This is a division of application Serial No. 08/388,933 filed February
15, 1995 entitled "Method and Apparatus for Terahertz Imaging".
This invention relates to spectroscopy in the terahertz frequency
range and, more particularly, to a method and apparatus for creating images of
objects with signals in this frequency range.
Background of the Invention
Terahertz time-domain spectroscopy ("THz-TDS") is a very powerful
spectroscopic technique in the far-infrared spectral region. Terahertz radiation
has been generated and detected using optically gated transmitters and receivers
such as photoconducting dipole antennae as described in P. Smith et al.,
IEEE J. of Quantum Electronics, Vol. 24, No. 2, pp. 255-260 (1988) and N.
Katzenellenbogen et al., Appl. Phys. Lett., Vol. 58, No. 3, pp. 222-224
(1991). With these techniques, terahertz spectroscopy offers a reasonably good
signal-to-noise ratio ( up to approximately 104); it can be performed
without special thermal stabilization apparatus such as cooled detectors; it can
be realized in a compact system; and it offers a transmitter and detector technology
which is compatible with integrated circuit technology.
Numerous experiments using terahertz time domain spectroscopy have
been performed on solids, liquids, and gases. Some experiments have analyzed the
spectrum of a terahertz signal affected by carriers in semiconductors and superconductors.
Other experiments have performed terahertz time domain spectroscopy on water vapor
as well as N2O gas. Still other experiments have reported terahertz
time domain spectroscopy of chemical compounds in the liquid phase. In all these
experiments, the terahertz signal was transmitted through the object under study
in a single illuminated volume region (usually 25 mm in diameter) to provide the
spectral information about that homogeneous region.
Summary of the Invention
I have recognized that the time domain spectroscopy and, more particularly,
terahertz signals can be used for imaging objects by collecting individual signals
propagating through distinct (spatially separate) points on the object and processing
these signals to create the image of the object. It is also possible to focus the
signal source on the object at distinct points and scan the source and detector
in synchronism across the object in a pattern transverse to the propagation direction.
Additionally, it is possible to cause to source to bathe the entire object with
substantially parallel beams which could then be sampled by a detector scanning
the object. Of course, in an alternative embodiment it would be possible to translate
the object in the appropriate transverse directions while holding the focused transmitter
and receiver in substantially fixed positions.
Brief Description of the Drawing
A more complete understanding of the invention may be obtained by
reading the following description of specific illustrative embodiments of the
invention in conjunction with the appended drawing in which:
- FIG. 1 shows a simplified block diagram of an illustrative terahertz imaging
system in accordance with the principles of the present invention;
- FIGs. 2 and 3 show comparisons between input terahertz waveforms and the output
waveform after propagating through a known material;
- FIGs. 4 through 6 show illustrative embodiments for insuring a desired amount
of scanning for the object to be scanned by the system of FIG. 1;
- FIG. 7 shows a portion of an illustrative terahertz focal plane array useful
in the embodiment in FIG. 6; and
- FIG. 8 shows an image of a semiconductor dual-in-line packaged chip produced
by the illustrated terahertz imaging system.
The THz imaging system of FIG. 1 in accordance with the present invention
includes a source 1 of repetitive, femtosecond duration, optical pulses, an optically
gated transmitter 2 of THz transients having a broad spectral bandwidth, imaging
optics 3 comprising lenses and/or mirrors, an object 4 to be investigated, a time-gated
detector or detector array 5, a scanning delay 6 capable of changing the delay
between the femtosecond gating pulses on the transmitter and detector(s) at a rate
of a few Hz to hundreds of Hz for the purpose of temporally heterodyning the THz-frequency
transients down into the acoustic (Hz) range so that they can be processed by
electronic techniques, a digital signal processing unit 7 including a digital signal
processor and an A/D converter to process the time-domain data and extract the
desired information, and a display 8 to view the image.
Certain material and objects can be characterized by a frequency-dependent
absorption, dispersion, and reflection of terahertz transients in signals which
pass through the material or object. The present terahertz imaging system analyses
that frequency dependence in the time-domain by collecting that transmitted signal
propagating through the object and then processing the information contained in
those signals for every point or "pixel" on that object. This is a non-invasive
imaging technique that is capable of differentiating between different materials,
chemical compositions, or environments. This technique has applications not solely
limited to biomedical imaging of tissue, "safe X-rays", chemical reaction analysis,
environmental and pollution control, process control, materials inspection, fault
detection, non-contact mapping of the doping in semiconductor wafers, profiling
of doping and defects in laser crystals, and packaging inspection.
A typical terahertz transmitter emits a single cycle of electromagnetic
radiation centered at 1 THz after being illuminated by a 100 fs laser pulse from
either a modelocked dye laser operating around 620 nm or a modelocked Ti:Sapphire
or Cr:LiSAF laser operating around 800 nm. Because of the short duration of the
THz-transient, the spectrum is broadband, typically extending from less than 100
GHz to several THz.
No electronic circuit is capable of measuring and processing THz
bandwidth electrical signals directly at this time. Sampling techniques based on
the repetitive nature (typically ≈100 MHz repetition rate) of the optical and
THz pulses can be used to measure the THz waveforms provided that the sampling
window is shorter than any THz transient to be measured. Typical photoconducting
sampling gates have sampling times shorter than 0.5 ps and are thus able to measure
frequency transients in excess of 2 THz. No fast electronics is needed in the sampling
technique, and only the average photocurrent in the dipole antenna is measured.
Similar to a sampling scope, the delay between the THz waveform and the detector
gating pulse is scanned slowly at a rate of about 10-100 Hz. Thus, each sampling
pulse samples the THz pulse at a somewhat different time, until the entire THz
waveform has been reconstructed from the samples. This leads to a "temporal down
conversion" of the THz waveform into the kHz range, where it can readily be processed
by electronics. This sampling technique is also known as Equivalent-Time-Sampling
but is otherwise used in any commercial digital sampling oscilloscope. This isochronous
sampling technique has been described for picosecond optical sampling by K. Weingarten
et al. in IEEE J. of Quantum Electronics, Vol. 24, No. 2, pp. 198-220 (1988).
Many, if not most, compounds show very strong frequency-dependent
absorption or reflection within the frequency range covered by these THz transients.
Also, molecules and chemical compounds, at least in the gas phase, but also ions
in certain crystals, have strong and sharp absorption lines in the THz spectral
regions. The absorption lines are characteristic of the material under study such
as a water molecule and its environment and can serve as a "fingerprint" of the
molecule. Each chemical substance hence leads to a characteristic THz waveform
that identifies the chemical composition and environment of the sample. There are
also materials that are completely opaque to THz radiation such as metals and other
materials with high electrical conductivity.
In the present THz imaging system, the spectra described above need
not be computed or directly measured. Instead, the relevant information can be
extracted right from the time-domain data, in a manner similar to speech recognition
and processing. In FIGs. 2 and 3 are the input THz waveforms (dashed) and the waveforms
after propagation through the doped silicon sample (FIG. 2) and water vapor (FIG.
The digital signal processor can recognize the characteristic shapes
of the transmitted THz waveforms (specific shape change and attenuation in one
case for silicon and ringing with characteristic frequency in the other case for
water vapor), to determine the particular material at the spot illuminated by the
THz beam. This requires training (or loading) the DSP with these specific waveforms
in advance. Such a procedure is well within the knowledge of persons skilled in
the art and will not be repeated here.
In a particular embodiment shown in FIG. 4 for the transmitter, receiver,
and optics of FIG. 1, the THz beam emerging from the transmitter is focused to
a diffraction-limited spot of 0.3-0.5 mm diameter. This is the diffraction-limited
spot size for 1 THz radiation and close to the best spatial resolution possible
with this technique. This spot is then imaged onto a single THz detector. The sample
is placed in the focal plane of the THz beam and scanned in x and y in a zigzag
pattern using two orthogonal, motor driven translation stages (shown pictorially
by the x and y arrows).
The delay between transmitter and detector gating pulses is continuously
scanned by a 10 Hz scanning delay line. The amplitude of the scanning delay line
can be adjusted and determines the time window of data acquisition - a 1 mm amplitude
corresponds to a 6.7 ps time window. The average photocurrent induced in the photoconducting
dipole detector is measured with a current-to-voltage converter and then fed to
a A/D-converter and DSP processor card. We use an A/D-converter capable of a 50
kHz conversion rate, and a DSP processor that can Fourier-transform the waveforms
at a rate of 100 FFTs each second. Thus, with this system we can easily obtain
the FFT spectrum of each THz waveform synchronously with the 10 Hz scan rate.
In an example from practice, the FFT spectrum is represented on the
display screen as a colored dot with the frequency components of the THz spectrum
represented by the frequencies of the visible (rainbow) spectrum. That is, the
terahertz spectrum is mapped onto the visible spectrum and only those frequency
components which propagate through the object under study can contribute to the
Since DSPs are used in this system, it is also possible to utilize
time domain techniques by computing the convolution (correlation) between the
received terahertz signals and stored patterns which are related to particular
elements, compounds, etc. The signals which most closely match the received signals
will identify the point of the object being scanned.
In another example from experimental practice, the DSP processor
looks for certain absorption lines which are characteristic of a specific molecule
and assigns a specific color and intensity to this absorption pattern. After each
scan, the sample is moved by one "pixel" (preferably roughly the spot size of the
THz beam on the sample) and the display is updated for that particular pixel. With
the above system, a 50x50 image can be acquired and displayed in just over 4 minutes.
FIG. 8 shows a preliminary result for a THz image obtained as described
above. The picture is a so-called "THz X-Ray" of a packaged semiconductor chip.
In another embodiment shown in FIG. 5 for the transmitter, receiver,
and optics of the imaging system in FIG. 1, the sample remains stationary and
the THz beams are scanned across the sample. This can be done either by mechanically
steering the THz beam with mirrors or by optical steering of the THz beam (in which
case, steering of the optical beams causes a steering of the THz beams).
In the embodiment shown in FIG. 6 for the transmitter, receiver, and
optics of the imaging system in FIG. 1, THz waveforms for the entire sample are
acquired simultaneously by using a focal-plane THz detector array as shown in FIG.
7. Here, the entire sample is flood-illuminated by a THz beam, and the sample is
imaged onto the focal-plane detector array using a lens system. The flood-illumination
causes the illumination to appear as parallel beams from individual point sources.
The focal plane THz detector array consists of a two-dimensional array
of THz dipole antennas (in this case 50 pm on each side) which are lithographically
defined on a low temperature (LT)-GaAs or radiation-damaged Silicon-on-Sapphire
(SOS) chip so that the gating time is subpicosecond. MSM photoconductive switches
using interdigitated finger contacts are defined between the antenna chip. The
size of the interdigitated photoconductive MSM switch is roughly 10 pm square.
Each of the antenna/MSM elements constitutes a THz image pixel. The MSM photoconductive
switches are gated by a short optical pulse derived from a beam that covers the
entire area of the chip and is focused onto the MSM detectors using a microlens
array. The microlens array and the gate pulse can either come from the same side
as the THz radiation (with a beam splitter), or from opposite sides (in this case
the THz beam travels through the chip substrates before it is detected by the antennas).
Only 1 pJ of readout energy is required for each MSM gate, so that a 10 nJ optical
pulse can gate a 100x100 focal plane array. The antenna chip is solder bump-bonded
to another chip underneath with one contact on each antenna pad that carries the
detected photocurrent off the chip and to the DSP processor. Preferably, the underlying
chip contacted by the solder bumps is a CCD array, so that all pixels can be read
out sequentially like a video camera. The photogenerated charges are accumulated
in the CCD array over many optical pulses before the charge is read out.