Narrow-band terahertz wave generation and detection in one periodically poled lithium niobate crystal

In this article, we present studies on therahertz (THz) wave generation and frequency up-conversion in a periodically poled lithium niobate (PPLN) crystal. A frequency at 1.37 THz was generated as femtosecond pump pulses passed through a PPLN crystal with grating periods of 30 μm. The pump-induced THz wave interacts with the probe wave in the crystal by frequency mixing. The frequency up-converted THz wave is easily detected by a normal photodiode. A new scheme for generation and detection of THz wave in one non-linear crystal was proposed.     

级联参量振荡产生太赫兹辐射的理论研究

针对光学参量振荡产生太赫兹波转换效率低的缺点, 提出了级联参量振荡产生太赫兹波的新机理以提高转换效率. 以周期极化铌酸锂晶体为例, 对级联参量振荡产生太赫兹波的原理和过程进行了理论研究. 分析了抽运光波长、周期极化铌酸锂晶体极化周期和工作温度对产生一阶、二阶闲频光频率的影响. 推导了三波共线相互作用条件下太赫兹波的增益特性和吸收特性. 计算结果表明, 通过级联参量振荡可以有效提高太赫兹波的转换效率, 并可以得到宽调谐的太赫兹波输出. 基于分析结果, 设计了周期极化铌酸锂晶体级联参量振荡产生高效率、宽调谐、窄线宽、连续太赫兹波的实验. 关键词： 太赫兹波 太赫兹波参量振荡 级联参量振荡     

Surface-emitted terahertz-wave difference-frequency generation in two-dimensional periodically poled lithium niobate

We report on the demonstration of surface-emitted terahertz- (THz-) wave difference-frequency generation from two-dimensional (2D) periodically poled lithium niobate (PPLN). The two orthogonal periodic structures individually compensate for both the phase mismatch of the launched lasers and the generated THz wave. Tunable 1.5-1.8 THz wave generation with a bandwidth of 10-GHz was obtained by use of two 2D PPLN crystals. We also confirmed that THz waves were simultaneously generated into two opposite directions, which suggests the possibility of higher THz-wave output power.     

周期极化铌酸锂THz波产生理论分析

首先采用光参量振荡的机理研究了THz波在准相位匹配媒质周期极化铌酸锂晶体中同向传播与反向传播情况下,以及THz波从晶体表面辐射产生的特性;其次研究了THz波的温度调谐特性、极化反转光栅周期调谐以及改变入射泵浦光与光栅波矢夹角的调谐特性;然后分析了THz波谱宽特性,给出解析表达式,并与实验结果进行了比较,证明理论分析结果与Y.S.Lee等人所给出的实验结果符合很好;最后分析了THz波的稳定性与温度、光栅周期、泵浦入射角以及辐射角的关系.     

Second-harmonic generation of an optical frequency comb at 1.55 microm with periodically poled lithium niobate

To expand the span of the optical frequency comb (OFC), we generated the second harmonics of an OFC at 1.55microm , using a multiperiod periodically poled lithium niobate (PPLN) crystal. A coupled-cavity OFC generator with an average output power of 0.2 mW was amplified and expanded with a fiber amplifier and a dispersion-flattened fiber. The fundamental OFC average power and span were 100 mW and 45 THz, respectively. The second-harmonic comb's span was 3.2 THz; however, we tuned the center frequency over 30 THz by changing the poling period. We also demonstrated that the second-harmonic comb can be used for frequency-difference measurement.     

周期极化铌酸锂中光整流THz波辐射

从理论上详细研究飞秒激光在周期极化铌酸锂(PPLN)晶体中由光整流效应产生的THz波辐射.着重研究THz波辐射场的频域场、时域场和频谱宽度的分布.并详细讨论辐射场脉冲持续时间、幅度、频谱宽度随晶体长度和辐射角的变化. 关键词： PPLN 光整流 THz波     

Temperature dependent narrow-band terahertz pulse generation in periodically poled crystals via difference frequency generation

Femtosecond optical pulse is used to generate narrow-band terahertz pulses depending on a quasi-phase-matched condition in periodically poled lithium niobate (PPLN) and stoichiometric lithium tantalate (PPSLT) crystals by difference frequency generation. The origin of narrow-band THz generation proved that the two frequency components of the fs pulse contribute to the frequency mixing. By cryogenic cooling, the absorption of THz waves in the crystal is significantly reduced which results in efficient THz generation. Simultaneously generated forward and backward THz pulses were 1.38 and 0.65 THz with as narrow as the bandwidth of 32 GHz in the PPSLT sample. Temperature dependence of the generated THz waveforms had good agreement with the simulation result using one dimensional plane-wave propagation model.     

Terahertz-wave generation in a conventional optical fiber

We theoretically propose surface-emitted and collinear phase-matched terahertz (THz)-wave generation in a conventional optical fiber. The third-order nonlinear effect, four-wave mixing (FWM), is used to generate THz waves in an optical fiber. Surface-emitted THz-wave generation via FWM is realized using a single-mode fiber. Perfect phase matching is obtained at ~800 nm and 1.5 microm pumping, and it follows that third-order polarization in an optical fiber has the same phase at any point. In this situation, the optical fiber acts like a phased array antenna of the THz wave. Collinear phase-matching THz waves are obtained under the same conditions as for surface-emitted THz waves, and the THz wave is propagated in the silica cladding of the optical fiber. This is a promising method for realizing a reasonable THz-wave source.     

Tunable terahertz waves generated by mixing two copropagating infrared beams in GaP

By mixing two copropagating coherent beams near 1 microm in a zinc blende GaP crystal, we have efficiently generated coherent terahertz (THz) waves. Such efficient conversion is made possible by use of a rest-strahlen band in the THz region to achieve phase matching in an isotropic crystal. A tuning range as wide as 71.1-2830 microm (0.106-4.22 THz) was achieved, whereas the highest output peak power reached 15.6 W at 173 microm. To obtain such a tuning range we continuously tuned the wavelength of one coherent infrared beam within a bandwidth of approximately 15.3 nm.     

Generation of backward terahertz waves in GaSe crystals

We have observed a backward-propagating terahertz wave by mixing two coherent infrared beams in two GaSe crystals. For the 47 mm long crystal, the output wavelength can be tuned in the wide range of 167.6 to 2060 microm (0.146 to 1.79 THz), whereas the output peak power reaches 217 W. The corresponding power conversion efficiency is approximately 0.03%.     

Creation of multi-frequency terahertz waves by optimized cascaded difference frequency generation

A new scheme which generates multi-frequency terahertz (THz) waves from planar waveguide by the optimized cascaded difference frequency generation (OCDFG) is proposed. A THz wave with frequency ω_T1 is generated by the OCDFG with two infrared pump waves, and simultaneously a series of cascaded optical waves with a frequency interval ω_T1 is generated. The THz wave with a frequency of M-times ω_T1 is generated by mixing the m-th-order and the (m+M)-th-order cascaded optical wave. The phase mismatch distributions of cascaded difference frequency generation (CDFG) are modulated by changing the thickness of planar waveguide step by step, thereby satisfying the phase-matching condition from first-order to high-order cascaded Stokes process step by step. As a result, the intensity of THz wave can be enhanced and modulated by controlling the cascading order of OCDFG.     

Optical terahertz wave generation in a planar GaAs waveguide

We report generation of terahertz (THz) radiation in a planar 61-microm-thick GaAs waveguide with a TM0 propagation mode, achieved by phase-matched difference frequency mixing. The THz output was centered near 2 THz and had 1 microW average power. As a pump source we utilized both the signal and the idler outputs of a near-degenerate type II synchronously pumped optical parametric oscillator operating near 2 microm with the average powers of 250 and 750 mW, correspondingly.     

Periodically poled lithium niobate optical parametric amplifier seeded with the narrow-band filtered output of an optical parametric generator

The results of a simple scheme to generate continuously tunable pulsed narrow-bandwidth (less than 0.1 cm (-1)) light in the infrared are presented. A periodically poled lithium niobate (PPLN) optical parametric amplifier is seeded with the filtered output of a PPLN optical parametric generator. A high-finesse Fabry-Perot etalon is used as the filtering element, giving bandwidths as narrow as 0.08 cm (-1) and tunable over 18 cm (-1) without any adjustments to the PPLN crystals. High efficiency is obtained with a 15-ns 1-kHz Nd:YAG laser, giving energies of up to 180 microJ of signal at 1.6 microm and 60 microJ of idler at 3.3 microm .     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array

Real-time, continuous-wave terahertz imaging is demonstrated with a 10 mW, 2.52 THz (118.8 microm) far-infrared gas laser and a 160 x 120 element microbolometer camera. The microbolometer camera is designed for wavelengths of 7.5-14 microm but retains sensitivity at terahertz (THz) frequencies. The setup has no moving parts, and transmission-mode THz images can be obtained at the video rate of 60 frames/s. The peak signal-to-noise ratio is estimated to be 13 dB for a single frame of video, acquired in 16 ms. With this setup, THz imaging through a FedEx envelope is demonstrated, showing the feasibility of real-time mail screening.     

Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications

We review the generation of broadband THz radiation from femtosecond photo‐induced gas plasmas, with an emphasis on the highly efficient “AC‐bias” case where the plasma is generated and driven by a superposition of fundamental and second‐harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi‐quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four‐wave mixing model. We also employ these models to the case of few‐cycle pulses, where the observed THz emission is related directly to the carrier‐envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.}     

Electro-optic periodically poled lithium niobate Bragg modulator as a laser Q-switch

We report an electro-optic Bragg modulator using a periodically poled lithium niobate (PPLN) crystal. We measured a half-wave voltage of 160 V when transmitting a 1064 nm laser through a 14.2 mm long, 780 microm thick, 20.13 microm period PPLN crystal at the Bragg angle. We also demonstrated a Q-switched Nd:YVO(4) laser using such a PPLN Bragg modulator as its Q-switch, producing 7.8 ns, 201 microJ pulses at a 10 kHz repetition rate when pumped by a 19.35 W diode laser at 808 nm.     

Terahertz surface-wave resonant sensor with a metal hole array

A surface-wave sensor based on the resonant transmission characteristics of metal hole arrays is demonstrated in the terahertz (THz) region. Since the frequency of the transmission peak of a metal hole array, which corresponds to the resonant frequency of the surface waves, is particularly sensitive to the refractive index in the vicinity of the metal surface, a very small change in the substances attached to the surface can be detected by monitoring the transmission spectrum. By attaching a layer of substance (thickness t < 5 microm) much thinner than the wavelength of the THz wave (lambda(THz) = 1 mm at 0.3 THz) to the surface of a metal hole array, we demonstrated that the existence of such a small amount of substance can be detected more easily than without the metal hole array. This demonstration of THz sensing with metal hole arrays indicates the possibility of realizing THz surface-wave sensors for biochemical molecules in the THz region.     

红外低发射率隐身涂层对太赫兹波的反射光谱研究

为研究红外低发射率隐身涂层对太赫兹波的反射特性,制备了红外低发射率隐身涂料,测试了其可见光效果、红外热像图及红外发射率等特性参数。以土黄色红外低发射率涂料为测试样品,利用透射式太赫兹时域光谱系统获得了样品在太赫兹波段的复折射率。分析了特征矩阵理论,并利用特征矩阵理论计算了涂层厚度(0.3~0.5 mm）与入射角度(0°~60°)的变化对入射太赫兹波反射特性的影响。结果表明,在相应厚度及入射角度范围内,太赫兹波在0.8 THz频率下具有多个反射峰值,最高值可达90%以上,有利于实现太赫兹波对红外低发射率隐身涂层下金属目标的探测。此外,涂层厚度变化对入射太赫兹波反射率具有较大影响,涂层越厚,太赫兹波的反射振荡越多,反射峰值越大。入射角度对太赫兹波的反射特性具有一定的影响,但整体影响不大,有利于太赫兹波实现多角度目标的探测。最后,以表面均匀涂覆0.42 mm厚涂料的金属板为测试样品,实验测量了样品在0.1~1.5 THz频率范围内的反射特性,并与部分理论计算结果进行对比。结果表明：实验测量结果与理论计算结果在数值和趋势上较为吻合,但也存在一定的偏差。究其原因,主要由样品厚度和样品参数误差导致,但依然可利用特征矩阵理论研究红外低发射率涂层对太赫兹波的反射光谱特性。     

半导体激光直接倍频的488nm蓝光激光器

利用波导型准相位匹配周期极化反转铌酸锂(PPLN)晶体直接倍频波长为976 nm的连续半导体激光二极管,在最佳晶体工作温度(28℃)下,获得了波长为488 nm的连续蓝光输出,最大输出功率大于20 mW。所用的晶体尺寸为8 mm×1.4 mm×1 mm,波导截面为4.5μm×3.5μm,极化周期为5.2μm。研究了波导型周期极化反转铌酸锂晶体的倍频效率与温度的关系,与普通的周期极化反转铌酸锂相比,倍频效率与温度关系的敏感度较低。同时,由于晶体可以在室温下工作,简化了加温与温控部件,提高了整机的工作效率。在此实验的基础上,制成了一台小型的全固态488 nm连续蓝光激光器。