Measurements of High-Field THz Induced Photocurrents in Semiconductors

THz pulses have provided a useful tool for probing, with time resolution, the free carriers in a system. The development of methods to produce intense THz radiation has been slow since spectroscopists and condensed matter physicists first began probing materials with THz pulses. We have developed a method for producing intense ultra-short THz pulses, which have full width half maximum of 300 fs - approximately a half cycle of THz radiation. These intense half cycle pulses (HCPs) allow us to use THz radiation not only as a probe of the free carriers in a system but also as a source of excitation to alter a system in some way. In particular, HPCs perturb free carriers considerably in short time scales but show minimal effect to individual free carriers over long time. By exposing the semiconductor indium antimonide (InSb) to our intense THz HCP radiation, we have observed non-linear optical effects which suggest the generation of new free carriers by below band-gap THz photons. This generation of free carriers appears to be caused by an avalanche multiplication process, which should amplify the number of free carriers already in the system and then induce a current in the timescale of our THz pulse. This amplification on such a short timescale suggests the possibility of an ultra-fast detector of weak above band-gap radiation. We constructed a device which detects these currents by painting an electrode structure on the surface of the semiconductor. The currents induced across the electrodes by this avalanche multiplication process were measured and compared with other measurements of this non-linear optical process. We successfully measured THz induced currents in InSb, which indicate promise towards the development of an ultra-fast detector, and we gain insight into a possible physical explanation of the THz induced free carriers we observe in InSb

Measurements of High-Field THz Induced Photocurrents in Semiconductors

THz pulses have provided a useful tool for probing the time-resolved dynamics of free carriers in a system. However, the development of methods to produce intense THz radiation has been slow. We have developed a method for producing intense ultra-short THz pulses, which have a full width at half maximum of 300 fs — approximately a half cycle of THz radiation. These intense half cycle pulses (HCPs) allowed us to use THz radiation as a source of excitation. By exposing the semiconductor indium antimonide (InSb) to intense THz HCP radiation, we have observed non-linear optical effects which suggest the generation of new free carriers by below band-gap THz photons. This generation of free carriers appears to be due to an avalanche multiplication process which then induces a current in the time-scale of our THz pulse. This amplifi cation on such a short timescale suggests the possibility of an ultrafast detector of weak above band-gap radiation. We constructed a device which detects these currents by painting an electrode structure on the surface of the semiconductor. The currents induced across the electrodes by this avalanche multiplication process were measured and compared with other measurements of this non-linear optical process. We successfully measured THz induced currents in InSb, suggesting promise towards the development of an ultra-fast detector. Further, we have gained insight into a possible physical explanation of the THz induced free carriers we observe in InSb

An analysis of commercial opportunities between Brazil and Russian Federation

Thesis (S.M.)--Massachusetts Institute of Technology, Sloan School of Management, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. [59]-[62]).This paper begins with the assumption that trade between Brazil and the Russian Federation is underdeveloped. It thus takes the view that there are other product categories, which presents great chances for further development resulting not only in financial gains, but also in closer collaboration between the governments. Through a series of analysis of public data on international trade between countries, literature and news review it is found that currently bilateral trade between Brazil and Russia is concentrated in a restricted number of products, mainly quasi-commodities. In order to stimulate the development of further bilateral trade, the paper focus on a few products that have potential for further commercial exploration.by Nicholas Vana Ferenczi Semelman Wiczer.S.M

Equilibrium sovereign default with endogenous exchange rate depreciation

Sovereign default is often associated with disturbances in a country’s trade relations. Often the defaulter’s currency depreciates while trade volume falls drastically. This paper develops a model to incorporate real depreciation along with sovereign bankruptcy. The exchange rate is determined in equilibrium as the relative price of imports. We demonstrate that a default episode can imply up to a 30% real depreciation. This matches the depreciations observed in crisis events for developing countries. We argue that much of the exchange rate movement is explained by market clearing adjustments to trade disruptions in the aftermath of default

Equilibrium sovereign default with endogenous exchange rate depreciation

Sovereign default is often associated with disturbances in a country’s trade relations. Often the defaulter’s currency depreciates while trade volume falls drastically. This paper develops a model to incorporate real depreciation along with sovereign bankruptcy. The exchange rate is determined in equilibrium as the relative price of imports. We demonstrate that a default episode can imply up to a 30% real depreciation. This matches the depreciations observed in crisis events for developing countries. We argue that much of the exchange rate movement is explained by market clearing adjustments to trade disruptions in the aftermath of default

Enhanced Fatty Acid Oxidation and FATP4 Protein Expression after Endurance Exercise Training in Human Skeletal Muscle

FATP1 and FATP4 appear to be important for the cellular uptake and handling of long chain fatty acids (LCFA). These findings were obtained from loss- or gain of function models. However, reports on FATP1 and FATP4 in human skeletal muscle are limited. Aerobic training enhances lipid oxidation; however, it is not known whether this involves up-regulation of FATP1 and FATP4 protein. Therefore, the aim of this project was to investigate FATP1 and FATP4 protein expression in the vastus lateralis muscle from healthy human individuals and to what extent FATP1 and FATP4 protein expression were affected by an increased fuel demand induced by exercise training. Eight young healthy males were recruited to the study. All subjects were non smokers and did not participate in regular physical activity (<1 time per week for the past 6 months, VO2peak 3.4±0.1 l O2 min−1). Subjects underwent an 8 week supervised aerobic training program. Training induced an increase in VO2peak from 3.4±0.1 to 3.9±0.1 l min−1 and citrate synthase activity was increased from 53.7±2.5 to 80.8±3.7 µmol g−1 min−1. The protein content of FATP4 was increased by 33%, whereas FATP1 protein content was reduced by 20%. Interestingly, at the end of the training intervention a significant association (r2 = 0.74) between the observed increase in skeletal muscle FATP4 protein expression and lipid oxidation during a 120 min endurance exercise test was observed. In conclusion, based on the present findings it is suggested that FATP1 and FATP4 proteins perform different functional roles in handling LCFA in skeletal muscle with FATP4 apparently more important as a lipid transport protein directing lipids for lipid oxidation