Basic Detection Techniques Front-end Detectors for the Submm Andrey Baryshev/Wolfgang Wild Lecture on 21 Sep 2006

Basic Detection Techniques – Submm receivers2 Contents overview Submm / THz regime Submm / THz regime Definition and significance Definition and significance Science examples Science examples Submm detection: direct + heterodyne Submm detection: direct + heterodyne Heterodyne receiver systems Heterodyne receiver systems Signal chain, block diagram Signal chain, block diagram Heterodyne principle Heterodyne principle Noise temperature and sensitivity Noise temperature and sensitivity Heterodyne frontend Heterodyne frontend Mixers Mixers Local oscillators Local oscillators IF amplifiers IF amplifiers Spectrometers: Filterbank, AOS, Autocorrelator, FFT Spectrometers: Filterbank, AOS, Autocorrelator, FFT Overview submm astronomy facilities Overview submm astronomy facilities Examples of heterodyne receiver systems Examples of heterodyne receiver systems ALMA 650 GHz ALMA 650 GHz HIFI space instrument HIFI space instrument Direct detection systems Direct detection systems Signal chain, block diagram Signal chain, block diagram Types of direct detectors and operating principles Types of direct detectors and operating principles Noise equivalent power (NEP) Noise equivalent power (NEP) Examples of a direct detection system Examples of a direct detection system Quasi optics Quasi optics Practical work at SRON Measurement of sensitivity of heterodyne and direct detection system.

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4 Submillimeter/THz Wavelength Regime I λ ~ 0.1 … 1 mm λ ~ 0.1 … 1 mm Photon energy corresponds 2-20 K in temperature scale (hF =kT) Photon energy corresponds 2-20 K in temperature scale (hF =kT) Between infrared/optical and radio waves Between infrared/optical and radio waves Submm technology is relatively new (~ 20 years) Submm technology is relatively new (~ 20 years) (Compare to optical technology: ~ 400 years) Submm astronomy is crucial for understanding star and planet formation Submm astronomy is crucial for understanding star and planet formation Range of 0.1… 0.3 mm is one of the last unexplored regimes in astronomy Range of 0.1… 0.3 mm is one of the last unexplored regimes in astronomy

Basic Detection Techniques – Submm receivers5 Submillimeter Wavelength Regime II Technically challenging and interesting Technically challenging and interesting  Challenging: small λ means high precision fabrication  Interesting: Combination of optical and electronic techniques Submm astronomy and technology are very dynamic fields Submm astronomy and technology are very dynamic fields

Basic Detection Techniques – Submm receivers6 Advantages of THz radiation Definition Frequency range THz Emerging field (largely unexplored) Unique properties Many spectral features in THz region See through many materials Sensitive to water Presently used for astronomy, Earth observation Image of a galaxy Water, gas Spectrum of ethanol and water THz imageTHz radar image Image made by A. Baryshev

Basic Detection Techniques – Submm receivers7 Why submillimeter ? Sub-/Millimeter vs. optical astronomy ItemSub-/Millimeter Optical / IR WavelengthFrequency 0.1 mm to 3 mm 100 GHz to 3 THz 0.4 to 30 μm 10 to 600 THz Targets Cold medium (10-100K) Molecular clouds Extended structures Hot medium (a few 1000K) Stars Point sources Sub-/Millimeter astronomy studies the Cold Universe. And most of the sky is dark and cold …

Basic Detection Techniques – Submm receivers8 Radiation at (sub)mm wavelengths  Continuum: cold dust at K (black body of 30K peaks at 0.1 mm)  Lines: pure rotational transitions of molecules Sub-/mm radiation probes cold molecular clouds of gas and dust Energy levels of CO and CS

Basic Detection Techniques – Submm receivers9 The Earth atmosphere at submm wavelenghts The Earth atmosphere is only partially transparent for submillimeter wave radiation The Earth atmosphere is only partially transparent for submillimeter wave radiation Several atmospheric “windows” exist Several atmospheric “windows” exist Water vapor and oxygen cause strong absorption Water vapor and oxygen cause strong absorption  dry, high observatory sites  airplane, balloon and space platforms

Basic Detection Techniques – Submm receivers10 Atmospheric transmission at 5000m altitude pwv = precipitable water vapour, i.e. the column height of condensed water vapour

Basic Detection Techniques – Submm receivers11 Submillimeter astronomy – star formation New stars form in molecular clouds New stars form in molecular clouds These clouds are best observed in the infrared and submm regime since they are cold and have high optical extinction These clouds are best observed in the infrared and submm regime since they are cold and have high optical extinction Star and planet formation is associated with a rich interstellar chemistry  many lines observable in IR/submm/mm Star and planet formation is associated with a rich interstellar chemistry  many lines observable in IR/submm/mm Spectral Survey IRAS JCMT Spectral Survey IRAS

Basic Detection Techniques – Submm receivers12 Cazaux et al. 2003

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Basic Detection Techniques – Submm receivers14 Optical vs. Submm/Far-Infrared Orion Trapezium Region at Optical WavelengthsHighlighted Region at IR

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Basic Detection Techniques – Submm receivers16 Molecular gas in M31 CO line emission traces molecular gas. This is where new stars form. Nieten et al. 2005

Basic Detection Techniques – Submm receivers17 Dust and CO at z=6.4 ! IRAM 30m MAMBO Bertoldi et al Sloan survey: optical image Contours: dust => Heavy elements formed shortly after Big Bang Z=6.4

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Design of a Scientific Instrument 06 June Two Main Detection Schemes for Sub-/mm Radiation Incoherent detection  direct detectors (bolometer) total power detection no phase information  used on single antenna low spectral resolution Coherent detection  heterodyne receiver frequency down conversion high spectral resolution phase information  single antenna and interferometer Heterodyne technique and receivers will be treated here.

Design of a Scientific Instrument 06 June Heterodyne Signal Chain Convert incoming radiation into electronic signal (IF) for further processing Spectral information is preserved (spectral resolution Δf/f determined by backend) Heterodyne detection achieves spectral resolution > 10 6 Heterodyne Instrument Spectrometer/ Correlator Data acquisition “Front End”“Backend” Intermediate Frequency (IF) optical electrical

Design of a Scientific Instrument 06 June Principle of Heterodyne Mixing // 0f IF LO RFIF Heterodyne principle = mixing of two frequencies (signal + local oscillator) to produce (sum and) difference signal (intermediate frequency = IF) Mixing needs non-linear element (e.g. diode, SIS junction) = mixer RF Lower sideband (LSB) Upper sideband (USB) freq f IF = | f LO - f RF | Double sideband mixer: both sidebands converted to same IF Single sideband mixer: Only one sideband converted to IF Sideband separating mixer: two sidebands converted to different IF outputs

Design of a Scientific Instrument 06 June Combine strong LO signal V LO = cos(  LO t) (e.g. 996 GHz) + A weak RF signal V S = cos(  S t+  ) (e.g GHz) Gives total power absorbed P ~ V S V LO cos((  S -  LO )t +  )+…. Amplitude and phase information conserved in IF signal Detect radiation at frequencies where no amplifiers are available IF signal Heterodyne Mixing 996 Local Oscillator Mixing needs strong non-linear detector charcteristic

Design of a Scientific Instrument 06 June Block Diagram of a Heterodyne Receiver to correlator or spectrometer Astronomical RF signal (e.g. 650 GHz) Optics Mixer Local oscillator IF amp(s) LO ref in 4 K IF signal out (e.g. 4 GHz) LO signal (e.g. 646 GHz) Components:  Optics Mixer Local Oscillator (LO) Calibration source IF amplifier(s) Dewar and cryogenics Bias electronics Spectrometer(s) Cal source

Design of a Scientific Instrument 06 June A Heterodyne Receiver

Design of a Scientific Instrument 06 June A heterodyne receiver for space 7 LO Beams Telescope Beam ~ 50 cm HIFI = Heterodyne Instrument for the Far-Infrared Will fly on the Herschel Space Observatory in 2008

Design of a Scientific Instrument 06 June HIFI Signal Path mixer optics WBS IF LOU LSUHRS ICU IF spectrometers Instrument Control Unit Local Oscillator Unit Local Oscillator Source Unit Focal Plane Unit Telescope To Astronomer

Basic Detection Techniques – Submm receivers27 Main components of a heterodyne front-end Optics  last part of this college Optics  last part of this college Submillimeter wave mixer Submillimeter wave mixer  SIS = Superconductor-Insulator-Superconductor  HEB = Hot-Electron-Bolometer  (Schottky = Semiconductor-metal contact diode) Local Oscillator Local Oscillator  Multiplier chain  Quantum-Cascade-Laser (QCL) Intermediate frequency (IF) amplifiers Intermediate frequency (IF) amplifiers

Basic Detection Techniques – Submm receivers28 Sensitivity and Noise Temperature In radio and submm astronomy, the signal unit “Temperature” is used. In radio and submm astronomy, the signal unit “Temperature” is used. This is really a signal power W = k T Δν (k Boltzman constant) This is really a signal power W = k T Δν (k Boltzman constant) Usually the signal power is much smaller than the noise power (“noise temperature”) of the receiving system. Usually the signal power is much smaller than the noise power (“noise temperature”) of the receiving system. The noise temperature of a system is defined as the physical temperature of a resistor producing the same noise power. The noise temperature of a system is defined as the physical temperature of a resistor producing the same noise power. Difference measurements are used to detect the signal, e.g. Difference measurements are used to detect the signal, e.g. (sky + signal source) minus (sky) (sky + signal source) minus (sky)

Basic Detection Techniques – Submm receivers29 The “ideal” submillimeter wave receiver Converts all incoming radiation into an electric signal  no photons “lost”  has no own noise contribution However: Heisenberg’s uncertainty principle (ΔE x Δt ≥ h/2π) makes such a noiseless mixer impossible. Why ? – A heterodyne mixer measures signal amplitude and phase. This corresponds to number of photons and time in the photon picture which – according to the uncertainty principle – cannot be measured simultaneously with infinite precision. This uncertainty results in a minimum noise of a heterodyne mixer, the “quantum limit”. Current best mixers are ~few times worse than the quantum limit.

Basic Detection Techniques – Submm receivers30 Sensitivity of a receiving system The answer is the Radiometer formula (Sensitivity):  T min = c 1 T sys / (t  ) 1/2 Received noise power from an antenna / receiver system: Noise powerW sys = W A + W rx = k T sys  = k (T A + T rx )  T sys = T A + T rx receiver noise temperature antenna temperature (signal, atmosphere, antenna losses) system temperatureintegration time system bandwidth Question: What is the smallest detectable signal ?

Basic Detection Techniques – Submm receivers31 Noise Contributions from Receiver Components Receiver as a series of linear two-ports: T 1, G 1 T 2, G 2 T 3, G 3 T n, G n To detector T rx = T 1 + T 2 / G 1 + T 3 / (G 1 G 2 ) + … + T n / ( G 1 G 2 …. G n )  Receiver noise temperature determined by first few elements  Cooled optics for high frequencies OpticsMixer1 st IF amplifier Question: What is the noise contribution from different receiver components ? T: noise temp G: Gain

Basic Detection Techniques – Submm receivers32 HIFI signal chain

Basic Detection Techniques – Submm receivers33 Sub-/millimeter Optics Main function: coupling of the antenna signal into mixer Used components: Lenses (e.g. PTFE, quartz) Mirrors (plane and focusing) Feed horn Grids (polarization separation) quarter / half-wave plates Martin-Puplett Interferometers Gaussian optics used in sub-/mm regime (separate lecture)

Basic Detection Techniques – Submm receivers34 Cryogenic submillimeter mixers SIS = Superconductor-Insulator-Superconductor - used in mm and submm from ~70 GHz to ~1200 GHz - very good performance - theory well understood - submm detector of choice at ground-based and space telescopes telescopes HEB = Hot-Electron-Bolometer - used above ~1200 GHz into THz regime - performance better than SIS above 1200 GHz - theory not well understood - active research on-going

Basic Detection Techniques – Submm receivers35 The SIS mixer The SIS mixer (Superconductor-Insulator-Superconductor) element is a sandwich structure with a very thin insulator. Superconductor-Insulator-Superconductor (SIS) Tunnel Junctions SEM view of junction top electrode (1x1 µm²) Cross section of a typical Niobium SIS tunnel junction insulator thickness <= 1nm : tunneling SSI

Basic Detection Techniques – Submm receivers36 Bandgap structure of an SIS mixer Energy gap  in density of states   no current below V bias = 2  /e  low shot noise root singularity in density of states:  large current flow at V Gap  extremely sharp nonlinearity Superconductor 1 at V ~ V Gap Superconductor 2 grounded Ins. „Semiconductor“ model for SIS „Quasiparticle Excitations“ ~ Electrons (Cooper pair tunneling effects not shown !)

Basic Detection Techniques – Submm receivers37 SIS mixer principle = photon assisted tunneling Photon assisted tunneling (Dayem&Martin) series of steps at V = U Gap – nh /e Frequency limit for mixing at h = 4  (1400 GHz for Nb) LO power: P LO ~ (h /e)²/R N (800 GHz, 20 Ohms: 0.5µW)

Basic Detection Techniques – Submm receivers38 Some formulas

Basic Detection Techniques – Submm receivers39 300, 400, 800 GHz photon steps

Basic Detection Techniques – Submm receivers40 Different RF power Load line

Basic Detection Techniques – Submm receivers41 Typical SIS mixer responce

Basic Detection Techniques – Submm receivers42 SIS mixer implementation Task: Couple the astronomical signal to the (very small, ~1 μm 2 ) tunnel junction. Two ways are used: Feedhorn and waveguide (waveguide mixer) Feedhorn and waveguide (waveguide mixer)or A lens and antenna structure (quasi-optical mixer) A lens and antenna structure (quasi-optical mixer)

Basic Detection Techniques – Submm receivers43 Example of a waveguide SIS mixer ( GHz) Feed horn Magnet Junction holder Lens 10 mm

Basic Detection Techniques – Submm receivers44 Precision machining Backshort cavity Mixer backpiece Terahertz mixer Human hair 0.1 mm With SIS chip and tunnel junction

Basic Detection Techniques – Submm receivers45 HIFI mixers GHz and GHz These mixers fly now on the Herschel Space Observatory

Basic Detection Techniques – Submm receivers46 HIFI mixer design magnet Device mount with backshort, substrate channel and alignment spring Magnet pole shoes IF-board Corrugated horn ESD protection, bias and LF filtering Pressure unit Re-alignment spring Cover for bias/ESD PCB

Basic Detection Techniques – Submm receivers47 Example of a quasioptical mixer structure Mixer chip Lens Antenna structure SIS junctionStripline 0,25 mm 10 mm

Basic Detection Techniques – Submm receivers48 Quasi-optical mixer implementation Silicon lensIF board Quasi-optical mixer for the Space instrument HIFI Chalmers Technical University Gothenburg, Sweden 1.5 THz Main challenges:- chip alignment on lens - optical properties, beam direction

Basic Detection Techniques – Submm receivers49 Hot electron bolometer (HEB) principle Thin superconducting film Square law power detector thermal time constant t = C/G C: thermal capacitance G: thermal conductivity Mixer operation: can detect beat frequency between LO and signal has to be very fast (ps) for few GHz IF (needed for spectroscopy)

Basic Detection Techniques – Submm receivers50 Hot electron bolometer (HEB) mixer Radiation heats electrons  R Cooling either by phonons or out-diffusion Direct or heterodyne detection Principle of operation 1  m x 0.15  m (W x L) Hot Electron Bolometer Limitations IF bandwidth, sensitivity

Basic Detection Techniques – Submm receivers51 Typical I-V cirves

Basic Detection Techniques – Submm receivers52 Submm mixer noise temperatures HIFI space instrument Jan 2006 Mixer noise increases with frequency (increased losses)