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

The THz band, ranging from 0.1 - 10 THz, has largely remained untapped due to challenges in the THz generation. This band occupies a unique position in the electromagnetic spectrum between RF and optical frequencies and can enable several applications such as high-speed communication, imaging, radar, and spectroscopy. However, THz frequencies lie beyond the fmax of silicon technologies, making direct THz generation unfeasible. This paper is an overview of approaches for efficient high-power THz generation in silicon by utilizing the reverse recovery of PIN diodes. We review the design of a THz pulse radiator and a Continuous wave (CW) radiator fabricated using GlobalFoundries 90 nm SiGe BiCMOS process. THz applications such as imaging, vibration sensing, gas spectroscopy, and communication, are demonstrated. THz channel characterization using a pulse radiator is also presented. We conclude the paper with a discussion on the potential for silicon-based THz generation beyond 1 THz.

Based on arsenic tri-sulfide films on titanium-diffused lithium niobate, we designed a hybrid optical waveguide for efficient mid-infrared emission by phase-matched difference frequency generation (DFG). The hybrid waveguide structure possesses a low-index magnesium fluoride buffer layer sandwiched between two high-index As(2)S(3) slabs, so that pump and signal waves are tightly confined by titanium-diffused waveguide while the DFG output idler wave at mid-infrared is confined by the whole hybrid waveguide structure. On a 1 mm-long hybrid waveguide pumped at 50 mW powers, a normalized power conversion efficiency of 20.52%W(-1)cm(-2) was theoretically predicted, which is the highest record for mid-infrared DFG waveguides based on lithium niobate crystal, to the best of our knowledge. Using a tunable near-infrared pump laser at 1.38-1.47 µm or a tunable signal laser at 1.95-2.15 µm, a broad mid-infrared tuning range from 4.0 µm to 4.9 µm can be achieved. Such hybrid optical waveguides are feasible for mid-infrared emission with mW powers and sub-nanometer linewidths.

We present the numerical results for the optimization of the pump-to-idler conversion efficiencies of nanosecond idler wavelength tunable cascaded optical parametric oscillators (OPO) in different wavelength tuning ranges, where the primary signals from the OPO process are recycled to enhance the pump-to-idler conversion efficiencies via the simultaneous difference frequency generation (DFG) process by monolithic aperiodically poled, magnesium oxide doped lithium niobate (APMgLN) crystals. The APMgLN crystals are designed with different chirp parameters for the DFG process to broaden their thermal acceptance bandwidths to different extents. The idler wavelength tuning of the cascaded OPO is realized by changing the temperature of the designed APMgLN crystal and the cascaded oscillation is achieved in a single pump pass singly resonant linear cavity. The pump-to-idler conversion efficiencies with respect to the pump pulse duration and ratio of OPO coefficient to DFG coefficient are calculated by numerically solving the coupled wave equations. The optimal working conditions of the tunable cascaded OPOs pumped by pulses with energies of 350 μJ and 700 μJ are compared to obtain the general rules of optimization. It is concluded that the optimization becomes the interplay between the ratio of OPO coefficient to DFG coefficient and the pump pulse duration when the idler wavelength tuning range and the pump pulse energy are fixed. Besides, higher pump pulse energy is beneficial for reaching higher optimal pump-to-idler conversion efficiency as long as the APMgLN crystal is optimized according to this pump condition. To the best of our knowledge, this is the first numerical analysis of idler wavelength tunable cascaded OPOs based on chirp-assisted APMgLN crystals.

High-efficiency THz generation in quasi-phase matched (QPM) optically contacted GaAs (OC-GaAs) with near-Brewster angle pumping is studied numerically. The effective nonlinear coefficients d(eff) for different incident angles and polarization directions are investigated. Compared with the normal incidence case, reflection loss of the pump energy at OC-GaAs interfaces can be reduced by propagating the pump at a near-Brewster angle (66.92?). The effect of the air-gap spacings between adjacent OC-GaAs layers to the number of optimal QPM periods and the efficiency of THz generation are calculated. In our study, the number of optimal QPM periods of OC-GaAs is increased significantly from 12 in the normal incidence configuration to 25 in the near-Brewster angle pumped configuration, while the efficiency of THz generation is enhanced by more than 16 times.

We obtained >1 W average power and up to 5μJ pulse energy at ∼3520 nm wavelength via difference-frequency generation (DFG) of an all-fiber, few-ns pulse, variable pulse-repetition-frequency laser source operating at 1064nm and 10mW, continuous-wave, 1525nm single-frequency diode laser, within a 5cm-long periodically poled lithium niobate (PPLN) crystal. To the best of our knowledge, this result amounts to the highest-average-power, spectrally bright mid-infrared radiation obtained via DFG.

We demonstrate a highly efficient method for the generation of a high-field terahertz (THz) pulse train via optical rectification (OR) in congruent lithium niobate (LN) crystals driven by temporally shaped laser pulses. A narrowband THz pulse has been successfully achieved with sub-percent level conversion efficiency and multi MV/cm peak field at 0.26 THz. For the single-cycle THz generation, we achieved a THz pulse with 373-μJ energy in a LN crystal excited by a 100-mJ laser pulse at room temperature. The conversion efficiency is further improved to 0.77 % pumped by a 20-mJ laser pulse with a smaller pump beam size (6 mm in horizontal and 15 mm in vertical). This method holds great potential for generating mJ-level narrow-band THz pulse trains, which may have a major impact in mJ-scale applications like terahertz-based accelerators and light sources.

The development of high-performance, room-temperature, continuous wave (CW), widely-tunable terahertz (THz) lasers has attracted considerable industrial interest and has shown great potential for applications in various fields ranging from THz spectroscopy and biomedical imaging to safety and security applications. Among the commonly used lasers, direct quantum cascade lasers are regarded as the most popular sources offering the highest-to-date wall-plug efficiency. However, such lasers generally require cryogenic cooling and suffer from the drawbacks in terms of the production complexity and the lack of broad tunability. On the other hand, high-performance laser sources based on semiconductor disk lasers (SDLs) and periodically-poled ferroelectric crystals can offer an access to new wavelengths not covered by available gain media, particularly in the THz spectral region. In this regard, vertical-external-cavity surface-emitting lasers (VECSELs), with remarkable output parameters, can allow the demonstration of highly-efficient THz emission in the 0.8 – 2 THz spectral window at room temperature based on difference frequency generation in advanced quasi-phase-matched crystals, such as periodically-poled lithium niobate (PPLN) and aperiodically-poled lithium niobate (aPPLN) crystals. In recent years, a few VECSEL-based THz sources, referred to as Terahertz-External-Cavity Surface-Emitting Lasers (TECSELs), have been demonstrated utilizing PPLN crystals [1–4], generating non-tunable multimode or, with certain efforts, even single-frequency THz signal.

Considerable work has been done to investigate the spectrum of the NO molecule and develop methods to measure trace concentration of this species. Detection of nitric oxide using tunable diode laser spectroscopy (TDLAS) has been previously reported. A sensitivity of 300 ppt was obtained, which demonstrated the potential of high-resolution infrared spectroscopy for NO detection. However, at present time, the absence of compact and room-temperature tunable infrared sources at NO absorption region (5.1 to 5.7 /spl mu/m) greatly limits the use of laser spectroscopy for potential applications. In this work, difference-frequency generation (DFG) is suggested as a convenient means to obtain such an infrared source, while periodically poled lithium niobate (PPLN) is selected as the nonlinear medium. Primarily, this material can be engineered for noncritical quasi-phase-matching of any combination of visible or near-IR lasers to produce DFG within its transparency range. It also features a large nonlinear coefficient and an optical transparency from visible to 5.4 /spl mu/m.

We propose a general routine, Genetic-Simulated Annealing method (GSA) for the optimal design of quasi-phase matched (QPM) non-uniform grating. Here this method is applied to the design of the flat broadened wavelength conversion bandwidth of 40-mm aperiodically poled lithium niobate (APLN) based on difference-frequency generation (DFG). The flat conversion bandwidth of APLN is broadened from the original 65nm in a 40-mm periodically poled lithium niobate (PPLN) to the pre-designated 130 nm in a 40-mm APLN, which conversion efficiency is 6.7 dB higher than that of 12.4-mm PPLN under the same conversion bandwidth.

We report on the fabrication of ridge waveguides formed in congruent periodically poled lithium niobate (PPLN). The waveguides were fabricated by periodic poling, proton exchange, subsequent annealing and reverse proton exchange followed by diamond blade dicing. Up to 1 W of single-pass second-harmonic generation at 775 nm has been realized in 50 mm long ridge waveguides with internal conversion efficiency of 70%. Furthermore, difference frequency generation at 3.3 μm of two pump waves at 1.05 μm and 1.55 μm has been obtained in similarly fabricated PPLN ridge waveguides with an appropriate poling period.

Periodically poled lithium niobate (PPLN) is a relatively new non-linear optical material which can be used for such processes as second harmonic generation, sum and difference frequency generation and optical parametric oscillation. The use of periodically-poled lithium niobate in spectroscopy and chemical sensing offers many potential advantages over systems employing more traditional laser sources. When pumped by the fundamental of a Nd:YAG laser PPLN offers the promise of high efficiency, high power, broad tunability (1.5–3.5 µm) and compact size. The broad tunability over the C-H stretch region is an important advantage for many chemical sensing applications. At Sandia, we are developing IR sources based on PPLN for both remote and in-situ chemical sensing.

Introduction: Within the framework of the phenomenological theory of phase transitions of the second kind of Ginzburg-Landau, the kinetics of ordering of a rapidly quenched highly nonequilibrium domain structure is considered using the lithium tantalate and lithium niobate crystals as an example. Experimental: Using the statistical approach, evolution equations describing the formation of the domain structure under the influence of a high-frequency alternating electric field in the form of a standing wave were obtained. Numerical analysis has shown the possibility of forming thermodynamically stable mono- and polydomain structures. It turned out that the process of relaxation of the system to the state of thermodynamic equilibrium can proceed directly or with the formation of intermediate quasi-stationary polydomain asymmetric phases. Results: It is shown that the formation of Regular Domain Structures (RDS) is of a threshold character and occurs under the influence of an alternating electric field with an amplitude less than the critical value, whose value depends on the field frequency. The conditions for the formation of RDSs with a micrometer spatial scale were determined. Conclusion: As shown by numerical studies, the RDSs obtained retain their stability, i.e. do not disappear even after turning off the external electric field. Qualitative analysis using lithium niobate crystals as an example has shown the possibility of RDSs formation in high-frequency fields with small amplitude under resonance conditions

The study and control of materials with extremely strong THz fields [1, 2], or the acceleration and manipulation of electrons [3] and protons [4] are emerging applications which require THz sources with unprecedented parameters. Optical rectification of femtosecond pulses with tilted pulse front in lithium niobate (LN) [5] has become a standard technique for efficient THz generation. Conventionally [1, 5, 6], a prism-shaped LN crystal is used with a large wedge angle equal to the pulse-front tilt (63°). This results in a nonuniform pump propagation length across the beam, which lead to a spatially varying interaction length for THz generation. This negatively affects the THz beam quality and focusability, thereby limiting the achievable field strength. In order to provide uniform interaction length across large pump and THz beams, we have proposed a setup, where the nonlinear medium is a plane-parallel LN slab with an echelon structure on its input surface [7] (NLES, Fig. 1). Inside the LN slab, a segmented tilted pulse front is formed with an average tilt angle as required by phase matching.

In the present investigation, a high intense ultra-short pulse configuration has been proposed for plasma induced THz generation in carbon nano tubes. When a high intense ultra-short electromagnetic wave interacts with the periodically structured carbon nano tubes possessing second order susceptibility, the electromagnetic wave imparts oscillatory velocities to electrons that produce the heat. The generation of excess heat may deform the structure of the carbon nano tubes which lead to the reduction in the efficiency of THz generation. Therefore, the pulse parameters evolution such as normalized energy of the pulse, pulse width, position of the pulse center and reflectivity coefficient of the medium is investigated theoretically on plasma generation. The outcome of the theoretically investigated reveals that the increment in the heating rate can be caused by decrease in the pulse width, increment in reflectivity coefficient and the pulse peak power. Finally, an array of ultra-short pulse configurations has been proposed for efficient THz generation. The present work is helpful to regulate the heating of nanostructure during ultra-short pulse interaction.

In this paper, mid-IR light generation based on difference frequency generation (DFG) in a single piece of PPLN is proposed and studied. In the studies, a Yb doped fiber laser at 1.092 μm and a tunable laser around 1.55 μm were used. The output mid-IR laser with the wavelength tunable around 3.7 μm was generated. Since compact Yb doped fiber lasers at 1.092 μm and tunable semiconductor laser diodes around 1.55 μm are available on the market, the proposed mid-IR laser is potentially portable. Our simulations show that tunable mid-IR light as broad as 800 nm can be obtained from a single PPLN chip simply by tuning temperature (from 30 ^oC to 500 ^oC) and signal wavelength (from 1.48 μm to 1.62 μm).