Frequency-domain interferometric second-harmonic spectroscopy

Contrast-enhanced phase-resolved second harmonic generation microscopy

The characterization of inverted structures (crystallographic, ferroelectric, or magnetic domains) is crucial in the development and application of novel multi-state devices. However, determining these inverted structures needs a sensitive probe capable of revealing their phase correlation. Here a contrast-enhanced phase-resolved second harmonic generation (SHG) microscopy is presented, which utilizes a phase-tunable Soleil-Babinet compensator and the interference between the SHG fields from the inverted structures and a homogeneous reference. By this means, such inverted structures are correlated through the π-phase difference of SHG, and the phase difference is ultimately converted into the intensity contrast. As a demonstration, we have applied this microscopy in two scenarios to determine the inverted crystallographic domains in two-dimensional van der Waals material MoS2. Our method is particularly suitable for applying in vacuum and cryogenic environments while providing optical diffraction-limited resolution and arbitrarily adjustable contrast. Without loss of generality, this contrast-enhanced phase-resolved SHG microscopy can also be used to resolve other non-centrosymmetric inverted structures, e.g. ferroelectric, magnetic, or multiferroic phases.

Evidence for moiré excitons in van der Waals heterostructures

Recent advances in the isolation and stacking of monolayers of van der Waals materials have provided approaches for the preparation of quantum materials in the ultimate two-dimensional limit^1,2. In van der Waals heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moiré superlattice³. It is widely recognized that the moiré superlattice can modulate the electronic band structure of the material and lead to transport properties such as unconventional superconductivity⁴ and insulating behaviour driven by correlations^5-7; however, the influence of the moiré superlattice on optical properties has not been investigated experimentally. Here we report the observation of multiple interlayer exciton resonances with either positive or negative circularly polarized emission in a molybdenum diselenide/tungsten diselenide (MoSe₂/WSe₂) heterobilayer with a small twist angle. We attribute these resonances to excitonic ground and excited states confined within the moiré potential. This interpretation is supported by recombination dynamics and by the dependence of these interlayer exciton resonances on twist angle and temperature. These results suggest the feasibility of engineering artificial excitonic crystals using van der Waals heterostructures for nanophotonics and quantum information applications.

Charge Transfer to Solvent Dynamics at the Ambient Water/Air Interface

Electron-transfer reactions at ambient aqueous interfaces represent one of the most fundamental and ubiquitous chemical reactions. Here the dynamics of the charge transfer to solvent (CTTS) reaction from iodide was probed at the ambient water/air interface by phase-sensitive transient second-harmonic generation. Using the three allowed polarization combinations, distinctive dynamics assigned to the CTTS state evolution and to the subsequent solvating electron-iodine contact pair have been resolved. The CTTS state is asymmetrically solvated in the plane of the surface, while the subsequent electron solvation dynamics are very similar to those observed in the bulk, although slightly faster. Between 3 and 30 ps, a small phase shift distinguishes an electron bound in a contact pair with iodine and a free hydrated electron at the water/air interface. Our results suggest that the hydrated electron is fully solvated in a region of reduced water density at the interface.

Stabilized phase detection of heterodyne sum frequency generation for interfacial studies

We present a collinear-geometry heterodyne sum frequency generation (HD-SFG) method for interfacial studies. The HD detection is based on a collinear SFG configuration, in which picosecond visible and femtosecond IR beams are used to first produce a strong local oscillator and then to generate weak SFG signals from an interface. A time-delay compensator, consisting of an MgF2 window, is placed before the sample to introduce the time delay between the local oscillator and the interfacial SFG signals for spectral interferometry. Our HD-SFG method exhibits advantages of long-time phase stability. It is not sensitive to sample heights, does not require reflection correction, and is easy to implement.

Enhancement of second-order nonlinear-optical signals by optical stimulation

Second-order nonlinear optical interactions such as sum- and difference-frequency generation are widely used for bioimaging and as selective probes of interfacial environments. However, inefficient nonlinear optical conversion often leads to poor signal-to-noise ratio and long signal acquisition times. Here, we demonstrate the dramatic enhancement of weak second-order nonlinear optical signals via stimulated sum- and difference-frequency generation. We present a conceptual framework to quantitatively describe the interaction and show that the process is highly sensitive to the relative optical phase of the stimulating field. To emphasize the utility of the technique, we demonstrate stimulated enhancement of second harmonic generation (SHG) from bovine collagen-I fibrils. Using a stimulating pulse fluence of only 3  nJ/cm2, we obtain an SHG enhancement >10(4) relative to the spontaneous signal. The stimulation enhancement is greatest in situations where spontaneous signals are the weakest--such as low laser power, small sample volume, and weak nonlinear susceptibility--emphasizing the potential for this technique to improve signal-to-noise ratios in biological imaging and interfacial spectroscopy.

Heterodyne-detected and ultrafast time-resolved second-harmonic generation for sensitive measurements of charge transfer

In organic photovoltaics many key ultrafast processes occur at the interface between electron donor and acceptor molecules. Traditional ultrafast spectroscopies, such as pump-probe or time-resolved fluorescence, are not ideal for studying the interface because most of their signal is from the bulk material. Time-resolved second-harmonic generation (TRSHG) spectroscopy solves this problem by only generating signal from the interface. We demonstrate an optically heterodyned TRSHG to reduce the impact of stray light, enhance sensitivity, and detect the full complex signal field.

Probing surface and interface structure using optics

Optical techniques for probing surface and interface structure are introduced and recent developments in the field are discussed. These techniques offer significant advantages over conventional surface probes: all pressure ranges of gas-condensed matter interfaces are accessible and liquid-liquid, liquid-solid and solid-solid interfaces can be probed, due to the large penetration depth of the optical radiation. Sensitivity and discrimination from the bulk are the two challenges facing optical techniques in probing surface and interface structure. Where instrumental improvements have resulted in enhanced sensitivity, conventional optical techniques can be used to characterize heterogeneous adsorbed layers on a substrate, often with sub-monolayer resolution. Nanoscale lateral resolution is possible using scanning near-field optics. A separate class of techniques, which includes reflection anisotropy spectroscopy, and nonlinear optical probes such as second-harmonic and sum-frequency generation, uses the difference in symmetry between the bulk and the surface or interface to suppress the bulk contribution. A perspective is presented of likely future developments in this rapidly expanding field.

Resonant frequency-domain interferometry via third-harmonic generation

A method for measuring resonances using a combination of third-harmonic generation and frequency-domain interferometry is described and demonstrated in an index-matched dielectric material. The phase of the third-harmonic spectrum of a pulse generated from a resonant NdAlO(3) thin film and a temporally displaced sapphire substrate pulse was measured by analyzing the spectral interference pattern. The appropriate combination of substrate and film signals was obtained by translating the sample through the laser focus while observing the third-harmonic intensity.

New information on water interfacial structure revealed by phase-sensitive surface spectroscopy

A phase-sensitive sum-frequency vibrational spectroscopic technique is developed to study interfacial water structure of water/quartz interfaces. Measurements allow deduction of both real and imaginary parts of the surface nonlinear spectral response, revealing an unprecedentedly detailed picture of the net polar orientations of the water species at the interface. The orientations of the icelike and liquidlike species appear to respond very differently to the bulk pH change indicating the existence of different surface sites on quartz with different deprotonation pK values.

Measuring ultrashort optical pulses in the presence of noise: an empirical study of the performance of spectral phase interferometry for direct electric field reconstruction

We have measured the performance of a real spectral phase interferometry for direct electric field reconstruction (SPIDER) apparatus operating under suboptimal conditions. We analyzed the errors in SPIDER's measurements of the temporal phases and intensities of 50-fs ultrashort laser pulses as a function of the additive noise in the detected signal. It was found that SPIDER performs exceptionally well, particularly in the case of additive noise. Specifically, a signal with 10% noise yields a pulse that has a mere 2% error in its intensity profile and a phase that differs from the nominal value by 0.2 rad. Furthermore, we quantified SPIDER's performance with limited detector resolution and as a function of signal averaging.

Phase modulation of ultrashort light pulses using molecular rotational wave packets

We demonstrate experimentally how the time-dependent phase modulation induced by molecular rotational wave packets can manipulate the phase and spectral content of ultrashort light pulses. Using impulsively excited rotational wave packets in CO2, we increase the bandwidth of a probe pulse by a factor of 9, while inducing a negative chirp. This chirp is removed by propagation through a fused silica window, without the use of a pulse compressor. This is a very general technique for optical phase modulation that can be applied over a broad spectral region from the IR to the UV.

Electric-field-induced second-harmonic generation in GaN devices

Electric-field-induced second-harmonic generation is used to detect electric fields in a GaN UV Schottky photodiode and in a GaN light-emitting diode. The second-harmonic signal is measured as a function of bias voltage and incident laser power. This technique is sensitive to small applied voltages and can be used to track electronic waveforms. The photocurrent generated by this technique is found to be less than 100 pA when the fundamental and second-harmonic frequencies are both below the device bandgap.

Strong surface state effects in nonlinear magneto-optical response of Ni(110)

Spectroscopic magnetization-induced optical second harmonic generation (MSHG) measurements from a clean Ni(110) surface reveal strong resonance effects near 2.7 eV that can be attributed to the presence of an empty surface state. The good agreement with model calculations shows the potential of MSHG to probe spin-polarized interface band structures.