small, resulting in what appears to be a half-cycle THz field. The amplitude of the excited THz fields follow a cos² (θ) angular dependence due to the THz electric field being polarized parallel to the LT crystallographic c-axis over the entire ring. As the THz fields focus, the THz field amplitude increases and dominates the hyperpolarizability signal, making the single-cycle THz waveform more apparent. At the THz focus, the full width at half maximum dimensions of the THz spot at the focus are 30 µm horizontally and 60 µm vertically. The difference in the spot dimensions is due to the cos² (θ) dependence of the excited THz fields. (See Supplement 1 for more details.) After passing through the focus, the THz field diverges and the peak field strength decreases.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** THz buildup. Top row: Images of the THz field at various times. Scale bar = 200 µm. Bottom row: Line-out plots of the THz field amplitude along a horizontal dashed line going through the THz focus. All line-out plots have the same vertical scale. The arrows indicate the direction of THz field propagation. See Visualization 1 for a movie of the THz buildup.</figcaption></figure>

Figure 3(a),(b) show the time traces and spectra, respectively, of the focusing THz field at several distances *r* from the focus. As the THz field converges, the peak field strength increases due to the individual responses from the multiple pump rings interfering constructively and focusing. In addition, the THz spectrum narrows about broad peaks at 0.7 THz and 1.3 THz, suggesting optimal velocity-matching at the two different frequencies. The origin of these two frequencies is clarified by the space-time plot and dispersion plot shown in Fig. 3(c),(d) respectively. The space-time plot shows that the pump rings excite two sets of focusing THz fields. The main THz field arrives at the focus at *t* = 0 ps and a second weaker THz field arrives about 1 ps afterward. The dispersion plot reveals that spectral peaks at 0.7 THz and 1.3 THz correspond to THz fields in the TE₀ and TE₁ dielectric waveguide modes respectively. (See Supplement 1 for a brief discussion of the TE waveguide modes.) Due to the slightly different group velocities of the TE₀ and TE₁ waveguide modes, the TE₀ and TE₁ waveguided THz fields focus at slightly different times. Note that the propagation time of the pump light from front to back of the LT slab is *dn*₈₀₀/*c* = 360 fs, where *d* = 50 µm is the slab thickness and *n*₈₀₀ = 2.157 is the LT optical refractive index at the 800-nm pump wavelength [39]. The TE₀ mode has no nodes in the plane of the slab, while the TE₁ mode has a node in the center. For frequencies below 1 THz, i.e., field cycles longer than 1 ps, the pump pulse traverses the slab in less than half of a cycle, so the THz response generated by the pump pulse is approximately in phase throughout the slab thickness. For higher frequencies, the THz field generated by the pump pulse near the front of the slab is significantly out of phase with the THz field generated near the back,
so the response has a significant TE₁ mode component. A thinner slab would have less TE₁ mode response and a TE₀ response that would extend to higher frequencies. Note also that the imaging of the THz fields depends on the field-induced phase delay of the 532-nm probe light (n₅₃₂ = 2.207) [39], integrated through the slab thickness. The opposite signs of the TE₁ mode field in the front and back of the slab diminish the contributions of this mode to the images we record.

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** THz field at the focus. (a) Time traces of the THz field at various distances from the focus. The THz fields reach the focus at t = 0 ps. The pump energy was 225 µJ. Saturation of the THz signal was not observed. (See the Supplement 1 for details.) (b) THz spectra at the same positions as in (a). For the time traces away from the focus (r ≠ 0 µm), only the converging THz wave is considered when calculating the spectrum. The plots in (a) and (b) have been vertically offset for clarity. (c) Space-time plot constructed from lineout plots taken through the center of the focus. (d) Dispersion plot obtained from a 2D Fourier transform of (c). The calculated TE₀ (red solid line) and TE₁ (dashed red line) dielectric waveguide dispersion curves are overlaid.</figcaption></figure>

**4. Optimization**

In order to optimize the performance of the TREx scheme, we investigated how the peak THz field strength varies with the scan speed of the pump rings and the number of pump rings used to excite the THz fields. Simulations were performed using the finite-difference time-domain (FDTD) method with the open-source software package MEEP [38] to corroborate the experimental results. See Supplement 1 for simulation details along with simulations investigating the effect of material anisotropy and pump ring ellipticity.

First, we optimize the pump scan speed. The peak THz field strength was measured at various pump scan speeds, which were controlled by adjusting the demagnification *M* of the zoom lens, while the number of rings used was held constant at 30. The demagnification of the zoom lens
was adjusted by moving the position of the second lens. The position of the LT slab was then adjusted to place it in the image plane of the zoom lens. (See Supplement 1 for details of the zoom lens.) Note that LT was chosen because the THz-frequency refractive index is very nearly isotropic in LT, with a value of approximately 6.2. If the far more anisotropic LN were used instead, cylindrical lenses would be required to reshape the pump rings into ellipses so that the THz fields focus properly. See Supplement 1 for details on correcting for anisotropy. The experimental and simulated results are shown in Fig. 4(a). The THz field strength reaches a maximum at *M* = 12.4, which is in good agreement with the predicted demagnification ratio. This behavior can be explained by comparing the dispersion plots at different scan speeds shown in Fig. 4(b),(c) (*M* = 12.4 and 11.6). The velocity-matching condition described by Equation 1 is graphically represented in the dispersion plots by the overlaid TREx light line. At optimal velocity-matching, the TREx light line approaches the TE₀ waveguide dispersion curve over a wide frequency range. Although perfect velocity-matching is not achieved for all frequencies, THz fields are still excited over a large frequency range and constructive interference between the many frequency components results in a large peak field strength. When a faster scan speed is used (lower demagnification), the slope of the TREx light line increases and the excited bandwidth is significantly smaller, leading to a lower peak field strength. In both dispersion plots, the TREx scheme also excites TE₁ waveguided THz fields at higher frequencies. However, the curvature of the TE₁ dispersion prevents broadband velocity-matching. Thus, the optimal scan speed is that which optimizes the overlap between the TREx light line and the TE₀ dispersion, which in turn maximizes the excited THz bandwidth.

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Optimizing TREx performance. (a) Plot of THz field strength at the focus at varying zoom lens demagnification ratios. (b),(c) Dispersion plots showing the spectral content of the THz pulse at the data points labeled in (a). (d) Plot of the THz field strength at the focus at varying numbers of pump rings. (e),(f) Simulated space-time plots of the focusing THz field at the data points labeled in (d).</figcaption></figure>

Next, we optimize the number of pump rings used at constant pump fluence while keeping the demagnification at the optimized value (12.4). The pump beam was expanded using a 3 × telescope to illuminate the entire echelon mirror with an approximately flat beam profile. An iris was used to set the beam diameter and the number of pump rings. The iris was placed right before the echelon mirror to minimize diffraction effects. The experimental and simulated results
are shown in Fig. 4(d). The peak THz field strength increases roughly linearly with the number of rings and flattens at 35 rings. Simulated space-time plots of the focusing THz fields using 30 and 80 pump rings are shown in Fig. 4(e),(f) respectively. When 30 pump rings are used, the THz field at the focus is single-cycle. However, when using 80 pump rings, more THz cycles appear. This suggests that spatial walkoff limits the number of pump rings that could be used. The walkoff originates from the nonlinear shape of the TE₀ dispersion curve, resulting in a slight difference between the THz phase and group velocities. Because the THz phase velocity is slightly faster than the group velocity, the pump rings (which are velocity-matched to the THz phase velocity) scan slightly faster than the THz pulse speed. As the THz pulse lags behind the pump light, additional lobes appear, with the pump light always driving the leading lobe of the THz field. The optimal number of pump rings can be estimated by considering the walkoff between the pump rings and the excited THz field. Assuming that pump rings only contribute to buildup if they are spatially within one wavelength of the peak of the THz field, we get the condition

$$ \frac{2H}{c} (v_{ph} - v_{gr}) n_{\text{rings}} = \lambda_{\text{THz}} \quad (3) $$

where $n_{\text{rings}}$ is the optimal number of pump rings, $\lambda_{\text{THz}}$ is the THz wavelength in the LT slab, and $v_{ph}$ and $v_{gr}$ are the THz phase and group velocities, respectively. Plugging in the values for $v_{ph}$ and $v_{gr}$ in a 50-μm LT waveguide at $\lambda_{\text{THz}} = 48\mu m$ (1 THz) gives an estimate of 35 rings, which matches well with the experimental and simulated data.

Using the optimized parameters for demagnification ($M = 12.4$) and number of pump rings ($n_{\text{rings}} = 35$), the pump energy was increased to determine the maximum observable field strength. The pump energy was adjusted using a half waveplate and linear polarizer. When the pump fluence was above the optical damage threshold of LT at 1 kHz (approximately 96 mJ/cm²), the laser repetition rate was reduced to 10 Hz. Fig. 5 shows the pump pulse energy-dependence results at room temperature. The peak THz field strength increases monotonically with pump energy, with a decreasing slope at higher pump energies. The optical-to-THz generation efficiency shows a linear increase before saturating at higher pump energies due to self-phase modulation [40] and three-photon absorption [41]. The largest observed field strength achieved with no damage to the LT slab was 175 kV/cm using 0.56 mJ of pump energy, corresponding to a 0.0055% conversion efficiency. Since the THz field is generated in a thin LT slab, the total THz pulse energy (and corresponding conversion efficiency) is low compared to what is routinely achieved in a bulk LN crystal [41]. At higher pump energies, there was gradual damage to the surface of the slab. Optical damage is mediated by photoexcited electrons that become trapped and accumulate at defect sites [42]. Therefore, the damage threshold can be increased by lowering the laser repetition rate, which may allow photoexcited carriers to relax between laser shots [43]. When the laser repetition rate was decreased to 10 Hz, we observed several small damage spots that slowly grew with each laser shot at pulse energies >0.70 mJ. The rate of damage accumulation was slow enough that we could image the THz fields prior to severe damage. The largest observed field strength under these conditions was 300 kV/cm using 1.3 mJ of pump energy (0.007% conversion efficiency). We expect that various steps could be taken to further improve the maximum field strength, such as ensuring a dust-proof environment, pumping with a longer wavelength such as 1030 nm at which 3-photon absorption does not take place in LT [44], and using MgO-doped LT which would likely have a higher damage threshold than the undoped LT used here as is well known to be the case for lithium niobate [45]. To improve the conversion efficiency, one possibility would be to implement a lateral pumping scheme similar to what was used in Ref. [46] to dramatically increase the interaction length over which the THz field is generated. Taking inspiration from the multi-bounce geometry used in Ref. [36], a LT slab could be shaped into a conical frustum and a single ring of pump light would be focused onto the beveled edge. The bevel angle would be chosen so that the pump light converges inward at the same rate as the focusing THz field.
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Pump pulse energy dependence. (a) Measured THz field strength at varying pump pulse energies. (b) Calculated optical-to-THz conversion efficiency. The black dashed line represents the field strength and conversion efficiency obtained from a single line-focused pump beam. The grey shaded region indicates pump fluences where the laser repetition rate was reduced to 10 Hz to slow down optical damage to the LT slab.</figcaption></figure>

We compare the results against a reference measurement performed by using a cylindrical lens to focus the pump beam as a single line of pump light on the LT slab, as is typically done in a polaritonics experiment. This method generates THz electric field strengths up to 10 kV/cm, measured 100 µm away from the line pump, when the pump fluence is just below the damage threshold [20]. The line pump reference data are represented in Fig. 5 as horizontal dashed lines. The TREx scheme produces much larger electric field amplitudes using much lower pump fluences. At the same pump fluence, we see an 18-fold increase in the THz field strength, a notable increase compared to previous works.

From Fig. 4 and 5, we see that the peak THz field strength depends approximately linearly on both the number of pump rings and the pump fluence. If the total pump energy is fixed, then a linear increase in the number of pump rings is accompanied by a quadratic decrease in the pump fluence, leading to an overall decrease in peak field strength. Thus, the maximum field strength is found by maximizing the pump fluence. In practice, this means choosing the number of rings such that the pump fluence is just below the optical damage threshold.

## 5. Magnetic field behavior

Unlike most experimental geometries, this method generates two counter-propagating focusing THz fields. Thus, while the THz electric field components interfere constructively at the focus, the magnetic field components interfere destructively to reach nearly zero amplitude. Although the magnetic field cannot be imaged directly in our experimental geometry, its behavior can be inferred by reversing the LT *c*-axis on one side of the THz focus. To do this, a single-crystal LT slab was cut parallel to the *c*-axis using femtosecond laser machining [22] and one half was flipped over to reverse the *c*-axis orientation. The two pieces were then rotated by 180 degrees so that their edges at the interface were pristine (i.e. not laser cut), brought as close together as possible on a flat surface, and then glued together at the corners using nail polish to make a “split” slab. In order to facilitate imaging of the THz field at the focus, the center of the pump rings was shifted slightly away from the cut so that the THz signal was not obscured. Fig. 6(b),(d) show the electric and magnetic field polarizations in both a normal uncut LT slab and a “split” slab. By reversing the *c*-axis in half of the slab, the polarity of the electric (magnetic) fields is reversed,
resulting in destructive (constructive) interference in the new slab. Simulations comparing the electric and magnetic fields in the normal and “split” slabs are shown in Supplement 1.

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Imaging the electric fields in a normal and “split” LT slab. (a),(c) Space-time plots of the THz electric field excited in the geometries shown in (b) and (d). The sign of the electric field shown in (c) is inverted on one side of the cut due to the reversed LT *c*-axis orientation. The position of the THz focus is marked by an arrow in both plots. (b),(d) Schematic illustrations of the electric and magnetic field polarizations of the excited THz fields in (b) a normal LT slab and (d) a modified LT slab where the *c*-axis is reversed on opposite sides of the THz focus. The dashed line in (d) indicates the cut in the “split” LT slab.</figcaption></figure>

Experimental space-time plots for both the normal and “split” slabs are shown in Fig. 6(a),(c) respectively. The normal slab shows constructive interference of the electric fields at the focus. In contrast, the “split” slab shows destructive interference, reducing the THz field strength at the center of the focus by a factor of 6. By observing destructive interference in the electric field in the “split” slab, we deduce that the magnetic field destructively interferes in the normal slab. There are a few reasons that could explain why the destructive interference was not perfect in the experiment. Inhomogeneities in the intensity profile of the pump beam prevent the destructive interference from perfectly cancelling the electric fields in the “split” slab. In addition, the TREx focus was slightly offset from the cut, which leads to a small imbalance in the THz field strength generated on either side of the cut. The air gap in the cut was measured to be <5 µm wide, which is much smaller than the THz wavelength in air, so we expect negligible losses in THz field amplitude due to tunneling through the air gap. However, slight out-of-plane misalignment of the two slabs could diminish how efficiently the THz field tunnels through the gap. We note that constructive interference of the THz magnetic field could also be achieved in a single LT slab by including a temporal delay between the two sides of the optical pump light, for example by inserting different thicknesses of glass in front of the left and right-hand sides of the circular echelon. A shift of one-half of the peak THz wavelength would optimize the magnetic field superposition. Although the superposition is not optimized for the entire THz bandwidth, this approach could permit switching between enhanced electric or magnetic field components at the focus.
**6. Summary and outlook**

We have demonstrated a novel compact scheme for generating large THz field strengths in thin LT waveguide slabs using a conical pulse front. The largest THz field strengths recorded using this technique without damage to the LT were 175 kV/cm at a 1 kHz laser repetition rate. Higher field strengths up to 300 kV/cm were observed at 10 Hz repetition rate. We expect that the peak THz field strength could be further increased by pumping at a longer wavelength such as 1030 nm where THz generation is more efficient [44]. This method enables versatile waveguide-based THz spectroscopy and photonics by allowing strong THz fields to be delivered without any free-space THz generation to samples that could be on the LT surface or in a laser-machined hole at the focus and perhaps located inside a cryostat, cryomagnet, or XFEL chamber. The use of a thin slab could also circumvent thermal management issues encountered when pumping a bulk THz generation crystal at high repetition rate [47]. Furthermore, we showed that the magnetic field component of the focusing THz field is significantly diminished at the focus ordinarily, and that measures can be taken to reverse the enhancement to generate a strong magnetic field and a diminished electric field. This method could be useful for studying materials that respond to both a THz electric and magnetic field, such as multiferroics, without using THz enhancement structures to selectively enhance one of the field components.

**Funding.** Deutscher Akademischer Austauschdienst; Direction Générale de l'Armement (ERE 2017 60 0040); U.S. Department of Energy (DESC0019126).

**Acknowledgments.** We thank Jet Lem for his assistance with the experiments and manuscript preparation. The research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award No. DESC0019126. T.P. acknowledges financial support from DGA (Direction Générale de l'Armement) under grant ERE 2017 60 0040. Y.K. acknowledges support from the DAAD (German Academic Exchange Service) fellowship.

**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.

**Supplemental document.** See Supplement 1 for supporting content.

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