NIRT: Terahertz Spectroscopy On A Chip

1
NIRT Terahertz Spectroscopy On A Chip

• Development of nanoscale frequency tunable THz
  sources and sensors for compact spectroscopic
  systems
• A. G. Markelz, J. P. Bird, University at Buffalo,
  SUNY, S. J. Allen, UCSB, G. Aizin, Kingsborough
  College, L. Murokh, Queens College

THz Spectroscopy and Applications
.

Recent Progress


Enhanced Response from Tunable Plasmonic
Terahertz Detectors
Photonic Detectors Quantum Point Contacts (QPCs)
Detect Ligand binding

• Molecular Conformational Vibrational Modes in THz
  range
• Conformational monitoring for pharmaceuticals
• ? Rapid tagless biomolecular binding assays
• - Pharmaceutical development
• - Pathogen detection
• - Biosensors
• Packaging materials transparent in THz range
• - Package content monitoring
• - Security screening
• Molecular gas rotational modes in THz range
• - Emissions monitoring

• Previously we have shown that QPCs may be used
  as a tunable barrier with an energy gap in the
  meV range
• This suggests that these devices should serve as
  tunable detectors of radiation in the THz range
• We are exploring the THz photo-conductance of
  QPCs using a molecular-gas laser to generate
  radiation at 1.4 2.6 THz (collaboration with Y.
  Kawano K. Ishibashi, RIKEN)

Finger gate biased to depletion for
sensitivity
0.69 THz
Grating gated FET
Grating gates biased for tuning
Resonant response exhibiting 50 GHz line width,
well matched to gas phase spectra at atmospheric
pressure or line widths in ceramic solids
Plasmon harmonic resonances
E.A Shaner, A.D. Grine, M.C. Wanke, M. Lee, J.L.
Reno and S.J. Allen IEEE Phot. Tech. Lett., 18,
1925 (2006).
1000 fold increase in sensitivity with finger
gate bias to depletion
Broad application of THz spectroscopy and imaging
is limited by large spectroscpic systems and lack
of imaging arrays. Need for compact cheap
spectroscopic systems to fully utilize potential
of THz range.
Approach .
Barrier characterization I-V well modelled with
theory
Experiment
2D Schottky diode model
Tunable charge densities and barriers allow for
THz spectroscopic scanning Initial focus on two
classes of detector photonic and
plasmonic. Initial photonic detection focuses on
detectors based on quantum point contacts, which
have single electron sensitivity. Key innovation
is to incorporate quantum wires coupled to the
QPC barrier to overcome the dominance of free
carrier absorption. Initial plasmon detectors
focus on improvement of sensitivity of 1D grating
array photoconductive detectors by directly
sensing the electron gas heating and developing
2D grating array plasmonic photoconductive
detector with tunability through charge density,
magnetic field and inter-dot coupling. All
initial devices in MBE grown III-V materials with
e-beam and photo lithography fabrication
techniques. Future devices use colloidal quantum
dots and carbon nanotubes.

• I-V curves can be explained in the 2D Schottky
  diode model with both thermionic emission and
  thermionic-field emission included.
• The asymmetry is due to split-finger gate voltage
  referenced with respect to the drain

Barrier characterization Thermally activated
response dependent on source drain bias
(a) Calculated absorption spectra for a
double-well grating-gate detector, at several
different THz frequencies and as a function of
the grating-gate voltage. (b) THz photoresponse
measured as a function of the gate voltage for
four different frequencies at 25 K .
Typical responsivity, and response time vs bias
current
Upper panel shows the key features of a QPC,
with source (S) and drain (D) and the QPC gates.
Middle panel is a cross section of the
heterostructure on which the QPC gates (black)
are deposited. Lower panel shows schematically
the saddle form of the QPC potential.
Vfinger -1.000 V
Grating-gated device with an integrated
bolometric detector that is implemented by
control of a separate bias applied to the
pinch-off gate. The period of the gates is
typically a couple of microns.
The effective barrier height of a QPC and its
variation with Vg. The inset shows the activated
dependence of the current used to calculate the
values of the barrier .
2D plasmon arrays
Team .
2D quantum dot arrays fabricated at Chiba
University using e-beam exposure and wet etching.
Measurements using THz Time Domain Spectroscopy
show temperature dependent resonances. New
structures using gated grids will allow
controllable carrier density and dot-dot
interaction and remove concerns about carrier
depletion due to wet etching.
Broader Impact
Grad Student Researchers Nafees Kabir and Jongwoo
Song, UB EE Joseph Knab and Imtiaz Tanveer, UB
Physics Greg Dyer, UCSB Physics Undergrad
research interns Jess Crossno, Santa Barbara City
College, (potential transfer to UCSB, INSET
program, 2007 Sigma Xi Best Student Poster
winner) Sean Haney, UCSB Physics also SURF Bill
Sowerwine, UCSB Physics also SURF Susanna Kohler,
UCSB Physics also SURF High School
Students Wendell Hamner, UB NIRT GEMS on th
GO Jerrell Bradley , UB NIRT GEMS on the GO
Photonic Detectors THz-assisted FRET in coupled
colloidal quantum dots
GGEMS on the GO presentation at the Buffalo
Museum of Science
Jess Crossno and Greg Dyer
References 1 For further information see
http//www.physics.buffalo.edu/THz_on_a_chip/ or
email amarkelz_at_buffalo.edu 2 N. A. Kabir, Y.
Yoon,J. R. Knab, J.-Y. Chen, A. G. Markelz, J. L.
Reno,Y. Sadofyev, S. Johnson ,Y.-H. Zhang, and J.
P. Bird, Appl. Phys. Lett. 89, 132109 (2006). 3
X. G. Peralta, S. J. Allen, M. C. Wanke, N. E.
Harff, J. A. Simmons, M. P. Lilly, J. L. Reno, P.
J. Burke, and J. P. Eisenstein, Applied Physics
Letters 81, 1627-1629 (2002). 4 E.A. Shaner,
M.C. Wanke, A.D. Grine, S.K. Lyo SK, J.L. Reno,
S.J. Allen, Appl.Phys. Lett., 90, 181127 (2007).
5E. A. Shaner, A. D. Grine, M. C. Wanke,M.
Lee, J. L. Reno,S. J. Allen, IEEE Photonics
Technology Letters, 18, 1925 (2006). 6 G. R.
Aizin, D. V. Fateev, G. M. Tsymbalov, V. V.
Popov, APL, 91, 163507 (2007)