Directional Terahertz Radiation from GalnP Lateral Superlattice.

We devised directionally controllable THz emission sources based on lateral composition modulation (LCM) structures. LCM structures were composed of In-rich Ga0.47In0.53P and Ga-rich Ga0.51In0.49P layers whose period was in quantum scale of ~`5 nm. The inherent type II band alignment in these structures leads to electron-hole (e-h) separation and plays a key role in generating later- ally polarized dipole ensembles, thus concomitantly emitting enhanced transmissive THz waves as compared to bulk sample. On the other hand, in lateral geometry, changes in THz fields between LCM and bulk structures turned out to negligible since the vertical electronic diffusion was allowed in both samples.

Polarized Terahertz Waves Emitted from In0.2Ga0.8As Nanowires.

We investigate the polarizability of terahertz (THz) waves emitted from undoped In0.2Ga0.8As nanowires (NWs). THz emission amplitude shows strong enhancement in vertically aligned NWs compared to less-aligned NWs. In particular, polarized THz waves are clearly demonstrated in aligned NWs via a drastic variation of amplitudes as a function of the axis angle in polarization-sensitive photoconductive antenna. In addition, phase reversal between aligned and less-aligned NWs substantiates the geometrical dependence of electronic diffusion in generating the transient THz electric fields.

Universal Efficiency Improvement in Organic Solar Cells Based on a Poly(3-hexylthiophene) Donor and an Indene-C60 Bisadduct Acceptor with Additional Donor Nanowires.

With poly(3-hexylthiophene) (P3HT) nanowire (NW) inclusion in active layers (ALs), organic solar cells (OSCs) based on P3HT donor and indene-C60 bisadduct (ICBA) acceptor showed power conversion efficiency (PCE) improvements for both bulk heterojunction (BHJ)- and bilayer (BL)-structure AL devices. The PCE increase was approximately 14 % for both types of P3HT:ICBA OSCs. However, improvements in short-circuit current density (Jsc ) were about 4.4 and 6.4 % for BHJ- and BL-type AL devices, respectively. A systematic study showed that the addition of P3HT NWs did not result in enhanced internal quantum efficiencies for either type of device. However, the difference in light-harvesting efficiency was important in accounting for Jsc variations. Interestingly, there was no correlation between Jsc and PCE variations, whereas the open-circuit voltage (Voc ) and fill factor (FF) showed correlations with the PCE. The variation in FF is discussed in terms of Voc and equivalent-circuit parameters based on a nonideal diode model.

Enhanced performance using an SU-8 dielectric interlayer in a bulk heterojunction organic solar cell.

The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility-lifetime (µ-τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.

Fully solution-processed semitransparent organic solar cells with a silver nanowire cathode and a conducting polymer anode.

We report the fabrication of efficient indium-tin-oxide-free organic solar cells based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). All layers of the devices from the lowermost silver nanowire cathode to the uppermost conducting polymer anode are deposited from solution and processed at plastic-compatible temperatures<200 °C. Owing to the absence of an opaque metal electrode, the devices are semitransparent with potential applications in power-generating windows and tandem-cells. The measured power conversion efficiencies (PCEs) of 2.3 and 2.0% under cathode- and anode-side illumination, respectively, match previously reported PCE values for equivalent semitransparent organic solar cells using indium tin oxide.

Directional terahertz emission from corrugated InAs structures.

The terahertz (THz) radiation from transient dipoles, formed by distinct diffusion coefficients between oppositely charged carriers as often observed in low band gap semiconductors, propagates with an anisotropic amplitude distribution perpendicular to the dipole axis along the diffusive motion. By directionally adjusting the electronic diffusion, we conceptualize groove-patterned THz emitters based on (100) InAs thin films and demonstrate the unidirectional radiation. Line-of-sight emission along the surface-normal direction is greatly enhanced in a distributed asymmetric trapezoid with its period similar to the electronic diffusion length of InAs. This directional enhancement is in clear contrast to the constant emission amplitude along the lateral direction, regardless of pattern scale, which manifests the role of groove patterns as microscale reflectors in laterally corrugating the carrier density. In contrast to the rather limited nonlinearity in (100) plane, the azimuthal angle dependence of the THz field amplitude in corrugated samples shows a combined effect of diffusive transport and second-order nonlinearity, whose compositional contributions varies in different structures.

Sub-100 fs single-walled carbon nanotube saturable absorber mode-locked Yb-laser operation near 1 microm.

Transmission- and reflection-type single-walled carbon nanotube saturable absorbers (SWCNT-SAs) were designed and fabricated for passive mode-locking of bulk lasers in the 1 microm spectral range. Mode-locked laser operation based on a diffusion-bonded Yb:KYW/KYW crystal was demonstrated, and pulses as short as 83 fs and 140 fs were achieved applying reflection-type and transmission-type SWCNT-SA, respectively. The nonlinear parameters of the absorbers were measured to be in close vicinity to those of a semiconductor saturable absorber mirror for the same wavelength range. Mode-locking performance with SWCNT-SAs and the SESAM was compared utilizing the same cavity, with the SESAM resulting in only slightly shorter pulses of 66 fs duration. The nearly identical performance indicates that well-optimized SWCNT-SAs can substitute SESAMs even in the 1 microm region.

Passive mode-locking of a Tm-doped bulk laser near 2 microm using a carbon nanotube saturable absorber.

Stable and self-starting mode-locking of a Tm:KLu(WO(4))(2) crystal laser is demonstrated using a transmission-type single-walled carbon nanotube (SWCNT) based saturable absorber (SA). These experiments in the 2 microm regime utilize the E11 transition of the SWCNTs for nonlinear saturable absorption. The recovery time of the SWCNT-SA is measured by pump-probe measurements as approximately 1.2 ps. The mode-locked laser delivers approximately 10 ps pulses near 1.95 microm with a maximum output power of up to 240 mW at 126 MHz repetition rate.

Mode-locked self-starting Cr:forsterite laser using a single-walled carbon nanotube saturable absorber.

We report what we believe to be the first passive mode-locking of Cr:forsterite laser using a single-walled carbon nanotube saturable absorber (SWCNT-SA). The dispersion-compensated Cr:forsterite laser in a self-starting configuration produces nearly Fourier transform-limited pulses as short as 120 fs near 1.25 microm. The maximum average output power of 202 mW obtained with a 5% output coupler at a repetition rate of 79.1 MHz represents, to the best of our knowledge, the highest power level ever reported for SWCNT-SA mode locking of solid-state lasers.

Passive mode locking of Yb:KLuW using a single-walled carbon nanotube saturable absorber.

Mode locking of an Yb-doped bulk laser in the 1 microm spectral range using a single-walled carbon nanotube saturable absorber (SWCNT-SA) is demonstrated for the first time, to our knowledge. Passive mode locking of an Yb:KLuW laser resulted in nearly transform-limited pulses as short as 115 fs at 1048 nm. In addition, the nonlinear response of the SWCNT-SA was measured, yielding a modulation depth of 0.25% and a relaxation time of 750 fs.