Materials Science Section

Research Highlights

1.Development of customized GaAs p-i-n photodetectors as per user requirements

15 Nos of the specified single and dual elements based GaAs p-i-n photodetectors are developed by performing the device structure growth by MOVPE, multistage processing, bonding, testing and packaging. The developed detector works in the wavelength range of 325 to 850 nm with maximum responsivity of 0.58 A/W and the dark current remains < 1 nA. Temporal response of the developed detector is < 2.5 ns measured from ultrashort laser pulses. The integrated electronic circuit based on preamp, comparator and microcontroller along with in-house developed detectors are used for early arc fault detection which prevents the damage of the system by powering -off the equipment in < 1 ms. Three sets of developed detectors along with electronics are deployed in the RF circulator. In which a provision for early arc detection and the feedback mechanism for power control have been provided. No arc is detected in the circulator that was operated upto 37.5 kW power. The dual element based balanced photodetectors are also developed. The developed balanced photodetector along with amplifier is used for identifying the two laser beam originated from the same source having 1micro watt power difference. Typically 38mV amplified signal for 1micro watt power difference between the two beams is detected in the oscilloscope. Experiment is demonstrated to users and results are discussed. These balanced detectors with electronics are being packaged in a suitable metal carrier box and will be given to users.

Fig. A. 1 Photographs of indigenously developed a) single b) & c) dual element GaAs based p-i-n photodetectors, d) balanced detector along with electronics
Fig. A. 1 Photographs of indigenously developed a) single b) & c) dual element GaAs based p-i-n photodetectors, d) balanced detector along with electronics
Status on 20 March, 2020

2.Establishment of a setup for de-coherence measurements down to 100 femtosecond time scale

A setup for the measurement of de-coherence time down to 100 femtosecond time scale has been established. A linear polarized, 680 nm beam, which was generated by using a photonic crystal fiber and selected by prism pair-slit combination was used for the excitation of sample. The temperature of the sample can be varied in the range of 12 to 300 K. A wse2 mono-layer flake of dimensions ~10 μm mounted in a cryostat at temperature 12 K was used as the sample. Using a 50 X objective, a microscope was assembled to view samples of size less than 3 μm. The same microscope was also used to collect the PL signal emitted by the sample. Rotation angle dependent polarization measurements showed emission of coherent PL from the wse2 monolayer sample. Transient reflectivity measurement on a standard fast responding sample indicated that the setup can measure de-coherent time down to 90 fs. Further measurements on the coherent PL detection are in progress.

Fig. A.2 Photograph of the setup developed for the detection of coherent PL along with a photograph of imaged flake, graphs showing the time resolution and de-coherence measurements are also shown
Fig. A.2 Photograph of the setup developed for the detection of coherent PL along with a photograph of imaged flake, graphs showing the time resolution and de-coherence measurements are also shown
Status on 12 March, 2020

3.Role of Hot Electrons in the Development of GaAs-Based Spin Hall Devices with Low Power Consumption

Strong Inverse Spin Hall Effect (ISHE) signal can be realized in GaAs via the electrical injection of hot electrons. In a recent article [P. Mudi et al., J. Appl. Phys. 126, 065703 (2019)], we have demonstrated that optically induced hot electrons can also lead to the establishment of ISHE in GaAs but at much lower electric field. In this letter, we examine the combined role of high electric field and high energy optical injection of hot electrons in GaAs. An anomalous behavior of ISHE voltage (VISHE) is observed. Spin polarized hot-electrons are optically injected in the conduction band of n-GaAs which are then transferred from Γ to L valley with or without applied bias. At 2.33 eV excitation energy (Eex), a monotonous increase of VISHE is seen with electric field which starts to fall beyond a critical electric field. A theoretical framework based on the rate equations governing inter valley scattering is proposed, which successfully explains the observed behavior. Utilizing optically injected hot-electrons at Eex=2.33 eV, a Spin Hall device is demonstrated, that consumes less power and yield better signal-to-noise ratio. These findings are highly encouraging for the development of next generation spin-optoelectronic devices.

Figure A.3.1 (a) Longitudinal current (I<sub>||</sub>) plotted as a function of applied electric field (E<sub>||</sub>) for S1 and S2, (b) ISHE voltage plotted with applied electric field (E<sub>||</sub>) for E<sub>ex</sub> = 1.65 eV and 12 mW laser for S1.
Figure A.3.1 (a) Longitudinal current (I||) plotted as a function of applied electric field (E||) for S1 and S2, (b) ISHE voltage plotted with applied electric field (E||) for Eex = 1.65 eV and 12 mW laser for S1.
Figure A.3.2 (a) V<sub>ISHE</sub> measured as a function of external electric field (E<sub>||</sub>) at E<sub>ex</sub> = 2.33 eV, 12 mW laser power for S1, the dotted curves show the contribution of the first and second term of right hand side of Equation. 1 (b) Numerical estimation of L valley spin polarization (PL), occupation probability (NL) and V<sub>ISHE</sub> against external electric field at E<sub>ex</sub> = 2.33 eV.
Figure A.3.2 (a) VISHE measured as a function of external electric field (E||) at Eex = 2.33 eV, 12 mW laser power for S1, the dotted curves show the contribution of the first and second term of right hand side of Equation. 1 (b) Numerical estimation of L valley spin polarization (PL), occupation probability (NL) and VISHE against external electric field at Eex = 2.33 eV.

Priyabrata Mudi, Shailesh K. Khamari, and Tarun K. Sharma Phys. Stat. Sol. RRL Appeared online on 16 Apr. 2020 {https://doi.org/10.1002/pssr.202000097}
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