Stimulated Polariton Scattering Research Articles

Cascaded Stimulated Polariton Scattering in a Single-Pass KTP Crystal under Picosecond Pumping

Cascaded stimulated polariton scattering (SPS) under powerful picosecond pumping of 532 nm wavelength was investigated in a single pass of a KTP (KTiOPO4) crystal. Under ordinary polarization of the pump wave (relatively XOZ-plane) parametric scattering in the KTP crystal was observed. By rotation of the pump wave polarization (from ordinary to extraordinary), this scattering gradually transforms into polariton scattering. Polariton signal components (spots) with gaps between them were registered at external angles of ~2° between the neighboring spots. Five polariton signal components were detected above the pump beam, with a frequency difference between neighboring cascaded components of ~188 cm−1 (5.64 THz). The wavelength of each next component increased regularly with the angle between this component and the pump wave, so this regular sequence of polariton signal components can be regarded as a cascaded SPS.

Cavity design with single-mirror THz frequency tuning for polariton lasers.

We demonstrate a new, to the best of our knlowledge, cavity design for terahertz (THz) lasers based on stimulated polariton scattering (SPS). The design simplifies the angle tuning of these lasers, which require non-collinear cavity fields at fundamental and Stokes wavelengths to cross in an SPS crystal with adjustable crossing angle. A mirror shared by both the fundamental and Stokes cavities ensures stationary overlap of the fields within the crystal, with the angle between the fields tunable by adjustment of one axis of a single mirror. We demonstrate the design for an intracavity SPS laser using a rubidium titanyle phosphate (RTP) crystal, and achieve single-mirror tuning of the THz output in bands between 3 and 5.8 THz, with a maximum output of 78 μW at 4.08 THz.

Continuously Tunable and Energy-Enhanced Injection Pulse-Seeded Terahertz Parametric Generator Based on KTP Crystal

A continuously tunable, energy-enhanced injection, pulse-seeded THz parametric generator based on a KTP crystal (KTP ips-TPG) has been demonstrated in this paper. Theoretical analysis denotes that improving the initial Stokes intensity is favorable to achieve a continuously tunable, high-energy THz generation in a KTP crystal, which is clarified to originate from the enhancement of stimulated polariton scattering (SPS) process in the photon-like region of polariton and parametric amplification process in the phonon-like region of polariton. In the experiment, the THz continuously tunable range was from 2.93 to 6.21 THz, and the THz energies were 6.27, 2.62, 5.43, 4.59, and 5.88 μJ at the output peaks of 3.33, 3.83, 4.41, 4.85, and 5.69 THz, respectively. Compared with the traditional KTP THz parametric oscillator (KTP TPO) with discontinuous frequency tuning, the KTP ips-TPG has great improvement in terms of continuous frequency tuning range and THz output energy, where the enhancement ratios of THz output energy were 6.88× at 3.36 THz, 5.25× at 4.35 THz, 18.21× at 4.79 THz, and 1.82× at 5.69 THz. This injection pulse-seeded THz parametric source based on a KTP crystal is not only suitable to the practical applications in THz high frequency range, but also provides a technical solution for the KTP-family crystals to achieve a continuously tunable and high energy THz generation in a wide THz region.

Simultaneous generation of two THz waves with bulk LiNbO3 and four THz waves with PPLN by coupled optical parametric generation

We present a theoretical research concerning simultaneous generation of two terahertz (THz) waves with bulk LiNbO3 and four THz waves with periodically poled LiNbO3 (PPLN) by coupled optical parametric generation (COPG). First, we investigate collinear phase matching of COPG generating two orthogonally polarized THz waves with two types of phase matching of o = e + e and o = e + o with bulk LiNbO3. The two orthogonally polarized THz waves are generated from stimulated polariton scattering (SPS) with A1 and E symmetric transverse optical (TO) modes in bulk LiNbO3, respectively. Then, we find that perturbations of phase mismatch for o = e + e and o = e + o can be compensated by a same grating vector of PPLN. As a result, four THz waves are simultaneously generated with a PPLN crystal and a pump laser. We calculate third-order nonlinear optical coefficients of o = e + o generating THz waves from E symmetric TO modes. The intensities of four THz waves are calculated by solving coupled wave equations. The calculation results demonstrate that the COPG generating four THz waves have high photon conversion efficiencies.

Tunable Stokes laser based on the cascaded stimulated polariton scattering and stimulated Raman scattering in RbTiOPO4 crystal.

Stimulated polariton scattering (SPS) and stimulated Raman scattering (SRS) in ${{\rm RbTiOPO}_4}$RbTiOPO4 (RTP) crystal are combined in an intracavity-pumped Stokes parametric oscillator (SPO) to extend the tunable Stokes laser spectral range. The pumping laser wavelength is 1064 nm from a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. By the SPS process in the SPO, the SPS-Stokes wave can be discontinuously tuned in the range of 1075.7-1076.0 nm, 1077.7-1080.4 nm, 1081.8-1082.2 nm, and 1084.8-1087.8 nm, respectively. By the following SRS process in the same RTP crystal, the laser wavelength is further shifted in the range of 1107.7-1108.1 nm, 1109.0-1112.7 nm, 1114.3-1115.1 nm, and 1117.8-1121.1 nm, respectively. A maximal average output power of 970 mW is achieved for the SRS-Stokes wave at the peak wavelength of 1118.8 nm. It is obtained when the diode power is 7.9 W, and the pulse repetition frequency (PRF) is 10 kHz.

Enhancement of Intracavity-Pumped Terahertz Parametric Oscillator Power by Adopting Diode-Side Pumped Configuration Based on KTiOPO4 Crystal

In this paper, a KTiOPO4 (KTP) crystal is used as the nonlinear medium in an intracavity-pumped terahertz parametric oscillator (TPO) based on stimulated polariton scattering (SPS). Almost all the reported intracavity SPS sources adopt the diode-end pumped configuration, and the THz output power is limited by the serious thermal effect and small fundamental beam size. For improving the THz output power, we propose diode-side pumping for the laser medium to get a higher fundamental power and a larger fundamental beam size. A convex–plane fundamental laser resonator is used to further offset the thermal effect and increase the fundamental beam size. The THz frequency of the intracavity-pumped KTP terahertz parametric oscillator can be discontinuously tuned from 3.19 THz to 5.94 THz with three gaps. The fundamental beam diameter in the KTP crystal is about 1.3 mm. The maximum average THz power is 166 μW at 5.74 THz under a pulse repetition frequency (PRF) of 6 kHz and a diode pump power of 98 W. By means of the diode-side pumped configuration, the maximum THz output power is more than two-fold higher compared to the diode-end pumped configuration reported using the KTP crystal.

BaTeMo2O9 crystals: optical properties and applications in the terahertz range

The optical properties of the α/β-BaTeMo2O9 (α/β-BTM) crystal in the terahertz range were characterized by the terahertz time domain spectroscopy (TDS) system. Frequency-dependent refractive indices and absorption coefficients of two crystals were measured from 0.2 to 2 THz, and discussions and comparisons were made on birefringence, absorption and phonon modes by referring to their structures. The Sellmeier equations for both crystals were also fitted in their transparent ranges. Based on the mode properties and parameters, a feasible scheme for terahertz generation via stimulated polariton scattering (SPS) with β-BTM was proposed and the angle tuning characteristics were calculated. Simulations show that β-BTM has great potential and should be more advantageous than LiNbO3 in generating high-frequency terahertz waves.

Injection pulse-seeded terahertz-wave parametric generator with gain enhancement in wide frequency range.

An injection pulse-seeded terahertz-wave parametric generator (ips-TPG) has been demonstrated with gain enhancement in wide tuning range. Theoretical analysis denotes that the compensation of initial Stokes energy is favorable to the THz gain enhancement in wide frequency range, which is attributed to the improvement on interaction of stimulated polariton scattering (SPS) and difference frequency generation (DFG) processes. In the experiment, the THz frequency tuning range from 1.04 THz to 5.15 THz was achieved based on near-stoichiometric LiNbO3 (SLN) crystal. Compared with the traditional terahertz parametric oscillator (TPO) under the same experimental conditions, a significant enhancement of THz output energy was occurred in high frequency range. As the THz frequency increased from 1.9 THz to 3.6 THz, the enhancement ratios from 1.6 times to 34.7 times were obtained. Besides, the 3dB bandwidth of ips-TPG was measured to be 2.1 THz, which was about 2.6 times that of SLN-TPO. This THz parametric source with a relative flat gain in wide frequency range is suitable to a variety of practical applications.

Intracavity THz Polariton Source Using a Shallow-Bounce Configuration

We demonstrate a frequency-tunable THz polariton source based on stimulated polariton scattering (SPS) in MgO:LiNbO3. The system produces THz emission using an intracavity shallow-bounce configuration. Continuously tunable THz emission across the frequency range of 1.05–2.89 THz is demonstrated, and a maximum average THz power of 67.5 μ W is detected at 1.34 THz, when pumped with 15.1 W from a continuous-wave laser diode at 880 nm. In contrast to conventional linear configurations, this shallow-bounce configuration offers enhanced THz output power across the systems' entire THz frequency-tuning range, especially at higher frequencies. This is achieved through careful consideration of the THz generation volume, and locating it as close as possible to the emitting surface of the polariton-active crystal, thereby minimizing the deleterious effect of absorption within the SPS crystal. In addition to experimental data, the degree of THz enhancement offered using this configuration, in contrast to a linear configuration is also investigated mathematically.

Two parallel polarized terahertz waves generation from quasi-phase-matching stimulated polariton scattering with periodically-inverted GaAs

In this work, we theoretically analyze the generation of two parallel polarized terahertz (THz) waves from coupled optical parametric oscillations (OPOs) with periodically-inverted GaAs by stimulated polariton scattering (SPS). We analyze collinear phase-matching (CPM) conditions for bulk GaAs generating a THz wave and quasi-phase-matching (QPM) conditions for periodically-inverted GaAs generating two THz waves. The analysis results indicate that the frequencies of the two THz waves and the frequency differences between the two THz waves are efficiently tuning by varying grating periods of periodically-inverted GaAs. The photon flux densities (PFD) of the two THz waves are calculated. The generating PFD of the two THz waves are efficiently enhanced by injecting a Stokes seed laser or increasing pump intensity. The scheme proposed in this work enable new applications of THz wave in imaging and spectroscopy.

Efficient Ring-Cavity Terahertz Parametric Oscillator With Pump Recycling Technique

The energy scaling of THz wave in wide range based on surface-emitted (SE) ring-cavity terahertz parametric oscillator (TPO) with pump recycling technique is demonstrated in this paper. The continuously tunable range from 1.24 to 3.77 THz was achieved with full pump beam recycling, whereas the tunable range was 1.24-2.72 THz without pump beam recycling. The terahertz output energy and conversion efficiency increased more than seven times throughout the whole tuning range. The maximum enhancement ratio of the terahertz output energy was 12.8 times at 2.72 THz. Additionally, under the same experimental conditions, THz conversion efficiency was increased from 7.77 × 10 -6 to 7.85 × 10 -5 and the threshold energy was significantly reduced by 41% at 1.63 THz. Moreover, through the theoretical analysis, the enhancement mechanism of TPO by pump recycling was verified to originate from the threshold reduction of the stimulated polariton scattering (SPS) process and the interaction improvement between the pump and Stokes waves in the difference frequency generation (DFG) process.

Simultaneous Generation of Two Pairs of Stokes and Terahertz Waves from Coupled Optical Parametric Oscillations with Quasi-Phase-Matching

We present a theoretical investigation on simultaneous generation of two pairs of Stokes and terahertz (THz) waves from coupled optical parametric oscillations (OPOs) with a quasi-phase-matching (QPM) scheme. The two pairs of Stokes and THz waves are generated by stimulated polariton scattering (SPS) from periodically-inverted GaP. By analyzing the QPM conditions of coupled OPOs we find that the two THz waves with any frequency below the transverse optical (TO) mode frequency of GaP can be simultaneously generated with a suitable pump wavelength. We calculate the photon flux densities of the two THz waves by solving the coupled wave equations. The calculation results indicate that the two THz waves can be efficiently generated with high pump intensities, particularly in lower THz frequency band.

Simultaneous Generation of Two Orthogonally Polarized Terahertz Waves by Stimulated Polariton Scattering with a Periodically Poled LiNbO3 Crystal

We present a theoretical investigation of the simultaneous generation of two orthogonally polarized terahertz (THz) waves by stimulated polariton scattering (SPS) with a periodically poled LiNbO3 (PPLN) crystal. The two orthogonally polarized THz waves are generated from SPS with A1 and E symmetric transverse optical (TO) modes in a LiNbO3 crystal, respectively. The parallel polarized THz wave is generated from A1 symmetric TO modes with type-0 phase-matching of e = e + e, and the perpendicular polarized THz wave is generated from E symmetric TO modes with type-I phase-matching of e = o + o. The two types of phase-matching of e = e + e and e = o + o can be almost satisfied simultaneously by accurately selecting the poling period of the PPLN crystal. We calculate the photon flux density of the two orthogonally polarized THz waves by solving the coupled wave equations. The calculation results indicate that the two orthogonally polarized THz waves can be efficiently generated, and the relative intensities between the two orthogonally polarized THz waves can be modulated.

Discovery of high-gain stimulated polariton scattering near 4 THz from lithium niobate.

Lithium niobate is the most popular material for terahertz wave generation via stimulated polariton scattering (SPS), previously known to have a gain peak near 2THz. Here we report the discovery of another phase-matched gain peak near 4THz in lithium niobate, which greatly extends the useful gain spectrum of lithium niobate. Despite the relatively high 4THz absorption in lithium niobate, the 4THz SPS becomes dominant over the 2THz one in an intensely pumped short lithium niobate crystal due to less diffraction-induced absorption and mode-area mismatch. We also demonstrate a signal-seeded OTPO that generates 1.4nJ at 4.2THz from lithium niobate with 17.5mJ pump energy.

Theoretical analysis of stimulated polariton scattering from the A1-symmetry modes of KNbO3 crystal

Stimulated polariton scattering (SPS) based on noncollinear phase matching scheme from the A1-symmetry modes of KNbO3 crystal is investigated for generating terahertz (THz) wave. Frequency tuning characteristics of THz wave by varying the phase matching angle and pump wavelength are analyzed. The expression of the effective parametric gain length under the noncollinear phase matching condition is deduced. Parametric gain and absorption characteristics of THz wave in KNbO3 are theoretically simulated. The characteristics of KNbO3 for parametric oscillator (TPO) are compared with those of MgO:LiNbO3. The analysis results indicate that KNbO3 is an excellent optical crystal for TPO to enhance the output of THz wave.

Tunable THz polariton laser based on 1342 nm wavelength for enhanced terahertz wave extraction.

We detail the operation of a THz laser source based on non-linear stimulated polariton scattering (SPS) in Mg:LiNbO3. This system utilizes a fundamental wavelength of 1342nm to completely avoid the negative effect of free-carrier generation within high-resistivity silicon (Si) prisms used to extract THz radiation from the Mg:LiNbO3 crystal. THz power of up to 23.6μW (62.3μW when chopped at 50% duty cycle) was detected at 1.33THz, and frequency tunability across the range 1.05-2.2THz was achieved.

Stimulated polariton scattering in an intracavity RbTiOPO4 crystal generating frequency-tunable THz output.

A high power, frequency-tunable THz source based on intracavity stimulated polariton scattering (SPS) in RbTiOPO4 (RTP) is demonstrated for the first time. Frequency tunable THz output was obtained from 3.10 to 4.15 THz, with a gap at 3.17 to 3.49 THz, arising from the 104 cm-1 A1 mode in RTP. A maximum average output power of 16.2 µW was detected at 3.8 THz. This is the highest average output power ever reported for an intracavity polariton laser.

Tunable KTA Stokes laser based on stimulated polariton scattering and its intracavity frequency doubling.

This paper presents the tunable Stokes laser characteristics of KTiOAsO4 (KTA) crystal based on stimulated polariton scattering (SPS). When the pumping laser wavelength is 1064.2 nm, the KTA Stokes wave can be discontinuously tuned from 1077.9 to 1088.4 nm with four gaps from 1079.0 to 1080.1 nm, from 1080.8 to 1082.8 nm, from 1083.6 to 1085.5 nm, and from 1085.8 to 1086.8 nm. When a frequency doubling crystal LiB3O5 (LBO) is inserted into the Stokes laser cavity, the frequency-doubled wave can be discontinuously tuned from 539.0 to 539.5 nm, from 540.1 to 540.4 nm, from 541.3 to 541.8 nm, from 542.7 to 542.9 nm and from 543.4 to 544.2 nm. With a pumping pulse energy of 130.0 mJ and an output coupler reflectivity of about 30%, the obtained maximum Stokes laser pulse energy at 1078.6 nm is 33.9 mJ and the obtained maximum frequency-doubled laser pulse energy at 543.8 nm is 15.7 mJ. By using the most probably coupled transverse optical modes obtained from the literature, the polariton refractive indexes, and the simplified polariton Sellmeier equations, the polariton dispersion curve is obtained. The formation of the Stokes frequency gaps is explained.

Multiwavelength ultrafast LiNbO(3) Raman laser.

We present a multiwavelength ultrafast Raman laser based on lithium niobate which uses polariton scattering in combination with Raman scattering to selectively generate new wavelengths from a nanojoule-scale picosecond pump laser. Pumped by a 1064 nm pump laser, the system generates 1123 nm by stimulated polariton scattering (SPS) and 1140 nm by stimulated Raman scattering (SRS). Cascading of these intracavity fields generates 1155 nm and 1174 nm, as well as generating THz output.

Cascaded stimulated polariton scattering in a Mg:LiNbO(3) terahertz laser.

Cascaded Stimulated Polariton Scattering (SPS) to the fourth-Stokes order is observed experimentally in an intracavity THz polariton laser utilising Mg:LiNbO(3). The performance of the cascaded laser is presented, the origin of the cascaded fields is explained and compared to theory, and the potential consequences for using cascading to enhance the THz output from this type of device are discussed.