We derive an evolutional equation incorporating the processes of spin-polarization transfer from an electron to a magnetic ion subsystem of a diluted magnetic semiconductor along with spin-lattice relaxation and spatial spin diffusion. Above equation has been obtained for nonequilibrium magnetization due to exchange scattering of photoexcited charge carriers by magnetic ions. We show that the mechanism of a band gap narrowing due to exchange scattering requires relatively low optical power to reach an optical bistability for pump frequency range close to crystal band gap. In a bulk crystal, only relatively small local area with essential magnetization enhancement can absorb optical power, thus forming a photoinduced magnetization wave. Spatial spin diffusion can be responsible for a motion of such magnetization wave. We solve above derived equation both analytically for one-dimensional case and numerically otherwise and perform its stability analysis. We also evaluate numerically possible threshold of photoinduced magnetization wave excitation for typical diluted magnetic semiconductor A_1-x^IIMn_xB^VI and estimate its length and velocity of propagation.

The injection of nonequilibrium carriers in a paramagnetic diluted magnetic semiconductor in the presence of magnetic field may induce a spontaneous magnetization patterning. This phenomenon is studied in CdMnTe quantum wells using time resolved Kerr rotation experiments and the model based on the coupled rate equations for interacting electron and magnetic ion spins is developed.

Exciton spin relaxations in bulk GaN were directly observed with sub-picosecond's time resolution. The obtained spin relaxation times of A-band free exciton are 0.47 ps - 0.25 ps at 150 K - 225 K. The spin relaxation time of the acceptor bound exciton at 15K is measured to be 1.1 ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin relaxation time of A-band free exciton is found to be proportional to T ^-1.4, where T is the temperature. The fact that the spin relaxation time in GaN is shorter than that in GaAs, in spite of the small spin-orbit splitting, suggests that the spin relaxation is dominated by the defect-assisted Elliot-Yafet process.

A brief review is given of our recent experimental results from in-depth investigations of spin depolarization and underlying physical mechanisms within semiconductor spin detectors based on II-VIs (e.g. Zn(Cd)Se quantum wells) and III-Vs (e.g. InGaN quantum wells), which are relevant to applications for spin-LEDs based on ZnMnSe/Zn(Cd)Se and GaMnN/InGaN structures. By employing cw and time-resolved magneto-optical and optical spin orientation spectroscopy in combination with tunable laser excitation, we show that spin depolarization within these spin detectors is very efficient and is an important factor limiting efficiency of spin detection. Detailed physical mechanisms leading to efficient spin depolarization will be discussed.

Ultrafast Extreme Ultraviolet (EUV) radiation is used to probe transient surface phenomenon in three experimental geometries. Optical irradiation of the sample surface generates thermal and acoustic transients that are subsequently probed with a time-delayed EUV pulse. In all experimental geometries we show excellent signal-to-noise ratios (>10:1) and increased sensitivity to surface deformations (<.02nm) directly attributable to the reduced wavelength of the probing light.

Thermal effects are unavoidable in laser material processing and are present, to some extent, even in the case when ultra-short (sub-picosecond) pulsed irradiation is used. We discuss here the matters of high-precision energy delivery into micrometer-sized volumes for three-dimensional (3D) laser microfabrication. Precise account of the absorbed energy, pulse duration, and focal spot size allows to optimize laser processing parameters. As an example, a 3D micro-structuring of silica with better than 15 μm resolution is demonstrated by pulses of 11 ns duration and 266 nm wavelength (for a focusing by a low numerical aperture NA = 0.029 lens). The two photon absorption coefficient of silica, β ≃ 60 ± 10 cm/GW, at 266 nm has been determined. The thermal black-body type emission of non-equilibrated electrons is discussed as a possible light source for 3D modification and structuring of photo-sensitive and photo-polymerizable materials. It is also demonstrated that optical properties of ionized dielectrics can be used to determine the temperature.

With recent advances in ultrafast laser technology, ablation using femtosecond laser pulses has found a wide range of applications. It is widely believed that ablation using femtosecond lasers has many unique advantages compared to longer-pulse ablation. One of the most significant features is believed to be that a negligible amount of energy should remain in the sample following ablation because the deposited heat does not have enough time to travel into the bulk sample during the femtosecond laser pulse. In this paper, we discuss our new findings on thermal effects in femtosecond laser ablation of metals. In contrast to the previous common belief, we find that a significant amount of residual thermal energy is deposited in metal samples following multi-shot femtosecond laser ablation. We further discuss how the absorptance of metals depends on the structural modifications. Lastly, we discuss the formation of periodic surface structures on metals induced by femtosecond laser pulses.

Efficient second harmonic generation of femtosecond and picosecond pulses into the visible and near-ultraviolet is reported using the quasi-phase-matched nonlinear material, periodically-poled KTP, and the new birefringent nonlinear crystal, BiB₃O₆. Second harmonic average powers as much as 1 W in the blue, tuning throughout 370-450 nm, single-pass conversion efficiencies in excess of 50% and pulse durations from 200 fs to 3 ps have been achieved.

In this talk, we will review our recent works about the GHz-repetition-rate (GRR) femtosecond lasers. These works make GRR femtosecond lasers more flexible in the manipulation of pulse repetition-rate and the operating wavelength. We first demonstrate a phase insensitive way to multiply the repetition-rate of a passive mode-locked laser in femtosecond regime. By inserting an intracavity flat surface with low reflectivity, we multiplied the repetition-rate of a femtosecond Cr:forsterite laser by ten times. It provides a simple and stable way to modify MHz-repetition-rate femtosecond lasers into GRR lasers. To achieve desired wavelength, which can't be directly generated by a gain medium, nonlinear conversion is required. But for GRR femtosecond lasers, the efficiency of single-pass conversion is low due to its low pulse energy. In order to increase the yield, we adopt the method of resonant-enhanced external cavity. With a resonant cavity matched to a 2-GHz repetition-rate Ti:sapphire laser, we demonstrated a high power femtosecond blue source at 2-GHz repetition-rate.

We perform fs degenerate pump-probe experiments on an InGaN/GaN quantum-well sample and an InGaN thin film of 800 nm in thickness, in which nm-scale cluster structures have been identified. In the InGaN/GaN quantum-well sample, we can identify three stages of carrier relaxation. The fast-decay time, ranging from several hundred fs to one ps, corresponds to the process reaching a local quasi-equilibrium condition, in which carriers reach a thermal distribution within one or a few nearby indium-rich clusters. The slow-decay time, ranging from tens to a couple hundred ps, corresponds to the process reaching a global quasi-equilibrium condition, in which carriers reach a thermal distribution among different clusters of various potential minima. In this stage, the mechanism of carrier transport over barriers between clusters dominates the relaxation process. Finally, carrier recombination dominates the relaxation process with the carrier lifetime in the range of a few ns. In the InGaN thin film sample, we can identify the variation of the space-averaged density of state with energy level in this sample. The carrier dynamics is controlled by the shift of effective bandgap and hence the behavior of band filling, which are determined by the combined effect of bandgap renormalization and phonon effect (bandgap shrinkage with increasing temperature). Two-photon absorption and free-carrier absorption can be observed when the corresponding density of state is low and hence the band-filling effect is weak. The variation of the space-averaged density of state with energy level can be due to the existence of indium-composition-fluctuation nanostructures, which is caused by the spinodal decomposition process.

We have investigated the ultrafast coherent dynamics of the intersubband transition in GaN/AlN multiple quantum wells, using a spectral resolved two-color pump-probe technique. We have found a significant spectrum change as a function of the delaytime τ. For negative delaytimes corresponding to τ < 0, coherent spectrum oscillations have been observed. At τ ~ 0, asymmetric dispersion has been observed. For positive delaytimes, τ > 0, no measurable spectrum change has been observed. From the analysis of these results, we have estimated that intersubband transitions dephasing time is more than 100 fs measured at room temperature.

Transient Raman spectroscopy has been used to study electron transport in a thick InN film grown on GaN at T = 300 K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity in the Γ valley, which reaches as high as 7.5x10⁷ cm/sec, can exceed its steady state value by as much as 40%. Electron velocities have been found to cut off at around 2x10⁸ cm/s, significantly larger than those observed for other III-V semiconductors such as GaAs and InP. Our experimental results suggest that InN is potentially an excellent material for ultrafast electronic devices.

Mg-doped AlN epilayers grown by metalorganic chemical vapor deposition have been studied by deep UV time-resolved photoluminescence (PL) spectroscopy. A PL emission line at 6.02 eV has been observed at 10 K in Mgdoped AlN, which is about 40 meV below the free-exciton (FX) transition in undoped AlN epilayer. Temperature dependent measurement of the PL intensity of this emission line also reveals a binding energy of 40 meV. This transition line is believed to be due to the recombination of an exciton bound to neutral Mg acceptor (I₁) with a binding energy, E_bx of 40 meV. The recombination lifetime of the I₁ transition in Mg doped AlN have been measured to be 130 ps, which is close to the expected value. Excitation intensity dependence of time-resolved PL for Mg-doped AlN epilayer is also measured to understand carrier and exciton dynamics.

Femtosecond high-field transport in GaAs is investigated tracing ultrafast modifications of the Franz-Keldysh absorption spectrum of AlGaAs heterostructure diodes. A sophisticated sample design allows to isolate unipolar transport contributions in combination with a nanometer scale definition of layers for both photoexcitation and detection of the propagating carriers. This novel all-optical technique is applied to various aspects of ultrafast charge dynamics in semiconductors: (i) Isolating the contribution of holes, we directly measure transient carrier velocities for electric fields between 15 kV/cm and 200 kV/cm. Especially, we compare room temperature operation to results for T_L = 4 K. Transient hole velocities are found not to exceed a value of 1.2 x 10⁷ cm/s which is a result of ultrafast optical phonon emission with a scattering time below 25 fs. (ii) For the case of unipolar electron transport, we find an ultrafast velocity overshoot and a quasi-ballistic electron motion with an average velocity of 4 x 10⁷ cm/s over distances as large as 200 nm. (iii) For electric fields F > 350 kV/cm the dynamical build up of a nonequilibrium carrier avalanche due to impact ionization is directly analyzed in the time domain. Most interestingly, the timescale of the carrier multiplication is found to be in the order of 10 ps.

The time-resolved photoluminescence of free and bound excitons in bulk single-crystal CuInS₂ grown by the traveling heater method is examined. It is found that radiative decay of the free exciton at 1.535 eV and the bound exciton at 1.530 eV is exponential with two characteristic decay-times while that of the bound excitons at 1.525 and 1.520 eV is well-represented by a single exponent at low temperatures. The radiative lifetimes of the free exciton and the bound excitons at 1.530, 1.525, and 1.520 eV are obtained to be 320 ps, 500 ps, 2.1 ns, and 3.5 ns, respectively. A thermal release process of the observed bound excitons is discussed in terms of the obtained activation energy. The capture center cross-section for free exciton is also estimated. According to our estimates, a neutral charge is to be assigned to the defect centers associated with the observed bound excitons.

We study the ultrafast transition of a pure longitudinal optical phonon resonance to a coupled phonon-plasmon system. Following 10-fs photoexcitation of intrinsic indium phosphide, ultrabroadband THz opto-electronics monitors the buildup of coherent beats of the emerging hybrid modes directly in the time domain with sub-cycle resolution. Mutual repulsion and redistribution of the oscillator strength of the interacting phonons and plasmons are seen to emerge on a delayed femtosecond time scale. Both branches of the mixed modes are monitored for various excitation densities N. We observe a pronounced anticrossing of the coupled resonances as a function of N. The characteristic formation time for phonon-plasmon coupling exhibits density dependence. The time is approximately set by one oscillation cycle of the upper branch of the mixed modes.

We investigated the radiation mechanisms of THz radiation from semiconductor surfaces at high-density excitation under a magnetic field. Excitation density dependences of radiation intensity and the waveforms of the terahertz radiations from InAs and semi-insulating InP surfaces were investigated with and without magnetic fields (0, 2T, and - 2T). Substantial changes of the intensity and the waveforms including a polarity reversal were observed by changing the excitation densities. In InAs, the enhancement of the radiated energy is observed under a magnetic field of ±2 T and the radiated energy increases quadratically with increasing the excitation density below 0.1 μJ/cm². The behavior of the dependence for ±2 T changes clearly above 1 μJ/cm². The drastic change of the wave forms was observed at high density excitation and was explained by the polarity reversal of the THz wave induced by the magnetic field. The reversal originates from the crossover of the radiation mechanism of the magnetic induced component from the electrons in the accumulation layer to the diffusion current by the photogenerated electrons at high-density excitation under a magnetic field. In InP, the characteristic behavior including the polarity reversal of the angle independent component was observed in the crystal orientation angle dependence by changing the excitation density. These facts indicate that three different radiation mechanisms co-exist and that the dominant radiation mechanism changes with increasing the excitation density from the drift current for low-excitation density to the diffusion current and the optical rectification for high-excitation density.

We have experimentally measured the terahertz radiation from a series of ion-implanted semiconductors, both from the bare semiconductor surface and from photoconductive switches fabricated on them. GaAs was implanted with arsenic ions, and InGaAs and InP with Fe⁺ iron ions, and all samples were annealed post implantation. An increase in emission power is observed at high frequencies, which we attribute to the ultrafast trapping of carriers. We use a three-dimensional carrier dynamics simulation to model the emission process. The simulation accurately predicts the experimentally observed bandwidth increase, without resorting to any fitting parameters. Additionally, we discuss intervalley scattering, the influence of space-charge fields, and the relative performance of InP, GaAs and InAs based photoconductive emitters.

We investigate on-axis terahertz transmission through random collections of sub-wavelength sized metallic particles. Despite both the inherent opacity of the metallic particles at terahertz frequencies and the absence of straight-line photon trajectories through the samples, we observe significant, polarized terahertz transmission through dense metallic particle collections, which are five orders of magnitude thicker than the radiation skin depth. The effects of sample length, particle size, particle shape, and conductivity on the enhanced transmission are explored experimentally and numerically. Our findings show that the polarized terahertz transmission is mediated by coherent near-field coupling of surface electromagnetic waves across the ensemble.

We experimentally investigate terahertz pulse transmission through dense, random ensembles of sub-wavelength ferromagnetic Co particles. Due to the magnetoresistance inherent to the ferromagnetic metal, the terahertz optical properties of the Co particle ensembles are shown to be strongly dependent on the strength and orientation of an external magnetic field.

Local modification of Eu³⁺ ions doped in SnO₂- and Al₂O₃-SiO₂ glasses was investigated using a femtosecond laser pulse. SnO₂ nanocrystals were successfully precipitated in transparent glasses by irradiation with an 800-nm femtosecond laser. Upon laser irradiation, the Sn atoms were activated to react with oxygen, resulting in the formation of SnO₂ nanocrystals. The precipitated SnO₂ crystals grew up to ca. 5 nm size by the Joule-heating effect of the laser. The fluorescence intensities of the codoped-Eu³⁺ ions were enhanced higher than 100 times that of the glass without nanocrystals by exciting with the energy corresponding to the absorption edge of the SnO₂ nanocrystals, the energy of which is effectively transferred to the Eu³⁺ ions. Near-infrared to visible up-converted fluorescence was observed in the Eu⁺³ ions doped in the Al₂O₃-SiO₂ glasses during femtosecond laser irradiation. The dependence of the intensity of the Eu³⁺-emission on the pump power reveals that the three-photon excitation is dominant in the up-conversion process. These glasses can provide a bright prospect for their optical applications.

Z-scan and pump-probe measurements with ultra-fast 800 nm laser pulses were used to compare the ultrafast third-order optical nonlinearities of VO₂ nanoparticles and thin films in both semiconducting and metallic states. It is found that when the samples are hold at temperatures above 67^oC in their metallic state, both nanocrystals and thin films present a positive intensity-dependent nonlinear index of refraction. In this metallic state the nanocrystals exhibit a saturable optical nonlinearity and enhancement of the nonlinear effects larger than those found in thin films. Below the transition temperature, the optical nonlinearities are more complex, since they arise from alterations in the VO₂ that arise both from the phase transition and from unrelated third-order nonlinear effects. Under these conditions, thin films exhibit a complete reversal to a negative nonlinear index of refraction while the nanocrystals, remarkably, show a smaller but still positive index. Pump-probe measurements on vanadium dioxide nanocrystals and thin films show they both exhibit an ultrafast response, undergoing the phase transition induced by a single laser shot in less than 120 fs. The speed of the solid-solid transformation, along with the striking reversal of the nonlinear properties across the phase transition, puts vanadium dioxide in a unique category among nonlinear materials.

Using time-resolved and time-integrated photoluminescence and spectrally resolved two-colour three-pulse photon echo spectroscopy we study the quantum confinement and dephasing properties of near spherical Si QDs with an average size of 4.3 nm. Filling of the low energy states and parabolic confinement of the quantum dot structures can be inferred from the dependence of the photoluminescence intensity on the detection wavelengths. A dephasing time of 1 - 1.8 ps which is slightly dependent on the quantum dot energy states can be measured. We show that the dephasing time of the electrons in the quantum dots is strongly influenced by the density of excited carriers.

We have investigated the electron and hole spin dynamics in p-doped semiconductor InAs/GaAs quantum dots by time resolved photoluminescence. We observe a decay of the average electron spin polarisation down to 1/3 of its initial value with a characteristic time of T_Δ ≈ 500ps. We attribute this decay to the hyperfine interaction of the electron spin with randomly orientated nuclear spins. Magnetic field dependent studies reveal that this efficient spin relaxation mechanism can be suppressed by a field in the order of 100mT. In pump-probe like experiments we demonstrate that the resident hole spin, "written" with a first pulse, remains stable long enough to be "read" 15ns later with a second pulse.

Glass containing spherical silver nanoparticles shows a strong extinction band in the visible range due to the surface plasmon resonance (SPR) of the particles. Irradiating this material with intense, ultrashort laser pulses with a wavelength close to the SPR leads to permanent changes of its optical properties. In particular, using linearly polarized pulses, we observed strong dichroism; the latter is nanoscopically caused by deformation of the particles to ellipsoidal shapes with an additional halo of small silver particles around the central one, with a preferential orientation. In case of a single laser shot of sufficient intensity this orientation is orthogonal to the laser polarization, whereas multi-shot irradiation usually causes preferential orientation along the laser polarization. This effect is quite useful for the production of dichroitic or polarizing microstructures, and optical elements or optoelectronic devices. In this paper we describe the results of a variety of experimental studies (mostly femtosecond laser pump-probe, electron microscopy, photoluminescence) on the understanding of the physical processes, which show clearly that ultrafast ejection of electron and silver ions into the glass matrix is the starting mechanism, whereas in the course of deformation diffusion processes controlled by the local temperature play a decisive role for the final particle shapes (and thus the optical properties after laser treatment).

Metallic nanoparticles (NP) draw intense scientific interest due to their unique physical properties, which differ from those of bulk and atomic species. Nowadays, the aim of this research area is focused, for example, in the nonlinear (NL) optical properties of NP, the dynamics of confined electrons and the possibility of photonic applications. In this work the transient response of non-spherical nanoparticles are studied. Two differently prepared colloids are examined: one sample has a broad distribution of sizes and shapes of silver nanoparticles, while the second sample, which is processed by laser ablation, presents a more uniform size and shape distribution (greater fraction of spherical particles). The experiments are performed with a Ti:sapphire femtosecond laser using a two-color, polarization-resolved, pump-probe setup. The pump pulse at ≈ 800 nm excites intraband transitions, heating the electronic distribution, while the probe pulse is scanned around the peak of the Surface Plasmon Ressoance (SPR): ≈ 400 nm. It is observed that for the pristine colloid (non-spherical particles) the signal amplitudes of the probe polarizations parallel and perpendicular to the pump polarization present different behaviors as a function of the probe photon energy. The same effect is not observed for the laser ablated samples, which are isotropic. The temporal response of the samples is also different, with the laser ablated samples presenting faster electronic cooling rates. A model describing the induced dichroism that takes into account the shape of the colloid particles is presented. The effects of the size distribution are also discussed.

Magnetization-induced second-harmonic generation (MSHG) technique in transverse Kerr configuration is used to explore the nonlinear magneto-optical properties of cobalt and cobalt oxide nanoparticles at room temperature. The nanoparticles are deposited on Si (100) substrate by a RF magnetron sputtering system at room temperature. The STM image of the studied sample indicates that the cobalt particle size is in the range of 10 nm to 30 nm. MSHG technique investigates their magnetic anisotropy property and determines their crystal and magnetic symmetry. The pure cobalt nanoparticle thin films have a large nonlinear Kerr rotation response with a low external magnetic field. The increase of coercivity field, H_c, in the cobalt oxide nanoparticle thin films, can be attributed to the strong interaction between the ferromagnetic cobalt and anti-ferromagnetic cobalt oxide interface. It is found that the magnetic anisotropy is due to the shape anisotropy of cobalt nanoparticle thin film and their interaction. The magnetization reversal process of cobalt nanoparticles involves the rotation of the magnetic moment in the surface plane but not the surface normal plane. This study aims to reveal the important information regarding the magnetization orientation in the ferromagnetic materials and their heterostructures.

An ultrafast optical pump and probe technique known as picosecond ultrasonics is used to generate and detect coherent acoustic phonon pulses in nanostructured films grown on Si wafers. By detecting the phonons after they have diffracted across a millimeter thick wafer, it is possible to measure the scattered phonons in the acoustic far field. Numerical backpropagation algorithms can then be used in order to reconstruct the object which scattered the acoustic phonon pulses. We describe measurements and simulations of experiments performed on surface and sub-surface nanostructures. Results with ~500 nm image resolution are shown, and plans for improving that resolution by an order of magnitude will be described.

Zone folded coherent acoustic phonons were generated in a multilayered GaSb/InAs epitaxial heterostructure via rapid heating by femtosecond laser pulses. These phonons were probed by means of ultrafast x-ray diffraction. Phonons both from the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the absorption index due to photogenerated free carriers.

We study coherent plasmons in the narrow-gap semiconductor InSb by measuring the far-infrared electromagnetic radiation they emit. These collective oscillations are excited with ultrashort, near-infrared laser pulses having photon energy far above the semiconductor band gap. Coherent plasmon behavior is characterized as a function of temperature, doping density, optically injected carrier density, and spatial confinement.

The nonradiative recombination effect on the photoluminescence (PL) decay dynamics in GaInNAs/GaAs quantum wells is studied by photoluminescence and time-resolved photoluminescence under various excitation intensities and temperatures. It is found that the PL decay dynamics strongly depends on the excitation intensity. In particular, under the moderate excitation levels the PL decay curves exhibit unusual non-exponential behavior and show a convex shape. By introducing a new concept of the effective concentration of nonradiative recombination centers into a rate equation, the observed results are well simulated. In the cw PL measurement, a rapid PL quenching is observed even at very low temperature and is of the excitation power dependence. These results further demonstrate that the non-radiative recombination process plays a very important role on the optical properties of GaInNAs/GaAs quantum wells.

We report on experiment and theory of the evolution of the electric field in undoped GaAs/AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. We performed time-resolved pump-probe experiments and measured the photocurrent generated by spectrally narrowed and wavelength-tuned probe pulses as a function of delay time, pump power and bias field. The drift of the charge carriers, subsequent to the optical excitation, leads to the buildup of an inhomogeneity of the electric field which was traced via the temporal changes of the Wannier-Stark spectra. Although the photocurrent spectra by themselves only yield information on the absorption integrated spatially over the superlattice, we extract information on the local electric fields and the charge-carrier densities by a comparison of the measured data with the results of Monte-Carlo simulations. We find that at moderate excitation densities (10¹⁶-cm^-3 range) the superlattice within a few picoseconds splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. The largest field differences are found just when the last electrons are swept out after 10-30 ps, the exact time depending on the superlattice parameters and excitation conditions. The initial homogeneous field is restored on a much longer time scale of hundreds of picoseconds which is defined basically by the drift of the heavy holes. Our calculations show that Bloch gain in optically excited semiconductor superlattice is expected in spite of the inhomogeneous field if the field in the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep-out of the electrons.

We study free-space terahertz pulse propagation through samples of densely packed Cu microparticles that are coated with Au nano-layers. By coating the Cu particles with Au nano-layers, the terahertz transmission is dramatically attenuated. The substantial attenuation cannot be reconciled by the inherent resistivities of the Cu and Au metals. The experimental results strongly show that the transmission attenuation arises from contact resistance between the Au and Cu metals.