UFO/NTU

THz Optoelectronics

Chi-Kuang Sun

UltraFast Optics Laboratory (UFO)Graduate Institute of Electro-Optical Engineering and

Department of Electrical EngineeringNational Taiwan University

Taipei, TAIWAN

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Outline

Introduction (Motivation)MSM TWPD for 800 nmMSM TWPD for 1300 and 1550 nmTHz emitter (photomixer) based on MSM TWPDTHz Imaging SystemTHz biochipsSummary

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Demands of Ultrahigh Bandwidth Photonics

>10 Tbit/s Fiber Communication Backbone SystemWDM vs. TDM: Hybrid System

Local Fiber Communication Network

Microwave Photonics SystemFiber-Radio System

THz-Millimeter Wave Photonic Emitter

Ultrahigh Speed Photonic MeasurementActive microwave probe

Time domain Network-analyzer

Ultrahigh Speed Electronic Gating

Short Electrical Pulse Generator

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Traditional Ultrahigh Speed Vertical InGaAs/InP p-i-n Photodetector

50 nm p+ InGaAsand 400 nm p-InP

500 nm n-InP

6 µm

180 nm i-InGaAs

2 µm

p

in

alloyed p-metalair-bridged metal

metal reflector

air

InP:Fe substrate

light input

PMGIn-metal

SiNx anti-reflective coating

air-bridged metal

n-metal

p-mesa

n-mesa

I-H. Tan, C.-K. Sun, K. S. Giboney, J. E. Bowers, E. L. Hu, B. I. Miller and R. J. Capik, IEEE Photon.Technol. Lett., vol. 7, 1477-9 (1995).

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3 dB BANDWIDTH

2 Pm device, - 2 V bias

1.00.8

0.60.4

0.20.0

Nor

mal

ized

EO

Sig

nal

20151050Delay (ps)

FWHM 2.7 ps

-10

-8

-6

-4

-2

0

Mag

nitu

de (d

B)

250200150100500Frequency (GHz)

120 GHz

142 GHz (Deconvolved)

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Limiting Factors For Ultrahigh Speed Vertical p-i-n Photodetector

Transit timeBandwidth-efficiency product

- limited by absorbing layer thickness and device areaDiode RC time constant

layer thickness Cdevice area C R

Parasitic Capacitanceimportant when device area < 25 Pm2

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Early Approach

Thin absorption layer9 Decrease carrier transit time

- Not too thin to cause quantum confinementLow quantum efficiencyHigh capacitance

Small device area9 Decrease device capacitance

- Not too small to affect light couplingIncrease resistance

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SPACE CHARGE SCREENING EFFECT

Increase Pump Intensity1 .5

1 .0

0 .5

0 .0

2 01 51 050

2 .0

1 .5

1 .0

0 .5

0 .0

2 01 51 050

E

X

E

X

T (ps)

I

I

T (ps)

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SPACE CHARGE SCREENING EFFECT

13 fC 29 fC 43 fC 68 fC

-20

-15

-10

-5

0

20100

Time Delay (ps)

-2V

-3V

-4V

-5V-6

-5

-4

-3

-2

-1

0

1

EO

Sig

nal (

A.U

.)

20100

Time Delay (ps)

-2V

-1V

-3V

-4V

-5V-12

-10

-8

-6

-4

-2

0

2

20100

Time Delay (ps)

-1V

-2V

-3V

-4V

-5V -15

-10

-5

0

5

20100

Time Delay (ps)

-2V

-3V

C.-K. Sun, I-H. Tan, and J. E. Bowers, "Ultrafast transport dynamics of p-i-n photodetectors under high power illumination," IEEE Photonic Technology Letters 10(1), 135 (1998).

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How about Large Area device Instead of NanoDevices?

For THz Bandwidth

RC time constantReplace Lumped Circuit Model with Microwave Circuit Model

Device Transit TimeUse short carrier lifetime material

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The First p-i-n Traveling-Wave Photodetector

Light

pin

SI Substrate

Isolated2Z 0

2Z 0

1.47 ps FWHM

EO

Sig

nal (

AU

)

0 2 4 6 8 10

Time (ps)

-40

-30

-20

-10

0

10

Mag

nitu

de (d

B)

External Quantum

Efficiency (%)

1

10

100

0 200 400 600 800 1000

-3 dB @ 172 GHz

10% @ 400 GHz

1% @ 800 GHz

Frequency (GHz)

42

830 nm

QE ~ 50%

K. S. Giboney, M. J. W. Rodwell, and J. E. Bowers, IEEE J. Selected Topics in Quantum. Electron., vol. 2, 1996, pp. 622-629

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Advantage of Traveling-Wave Photodetector

Enhance quantum efficiency by decoupling carrier transport from light propagationReplace RC time constant limitation by velocity-mismatch bandwidth

C.-K. Sun and J. E. Bowers, "High Bandwidth Photodetectors," in The Femtosecond Technology, T. Kamiya, F. Saito, H. Yajima, and O. Wada, ed., Berlin Heidelberg, Springer-Verlag, pp. 134-151 (1999).

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low-temperature-grown GaAs p-i-n traveling wave photodetector

D.C. external quantum efficiencies as high as 8 % were obtained

Measurement

-30

-25

-20

-15

-10

-5

0

5

0 500 1000 1500 2000

CorrectionRel

ativ

e re

spon

se (d

B)

Freq(GHz)

~ -3dB 560 GHz

520 GHz

0 1 2 3 4 5 6 7 8

TheoryMeasurementCorrection~ 570 fs

530 fs

Time (ps)

Rel

ativ

e Ph

otoc

urre

nt re

spon

se

Y.-J. Chiu, S. B, Fleischer, and J. E. Bowers, “High-speed low-temperature-grown GaAs p-i-n traveling-wave photodetector,” IEEE Photon. Tech. Lett. 10, 1012 (1998)

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Bandwidth Limitation Factor of p-i-n TWPD

Carrier transport time or carrier lifetime

Velocity mismatch bandwidth

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Velocity-Mismatch Bandwidth in TWPD

The velocity mismatch 3 dB bandwidth for long TWPDs with matched input termination (J=0) is given by

)(2/ eoeovm vvvvB �* SD

22 52/2 eoeovm vvvvB �* SD

while the velocity mismatch 3dB bandwidth for open-circuit input termination (J=1) is given by

SD 3/ evm vB *|

which can be closely approximately by a single-pole response with a bandwidth of over the entire practical range of the velocity mismatch for slow-wave mode devices.

JJ

C.-K. Sun and J. E. Bowers, "High Bandwidth Photodetectors," in The Femtosecond Technology, T. Kamiya, F. Saito, H. Yajima, and O. Wada, ed., Berlin Heidelberg, Springer-Verlag, pp. 134-151 (1999).

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Microwave loss in TWPD

The measured field loss coefficiency in LTG-GaAs p-i-n TWPD

Frequency (GHz)

10 100 100010-5

0.0001

0.001

0.01

0.1

Fiel

d at

tenu

atio

n co

effic

ienc

y (P

m-1

)

Y.-J. Chiu, S. B, Fleischer, and J. E. Bowers, “High-speed low-temperature-grown GaAs p-i-n traveling-wave photodetector,” IEEE Photon. Tech. Lett. 10, 1012 (1998)

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Challenges for Ultrahigh Power/Speed PDs

6ORZ�ZDYH�PLFURZDYH�PRGH�LQ�SLQ�7:3'slow microwave velocityhigh microwave loss

Low saturation power

Our Solution:

9MSM TWPD¾Quasi-TEM microwave mode¾ high internal field (external Bias)

* J.-W. Shi, et al., IEEE Photon. Techno. Letters, 13, pp. 623-625, June, 2001.

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Cross Sectional View of MSM TWPD

LTG-GaAs

S. I. GaAs

300nm

200nm

400nm

1μm

3μm

Optical cladding layer

Optical isolation layer Al0.5Ga0.5As

Al0.2Ga0.8As

Al0.3Ga0.7As

2μm

Metal

(110)

J.-W. Shi, K.-G. Gan, Y.-J. Yang, Y.-H. Chen, C.-K. Sun, Y.-J. Chiu, and J. E. Bowers, “Metal-semiconductor-metal traveling-wave photodetectors,” IEEE Photonic Technology Letters 13 (6), 623-625 (2001).

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Top View of MSM TWPD

Integrated CPW line

Self aligned photo-absorption region

Flared out CPW region for microwave probe

hv hv

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Traveling-wave structure5HPRYH�5&�WLPH�FRQVWDQW�OLPLWDWLRQ(QKDQFH�TXDQWXP�HIILFLHQF\�E\�GHFRXSOLQJ�FDUULHU�WUDQVSRUW�IURP�OLJKW�SURSDJDWLRQLonger absorption length9 Lower carrier density9 higher output power9 Less bandwidth saturation

MSM structureQuasi-TEM Mode vs. Slow wave mode (p-i-n)9 Low microwave loss 9 High microwave velocity and high velocity-matching bandwidth

Easier impedance matching 9 Less Boundary reflection*

Higher Bias9 Less output voltage saturation

Low-temperature-grown GaAsShort carrier trapping time 9 Fast response9 Less space charge screening

*J.-W. Shi, C.-K. Sun, Journal of LightwaveTechno., 18, pp. 2176-2187, 2000.

Advantages of MSM TWPD

oe vv !

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800 nm DC Measurement

1.0 1.5 2.0 2.5 3.0

40

60

80

100

120

140

8.1% Fitting line

Bias 15V

Phot

ocur

rent

(PA)

Optical Power (mW)

• Similar to p-i-n TWPD*

• Much better than MSM VPD • Output photocurrent is linear vs. input optical power

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EO Sampling System

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Record Bandwidth Performance of MSM TWPD at 800 nm

-8 -6 -4 -2 0 2-0.2

0.0

0.2

0.4

0.6

0.8

1.0 FWHM:0.8psphoto-generated charge:120 fC

E-O

sig

nal (

A.U

.)

Time (ps)0 200 400 600 800 1000

0.0

0.5

1.0

1.5

2.0

-3dB @ 570GHz

Freq

uenc

y R

espo

nse

Frequency (GHz)

LTG-GaAs MSM TWPD vs. pin TWPD• Similar QE (~8%)• Improved bandwidth and output power

570GHz/120fC vs. 520GHz/7fC* J.-W. Shi, et al., IEEE Photon. Techno. Letters, 13, pp. 623-625, June, 2001.

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Record-High Peak-Output-Power-Bandwidth Product

0 5 10 15 20-200

0

200

400

600

1.8 ps

Pho

tocu

rren

t (m

A)

Time (ps)

Vp (peak voltage: ~30V) X Electrical Bandwidth(190GHz) = 5.7 THz-V

Limited by the bias voltage (30V)

Bias Voltage: 30VPeak Output Voltage: ~30V

Photo-charge:~2100 fC

0 50 100 150 200 250-5-4-3-2-1012

-3dB@190 GHz

Frequency (GHz)P

ower

(dB

)

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Record-High Peak-Output-Power-Bandwidth Product

Vp (V) /Photo-charge (fC)

Electrical bandwidth (GHz) / Response time(ps)

LTG-GaAs pin TWPD1 1400 fC 6 ps

GaAs pin TWPD2 59 fC ~5.5 ps

InGaAs pin VPD3 ~68 fC 7.2 ps

Uni-Traveling Carrier PDs (UTC-PD)4

4.6 V 94GHz/4.6 ps

Velocity Match Distributed PD5

2.5V 40~50GHz

LTG-GaAs MSM TWPD

30V/2100fC 190GHz/1.8 ps

1. Y. J. Chiu, et al., IEEE Photon. Tech. Lett., 10, pp.1012-1014, 1998. 2. K. S. Giboney, et al., IEEE Journal Of Selected Topics In Quantum Electronics.,2, pp. 622, 1996.3. C.-K. Sun, I.-H. Tan, and John E. Bowers, IEEE Photon. Techno. Letters, 10, pp. 135-137, 1998.4. K. Kato, IEEE Trans. Microwave Theory Tech., 47, pp.1265,19995. L. Y. Lin, et al., IEEE Trans. Microwave Theory Tech., 45, pp. 1320-1331, 1997.

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Record-High Peak-Output-Power-Bandwidth Product

UCLA†

• 0.125 THz-V• 50 GHz• 2.5 V• VMPD

NTT*

• 0.432 THz-V• 94 GHz• 4.6 V• UTC-PD

NTU-UFO Group

• 5.7 THz-V• 190 GHz• 30 V• MSM TWPD

* Microwave Photonics Conference, paper T-5.1 (1999)† IEEE Tran. Microwave Theory and Tech. 45, 1320 (1997)

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Advantages of LTG-GaAs Based TWPD in Long Wavelength Regime (1.3~1.55Pm)

Lower cost of GaAs than InPLarger wafer size of GaAsMature processing technique

Sub-picosecond electron trapping time in LTG-GaAs vs. several-picosecond carrier trapping time in LTG-InGaAs

Ultra-high speed performance

Low photo-absorption constantLarge photo-absorption volumeUniform absorption High output power

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The Absorption Mechanism of LTG-GaAs in Long Wavelength Regime

C.-K. Sun, et al., “Electron relaxation and transport dynamics in low-temperature-grown GaAs under 1eV optical excitation,” Applied Physics Letters 83, pp. 911-913 (2003) .

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Impulse response of MSM TWPD at ~1.3Pm Wavelength

E-O sampling system: Cr4+:forsterite laserWavelength: ~1.3Pm (1230nm)

0 5 10 150

5

10

15

20

25

1.28 ps

Out

put C

urre

nt (m

A)

Time (ps)0 50 100 150 200 250 300

-6

-4

-2

0

2

-3dB @ 234GHz

Elec

trica

l Res

pons

e (d

B)Frequency (GHz)

70Pm device length (absorption volume a���Pm3 ) + Superior microwave guiding

= 234 GHz Ultrahigh Bandwidth

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Record Power-Bandwidth Product for Telecommunication Wavelength

0 5 10 15

0

15

30

45

60

75

2.1ps

Out

put C

urre

nt (m

A)

Time (ps)0 50 100 150 200 250 300

-6

-4

-2

0

2

-3dB@160GHz

Frequency (GHz)P

ower

(dB

)• Optimum bias voltage and Input optical energy: 10V, 27pJ/pulse• Record peak output voltage (3.55V)-bandwidth (160GHz) product (in

telecommunication wavelength regime: 570 GHz-V*vs. **UTC-PD: 430 GHz-V (~4.8 ps, 94GHz, Vp:4.6V).

*J.-W. Shi, et al., IEEE Photon. Techno. Lett., 14, pp. 363-365, 2002.**Microwave Photonics Conference, paper T-5.1, 1999

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Applications of Ultra-High Bandwidth-Output Photodetector

9Photo-receiver circuit without electrical amplifier *optical-fiber amplifier

9 Photomixer**

RF source with high tunability and output power

*K. Kato, IEEE Trans. Microwave Theory Tech., 47, pp. 1265-1281, 1999**S. Verghese, et al., IEEE Trans. Microwave Theory Tech., 45, pp. 1301-1309, 1997.

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What is THz Waves

1 Tera = Occupies the 100GHz – 10THz spectrumSubmillimeter or millimeter waves

1210

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Properties of THz Waves

Material-dependent transmissionPolar liquid => strong absorptionNon-polar substance => nearly transparentMetal => opaqueDielectrics => characteristic absorption

Polar molecular recognitionMaterial characterizationQuality controlSecurity Checketc..

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Applications of THz Technology

Daniel van der Weide, “Electronic Terahertz Technology,”Optics & Photonics News, pp. 49-53, April 2003.

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Collective Vibrational Modes of Biological Molecules

“Collective Vibrational Modes in Biological Molecules Investigated by Terahertz Time-Domain Spectroscopy,” M. WALTHER, P. PLOCHOCKA, B. FISCHER, H. HELM, P. UHD JEPSEN, Biopolymers (Biospectroscopy) 67: 310–313, 2002

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Methods to Generate THzPhotoconductive switch1

Difference frequency in nonlinear crystal2Optical rectification

Resonant tunnel diode(RTD) oscillator arrays3

Fixed low oscillation frequency Complex structure

P-type Ge-strained laser4/ Quantum cascade laser

Operating at cryogenic temperatureFree electron laser5

Highest power but expensivePhotonic transmitters based on photodetectors

1. D. H. Auston, et al., Appl. Phys. Letters, 45, pp. 284-286, 1984.2. Wei Shi, and Y. J. Ding, CLEO 2002 Technical Digest, pp. 145-146.3. M. Reddy, et al., IEEE Electron Device Letters, 18, pp. 218-221, 1997. 4. E. Brundermann, et al., Appl. Phys. Letters, 68, pp. 1359-1361, 1996.5. G. P. Williams, Review of Scientific instruments, 73, pp. 1461-1463, 2002.

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Photonic Transmitter (Photomixer)

Room temperature operationCompared with Quantum Cascade Laser1

Tunable THz wavelengthCompared with resonant tunnel diode(RTD) array2

Easily integrated with other semiconductor devices (such as semiconductor laser, amplifier…)Compact (<< 1mm2)

1. R. Kohler, et al., CLEO 2002 Postdeadline Papers, CPDC12-1, 2002.

2. M. Reddy, et al., IEEE Electron Device Lett. Vol. 18, pp.218-221, 1997.

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Excitation of Photonic Transmitters

CW excitation1

pulse shaper or Fabry-Perot filter

photonic transmitters

cw laser @ f1

cw laser @. f2 f1-f2

Quasi-CW excitation2

Pulselaser

1. Sean M. Duffy, et al., IEEE trans. Microwave Theory Tech. vol.49, pp.1032-1038, 20012. A. S. Weling, et al., Appl. Phys. Lett. vol. 64, pp.137-139, 1994

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Top View of Photonic Transmitters

Bow-tie antenna type

MSM-TWPD

Low pass filter

CPW fed Bow-tie antenna

Optical excitation beam

100 μm

dc probe pad

100 μm

MSM-TWPD CPW fed slot dipole antenna

dc probe pad

Optical excitation beam

Low pass filterSlot dipole antenna

type

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Characteristics of Membrane Photonic Transmitters

100μm GaAs substrate

THz emitter

100μm glass substrate

THz emitter

Do not need a Si lens to improve radiation efficiency!

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Expected Overall Frequency Response

Combing simulated radiation loss with frequency response of MSM-TWPD.Æ to optimize the overall frequency tuning range

400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

Sim

ulat

ed T

Hz

Pow

er (a

.u.)

Frequency (GHz)

Slot dipole type Bow-tie type

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Setup of the Measurement System

Operation at room temperature

Photonic transmitter

THz radiation

Central wavelength : 850 nmRepetition rate : 82 MHz

Pulse width : 100 fs

Ti:Sapphire

Bolometer

UFO/NTUOperation at room temperature

Photonic transmitter

THz radiation

Central wavelength : 850 nmRepetition rate : 82 MHz

Pulse width : 100 fs

Ti:Sapphire

Setup of the Measurement System

Bolometer

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400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0

400 GHz

THz

Pow

er (a

.u.)

Frequency (GHz)

Simulated response Measured response

800 GHz

Both transmitters have wider bandwidth than the previous one.Æ Frequency tuning range increases!

Measured Frequency Responses

at a dc bias voltage of 8V

under excitation power of 1.33 mW

Slot dipole antenna type

400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0 400 GHz

THz

Pow

er (a

.u.)

Frequency (GHz)

Simulated response Measured response

700 GHz

Bow-tie antenna

type

600 700 800 900 1000 11000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

TH

z O

utpu

t Pow

er (a

.u.)

Frequency (GHz)

Dipole Bow-tie Previous

0.5

Both high power efficiency and ultrawide frequency tuning range can be achieved by using the slot dipole antenna.

Comparison of

bandwidth

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Maximum light-THz power conversion efficiency : 0.33％

0 2 4 6 8 10 12 14 16 18 200

3

6

9

12

0

10

20

30

40

Squ

ared

Cur

rent

(X10

-10 A

2 )

Tera

hertz

Pow

er ( P

W)

Bias Voltage (V)

Squared photocurrent fitting Terahertz power

6.11μW1.87 mW ＝ 0.33 %

3.53×1014

400×109 ＝× 0.33 % 291%at 400GHz Excitation

under 1.87 mW Excitation Power

Bias Dependency of THz Output Power

Corresponding toExternal quantum efficiency : 291%

Two curves match well when dc bias voltage is below 15V.

＞100 %!!!

*N. Zamdmer, et.al., Appl. Phys. Lett. Vol. 75, pp.2313-2315, 1999.

Carrier lifetime Increasing effect*

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NTU-UFO Group• MSM TWPD• Side illumination• 2x10-4 Conversion Efficiency(1.6 THz)

• 0.11% Conversion Efficiency (645 GHz)

• 0.33% Conversion Efficiency 291% Quantum Efficiency(404 GHz)

• Allow integration with edge-emitting 2-O laser diode

MIT/UCSB/CalTech Group*• Photoconductive switch• Vertical illumination• 9x10-6 Conversion Efficiency(1.6 THz)

• 6.7x10-5 Conversion Efficiency(645 GHz)

*Sean M. Duffy, et al., IEEE Trans. Microwave Theory Tech. vol. 49, pp.1032-1038, 2001.

Edge-coupled vs. Vertically illuminated

THz radiation by photomixing

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Quasi-CW THz imaging system

PC

Lock-in amplifier

2D Translation stage

Bolometer Quasi-CWTHz source

sample

Scanning stepsize ~ 500PmParabolic mirror

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Spatial resolution

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Imaging of Biological Tissues Using 1THz Waves

• Dark area : Reduced THz transmission Organic material absorption

• Size 6cm x 4.5cm• At 1THz• SNR>100

Dried seahorse in an opaque plastic box

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Imaging of Water Contents Using 945GHz Waves

Fresh flowers inside an opaque plastic box

• Water distribution* P. Y. Han, G. C. Cho and X. C. Zhang, Opt. Lett. 25, 242 (2000).

• Size 1.6cm x 5cm THz frequency : 945GHz

• Acquisition time~ 16min (integration time constant 0.3s/pixel)

• From Bear’s law, effective attenuation constant~131 cm-1*

0LI I e D�

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Imaging of Metallic Materials Using 465GHz Waves

A part of a metal blade inside an opaque plastic box

2x4 cmContrast of imaging originated from reflection of THz

waves by metals

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Summary

We proposed & demonstrated LTG-GaAs MSM TWPD Record bandwidth at 800 nm (570 GHz)Record bandwidth-output-power product at 800 nm (5.7 THz-V)Record bandwidth-output-power product at telecommunication

wavelength (570 GHz-V)

We demonstrated an edge-coupled membrane THz photonic transmitters based on the MSM TWPD

Record conversion efficiency (2x10-4 @ 1.6 THz)Record conversion efficiency (0.11% @ 0.57 THz)Record conversion efficiency (0.33% @ 0.4 THz)Record quantum efficiency (291% @ 0.4 THz)

Compact THz imaging system is realized.Rich device physics.THz biochip and THz sensing.

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Acknowledgement

UFO/NTUYoung-Liang HuangYen-Hung ChenMing-Chun TienJa-Yu LuHsu-Hao ChangLi-Jin ChenTzeng-Fu Kao

UCSBKian-Giap GanYi-Jen Chiu (NSYSU,

TAIWAN)Prof. John E. Bowers

National Central University/EE

Prof. Jeng-Inn ChyiProf. Jin-Wei ShiWei-Shen Liu

National Taiwan University/EE

Prof. Ruey-Beei Wu An-Shyi Liu Yi-Chun Yu

UFO/NTU

MERCI BEAUCOUP

Project Supported by National Science Council, Taiwan

Academia Sinica, TaiwanNational Science Foundation, USA