Laser Terahertz Emission Research Articles

Optical damage thresholds of single-mode fiber-tip spintronic terahertz emitters.

Spintronic terahertz emitters (STEs) are gapless, ultrabroadband terahertz sources that can be driven within a wide pump-wavelength and repetition-rate range. While STEs driven by strong pump lasers operating at kilohertz repetition rates excel in generating high electric field strengths for terahertz spectroscopy or ellipsometry, newly advancing technologies such as ultrafast modulation of terahertz polarization, scanning tunneling microscopy, laser terahertz emission nanoscopy, and fully fiber-coupled integrated systems demand an STE pumping at megahertz repetition rates. In all these applications the available terahertz power is ultimately limited by the STE's optical damage threshold. However, to date, only very few publications have targeted this crucial topic and investigations beyond the kilohertz repetition-rate regime are missing. Here, we present a complete study of our single-mode fiber-tip STEs' optical damage thresholds covering the kilohertz, megahertz, and gigahertz repetition-rate regimes as well as continuous-wave irradiation. As a very important finding, we introduce the necessity of classifying the optical damage threshold into two regimes: a low-repetition-rate regime characterized by a nearly constant fluence threshold, and a high-repetition-rate regime characterized by an antiproportional fluence dependence ("average-power threshold"). For our single-mode fiber-tip STEs, the transition between these regimes occurs around 4 MHz. Moreover, we present a cohesive theory of the damaging thermodynamical processes at play and identify temperature-driven inter-layer diffusion as the primary cause of the STE failure. These findings are substantiated by atomic force microscopy, infrared scattering-type scanning near-field optical microscopy, and scanning transmission electron microscopy measurements. This new level of understanding offers a clear optimization lever and provides valuable support for future advancements in the promising field of spintronic terahertz emission.

Spintronic terahertz metasurface emission characterized by scanning near-field nanoscopy

Abstract Understanding the ultrafast excitation, detection, transportation, and manipulation of nanoscale spin dynamics in the terahertz (THz) frequency range is critical to developing spintronic THz optoelectronic nanodevices. However, the diffraction limitation of the sub-millimeter waves – THz wavelengths – has impaired experimental investigation of spintronic THz nano-emission. Here, we present an approach to studying laser THz emission nanoscopy from W|CoFeB|Pt metasurfaces with ∼60-nm lateral spatial resolution. When comparing with statistic near-field THz time-domain spectroscopy with and without the heterostructures on fused silica substrates, we find that polarization- and phase-sensitive THz emission nanoscopy is more sensitive than the statistic THz scattering intensity nanoscopy. Our approach opens explorations of nanoscale ultrafast THz spintronic dynamics in optically excited metasurfaces.

Probing photocarrier dynamics in a Bi2Te3–Te eutectic p–n junction with a laser terahertz emission microscope

Bismuth telluride (Bi2Te3)-based heterostructures have attracted considerable attention owing to their interesting anisotropic properties and expected higher thermoelectric performance. Therefore, exploring the nature of the carrier dynamics in these heterostructures has been an important subject in the design and optimization of advanced materials. In the present study, hot carrier injection and its subsequent spatiotemporal behavior in a multilayered crystalline Bi2Te3–Tellurium (Te) eutectic composite were studied using a laser terahertz (THz) emission microscopy (LTEM). The THz emission electric fields at the Bi2Te3–Te interface were polarized perpendicular to the interface. The polarities of these waveforms reveal the direction of the electric field between the Bi2Te3 and Te regions, indicating the carrier types of these components and the p–n junction formed at the interface. In addition, in the Te region, a strong THz emission with an electric field polarized parallel to the interface was observed. This unique THz emission can be qualitatively explained through hot photocarrier anisotropic transport by considering the effective mass of electrons and holes. LTEM clarified the local carrier dynamics in the microstructures and revealed the potential distribution and anisotropic transport properties. These findings contribute to the exploration of eutectic heterostructures as new functional materials and provide new avenues for cutting-edge thermoelectric and photovoltaic devices.

NH3 laser THz emission under optical pumping by “long” (∼100 µs) CO2 laser pulses

Terahertz (THz) NH3 lasing with optical pumping by electron-beam-sustained discharge “long” (∼100 µs) CO2 laser pulses was obtained. The NH3 laser emission pulses and the “long” pulses of the CO2 pump laser were simultaneously measured with nanosecond response time. The NH3 lasing duration and its delay with respect to the pump pulse were measured for various CO2 laser pulse energies. For the CO2 laser pump line 9R(30), three wavelengths of 67.2, 83.8, and 88.9 µm were recorded. For the CO2 laser pump line 9R(16), only a single NH3 laser line with a wavelength of 90.4 µm was detected.

Laser Terahertz Emission Microscope for Imaging Grain Heterogeneity: A Case Study of CH3NH3PbI3 Perovskite Semiconductors

Strong terahertz (THz) emission from the methylammonium lead iodide (MAPbI3) perovskite semiconductors has been observed following above-bandgap photoexcitation, yet local THz responses of crystalline microstructures are absent. We implement laser THz emission microscope (LTEM), yet-to-be applied to the perovskite semiconductors, as a novel and complementary tool to evaluate the electronic and grain heterogeneity of MAPbI3 thin films. Two MAPbI3 samples with different grain sizes are studied. Using this approach, we show that the one with a larger grain size gives more uniform THz radiation. More significant spatial THz intensity fluctuation is observed for the sample with a smaller grain size.

Laser terahertz emission microscopy of nanostructured spintronic emitters

Laser terahertz (THz) emission spectroscopy has broken the diffraction limit of THz frequencies and offers multifaceted spectroscopic and imaging capabilities for understanding the light–matter interaction in various quantum and energy materials. However, this advanced technique has not yet been applied in the recently extensively studied spintronic THz emission process, in which the material surface morphology may play an important role. Here, we conduct THz emission microscopy on 5.4-nm thick Pt/CoFeB/W heterostructures and obtain twice enhanced THz by tightly focusing the pumping laser, delicately choosing the radiation location on nanofilms and coating gold nanorods. Through THz emission mapping, the material surface morphology and its modification have a strong correlation with THz emission performance from spintronic emitters. Our proposed femtosecond fiber laser driven spintronic THz emission microscopy can provide exciting possibilities for studying surface morphology sensitive THz emission materials and microdomain ultrafast dynamics for low-dimensional small samples via further coupling optical microscopy.

Laser terahertz emission microscopy revealing the local fluctuation of terahertz generation induced by Te inclusion

As a state-of-the-art electro-optical crystal, zinc telluride (ZnTe) is widely used in terahertz (THz) emission by optical rectification. However, the efficiency of THz generation in ZnTe is usually degraded by factors such as harmonic generation, two-photon absorption, and free-carrier absorption. In this work, we first report the effect of micro-scale Te inclusions in ZnTe on the local area THz emission. Unusual THz time-domain waveforms emitted from Te inclusions are observed to last longer than 400 ps, which is attributed to the photocurrent surge (PS) effect. This Te inclusion-associated THz radiation is proved consistent with the incident power and polarization angle dependences. Simultaneously, the Te inclusions embedded in the ZnTe matrix are visualized by laser THz emission microscopy (LTEM). According to the LTEM images, the driving force of the carrier movement is revealed, which is ascribed to the heterojunction field composed of Te and p-ZnTe. Our results not only afford a comprehensive understanding of the THz characteristics of Te inclusions but also put forward LTEM as a promising non-contact evaluation method for evaluating the bulk defects in semiconductors.

Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor β-Ga2O3

Although gallium oxide Ga2O3 is attracting much attention as a next-generation ultrawide bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped β-Ga2O3 wafers. Femtosecond (fs) laser illumination generated THz waves based on the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga2O3. TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and diffusion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them.

Simplified formulas for the generation of terahertz waves from semiconductor surfaces excited with a femtosecond laser

We derive simple formulas to explain terahertz (THz) emission from semiconductor surfaces excited by a femtosecond (fs) laser. Femtosecond optical pulses with energies larger than the bandgap create photocarriers that travel and generate THz radiation, according to the time derivative of the photocurrent. By assuming that only electrons traveling in an ultrafast time scale, less than a few hundred fs, contribute to THz radiation, one can obtain simple expressions for the emission originating from the photocarrier drift accelerated with a built-in field or from the photocarrier diffusion. The emission amplitude of the former is in proportion with the electron mobility, the Schottky-barrier height, and the laser intensity and one of the latter with the laser intensity and diffusion coefficient squared. We also discuss the formula for emission from metal–insulator–semiconductor structures. The derived expressions are useful in understanding the THz emission properties observed by a laser THz emission microscope (LTEM), bringing the LTEM into real applications in the field of semiconductor research and development.

Laser THz emission nanoscopy and THz nanoscopy

We present an experimental and theoretical comparison of two different scattering-type scanning near-field optical microscopy (s-SNOM) based techniques in the terahertz regime; nanoscale reflection-type terahertz time-domain spectroscopy (THz nanoscopy) and nanoscale laser terahertz emission microscopy, or laser terahertz emission nanoscopy (LTEN). We show that complementary information regarding a material's charge carriers can be gained from these techniques when employed back-to-back. For the specific case of THz nanoscopy and LTEN imaging performed on a lightly p-doped InAs sample, we were able to record waveforms with detector signal components demodulated up to the 6th and the 10th harmonic of the tip oscillation frequency, and measure a THz near-field confinement down to 11 nm. A computational approach for determining the spatial confinement of the enhanced electric field in the near-field region of the conductive probe is presented, which manifests an effective "tip sharpening" in the case of nanoscale LTEN due to the alternative geometry and optical nonlinearity of the THz generation mechanism. Finally, we demonstrate the utility of the finite dipole model (FDM) in predicting the broadband scattered THz electric field, and present the first use of this model for predicting a near-field response from LTEN.

Generation of highly efficient terahertz radiation in ferromagnetic heterostructures and its application in spintronic terahertz emission microscopy (STEM)

The laser terahertz emission microscopy (LTEM) technique, which breaks through the resolution limitation of terahertz waves from millimeter to micrometer scales, has been widely used in many real application circumstances, such as contactless chip nondestructive testing, biosensing, imaging, and so on. Recently developed spintronic terahertz emitters featuring many unique properties such as high efficiency, easy integration, low cost, large size and so on, may also have great applications in LTEM, which can be called spintronic terahertz emission microscopy (STEM). To achieve high efficiency and good performance in STEM, we propose and corroborate a remnant magnetization method to radiate continuous and stable terahertz pulses in W/CoFeB/Pt magnetic nanofilms without carrying magnets on the transmitters driven by nJ femtosecond laser pulses. We systematically optimize the incidence angle of the pumping laser and find the emission efficiency is enhanced under oblique incidence, and we finally obtain comparable radiation efficiency and broadband spectrum in W/CoFeB/Pt heterostructures compared with that from 1 mm thick ZnTe nonlinear crystals via optical rectification under the same pumping conditions of 100 fs pulse duration from a Ti:sapphire laser oscillator, which was not previously demonstrated under such long pulse duration. We believe our observations not only benefit for a deep insight into the physics of femtosecond spin dynamics, but also help develop novel and cost-effective broadband spintronic terahertz emitters for the applications in STEM.

Nanoscale Laser Terahertz Emission Microscopy

Laser terahertz emission microscopy (LTEM) has become a powerful tool for studying ultrafast dynamics and local fields in many different types of materials. This technique, which relies on acceleration of charge carriers in a material upon femtosecond excitation, can provide insight into the physics of charge transport, built-in fields, grain boundaries or surface states. We describe a new implementation of LTEM with a spatial resolution in the nanoscale regime based on a scattering-type near-field tip-based approach. We observe a spectral reshaping of the signal compared to conventional LTEM, which is analyzed using a resonant antenna model. Our experimental and computational results clarify the importance of the mechanisms for both the plasmonic in-coupling of the near-infrared pulses into the near field and the out-coupling of the generated terahertz pulses. We demonstrate a tip-size-limited spatial resolution of ∼20 nm by imaging a gold nanorod using terahertz emission from the underlying substrate. T...

Probing the surface potential of oxidized silicon by assessing terahertz emission

Using laser terahertz emission microscopy, we measured laser-excited terahertz (THz) emission from silicon wafers with silicon-oxide passivation layers, revealing a strong correlation between the THz waveform and the surface potential. The surface potential was electrically tuned by a semitransparent top electrode disc and evaluated by measuring capacitance–voltage characteristics. The waveform changed with external bias and inverted near the flatband voltage, and changes appeared in the peak amplitude were similar to the capacitance–voltage characteristics. These results indicate that by analyzing the waveform of laser-excited THz emission generated by laser terahertz emission microscopy, we could quantitatively measure and map the internal field of surface band bending in semiconductors.

Polarization imaging of imperfect m-plane GaN surfaces

Abstract

Comparison between laser terahertz emission microscope and conventional methods for analysis of polycrystalline silicon solar cell

A laser terahertz emission microscope (LTEM) can be used for noncontact inspection to detect the waveforms of photoinduced terahertz emissions from material devices. In this study, we experimentally compared the performance of LTEM with conventional analysis methods, e.g., electroluminescence (EL), photoluminescence (PL), and laser beam induced current (LBIC), as an inspection method for solar cells. The results showed that LTEM was more sensitive to the characteristics of the depletion layer of the polycrystalline solar cell compared with EL, PL, and LBIC and that it could be used as a complementary tool to the conventional analysis methods for a solar cell.

Laser Terahertz Emission Spectroscopy of Graphene/InAs Junctions

ABSTRACTWe applied laser THz emission spectroscopy to study the effects of monolayer graphene on the THz emission from InAs. THz emission from graphene/InAs varies linearly with the laser excitation power in the low-intensity excitation regime. We found that unlike in graphene/SI-InP junctions, graphene and O2 adsorbates on graphene have no significant effect on the THz emission from graphene/InAs junctions because the THz radiation mechanism in InAs is by the photo-Dember effect, whereas for SI-InP is by the surge current effect. There is also a slight enhancement in the THz emission from both bare InAs and graphene/InAs by UV illumination, which is probably due to the additional photoexcited carriers by UV that somehow enhances the photo-Dember field.

Recent progress on the development of a laser terahertz emission microscope and its prospects for industrial applications

Recent progress on the development of a laser terahertz emission microscope and its prospects for industrial applications

Scanning laser THz imaging system

The laser terahertz (THz) emission microscope (LTEM) is a unique inspection tool which can directly two-dimensionally map the THz pulse emission from a variety of electric materials and devices. Thus, two-dimensional mapping directly yields various physical information by visualizing the distributions of electric field, supercurrent, ferroelectric domain structures, magnetic fluxes, etc. Based on techniques created for conventional LTEM, we have developed a more highly functional laser THz imaging system, including a scanning laser THz (near-field) imaging system having the performance of high-speed and high-resolution, a scanning probe LTEM coupled with an atomic force microscope for high-resolution imaging, and a dynamic THz emission microscope to investigate the ultrafast carrier dynamics two-dimensionally in the sample. These systems offer diverse characteristics, making them suitable for various applications. We take a look at these systems and some typical applications.

Laser terahertz emission microscopy studies of a polysilicon solar cell under the illumination of continuous laser light

Terahertz wave-emission properties of a polysilicon solar cell excited by femtosecond laser pulses were visualized by laser terahertz emission microscopy (LTEM). The effects on the terahertz emissions of continuous-wave (cw) laser illumination of the solar cell at two wavelengths (808 and 365 nm) were examined under conditions of reverse and zero bias. The amplitude of the terahertz pulses in the presence of illumination was lower than that in its absence, and it decreased with increasing illumination power. The photovoltaic effect was smaller for cw laser illumination at the shorter wavelength. These phenomena can be explained in terms of screening of the electric field in the depletion layer of the p−n junction, as a result of the presence of photoexcited carriers. In addition, the LTEM images permitted the visualization of the crystalline grain structure of the solar cell and the dynamics of photocarriers, and they reflected the local electric field distribution in the cell. LTEM was therefore shown to be a promising technique for the evaluation and inspection of solar cells.

Observation of impurity diffusion defect in IGBT using a laser terahertz emission microscope technique

This paper describes a technique for observing impurity diffusion defects in IGBTs using laser terahertz emission microscopy (LTEM). In this method, femtosecond laser pulses are applied to the p-n junction of a trench IGBT structure and terahertz (THz) waves emitted from the wiring connected to the p-n junction are observed by a photoconductive antenna. Furthermore, covering the measurement sample with aluminum foil blocks opposite-phase THz waves emitted from the sample and improves the detection sensitivity of the THz waves emitted from the IGBT wiring. This technique enables impurity diffusion defects, which were created by deliberately omitting a portion of the resist pattern used in the IGBT emitter n+ ion implantation process, to be recognized as contrast differences on an LTEM image.