Thin Nonlinear Crystal Research Articles

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals.

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS2) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS2 relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

Spectra of Ultrabroadband Squeezed Pulses and the Finite-Time Unruh-Davies Effect.

We study spectral properties of quantum radiation of ultimately short duration. In particular, we introduce a continuous multimode squeezing operator for the description of subcycle pulses of entangled photons generated by coherent-field driving in a thin nonlinear crystal with second-order susceptibility. We find the ultrabroadband spectra of the emitted quantum radiation perturbatively in the strength of the driving field. They can be related to the spectra expected in an Unruh-Davies experiment with a finite time of acceleration. In the time domain, we describe the corresponding behavior of the normally ordered electric field variance.

Broadband terahertz spectroscopy (Invited Paper)

An overview of the major techniques to generate and detect THz radiation so far, especially the major approaches to generate and detect coherent ultra-short THz pulses using ultra-short pulsed laser, has been presented. And also, this paper, in particularly, focuses on broadband THz spectroscopy and addresses on a number of issues relevant to generation and detection of broadband pulsed THz radiation as well as broadband time-domain THz spectroscopy (THz-TDS) with the help of ultra-short pulsed laser. The time-domain waveforms of coherent ultra-short THz pulses from photoconductive antenna excited by femtosecond laser with different pulse durations and their corresponding Fourier-transformed spectra have been obtained via the numerical simulation of ultrafast dynamics between femtosecond laser pulse and photoconductive material. The origins of fringes modulated on the top of broadband amplitude spectrum, which is measured by electric-optic detector based on thin nonlinear crystal and extracted by fast Fourier transformation, have been analyzed and the major solutions to get rid of these fringes are discussed.

Dynamics of a paired optical vortex generated by second-harmonic generation

We study the dynamics of a paired optical vortex (OV) generated by second-harmonic generation (SHG) using sub-picosecond pulses. By changing the position of a thin nonlinear crystal along the propagation direction, we observe a rotation of two vortices with changing separation distance. The dynamics is well explained by SHG with a beam walk-off, which introduces a contamination of zero-order Laguerre-Gaussian beam (LG(0)) together with topological charge doubling. The quantitative analysis indicates that the rotation angle of the OVs manifests the Gouy phase while the splitting provides the walk-off angle of the crystal. We also show that the subtraction of LG(0) is realized by the superposition of LG(0) with an anti-balanced phase in the pump.

Characterization of Supercontinuum and Ultraviolet Pulses by Using XFROG

We present cross-correlation frequency-resolved optical gating (XFROG) measurements of supercontinuum pulses generated by using a photonic crystal fiber (PCF), and ultraviolet (UV) pulses generated by frequency doubling of infrared ultra-short pulses. Since supercontinuum pulses have broad spectra, XFROG measurement typically requires using an extremely thin nonlinear crystal which has a thickness of sub-ten microns. Instead of using such a thin crystal, we employed a relatively thick crystal which was mounted on a galvanometer in order to achieve a phase-matching over the whole spectral bandwidth of the supercontinuum pulses by a crystal-dithering technique. Experimental results of the retrieved phase and intensity were in fair agreement with the independently measured data.

Two color laser ablation: Enhanced yield, improved machining

Laser ablation of semiconductors with nano- and picosecond lasers can be significantly improved, both in terms of yield and surface quality, by simultaneous irradiation of the sample with the fundamental beam ( λ = 1064 nm) and a small amount of its second harmonic (SH) produced in a thin nonlinear crystal. While the total energy fluence is conserved, the small fraction of the second harmonic serves to excite electrons into the conduction band to get the ablation process started. For femtosecond laser pulses, the effect becomes insignificant, since sufficient conduction band population is provided by multiphoton absorption.

Enhancement of laser ablation yield by two color excitation

We present ablation results of silicon obtained by simultaneous irradiation of the sample with the fundamental beam of a picosecond-neodymium-vanadate (Nd:VAN) laser (1064 nm, 10 ps pulse duration) and a small amount of second harmonic (SH) produced in a thin nonlinear crystal. In this fashion, the ablation yield could be increased by 70%. In addition, the ablation quality was improved in terms of surface smoothness. The underlying mechanism can be attributed to a ‘seeding’ of the target area with free carriers by the 532 nm radiation.

Increased polarization-entangled photon flux via thinner crystals

We analyze the scaling laws that govern the production of polarization-entangled photons via type-II spontaneous parametric down-conversion (SPDC). We demonstrate experimentally that thin nonlinear crystals can generate a higher number of entangled photons than thicker crystals, basically because they generate a broader spectrum.

The orbital angular momentum of down-converted photons

We calculate the relative amplitude of the orbital angular momentum (OAM) of entangled photon pairs from the spontaneous parametric down-conversion of a thin quadratic nonlinear crystal. The results show that the amplitude depends on both of the two Laguerre indices l, p. We also discuss the influences of the commonly used holograms and mono-mode fibres for mode analysis. We conclude that only a few dimensions can be explored from the infinite OAM modes of down-converted photon pairs.

Bell-state preparation using pulsed nondegenerate two-photon entanglement

We report a novel Bell state preparation experiment. High-purity Bell states are prepared by using femtosecond pulse pumped \emph{nondegenerate} collinear spontaneous parametric down-conversion. The use of femtosecond pump pulse {\em does not} result in reduction of quantum interference visibility in our scheme in which post-selection of amplitudes and other traditional mechanisms, such as, using thin nonlinear crystals or narrow-band spectral filters are not used. Another distinct feature of this scheme is that the pump, the signal, and the idler wavelengths are all distinguishable, which is very useful for quantum communications.

Femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd3+:YAG laser

A femtosecond laser (pulse duration 30 fs, centre of the emission spectrum at λ = 1260 nm) with a chromium-doped forsterite crystal was developed. This laser operated on the basis of Kerr-lens mode locking. Synchronous pumping with an Nd3+:YAG picosecond laser resulted in forced generation of femtosecond pulses characterised by an enhanced stability of the output power, which was 5–7 times better than the stability of the pump laser. The output radiation of the forsterite laser was converted efficiently to the second harmonic (central wavelength 630 nm) in a thin nonlinear BBO crystal. The average power of the second-harmonic radiation was 15 mW.

Conversion of ultrashort laser pulses into an electromagnetic signal in dispersive waveguides

An analysis is made of the principal features of the conversion of short laser pulses (by the optical rectification effect) into an electromagnetic signal in a guiding system composed of two parallel metal planes with a thin nonlinear crystal between them. It is shown that the dispersion of such a system may increase the length of a signal well in excess of the duration of the original pulse. Estimates of the amplitudes of the excited oscillation modes are given for KDP and lithium niobate crystals. It is pointed out that the proposed conversion method can be used in measurements of the parameters of the output of self-mode-locked lasers.