Terahertz (THz) imaging is promising for nondestructive evaluation, since many optically opaque dielectrics are
transparent to THz waves. Conventional THz imaging systems employ focusing elements such as spherical lenses and
off-axis parabolas, but their fixed focal length produces an inherent trade-off between lateral resolution and depth of
focus. Furthermore, image quality suffers when imaging objects located inside a dielectric medium. The air-dielectric
interface introduces significant spherical aberration that degrades spatial resolution. Bessel beams are known to produce
a small spot size over a large depth of focus. The contribution of our work is two-fold: (1) We demonstrate THz imaging
with a significantly improved depth of focus using a zero-th order Bessel beam produced by an axicon lens. (2) We also
demonstrate, for the first time to our knowledge, that Bessel beams experience reduced spherical aberration when
imaging objects embedded in a dielectric medium. Imaging experiments are performed with a time-domain THz system,
where a zero-th order quasi-Bessel beam is formed with an axicon lens made from high density polyethylene (HDPE).
The HDPE axicon has a 50 mm diameter and an apex angle of 120 degrees. Point spread function (PSF) measurements
confirm that lateral resolution is maintained over a 25 mm depth of field in air. The same lateral resolution is achieved
over a 35 mm range inside a HDPE substrate. Needle objects embedded inside a thick HDPE substrate are imaged with
high spatial resolution. Image contrast is significantly improved by digital filtering to reduce sidelobe levels. These
promising results suggest that Bessel beams are well suited for terahertz nondestructive imaging of thick dielectric
Spectroscopic photoacoustic microscopy (PAM) requires a pulsed nanosecond laser with tunable wavelength, but such
lasers are expensive and have poor wavelength switching speed. We are developing a rapidly tunable system based on a
high repetition rate supercontinuum source. A supercontinuum is produced by propagating 0.6 ns duration pulses from
an 7.5 kHz Q-switched Nd:YAG microchip laser through 7 meters of photonic crystal fiber (PCF). Wavelength
selection is achieved with a rapidly tunable prism-based monochromator, where an actuator-controlled mask selects the
desired wavelength band. Ten different wavelength bands (570 to 930 nm) are acquired in less than 1 second for each
image pixel. Each wavelength has a bandwidth of 40 nm. The PAM system employs optical focusing of the excitation
beam and detection with a 25 MHz spherically focused f/3 transducer. Multiwavelength imaging is tested on phantoms
with different color inks. The inks were correctly identified by processing the multiwavelength images with a linear
discriminant analysis. A major advantage of our tunable source is the high repetition rate and rapid access to widely
separated wavelengths. These promising results suggest the potential of our wavelength agile source for spectroscopic
Photoacoustic microscopy (PAM) provides excellent image contrast based on optical absorption. Microchip lasers are
attractive optical sources for PAM, as they are compact and provide nanosecond pulse durations at several kHz
repetition rates. However, spectroscopic imaging is not possible with microchip lasers due to their fixed wavelength
output. We are investigating multispectral PAM with a supercontinuum source based on a photonic crystal fiber (PCF)
pumped with a microchip laser. The Q-switched Nd:YAG microchip laser produces 0.6 ns duration pulses at 1064 nm
with 8 uJ of energy at a 6.6 kHz repetition rate. These pulses are sent through 7 meters of PCF with a 5 um diameter core
and a zero dispersion wavelength of 1040 nm. The supercontinuum is sent through a tunable band-pass filter before
being focused into the object. Photoacoustic detection is performed with a 25 MHz spherically focused f/2 transducer.
En-face imaging experiments were performed on ink phantoms. Images are acquired at seven different wavelengths from
575 to 875 nm. A simple discriminant analysis of the multispectral photoacoustic data produces images that clearly
distinguish the different absorbing regions of the sample. These preliminary results suggest the potential of the
supercontinuum PCF source for multispectral PAM.
Sparse arrays are highly attractive for implementing two-dimensional arrays, but come at the cost of degraded image
quality. We demonstrate significantly improved performance by exploiting the coherent ultrawideband nature of singlecycle
THz pulses. We compute two weighting factors to each time-delayed signal before final summation to form the
reconstructed image. The first factor employs cross-correlation analysis to measure the degree of walk-off between timedelayed
signals of neighboring elements. The second factor measures the spatial coherence of the time-delayed delayed
signals. Synthetic aperture imaging experiments are performed with a THz time-domain system employing a
mechanically scanned single transceiver element. Cross-sectional imaging of wire targets is performed with a onedimensional
sparse array with an inter-element spacing of 1.36 mm (over four λ at 1 THz). The proposed image
reconstruction technique improves image contrast by 15 dB, which is impressive considering the relatively few elements
in the array. En-face imaging of a razor blade is also demonstrated with a 56 x 56 element two-dimensional array,
showing reduced image artifacts with adaptive reconstruction. These encouraging results suggest that the proposed
image reconstruction technique can be highly beneficial to the development of large area two-dimensional THz arrays.
We report spectral domain optical coherence tomography (SDOCT) with a supercontinuum source based on a photonic
crystal fiber pumped with nanosecond laser pulses. The Q-switched Nd:YAG microchip laser produces 0.6 ns duration
pulses at 1064 nm with 8 μJ of energy at a 6.6 kHz repetition rate. These pulses are sent through 3 m of photonic crystal
fiber with a zero dispersion wavelength of 1040 nm. The fiber output is coupled into a fiber-based SDOCT system
operating at a central wavelength of 800 nm. The A-line acquisition rate is 6.6 kHz, where each A-line is produced by a
single supercontinuum pulse. Point spread function measurements show excellent resolution, but sensitivity is degraded
by spectral fluctuations of individual supercontinuum pulses. Test images show less dynamic range compared to a
Ti:Sapphire femtosecond laser based system. However, this supercontinuum source has potential for stroboscopic
illumination in time-resolved low coherence interferometry.
High-quality crystals of gallium selenide (GaSe) and thallium arsenic selenide (Tl3AsSe3) were successfully grown. The refractive indices were measured in the subterahertz spectral region using time-domain spectroscopy. GaSe has a refractive index of 3.2 and an absorption coefficient of 1 cm–1, along with an absorption peak at 0.6 THz. Tl3AsSe3 clearly shows birefringence, where the refractive indices are 5.0 and 5.4 along the fast and slow axes, respectively. The absorption coefficient is over 3 cm–1 at 0.3 THz, increasing steadily with frequency.
Multi-dimensional, high frequency ultrasound arrays are extremely difficult to fabricate from conventional piezoelectrics. For over a decade, our lab has explored optical detection as an alternate technology for high frequency applications. We have developed several different types of acoustically coupled optical resonators to provide the sensitivity and bandwidth required for biomedical imaging. Waveguide and fiber lasers, thin Fabry-Perot etalons constructed from polymers, and thin microring resonators imprinted into polymers have all been used as ultrasound transducer arrays. Their performance rivals the theoretical conversion efficiency of piezoelectric devices but with bandwidths approaching 100 MHz, array element dimensions approaching 10 um, and no electrical interconnects. In this paper we present results on several resonant optical ultrasound transducer (ROUT) arrays, emphasizing their potential use in photoacoustic imaging. These results strongly suggest that a high resolution photoacoustic microscope can be constructed using a ROUT in a footprint appropriate for endoscopic and minimally invasive applications.
Using a scanned laser to generate ultrasound, via the thermoelastic effect, offers an alternative approach for realizing high density, high frequency ultrasound imaging arrays. The approach bypasses the complexity and intricacy required for forming conventional piezoelectric array elements and their associated electrical connections. Thus, it is particularly well suited to 2D arrays. In this paper, the devices considered comprise a carbon black loaded PDMS polymer layer on top of a glass or PDMS substrate. PZFlex Finite Element Analysis (FEA) was used to investigate the impact of a variety of design variables including: laser spot size, substrate material and thermoelastic coupling medium. Predicted single element angular response broadly matched responses obtained by experiment. Specifically, if a low acoustic loss glass substrate is used then measurable sidelobes occur at approximately 40 degrees. However, if the glass substrate is replaced by a PDMS material, then the traveling waves that give rise to sidelobes are no longer supported and a smooth single element angular response is obtained in both experiment and FEA simulation. FEA suggests that there are other modes in addition to the Rayleigh mode observed in the experiment. It is believed that these modes are more quickly damped in the experimental case. Therefore, while FEA provides a very versatile and valuable analysis tool, the accuracy of its predictions are contingent on accurate knowledge of device geometry and relevant material properties.
A promising alternative to piezoelectricity for high frequency array applications is optical generation and detection of ultrasound. An array element is defined by the size and location of a laser beam focused onto a suitable surface. We've built a two-dimensional synthetic receive array, where a HeNe laser probes the surface displacements of a thin reflective membrane. Using a conventional transducer as the ultrasound source, images with near optimal resolution and wide fields of view have been produced at 10 - 50 MHz. We are currently exploring a different form of optical detection where the incident ultrasound modulates the thickness of an etalon (a Fabry-Perot interferometer). Preliminary experiments demonstrate improved sensitivity using a high finesse etalon. Our work in optical generation of ultrasound uses the thermoelastic effect. A major drawback to thermoelastic generation has been poor conversion efficiency. We obtained an increase in conversion efficiency of nearly 20 dB using an optical absorbing film consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black. Radiation pattern measurements indicate that we have produced a 75 MHz two-dimensional array element. These results demonstrate the potential of optoacoustic arrays for high frequency ultrasound imaging.
We present time- and space-resolved XUV spectra of boron and carbon plasmas created by focusing 100-fs laser pulses on a solid target to an intensity of 1017 W/cm2. Emission lines originating from He-like and H-like excited states from n equals 2 to the ionization limit are observed with a spatial resolution of 100 micrometers in the direction normal to the target plane and with a temporal resolution of up to 4 ps. The position of the ionization limited is seen to depend very crucially on the plasma parameters of density and temperature, and is explained through continuum lowering effects. We observed the dynamics of the continuum lowering for plasma slices at different distances from the target, and record a maximum lowering of 40 eV in He-like carbon (10% of the ionization potential) from the disappearance of the 1snp - 1s2 line and from the position of the continuum edge.