1. Quantum engineered field-effect transistors for warm, wide-bandwidth, near quantum-limited Terahertz heterodyne receivers Mark Sherwin UCSB Physics Department and Institute for Terahertz Science and Technology

2. Herschel Space Observatory Bilpratt et. al., A&A 518 L1 (2010)

3. Heterodyne Instrument for Far Infrared (HIFI) Th. De Grauw, et. al., A&A 518 L6 (2010)

4. HIFI spectrum of Orion hot core Th. De Grauw, et. al., A&A 518 L6 (2010)

5. 35 Cl/ 37 Cl ratio in dense molecular clouds J. Cernicharo et. al., A&A L115 (2010)

6. H 2 O in cometary atmosphere 3x10 5 km P. Hartogh et. al., A&A 518, L150 (2010)

7. Coherent detection (e. g., heterodyne mixing) Frequency Signal f LO = 1 THz to 5 THz f sig f IF = 1 - 10 GHz Receiver Noise: T op = T A + T M + T Amp /  M Integration Time:   (T R ) 2 Mixer TMTM MM P LO f LO T Amp f IF IF Amp Backend Spectrometer f IF = | f LO - f sig | HETERODYNE RECEIVER f /  f = 10 7 - 10 8 Quantum Limit (double sideband) T M ≥hf/2k B T Q = 25K @ 1 THz TATA

8. DSB system noise temperature on HIFI Th. De Grauw, et. al., A&A 518 L6 (2010) Operating temperature 1.7K

9. Heterodyne receivers for future missions Higher frequency (HD @ 2.7 THz, OI@4.7 THz) Lower noise High temperature operation (40-100K) Wider bandwidth (>15 GHz vs. 4 GHz for phonon-cooled HEB) Low LO power Arrays

10. Superconducting hot-electron bolometers

11. Theory, proposed mixers

12. Proposed mixers Mixer noise temperature (classical theory): 200K DSB Operating temperature: T op =30-100K LO frequency: 1.5-5 THz IF bandwidth: >15 GHz LO power requirement: –~1 µW@ T op =30K –~100µW@ T op =100K Planar, suitable for arrays

13. Quantum-engineered FET as hot-electron bolometer SourceDrain IF THz Front gate (to antenna) Back gate (to antenna) absorber detector “Tunable antenna-coupled intersubband terahertz (TACIT) sensor”

14. Relevant physics in quantum wells Absorption: intersubband transition in quantum well Detection: temperature-dependent mobility of 2-D electron gas IF THz absorber IF THz detector

15. Example: intersubband absorption in square quantum well

16. Intersubband absorption in square well 0 20 40 60 80 Position (nm) Energy (meV) 250 150 50 0 100 200 Al 0.3 Ga 0.7 As GaAs in-plane wavevector k Energy

17. Absorption vs. dc electric field, constant N s 2.4 THz4.8 THz

18. Absorption peak vs. dc electric field, charge density Experiment: Williams et. al., PRL, 2001) Theory: Ullrich and Vignale Ibid. n s =10 10 cm -2 n s =13x10 10 cm -2 Time-dependent Local density approximation All parameters from experiment Frequency (THz) 2.4 4.8

19. Intersubband absorption below 1 THz

20. Detection: temperature-dependent mobility IF THz detection Mobility of 2-D electron gases vs. temperature Mobility determined by electron temperature T e.

21. Detection Hot-electron bolometric IF THz detection

22. Speed Phonon cooling Diffusion cooling –20 GHz IF bandwidth demonstrated in 4µm long millimeter-wave mixer. (M. Lee et. al., APL 2001) J. N. Heyman et. al., PRL 74, 2682 (1995) 50 2510 T (K) IF bandwidth (GHz) 0.16 1.6 16

23. Coupling efficiency: impedance of active region L V(t) Area A Charge Q + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2-DEG= Dipole sheet on springs On resonance R can be 20  in future designs C can be tuned out by rf embedding circuit design Source-Load coupling can be>90%!

24. The math M. Sherwin et. al., Proceedings of 2002 Monterey Submm workshop L V(t) Area A Charge Q + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

25. Can couple efficiently to ~10,000 electrons! L V(t) Area A Charge Q + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2-DEG= Dipole sheet on springs On resonance 49 Ohms* 18 Ohms* *on resonance for high-quality 40 nm square well, T e =50 K, f=2.3 THz, A=5µm 2,L=0.15 µm

26. Theory for NEP, T M NEP (direct detector) –  =insertion loss,  =1/R(dR/dT), N S = sheet density, A=active region area Double sideband noise temperature (mixer) M. Sherwin et. al., Proceedings of 2002 Monterey Submm workshop Johnson noiseThermal conduction noise

27. Performance limits for TACIT mixer  10,000 electrons 10 6 cm 2 V-s @ low temp. Bulk LO phonon scattering

28. First-generation devices Twin slot coupled to coplanar waveguide Demonstrated direct detection, electric field tuning Difficult fabrication with very low yield Quantum well far from optimal Microwave embedding circuit far from optimal Collaboration with W. R. McGrath, Paolo Focardi

31. Completed TACIT detector Source Drain Front gate bias line Back gate bias line

32. 10nm 130 nm 10.8 nm 197 nm 500 nm 1000 nm 200 nm 70 nm 9 nm 70 nm 3 nm 0.5 mm substrate buffer etch stop cap Si delta Doping (10 12 cm -2 ) Coupled quantum wells barriers cap GaAsAl 0.3 Ga 0.7 As Sample structure design and growth

33. Intersubband absorption characterization THz in (FTIR)THz out bolometer 1.6 THz Design frequency theory Electrons in quantum well

34. Fabrication of 1 st generationTACIT detector Epoxy Bond and Stop-Etch (EBASE), Sandia 1. Process front side2. Epoxy bond to GaAs wafer 3. Etch away substrate 4. Process back side, etch vias

35. Experimental Set Up GaAs Host Substrate Silicon Lens UCSB Free Electron Laser at 40-80 cm-1 Sample in a Helium Flow Cryostat, T=20-100K Current amplifier Sources and measures channel oscilloscope 0 V Gate Voltage 5  s, 1mW

36. Response to fast THz pulses 1.5 ns 3.5 ns

37. Tunability with gate voltage Sample A T b =100K T e ~120K f=1.53 THz FEL Sample B T b =77K T e ~100K f=1.6 THz Molecular gas laser Photocurrent (nA) Photovoltage (mV) Charge density held constant.

38. Origin of “double peak” -0.2 -0.1 0 0.1 0.2 Electric field (mV/Angstrom) Intersubband absorption frequency (THz)

39. New design* Easier fabrication Eliminate some parasitics Can rapidly iterate to optimize Inspired by Chris McKenney’s thesis, Cleland group.

40. TACIT mixer development strategy Microwave engineering MBE growth of wafers with high mobility, narrow intersubband linewidth. Develop fabrication process Test and iterate 40

41. TACIT specs to enable new missions High-temperature operation Wide bandwidth High frequency Low noise 41

42. Herschel cryogenics Herschel – Large dewar drives up mission costs – Duration limited to 3.5 years 42

43. Long-duration mission with cryocooler Atmospheric Infrared Sounder (AIRS) – Detectors @ 58K – Long-lifetime cryocooler – Launched 2002 43

44. Potential platforms for TACIT mixers Explorer-class missions – Astrophysics – Planetary science Long-duration ballooning SOFIA 44

45. Summary and conclusions THz heterodyne spectroscopy: important science TACIT mixers offer improvements over state of art Timely to develop TACIT mixers in concert with new mission concepts 45 Superconducting HEB actual TACIT mixer theory Operating temp.2K30-100K Bandwidth4 GHz>15 GHz Noise temperature @ 2.5 THz 1000K200K

46. Acknowledgments W. R. McGrath (JPL): Antenna design P. Focardi (JPL): Antenna impedance, mode matching theory G. B. Serapiglia (UCSB->law school): processing, characterization, experiments Sangwoo Kim (UCSB-> Tanner Research Labs): room- temperature devices. M. Hanson (UCSB): sample growth A. C. Gossard (UCSB): sample growth Funding: NASA, NSF 46

47. Insertion losses for this device and experiment Matching antenna pattern – Only 4% coupled into antenna mode* – Device fried before could be improved – Mode matching can be increased to >90% + Antenna pattern Illumination pattern *Computed following Goldsmith, “Quasioptical systems” + Focardi, McGrath and Neto, IEEE MTT 2004. Z load = (1.7-i39)  P in Z source =(20-i40)  Matching antenna and load impedances - Only 2.5% coupled from antenna into load

48. Responsivity at T bath =80K R optical =V/P in =0.1 V/W R internal =R optical /(total insertion loss)=107±75 V/W R electrical Theory: R internal =(1100±400) V/W

49. Electrical transport data This sample 10 6 mobility 2-DEG

50. Electrical transport data