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Overview:
Ingrid Wilke's terahertz (THz) and ultra-fast Spectroscopy
lab is focused on biological and medical applications of time-domain
THz methods; applications of time-domain THz methods in accelerator
physics; single-shot femtosecond electron beam bunch length
measurements; and dielectric and superconducting THz properties
of transition metal oxide thin films.
In the last decade time-domain terahertz
transmission spectroscopy (TDTTS) has become a powerful method
for studying properties of various materials from dielectrics
to semiconductors and superconductors. TDTTS operates with
sub-picosecond pulses of electromagnetic radiation, which
in the frequency domain implies the coverage of a very broad
range spanning from tens of gigahertz to a few terahertz.
Thus, explains Wilke, TTDTS bridges a large frequency gap
between microwave and conventional infrared spectroscopy.
Technical Description:
A time-domain THz-transmission spectrometer is typically powered
by a femtosecond Ti:sapphire laser and operates according
to the pump-probe scheme. A schematic of the experimental
arrangements is displayed in Fig. 1. The pump-probe principle
is characterized by the splitting of the initial Ti:sapphire
laser beam into two parts – the pump beam and the probe
beam. The pump beam hits an emitter, which in response to
the optical pulse releases a sub-picosecond pulse of THz-radiation.
The probe beam gates the detector whose response is proportional
to the amplitude and the sign of the electric field of the
THz-pulse. By varying the delay between pump and probe pulses
the whole time profile of the THz-pulse is traced. The complex
transmittance of a sample is then placed in the focus of the
THz-beam and given as the ratio of the Fourier transforms
of a THz-pulse transmitted through the sample and a reference,
for example a freely propagating THz-pulse.
Wilke's lab employs time-domain Thz-spectroscopy
to investigate the electro-magnetic properties – electrical
conductivity, dielectric properties – of thin films and
bulk materials. A recent focus of the lab's research has been
the investigation of superconducting thin films. TDTTS measurements
of superconducting thin films are motivated by a basic understanding
of quasi-particle excitations and pairing mechanisms as well
as an assessment of the performance of high-temperature superconductors
in passive electronic devices operating at microwave and THz-frequencies.
Fig. 1
Relativistic electron beam diagnostics
Time-domain THz methods are also a unique new method to measure
the length and shape of single relativistic electron bunches
in linear accelerators. Accelerators employed in next generation
TeV linear electron-positron colliders for high energy physics,
or used as drivers for new femtosecond X-ray free electron
lasers (FELs), require dense relativistic electron bunches
with bunch lengths shorter than a picosecond. Precise measurements
of the electron bunch length and its longitudinal charge distribution
are necessary to monitor the preservation of the beam quality
while the electron bunch train travels through the beam pipe,
as well as to tune and to operate a linear collider or a FEL
(Fig.2).
 |
Fig.
2. Experimental arrangements for electron bunch length
measurements by electro-optic sampling with chirped optical
pulses. The electron bunch length is measured by using
an electro-optic crystal of ZnTe placed inside the vacuum
pipe at the entrance of the undulator. The shaded parts
indicate the vacuum housing of the electron beam. |
Ultra-fast spectroscopy
A new area of research underway in Wilke's lab is the interaction
of femtosecond optical laser pulses with biological cells,
in particular the generation of pores in the cell membrane
as well as the basic understanding of this process. Femtosecond
optoporation offers great potential for targeted transfection
of cells with high transfection efficiency, opening up possible
applications in drug delivery and genetic engineering fields.
Contact Information:
Ingrid Wilke
Assistant Professor of Physics
Rensselaer Polytechnic Institute
110 8th Street
Troy, NY 12180-3590
(518) 276-6318
wilkei@rpi.edu
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