共查询到20条相似文献,搜索用时 15 毫秒
1.
The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity
of 1013 W/cm2 in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth
increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp
decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has
been detected in BaF2 crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The
generation of coherent phonons during local laser excitation in CaF2 and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively. 相似文献
2.
V. I. Bezborodov V. K. Kiseliov P. K. Nesterov M. S. Yanovsky 《International Journal of Infrared and Millimeter Waves》2006,27(5):725-734
A phase frequency changer (PFC) for quasi-optical transmission line with phase sections made of crystal quartz and polarizers
executed on the basis of small-period wire gratings has been considered. The PFC is intended for operation in terahertz (THz)
frequency region. It has been examined at the frequency f
o= 0.89THz. The influence of the differential phase shift deviation in the sections on the output signal spectrum and the influence
of the sections mismatch on the reflection signal spectrum have been considered. In the frequency region ± 10% f
o the levels of the spurious spectral components of output signal are less than −40 dB with regard to the level of the useful
signal of the shifted frequency. The levels of all spectral components of the reflection signal are less than −60 dB with
regard to the level of the useful component of the PFC output signal. 相似文献
3.
The characteristics of a coherent tunable terahertz-wave source have been experimentally investigated in detail. By using
a difference frequency generation as a result of stimulated Raman scattering by polaritons in MgO:LiNbO3 crystals, the signal wave was continuously tuned in the range of 1.069–1.075 μm, which corresponded to a terahertz-wave in
the wide range from 227 to 104 μm (1.3 to 2.9 THz). The highest output signal pulse energy at the pumping level of 44 mJ/pulse
was 1.8 mJ/pulse with 20 mW seeder injection, and the terahertz peak power was 139 mW at the wavelength of 143 μm. This source
has the advantages of simple alignment, simplicity of tuning, and compactness that makes the device more attractive. 相似文献
4.
Yonghong Zhang Xiaohong Tang Yong Fan Zhengde Wu 《International Journal of Infrared and Millimeter Waves》2008,29(2):142-147
A W-band coherent stepped-frequency pulsed radar front-end is developed. It consists of a millimetre wave transmitting source,
a mm-wave local source, a DDS with multi frequency points output and two microwave sources serving as local oscillators. All
the sources are coherent with the 120 MHz referenced crystal oscillator. The mm-wave sources are realized by frequency multiplier
chain, up-conversion and injection locking. The phase noise of fundamental-wave injection-locked W-band harmonic Gunn oscillator
output signal achieves −98 dBc/Hz at 10 kHz offset and the spurious output is less than −50 dBc. The received intermediate
frequency signal is also presented. 相似文献
5.
U. Bovensiepen 《Applied Physics A: Materials Science & Processing》2006,82(3):395-402
The Gd(0001) surface is investigated by pump–probe experiments using femtosecond laser pulses at 740–860 nm wavelength. Employing
optical second-harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions
that are even and odd with respect to magnetization reversal. A coherent phonon–magnon mode at a frequency of 3 THz that is
excited through the exchange-split surface state is observed in the time domain. A magneto-elastic phonon–magnon interaction
based on spin–orbit coupling is weak for Gd and a modulation of the exchange interaction mediated by the lattice vibration
is proposed as a microscopic interaction mechanism of this coupled mode. In parallel, electron–electron and electron–phonon
interactions and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation
of the optical wavelength shows that for longer wavelengths up to 860 nm the coherent mode dominates, while for shorter ones
(≥740 nm) incoherent contributions prevail. This dependence indicates that selective depopulation of the occupied surface
state component drives the coherent excitation. However, temperature-dependent studies show that the oscillation amplitude
of even and odd contributions scales with the spin polarization of the surface state, suggesting that the spin dependence
of the ion potentials contributes as well. Furthermore, the frequency of the coherent mode presents a blue shift with a delay
of 0.17 THz/ps that saturates at the static frequency of the respective bulk phonon. This behavior is a consequence of equilibration
of the screened ion potential at the surface subsequent to the intense laser excitation.
PACS 78.47.+p; 63.22.+m; 63.20.Ls; 75.30.Ds 相似文献
6.
《中国光学快报(英文版)》2015,(9)
In order to realize homodyne reception and Doppler frequency shift tracking in ground-to-satellite coherent laser communication, a local laser is experimentally demonstrated in this Letter. It is realized based on modulationsideband injection locking, and has a 10 GHz tuning range, a 1 THz/s tuning rate, a 5 k Hz linewidth, and 16 m W of output power. When applied to a Costas loop in a coherent laser communication system, the local laser can achieve ?5 GHz Doppler frequency shift tracking with a 20 MHz/s frequency shift rate, which is sufficient for the ground-to-satellite coherent laser communication. 相似文献
7.
The article presents a proposal for a new method of automatic quality control of microlenses arrays, which is based on a semiderivative
real filter. The use of the semiderivative filter for examining pure-phase objects involves modifying the spatial frequency.
The basis of the proposed setup is a 4f correlator setup with coherent light. The phase object examined is placed in the input
plane of the correlator. Next, the light passes through a filter located in the frequency plane, which gives an intensity
signal. In the output plane a charge-coupled device (CCD) camera registers the light intensity, the range of which informs
the shape of the phase object. The proposed method is shift invariant, so it allows for examination of single elements or
a set of micro-optical elements simultaneously. Additionally, the same setup allows for measuring the phase of objects whose
thickness is either considerably smaller or much bigger than 2π. 相似文献
8.
Q. Xu X. M. Hu J. W. Yin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,51(2):267-273
The atomic coherence in a three-level Λ atom is studied, in which each optical transition is driven by a coherent field and
the metastable states are coupled to each other via a microwave field. It’s shown that the atomic coherence crucially depends
on the relative phase delay between the envelopes of the amplitudes of the three coupling fields. In particular, when the
phase delay is adjusted to 0 or π, the maximal atomic coherence arises, while the maximal atomic coherence doesn’t occur once
the phase delay is changed to π/2. The maximal atomic coherence is attributed to the trapping of the population in the lower
sublevels. 相似文献
9.
U. Madvaliev T. Kh. Salikhov D. M. Sharifov N. A. Khan 《Journal of Applied Spectroscopy》2006,73(2):185-193
We have theoretically studied the effect of thermal nonlinearity, due to the temperature dependence of the thermophysical
and optical parameters for thermally thick opaque media, on the characteristics of the fundamental photoacoustic signal when
the signal is detected by a gas microphone. We have shown that the dependence of the amplitude of the nonlinear component
of the signal on the intensity of the incident radiation I0 is expressed by means of the dependence of the temperature rise for the irradiated sample surface Θ0 on I0, and the thermal nonlinearity does not affect the phase of the photoacoustic signal. We propose a theory for generation of
the second harmonic of the photoacoustic signal. We have established that the phase shift of the photoacoustic signal is equal
to 3π/4, while its amplitude depends on the frequency (∼ω−3/2) and the intensity (∼ I
0
2
).
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 170–176, March–April, 2006. 相似文献
10.
Jun-ichi Nishizawa Tetsuo Sasaki Ken Suto Masahiko Ito Takashi Yoshida Tadao Tanabe 《International Journal of Infrared and Millimeter Waves》2008,29(3):291-297
We developed high-resolution GaP THz signal generator using Cr:Forsterite lasers with gratings as both a pump and a signal
beam for difference-frequency generation. A line width of less than 500 MHz and a wide tunable frequency range (0.6–6.2 THz)
provide sufficient resolution for measuring materials with sharp absorption bands using the generator as the light source
for a THz spectrometer. This is suitable for materials such as gases or solid samples at low temperatures. We demonstrated
the detection of defects in organic materials, as they appear as slight deviations in the absorption frequency in the THz
region. 相似文献
11.
This paper evaluates the coherent optical code-division multiple-access (OCDMA) techniques and examines the overall system
performance in terms of signal-to-noise ratio (SNR) penalty as a function of simultaneous users accommodated to maintain an
appropriate value of the bit-error rate (BER) for homodyne and heterodyne detections. As spreading codes, the proposed structures
are utilizing a recently introduced prime code family hereby referred to as double-padded modified prime code (DPMPC). As
a coherent modulation, binary phase-shift keying (BPSK) format is deployed. In homodyne detection, two different phase modulations
are studied including either an external phase-modulator or injection-locking methods. The phase limitation and the performance
for two methods plus multiple-access interferences (MAI) and receiver noise in a shot-noise limited regime are investigated.
In heterodyne detection, BER analysis of the system based on only external phase modulator is examined. It is found that by
maintaining BER = 10−9, this system can accommodate an increased number of simultaneous users to compare with systems which employ conventional
bipolar codes. 相似文献
12.
Time‐resolved two color pump‐probe polarization spectroscopy was performed at room temperature on SrTiO3 films grown directly on Si with film thickness varying from 2 nm to 7.8 nm. An E‐symmetry mode with a characteristic frequency of 0.2 THz is impulsively generated and measured in these coherently strained tetragonal phase SrTiO3 thin films. A superimposed exponentially decaying signal observed indicates the possible relaxational hopping of Ti ion between double potential wells. The dependence of the coherent phonon signal on pump and probe laser polarization helps to identify the symmetry of the phonon modes. 相似文献
13.
Wang F Cheever D Farkhondeh M Franklin W Ihloff E van der Laan J McAllister B Milner R Tschalaer C Wang D Wang DF Zolfaghari A Zwart T Carr GL Podobedov B Sannibale F 《Physical review letters》2006,96(6):064801
The generation of brilliant, stable, and broadband coherent synchrotron radiation (CSR) in electron storage rings depends strongly on ring rf system properties such as frequency and gap voltage. We have observed intense coherent radiation at frequencies approaching the THz regime produced by the MIT-Bates South Hall Ring, which employs a high-frequency S-band rf system. The measured CSR spectral intensity enhancement with 2 mA stored current was up to 10,000 times above background for wave numbers near 3 cm(-1). The measurements also uncovered strong beam instabilities that must be suppressed if such a very high rf frequency electron storage ring is to become a viable coherent THz source. 相似文献
14.
C. C. Santos I. Guedes J. P. Siqueira L. Misoguti S. C. Zilio L. A. Boatner 《Applied physics. B, Lasers and optics》2010,99(3):559-563
In a laser coherent combination, the phase detection and control is the most critical. Using the beat frequency method, consistence
of the phase in output beams can be ensured through real-time detection and correction for the phase change. Phase noises
are controlled by a liquid crystal phase modulator. At the same time, the liquid crystal polarization controller is used to
make the polarization state stable, which is good for the improvement of the combining efficiency. The wave length of the
main oscillation laser is 532 nm. The output power of laser can be adjusted continuously from 0 to 6 W. The shifted frequency
of the system is 40 MHz. The accuracy for phase control is superior to λ/70 RMS. In the process of closed-loop control, using the liquid crystal phase and polarization controller, a better signal
phase correction of optical path has been achieved for the coherent combination of high power laser arrays. 相似文献
15.
F. Hindle G. Mouret S. Eliet M. Guinet A. Cuisset R. Bocquet T. Yasui D. Rovera 《Applied physics. B, Lasers and optics》2011,104(4):763-768
The generation of cw-THz radiation by photomixing is particularly suited to the high resolution spectroscopy of gases; nevertheless,
until recently, it has suffered from a lack of frequency metrology. Frequency combs are a powerful tool that can transfer
microwave frequency standards to optical frequencies and a single comb has permitted accurate (10−8) THz frequency synthesis with a limited tuning range. A THz synthesizer composed of three extended cavity laser diodes phase
locked to a frequency comb has been constructed and its utility for high resolution gas phase spectroscopy demonstrated. The
third laser diode allows a larger tuning range of up to 300 MHz to be achieved without the need for large frequency excursions,
while the frequency comb provides a versatile link to be established from any traceable microwave frequency standard. The
use of a single frequency comb as a reference for all of the cw-lasers eliminates the dependency of synthesized frequency
on the carrier envelope offset frequency. This greatly simplifies the frequency comb stabilization requirements and leads
to a reduced instrument complexity. 相似文献
16.
Hsiang-Chen Chui Ming-Sheng Ko Yi-Wei Liu Tyson Lin Jow-Tsong Shy Sen-Yen Shaw Rostislav V. Roussev Martin M. Fejer 《Optics and Lasers in Engineering》2006,44(5):479-485
A 197.2 THz (1520.2 nm) ITU-T grid distributed feedback (DFB) diode laser is frequency stabilized at 197.198 THz by 1ocking its second harmonic (SH) signal on the rubidium 5S1/2→7S1/2 two-photon transition at 394.396 THz (760.1 nm). With 100 mW from the DFB diode laser and amplifying by an erbium-doped fiber amplifier, we obtain an SH power of 15 mW using a periodically poled lithium niobate (PPLN) waveguide frequency doubler. The stability was 2×10−11 (10 s), corresponding to a frequency variation of 4 kHz at 1520.2 nm. Our scheme provides a compact and high performance frequency reference in the communication band. 相似文献
17.
Phase-alternated compositeπ/2 pulses have been constructed for spinI=1 to overcome quadrupole interaction effects in solid state nuclear magnetic resonance (NMR) spectroscopy. Magnus expansion
approach is used to design these sequences in a manner similar to the NMR coherent averaging theory. It is inferred that the
symmetric phase-alternated compositeπ/2 pulses reported here are quite successful in producing quadrupole echo free from phase distortions. This effectiveness
of the present composite pulses is due to the fact that most of them are of shorter durations as compared to the ones reported
in literature. In this theoretical procedure, irreducible spherical tensor operator formalism is employed to simplify the
complexity involved in the evaluation of Magnus expansion terms. It has been argued in this paper that compositeπ/2 pulse sequences for this purpose can also be derived from the broadband inversionπ pulses which are designed to compensate electric field gradient (efg) inhomogeneity in spinI=1 nuclear quadrupole resonance (NQR) spectroscopy. 相似文献
18.
针对二氧化钒(VO2)薄膜在可调谐太赫兹功能器件中的应用, 采用磁控溅射法在K9玻璃衬底上制备了VO2薄膜, 并用X射线衍射(XRD)对薄膜的晶相进行表征。利用配备加热装置的太赫兹时域光谱系统(THz-TDS)研究了薄膜样品在变温过程中的THz反射、透射光谱特性及其变化规律。实验结果表明, 随着加热温度的升高, VO2薄膜发生半导体-金属相变并对宽频段THz波产生显著的调制作用。调制深度明显依赖于THz频率, 薄膜样品对THz波反射功率、透射率的幅度调制深度在0.3~0.5 THz范围波动较大;对THz波的透射率在低频处较大, 高频处较小, 调制深度在35%~65%之间变化。该薄膜制备简单, 质量高, 可应用于太赫兹开关和调制器等功能器件。 相似文献
19.
G. Chen J. G. Chen Y. J. Yang J. Q. Lin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2010,57(1):145-149
We numerically calculate the high-order harmonic generation (HHG)
power spectra from a one-dimensional model atom irradiated by linearly
polarised 12 fs two-colour laser pulses composed of a fundamental pulse from
Ti:sapphire laser and its second harmonic. It is found that a distinct
double plateau structure appears when the relative phase of the two pulses
is set as π/8, 2π/8 or 3π/8, and the double plateau structure
disappears when the relative phase is set as 4π/8, 5π/8, 6π/8
or 7π/8. The relative-phase-dependent plateau structure is explained by
the temporal profile of the synthesised electric fields as well as the
semi-classical “three-step” model. Moreover, our numerical result shows
that cut-off frequencies of the two-colour pulse HHG spectra can be exactly
predicted by use of the semi-classical “three-step” model. 相似文献
20.
利用低频光通信波段真空压缩态光场可实现基于光纤的量子精密测量.本文利用简并光学参量振荡器实验制备出1550 nm低频真空压缩态光场.在分析频段10—500 kHz范围内压缩态光场的压缩度均达3 dB.用实验制备的1550 nm真空压缩态光场填补光纤马赫-曾德尔干涉仪的真空通道,实现了量子增强型光纤马赫-曾德尔干涉仪,完成了突破标准量子极限的相位调制频率为500 kHz的低频信号测量.与光纤马赫-曾德尔干涉仪相比,测量信噪比提高了2 dB. 相似文献