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Yang C.-S.,National Tsing Hua University | Yang C.-S.,Lawrence Berkeley National Laboratory | Tang T.-T.,Taiwan Semiconductor Manufacturing Company | Chen P.-H.,National Chiao Tung University | And 4 more authors.
Optics Letters | Year: 2014

Transmittance of the device was as high as ∼75%. Phase shift exceeding φ/2 at 1.0 THz is achieved in a ∼500 μm-thick cell. The driving voltage required for the device operating as a quarter-wave plate was as low as 17.68 V (rms), an improvement of nearly an order of magnitude over previous work. © 2014 Optical Society of America.


Yang C.-S.,National Tsing Hua University | Chang C.-M.,National Tsing Hua University | Chen P.-H.,National Chiao Tung University | Yu P.,National Chiao Tung University | And 2 more authors.
Optics Express | Year: 2013

Indium-tin-oxide (ITO) nanorods (NRs) and nanowhiskers (NWhs) were fabricated by an electron-beam glancing-angle deposition (GLAD) system. These nanomaterials are of interests as transparent conducting electrodes in various devices. Two terahertz (THz) time-domain spectrometers (TDS) with combined spectral coverage from 0.15 to 9.00 THz were used. These allow accurate determination of the optical and electrical properties of such ITO nanomaterials in the frequency range from 0.20 to 4.00 THz. Together with Fourier transform infrared spectroscopic (FTIR) measurements, we found that the THz and far-infrared transmittance of these nanomaterials can be as high as 70% up to 15 THz, as opposed to about 9% for sputtered ITO thin films. The complex conductivities of ITO NRs, NWhs as well films are well fitted by the Drude-Smith model. Taking into account that the volume filling factors of both type of nanomaterials are nearly same, mobilities, and DC conductivities of ITO NWhs are higher than those of NRs due to less severe carrier localization effects in the former. On the other hand, mobilities of sputtered ITO thin films are poorer than ITO nanomaterials because of larger concentration of dopant ions in films, which causes stronger carrier scattering. We note further that consideration of the extreme values of Re{σ} and Im{σ} as well the inflection points, which are functions of the carrier scattering time (τ) and the expectation value of cosine of the scattering angle (γ), provide additional criteria for accessing the accuracy of the extraction of electrical parameters of non-Drude-like materials using THz-TDS. Our studies so far indicate ITO NWhs with heights of ∼1000 nm show outstanding transmittance and good electrical characteristics for applications such as transparent conducting electrodes of THz Devices. © 2013 Optical Society of America.


Lo K.-H.,National Tsing Hua University | Li M.-C.,National Tsing Hua University | Ho R.-M.,National Tsing Hua University | Ho R.-M.,Frontier Research Center on Fundamental and Applied science of Matters | And 6 more authors.
Langmuir | Year: 2013

This work presents a simple method to generate ordered chromophore/ dispersant nanoarrays through a pore-filling process for a nanoporous polymer template to enhance chromophore luminescence. Fluorescence results combining with the morphological evolution examined by scanning probe microscopy reveal that the enhanced luminescence intensity reaches the maximum intensity as the nanopores of the template are completely filled by the chromophore/dispersant mixture. The variation is attributed to nanoscale spatial effect on the enhanced mixing efficiency of chromophore and dispersant, that is, the alleviation of self-quenching problem, as evidenced by the results of attenuated total reflection Fourier transform IR spectroscopy combining with grazing incident wide-Angle X-ray diffraction. The enhanced luminescence of the chromophore/dispersant nanoarrays driven by the nanoscale spatial effect is highly promising for use in designing luminescent nanodevices. © 2013 American Chemical Society.


Yang C.-S.,National Tsing Hua University | Yang C.-S.,Lawrence Berkeley National Laboratory | Tang T.-T.,Taiwan Semiconductor Manufacturing Company | Pan R.-P.,National Chiao Tung University | And 3 more authors.
Applied Physics Letters | Year: 2014

Indium Tin Oxide (ITO) nanowhiskers (NWhs) obliquely evaporated by electron-beam glancing-angle deposition can serve simultaneously as transparent electrodes and alignment layer for liquid crystal (LC) devices in the terahertz (THz) frequency range. To demonstrate, we constructed a THz LC phase shifter with ITO NWhs. Phase shift exceeding π/2 at 1.0 THz was achieved in a ∼517 μm-thick cell. The phase shifter exhibits high transmittance (∼78%). The driving voltage required for quarter-wave operation is as low as 5.66 V (rms), compatible with complementary metal-oxide-semiconductor (CMOS) and thin-film transistor (TFT) technologies. © 2014 AIP Publishing LLC.


Yang C.-S.,National Tsing Hua University | Pan R.-P.,National Chiao Tung University | Yu P.,National Chiao Tung University | Pan C.-L.,National Tsing Hua University | Pan C.-L.,Frontier Research Center on Fundamental and Applied Science of Matters
2014 31th URSI General Assembly and Scientific Symposium, URSI GASS 2014 | Year: 2014

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 (ITO) nanowhiskers (NWhs) were employed as transparent electrodes. The driving voltage required for a 2π wave plate is as low as 5 Vrms. © 2014 IEEE.


Lin J.-W.,National Tsing Hua University | Wun J.-M.,National Central University | Shi J.-W.,National Central University | Pan C.-L.,National Tsing Hua University | Pan C.-L.,Frontier Research Center on Fundamental and Applied Science of Matters
Journal of Infrared, Millimeter, and Terahertz Waves | Year: 2014

We conducted a comparative study of two schemes of photonic generation and switching of few-cycle sub-THz or millimeter wave (MMW) pulses by use of a photonic-transmitter-mixer (PTM) module with a broadband and high-power near-ballistic uni-traveling carrier photodiode (NBUTC-PD). In the first scheme, we performed all-optical ultra-fast switching (bias modulation) of the PTM injected with a 93 GHz optical local-oscillator signal. Sub-2-cycle short MMW pulses with central frequency at 93 GHz were generated. To compare, in scheme 2, we employed femtosecond optical short pulses to directly excite the PTM under a DC bias (optical modulation). The former approach is shown to be capable of providing much less signal distortion and much shorter pulse duration than the latter. © 2014 Springer Science+Business Media New York.


Yang C.-S.,National Tsing Hua University | Lin C.-H.,National Tsing Hua University | Zaytsev A.,National Tsing Hua University | Teng K.-C.,National Tsing Hua University | And 3 more authors.
Applied Physics Letters | Year: 2015

We have demonstrated the laser ablation of PMMA using dual-color waveform synthesis of the fundamental (ω) and its second-harmonic (2ω) of a femtosecond Ti:Sapphire laser. A modest and yet clear modulation (∼22%) in ablated area versus relative phase between the 2ω and ω beams with a power-ratio of 15% (28/183 mW) is revealed. This is explained qualitatively by the dependence of ablation on multiphoton ionization of which the rate is related to the relative phase of the synthesized waveform. At higher peak power ratios, the modulation decreases rapidly, as the two-photon-ionization rate of the 2ω dominates over that of the three-to four-photon ionization of the ω beam. This technique demonstrates the feasibility of phase-controlled laser processing of materials. © 2015 AIP Publishing LLC.


Li M.-C.,National Tsing Hua University | Ho R.-M.,National Tsing Hua University | Ho R.-M.,Frontier Research Center on Fundamental and Applied science of Matters | Lee Y.-D.,National Tsing Hua University
Journal of Materials Chemistry C | Year: 2013

Here, we aim to systematically examine the mechanism of photo-induced pyrene association in pyrene-labeled polymers (PLPs) for optical recording. By taking advantage of polymer glass transition, the associated pyrene moieties can be frozen in the glassy matrix of the PLP to give the emission contrast for optical recording, as evidenced by the phenomenon of room-temperature phosphorescence. In situ time profiles of the fluorescence emission during recording reflect that the association of pyrene moieties in PLPs is a diffusion-controlled process, and it is highly dependent on the mobility of the polymer chain. For crystallizable PLPs, the polymer chain can also be frozen in the crystalline state that inhibits the association of pyrene moieties in PLPs. As a result, Tg and Tm of the PLPs can be exploited as energy barriers for the induction of the association of pyrene moieties in PLPs and the recording capabilities of the PLPs will be dependent upon the thermal properties of the PLPs. This journal is © The Royal Society of Chemistry 2013.


Li C.-L.,National Tsing Hua University | Li M.-C.,National Tsing Hua University | Ho R.-M.,National Tsing Hua University | Ho R.-M.,Frontier Research Center on Fundamental and Applied science of Matters
Macromolecules | Year: 2011

A series of poly(4-vinylpyridine)-b-poly(ε-caprolactone) (P4VP-PCL) diblock copolymers have been synthesized and used for the formation of nanostructures with tunable colors arising from the association of chromophores with P4VP block in P4VP-PCL. The association of chromophores leads to the bathochromical shifts of charge transfer absorption peaks, resulting in the color appearance into the visible region. To achieve the formation of well-defined nanostructured materials, the phase behavior of the mixtures of chromophore/P4VP-PCL was systematically examined. As evidenced by transmission electron microscopy and small-angle X-ray scattering (SAXS), the phase transformation of self-assembled nanostructures can be easily induced by introducing chromophores due to the association of 2-methylidenepropanedinitrile in the chromophores with the lone-pair electron of nitrogen in P4VP block (that is the increase on the effective volume fraction of P4VP, as identified by SAXS experiments through the analysis of one-dimensional correlation function). As a result, by taking advantage of charge transfer and corresponding morphologies from transformation, well-defined nanostructured films resulting from mixing of chromophore and P4VP-PCL offer the possibility to create stimuli-responsive nanomaterials with tunable color. © 2011 American Chemical Society.


Liu K.-W.,National Tsing Hua University | Li C.-L.,National Tsing Hua University | Li M.-C.,National Tsing Hua University | Ho R.-M.,National Tsing Hua University | Ho R.-M.,Frontier Research Center on Fundamental and Applied science of Matters
Langmuir | Year: 2013

Here, we develop a method to fabricate stimuli-responsive color films using block copolymer, poly(4-vinylpyridine)-b-poly(ε-caprolactone) (P4VP-PCL), as a template complexed with functionalized chromophores. The P4VP block in the P4VP-PCL can be associated with a cyano end-capped chromophore via charge transfer, which is a noncovalent interaction that can be conveniently manipulated by external stimuli, giving a specific color. The color of the film can be switched by tuning the charge transfer interaction between the chromophore and P4VP with controlled environmental conditions, such as pH, temperature, and moisture, while maintaining high transmittance for visible light due to the formation of the nanostructure of chromophore/P4VP-PCL complex. However, the association/dissociation process between chromophore and P4VP is diffusion-dominated, which may limit the kinetic response time for color change. A way to create quick and reversible color switching can be achieved by a combination of stimuli. The contrasting color change of the responsive chromophore/P4VP-PCL thin films which exhibit RGB primary colors can provide a sensor film that is flexible, fast-responsive, and convenient. © 2013 American Chemical Society.

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