Yu-Ting Wang

Ultrafast Multi-Level Logic Gates with Spin-Valley Coupled Polarization Anisotropy in Monolayer MoS2

The inherent valley-contrasting optical selection rules for interband transitions at the K and K′ valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and ~300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives.

Use of Ultrafast Time-Resolved Spectroscopy to Demonstrate the Effect of Annealing on the Performance of P3HT:PCBM Solar Cells

The organic solar cells of heterojunction system, ITO/PEDOT:PSS/P3HT:PCBM/Al, with a thermal annealing after deposition of Al exhibit better performance than those with an annealing process before deposition of Al. In this study, ultrafast time-resolved spectroscopy is employed to reveal the underlying mechanism of annealing effects on the performance of P3HT:PCBM solar cell devices. The analyses of all decomposed relaxation processes show that the postannealed devices exhibit an increase in charge transfer, in the number of separated polarons and a reduction in the amount of recombination between excited carriers. Moreover, the longer lifetime for the excited carriers in postannealed devices indicates it is more likely to be dissociated into photocarriers and result in a larger value for photocurrent, which demonstrates the physical mechanism for increased device performance.

Environment-dependent ultrafast photoisomerization dynamics in azo dye.

Azoaromatic dyes have been extensively investigated over the past decade due to their potential use in a variety of optical devices that exploit their ultrafast photoisomerization processes. Among the azoaromatic dyes, Disperse Red 19 is a commercially available azobenzene nonlinear optical chromophore with a relatively high ground-state dipole moment. In the present study, we used ultrafast time-resolved spectroscopy to clarify the dynamics of a push-pull substituted azobenzene dye. Solution and film samples exhibited different ultrafast dynamics, indicating that the molecular environment affects the photoisomerization dynamics of the dye.

Controlling the carrier-envelope phase of single-cycle mid-infrared pulses with two-color filamentation

Carrier-envelope phase (CEP) of single-cycle pulses generated through two-color filamentation has been investigated. We have observed a particular behavior of the phase: the phase of high-frequency components of the generated pulses changes continuously and linearly with the relative phase between the two-color input pulses, whereas the phase of the low-frequency components takes only two discrete values. The transition of the phase behavior has been clearly observed by using frequency-resolved optical gating capable of CEP determination. We have found out that such a phase behavior is a unique feature of single-cycle pulses generated with a passive CEP stabilization scheme.

Carrier-envelope phase of single-cycle pulses generated through two-color laser filamentation

Carrier-envelope phase (CEP) control of the pulses from two-color filamentation has been investigated. The CEP variation with the relative phase between the two-color pulses is explained with a four-wave mixing model.

Single-shot detection of mid-infrared spectra by chirped-pulse upconversion with four-wave difference frequency generation in gases

Summary form only given. Single-shot detection of an entire MIR spectrum (500-5000cm-1) has been required for advanced molecular spec-troscopy such as pump-probe spectroscopy to trace ultrafast structural dynamics of molecules, real-time molecular imaging of biological tissues, etc. However, due to low pixel numbers, low sensitivity, and high cost of multi-channnel MIR detectors, the bandwidth has been limited to ~500 cm-1 at direct measurement of MIR spectra by using dispersive infrared spectrometers.Chirped-pulse upconversion is an alternative approach to detect an MIR spectrum with single-shot. By converting the wavelength of coherent MIR pulse to visible range, it becomes possible to detect MIR spectra with a visible spectrometer, which has much higher performance than MIR spectrometers. However, the bandwidth of the chirped-pulse upconversion has still been limited to ~1000 cm-1 because of the limited transmission range of the nonlinear crystals [1]. In this contribution, we have demonstrated ultrabroadband detection of MIR spectra on a single-shot basis using chirped-pulse upconversion with four-wave difference frequency generation (FWDFG) in gases. The schematic of the method is shown in Fig. 1(a). By using a gas as a nonlinear medium, the detection bandwidth becomes dramatically broad due to wide transmission range of gas media. Experimental demonstration of the scheme was realized with the system described as follows. We generated sub-single-cycle MIR pulses by using four-wave mixing of the fundamental and the second harmonic of Ti:sapphire amplifier (Femtopower compactPro, FEMTOLASERS) output through filamentation in air, which is basically the same generation scheme as that reported in Ref. 2 and 3. A small portion of the fundamental pulse (0.1 mJ) before the compressor of the Ti:sapphire amplifier was used as a chirped pulse, whose pulse duration was 10.3 ps. The chirped pulse Eref(t - τ) and the MIR pulse (EIR(t), 0.5 μJ) were focused into xenon with a parabolic mirror (f=50 mm) and generated a FWDFG signal, E2 ref(t - τ)EIR(t), which spread from 400 nm to 550 nm. The spectrum of the FWDFG signal was measured with a conventional spectrometer with a camera EMCCD (ProEM+1600, Princeton Instruments). The camera was synchronized with the repetition rate (1 kHz) of the laser and the spectrum was measured with a single shot, namely within 1 ms. A typical spectrum is shown as the upper curve in Fig. 1(b). By using retrieval algorithm from the upconverted spectrum to the original MIR spectrum [4] including the nonlinear chirp of the reference pulse, it was possible to retrieve the MIR spectrum shown as the lower curve in Fig. 1(b). Fine structure due to absorption of carbon dioxide (~2300 cm-1) and water vapor (~1600 cm-1 and ~3700 cm-1) in air was clearly observed. At the conference, we plan to show the application of the system to MIR absorption spectroscopy.

Carrier-envelope phase of ultrashort pulses generated by optical rectification process

Passive stabilization of carrier-envelope phase (CEP) of few-cycle pulses by use of difference frequency generation (DFG) is a key technology for frequency comb and attosecond science [1,2]. Similarly, the CEP of the ultrashort terahertz pulse generated by optical rectification is also passively stabilized. The optical rectification process has been considered as just a special case of DFG, where the frequencies of the two input pulses are the same.In this contribution, we report how the CEP of the generated pulse is determined through the DFG and the optical rectification. We have found a clear difference of the CEP determination between these two processes. Assuming the two complex input electric fields are E1(t) = E (t)exp(iω1t + iφ1) and E2(t) = E (t)exp(iω2t + iφ2), the DFG field generated through the process ω1 - ω2 → ω0 can be described as ∂ E1(t)E 2(t) = ∂ E (t)E (t)expi(ω1 - ω2)t + i(φ1 - φ2) E0(t) o = E0(t)exp(iω0t + iΔφ), (1) ∂t ∂t where E0(t) = ∂ E (t)E (t) + iω0E (t)E (t), ω1 - ω2 = ω0, and φ1 - φ2 = Δφ. When ω0 ~ 0, namely, the ∂t process is called as optical rectification, the real field of the DFG reduces to ∂ E (t)E (t)cosΔφ. As a result, Δφ ∂t does not contribute to the phase but to the amplitude of the output field. On the other hand, when ω0 0, the real output field can be written as ω0E (t)E (t)cos(ω0t + Δφ + π/2). In this case, the CEP of the field is Δφ + π/2, which means the relative phase between the two input pulses directly affect the CEP of the output pulse. To experimentally investigate the CEP variation, we have generated phase-stable mid-infrared (MIR) pulses by using four-wave mixing (ω1 + ω1 - ω2 → ω0) through filamentation [3] and characterized the pulse including CEP information by measuring FROG and electro-optic sampling [4]. Figure 1(a) and (b) shows waveforms and spectral phases of the MIR pulses at different relative phases of the input pulses, respectively. Figure 1(c) shows phase change for each frequency components of the MIR pulses. The phase of the high frequency components (ω0 >3000 cm-1) changes continuously and linearly with respect to the relative phase. On the other hand, the phase of the low frequency components (ω0 <;3000 cm-1) changes by 0 or π like a step function, which means that Δφ basically affect only the amplitude. The π phase jump means that only the sign of the amplitude (the sign of cosΔφ) changes. The phase variation is understood with the above mentioned simple theory. At the conference, we plan to show the detail of the physics of the phase variation with numerical simulation results.

Chirped-pulse upconversion of mid-infrared pulses with four-wave difference frequency generation in gases

Single-shot detection of mid-infrared spectra from 250 to 5500 cm-1 with 5 cm-1 resolution was demonstrated by using chirped-pulse upconversion with four-wave difference frequency generation in gases.

Mid-infrared chirped-pulse upconversion with four-wave difference frequency generation in gases

Chirped-pulse upconversion of sub-single-cycle mid-infrared pulses with gaseous media has been realized. Single-shot detection of mid-infrared spectra from 250 to 5500 cm-1 with 5 cm-1 resolution was demonstrated.