A tunable terahertz (THz) filter is reported, which was based on surface plasmonic effects from a subwavelength copper hole array, and the tunability was made possible by a substrate of phase-transition vanadium dioxide (VO2) film. The phase transition of the VO2 film was induced by femtosecond laser pulses and the modulation depth of the THz pulse peak by the VO2 film was measured by THz time-domain spectroscopy to be 80% under optimal fabrication conditions. The change of the conductivity of the substrate film could lead to a shift of the center resonance frequency from 1.33 to 0.92 THz, a relative shift of 30.8%. This tunable THz filter holds great promise for various applications.
A novel ring fiber laser based on Er-doped photonic crystal fiber (EDPCF) is demonstrated. EDPCF is used as gain
medium to implement stable CW operation. The laser output wavelength can be continuously tuned over the range from
1529 to 1535 nm by adjusting the polarization state of the light inside the cavity. Stable dual-wavelength CW operation
is also observed with proper settings of the intracavity polarization controllers. The laser stability in terms of the output
power and the operation wavelength is characterized, and less than ±0.05 dB power fluctuation as well as 18 pm
wavelength perturbation are obtained.
Nano-particles and nano-period ripples can be used to form some micro-nano-instruments in microelectronics. In this
paper a femtosecond (fs) pulse laser (148fs, 1KHz, center wavelength=775nm) is used to irradiate on metal surface, then
nano-particles and nano-period ripples can be obtained in the focal scale, the size of which is shorter than the center
wavelength. For describing the formation time and characteristic of nano-particles and nano-period ripples, we have
increased the laser fluence from the threshold of material to a higher value and increased the laser exposure time in fixed
laser fluence on different materials. Experimental results show that nano-particles occur earlier than nano-period ripples.
And as the fs laser pulse number being increased, the periodical nano-ripples is clearer and clearer and the depth of
ripples is deeper and deeper. Finally, permanent ablation will occur in the focal scale basing on the nano-ripples. After
surface plasmon polaritons theory is used, the period of nano-ripples can be explained exactly. This work is the basic
research for micro-nano-instruments by fs laser ablation technique on metal surface.
We demonstrate a passively mode-locked all fiber ring laser based on all-solid Yb-doped Photonic Bandgap fiber (AS-Yb-
PBGF) which is designed to simultaneously provide laser gain and anomalous dispersion. The laser is comprised of
0.42m AS-Yb-PBGF and a segment of single mode fiber (SMF). By varying the length of the SMF from 0.8m to 3.2m,
three operation schemes, namely soliton operation, gain-guided soliton operation, and self-similarity operation, have
been obtained by means of numerical simulations in split-step Fourier method in the ring laser. In the case of net
anomalous cavity group-velocity dispersion (GVD) when the SMF is 0.8m, we obtain soliton pulse with the balance
between the nonlinear self-phase modulation (SPM) and the anomalous GVD. The soliton pulse has the pulse duration of
355fs and the bandwidth of 1.9nm, corresponding to the time-bandwidth product of 0.33, nearly a transform-limited
pulse. As the length of the SMF is changed to 2m, self-similar propagation is generated with the net cavity GVD of
+0.014ps2. The pulse has strong frequency chirp and high pulse energy up to 4nJ. The pulse duration is 2.5ps with the
bandwidth of 2.1THz. To obtain gain-guided soliton, we change the SMF to 3.2m leading to a large net positive cavity
GVD of +0.04 ps2 and enforce the spectral limit of the gain fiber. A chirped gain-guided soliton pulse is observed with
pulse energy of ~1nJ, the pulse duration of 2.5ps and the bandwidth of 2.5 THz.
Although the ZnTe crystal used the generation of THz radiation has a favorable phase matching properties at 820nm, it
has strong third-order nonlinearity, such as two-photon absorption effect. And we found experimentally that, the effect of
two-photon absorption in ZnTe on the THz radiation was not neglect. In addition, due to optical rectification and
diffraction effect, the generated efficiency of THz radiation is also related to the optical excitation size in the emitter. In
this paper, by taking into account optical rectification, diffraction and two-photon absorption effects, the theoretical
model is established to describe the emitting field intensity of THz radiation. There is excellent agreement between the
theoretical results and the experimental data. The good agreement demonstrates that there is a trade-off between these
three effects for THz radiation.
In this paper, we applied wavelet-transform to analysis the water vapor absorption spectroscopy in terahertz range, and
compared with conventional Fourier-transform analysis in terahertz time-domain spectroscopy. The extracted absorption
lines are in excellent agreement with the results of using Fourier-transform analysis. The results show that
wavelet-transform analysis of the absorption spectrum delivers accurate absorption lines in THz range. Because the
results are shown in a time-frequency domain, it gives a more intuitive image on when the absorption happens at which
frequency. Combining wavelet-transform technique with THz-TDS, we hope there births a new spectroscopy: the
wavelet-transform terahertz time-domain spectroscopy.
As self-frequency-doubled (SFD) lasers are potentially more compact and less costly, the novel high efficient SFD lasers
attract great interest. Here we report the spectroscopic properties under room temperature of a new self-frequency-double
Yb3+ doped Gd0.2Y0.75(BO3)4 (Yb:GdYAB) single crystal grown in Shandong University. Both foundation and
self-double-frequency continuous-wave laser were operated with a high-power diode. A foundation laser slope
efficiency of 82% with respect to the absorbed 975 nm pump power of 2.62 W was obtained. The center wavelength at
1040nm was generated and the laser wavelength could be tuned from 1020 to 1064 nm. And thus, the broadly emission
spectra was advantageous for the production of short-pulse diode-pumped solid state lasers. Self-double-frequency green
light was also obtained under pump power of only 274mW with a flat-concave cavity. The maximal output power was
330mW when pumped power was 2.62 W. A diode-to-green optical conversion efficient of 12.6% was achieved. The
results demonstrated that the Yb:GdYAB crystals are potential candidates for efficient microchip both 1 μm and visible
laser media.
To study the reflection and transmission properties of the gratings, a novel system based on terahertz time domain spectroscopy was established. With this system, we have synchronously measured both the reflection and transmission spectra of the deep zero-order grating with subwavelength slits. In the experiments we found that the enhanced transmission spectra of the grating are complementary to the reflection spectra, which strongly depend on the geometry of the sample and the grating pose relative to the incident THz radiation, such as leaning from incident THz radiation and slits direction of the gratings out of p-polarized radiation. The experimental results agree with the theoretical analysis based on the surface plasmon polaritons and waveguide theory.
Using a plane wave expansion method, we numerically show that the bandgaps of honeycomb photonic bandgap fibers can be effectively tuned by interstitial air holes. It is shown that the number of bands varies with the filling fraction of interstitial air holes, and also that the width of the bands can be substantially increased by interstitial air holes. For a honeycomb photonic bandgap fiber with a large air filling fraction, the widths of the primary and secondary bandgaps are increased twice by interstitial air holes. In the case of a small air filling fraction, the relative sizes of the two gaps can reach up to 11.7% and 6.4% using interstitial air holes. Therefore, using interstitial air holes proves to be an effective way to tune the bandgaps of honeycomb photonic bandgap fibers.
We present a new method of diagnosing cancer, femtosecond laser in vivo HpD (haematoporphyrin derivatives) two-photon fluorescence, and observations of in vivo HpD two-photon fluorescence of cancer tissue of little mice using excitation of femtosecond laser pulse generated by self-mode-locked Ti:sapphire laser. The narrow pulse width, the high peak power of the excitation source and the central wavelength of 810 nm that is the window wavelength of biological tissue show that biomedical signal induced by this light source must have high signal-noise ratio. This femtosecond laser pulse and the two-photon fluorescence technique do no harm to normal tissue surrounding the cancer. The result of our experiments shows that the cancer tissue can be distinguished and diagnosed with the method of in vivo HpD two-photon fluorescence excited by femtosecond laser pulses with suitable wavelength.
A dynamic equation of passive mode locking with the equivalent fast saturable absorber in self-mode-locked solid state laser is presented. With the linearized rate equations , we obtained the self-starting condition .Furthermore, the relation between the growth rate of coupling modes and the starting factor is presented. The theory is in good agreement with the experimental results obtained by other researchers.
We demonstrate a femtosecond Kerr-lens mode-locked Cr:LiSGaF laser that based on a compact cavity geometry. For the first time to our knowledge, with a combination of 488-, 476- and 458-nm lines from Argon-ion laser as a pump source, 58-fs pulses centered at 845 nm were generated. The average output power is 30 mW, and the time-band product is 0.331, assuming a sech2 pulse shape. Neither Acousto-modulator nor physical aperture or slit were used in this cavity to start and sustain the Kerr-lens mode-locking operation.
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