Bulk Nonlinear Crystals Research Articles

Simultaneous label-free autofluorescence multi-harmonic microscopy driven by the supercontinuum generated from a bulk nonlinear crystal.

Nonlinear microscopy encompasses several imaging techniques that leverage laser technology to probe intrinsic molecules of biological specimens. These native molecules produce optical fingerprints that allow nonlinear microscopes to reveal the chemical composition and structure of cells and tissues in a label-free and non-destructive fashion, information that enables a plethora of applications, e.g., real-time digital histopathology or image-guided surgery. Because state-of-the-art lasers exhibit either a limited bandwidth or reduced wavelength tunability, nonlinear microscopes lack the spectral support to probe different biomolecules simultaneously, thus losing analytical potential. Therefore, a conventional nonlinear microscope requires multiple or tunable lasers to individually excite endogenous molecules, increasing both the cost and complexity of the system. A solution to this problem is supercontinuum generation, a nonlinear optical phenomenon that supplies broadband femtosecond radiation, granting a wide spectrum for concurrent molecular excitation. This study introduces a source for nonlinear multiphoton microscopy based on the supercontinuum generation from a yttrium aluminum garnet (YAG) crystal, an approach that allows simultaneous label-free autofluorescence multi-harmonic imaging of biological samples and offers a practical and compact alternative for the clinical translation of nonlinear microscopy. While this supercontinuum covered the visible spectrum (550-900 nm) and the near-infrared region (950-1200 nm), the pulses within 1030-1150 nm produced label-free volumetric chemical images of ex vivo chinchilla kidney, thus validating the supercontinuum from bulk crystals as a powerful source for multimodal nonlinear microscopy.

Compact Diode-Pumped Solid-State Laser with Intracavity Pump-Enhanced DFG Emitting at ~3.5 Microns

We report a diode-pumped solid-state (DPSS) laser used for intracavity pump-enhanced difference frequency generation (DFG) to create a 3.5-micron laser. Using a 50 mm-long periodically poled lithium niobate (PPLN) crystal inside the cavity of an Nd:YVO4 solid-state laser at 1064 nm with 4.5 W pump power at 808 nm, and a 310 mW C-band signal at 1529 nm, up to 31 mW of mid-infrared output power at 3499 nm is obtained. The cavity requires no active stabilization and/or locking, and the entire cavity is <8 cm in length. The obtained output power corresponds to a black-box efficiency of 2.20%W−1, which is the highest value reported to date for continuous-wave DFG based on a bulk nonlinear optical crystal with no active stabilization. Potential future applications in free-space optical communication are also discussed.

Entangled photon-pair source using a wedge-shaped nonlinear crystal

Entangled photon pairs constitute a crucial resource in quantum technologies. The current technique for preparing photon pairs involves performing spontaneous parametric down-conversion (SPDC) in bulk nonlinear crystals. The increased prominence of quantum technologies has led to a growing interest in high performance entangled photon-pair sources. However, the birefringence walk-off caused by nonlinear crystals affects the quality of the entangled photon pairs generated in SPDC. This letter presents an innovative approach for producing entangled photon pairs using a wedge-shaped nonlinear crystal to compensate for the birefringence walk-off. The proposed scheme features a basic entanglement source path, which can be applied to other methods of preparing entangled photon pairs. An experiment demonstrated that this method can produce high-quality entangled photon pairs with a high degree of entanglement, paving the way for the development of practical quantum communication, sensing, and computation.

Efficient Second-Harmonic Generation from Dielectric Inter-subband Polaritonic Metasurfaces Coupled to Lattice Resonance.

Nonlinear optical metasurfaces offer a possibility to perform frequency mixing without the phase-matching constraints of bulk nonlinear crystals and with control of the local nonlinear response at a sub-wavelength scale. Nonlinear inter-subband polaritonic metasurfaces created by combining the semiconductor heterostructures with quantum-engineered inter-subband nonlinear response and electromagnetically engineered metal-clad nanoresonators offer by far the largest second-order nonlinear response of all condensed matter systems reported to date. However, the nonlinear optical response of these metasurfaces is limited by optical intensity saturation in the nanoresonator hot spots that prevented the achievement of power conversion efficiencies over 0.2% in three-wave mixing experiments. In this study, we propose and experimentally demonstrate dielectric inter-subband polaritonic metasurfaces for second-harmonic generation that achieve 0.37% power conversion efficiency. Our structure is created by a new design approach that combines dielectric resonators inducing Mie resonant modes with a lattice resonance to achieve a uniform and high field enhancement throughout the meta-atom volume.

Bismuth telluride iodide monolayer flakes with nonlinear optical response obtained via gold-assisted mechanical exfoliation

Bismuth telluride iodide (BiTeI) is a layered material known as Rashba semiconductor, which is promising for several applications such as spintronics, nonlinear optics, and energy-related devices. Due to its layered structure and low cleavage energy, BiTeI is particularly suitable for producing two-dimensional (2D) BiTeI crystals, allowing for the development of devices at the nanoscale. However, traditional methods of mechanically exfoliating BiTeI have encountered significant challenges. To address this, we used the so-called gold-assisted mechanical exfoliation to successfully obtain 2D BiTeI flakes. Through extensive characterization employing a range of techniques including Raman spectroscopy and second-harmonic generation (SHG) measurements, we examine the morphological, structural, and chemical properties of the gold-assisted mechanically exfoliated BiTeI flakes. We confirm their crystalline nature while also providing information about strain and defects present within the flakes. Moreover, SHG measurements revealed a significant nonlinear optical response, and the non-centrosymmetric structure observed in these flakes can be attributed to quantum confinement effects and the absence of phase-matching requirements typically found in bulk nonlinear crystals.

Resolving sub-cycle signatures: A perspective on hallmarks of terahertz metrology

Frequency up-conversion has become amongst the most prevalent techniques for detection of terahertz waves in metrology systems. State-of-the-art up-conversion techniques rely on the coherent transferring of the information encoded in all degrees of freedom of a terahertz wave to either the near-infrared or visible domain, where detectors are readily accessible. This allows for an indirect reconstruction of the terahertz wave. However, unlike most up-conversion methods employed in photonics which are concentrating on narrowband tones (at both terahertz and near-infrared frequencies), a broadband, hence temporally constrained, terahertz transient is sampled on time-scales shorter than its oscillation period. Here, femtosecond laser pules serve as temporal gates. In this perspective, we highlight several hallmarks of terahertz metrology that originate from these sub-cycle measurement capabilities and elaborate why this enables studies in fundamental and applied science, with a particular focus on novel measurement concepts in classical and quantum. We focus on so-far demonstrated detection performance in bulk non-linear crystals. Finally, we discuss current challenges and the most pressing questions ahead.

Classical imaging with undetected photons using four-wave mixing in silicon core fibers

Undetected-photon imaging allows for objects to be imaged in wavelength regions where traditional components are unavailable. Although first demonstrated using quantum sources, recent work has shown that the technique also holds with classical beams. To date, however, all the research in this area has exploited parametric down-conversion processes using bulk nonlinear crystals within free-space systems. Here, we demonstrate undetected-photon-based imaging using light generated via stimulated four-wave mixing within highly nonlinear silicon fiber waveguides. The silicon fibers have been tapered to have a core diameter of ∼ 915 nm to engineer the dispersion and reduce the insertion losses, allowing for tight mode confinement over extended lengths to achieve practical nonlinear conversion efficiencies ( ∼ − 30 dB ) with modest pump powers ( ∼ 48 mW ). Both amplitude and phase images are obtained using classically generated light, confirming the high degree of spatial and phase correlation of our system. The high powers ( > 10 nW ) and long coherence lengths ( > 4 km ) associated with our large fiber-based system result in high contrast and stable images.

Frequency characteristics of strip modules of the quasi-chaotic signals formation based on bulk nonlinear optical crystals

In this paper the experimental study of frequency dependences of scattering matrix coefficients of strip modules based on a coplanar strip line with partial dielectric filling in the form of bulk non-linear optical crystals of quartz, potassium titanium phosphate (KTP), zinc germanium diphosphide, lithium niobate has been performed. The appearance of the effect of forming quasi- chaotic oscillations in modules containing KTiOPO4, ZnGeP2, LiNbO3 bulk crystals when LFM signals are fed at fast switching of incident wave direction in the receiving and transmitting tract of the vector network analyzer is demonstrared. Parameters of comparative characterization of crystal materials are introduced. The complete phase of the modulus transmission coefficient is determined. The analysis of its physical sense is made, followed by processing complete phase shift, which provides the possibility to develop the algorithm of estimating the frequency dependence of crystal’s relative permittivity. The studied modules may be used as formers of quasi- chaotic microwave signals.

Nonlocal and cascaded effects in nonlinear graphene nanoplasmonics.

The ability of plasmons to focus light on nanometer length scales opens a wide range of enticing applications in optics and photonics, among which the enhancement of nonlinear light-matter interactions for all-optical modulation and spectral diversification emerges as a prominent theme. However, the subwavelength plasmonic near-field enhancement in good plasmonic materials such as noble metals is hindered by large ohmic losses, while conventional phase-matching of fields in bulk nonlinear crystals is not suitable for realizing nonlinear optical phenomena on the nanoscale. In contrast, anharmonic electron motion of free charge carriers in highly-doped graphene, which supports long-lived, highly-confined, and actively-tunable plasmons, renders the carbon monolayer an excellent platform for both plasmonics and nonlinear optics. Here we theoretically explore the enhancement in nonlinear response that can be achieved by interfacing multiple graphene nanostructures in close proximity to trigger nonlocal effects associated with large gradients in the electromagnetic near field. Focusing on second- and third-harmonic generation, we introduce a semianalytical formalism to describe interacting graphene nanoribbons with independent width, location, and electrical doping, so as to realize configurations in which plasmonic resonances may simultaneously enhance both the fundamental optical excitation frequency and harmonic intermediary and/or output frequencies. Our findings reveal the importance of both passive and active tuning in the design of atomically-thin nanostructures for nonlinear optical applications, and in particular emphasize the role played by nonlocal effects in generating an even-ordered nonlinear response that may contribute to other nonlinear optical processes through a cascaded interaction. We anticipate that our findings can aid in the design of actively-tunable nonlinear plasmonic resonators and metasurfaces.

Nonlinear optical imaging by detection with optical parametric amplification (invited paper).

Nonlinear optical imaging is a versatile tool that has been proven to be exceptionally useful in various research fields. However, due to the use of photomultiplier tubes (PMTs), the wide application of nonlinear optical imaging is limited by the incapability of imaging under ambient light. In this paper, we propose and demonstrate a new optical imaging detection method based on optical parametric amplification (OPA). As a nonlinear optical process, OPA intrinsically rejects ambient light photons by coherence gating. Periodical poled lithium niobate (PPLN) crystals are used in this study as the media for OPA. Compared to bulk nonlinear optical crystals, PPLN crystals support the generation of OPA signal with lower pump power. Therefore, this characteristic of PPLN crystals is particularly beneficial when using high-repetition-rate lasers, which facilitate high-speed optical signal detection, such as in spectroscopy and imaging. A PPLN-based OPA system was built to amplify the emitted imaging signal from second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy imaging, and the amplified optical signal was strong enough to be detected by a biased photodiode under ordinary room light conditions. With OPA detection, ambient-light-on SHG and CARS imaging becomes possible, and achieves a similar result as PMT detection under strictly dark environments. These results demonstrate that OPA can be used as a substitute for PMTs in nonlinear optical imaging to adapt it to various applications with complex lighting conditions.

Spontaneous parametric down-conversion in bottom-up grown lithium niobate microcubes

Nonclassical light sources are highly sought-after as they are an integral part of quantum communication and quantum computation devices. Typical sources use bulk nonlinear crystals that rely on stringent phase-matching conditions, limiting the operating wavelength and bandwidth. In this work, we demonstrate the generation of photon pairs from a free-standing lithium niobate microcube at the telecommunication wavelength of 1.56 µm through the spontaneous parametric down-conversion process. The maximum photon pair generation rate obtained from a single microcube with the size of 3.6 µm is 490 Hz, resulting in an efficiency of 20.6 GHz/Wm, which is three orders of magnitude larger than the efficiency of biphoton generation in bulk nonlinear crystals. The microcubes are synthesized through a solvothermal method, offering the possibility for scalable devices via bottom-up assembly on any substrates. Our work constitutes an important step forward in the realization of compact nonclassical light sources with a wide bandwidth for various quantum applications.

Liquid-Phase Exfoliation of Bismuth Telluride Iodide (BiTeI): Structural and Optical Properties of Single-/Few-Layer Flakes.

Bismuth telluride halides (BiTeX) are Rashba-type crystals with several potential applications ranging from spintronics and nonlinear optics to energy. Their layered structures and low cleavage energies allow their production in a two-dimensional form, opening the path to miniaturized device concepts. The possibility to exfoliate bulk BiTeX crystals in the liquid represents a useful tool to formulate a large variety of functional inks for large-scale and cost-effective device manufacturing. Nevertheless, the exfoliation of BiTeI by means of mechanical and electrochemical exfoliation proved to be challenging. In this work, we report the first ultrasonication-assisted liquid-phase exfoliation (LPE) of BiTeI crystals. By screening solvents with different surface tension and Hildebrandt parameters, we maximize the exfoliation efficiency by minimizing the Gibbs free energy of the mixture solvent/BiTeI crystal. The most effective solvents for the BiTeI exfoliation have a surface tension close to 28 mN m–1 and a Hildebrandt parameter between 19 and 25 MPa0.5. The morphological, structural, and chemical properties of the LPE-produced single-/few-layer BiTeI flakes (average thickness of ∼3 nm) are evaluated through microscopic and optical characterizations, confirming their crystallinity. Second-harmonic generation measurements confirm the non-centrosymmetric structure of both bulk and exfoliated materials, revealing a large nonlinear optical response of BiTeI flakes due to the presence of strong quantum confinement effects and the absence of typical phase-matching requirements encountered in bulk nonlinear crystals. We estimated a second-order nonlinearity at 0.8 eV of |χ(2)| ∼ 1 nm V–1, which is 10 times larger than in bulk BiTeI crystals and is of the same order of magnitude as in other semiconducting monolayers (e.g., MoS2).

Overcoming Intensity Saturation in Nonlinear Multiple-Quantum-Well Metasurfaces for High-Efficiency Frequency Upconversion.

Engineered intersubband transitions in semiconductor heterostructures featuring multiple quantum wells (MQWs) are shown to support record-high second-order nonlinear susceptibilities. By integrating these materials in metasurfaces with tailored optical resonances, it is possible to further enhance photonic interactions, yielding giant nonlinear responses in ultrathin devices. These metasurfaces form a promising platform for efficient nonlinear processes, including frequency upconversion of low-intensity thermal infrared radiation and harmonic generation, free of phase-matching constraints intrinsic to bulk nonlinear crystals. However, nonlinear saturation at moderately large pump intensities due to the transfer of electron population into excited subbands facilitated by strongly enhanced light-matter interactions in metasurfaces fundamentally limits their overall efficiency for various nonlinear processes. Here, the saturation limits of nonlinear MQW-based metasurfaces for mid-infrared frequency upconversion are significantly extended by optimizing their designs for excitation with a strong pump coherently coupled with unpopulated upper electron subbands. This counterintuitive pumping scheme, combined with tailored material and photonic engineering of the metasurface, avoids saturation at practical levels of continuous-wave pump intensities, yielding significantly larger upconversion efficiencies than in conventional approaches. The present results open new opportunities for nonlinear metasurfaces, less limited by saturation mechanisms, with important implications for night-vision imaging and compact nonlinear wave mixingsystems.

Tunable unidirectional nonlinear emission from transition-metal-dichalcogenide metasurfaces

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

Parameter estimation of time and frequency shifts with generalized Hong-Ou-Mandel interferometry

Hong-Ou-Mandel interferometry takes advantage of the quantum nature of two-photon interference to increase the resolution of precision measurements of time-delays. Relying on few-photon probe states, this approach is applicable also in cases of extremely sensible samples and it achieves attosecond (nanometer path length) scale resolution, which is relevant to cell biology and two-dimensional materials. Here, we theoretically analyze how the precision of Hong-Ou-Mandel interferometers can be significantly improved by engineering the spectral distribution of two-photon probe states. In particular, we assess the metrological power of different classes of biphoton states with non-Gaussian time-frequency spectral distributions, considering the estimation of both time- and frequency-shifts. We find that grid states, characterized by a periodic structure of peaks in the chronocyclic Wigner function, can outperform standard biphoton states in sensing applications. The considered states can be feasibly produced with atomic photon sources, bulk non-linear crystals and integrated photonic waveguide devices.

Reversible and Tunable Second‐Order Nonlinear Optical Susceptibility in PZT Thin Films for Integrated Optics

AbstractSecond‐order nonlinear optical processes enable a wide range of applications used in research and industry. The majority of available second‐order nonlinear devices however relies on bulk nonlinear crystals with low second‐order nonlinearity. By exploiting the advancements made in integrated optics, materials with large second‐order nonlinearity can enable efficient and small‐sized on‐chip nonlinear devices at low cost. Unfortunately, silicon and silicon nitride, mostly used for photonics integrated circuits exhibit negligible second‐order nonlinearity (χ(2)) and alternate materials have to be investigated. Lead zirconate titanate (PZT) thin films with high second‐order nonlinearity stand as a good candidate for on‐chip nonlinearity. An electric‐field induced tuning of χ(2) is demonstrated here in PZT thin films grown on glass substrates with a tuning efficiency of 3.35 pm V−2. Strong second‐harmonic generation is recorded and a very high dominant tensor component of 128 pm V−1 is reported. The χ(2) of the PZT thin films can be reversed by poling with a DC electric field at room temperature. This opens avenues for highly efficient and tunable on‐chip nonlinear devices.

Entangled photon-pair sources based on three-wave mixing in bulk crystals.

Entangled photon pairs are a critical resource in quantum communication protocols ranging from quantum key distribution to teleportation. The current workhorse technique for producing photon pairs is via spontaneous parametric down conversion (SPDC) in bulk nonlinear crystals. The increased prominence of quantum networks has led to a growing interest in deployable high performance entangled photon-pair sources. This manuscript provides a review of the state-of-the-art bulk-optics-based SPDC sources with continuous wave pump and discusses some of the main considerations when building for deployment.

Broadband terahertz wave generation from an epsilon-near-zero material

Broadband light sources emitting in the terahertz spectral range are highly desired for applications such as noninvasive imaging and spectroscopy. Conventionally, THz pulses are generated by optical rectification in bulk nonlinear crystals with millimetre thickness, with the bandwidth limited by the phase-matching condition. Here we demonstrate broadband THz emission via surface optical rectification from a simple, commercially available 19 nm-thick indium tin oxide (ITO) thin film. We show an enhancement of the generated THz signal when the pump laser is tuned around the epsilon-near-zero (ENZ) region of ITO due to the pump laser field enhancement associated with the ENZ effect. The bandwidth of the THz signal generated from the ITO film can be over 3 THz, unrestricted by the phase-matching condition. This work offers a new possibility for broadband THz generation in a subwavelength thin film made of an ENZ material, with emerging physics not found in existing nonlinear crystals.

Tunable third harmonic generation in the vacuum ultraviolet region using dielectric nanomembranes

Tunable coherent light sources operating in the vacuum ultraviolet (VUV) region in the 100–200-nm (6–12 eV) wavelength range have important spectroscopic applications in many research fields, including time-resolved angle-resolved photoemission spectroscopy. Recent advances in laser technology have enabled the upconversion of visible femtosecond lasers to the vacuum and extreme ultraviolet regions. However, the complexity of their experimental setups and the scarcity of bulk nonlinear crystals for VUV generation have hampered its widespread use. Here, we propose the use of a free-standing dielectric nanomembrane as a simple and practical method for tunable VUV generation. We demonstrate that third harmonic VUV light is generated with sufficient intensity for spectroscopic applications from commercially available SiO2 nanomembranes of submicron thicknesses under excitation with visible femtosecond laser pulses. The submicron thickness of the nanomembranes is optimal for maximizing VUV generation efficiency and prevents self-phase modulation and spectral broadening of the fundamental beam. The observed VUV photons are up to 107 photons per pulse at 157 nm with a 1-kHz repetition rate, corresponding to a conversion efficiency of 10−6. Moreover, the central VUV wavelength can be tuned in the 146–190-nm wavelength range by changing the fundamental wavelength. We also explore material and thickness dependence with experiments and calculations. The presented results suggest that dielectric nanomembranes can be used as practical nonlinear media for VUV spectroscopic applications.

Multimode single-pass spatio-temporal squeezing.

We present a single-pass source of broadband multimode squeezed light with potential application in quantum information and quantum metrology. The source is based on a type I parametric down-conversion (PDC) process inside a bulk nonlinear crystal in a non-collinear configuration. The generated squeezed light exhibits a spatio-temporal multimode behavior that is probed using a homodyne measurement with a local oscillator shaped both spatially and temporally. Finally we follow a covariance matrix based approach to reveal the distribution of the squeezing among several independent temporal and spatial modes. This unambiguously validates the multimode feature of our source.