Generation of tunable narrow-band surface-emitted terahertz radiation in periodically poled lithium niobate

Terahertz waveform synthesis in integrated thin-film lithium niobate platform

Bridging the "terahertz gap" relies upon synthesizing arbitrary waveforms in the terahertz domain enabling applications that require both narrow band sources for sensing and few-cycle drives for classical and quantum objects. However, realization of custom-tailored waveforms needed for these applications is currently hindered due to limited flexibility for optical rectification of femtosecond pulses in bulk crystals. Here, we experimentally demonstrate that thin-film lithium niobate circuits provide a versatile solution for such waveform synthesis by combining the merits of complex integrated architectures, low-loss distribution of pump pulses on-chip, and an efficient optical rectification. Our distributed pulse phase-matching scheme grants shaping the temporal, spectral, phase, amplitude, and farfield characteristics of the emitted terahertz field through designer on-chip components. This strictly circumvents prior limitations caused by the phase-delay mismatch in conventional systems and relaxes the requirement for cumbersome spectral pre-engineering of the pumping light. We propose a toolbox of basic blocks that produce broadband emission up to 680 GHz and far-field amplitudes of a few V m^-1 with adaptable phase and coherence properties by using near-infrared pump pulse energies below 100 pJ.

Generation of narrowband terahertz radiation via phonon mode enhanced nonlinearities in a BaGa4Se7 crystal

We report on narrowband terahertz (THz) radiation generation via optical rectification from a BaGa₄Se₇ crystal. The dense phonon mode distribution of the BaGa₄Se₇ crystal causes narrow transmission bands in the THz frequency range with enhanced nonlinear susceptibility magnitudes, thus permitting strong narrowband THz radiation generation at the frequencies of 1.97 and 2.34 THz. In comparison to THz radiation generated from a ZnTe crystal, the narrowband THz radiation produced by the BaGa₄Se₇ crystal is 4.5 times higher at 1.97 THz and 63% higher at 2.34 THz, thus making BaGa₄Se₇ a viable crystal for use in such areas as security and medicine.

Narrowband terahertz generation with chirped-and-delayed laser pulses in periodically poled lithium niobate

We generate narrowband terahertz (THz) radiation in periodically poled lithium niobate (PPLN) crystals using two chirped-and-delayed driver pulses from a high-energy Ti:sapphire laser. The generated frequency is determined by the phase-matching condition in the PPLN and influences the temporal delay of the two pulses for efficient terahertz generation. We achieve internal conversion efficiencies up to 0.13% as well as a record multicycle THz energy of 40 μJ at 0.544 THz in a cryogenically cooled PPLN.

Pulse sequences for efficient multi-cycle terahertz generation in periodically poled lithium niobate

The use of laser pulse sequences to drive the cascaded difference frequency generation of high energy, high peak-power and multi-cycle terahertz pulses in cryogenically cooled (100 K) periodically poled Lithium Niobate is proposed and studied. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves (or pump depletion), show that unprecedented optical-to-terahertz energy conversion efficiencies > 5%, peak electric fields of hundred(s) of mega volts/meter at terahertz pulse durations of hundred(s) of picoseconds can be achieved. The proposed methods are shown to circumvent laser induced damage limitations at Joule-level pumping by 1µm lasers to enable multi-cycle terahertz sources with pulse energies >> 10 milli-joules. Various pulse sequence formats are proposed and analyzed. Numerical calculations for periodically poled structures accounting for cascaded difference frequency generation, self-phase-modulation, cascaded second harmonic generation and laser induced damage are introduced. The physics governing terahertz generation using pulse sequences in this high conversion efficiency regime, limitations and practical considerations are discussed. It is shown that varying the poling period along the crystal length and further reduction of absorption can lead to even higher energy conversion efficiencies >>10%. In addition to numerical calculations, an analytic formulation valid for arbitrary pulse formats and closed-form expressions for important cases are presented. Parameters optimizing conversion efficiency in the 0.1-1 THz range, the corresponding peak electric fields, crystal lengths and terahertz pulse properties are furnished.

Efficient narrowband terahertz generation in cryogenically cooled periodically poled lithium niobate

We present an efficiency scaling study of optical rectification in cryogenically cooled periodically poled lithium niobate for the generation of narrowband terahertz radiation using ultrashort pulses. The results show an efficiency and brilliance increase compared to previous schemes of up to 2 orders of magnitude by exploring the optimal pump pulse format at around 800 nm, and reveal saturation mechanisms limiting the conversion efficiency. We achieve >10⁻³ energy conversion efficiencies, μJ-level energies, and bandwidths <20  GHz at ∼0.5  THz, thereby showing unprecedented spectral brightness in the 0.1-1 THz range relevant to terahertz science and technology.

Long-range parametric amplification of THz wave with absorption loss exceeding parametric gain

Optical parametric mixing is a popular scheme to generate an idler wave at THz frequencies, although the THz wave is often absorbing in the nonlinear optical material. It is widely suggested that the useful material length for co-directional parametric mixing with strong THz-wave absorption is comparable to the THz-wave absorption length in the material. Here we show that, even in the limit of the absorption loss exceeding parametric gain, the THz idler wave can grows monotonically from optical parametric amplification over a much longer distance in a nonlinear optical material until pump depletion. The coherent production of the non-absorbing signal wave can assist the growth of the highly absorbing idler wave. We also show that, for the case of an equal input pump and signal in difference frequency generation, the quick saturation of the THz idler wave predicted from a much simplified and yet popular plane-wave model fails when fast diffraction of the THz wave from the co-propagating optical mixing waves is considered.

T-Ray Sensing and Imaging

Terahertz (THz) radiation occupies part of the electromagnetic spectrum between the infrared and microwave bands. Until recently, technology at THz frequencies was under-developed compared to the rest of the electromagnetic spectrum, leaving a gap between millimeter waves and the far-infrared (FIR). In the past decade, interest in the THz gap has been increased by the development of ultrafast laser-based T-ray systems and their demonstration of diffraction-limited spatial resolution, picosecond temporal resolution, DC-THz spectral bandwidth and signal-to-noise ratios above 104.

This chapter reviews the development, the state of the art and the applications of T-ray spectrometers. Continuous-wave (CW) THz-frequency sources and detectors are briefly introduced in comparison to ultrafast pulsed THz systems. An emphasis is placed on experimental applications of T-rays to sensing and imaging, with a view to the continuing advance of technologies and applications in the THz band.

Bandwidth tunable THz wave generation in large-area periodically poled lithium niobate

A new scheme of optical rectification (OR) of femtosecond laser pulses in a periodically poled lithium niobate (PPLN) crystal, which generates high energy and bandwidth tunable multicycle THz pulses, is proposed and demonstrated. We show that the number of the oscillation cycles of the THz electric field and therefore bandwidth of generated THz spectrum can easily and smoothly be tuned from a few tens of GHz to a few THz by changing the pump optical spot size on PPLN crystal. The minimal bandwidth is 17 GHz that is smallest ever of reported in scheme of THz generation by OR at room temperature. Similar to the case of Cherenkov-type OR in single-domain LiNbO₃, the spectrum of THz generation extends from 0.1 THz to 3 THz when laser beam is focused to a size close to half-period of PPLN structure. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 220 nJ/THz for ~1 W pump power at 1 kHz repetition rate.

Effects of uncertain phase-matching wave vectors of rotating fan-out type poled LiNbO3 on THz generation

We investigated terahertz pulses from a rotating fan-out type poled lithium niobate (LiNbO(3)) pumped by femtosecond laser pulses. In particular, the rotating fan-out type poled sample produces an uncertain phase-matching wave vector perpendicular to input laser pulses. Such a wave vector allowed us to observe terahertz pulses normally unobservable from bulk or periodically poled LiNbO(3) at large rotation angles because of the terahertz wave critical angle of LiNbO(3). Further, we explained center frequency dependence on rotation angles by difference frequency generation process with the uncertain wave vector. We also discussed bandwidth dependence and terahertz pulse power regarding rotation angles.

Terahertz-wave surface-emitted difference-frequency generation without quasi-phase-matching technique

A scheme of terahertz (THz)-wave surface-emitted difference-frequency generation (SEDFG), which lacks the drawbacks associated with the usage of periodically orientation-inverted structures, is proposed. It is shown that both material birefringence of the bulk LiNbO(3) crystal and modal birefringence of GaAs/AlAs waveguide are sufficient to obtain SEDFG up to a frequency of approximately 3THz. The simplicity of the proposed scheme, along with the fact that there is a much smaller THz-wave decay in nonlinear crystal, makes it a good candidate for the practical realization of efficient THz generation. The use of a GaAs waveguide with an oxidized AlAs layer is proposed for enhanced THz-wave SEDFG in the vicinity of the GaAs polariton resonance at 8THz.

Intracavity parametric generation of nanosecond terahertz radiation using quasi-phase-matching

We report the use of quasi-phase-matching techniques based on periodically-poled MgO:LiNbO(3) for the generation of nanosecond duration pulses of terahertz radiation in intracavity optical parametric oscillators. Multiple idler-waves are generated with temporal studies indicating that the initiating process is the expected parametric down-conversion, but followed by cascaded difference frequency generation. A number of grating geometries have been explored, revealing the presence of dual solutions for the quasi-phase-matching process in the general case. Choice of grating parameters so as to minimize oscillation threshold while simultaneously ensuring effective extraction of the THz radiation is considered.

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.

Orbital angular momentum of entangled counterpropagating photons

We elucidate the paraxial orbital angular momentum of entangled photon pairs generated by spontaneous parametric downconversion in different noncollinear geometries in which the entangled photons counterpropagate. We find, in particular, the orbital angular momentum of entangled pairs generated in transverse-emitting configurations, in which none of the known rules for selecting orbital angular momentum holds.

Tuning characteristics of narrowband THz radiation generated via optical rectification in periodically poled lithium niobate

The tuning properties of pulsed narrowband THz radiation generated via optical rectification in periodically poled lithium niobate have been investigated. Using a disk-shaped periodically poled crystal tuning was easily accomplished by rotating the crystal around its axis and observing the generated THz radiation in forward direction. In this way no beam deflection during tuning was observed. The total tuning range extended from 180 GHz up to 830 GHz and was limited by the poling period of 127 microm which determines the maximum THz frequency in forward direction.