# Field evaporation of germanium under THz pulses: application to atom probe tomography

M. Karam, M. De Tullio, J. Houard, I. Blum, Angela Vella

► To cite this version:

M. Karam, M. De Tullio, J. Houard, I. Blum, Angela Vella. Field evaporation of germanium under THz pulses: application to atom probe tomography. APL Materials, 2026, 14 (2), pp.021103. 10.1063/5.0306293 . hal-05529497

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# Field Evaporation of Germanium under THz Pulses: Application to Atom Probe Tomography

M.Karam,¹ M.De Tullio,¹ J.Houard,¹ I.Blum,¹ and A.Vella¹

¹Univ. Rouen Normandie, INSA Rouen Normandie, CNRS, Normandie Univ., GPM UMR6634, F-76000 Rouen, France

(*angela.vella@univ-rouen.fr)

(Dated: January 14, 2026)

## Abstract

This study investigates the field evaporation behavior of germanium (Ge) under monocycle terahertz (THz) excitation using atom probe tomography (APT). The influence of THz pulse polarity, specimen geometry, and spectral content is systematically examined. Dual-polarity experiments reveal the coexistence of two emission mechanisms: field-driven and thermally assisted. Delayed evaporation features are observed, and their dependence on specimen geometry is analyzed using a nanowire heat diffusion model. Spectral filtering of the THz radiation further confirms the role of the pulse spectral content in shaping the thermal response of Ge samples. These findings identify key parameters governing THz-assisted APT of low-conductivity materials and provide guidelines for optimizing experimental conditions.
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0306293

INTRODUCTION

Over the last decades, advances in microscopy techniques have continually pushed the limits of spatial and chemical resolution at the nanoscale. Among these, APT stands out for its ability to reconstruct three-dimensional images with near-atomic resolution and chemical sensitivity. The technique relies on the field evaporation of atoms from a needle-shaped specimen under intense electric fields, originally triggered by short high-voltage pulses [1, 2]. This operating mode, however, restricted APT to conductive materials. A series of instrumental advances have since broadened the range of materials accessible to APT. Among these advances, the introduction of laser-assisted APT (La-APT) [3–6] enabled the analysis of semiconductors and insulating materials through the use of ultrafast laser pulses [7–9]. However, the absorption of laser energy often introduces significant thermal effects, which degrade both spatial and chemical resolution [10–14].

Recent developments have demonstrated that intense single-cycle terahertz (THz) pulses can also trigger the emission of charged particles [15, 16]. In metallic and semi-metallic systems such as aluminum and LaB<sub>6</sub>, THz-assisted APT has already shown clear advantages over laser-assisted approaches, providing sharper spectra and improved charge-state distributions [17–19]. The efficiency of THz-assisted APT is closely linked to the strong enhancement of THz fields at nanostructured surfaces. Several studies have shown that when THz radiation interacts with metallic nanoscale features, confinement effects lead to significant local field amplification [17, 20–22]. The magnitude of this enhancement depends strongly on both the geometry and the material properties of the nanostructure, and can bring the local electric field at the tip apex into the range of 1–60 V/nm, comparable to interatomic fields. This amplification mechanism provides a direct explanation for the ability of intense THz transients to initiate field evaporation processes. These field values are typically associated with metallic or semi-metallic tips, where free carriers ensure efficient confinement. However, bandgap materials may respond differently. A notable demonstration of this was recently provided by De Tullio et al., who showed that intense THz transients can trigger ion emission from amorphous silica, a wide band-gap material [23]. While these results establish the feasibility of THz-APT on such insulators, they also highlight the crucial role of excitation parameters in governing the evaporation process, with a pronounced dependence on pulse polarity. By contrast, semiconductors represent an intermediate class of materials: their low density of free carriers sets them apart from metals, yet their relatively narrow band gaps distinguish them from insulators. Despite their technological importance, their response to THz excitation has
<figure><img src="image_2.png" /></figure>

remained largely unexplored, in the case of field emission and field evaporation.

In this work we study the interaction of intense single-cycle THz pulses with germanium nanoneedles, in the case of field evaporation. The germanium is a semiconductor with a 0.66 eV band gap at room temperature. By varying THz pulse polarity, spectral content, and tip geometry, we analyze how these parameters govern the interplay between field-driven and thermally assisted emission, and evaluate their impact on the resulting mass spectra.

## MATERIALS AND METHODS

### Laser systems

The optical setup for the generation of monocycle THz pulses relies on the generation of plasma in air by two-color femtosecond laser [24, 25]. Specifically, it consists of the fundamental harmonic (FH) and the second harmonic (SH) of a Ti:Sapphire laser operating at a central wavelength of 800 nm, with a repetition rate of 1 kHz and a pulse energy of 2.1 mJ. THz emission arises primarily from the creation of an asymmetric photocurrent generated through the ionization and acceleration of electrons in air. The amplitude of the THz field is adjusted by tuning the power of the fundamental beam. The resulting THz transients are characterized via electro-optic sampling in a 100 µm GaP crystal and subsequently focused onto the specimen apex inside the ultrahigh vacuum APT chamber. Further details on the optical setup and THz beam path can be found in Ref [18]. In addition to amplitude control, the polarity of the THz pulses can be inverted by changing the relative phase between the BBO crystal used for the generation of the SH and the plasma. This affects the phase delay between the FH and SH components, allowing the generation of either positively or negatively directed THz pulses.

Here, positive (respectively negative) THz polarity refers to the orientation of the dominant half-cycle of the transient electric field relative to the DC extraction field at the tip apex. Figure 1 shows the electro-optic waveforms of both pulse types (a), along with their corresponding Fourier spectra (b), demonstrating that the spectral content remains unchanged.

For UV analyses, the femtosecond laser pulse system used is an amplified ytterbium-doped laser (AMPLITUDE SYSTEM s-pulse) delivering 350 fs pulses at 100 KHz. The laser wavelength of 1030 nm can be changed to green (515 nm) and UV (343 nm) by Second Harmonic Generation
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>FIG. 1. (a) THz transients with inverted field directions (polarity), detected using electro-optic (EO) sampling by GaP crystal. The pink curve corresponds to the positively polarized THz pulse, and the blue curve to the negatively polarized one. (b) Corresponding Fourier transforms showing that both pulses exhibit nearly identical spectral content despite the polarity inversion.</figcaption></figure>

and sum generation, respectively. For our work 343 nm was used.

APT systems

The atom probe experiments were carried out in a custom-built ultrahigh vacuum chamber with a base pressure of about $5 \times 10^{-11}$ mbar. The specimen was cooled to 50 K using a compression cryostat and biased with a positive DC voltage of several kilovolts, generating an intense static field at the tip apex. A transient THz or laser pulse is then applied and triggers the evaporation of surface atoms, which then travel along a 10 cm flight path toward the detector. The ions are collected on a position-sensitive detector composed of a chevron pair of microchannel plates followed by an 8 cm Roentdek delay-line detector [4]. The detector signals are digitized by a fast acquisition system, providing both the ion flight time and the impact position. From the time-of-flight (ToF) measurement, the mass-to-charge ratio (MCR) of the evaporated species is determined, enabling their chemical identification [26].

For THz analysis, the beam is focused on the sample using a spherical metallic mirror of focal length 2.5 cm. For UV analyses, the beam is focused using a lens having a focal length of 20 cm.
Sample preparation

Our Ge samples are prepared using a plasma FIB-SEM dual-beam system (Helios 5). Small volumes are extracted by lift-out, mounted on tungsten supports, and shaped into needles by annular milling [27, 28]. The milling was carried out at 30 kV with ion currents progressively reduced from 1 nA to 30 pA. Final cleaning steps are performed at 2 kV and 30 pA to minimize implantation and surface damage, yielding specimens with apex radii below 50 nm suitable for APT analysis.

DUAL THZ POLARITY ANALYSIS

The experiments were performed by sequentially applying positive and negative THz pulses to a single Ge specimen. To isolate the effect of THz pulse polarity, both configurations were conducted under identical conditions: the specimen was maintained at a fixed DC bias of 4.4 kV, and the pump laser operated at maximum power, producing a THz field amplitude of approximately 200 kV/cm. The detection rate was estimated at 0.02 ions per pulse for the positive polarity and 0.01 ions per pulse for the negative polarity. Each dataset comprises 20k ions, and the corresponding mass spectra are presented in Figure 2.

The mass spectra obtained under both positive and negative THz pulse configurations show similar species. Peaks corresponding to hydrogen and its molecular ion $H_2^+$ are visible, as well as water molecules ($(H_2O)^{2+}$). The presence of hydrogen and oxygen is likely due to the low repetition rate of our laser system (1 kHz). In the high-vacuum environment, the sample surface can be contaminated by residual gases between pulses, which were subsequently field evaporated or desorbed by the THz pulses [29]. As shown in Figure 2(a), the spectra acquired under both polarities exhibit comparable structural features. The Charge State Ratio is calculated as CSR = Ge$^{2+}$/Ge$^{1+}$ = 0.5 for both analyses. By using the simplified formula from Ref. [30]

$$F = 21.68 \left( 1 - \frac{a}{CSR^{0.3} + b + 0.256} \right) \quad (1)$$

with $a = 21.68$ V/nm and $b = 0.2051$ we obtain a value of $F = 17.5$ V/nm. Each spectrum displays a fast evaporation process, marked by sharp peaks corresponding to the Ge$^{1+}$ and Ge$^{2+}$ ions with their 5 isotopes, followed by a delayed contribution appearing as a broadened peak after the main peaks. Such delayed features, typically attributed to thermally assisted evaporation, have
<figure><img src="image_5.png" /></figure>

also been reported in previous studies, such as laser-assisted APT of MgO, Si and diamond, where
the authors linked this behavior to laser absorption occurring away from the tip apex [31–34].

<figure><img src="image_6.png" /><figcaption>FIG. 2. (a) Mass spectrum of a pure germanium sample acquired with negative (blue) and positive (pink) THz pulses. For both configurations, the DC voltage was maintained at 4.4 kV, corresponding to a field of 17.5 V/nm and the pump laser operated at full power, resulting in a detection rate of 0.01 ions per pulse for the negative polarity and 0.02 ions per pulse for the positive polarity. Each dataset comprises 20k collected ions, with a bin size of 0.1 amu. (b) and (c) Magnified views of the mass/charge intervals corresponding to the isotopes of Ge<sup>2+</sup> and Ge<sup>1+</sup>, respectively, using a bin size of 0.01 amu.</figcaption></figure>

By examining our results, we observe that both THz polarities produced the same thermal
response. This is expected, as changing the polarity does not affect the THz energy or the spot size
on the specimen. Consequently, the absorbed energy and thus the thermal contribution, remains
constant between the two analyses. However, while the thermal response remains unchanged,
differences emerge in the fast evaporation process. The sharp peaks of the germanium isotopes are
approximately three times higher when positive THz pulses are used. This can be explained by the
temporal profile of the THz pulses shown in Figure 1(a): the positive pulse exhibits a noticeably
higher peak in its positive half-cycle, compared to the corresponding positive half-cycle of the
negative THz pulse. Since this positive part of the waveform is directly responsible for triggering
<figure><img src="image_7.png" /></figure>

fast field evaporation, the larger amplitude under positive polarity accounts for the increased height of the sharp germanium peaks. The physical mechanisms of fast field-driven emission and slow thermal-driven emission are discussed in the following sections.

## FAST EVAPORATION: UV VERSUS TERAHERTZ ILLUMINATION

Laser-assisted APT analyses of semiconductors and oxides using UV, visible and near infrared light have revealed the presence of a fast evaporation process sometimes followed by the delayed evaporation. The physical mechanism of fast evaporation has been identified as a fast thermal process due to the absorption of the laser energy at the apex of the sample, where the intense electric field significantly increases absorption by band-bending and band-gap reduction [5, 35, 36]. In the case of THz radiation, photons have an energy of a few meV, which is thousands of times lower than that of UV, visible or IR photons. Their absorption at the apex will only be possible when the band-gap of the material is almost cancelled out by the static electric field, i.e. at the evaporation field of the material[36, 37]. However, the THz analyses of Fig. 2, 3 and 4(b) were performed at static field ranging between 14.5 and 18 V/nm, well below the evaporation field of germanium of 21.7 V/nm. Therefore, the increase in the absorption due to the intense electric field can be neglected. However, the optical properties of Ge can be changed by the defects implanted at the surface during the FIB sample preparation process. It has been experimentally proven that, at high doses, gallium implantation can strongly increase the absorption of infrared, visible or UV light in silicon and in larger oxides, such as silica[38, 39]. However, recent ab-initio simulations of amorphous silica show that only a high defects concentration exceeding 2% can induce the formation of a metallic state capable of absorbing low-energy photons such as THz photons[40]. However, such high concentration is never achieved during the sample preparation by FIB. Therefore, defect-assisted absorption of the THz energy can also be considered as negligible.

In the case of metallic and semi-metallic materials such as aluminum and LaB6, the electric field of the THz pulse was proved to act as an ultra-short electrical pulse due to its oscillation time in the ps range, as shown in Fig1 (a). In these cases, symmetric peak's shape is reported for ions having a mass-on-charge (m/n) ratio greater than 30 amu and an asymmetric shape with a tail on the left side for low m/n ratio [17, 18].

Figure 3 shows the mass spectra of Ge¹⁺ ions obtained using UV and THz pulses. Both analyses were performed on two different APT systems having the same path length and the same
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>FIG. 3. (a) Mass spectrum of Ge¹⁺ ions acquired with UV (black) and positive (green) THz pulses. The static field is fixed at 17.8 (14.5)V/nm for UV(THz) analysis. The UV energy is 8 nJ and the THz field of 200 kV/cm. Each dataset comprises 80k collected ions, with a bin size of 0.01 amu.</figcaption></figure>

detection system, the main difference being the illumination conditions. Both datasets have the same number of detected events (80k) and the same number of events in the range showed in the Fig 3, i.e. 45kevents.

The THz analysis was performed at the static field of 14.5 V/nm calculated from the CSR=0.01 [30]. For UV analysis the field was of 17.8V/nm from and the CSR=0.8. We set a higher static field in order to decrease the laser energy and reduce the thermal tail of the peaks in the mass spectrum. Working at 13 V/nm would require too much laser energy to achieve homogeneous evaporation and avoid surface diffusion. The UV spectrum shows asymmetric peaks with a beginning of a tail on the right side. However, the THz spectrum shows symmetric peaks. The mass resolving power at 10% (1%) of the maximum is of 370 (176) for THz and 200 (123) for UV laser.

The symmetric shape of the Ge¹⁺ peaks and the increase in the fast emission when the polarity of the THz pulse changes from negative to positive prove that fast emission is a field-driven emission.
DELAYED EVAPORATION AND THERMAL DIFFUSION

The delayed evaporation signal during laser-assisted APT analyses has already been reported on several materials such as Si, MgO, diamond, and ThO2, for example, when they are analyzed using UV, visible or near infrared light[5, 14, 31–33, 35]. In all these studies, delayed evaporation was correlated with the absorption of the laser energy far from the apex, because the location where the energy is absorbed inside the sample determines the origin of the thermal diffusion process that drives the delayed evaporation of surface atoms. It has been shown that the absorption maps depend on the laser wavelength and the sample geometry in terms of end radius, cone angle and tip length[5, 35, 41, 42]. For samples with low absorption, i.e. nearly transparent at the selected laser wavelength, the nanometric dimensions and the conical shape of the sample cause non-homogeneous absorption maps, with certain absorption maxima inside the sample, far from the apex. The position of these maxima was calculated using analytical models, based on the Mie diffraction theory [43], taking into account the conical shape of the sample [41], or numerically, solving the Maxwell’s equations in 3D [35, 42]. For a linear incident polarization of the laser along the main axis of the sample, the first maximum is located at[43]:

$$ \frac{d}{\lambda} \sim 1/4 \qquad (2) $$

where d is the local diameter of the sample and λ is the laser wavelength inside the material (λ = λ₀/n, where λ₀ is the free-space wavelength of the incident radiation and n is the real part refractive index of the material). Furthermore, experimental evidence of the presence of an absorption zone located far from the peak has been reported by observing the sample after illumination [41] or by measuring the absorption map by spatial modulation spectroscopy[34]. When the samples are a few microns long, the first maximum may be located outside the sample. In this case, the absorption of laser energy by the interface between the sample and the pre-tip causes the delayed evaporation, as discussed for MgO and diamond samples in refs.[31, 33]. In this study, in order to identify the origin of delayed evaporation, we analyze two Ge samples with different lengths of 3 (7.5) µm for tip 1 (tip 2), but with a similar tip radius of 50 nm and a similar cone angle of 1.5° at the base, i.e., 500 nm from the tip. However, at the apex, the two samples have different cone angles, 8° (5°) for tip 1 (tip2), as shown in Figure 4(a). Since the tips length is only a few micrometers and the THz beam waist on the sample is about 1 mm, the interface between the sample and the pre-tip is illuminated during THz-APT analysis. Measurements by Kaplunov et al. [44] on undoped single-crystal Ge show that the absorption coefficient (α) remains
low in the 0–300 µm wavelength range, which covers the spectral content of our THz pulses. In particular, α is about 1–2 cm⁻¹ in the 60–120 µm range, decreases to a minimum of approximately 0.5 cm⁻¹ between 160 and 220 µm, and stays below ~ 1 cm⁻¹ up to 300 µm. These values indicate that Ge is almost transparent to THz radiation. Therefore, according to Eq. 2, the first absorption maximum occurs at a distance from the apex where the local diameter is approximately 20 microns, calculated using the refractive index of n = 4, as reported in Ref. [45] in the range 4.2-7.7 THz at room temperature, and λ₀ = 300 µm, at 1 THz. Given the two values of the cone angles of the two tips, the position of the maximum, calculated from the apex, exceeds the length of the tip for both samples (it is greater than 100 µm). We can conclude that direct absorption in germanium is unlikely to cause the delayed evaporation observed. On the contrary, the energy is more likely to be deposited at the interface of the bonding point, which corresponds to previous observations [31, 33].

Given the absence of significant polarity-dependent effects as discussed earlier, both specimens were analyzed using the positive THz polarity to isolate the influence of geometry on the thermal response.

The corresponding mass spectra, presented in Figure 4(c), reveal a hump that consistently appears after the main Ge peaks. This hump does not overlap with the prompt field-evaporation peaks and exhibits a shift in position that depends on geometry. Rather than focusing on its amplitude, attention is placed on its position relative to the main peaks, which varies between the two samples. Therefore, the corresponding ToF spectra are shown in Figure 5. These spectra reveal a clear difference in the temporal delay (Δt) of the hump, defined as the time difference between the hump maximum and the ⁷⁴Ge⁺ peak. For the longer specimen, the delay is longer, suggesting that the interval between the prompt evaporation process and the hump is influenced by the distance over which thermal diffusion occurs before evaporation.

To verify this hypothesis, the sample can be approximated as a nanowire, an elongated structure in which heat conduction is primarily axial. Within this framework, the observed temporal separation (Δt) between the prompt and delayed components is attributed to the thermal diffusion from the region of energy deposition toward the apex. Assuming the sample is primarily heated in a region at a distance "z" from the apex, the time for heat diffusion to reach the apex (Δt) can be expressed as follows:

$$ \Delta t = \frac{z^2}{D} \qquad (3) $$
<figure><img src="image_10.png" /></figure>

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0306293

<figure><img src="image_11.png" /><figcaption>FIG. 4. FIB images of the two germanium samples, illustrating the difference in tip length and scale. (a) Sample with a tip length of 3 µm, displayed with a 1 µm scale bar; (b) Sample with a tip length of 7.5 µm, displayed with a 2 µm scale bar. (c) Corresponding mass spectra acquired from each sample, with the green curve representing Tip 1 (3 µm) and the purple curve representing Tip 2 (7.5 µm). Both samples were analyzed using a positive THz pulse polarity and a DC voltage of 4.5 kV, corresponding to a field of 17(13.6)V/nm for tip1(tip2). The resulting detection flux was 0.03 ions/pulse for the shorter tip and 0.2 ions/pulse for the longer tip. Each spectrum was normalized and obtained from a total of 38k detected ions, using a bin size of 0.1 amu.</figcaption></figure>

where *D* is the thermal diffusivity of germanium. In this model, a decrease in distance of the heated zone will result in a shorter temporal gap $\Delta t$ and bring the thermal hump closer to the main peak.

For the shorter tip (Sample 1), the delayed emission feature appears at a time interval of almost 62.5 ns from the main peak in the ToF spectrum. Using the known tip length of 2.5 µm and Equation (3), the thermal diffusivity is calculated to be approximately 1 cm²/s. In the case of the longer tip (Sample 2), with a length of 6 µm, the secondary feature is observed at a delay of 272 ns, yielding a thermal diffusivity of 1.3 cm²/s. Both values fall within the expected range for germanium at 50–150 K, corresponding to our experimental conditions, which typically spans
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

FIG. 5. ToF spectra of the two germanium tips with different lengths. (a) The green curve corresponds to the shorter tip (2.5 µm), and (b) the purple curve corresponds to the longer tip (6 µm). The temporal shift (Δt) indicated in each case corresponds to the delay between the main peak of $^{74}$Ge$^{1+}$ and the maximum of the subsequent thermal hump. To improve accuracy, (Δt) values were extracted from smoothed ToF curves using the Smooth function in OriginLab, allowing precise identification of the thermal hump maximum.

from 0.5 to 2.0 cm$^2$/s [46, 47]. The consistency between these values confirms that the long evaporation process, extending from tens to several hundred of nanoseconds, is likely associated with the absorption of THz energy at the substrate-specimen interface, probably by the glue used to attach the germanium sample to the tungsten pre-tip.

## OPTICAL FILTERS AND THERMAL RESPONSE

In the analyses presented previously, a silicon filter was systematically placed in the beam path to eliminate contributions from the NIR radiation which is used to generate the THz beam. This filter blocks visible and near-infrared wavelengths while transmitting the THz portion of the spectrum, ensuring that the observed thermal features originate solely from the THz-material interaction. The spectral purity of the generated THz pulse is therefore a critical factor in minimizing unwanted thermal contributions and ensuring the reliability of the analysis. In earlier configurations, THz pulse properties were adjusted using the BBO position for polarity control and by varying the laser power for amplitude tuning. To investigate the role of spectral content in thermal effects, the configuration with a single silicon filter was compared to a double-filter setup combining silicon and teflon. The impact of this additional filtering on the temporal and
<figure><img src="image_15.png" /></figure>

spectral structure of the THz pulse is shown in Figure 6, which presents the electro-optic traces (a) and their corresponding FFT spectra (b) for the two conditions, both measured at maximum laser power (corresponding to full THz energy).

<figure><img src="image_16.png" /><figcaption>FIG. 6. (a) Normalized EO traces of the THz pulse obtained with a silicon filter alone (blue) and with both silicon and teflon filters in the beam path (red), measured with the pump laser operated at maximum power. (b) Corresponding normalized fast Fourier transform (FFT) spectra. The comparison shows that the additional teflon filter reduces the relative contribution of higher-frequency components, resulting in a narrower effective spectral bandwidth compared to the silicon-only configuration.</figcaption></figure>

As shown in Figure 6(a), the normalized EO traces highlight differences in pulse duration between the two filtering conditions. The configuration with both silicon and teflon filters (red) produces a broader temporal profile compared to the silicon-only case (blue), consistent with the narrower frequency content visible in the corresponding FFT spectra in Figure 6(b). The normalized spectra show that the addition of the teflon filter reduces the relative contribution of higher-frequency components, with the spectral content dropping sharply above 5 THz. In contrast, the silicon-only configuration retains measurable components beyond 8 THz, which correspond to the phonon absorption of the GaP crystal used for the EO measurements. From the raw EO data (not shown here), the peak amplitude with the teflon filter was reduced from about 3.3 mV to 1.55 mV, i.e., by a factor of ~2.2, confirming the strong attenuation introduced by the additional filter.

Building on these insights, we performed APT analyses to evaluate how filtering conditions influence the evaporation response. Since previous results have shown that the polarity of the THz pulses does not affect the thermal response of Ge, the two configurations compared here were
<figure><img src="image_17.png" /></figure>

operated with different polarities without impact on the conclusions. The mass spectra in Figure 7 correspond to analyses with 20k detected ions each. In the silicon-only case (purple), the THz pulses were applied with negative polarity, a DC voltage of 5.8 kV, and the THz amplitude reduced by about 30% with respect to maximum, yielding an evaporation rate of 0.03 ions per pulse. In the configuration with both silicon and teflon filters (pink), the THz pulses were applied with positive polarity at a DC voltage of 6.7 kV, using the same reduced laser power, which resulted in a flux of 0.003 ions per pulse.

<figure><img src="image_18.png" /><figcaption>FIG. 7. Mass spectra obtained with a silicon filter only (purple) and with both silicon and teflon filters (pink). In the silicon-only case, the analysis was performed with negative THz polarity, a DC voltage of 5.8 kV, and the pump laser power reduced by about 30% of maximum, yielding a flux of 0.03 ions per pulse and 20k detected ions. In the silicon+teflon configuration, the analysis was performed with positive THz polarity at 6.7 kV, using the same reduced laser power, yielding a flux of 0.003 ions per pulse and 20k detected ions.</figcaption></figure>

In both configurations, the spectra are dominated by Ge²⁺, indicating that the local field is higher than 18 V/nm. However, it is not possible to calculate the value of the field because the peaks of Ge¹⁺ are masked by the delayed evaporation after the Ge²⁺ peaks. The main difference between the two mass spectra concerns the thermal contribution: with the silicon+teflon configuration, the hump appearing after the main Ge peaks is strongly reduced but a residual tail persists.

Because the configuration with both silicon and teflon filters not only reduces the THz spectral width but also decreases the THz amplitude by a factor of 2.2, the reduction of the hump could
<figure><img src="image_19.png" /></figure>

also be associated with the lower amplitude. However, for the Si filter alone, decreasing the THz amplitude from full power to 70% of the maximum does not remove the hump from the mass spectra, as shown in Figure 7. This experimental observation validates our hypothesis that delayed evaporation can be reduced by narrowing the spectral width of the THz pulse.

CONCLUSION

In this article, we conducted an experimental study to investigate how monocycle THz pulses influence the field evaporation behavior of germanium in APT. By examining the effects of pulse polarity, specimen geometry, and spectral content, we aimed to better understand the interplay between excitation conditions and thermal contributions in THz-assisted APT.

The polarity-dependent response confirmed that both THz polarities are capable of triggering ion emission. However, the results revealed the coexistence of two distinct emission mechanisms: a fast and delayed emission. Regarding the delayed contribution, the mass spectra show no dependence on the pulse polarity. In contrast, for peaks associated with fast emission, a strong effect was observed under positive pulses, with an increase in peak amplitude by about a factor of three.

In order to explore the mechanisms behind the fast emission and also to benchmark the THz-APT against conventional UV-APT, we compare THz and UV mass spectra. Under THz illumination, symmetric peaks are measured, whereas under UV illumination, the peaks are asymmetrical with a thermal tail to the right side. Therefore, THz radiation induces a “field-driven” emission from Ge, which improves the mass resolving power of the APT spectra compared to UV-APT.

To further explore the mechanisms behind delayed emission, we investigated the influence of tip geometry, specifically tip length. For the longer tip, a greater delay was observed between the field-driven peak and the thermal hump, indicating a geometry-dependent shift in emission timing. Using the nanowire approximation, this delay was attributed to heat diffusion from the region near the Ge-glue interface toward the apex. The longer the tip, the longer it takes for the absorbed energy to reach the apex and trigger thermal evaporation. This confirms that the thermal contribution arises from energy deposited in the pre-tip region and propagates via axial heat conduction, reinforcing the role of specimen geometry in shaping emission dynamics.

To complement this analysis, we explored how modifying the spectral content of the THz pulse affects thermal emission. The filtering study demonstrated that spectral shaping of the THz pulse directly impacts thermal effects. Adding a teflon filter led to a reduction in the thermal hump,
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0306293

consistent with the attenuation of high-frequency absorption.

Together, these results highlight the importance of carefully tuning both the excitation parameters and specimen geometry to control fast-driven and thermal-driven contributions when analyzing low-conductivity materials using THz-assisted APT. The present work shows that THz radiation can improve the mass resolving power of the APT spectra compared to UV-APT, but delayed thermal emission, due to THz absorption away from the sample apex, introduces very broad peaks that can mask other peaks. The present work demonstrates that the THz energy absorption occurs mainly at the junction between the Ge tip and the W pre-tip. Various strategies can be employed to reduce this absorption, such as covering the junction with a less absorbing layer, for example by depositing a thin metallic coating on the sample, including the glue-tip interface. However, the effectiveness of this approach strongly depends on the deposition technique and the choice of coating material. Other strategies involve preparing longer Ge samples, measuring several hundred microns, or reducing the spectral bandwidth of the THz radiation, keeping high THz energy at low frequencies.

The research leading to these results has received funding from European Union under Grant Agreement No. 101046651 project MIMOSA. This work was supported by the Regional Council of Normandie.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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