Two-Dimensional Photoacoustic Imaging of Blood Vessel Networks within Biological Tissues

Photoacoustic tomography （PAT） is a powerful imaging merits and most compelling features of light and sound, technique for medical diagnosis because it combines the We describe a PAT experimental system constructed in our laboratory which consists of a Q-switched Nd：YAG pulse laser operating at 532nm with a 8-ns pulse width to generate the photoacoustic signals from a biological sample. Two-dimensional photoacoustic imaging of blood vessel networks lcm below the tissue surface is achieved, We also successfully demonstrate that the system is capable of imaging the blood vessels over the ex vivo rat brain with skull and skin intact.

Broadband diffusion of terahertz waves by multi-bit coding metasurfaces

The terahertz region is a special region of the electromagnetic spectrum that incorporates the advantages of both microwaves and infrared light waves.In the past decade,metamaterials with effective medium parameters or gradient phases have been studied to control terahertz waves and realize functional devices.Here,we present a new approach to manipulate terahertz waves by using coding metasurfaces that are composed of digital coding elements.We propose a general coding unit based on a Minkowski closed-loop particle that is capable of generating 1-bit coding(with two phase states of 0 and 180°),2-bit coding(with four phase states of 0,90°,180°,and 270°),and multi-bit coding elements in the terahertz frequencies by using different geometric scales.We show that multi-bit coding metasurfaces have strong abilities to control terahertz waves by designing-specific coding sequences.As an application,we demonstrate a new scattering strategy of terahertz waves—broadband and wide-angle diffusion—using a 2-bit coding metasurface with a special coding design and verify it by both numerical simulations and experiments.The presented method opens a new route to reducing the scattering of terahertz waves.

Influence of lattice symmetry and hole shape on the light enhanced transmission through the subwavelength hole arrays

The golden films with various subwavelength hole arrays on the film surface are designed and fabricated on glass substrate by electron beam lithograply(EBL),focused ion beam(FIB),and reactive ion etching(RIE),respectively.The influences of the hole array symmetry and the hole shape on the light-enhanced transmission through the films are observed and simulated.The experimental results show that when the array lattice constant and the hole diameter are the same in the different array structures which are 1 μm and 350 nm respectively,the square hole arrays exhibit two transmission peaks at 1170 nm and 1580 nm with the transmissivities of 3% and 6%,respectively,while the hexagonal hole arrays exhibit an enhanced peak of 14% at 1340 nm;when the lattice constant and the duty cycle are the same for different array stucture,the transmission peaks are different for different hole shapes,which are at 763 nm with transmissivity of 12% for rectangular holes and at 703 nm with the one of 9%,respectively.The numerical simulation results by using the transfer matrix method(TMM) are consistent with the observed results.

Arbitrary Three-Phase Shifting Algorithm for Achieving Full Range Spectral Optical Coherence Tomography

An arbitrary three-phase shifting algorithm is introduced in order to achieve full range spectral optical coherence tomography imaging of biological tissue. Theoretical treatment behind this approach is given and experimentally verified. It is shown that this method is capable of eliminating the undesired auto-correlation and complex conjugate images, leading to the un-obscured full range spectral OCT imaging. An intact porcine eye is used to demonstrate the potential of such a method for biological imaging.

Feasibility of photoacoustic tomography for ophthalmology

For the eyeball composed of membrane and liquid, the contrast of ultrasound imaging is not high due to its small variance in acoustic impedance. As a new imaging modality, photoacoustic tomography combines the advantages of pure optical and ultrasonic imaging together and can provide high resolution, high contrast images. In this paper, the feasibility of photoacoustic tomography for ophthalmology is studied experimentally. A Q-switched Nd：YAG pulsed laser with 7-ns pulse width is used to generate photoacoustic signal of a porcine eyeball in vitro. The two-dimensional （2D） optical absorption image of the entire eyeball is reconstructed by time-domain spherical back projection algorithm. The imaging results agree well with the histological structure of the eyeball and show a high imaging contrast.

Terahertz generation based on four-wave mixing difference frequency by resonance-enhanced third-order nonlinear of media

Using nanosecond pulse near-infrared and mid-infrared laser pulses as the pump source, we obtain terahertz wave sources via four-wave difference frequency mixing. From the coupled wave theory, we analyze the four-wave mixing process of GaSe crystal and alkali metal vapor in detail, get the analytical expression of terahertz wave output power, and discuss the conditions for achieving phase matching. By adjusting the pump frequency, the third-order nonlinear polarization of alkali metal vapor is resonance-enhanced. This program offers a new type of high-power terahertz radiation source.

Widely and continuously tunable optical parametric generator based on MgO-doped periodically poled LiNbO_3 crystal

A widely and continuously tunable optical parametric generator （OPG） pumped by a 1064-nm acoustooptically Q-switched diode-end-pumped Nd：YAG laser based on MgO-doped periodically poled LiNbO3 crystal with a multigrating structure （29.2—30.4 μm） is reported. A broad continuous signal spectrum of 1513—1700 nm is obtained by changing the crystal grating periods from 29.2 to 30.4 μm and by tuning the crystal temperature from 30 to 180 ℃ simultaneously. When the average pump power is 1.82 W with pulse duration of about 70 ns operating at a repetition rate of 10 kHz, the maximum signal output power of the periodically poled MgO-doped lithium niobate （PPMgLN） OPG is about 210 mW corresponding to the idler and total powers of 118.4 and 328.4 mW respectively.

Study on 660-nm quasi-continuous-wave intracavity frequency-doubled NdrYAG laser

A quasi-continuous-wave intracavity frequency-doubled Nd:YAG laser which operates at 660 nm is studied. By using a flat-flat laser cavity, 2 Kr-lamps, KTP crystal and an acousto-optically Q-switch, 2-W output power at 660 nm is obtained. The relationship between laser cavity length and output power is analyzed.

Study on CW Nd:YAG infrared laser at 1319 nm

A continuous wave (CW) Nd:YAG infrared laser at 1319 nm is reported in this paper. The energy level of 1319-nm wave was analyzed. The repression of 1064-nm lasing and enhancement of 1319-nm output power were discussed. Mirror coating and cavity structure were studied and a maximum CW output power of 43 W at 1319 nm was achieved in experiments.