Frequency Generation Research Articles

Degenerate phase-matching for multi-wavelength nonlinear mixing in aperiodic lattice lasers

Holographically designed aperiodic lattices (ALs) have proven to be an exciting engineering technique for achieving electrically switchable single- or multi-frequency emissions in terahertz (THz) semiconductor lasers. Here, we employ the nonlinear transfer matrix modeling method to investigate multi-wavelength nonlinear (sum- or difference-) frequency generation within an integrated THz (idler) laser cavity that also supports optical (pump and signal) waves. The laser cavity includes an aperiodic lattice, which engineers the idler photon lifetimes and effective refractive indices. The key findings are the following: (i) the nonlinear conversion efficiency reveals resonant enhancement at those idler frequencies where the photon lifetime is high; (ii) the resonant phase-matching (PM) process between the pump and idler waves has a one-to-one link with the engineered effective index dispersion; and (iii) in the absence of any other dispersion, the lowest threshold, multi-wavelength defect modes of the aperiodic lattice laser have degenerate phase-matched pump frequencies. This set of results will potentially have a significant impact on the wavelength multiplexing in electronically switchable THz-over-fiber communication systems [U.S. patent application 20,150,248,047A1 (3 September 2015)].

Shaping picosecond quasi-triangular laser pulses during sum frequency generation

Shaping picosecond quasi-triangular laser pulses during sum frequency generation

Precise passive synchronization of dissipative soliton resonance lasers via cascaded cross-phase- and cross-absorption-modulation for mid-infrared mode-locked pulse generation

We present the first demonstration of utilizing cascaded cross-phase and cross-absorption modulation effects for synchronizing the pulse parameters of two mode-locked fiber lasers operating in the dissipative soliton resonance regime. By utilizing the cross-absorption modulation effect we achieve a significant improvement in pulse repetition frequency synchronization dynamic range (13.5 cm cavity optical path mismatch, or 1.87 kHz pulse repetition frequency mismatch) while simultaneously achieving passive pulse duration synchronization through injection of synchronization pulses into the gain fiber of one of the Slave lasers. This approach leads to a record pulse repetition frequency synchronization dynamic range. Subsequently, the synchronized DSR lasers at 1.06 µm and 1.56 µm are employed in a nonlinear difference frequency generation stage to generate mode-locked mid-infrared pulses with a pulse repetition rate of ∼ 2 MHz and an exceptional fractional instability of 3.60 × 10-13, measured for a 1000 s integration time.

Effect of para-substituents on NC bonding of aryl isocyanide molecules adsorbed on metal surfaces studied by sum frequency generation (SFG) spectroscopy.

The effect of para-substituents on the NC bond of aryl isocyanide molecules adsorbed on Au, Ag, Pt, and Pd surfaces was examined by vibrational sum frequency generation (VSFG) spectroscopy. The frequency of NC stretching vibration, υNC, depended on the electronic property of the substituents. When the substituents were more electron-withdrawing and more electron-donating, υNC shifted to lower and higher frequencies, respectively. However, υNC shifted to lower frequency again when the substituents were too electron-donating. The strong dependence of υNC on metal substrates was also observed, reflecting the difference in metal d-band energies, which leads to the difference in the contributions of σ-donation and π back-donation of the NC-metal coupling.

A Cascade Fractional-N Synthesizer Topology of DLL and Frequency Multiplier for 5G+ Communication Systems

This study presents a synthesizer topology based on a delay-locked loop (DLL) and programmable frequency multiplier for 5G+ communication systems. The proposed synthesizer comprises a 512-phase DLL, an intermediate frequency generator (IFG), and an RF frequency multiplier (RFFM). The 512-phase DLL provides 512 delayed pulses through a chain of 256 delay units and single-to-differential complementary converters (S2DCs). The IFG comprises I/Q-multiplexers, I/Q-accumulators, an XOR, and an S2DC. The I/Q-multiplexer outputs switch to the phase lag or lead waveforms at every rising or falling edge of the outputs, which makes the I/Q-multiplexer output frequency, fMX, programmable. The IF, fIF, is two times fMX, and fIF is up-converted to RF, fRF, through the RFFM. When the reference clock frequency, fref, is 156.25 MHz, the fIF range is 156.863–312.5 MHz and the fRF dynamic range is approximately 1.89–9.96 GHz. The channel resolution range is 3.698–38.609 MHz. Consequently, the proposed synthesizer provides a wide 134% output frequency bandwidth and a finer channel resolution smaller than fref. The presented synthesizer is fabricated in a 65 nm CMOS process. The total power consumption is 15 mW, and the rms jitter integrated from 1 kHz to 100 MHz measured as 107.6 fs.

Environmental Effects on the Interfacial Chemical Reactions at RTV Silicone-Silica Interfaces.

Silicone sealants and adhesives are extensively used in construction, automotive, industrial, and electronic applications because they exhibit excellent mechanical properties, strong adhesion, and good weather resistance. Room-temperature vulcanized (RTV) silicones develop good adhesion to many substrates and do not require heat for curing, which leads to flexible use in many applications. Although it is known that various factors such as relative humidity and temperature affect the curing of the RTV silicone adhesives, the interfacial chemistry that occurs during the curing process is still poorly understood but critical for success in adhesive applications. To address this, sum frequency generation (SFG) vibrational spectroscopy was used to probe the molecular details of the buried interface of the RTV silicone adhesive in situ. Time-dependent SFG experiments were conducted on two polydimethylsiloxane (PDMS) matrices, at three humidity levels, and with two kinds of silica surfaces to investigate the behavior of the methoxy groups at the interface and the impact of environmental conditions on the adhesion mechanism. It was found that both the methoxy groups from methyltrimethoxysilane (MTMS) and methoxy-terminated PDMS could segregate to the interface. The diffusion of MTMS and bulk rearrangement of methoxy-terminated PDMS lead to the segregation and ordering of methoxy groups at the interface. After comparing eight samples cured under different environmental conditions, the reactions of the interfacial methoxy groups were found to be facilitated by both the surface water on silica and moisture from the environment. The silylation treatment on the silica slows the reactions of the interfacial methoxy groups, while the high environmental humidity accelerates the consumption of the interfacial methoxy groups. These findings provide insightful information about the adhesion mechanism of RTV silicone adhesives and accelerate new product development.

Vibrational sum frequency generation (VSFG) spectroscopy of water adsorption on surfaces of yttria-stabilized cubic zirconia (YSZ).

Yttria-stabilized zirconia (YSZ) is found in a wide range of applications, from solid-oxide fuel cells to medical devices and implants. A molecular-level understanding of the hydration of YSZ surfaces is essential for optimizing its performance and durability in these applications. Nevertheless, only a limited amount of literature is available about the surface hydration of YSZ single crystals. In this study, we employ surface-sensitive non-linear vibrational sum frequency generation spectroscopy to investigate the hydration of YSZ(100), (110), and (111) single crystal substrates under ambient laboratory conditions. Three types of hydroxyl groups were identified at all three YSZ-D2O interfaces: (i) hydroxyls on the metal sites of Zr or Y resulting from the dissociative chemisorption of water, (ii) hydroxyls from proton adsorption to O sites formed from water dissociation, and (iii) hydroxyl groups as part of the physisorbed water at the interface.

Dynamic Phase Behavior of Amorphous Solid Dispersions Revealed with In Situ Stimulated Raman Scattering Microscopy.

This study reports the application of in situ stimulated Raman scattering (SRS) microscopy for real-time chemically specific imaging of dynamic phase phenomena in amorphous solid dispersions (ASDs). Using binary ritonavir and poly(vinylpyrrolidone-vinyl acetate) films with different drug loadings (0-100% w/w) as model systems, we employed SRS microscopy with fast spectral focusing to analyze ASD behavior upon contact with a dissolution medium. Multivariate unmixing of the SRS spectra allowed changes in the distributions of the drug, polymer, and water to be (semi)quantitatively imaged in real time, both in the film and the adjacent dissolution medium. The SRS analyses were further augmented with complementary correlative sum frequency generation and confocal reflection for additional crystallinity and phase sensitivity. In the ASDs with drug loadings of 20, 40, and 60% w/w, the water penetration front within the film, followed by both surface-directed and bulk phase separation in the film, was apparent but differed quantitatively. Additionally, drug-loading and phase-dependent polymer and drug release behavior was imaged, and liquid-liquid phase separation was observed for the 20% drug loading ASD. Overall, SRS microscopy with fast spectral focusing provides quantitative insights into water-induced ASD phase phenomena, with chemical, solid-state, temporal, and spatial resolution. These insights are important for optimal ASD formulation development.

6.7-8.1 µm tunable single-frequency difference frequency generation in ZGP for high-resolution spectroscopy.

Difference frequency generation (DFG) based tunable single-frequency mid-infrared (MIR) light sources are desirable for high-resolution spectroscopy, sensing, and imaging. In this work, we demonstrate a continuous-wave (CW) single-frequency DFG in a ZnGeP2 (ZGP) crystal driven by all-fiber near-infrared (NIR) fiber lasers, for the first time to our knowledge. The all-fiber NIR laser sources consist of a 1.5 µm erbium-doped fiber amplifier seeded by a CW tunable fast scanning single-frequency laser and a 1.9 µm CW tunable single-frequency thulium-doped fiber laser. Taking advantage of the high nonlinear coefficient and large birefringence of the ZGP crystal, single-frequency DFG in ZGP achieves a broad spectral tuning range from 6.7 to 8.1 µm, with an output power at the 10 µW level. Precise detection of C2H2 gas by continuously scanning the DFG source across a spectral range of 1.1 THz (∼34 cm-1) is also presented, highlighting the potential of the tunable DFG source for high-resolution optical spectroscopy applications. We anticipate this work will provide what we believe to be a new platform for spectroscopy in the molecular fingerprint spectral region.

A clinical study on the efficacy of high frequency therapy on nasolabial and periorbital wrinkles

ABSTRACT The high-frequency generator is considered a collagen stimulator and skin revitalizer, however there are few studies exploring its effects in the field of rejuvenation and aesthetics. Thus, this study aimed to investigate the impact of high frequency generator therapy on facial aging. A total of 26 participants aged between 50 and 60 years were selected and received high frequency generator treatment for eight weeks. The evaluation was conducted using a combination of photographic records and assessment tools, including Rosenberg scale, perception and satisfaction with treatment evaluation, overall aesthetic improvement scale, and modified Fitzpatrick scale. The analysis conducted by the Global Aesthetic Improvement Scale yielded significant results. The Modified Fitzpatrick Wrinkle Scale showed that high frequency therapy led to statistically significant improvements in the appearance of nasolabial wrinkles (right T0: 1.48, T8: 0.87; left T0: 1.51, T8: 1.05) and periorbital wrinkles (right T0: 1.69, T8: 1.05; left T0: 1.71, T8: 1.08). In conclusion, high frequency generator therapy can be a highly effective tool for treating skin aging on the face, however, we recommend that future research includes control groups, and adopts objective measures to expand knowledge about the effects of high frequency.

180 mW, 1 MHz, 15 fs carrier-envelope phase-stable pulse generation via polarization-optimized down-conversion from gas-filled hollow-core fiber

Gas-filled hollow core fibers allow the generation of single-cycle pulses at megahertz repetition rates. When coupled with difference frequency generation, they can be an ideal driver for generating carrier-envelope phase stable, octave-spanning pulses in the short-wavelength infrared. In this work, we investigate the dependence of the polarization state in gas-filled hollow-core fibers (HCF) on the subsequent difference frequency generation stage. We show that by adjusting the input polarization state of light in geometrically symmetric systems, such as hollow-core fibers, one can achieve precise control over the polarization state of the output pulses. This manipulation preserves the temporal characteristics of the generated ultrashort pulses, especially when operating at a near single-cycle regime. We leverage this property to boost the downconversion efficiency of the near single-cycle pulses in a type I difference frequency generation stage. Our technique overcomes the bandwidth and dispersion constraints of the previous methods that rely on broadband waveplates or adjustment of crystal axes relative to the laboratory frame. This advancement is crucial for experiments demanding pure polarization states in the eigenmodes of the laboratory frame.

3D hydrogen-like screening effect on excitons in hBN-encapsulated monolayer transition metal dichalcogenides

We observe both s-series and p-series excitons by using sum frequency generation spectroscopy on monolayer (1L-)MX2 (M = Mo, W, X = S, Se) encapsulated by hexagonal boron nitride (hBN). Moreover, we perform numerical calculations with the Rytova-Keldysh potential and obtain the relative dielectric constant of hBN among other parameters. The obtained relative dielectric constant can be approximated by the high-frequency limit of the infrared dispersion even though the exciton binding energies are almost on the phonon resonances in hBN. This suggests that the theoretically indicated modification of the exciton level structure due to the phonon resonances is negligible. The power-law scaling of exciton binding energies indicates that dielectric screening of 1L-MX2 exciton levels other than 1s can be approximated by that of a 3D hydrogen model with the dielectric constant of hBN.

A biomimetic approach towards a universal slippery liquid infused surface coating.

One biomimetic approach to surface passivation involves a series of surface coatings based on the slick surfaces of carnivorous pitcher plants (Nepenthes), termed slippery liquid-infused porous surfaces (SLIPS). This study introduces a simplified method to produce SLIPS using a polydopamine (PDA) anchor layer, inspired by mussel adhesion. SLIPS layers were formed on cyclic olefin copolymer, silicon, and stainless steel substrates, by first growing a PDA film on each substrate. This was followed by a hydrophobic liquid anchor layer created by functionalizing the PDA film with a fluorinated thiol. Finally, perfluorodecalin was applied to the surface immediately prior to use. These biomimetic surface functionalization steps were confirmed by several complimentary surface analysis techniques. The wettability of each surface was probed with water contact angle measurements, while the chemical composition of the layer was determined by X-ray photoelectron spectroscopy. Finally, ordering of specific chemical groups within our PDA SLIPS layer was determined via sum frequency generation spectroscopy. The hemocompatibility of our new PDA-based SLIPS coating was then evaluated by tracking FXII activation, fibrin generation time, clot morphology, and platelet adhesion to the surface. This hemocompatibility work suggests that PDA SLIPS coatings slow or prevent clotting, but the observation of both FXII activation and the presence of adherent and activated platelets at the PDA SLIPS samples imply that this formulation of a SLIPS coating is not completely omniphobic.

Orientational Disorder of Alcohol Molecules at Their Solution Surfaces in Low Concentration Range: A Molecular Dynamics Simulation Study.

Molecular dynamics (MD) simulations of short-chain alcohols (methanol, ethanol, and 1-propanol) in solution were carried out to examine the orientational disordering (randomizing) of alcohol molecules at the surface under diluted conditions. Recent vibrational sum frequency generation (VSFG) spectroscopy, combined with photoelectron spectroscopy, has successfully measured the disordering structure at low concentrations. The present MD simulations accurately reproduce this observation for the first time. To ensure reliable results through MD simulations, several widely used force field models for water and alcohol, including polarizable models, were examined. This examination involved a comparison of structural and thermodynamic quantities, such as surface density times orientation and surface excess concentration, which were obtained through surface-specific measurements like VSFG spectroscopy and surface tension measurements. It is found that the width of the density profile for alcohol molecules at the surface, along the surface normal, increases as the concentration decreases in the diluted condition, which is consistent with the results obtained from the previous neutron and X-ray grazing incidence reflection experiments. A molecular mechanism explaining the disordering of alcohol molecules with decreasing concentration is also discussed.

Turning four-wave mixing on and off in elliptically birefringent fibers via narrow frequency detuning.

Controlling four-wave mixing (FWM) is vital for several applications, including fiber optical communication, optical signal processing, optical amplification, and frequency generation. This paper presents a novel, to our knowledge, approach to control unidirectional FWM in elliptically birefringent fibers. By leveraging the frequency-dependent polarization eigenmodes of these fibers and detuning the optical frequency of one of the pump fields by a few megahertz, we can turn the FWM interaction on and off, thus controlling the generation of signal and idler fields. Moreover, this approach allows us to turn off the FWM interaction at any desired frequency, enabling all-optical switching and narrowband filtering applications.

Antiferromagnetic–ferromagnetic heterostructure-based spin Hall nano-oscillator

Spin oscillators relying on ferromagnetic (FM) materials have been limited to frequency generation in the range of only a few gigahertz. In contrast, antiferromagnetic (AFM) material-based oscillators have a potential for beyond gigahertz range oscillations. However, the use of AFM oscillators is limited due to challenges in detecting and controlling magnetic orientation. This arises from the inherent lack of significant net magnetization in AFMs. This work focuses on exploring the dynamic characteristics of a spin Hall nano-oscillator (SHNO) that addresses these challenges by leveraging the inter-layer exchange interaction between AFM and FM layers. The proposed design demonstrates stable and power-efficient oscillation in the FM layer, relying on the dynamics of the AFM layer. The proposed AFM–FM-based SHNO design achieves a maximum frequency of 16.4 GHz at ISOT = 180 μA. Furthermore, considering the thermal effects at 300 K, the stable oscillation frequency is achieved at 15.94 GHz. The proposed device exhibits robust and tunable oscillations over a wide frequency range with a power consumption of 4 μW. Moreover, this oscillator achieves 3.35× and 2.44× higher oscillation frequency compared to spin torque nano-oscillators and conventional SHNO-based oscillators, respectively.

Impact of electrolyte on the structure and orientation of water at air/water-polyethylene glycol polymer interface.

Polyethylene glycol (PEG) is a water soluble, non-ionic polymer with applications in drug delivery, protein precipitation, anti-biofouling, water-splitting, Li-ion batteries, and fuel cells. The interaction of PEG with water and electrolytes plays pivotal roles in such applications. Using interface-selective spectroscopy, heterodyne-detected vibrational sum frequency generation, and Raman difference spectroscopy with simultaneous curve fitting analysis, we show that water adopts different structures and orientations at the air/water-PEG interface, which depends on the molar mass of the PEG. At the air/water-PEG4000 (MW 4000u) interface, water is H-up oriented (i.e., water Hs are pointed away from the aqueous bulk) around 3200cm-1 and H-down oriented (i.e., water Hs are pointed toward the aqueous bulk) around 3470cm-1. Variation of the bulk concentration of PEG4000 does not change the dual orientation of interfacial water. The presence of an electrolyte (1.0M NaCl) selectively reduces the H-up oriented water without affecting the H-down oriented water at the air/water-PEG4000 interface. The selective reorganization of the interfacial water is assigned to the disruption of the asymmetric hydration around ether-oxygen of the surface-adsorbed PEG4000 by the Na+ ion of the electrolyte. Interestingly, in the case of low molar mass PEG (air/water-PEG200), the interfacial water neither shows the dual orientation nor is affected by 1.0M NaCl.

Noncollinear phase-matching of high harmonic generation in solids

We propose and experimentally demonstrate a scheme allowing reaching noncollinear phase-matching of high harmonic generation in solids, which may potentially lead to an enhancement of the generation efficiency. The principle is based on high-order frequency mixing of two light waves with identical frequencies but different directions of wavevectors. In this process, Nth harmonic frequency is produced by frequency mixing of N + 1 photons from a wave with high amplitude of electric field and a single photon from a wave with low field amplitude, which are propagating noncollinearly in optically isotropic media. We experimentally verify the feasibility of this scheme by demonstrating phase-matched generation of third and fifth harmonic frequency in sapphire.

Filamentation of 100-fs Pulses from a Ti:Sapphire Laser in Gases to Generate a Signal Wave of a Difference Frequency Generator

Filamentation of 100-fs Pulses from a Ti:Sapphire Laser in Gases to Generate a Signal Wave of a Difference Frequency Generator

Embedded systems as programmable square wave generator in wireless power transfer

<span>This study focuses on the design and development of programmable frequency generator using embedded devices that are able to produce square wave signals in the wireless power transfer (WPT) transmitter. We validate the accuracy of the output signal by measuring distance error. We validate that our system can change and sweep the frequency and produce high power by measuring the absorbed power in the load. We conduct the frequency sweep analysis to find optimal frequency and the frequency splitting phenomenon. The experiments show that the system can produce and sweep the square wave signals with less than 1% error. We also find that the frequency splitting occurred when distance among two coils in the range 0.5-6.5 cm and the splitting disappeared when the distance is above 7.5 cm. The frequency splitting shows that the measured optimum frequency differs from the calculation. The difference confirms that the programmable frequency generator is needed to adjust the frequency that can transfer maximum power to the load.</span>