Monday, June 26, 2017

Abstract-Flexibility and non-destructive conductivity measurements of Ag nanowire based transparent conductive films via terahertz time domain spectroscopy


Department of Physics, Yonsei University, Wonju-si, 26493, South Korea 2 Department of Electrical and Computer Engineering, University of Alabama, Tuscaloosa, AL 35487, USA 3 Korea Institute of Industrial Technology, Cheonan-si, Chungcheongnam-do, 31056, South Korea 4 Vault Creation, Seoul 02865, South Korea 5 Department of Nano-Optical Engineering, Korea Polytechnic University, Gyeonggi-do 15086, South Korea 6 These authors contributed equally to this work 7 8

Highly stable and flexible transparent electrodes are fabricated based on silver nanowires (AgNWs) on both polyethylene-terephthalate (PET) and polyimide (PI) substrates. Terahertz time domain spectroscopy (THz-TDS) was utilized to probe AgNW films while bended with a radius 5 mm to discover conductivity of bended films which was further analyzed through Drude-Smith model. AgNW films experience little degradation in conductivity (<3%) before, after, and during 1000 bending cycles. Highly stable AgNW flexible electrodes have broad applications in flexible optoelectronic and electronic devices. THz-TDS is an effective technique to investigate the electrical properties of the bended and flattened conducting films in a nondestructive manner.

Abstract-Ultrafast photodiodes under forward-bias conditions

High-speed performance and DC electrical power generation are simultaneously achieved with novel devices that include specifically designed absorption/collector junctions.
23 June 2017, SPIE Newsroom. DOI: 10.1117/2.1201703.006827
Driven primarily by the use of wireless mobile data and Internet videos, global network data traffic is continuing to increase. The information and communication technology sector thus takes up an ever-larger portion of global electricity consumption (now at about 10%).1 To minimize the demands of this growth, it is therefore necessary to increase the energy efficiency of high-speed network data processing.
To date, a number of processing techniques have been adapted to increase the energy efficiency of high-speed networks. For instance, optical interconnect (OI) techniques2 provide a revolutionary way to reduce the carbon footprint of data centers and their wired networks. The DC component of the high-speed optical data signal at the receiving end of an OI system, however, still produces waste heat energy. This energy is proportional to the product of the DC reverse bias of the photodiodes (PDs) and the output photocurrent,3 and this heating effect could thus be a serious issue for the next generation of OI systems. Such systems have densely packaged integrated circuits, with millions of optoelectronic components and optical channels for high-speed linking (i.e., at >50Gb/s). PDs that could sustain high-speed performance, even under zero (forward)-bias operation, would thus be a potentially effective solution for minimizing the OI thermal issue.
In this work, we describe our recently developed unitraveling carrier photodiodes (UTC-PDs).4, 5 We include type-II (i.e., staggered-jump) p-n absorption/collector (A/C) interfaces in these devices to further improve their speed under zero-bias operation.6, 7 In addition, we have designed and demonstrated7 our UTC-PD—with a gallium arsenide/indium gallium phosphide (GaAs/In0.5Ga0.5P) A/C junction—for application at 850nm because this is the most popular optical wavelength for very short reach linking (i.e., <300m) in modern data centers.2 To minimize the increase in the junction capacitance of our device under forward-bias operation, we have adopted—see Figure 1(a)—an under-cut mesa structure.7 We can thus achieve—see Figure 1(b)—an optical–energy (O–E) power conversion efficiency of nearly 25% for a bias of about +1V.
Figure 1. (a) Conceptual cross section of the proposed gallium arsenide/indium gallium phosphide (GaAs/In0.5Ga0.5P) unitraveling carrier photodiode (UTC-PD), which includes an undercut mesa structure. S. I.: Semi-insulating. (b) The DC optical–electrical (O–E) power conversion efficiency of the device at different biases.
The O–E frequency responses of a PD, both with (device A) and without (device B) our under-cut mesa structure, are shown in Figure 2(a) and (b). These results clearly indicate that the mushroom (i.e., under-cut) structure greatly enhances the O–E response of the device under forward-bias operation. Furthermore, we show the measured 10Gb/s eye pattern and bit-error-rate of device A, under forward bias, in Figure 2(c). We find that nearly 10Gb/s error-free performance can be sustained under +0.8V bias, with about 20% O–E power conversion efficiency.
Figure 2. Measured O–E frequency response of a PD (a) that includes the under-cut mesa—as shown in Figure 1(a)—and (b) that does not include the structure. Results are shown for a bias of +0.8V and –5V, under a fixed photocurrent of 90μA. (c) The bit-error-rate of device A as a function of the forward operation voltage (for output photocurrents of 100 and 140μA at 10Gb/s). The corresponding error-free eye patterns are shown in the inset.
We have also demonstrated an ultrafast PD that can be operated with a sub-terahertz bandwidth under zero bias and at a wavelength of 1.55μm. In this device we use an epilayer structure (similar to our under-cut mesa structure) in an indium-phosphide-based material system. In particular, we use a gallium arsenide antimony/indium phosphide (GaAs0.5Sb0.5/InP) type-II A/C collector interface to minimize the current-blocking effect, which is the major bandwidth-limiting factor of UTC-PDs with type-I (i.e., straddling gap) A/C (In0.53Ga0.47As/InP) junctions under zero-bias operation.4, 5 We use flip-chip bonding processing (see Figure 3) for this device so that we can achieve good heat sinking and optical coupling. The measured bias-dependent O–E frequency responses of the device under high (5mA) output photocurrent are shown in Figure 4. These results indicate that even under zero-bias operation, with a moderate output photocurrent (2mA), we can achieve an extremely wide 3dB O–E bandwidth (170GHz). To the best of our knowledge, this is the best high-speed performance that has yet been reported for any zero-bias photodiode.4, 5
Figure 3. Views (from above) of (a) the active PD, (b) co-planar waveguide (CPW) bonding pads on an aluminum nitride (AlN) substrate, and (c) the PD chip after flip-chip bonding. The inset to (a) shows an enlarged image of the fabricated PD, which has an active diameter of 6μm and measured DC responsivity at 0.09A/W. The CPW pad (b) provides a bandwidth of about 400GHz at 3dB. P: Anode. N: Cathode.
Figure 4. Bias-dependent frequency responses of the PD (with an active diameter of 6μm), measured at an output photocurrent of 5mA. Results are shown for a bias of 0, –0.5, and –1V.
In our work we have also compared our proposed PD structure with a traditional In0.53Ga0.47As/InP UTC-PD structure (i.e., that has an additional n-type charge layer close to the A/C interface).8 Both devices feature the same flip-chip-bonded packaging and A/C layer thicknesses of about 160/160nm. We find that our device exhibits a much smaller variation in sub-terahertz output power than the traditional UTC-PD (2.8 and 26.1dB, respectively) when the bias voltage swings from reverse to nearly forward. This result is thus a clear indication of our device's superior speed and power performance under zero-bias conditions. The photogenerated millimeter wave (MMW) power that we obtained with our PD under sinusoidal signal excitation (at an operating frequency of 170GHz) is shown in Figure 5 as a function of output photocurrent. We believe that the continuous wave output power we achieve is the highest to date for photonic generation of sub-terahertz waves from a PD under zero-bias operation.4, 5
Figure 5. Photogenerated millimeter wave power as a function of photocurrent, measured under sinusoidal signal excitation and different reverse bias voltages (i.e., 0, –0.5, and –1V) at an operating frequency of 170GHz. The solid black is the ideal trace for a 100% modulation and a 50 ohm (Ω) load.
In summary, we have have successfully demonstrated PDs in which we use either GaAs0.5Sb0.5/InP or GaAs/In0.5Ga0.5P A/C junctions. With our devices we can achieve record-breaking high-speed performance, under zero-bias (forward-bias) conditions at an optical wavelength of either 1.55 or 0.85μm.6 Our results thus overturn the commonly held belief that high-speed PDs must be a power-consuming device under reverse bias conditions. We now plan to further optimize the epilayer of our devices and the geometric structures of the GaAs/InGaP UTC-PD to boost its operation speed to 25Gb/s under forward-bias operation (and to thus meet the increase in the required data rate of the optical interconnect channel at 850nm). In addition, we will try to integrate some MMW passive components with our GaAsSb/InP UTC-PD operated at 1.55μm, to achieve unique MMW-over-fiber applications with bias-free requirements.

Jhih-Min Wun, Jin-Wei Shi
Department of Electrical Engineering
National Central University
Taoyuan, Taiwan
Jhih-Min Wun is a PhD student. His current research interests include high-speed optoelectronic device measurements and sub-terahertz high-speed photodiodes.

1. A guide to building the green Internet. Accessed 19 February 2017.
2. M. A. Taubenblatt, Optical interconnects for high-performance computing, J. Lightwave Technol. 30, p. 448-457, 2012.
3. H. Chen, A. Beling, H. Pan, J. C. Campbell, A method to estimate the junction temperature of photodetectors operating at high photocurrent, IEEE J. Quant. Electron. 45, p. 1537-1541, 2009.
4. H. Ito, S. Kodama, Y. Muramoto, T. Furuta, T. Nagatsuma, T. Ishibashi, High-speed and high-output InP-InGaAs unitraveling-carrier photodiodes, IEEE J. Sel. Topics Quant. Electron. 10, p. 709-727, 2004.
5. T. Umezawa, K. Akahane, N. Yamamoto, A. Kanno, K. Inagaki, T. Kawanishi, Zero-bias operational ultra-broadband UTC-PD above 110 GHz for high symbol rate PD-array in high-density photonic integration, Opt. Fiber Commun. Conf. Exhib., p. M3C.7, 2015.
6. J.-M. Wun, R.-L. Chao, Y.-W. Wang, Y.-H. Chen, J.-W. Shi, Type-II GaAs0 .5 Sb0 .5 /InP uni-traveling carrier photodiodes with sub-THz bandwidth and high-power performance under zero-bias operation, J. Lightwave Technol., 2016. doi:10.1109/JLT.2016.2606343
7. J.-W. Shi, C.-Y. Tsai, C.-S. Yang, F.-M. Kuo, Y.-M. Hsin, J. E. Bowers, C.-L. Pan, GaAs/In0 .5 Ga0 .5 P laser power converter with undercut mesa for simultaneous high-speed data detection and DC electrical power generation, IEEE Electron Device Lett. 33, p. 561-563, 2012.
8. J.-M. Wun, H.-Y. Liu, Y.-L. Zeng, S.-D. Yang, C.-L. Pan, C.-B. Huang, J.-W. Shi, Photonic high-power continuous wave THz-wave generation by using flip-chip packaged uni-traveling carrier photodiode and a femtosecond optical pulse generator, J. Lightwave Technol. 34, p. 1387-1397, 2016.

OT- LUNA Innovations Blog-Luna at Work to Purify Water Using Solar Energy

The availability of fresh water is recognized as a global issue of strategic relevance, with demand increasing due to factors like population growth and agricultural and industrial demands. Many conventional technologies such as reverse osmosis (RO) work well in established communities with infrastructure and wealth to fund their operation but are not cost-effective for poor or remote communities or when traditional infrastructure is down, as in the case of a natural disaster. Even with technologies like RO, there is also a pressing need for reducing the cost of larger-scale desalination.
Seawater desalination represents the most technologically reliable and sustainable way to produce drinkable water to supplement increasing demands on already overtaxed fresh water resources. The global desalination capacity is above 86.8 million m3/day[1] and is expected to reach about 180 million m3/day by 2024 and 280 million m3/day by 2030.[2] The market is dominated by reverse osmosis, which due to its high-pressure operation, (≥35 bars) requires that it utilize electricity and expensive high-pressure-rated pumps, tubing and membrane modules, being poorly suited in isolated impoverished environments.[3]
Membrane distillation (MD) is an emerging thermal, membrane-based separation process. The driving force is the vapor pressure difference across the hydrophobic macro-porous membrane, resulting in vapor flows from the feed to the permeate side, where it condenses. Because only vapor can permeate, non-volatile constituents like salt present in the influent water are almost 100% rejected. Furthermore, MD may be a promising alternative or at least a complement to RO, not being limited by osmotic phenomena and having the potential to produce desalted water at recovery factors higher than 85%. However, current MD technologies still suffer from high energy consumption and low water production rate due to low thermal efficiency caused, in part, by temperature polarization, heat loss through brine discharge, and feed temperature drop across the membrane module. One big hurdle to wide-spread commercialization of MD involves membrane fouling and degradation.

Figure 1: Solar MD Concept
Figure 1: Solar MD Concept

Luna is developing and demonstrating a third type of MD-based desalination processes –  nanophotonics enhanced direct solar membrane distillation (Figure 1). Our novel, green, solar powered water desalination process demonstrates high capability for seawater desalination with low energy consumption, driven entirely by the solar energy.
Luna’s nanophotonic technology is a unique approach to help resolve the current MD challenges. Luna combines varied nanosystems with commercially available, low-cost polymers to create a nanoenhanced MD membrane on an existing, commercially available MD membrane substrate.  This can then be housed in a very simple, low-cost plastic module to generate pure water from feed sources like seawater, brackish water, sewage/industrial run-offs and contaminated or otherwise undrinkable water. In this compact design, the high effeciency solor-thermal convesion material is integrated into the MD module for elimnation of the temperature polarization. Flat sheet membranes coated with our proprietery nanosystems are placed in a module with a transparent window and the hot water flux directly generated at the localized membrane surface under the solar irradiation. Unlike in traditional MD, where the driving force of vapor production (temperature difference) diminishes over the module length thereby leading to significantly lower water production in long and densely-packed MD modules, Luna’s process successfully overcomes this drawback. At laboratory demostration, our nanophotnic desalination process was able to achieve <175 ppm TDS (total dissolved solids) fresh water output at zero external energy consumption, which is <35% of the maximum TDS limit for drinking water of 500 ppm.[1]
Luna’s nanophotonic desalination technology uses inexpensive membranes already optimized for MD process. The membrane housing is also inexpensive due to its low-pressure requirements, as opposed to RO’s.  The housing is chemically stable and inert, thermally stable and durable, and adaptable to a variety of environments/conditions. Our all-in-one solar MD system does not need to have separate hot tanks, solar cells, membrane housings, or pumps in most cases.
The same enabling technology can also be used to make water of greater purity than even drinking water, such as for various medical applications. This is due to a combination of factors including almost complete rejection of non-volatile components and sterilization of the permeate vapor since the nanophotonic layer can create localized steam at above 121 °C.
Luna’s evolving nanophotonics enabling technology continues to be optimized to improve the performance and cost-effectiveness of this novel zero-energy water distillation process targeted to cost-effectively enhancing global RO systems and enabling broad utilization of small-scale distributed water production, meet the needs of remote/isolated communities in the military and civilian sectors, and help eliminate the lack of clean, drinking water suffered by many around the world.
3 “Analysis of Global Desalination Market,” Frost & Sullivan, September 2015 
4 Kalogirou, S. A. Seawater desalination using renewable energy sources. Progress in Energy and Combustion Science 2005, 31 (2005), 242–281.

Abstract-Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3

Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle inter-relation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights, by tracking the relaxation kinetics following excitation at frequencies related to the broken symmetry state. Here, we investigate the transient terahertz conductivity of BaPb1-xBixO3 - a material for which superconductivity is adjacent to a competing CDW phase - after optical excitation tuned to the CDW absorption band. In insulating BaBiO3 we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaPb0.72Bi0.28O3 (superconducting below Tc=7K), a similar response was also found immediately above Tc. This observation evidences the presence of a robust gap up to T=40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behaviour was observed in the same material fo T>40 K. Here, the photo-conductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above Tc in underdoped BaPb1-xBixO3, and provide new information on the dynamical interplay between superconductivity and charge order.

Sunday, June 25, 2017

Abstract-Analytical Characterisation of the Terahertz In-Vivo Nano-Network in the Presence of Interference Based on TS-OOK Communication Scheme

 Rui Zhang,  Ke Yang,  Qammer H. Abbasi,  Khalid A. Qaraqe, Akram Alomainy

This figure shows the average SINR versus communication distance for different node densities in human blood (red), skin (blue) and fat (green) tissues . High node density potentially impairs the system performance and SINR drops about 10 dB when the node density rises one order. The effective communication distance for dense in-vivo nano-networks is restrained to about 1 mm

The envisioned dense nano-network inside the human body at terahertz (THz) frequency suffers a communication performance degradation among nano-devices. The reason for this performance limitation is not only the path loss and molecular absorption noise, but also the presence of multi-user interference and the interference caused by utilising any communication scheme, such as time spread ON—OFF keying (TS-OOK). In this paper, an interference model utilising TS-OOK as a communication scheme of the THz communication channel inside the human body has been developed and the probability distribution of signal-to-interference-plus-noise ratio (SINR) for THz communication within different human tissues, such as blood, skin, and fat, has been analyzed and presented. In addition, this paper evaluates the performance degradation by investigating the mean values of SINR under different node densities in the area and the probabilities of transmitting pulses. It results in the conclusion that the interference restrains the achievable communication distance to approximate 1 mm, and more specific range depends on the particular transmission circumstance. Results presented in this paper also show that by controlling the pulse transmission probability and node density, the system performance can be ameliorated. In particular, SINR of in vivo THz communication between the deterministic targeted transmitter and the receiver with random interfering nodes in the medium improves about 10 dB, when the node density decreases one order. The SINR increases approximate 5 and 2 dB, when the pulse transmitting probability drops from 0.5 to 0.1 and 0.9 to 0.5.