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What Old Microwave Tricks Can Do in the New Nano Era?

Yuen-Wuu Suen

Department of Physics, National ChungHsing University

孫允武中興大學物理系

for semiconductor people of course

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OUTLINES:

1. What is so “NANO” in microwaves?

2. What do people think of using microwaves?

3. What I am doing about microwaves and makes them kind of “nano”!!

SCALES: time, length, energy

Some novel thinking!

Remember I am in the middle of Taiwan and between no where!

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About Microwaves:

Frequency: about 1GHz~40GHz

Sources up to about 1000 GHz are easily available!---so called millimeter (submillimeter) wave!

FROM ELVA-1, RUSSIA

http://www.elva-1.spb.ru/

Wavelength:NOTHING NANO????

100MHz 1GHz 10GHz 100GHz 1000GHz

3m 30cm 3cm 3mm 300m(in vacuum)

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Energy:

100MHz 1GHz 10GHz 100GHz 1000GHz

3m 30cm 3cm 3mm 300m(in vacuum)

hE

Something I can never remember:h=6.63×10-34J·s =4.136×10-15eV·skB =1.38×10-23J·K-1 =8.617×10-5eV·K-1

E/kB 5mK

(4.8)

50mK 0.5K 5K 50K

4×10-7eV 4×10-6eV 0.4meV0.04meV 4meVE

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Comparison Between Energy Scales

E/kB 5mK 50mK 0.5K 5K 50K

4×10-7eV 4×10-6eV 0.4meV0.04meV 4meVE

Bandgap of semiconductors about 0.2~3eV

Conduction-band or valence-band discontinuity

about 30~900meV

Fermi energy for electrons in a QW

Energy spacing for a electron in a typical quantum well (QW)

Energy of an optical phonon about 30~50meV

Ionization energy of shallow donors or aceptors

about 10~50meV

about 30~100meV

1GHz 100GHz

few meV

1THz

Exciton binding energy about 5~30meV

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Let’s talk about something related to microwavesFree carrier absorption E

k

EFphoton

phonon

m

Np

2 At low f (compared to 1/scatter), it is

just joule heating.

1~

4

)()(

22

22

2

cf

pcr

cp

m

en

cn

nf (cm-3) 1022 1018 1014 1010

fp (Hz) 0.9×1015 0.9×1013 0.9×1011 0.9×109

Some possibility here for quantum dots or low-dimensional systems

try some numbers:

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Besides microwave radars, satellite communications, ovens…, what else microwaves can do?

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Let’s try electron spins and magnets!!The famous 21 cm

line???(If you know quantum mechanics very well, it comes from the hyperfine interaction. H=AS1·S2)

H

0.53Å Very “NANO”!!

f0=1.42GHz Slower than your P4!

Electron spin splittings (electron spin resonance ESR)

For free electrons f0(GHz)=28.0B(tesla)In semiconductors E=g*BB

GaAs 0.44, GaN 1.98, InSb –51, Ge –2.5, Si 1.98

g=2

depend on host semiconductors and fields

Radio Astronomy

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Cyclotron resonance

m

eBc

Bz

fc(GHz)=28.0(m0/m*)B(tesla)

For electrons confined in a two-dimensional interface

BgnE Bc 2

1)

2

1(

Landau levels

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What can you do about the length scale?

Why is it so long compared to “NANO”?

The light travels so fast!! even divided by a reflective index.

Then let’s translate it to something (somewave) slower. Maybe it will become more “NANO”.

=c/nf

It comes out to be an acoustic wave.

Or you want it on a surface, and then it should be a surface acoustic wave (SAW).

cs~ 3000 m/s~ 10-5c

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100MHz 1GHz 10GHz 100GHz 1000GHz

3m 30cm 3cm 3mm 300m

E/kB 5mK

(4.8)

50mK 0.5K 5K 50K

4×10-7eV 4×10-6eV 0.4meV0.04meV 4meVE

SAW 30m 3m 300nm 30nm 3nm

1013Hz 1014Hz 1015Hz 1016Hz 1017Hzf for EM of the same as SAW

40meV 400meV 40eV4eV 400VE

Let’s see the scales again!

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How to generate SAWs?First you need a piezoelectric substrate.

Electric field Mechanical strain

You must place an inter-digit (comb-like) electrode on the surface as a transducer.

SAW

One can use optical or thermal methods---but less defined properties of SAWs.

OR

laserSAW

SAW

heat

You need an e-beam writer to get “NANO”.

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MORE about SAWs1. A SAW on the surface of a piezoelectric substrate travels with

a spatially modulated electric field, which gives a wave-like electric potential variation near the surface.

2. A SAW without an associated piezoelectric field is useless for microelectronics or nanoelectronics. For example, water wave.

3. The propagation properties of the SAW are very sensitive to the electrical or mechanical properties near the surface. Therefore, it is very useful for sensor applications.

4. The SAW is very useful on detecting the material properties of the length scale SAW.

5. The SAW can provide a controllable electrostatic-like field in the scale of SAW. Of course the distribution is flying.

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A SAW Delay Line as a Detector

Almost anything changed here can be detected.

You can build an oscillator including this SAW sensor, or you can hook up to an expensive vector analyzer to measure S’s.

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Anything GOOD to use SAW detectors?!

1. No contact!

2. Short wavelength compared to EM signals at the same frequency.

3. Low energy compared to EM signals at the same wavelength.

4. We can use SAWs to detect the special length scale in the sample via the size-resonance of SAWs and the sample.

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Besides those old goodies and odds, what’s new and “NANO”?

Some thinking……..

1. Make energy levels in nanostructures microwave active, so that one can use microwave doing something.

3. One can use SAW to drive electrons or holes to anywhere your want, where anywhere=nanostructures or quantum dots…….

2. One can use SAW sensor to detect some nano features in nanostructures.

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Microwave spectroscopy of a quantum-dot molecule

T. H. OOSTERKAMP et al Nature 395, 873 - 876 (1998)

The metallic gates (1, 2, 3 and F) are fabricated on top of a GaAs/AlGaAs heterostructure with a two-dimensional electron gas (2DEG) 100 nm below the surface.

Photon resonances in a double-dot sample.

Measured pumped current through the strongly coupled double-dot.   

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Determination of the complex microwave photoconductance of a single quantum dot

H. Qin, F. Simmel, R. H. Blick, J. P. Kotthaus, W. Wegscheider, and M. Bichler Phys. Rev. B 63, 035320 (2001)

Bias dependence of the quantum dot conductance in the vicinity of a single resonance.

Amplitude |A| of the photoconductance measurement obtained with the two-source setup.

Energy level alignment.

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Experimental setup for the two-source measurement:Two millimeter waves with a slight frequency offset generated by two phase-locked microwave synthesizers ( f =18.08 GHz and f =2.1 kHz) are added, doubled and filtered.

You can see some old microwave goodies in their setup.

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Single-electron acoustic charge transport on shallow-etched channels in a perpendicular magnetic field J. Cunningham et alPhysical Review B 62, pp. 1564-1567 (2000)

fSAW=2.716 25 GHz, I=nefSAW

They are trying to make a new electric current standard.

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Flying potential and flying electrons

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Quantum computation and spintronics

(a) Schematic diagram showing the effective potential due to a SAW passing across a Q1DC; (b) potential through the center of (a), parallel to the Q1DC.

Quantum computation using electrons trapped by surface acoustic waves C. H. W. Barnes, J. M. Shilton, and A. M. Robinson Physical Review B 62, pp. 8410-8419 (2000)

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Spin separation!! Spin operation!!

A Flying Qubitquantum bit

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Controlled-NOT gate

a SAW quantum-gate network

Toward a quantum computer

Probably you need to know some quantum mechanics before going any further!?

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Let’s talk about something opto….If I can drive electrons around, I think I can drive electrons and holes (or excitons) around. Then I want to select a place for them to recombine at the time I suggest…..and I want….

Acoustically Driven Storage of Light in a Quantum Well C. Rocke, S. Zimmermann, A. Wixforth, J. P. Kotthaus, G. Böhm, and G. Weimann, Phys. Rev. Lett. 78, 4099 (1997);

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What will happen if I drive electrons and holes to a quantum dot ( laser if you want)? Or to an array of quantum dots (lasers)… or …

Photon trains and lasing: The periodically pumped quantum dotC. Wiele, F. Haake, C. Rocke, and A. Wixforth

PHYSICAL REVIEW A 58, 2680 (1998)

Quantum-dot laser with periodic pumping

C.Wiele et alPhysical Review A 60, pp. 4986-4995 (1999)

(anything you can imagine)

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What are we doing in NCHU?

1. We have build a type-II phase lock loop (PLL) for pulsed microwave signal to detect very small phase variations due to absorption of microwave or SAW signals by electron systems. The resolution of the phase is better than 0.01 degree with average of –100dBm input power.The sensor is SAW delay line or coplanar waveguide.

2. We are setting up a simple e-beam writer.

3. We are fabricating high-frequency SAW transducers.

4. We are trying to digging small “nano” holes.

5. We are making lots of microwave connection cables.

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Pulsed RF/Microwave PLL and Gated Averaging System

Gated Average

Time Delay

RF or Microwave Generator

DC-Coupled Frequency Modulation (FM)

Directional Couplers

HighSpeed Diode Switch

Step Attenuator

Double-Balance Mixer

Pulse Generator

Amplifier

Power Splitter

Intensity Output

Power Detector

PLL

Integrator積分器

Precision Counter

(b1)

(I)

(C)

(M)

(D)

(E)

(P)

(G)

(H)

(s1)

(A)

(b2)

(s2)

(b3)

Stainless-StealSemirigid Coax

Stainless-StealSemirigid Coax

黑色 :低頻訊號橙色 :高頻訊號

(B)

Z Z

Cryogenic Environment

Impedance Match Network

Impedance Match Network

Active LDES Region

SAW Emitter IDT

SAW Receiver IDT

Sample

(F) (F)

(S)

(J)

Why pulsed?

1. Use low average power to prevent from heating

2. Use gated averaging technique to avoid direct EM interruption

3. Avoid the reflection and multiple reflection signals

What’s different from others:

We use type II PLL, home-brew sample-&-hold circuits, and cheap lock-in amplifiers.

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An improved homodyne amplitude detection scheme

0º 90º

Ref. Signal

Signal from the sample

90º hybrid

Power splitter

mixerTo PLL

To amplitude detection

~0

A home-made vector meter??

(if you still want to know some details, and still awake)

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Signal Gating & Averaging:

RF/Microwave pulse train

3~4 ms set by lock-in amp

~200 s set by lock-in amp

0.2~2 s set by pulse shaping circuit

s1(t)

s1(t)

sampling delay set by pulse generator

sampling gate set by a pulse generatorfed into the controlling node of a sample-and-hold circuit

s3(t)

time delays2(t) signal of mixer or power detector

Direct coupled EMReflected signals

s4(t) signal after SH

Peak power about –30dBm

fed into lock-in

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Our system is working------ Guess which one is the PLL system?

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A semiconductor chip attached on the SAW delay line

BeCu SR coax

IDT SAW transducer

Chip tied on the SAW delay line

He3 sample holder

5mm

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Detection by Phase Lock Loop (PLL)

Type-II PLL

Sample under detection

phase=1=11PLL system

s=ss

0 =1+ s =11

+s(B)s

0 =0=1+ s(B) =11+s(B)s

B:the parameter (magnetic field) changed in the experiment:velocity of the wave

can be measured very accurately.

sample

known

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Reference

From sample

Keep at a constant phase difference

Reference

From sample

Due to the change of sample conditions

Reference Tuning the frequency to match the phase

From sample

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SAW Delay-Line Sensor

2

2

00 )／(1

1

2 Mxx

effK

Lf

f

σ

2

2

SAW )/(1

/

2 Mxx

MxxeffKk

σσσσ

L

)( 210 M

GaAs:3.6×10-7 -1

GaAs/LiNO3(Y-Z):1.8×10-6 -1

2

,2, 00 fq xxxx

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Coplanar Waveguide (CPW) Sensor

Electric field

50 meandering CPWtotal length s

xx

0,0 qxx

m2

12

eff

D

dG

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Coplanar Waveguide (CPW) SensorSome formulae:

1m)()( CjGLjjj

m

F0 eff

LC ,

eff

r

C

LZ

0

00

11

m

12

12

eff

D

d

G

m

Np

2

m

rad

8

11

2

1

0

2 ln

Z

deffD

xx or 1

2/1

20

222 8

L

eff

s

Dxx Z

dfLf

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Data read from SAW delay line

0 2 4 6 8 10 12

SAW

Int

ensi

ty

0.001%

f

/f 0

B (Tesla)

=2

=1

ns=2.5×1011 cm-2f0=120MHzT=0.3KGaAs/AlGaAs 2DES

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0 2 4 6 8 10

0

2

4

6

8

10 A

mpl

itud

e (a

rb. u

nit)

200kHz=2 =1

f0=1.39GHz

B (T)

amplitude

f

Data read from CPW:

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0 2 4 6 8 10

f0=

T=0.3K

2.16G

=2 =1

1.74G

0.59G

0.96G

1.39G

500kHz

f

B (T)

2.92G

0 2 4 6 8 105

10

15

20

25

30

f0=

=2 =1

2.92G

2.16G

1.74G

1.39G

0.96G

0.59G

Am

plitu

de (

arb.

uni

t)

B (T)

More data:

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High frequency IDT pattern made by e-beam lithography

Still, not “nano” enough!

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Time delay response of a pulse microwave input

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So-Called

Flows ofMW modules,

Graduate students,

………..

Phys. Rev.

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1.<1m e-beam writer

UnderConstruction^0^

Nano…..Nano…..Nano…..Nano…..Nano…..Nano…..Nano…..Nano…..Nano…..Nano…..

3. Quantum dots, spins, spintronics

4. Buying source of higher frequency, maybe to THz.

2. Acoustoelectric effect V or A

spins