THz Optoelectronics. Chi-Kuang Sun

Transcription

1 THz Optoelectronics Chi-Kuang Sun UltraFast Optics Laboratory (UFO) Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering National Taiwan University Taipei, TAIWAN

2 Outline Introduction (Motivation) MSM TWPD for 800 nm MSM TWPD for 1300 and 1550 nm THz emitter (photomixer) based on MSM TWPD THz Imaging System THz biochips Summary

3 Demands of Ultrahigh Bandwidth Photonics >10 Tbit/s Fiber Communication Backbone System WDM vs. TDM: Hybrid System Local Fiber Communication Network Microwave Photonics System Fiber-Radio System THz-Millimeter Wave Photonic Emitter Ultrahigh Speed Photonic Measurement Active microwave probe Time domain Network-analyzer Ultrahigh Speed Electronic Gating Short Electrical Pulse Generator

4 Traditional Ultrahigh Speed Vertical InGaAs/InP p-i-n Photodetector 180 nm i-ingaas 50 nm p + InGaAs and 400 nm p-inp 6 µm 2 µm 500 nm n-inp alloyed p-metal air-bridged metal metal reflector PMGI n-metal air p i ir-bridged metal p-mesa n n-mesa n-metal light input InP:Fe substrate SiNx antireflective coating 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, (1995).

5 3 db BANDWIDTH 2 µm device, - 2 V bias Normalized EO Signal FWHM 2.7 ps Magnitude (db) GHz GHz (Deconvolved) Delay (ps) Frequency (GHz)

6 Limiting Factors For Ultrahigh Speed Vertical p-i-n Photodetector Transit time Bandwidth-efficiency product - limited by absorbing layer thickness and device area Diode RC time constant layer thickness C device area C R Parasitic Capacitance important when device area < 25 µm 2

7 Early Approach Thin absorption layer Decrease carrier transit time - Not too thin to cause quantum confinement Low quantum efficiency High capacitance Small device area Decrease device capacitance - Not too small to affect light coupling Increase resistance

8 SPACE CHARGE SCREENING EFFECT Increase Pump Intensity 1.5 I E T (ps) 20 X I E T (ps) 20 X

9 SPACE CHARGE SCREENING EFFECT 13 fc 29 fc 43 fc 68 fc EO Signal (A.U.) V -2V -3V -4V -5V V -2V -3V -4V -5V V -3V V -3V -4V -5V 20 Time Delay (ps) Time Delay (ps) Time Delay (ps) Time Delay (ps) 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).

10 How about Large Area device Instead of Nano Devices? For THz Bandwidth RC time constant Replace Lumped Circuit Model with Microwave Circuit Model Device Transit Time Use short carrier lifetime material

11 The First p-i-n Traveling-Wave Photodetector Isolated 2Z 0 EO Signal (AU) 1.47 ps FWHM 2Z 0 Magnitude (db) Time (ps) GHz GHz GHz External Quantum Efficiency (%) 830 nm Light p i n QE ~ 50% SI Substrate Frequency (GHz) K. S. Giboney, M. J. W. Rodwell, and J. E. Bowers, IEEE J. Selected Topics in Quantum. Electron., vol. 2, 1996, pp

12 Advantage of Traveling-Wave Photodetector Enhance quantum efficiency by decoupling carrier transport from light propagation Replace 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 (1999).

13 low-temperature-grown GaAs p-i-n traveling wave photodetector D.C. external quantum efficiencies as high as 8 % were obtained 5 Relative Photocurrent response Theory Measurement ~ 570 fs Correction 530 fs Time (ps) Relative response (db) ~ -3dB 560 GHz 520 GHz Measurement Correction Freq(GHz) 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)

14 Bandwidth Limitation Factor of p-i-n TWPD Carrier transport time or carrier lifetime Velocity mismatch bandwidth

15 Velocity-Mismatch Bandwidth in TWPD The velocity mismatch 3 db bandwidth for long TWPDs with matched input termination (γ=0) is given by B vm = Γαvove / 2π ( vo ve) while the velocity mismatch 3dB bandwidth for open-circuit input termination (γ=1) is given by B vm 2 2 2Γα vove /2 5v o ve = π which can be closely approximately by a single-pole response with a bandwidth of over the entire practical range of the velocity mismatch for slowwave mode devices. B Γα / 3π vm v e 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 (1999).

16 Microwave loss in TWPD The measured field loss coefficiency in LTG-GaAs p-i-n TWPD Field attenuation coefficiency (µm-1) Frequency (GHz) 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)

17 Challenges for Ultrahigh Power/Speed PDs Slow-wave microwave mode in pin TWPD slow microwave velocity high microwave loss Low saturation power Our Solution: MSM TWPD Quasi-TEM microwave mode high internal field (external Bias) * J.-W. Shi, et al., IEEE Photon. Techno. Letters, 13, pp , June, 2001.

18 Cross Sectional View of MSM TWPD Metal 2 m 300nm LTG-GaAs 200nm Optical cladding layer Al 0.2 Ga 0.8 As Al 0.3 Ga 0.7 As 400nm 1 m (110) Optical isolation layer Al 0.5 Ga 0.5 As S. I. GaAs 3 m 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), (2001).

19 Top View of MSM TWPD Integrated CPW line hv hv Self aligned photoabsorption region Flared out CPW region for microwave probe

20 Advantages of MSM TWPD Traveling-wave structure Remove RC time constant limitation Enhance quantum efficiency by decoupling carrier transport from light propagation Longer absorption length Lower carrier density higher output power Less bandwidth saturation MSM structure Quasi-TEM Mode vs. Slow wave mode (p-i-n) Low microwave loss High microwave velocity and high velocity-matching bandwidth Easier impedance matching Less Boundary reflection* Higher Bias Less output voltage saturation Low-temperature-grown GaAs Short carrier trapping time Fast response Less space charge screening *J.-W. Shi, C.-K. Sun, Journal of Lightwave Techno., 18, pp , v e > v o

21 800 nm DC Measurement Photocurrent (µa) Bias 15V 8.1% Fitting line Optical Power (mw) Similar to p-i-n TWPD * Much better than MSM VPD Output photocurrent is linear vs. input optical power

22 EO Sampling System

23 Record Bandwidth Performance of MSM TWPD at 800 nm E-O signal (A.U.) 1.0 FWHM:0.8ps 0.8 photo-generated charge:120 fc Time (ps) Frequency Response GHz 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 , June, 2001.

24 Record-High Peak-Output-Power-Bandwidth Product Photocurrent (ma) ps Time (ps) Bias Voltage: 30V Peak Output Voltage: ~30V Photo-charge:~2100 fc Power (db) V p (peak voltage: ~30V) X Electrical Bandwidth(190GHz) = 5.7 THz-V dB@190 GHz Frequency (GHz) Limited by the bias voltage (30V)

25 Record-High Peak-Output-Power-Bandwidth Product V p (V) /Photo-charge (fc) Electrical bandwidth (GHz) / Response time(ps) LTG-GaAs pin TWPD fc 6 ps GaAs pin TWPD 2 59 fc ~5.5 ps InGaAs pin VPD 3 ~68 fc 7.2 ps Uni-Traveling Carrier 4.6 V 94GHz/4.6 ps PDs (UTC-PD) 4 Velocity Match 2.5V 40~50GHz Distributed PD 5 LTG-GaAs MSM TWPD 30V/2100fC 190GHz/1.8 ps 1. Y. J. Chiu, et al., IEEE Photon. Tech. Lett., 10, pp , K. S. Giboney, et al., IEEE Journal Of Selected Topics In Quantum Electronics.,2, pp. 622, C.-K. Sun, I.-H. Tan, and John E. Bowers, IEEE Photon. Techno. Letters, 10, pp , K. Kato, IEEE Trans. Microwave Theory Tech., 47, pp.1265, L. Y. Lin, et al., IEEE Trans. Microwave Theory Tech., 45, pp , 1997.

26 Record-High Peak-Output-Power-Bandwidth Product NTU-UFO Group 5.7 THz-V 190 GHz 30 V MSM TWPD NTT* THz-V 94 GHz 4.6 V UTC-PD UCLA THz-V 50 GHz 2.5 V VMPD * Microwave Photonics Conference, paper T-5.1 (1999) IEEE Tran. Microwave Theory and Tech. 45, 1320 (1997)

27 Advantages of LTG-GaAs Based TWPD in Long Wavelength Regime (1.3~1.55µm) Lower cost of GaAs than InP Larger wafer size of GaAs Mature 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 constant Large photo-absorption volume Uniform absorption High output power

28 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 (2003).

29 Impulse response of MSM TWPD at ~1.3µm Wavelength E-O sampling system: Cr 4+ :forsterite laser Wavelength: ~1.3µm (1230nm) Output Current (ma) ps Time (ps) Electrical Response (db) GHz Frequency (GHz) 70µm device length (absorption volume 140µm 3 ) + Superior microwave guiding = 234 GHz Ultrahigh Bandwidth

30 Record Power-Bandwidth Product for Telecommunication Wavelength Output Current (ma) ps Time (ps) Power (db) dB@160GHz Frequency (GHz) 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, V p :4.6V). *J.-W. Shi, et al., IEEE Photon. Techno. Lett., 14, pp , **Microwave Photonics Conference, paper T-5.1, 1999

31 Applications of Ultra-High Bandwidth-Output Photodetector Photo-receiver circuit without electrical amplifier * optical-fiber amplifier Photomixer ** RF source with high tunability and output power * K. Kato, IEEE Trans. Microwave Theory Tech., 47, pp , 1999 ** S. Verghese, et al., IEEE Trans. Microwave Theory Tech., 45, pp , 1997.

32 What is THz Waves 1 Tera = Occupies the 100GHz 10THz spectrum Submillimeter or millimeter waves

33 Properties of THz Waves Material-dependent transmission Polar liquid => strong absorption Non-polar substance => nearly transparent Metal => opaque Dielectrics => characteristic absorption Polar molecular recognition Material characterization Quality control Security Check etc..

34 Applications of THz Technology Daniel van der Weide, Electronic Terahertz Technology, Optics & Photonics News, pp , April 2003.

35 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: , 2002

36 Methods to Generate THz Photoconductive switch 1 Difference frequency in nonlinear crystal 2 Optical rectification Resonant tunnel diode(rtd) oscillator arrays 3 Fixed low oscillation frequency Complex structure P-type Ge-strained laser 4 / Quantum cascade laser Operating at cryogenic temperature Free electron laser 5 Highest power but expensive Photonic transmitters based on photodetectors 1. D. H. Auston, et al., Appl. Phys. Letters, 45, pp , Wei Shi, and Y. J. Ding, CLEO 2002 Technical Digest, pp M. Reddy, et al., IEEE Electron Device Letters, 18, pp , E. Brundermann, et al., Appl. Phys. Letters, 68, pp , G. P. Williams, Review of Scientific instruments, 73, pp , 2002.

37 Photonic Transmitter (Photomixer) Room temperature operation Compared with Quantum Cascade Laser 1 Tunable THz wavelength Compared with resonant tunnel diode(rtd) array 2 Easily integrated with other semiconductor devices (such as semiconductor laser, amplifier ) Compact (<< 1mm 2 ) 1. R. Kohler, et al., CLEO 2002 Postdeadline Papers, CPDC12-1, M. Reddy, et al., IEEE Electron Device Lett. Vol. 18, pp , 1997.

38 Excitation of Photonic Transmitters CW excitation 1 cw f 1 cw f 2 f 1 -f 2 Quasi-CW excitation 2 photonic transmitters Pulse laser pulse shaper or Fabry-Perot filter 1. Sean M. Duffy, et al., IEEE trans. Microwave Theory Tech. vol.49, pp , A. S. Weling, et al., Appl. Phys. Lett. vol. 64, pp , 1994

39 Top View of Photonic Transmitters MSM-TWPD Optical excitation beam CPW fed slot dipole antenna Low pass filter dc probe pad Slot dipole antenna type MSM-TWPD Optical excitation beam 100 m CPW fed Bow-tie antenna dc probe pad Low pass filter Bow-tie antenna type 100 m

40 Characteristics of Membrane Photonic Transmitters THz emitter 100 m GaAs substrate THz emitter Do not need a Si lens to improve radiation efficiency! 100 m glass substrate

41 Expected Overall Frequency Response Combing simulated radiation loss with frequency response of MSM-TWPD. to optimize the overall frequency tuning range Simulated THz Power (a.u.) Slot dipole type Bow-tie type Frequency (GHz)

42 Setup of the Measurement System Bolometer Photonic transmitter THz radiation Ti:Sapphire Central wavelength : 850 nm Repetition rate : 82 MHz Pulse width : 100 fs Operation at room temperature

43 Setup of the Measurement System Bolometer Photonic transmitter THz radiation Ti:Sapphire Central wavelength : 850 nm Repetition rate : 82 MHz Pulse width : 100 fs Operation at room temperature

44 Measured Frequency Responses Both high transmitters power efficiency have wider and bandwidth ultrawidethan frequency the previous tuning range one. can be achieved Frequency by tuning using range the slot increases! dipole antenna. Normalized THz Output Power (a.u.) THz Power (a.u.) GHz GHz Frequency (GHz) Simulated Simulated response Dipole response Measured Measured Bow-tie response response Previous 800 GHz 700 GHz Comparison Slot Bow-tie dipole antenna antenna oftype bandwidth type at a dc bias voltage of 8V under excitation power of 1.33 mw

45 Bias Dependency of THz Output Power Two curves match well when dc bias voltage is below 15V. Terahertz Power (µw) Squared photocurrent fitting Terahertz power Bias Voltage (V) at 400GHz Excitation under 1.87 mw Excitation Power 0 Squared Current (X10-10 A 2 ) Maximum light-thz power conversion Carrier efficiency lifetime : 0.33 Increasing 6.11 W effect* 1.87 mw 0.33 % Corresponding to External quantum efficiency : 291% %!!! 0.33 % 291% *N. Zamdmer, et.al., Appl. Phys. Lett. Vol. 75, pp , 1999.

46 Edge-coupled vs. Vertically illuminated 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 edgeemitting 2-λ laser diode THz radiation by photomixing 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 , 2001.

47 Quasi-CW THz imaging system Lock-in amplifier PC Bolometer Quasi-CW THz source sample 2D Translation stage Scanning stepsize ~ 500µm Parabolic mirror

48 Spatial resolution

49 Imaging of Biological Tissues Using 1THz Waves Size 6cm x 4.5cm At 1THz SNR>100 Dark area : Reduced THz transmission Organic material absorption Dried seahorse in an opaque plastic box

50 Imaging of Water Contents Using 945GHz Waves Fresh flowers inside an opaque plastic box Size 1.6cm x 5cm THz frequency : 945GHz Acquisition time~ 16min (integration time constant 0.3s/pixel) From Bear s law, I = I0e α L effective attenuation constant~131 cm -1* Water distribution * P. Y. Han, G. C. Cho and X. C. Zhang, Opt. Lett. 25, 242 (2000).

51 Imaging of Metallic Materials Using 465GHz Waves A part of a metal blade inside an opaque plastic box 2x4 cm Contrast of imaging originated from reflection of THz waves by metals

52 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 1.6 THz) Record conversion efficiency 0.57 THz) Record conversion efficiency 0.4 THz) Record quantum efficiency 0.4 THz) Compact THz imaging system is realized. Rich device physics. THz biochip and THz sensing.

53 Acknowledgement Young-Liang Huang Yen-Hung Chen Ming-Chun Tien Ja-Yu Lu Hsu-Hao Chang Li-Jin Chen Tzeng-Fu Kao UCSB Kian-Giap Gan Yi-Jen Chiu (NSYSU, TAIWAN) Prof. John E. Bowers National Taiwan University/EE Prof. Ruey-Beei Wu An-Shyi Liu Yi-Chun Yu National Central University/EE Prof. Jeng-Inn Chyi Prof. Jin-Wei Shi Wei-Shen Liu

54 MERCI BEAUCOUP Project Supported by National Science Council, Taiwan Academia Sinica, Taiwan National Science Foundation, USA