Probing Semiconductor Nanostructures by a Pulsed Phase-Lock-Loop System

1. Probing Semiconductor Nanostructures by a Pulsed Phase-Lock-Loop System Yuen-Wuu Suen Department of Physics, National Chung Hsing University 孫允武 中興大學物理系 1

2. OUTLINES • How it works • The home-brewed pulsed phase-lock-loop system • Some preliminary results for two-dimensional electron systems (2DES) • What can we do next? 2

3. Detection by Phase Lock Loop (PLL) fs=bsls PLL system phase=f1=b1l1 sample f0 =f1+ fs =b1l1+bs(B)ls Df0 =0 (by tuning w)=Df1+Dfs(B) =Db1l1+Dbs(B)ls =Dwl1/u1+Dbs(B)ls B:the parameter (magnetic field, excitations, temperature, etc) changed in the experiment known Dw can be measured very accurately. 3

4. Basic scenario Sample under detection Type-II PLL SAW Delay-Line Sample under detection Coplanar Waveguide (CPW) 4

5. SAW Delay-Line L l GaAs:3.6×10-7W-1GaAs/LiNO3(Y-Z):1.8×10-6W-1 5

6. Coplanar Waveguide (CPW) 50Wmeandering CPWtotal length ls Electric field 6

7. Some formulae about lossy CPW: 7

8. Where to put the nanostructures (QDs, QWs, QXs…..) on the sensors? for CPW for SAW You don’t need to connect the QDs one by one! 8

9. What kind of information we can get? Microwave adsorption , dynamics at microwave frequencies… coming from: intraband adsorption cyclotron resonance spin flipping, spin rotations, spin-spin interaction, spin-orbit interaction---for “spintronics”?? spins E Magnetic field 9

10. FM Schematicof a homemade PLL system for microwave signals up to 18 GHz. The phase resolution is about 0.001 degree even under very low average input power (~-100dBm). A special designed homodyne amplitude detection scheme also allows us to detect very small microwave adsorption. 10

11. A homodyne amplitude detection scheme Ref. Signal (LO) 0º 90º mixer To PLL 90º hybrid ~0 To amplitude detection Power splitter A home-made vector meter?? Signal from the sample 11

12. Why pulsed? • Use low average power to prevent from heating • Use gated averaging technique to avoid direct EM interruption • Avoid the reflection and multiple reflection signals 12

13. s1(t) s2(t) signal of mixer time delay s4(t) signal after SH Direct coupled EM Reflected signals sampling delay set by a pulse generator s3(t) fed into lock-in sampling gateset by a pulse generatorfed into the controlling node of a sample-and-hold circuit Signal Gating & Averaging: ~200 ms set by a pulse generator Peak power about –30~-70dBm s1(t) RF/Microwave pulse train 0.2~2 ms set by a pulse shaping circuit 3~4 ms set by a lock-in amp 13

14. Typical data for CPW on 2DES: (b) (a) Re{sxx} Im{sxx} (a) The pattern of the meandering coplanar waveguide. (b) The amplitude and the frequency deviation Df vs magnetic field B are shown for f0=1.39GHz at T=0.3K. 14

15. More data: 15

16. Some plots for scaling analysis 16

17. Perspective We have developed a potent and very sensitive tool for studying microwave properties of low-dimensional systems. What Next? Put CPW on substrate with nanostructures. Put nanostructures on substrate with CPW. Add bias or other excitations. 17