[Reseach on THz Time Domain Spectrum of Photo-Induced Insulator-Metal Phase Transition of VO2 Films].

Vanadium dioxide (VO2) film will be phase-transitioned from insulator into metal, accompanied with dramatic change on conductivity, which is named as photo-induced insulator-metal phase transition. Such phase transition of VO2 film has important application potentials in modulators or other functional devices for terahertz waves. In this paper, the transmission spectrum variations before and after the photo-induced insulator-metal phase transition of vanadium dioxide film are investigated, and the phase transition properties in terahertz(THz) region are analyzed. In the experiment, the phase transition of the VO2 film was induced by a continuous wave (CW) laser source and a femtosecond (fs) laser source, respectively. Obvious changes on the THz waveforms were observed for the both mentioned means of excitation, and the amplitude attenuation, as well as the signal distortion, was intensified with the increase of the impinging optical power. The fast Fourier transform (FFT) spectra of the transmitted THz time-domain signals were analyzed and it was found that the amplitude of the transmitted spectrum decreased synchronously with the increase of the optical power, accompanied with deformation of the spectrum line shape at the same time. The reason was that the macroscopic dielectric properties of the VO2 film approached gradually to that of a metal as laser power was increased. A parameter, transmission modulation function, was defined in the paper as the amplitude difference between the transmission spectra of the VO2 film before and after the laser excitation, to describe the dispersivity of the photo-induced phase transition more clearly. From the curve of the transmission modulation function, strong frequency-dependent properties at THz frequencies were found to vary regularly with the incident light power. After furthermore comparison, it was found that, though the insulator-metal phase transition could be trigged by both CW laser source and fs laser source, the corresponding impinging optical power values were obviously alternative for the equivalent transmission modulation function. At the end of the paper, the difference of the phase transition efficiency between the two excitation methods was analyzed and discussed.

Experimental study on GaP surface damage threshold induced by a high repetition rate femtosecond laser.

The surface damage threshold of undoped bulk <110> GaP induced by a high repetition rate femtosecond pulse at 1040 nm with a duration of 61 fs was studied. The threshold value was obtained by a linear fit of the incident single pulse fluence and was confirmed with a breakdown test around the threshold level. The result will be useful in high intensity, high repetition rate laser applications and ultrafast processes.

Terahertz superconducting plasmonic hole array.

We demonstrate a superconductor array of subwavelength holes with active thermal control over the resonant transmission induced by surface plasmon polaritons. The array was lithographically fabricated on a high-temperature yttrium barium copper oxide superconductor and characterized by terahertz time-domain spectroscopy. We observe a clear transition from a virtual excitation of the surface plasmon mode to a real surface plasmon mode. The highly controllable superconducting plasmonic crystals may find promising applications in the design of low-loss, large- dynamic-range amplitude modulation and surface-plasmon-based terahertz devices.

Large dynamic resonance transition between surface plasmon and localized surface plasmon modes.

We present resonant terahertz transmission in a composite plasmonic film comprised of an array of subwavelength metallic patches and semiconductor holes. A large dynamic transition between a dipolar localized surface plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation. The reversal in transmission amplitude from a stop-band to a pass-band and up to pi/2 phase shift achieved in the composite plasmonic film make it promising in large dynamic phase modulation, optical changeover switching, and active terahertz plasmonics.

Application of terahertz time-domain spectroscopy in intracellular metabolite detection.

Using terahertz time-domain spectroscopy (THz-TDS), we have investigated the THz spectra of astaxanthin and riboflavin and the spectra of two kinds of cell, haenatcoccus plusivalis and bacillus subtilis, which could produce astaxanthin and riboflavin, respectively, during their metabolite process. Riboflavin was found to be much more absorptive to THz radiation and have richer spectral characteristics than astaxanthin. As an intracellular metabolite, riboflavin could be distinguished from the cells by using THz-TDS. The technique has potential applications in high-throughput screening of industrial strains.

[Joint time-frequency analysis for removing the spectral interference of terahertz].

Conventional Fourier-transform mixes the frequency components of the entire temporal terahertz waveform in one frequency domain; therefore, it cannot distinguish the terahertz frequency in the main pulse from the noise frequency in the pulse tail. Thus traditional Fourier-transform produces inconsistent spectra from different scanning lengths of terahertz pulse. And the interference spectrum appears when the THz echo pulse is recorded. The authors applied wavelet-transform and removed the inconsistent spectra and the interference spectra. Wavelet-transform technique exhibited the local frequency of THz in different time locations. This technique would find applications in THz time-resolved spectroscopy.

A close-ring pair terahertz metamaterial resonating at normal incidence.

We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by double-layer, self-aligned photolithography. Terahertz time-domain spectroscopy transmission measurements and numerical simulations have revealed negative index of refraction in the frequency range of 0.66-0.90 THz under normal wave incidence. The observed resonance behaviors can be well explained by a theoretical circuit model. The electromagnetic properties and the figure of merit of such close-ring metamaterials are also explored in terms of geometrical parameters of the unit cell with a goal of providing optimized design for three-dimensional metamaterials and potential device applications.

Broadband resonant terahertz transmission in a composite metal-dielectric structure.

We present a systematic numerical study of a metal-dielectric-metal sandwich plasmonic structure for broadband resonant transmission at terahertz frequencies. The proposed structure consists of periodic slotted metallic arrays on both sides of a thin dielectric substrate and is demonstrated to exhibit a broad passband transmission response. Various design considerations have been investigated to exploit their influence on the transmission passband width and the center resonance frequency. The structure ensures a broadband transmission over a wide range of incident angles.

Periodic optical delay line based on a tilted parabolic generatrix helicoid reflective mirror.

We report the design and testing of a novel linear scanning periodic optical delay line (ODL) by use of a helicoid reflective mirror based on a tilted parabolic generatrix that was driven by an electrical motor for a periodic change in the optical path length of the reflected light beam. The divergence and pulse front distortion of the optical beam reflected by the helicoid reflective mirror were simulated based on differential geometry. With a round-trip pass arrangement, a scanning range of delay time as large as 100 ps was obtained by spinning the helicoid reflective mirror with a pitch distance of 7.5 mm. This periodic ODL was used in an optical second-harmonic generation autocorrelator to test the linearity and temporal resolution in comparison with the autocorrelation signal obtained using an ODL structured with a motorized linear translation stage. Experiments demonstrate that our helicoid optical delay device may provide exceptional performance for optical interference, high-resolution terahertz time-domain spectroscopy, and general optical pump-probe experiments.

Two-dimensional coherent superposition of blue-shifted signals from an array of highly nonlinear waveguiding wires in a photonic-crystal fiber.

Frequency-shifted dispersive optical waves generated as a result of soliton dynamics of 30-fs Ti: sapphire-laser pulses in an array of waveguiding wires, implemented on a platform of a photonic-crystal fiber (PCF), are shown to produce regular stable interference patterns with high visibility, indicating a high coherence of frequency-shifted fields. For a hexagonal array of waveguides built into a silica PCF, the field intensity at the main peak of a six-beam interference pattern was found to be a factor of 22 higher than the intensity of a frequency-shifted signal from an individual waveguide in the array and 3.7 times higher than the field intensity attainable through an incoherent superposition of the same fields.

Design rules for phase-matched terahertz surface electromagnetic wave generation by optical rectification in a nonlinear planar waveguide.

The theory of guided waves in metal-dielectric planar multilayer structures is applied to reduce the loss and maximize optical nonlinearity for efficient terahertz-field generation in a surface electromagnetic wave by femtosecond laser pulses confined in a (chi)((2)) nonlinear planar waveguide. For typical parameters of thin-film polymer waveguides and metal-dielectric interfaces, the optimal size of the (chi)((2)) waveguide core providing the maximum efficiency of terahertz plasmon-field generation is shown to be less than the wavelength of the optical pump field.

Analysis and design of terahertz photonic crystal fibers by an effective-index method.

An effective-index method (EIM) is used to analyze and design photonic crystal fibers (PCFs) for the terahertz radiation. By building an analogy between a conventional optical fiber and a PCF, the EIM solves the effective index of the fiber cladding and the effective modal index of a PCF analytically. The EIM is first validated by comparison with available data in the reference, showing that the role of material dispersion is negligible at higher frequencies. Terahertz PCFs with flattened dispersion are designed based on this method and the scaling property of the Maxwell equations.

Effective surface plasmon polaritons on the metal wire with arrays of subwavelength grooves.

In this paper we explore the existence of electromagnetic surface bound modes on a perfect metal wire milled with arrays of subwavelength grooves. The surface modes are axially symmetric transverse magnetic (TM) waves and have the same polarization state with the dominant propagating surface plasmon polaritons on the real metal wires. The dispersion of the fundamental surface mode has close resemblance with the dispersion of the surface plasmon polaritons. Moreover, we note that for TM polarization this metallic structure can be equivalent to a dielectric coated metal wire with defined geometrical parameters and effective refractive index of the dielectric coating. This metallic structure is expected to have some potential applications in the optical research in microwave or THz region.

Time-resolved polarization to extract coded information from early ballistic and snake signals through turbid media.

Time-resolved polarization is used to extract coded information buried within the multiple scattering profiles from the early ballistic and snake components as they pass through turbid media. By polarization analysis the depolarized diffusive component and the natural-light background are significantly reduced to enhance the signal-to-noise ratio of a coded pulse train. This procedure has the potential to improve optical wireless communication in cloudy environments.