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High-dielectric constant glasses are prerequisite for developing terahertz (THz) components and systems. Oxyfluorosilicate (OFS) glasses have been developed and their THz properties have been characterized by using THz-time domain spectroscopy (THz-TDS) measurements. High-dielectric constant (8–13) and low loss (6–9/cm) properties in the THz region have been demonstrated and their dielectric properties have been studied using the single oscillator-based model through a comparison with other multi-component silicate oxide glasses. Unified single oscillator model, which can distinguish the electronic and ionic contributions to the dielectric property, has been applied in this analysis. The physical origin of the dielectric constant enhancement and the importance of interplay between the electronic polarizability and ionicity in high-dielectric constant glasses have been revealed. This study has demonstrated the usefulness of THz-TDS technique for characterizing dielectric properties of multi-component glasses in detail.

In this study, we report enhancement of terahertz (THz) radiation with indium-tin-oxide (ITO) thin-film deposited on semi-insulating gallium arsenide substrate (SI-GaAs). The amplitude of THz emission from both ITO/SI-GaAs and bare SI-GaAs substrate as a function of optical pump (i) incident angle, (ii) polarization angle, and (iii) power were investigated. The enhancement of peak amplitude of a THz pulse transmitted through the ITO/SI-GaAs sample in comparison to bare SI-GaAs substrate varied from 100% to 0% when the pump incidence angle changed from 0° to 50°. The maximum enhancement ratio of peak amplitude for a coated sample relative to the bare substrate is approximately up to 2.5 times at the minimum pump intensity of 3.6 TW/m2 and gradually decreased to one at the maximum pump intensity of 20 TW/m2. From outcomes of these studies, together with data on surface and material characterization of the samples, we show that THz emission originates from the ITO/GaAs interfaces. Further, both interface-field-induced transient current and field-induced optical rectification contribute to the observed THz signal. Observed enhancement was tentatively attributed to surface-plasmon-induced local field enhancement, coupled with constructive interference of forward and retro-reflected backward THz emission from the ITO/GaAs interfaces. The polarity-flip reported previously for very thin Au-coated GaAs was not observed. This was explained by the wide-bandgap, transparency and lower free carriers of ITO. For best results, the incident angle should be in the range of 0 to 30° and the incident polarization should be 0 to 45°. We further predict that the ITO thin film of suitable thickness or with engineered nanostructures, post-annealed under optimum conditions may lead to further enhancement of THz radiation from ITO-coated semiconductor surfaces.

A terahertz (THz) dielectric property characterization method based on a unified single oscillator model has been developed and applied to a variety of multi-component silicate oxide glasses. The experimental values of dielectric constant determined by THz time-domain spectroscopy (TDS) in the sub-THz region have been confirmed to agree well with the values calculated by the single oscillator model which incorporates the local field effects and the material’s ionicity. This has provided a unified formulation that enables systematic determination of the key physical parameters solely from the high-frequency (optical) and low-frequency (sub-THz) dielectric constants and characteristic resonance frequency in the (sub-)THz region. The low-frequency dielectric constant has been demonstrated to be fully determined by a single parameter of the microscopic total susceptibility. Also, the polarization ionicity, which is defined by the ionic fraction in the microscopic total susceptibility, has been found to be a good indicator to represent the ionic nature of the material. Through this analysis, an increasing trend of the effective ionic charge has been found in high-dielectric constant glasses such as oxyfluorosilicate glasses, and the physical mechanism of their dielectric constant enhancement has been discussed. The present method is expected to be applied to design and characterize dielectric properties of a wide range of multi-component glasses and other isotropic, insulating materials.

We investigate terahertz time-domain spectroscopy (THz-TDS) as a non-destructive and non-contact technique for depth profiling of dopants in semiconductors. THz temporal waveforms transmitted through silicon-ion-implanted semi-insulating gallium arsenide substrates, as-implanted or post-annealed by rapid thermal annealing, were analyzed by assuming a multi-layered Gaussian refractive index profile in the ∼sub-micrometer-thick implantation region. The implantation energy and dosages in this work were 200 KeV, 1014, 5 × 1014, and 1015 ions/cm2, respectively. The average values of real ( n) and imaginary ( κ) parts of refractive indices of an as-implanted sample in the depth range of 0–800 nm are 5.8 and 0.7, respectively, at 0.5 THz and are 6.2 and 0.2, respectively, at 1 THz. On the other hand, the refractive index profile of the post-annealed samples displays a prominent Gaussian-like form, and peak refractive indices ( n ∼ 25 and κ ∼ 32.7 at 0.5 THz and n, κ ∼17 at 1 THz) were found to be at the depth of 210 nm. Reconstructed dopant profiles in as-implanted, implanted, and post-annealed substrates were found to be in good agreement with measurements by secondary ion mass spectroscopy as well as simulation by the Monte Carlo method. We were also able to determine accurately the projected range ( Rp), straggle ( Rs), and concentration of dopants by the analysis of THz-TDS data. The spatial resolution, along the depth direction, of the THz-TDS technique for depth profiling of dopants was estimated to be as small as 8-nm. This work suggests the feasibility of using THz-TDS for nondestructive and non-contact diagnostics for profiling dopants in semiconductors.

We study optical rectification in GaSe by performing THz generation with femtosecond laser pulses whose wavelength is tuned from below to above the GaSe bandgap. As expected from a theory, we observed a first THz emission peak at 1.77 eV, where phase matching is realized. A second THz emission peak was recorded, when the pump photon energy reaches the crystal bandgap (2.205 eV). This can be attributed to a resonance of the GaSe nonlinearity. In crystals thinner than the coherence length, the bandgap peak is stronger than the phase-matched one.

Dielectric properties of oxyfluorosilicate (OFS) glasses have been characterized using Terahertz (THz)-time domain spectroscopy in the sub-THz region as well as optical reflection measurement. OFS glass containing 20 mol% of Nb2O5, which is termed ZNbKLSNd glass, has the highest refractive index of 3.70 in the sub-THz region. The THz and optical refractive indices of various silicate oxide glasses, including OFS glasses, have been confirmed to be correlated by a unified relationship utilizing a parameter defined by the ratio of ionic to electronic polarizability. Additionally, the frequency dependence of the THz dielectric constant has been interpreted by a single oscillator model for all silicate oxide glasses including OFS glasses. On the basis of the present unified dielectric model, the very high refractive index of ZNbKLSNd glass has been attributed to the lowering of oscillator resonance wavelength originated from the incorporation of Nb2O5 intermediate network former.

Poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is known for its potential to replace indium&ndash, tin oxide in various devices. Herein, when fabricating finger-type PEDOT:PSS electrodes using conventional photolithography, the cross-sectional profiles of the patterns are U-shaped instead of rectangular. The films initially suffer from non-uniformity and fragility as well as defects owing to undesirable patterns. Adding a small amount of hydrolyzed silane crosslinker to PEDOT:PSS suspensions increases the mechanical durability of PEDOT:PSS patterns while lifting off the photoresist. To further improve their microfabrication, we observe the effects of two additional oxygen (O2) plasma treatments on conventional photolithography processes for patterning PEDOT:PSS, expecting to observe how O2 plasma increases the uniformity of the patterns and changes the thickness and U-shaped cross-sectional profiles of the patterns. Appropriately exposing the patterned photoresist to O2 plasma before spin-coating PEDOT:PSS improves the wettability of its surface, including its sidewalls, and a similar treatment before lifting off the photoresist helps partially remove the spin-coated PEDOT:PSS that impedes the lift-off process. These two additional processes enable fabricating more uniform, defect-free PEDOT:PSS patterns. Both increasing the wettability of the photoresist patters before spin-coating PEDOT:PSS and reducing its conformal coverage are key to improving the photolithographic microfabrication of PEDOT:PSS.

We demonstrate electrically tunable twisted nematic (TN) aligned liquid crystal (LC) terahertz phase shifters with a novel structure, in which a single LC composite polymer thin film is inserted in the middle of the LC cell. For designing such types of phase shifters, we have applied the Mauguin formalism used in LC-based switches for applications in the visible. Experimental results are in good agreement with those predicted by the theoretical model. Using this design, the phase shift has been increased by 14.3° at 1.2 THz in comparison to the conventional design (98.1°). The threshold voltage has been lowered from 0.81 VRMS to 0.48 VRMS while the driving voltage for quarter-wave (π/2) phase shift operation has been reduced from 8.4 VRMS to 6.3 VRMS at 1.2 THz. The dynamic response of devices also enhances significantly. Such design has also successfully been adopted to demonstrate a 2π phase shifter with similar improvement. Full-wave operation is achieved at a bias of 25 VRMS at 1.2 THz using a highly birefringent LC material. Order parameter calculations show that the polymer film improves significantly LC molecular alignment in the thick LC cell for THz applications. We have also compared the figure of merits (FOMs) of the present devices with previously reported THz π/2 and 2π phase shifters.

We demonstrate indium-tin-oxide (ITO) finger-type pattern (FTP) as an effective transparent conducting electrode for terahertz (THz) application. The average transmittance of ITO-FTP is as high as 85% in the frequency range of 0.2–1.2 THz. A pair of fused silica substrates with ITO-FTP was employed for constructing an electrically tunable twisted nematic liquid crystal (LC) based THz phase shifter. The cross-orientation pair of ITO-FTPs served as wavelength-insensitive and transparent electrodes for the LC layer. A maximum phase shift exceeding π/2 at 1.2 THz was demonstrated for a device with a 600-μm-thick LC cell. The required driving voltage for quarter-wave operation and threshold voltage were 5.3 and 0.8 V RMS, respectively. Performance of the THz phase shifter is discussed by defining a figure of merit (FOM) for LC-based THz shifters and compared with previously reported devices.

We present a simple, reliable, and fast method to automatically generate noise-like or mode-locked pulses in a fiber laser. To generate the targeted pulses, we employed two-photon-induced current from a photocell as the feedback signal to control a set of rotatable wave plates in the laser. The effectiveness of using the two-photon absorption signal to discriminate noise-like pulses from mode-locked pulses is theoretically analyzed and experimentally confirmed. We demonstrate our method by using a dispersion-mapped ytterbium-doped fiber ring laser with nonlinear polarization evolution that can generate either noise-like or mode-locked pulses. The average time to attain the prescribed pulses is within a few minutes.

For the first time, ultrafast laser annealed BEOL gate-all-around (GAA) transistor and FeFET memory were demonstrated with monolithic 3D near-memory-computing (NMC) circuit. The GAA MOSFETs employing ultrafast picosecond visible laser dopant activation exhibit record-high Ion (nFETs=407 uA/um, pFETs=345 uA/um). The BEOL FeFETs memory exhibits large memory window ΔV th = 1.2V, more than 106 cycle endurance. Moreover, the 3D stackability of the GAA MOSFETs and FeFET memory bit cell enable reduces the area of the NMC circuitry and improve the readout throughput.

In this paper, we tune the terahertz (THz) optical and electrical properties of indium tin oxide (ITO) nano-thick film by rapid thermal annealing (RTA). Besides, thickness-dependent THz transmittance properties of as-deposited ITO thin film is investigated. The ITO film can be functioned as a THz reflective conductive electrodes by RTA at 600°C. THz power transmittance improved from 6 to 49 % at 0.2 THz by RTA at 800°C. Furthermore, we extracted complex refractive indices of as sputtered and annealed ITO films, and fitted the results with the Drude model. Additionally, the structural and morphological properties of ITO samples are presented.

In this work, we observed enhanced terahertz emission from the interface between indium tin oxide (ITO) and semi-insulating gallium arsenide (SI-GaAs) substrate. Further, we investigated the dependence of THz emission on pump power and angle of incident excitation beam. We found more than 3 times of THz emission power enhancement for incident angle below 70°. Besides, we found enhancement of THz radiation at low pump intensity and saturation with increasing peak excitation intensity beyond 20 TW/m2. The experimental results are well explained by plasmonic effects at the metal-semiconductor interface.

In this work, we analyzed silicon (Si) ion implanted semi-insulting gallium arsenide substrates (SI-GaAs) by using a terahertz time-domain spectroscopy (THz-TDS) system. The main goal is to determine the ion doping profile via a nondestructive approach by using THz-TDS spectroscopy. This approach will bring a new appliance to replace secondary ion mass spectroscopy (SIMS). For that, we have carried out the ion implantation experiment with an implantation energy of 200 keV at different doses. Then, we analyzed the THz-TDS experimental data of implanted SI-GaAs substrates by the Drude model to achieve depth-dependent dielectric and electrical properties at THz frequencies.

We studied terahertz pulse generation by a photoconductive dipole antenna excited by noise-like pulses both theoretically and experimentally.

Amorphous silicon was crystallized using a blue ultrafast Ti:Sapphire laser system. Polysilicon with average grain size of 280 nm was achieved with fluence 30 of mJ/cm2 and overlapping of 93.75% at room temperature.

The dielectric constant, energy band parameters and ionicity are key physical parameters for fully understanding the dielectric properties of glasses, but their mutual interrelationship and its applicability to a wide variety of multi-component glasses have not yet been confirmed. We have applied the optical and THz spectroscopic characterization to a set of multi-component silicate oxide glasses incorporating vastly different additive components. The key physical parameters evaluated in the optical region have been interpreted consistently by the single oscillator based dielectric model. The bandgap energy (Eg) and ionicity parameter (Wemple and DiDomenico's β parameter) estimated in the optical region have shown reasonable correspondence, respectively, with the homopolar energy (Eh) and the polarization ionicity (IP) which are evaluated from the THz dielectric constant measurement. The comprehensive interrelation thus confirmed has provided simple rules between the physical parameters without limitation of glass compositions. For example, a tradeoff relation between the oscillator strength and characteristic resonance wavelength and a simple correlation between the dielectric constant and bandgap energy have been determined for the present set of multi-component glasses. The energy band parameters have been used for deriving the joint density of state diagrams, and characteristic physical/chemical nature has been identified to each glass.

Through a numerical study, we proposed and demonstrated that the one-dimensional periodic plasmonic-grooves (1D-PPG) can give rise to a super chiral field inside grooves under normal illumination with off-groove linear polarization. The handedness of the optical chirality of chiral field can be reversed by modifying the incident polarization angle from + 45° relative to groove's period direction to its mirror-symmetrical angle -45°. This makes possible a novel sensing scheme that utilize this polarization-dependent local chiral field for circular dichroism detecting; meanwhile, the dielectric environment-sensitive plasmonic resonance of 1D-PPG can be employed for refractive index sensing. This dual-sensing strategy offers great potential for low-cost chirality sensing for chiral molecules.

By applying the well-known transient current model, theoretically investigating terahertz (THz) emission from gas plasma induced by multi-color femtosecond laser pulses has been developed in this work. Instead of static tunneling ionization model, we adopted Ammosov-Delone-Krainov (ADK) tunneling ionization model [1] to calculate the tunneling ionization rate. In three-color pumping schemes, the optimal power ratio tends to require a larger amount of fundamental field with the increment of total pump power. The optimal relative phases for the 2nd harmonic and 3rd harmonic field are $0.5\pi$ and $\pi$, respectively.

We designed and fabricated bi-polar type photoconductive antenna (PCA) for efficient generation of broadband terahertz (THz) radiation. The falling time of overlayingly generated photocurrent from bi-polar PCA can achieve 200 fs via superimposing two primarily opposite polarity photocurrent pulses having falling time of 100 ps with time delay of 200 fs. The simulation results disclosed a possibility to shift the central frequency from 0.1 to 1.3 THz and enhance the THz power by twice in the range 1.5-3.3 THz by using low cost substrate with long carrier life time such as semi-insulating gallium arsenide (SI-GaAs).

We simulate the generation of terahertz radiation by using photoconductive antenna excited by noise-like pulses (NLPs). Our simulation results show that the bandwidth of THz spectrum generated by NLPs is narrower than by mode-locked pulses which is from same laser system in the low frequency region. However, the spectrum generated by NLPs exhibits additional components extended to higher frequency, though the power is relatively low.

Development of the wideband and tunable quasi-optic terahertz (THz) components is in high demand. In this work, we demonstrate a tunable achromatic quarter-wave plate (AQWP) for the THz frequency range. The phase retardation of this device can be set at 90° ± 9° from 0.20 to 0.50 THz. The operation range from 0.20 to 0.50 THz can be tuned to from 0.30 to 0.70 THz by introducing three nematic liquid crystals phase retarders, of which the birefringence can be magnetically tuned. The frequency-dependent phase retardation is in good agreement with theoretical predictions.

For this study, we demonstrated three different types of twisted nematic (TN) liquid crystal (LC) terahertz (THz) phase shifters using pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) thin films as transparent conducting electrodes (TCEs). The transmittance of spin-coated pristine PEDOT: PSS thin film was as high as 92% in the frequency range of 0.2&ndash, 1.2 THz. This is among the highest reported. Several TN-LC cells were constructed in a comparative study, which confirmed the reliability of pristine PEDOT: PSS as a TCE layer for THz phase shifter applications. The highest phase shift, required root-mean-square (RMS) driving voltage, and threshold voltage achieved by devices tested were 95.2&deg, at 1 THz, 7.2 VRMS, and 0.5 VRMS, respectively. The thickness of the LC layer for the phase shifter was 250 &micro, m, approximately half as thick as previous designs. In addition, the pristine PEDOT: PSS-based TN-LC phase shifter exhibited a figure-of-merit (FOM) value of approximately 6.65 degree&middot, dB&minus, 1&middot, V&minus, 1. This compared favorably with previously reported homogeneously aligned phase shifters with an FOM of 2.19 degree&middot, 1. Our results indicated that a twisted nematic LC cell with pristine PEDOT: PSS thin films as electrodes is a good combination for a THz phase shifter and wave plates as well as other LC-based THz devices.

We have investigated tunable terahertz (THz) phase shifters that are based on a sandwiched liquid crystal (LC) cell with indium&ndash, tin&ndash, oxide (ITO) nanowhiskers (NWhs) as transparent electrodes. More than 360&deg, of phase shift at 1.0 THz was achieved at a driving voltage as low as ~2.6 V (rms). This is approximately 40 times smaller than that reported in previous works using an electrically tuned LC device. Significance of the NWhs in reducing the required voltage is demonstrated. Overall transmittance of the device is as high as 30%, which is accountable by absorption losses of ITO NWhs, quartz substrate and LC. Experimental results are in good agreement with a theoretical formulism while taking into account super-thick LC cells (~1 mm) and pretilt angles. We also propose and demonstrate a novel THz technique for measuring pretilt angles of liquid crystals.

Phase shift exceeding 2π at 1.0 THz with high transmittance was achieved in liquid-crystal THz phase shifters of three different designs. Indium-tin-oxide nanowhiskers and graphene were employed as transparent electrodes. The driving voltage required for a 2π wave plate is as low as 5 Vrms.

The enhanced broadband THz radiation excited by pre-chirping the femtosecond pumping laser pulse is explored from a photoconductive antenna made by the low-temperature molecular-beam-epitaxy (LT-MBE) grown Gallium Arsenide (GaAs). Nearly 3-dB enlargement on peak amplitude of THz radiation accompanied with broadened bandwidth is achieved with positively chirped pulse excitation. The laser with short pulsewidth assists the broadening of THz bandwidth, whereas the peak intensity of THz relies strictly on lengthened carrier lifetime. The enhancement of high-frequency radiation components at low pumping intensity also indicates the nature of power dependence in such a coherently controlled process. When photocurrent saturation occurs in the LT-MBE-grown GaAs under high-power pumping condition, the clipping effect on peak amplitude of current pulse lengthens the duration of current surge effect. Both the peak intensity and the frequency spectrum of the generated THz pulse concurrently declines. The physical mechanism related to pre-chirped pulse excitation dependent carrier dynamics is proposed to elucidate the spectral response as well as the power dependence in such a coherently controlled THz radiation process.

The design, analysis, and demonstration of flip-chip bonding packaged uni-traveling-carrier photodiodes (UTC-PDs) with THz (dc to 315 GHz) 3-dB bandwidth and high-power performance are reported. The high-frequency roll-off (up to 0.4 THz) of the flip-chip bonding structure and device heating under high power operation are both minimized through properly downscaling the area of the bonding pad and minimizing the solder distance to the active area of the miniaturized UTC-PD. In order to suppress the serious space-charge screening effect in miniaturized UTC-PDs under high-current density (~180 kA/cm 2 ) operation, an n-type charge layer is inserted into the collector. The detailed dynamic measurement results of these packaged PD modules indicate that non-equilibrium electron transport plays an important role in determining the maximum speed and THz output power. In addition, a femtosecond (fs) optical pulse train generator with a ~300 fs pulse-width output and repetition rate up to ~0.3 THz is also developed to further boost the photo-generated THz-power. Compared with using an optical signal with a sinusoidal envelope for PD excitation, the short-pulse approach can offer a 3-dB enhancement in output power under the same output photocurrent and operating frequency (around 0.3 THz). By utilizing such an fs light source and our PD module, a continuous wave output power as high as 1 mW at an operating frequency of ~0.3 THz is successfully demonstrated.

Quarter-wave operation or a phase shift of more than $\pi/2$ , which is approximately ten times greater than that reported in previous works using liquid crystals (LCs) and graphene electrodes, was demonstrated. The device is transparent to the terahertz (THz) wave, and the driving voltage required was as low as approximately 2.2 V (rms), which is also unprecedented. Experimental results supported a theoretical formalism adapted for LC cells with THz wavelength-scale thickness. The scattering rate, DC mobility, and carrier mean free path of bilayer graphene were also determined using THz spectroscopic techniques; the parameters were inferior to those of monolayer graphene. This observation can be attributed to the higher density of charged impurities in the bilayer graphene. The device performances of LC phase shifters using monolayer and bilayer graphene as electrodes were essentially identical.

Near ultraviolet (λ ≈ 400 nm) femtosecond laser annealing (400 nm-FLA) in a scanning mode was employed to crystallize amorphous silicon (a-Si) films at room temperature. The average grain size of polycrystalline silicon annealed was studied as a function of the incident laser fluence and beam overlap or the number of laser shots irradiated. In general, the grain size can be enlarged by either increasing the beam overlap at a fixed laser fluence or increasing the laser fluence for a fixed number of laser shots. An apparent threshold for the onset of rapid enlargement of grain size was observed for processing at ∼90% overlap and fluences above 25 mJ/cm2. A maximum grain size of ∼280 nm was attained at a laser fluence of 30 mJ/cm2 and overlap of 93.75%, beyond which the grain size attained was smaller, and eventually, ablation was observed at an overlap of 97.5% and higher. These trends and observed surface morphology of annealed samples suggest that the crystallization mechanism is like sequential lateral solidification, similar to 800 nm-FLA and excimer laser annealing. Raman spectroscopic studies show that the degree of crystallization achieved with 400 nm-FLA is even higher than that of 800 nm-FLA. Cross-sectional scanning electron microscopic images indicate that the 100 nm-thick a-Si film is not fully crystallized. This can be explained by the much shorter penetration depth of 400 nm light than that of 800 nm light in a-Si.