Pritam Khan

We report on circularly polarized Gaussian beam spin angular momenta that can be inverted upon scattering with quadrupole plasmon modes. The conditions for such conversion are met with high-angle collection, dark-field scattering microscopy on spherical plasmonic particles. We further report that silvered nanoporous silica microparticles exhibit a strong nonlinearity in their scattering, specifically a reverse saturated scattering (RSS), when exposed to high laser power densities on the sample of ca. 5 GW/cm². Handedness conversion by these microparticles is only observed at wavelengths tuned to the quadrupole modes. Conversely, the scattering remains linear, and the handedness is unchanged, when the same particles are illuminated with low laser power densities of ca. 10 W/cm². We infer that RSS tuned to the quadrupole modes sufficiently enhances their contribution so that they dominate the high-angle scattering, thereby justifying the light spin inversion. Moreover, the addition of a self-assembled monolayer of ethynylaniline (EA) on the microparticles results in handedness conversion for both low and high incident power, as expected from preferable dipole damping and plasmon mode red shift. This demonstrates that optical nonlinearity in scattering can be exploited for polarization tuning in plasmonic metamaterials.

In this Letter, we demonstrate for the first time, to the best of our knowledge, N i C o 2 O 4 (NCO) as a novel nonlinear optical material with straightforward potential applications in optical limiting. For the 532 nm nanosecond laser, excited state absorption (ESA) and free-carrier absorption give rise to large ESA coefficient ( β E S A ) and positive nonlinear n 2 . On the other hand, when excited with the 800 nm femtosecond laser, two-photon absorption (TPA) takes place, and bound carriers induce strong negative n 2 . The values of β and n 2 obtained for NCO are found to be higher compared to other conventional transition metal oxides and, therefore, are promising for optics and other photonics applications.

Amorphous chalcogenide glasses are intrinsically metastable, highly photosensitive, and therefore exhibit numerous light-induced effects upon bandgap and sub-bandgap illumination. Depending on the pulse duration of the excitation laser, ChGs exhibit a series of light-induced effects spanning over femtosecond to seconds time domain. For continuous wave (CW) illumination, the effects are dominantly metastable in terms of photodarkening (PD) and photobleaching (PB) that take place via homopolar to heteropolar bond conversion. On the other hand, under nanosecond and ultrafast pulsed illumination, ChGs exhibit transient absorption (TA) that is instigated from the transient bonding rearrangements through self-trapped exciton recombination. In the first part of the review, we pay special attention to continuous wave light-induced PD and PB, while in the second part we will focus on the TA and controlling such effects via internal and external parameters, e.g., chemical composition, temperature, sample history, etc.

Amorphous chalcogenide (ChGs) glasses are intrinsically metastable, highly photosensitive, and therefore exhibit numerous lightinduced effects upon bandgap and sub-bandgap illumination. Depending on the pulse duration of the excitation laser, ChGs exhibit a series of lightinduced effects spanning over femtosecond to seconds time domain. For continuous wave illumination, the effects are dominantly metastable in terms of photodarkening (PD) and photobleaching (PB) that takes place via homopolar to heteropolar bond conversion. On the other hand, under nanosecond and ultrafast pulsed illumination, ChGs exhibit transient absorption (TA) that is instigated from the transient bonding rearrangements through self-trapped exciton recombination. In the first part of the review, we pay special attention to continuous wave lightinduced PD and PB, while in the second part we will focus on the TA and controlling such effects via internal and external parameters e.g., chemical composition, temperature, sample history etc.

Second harmonic generation and sum frequency generation (SHG and SFG) provide effective means to realize coherent light at desired frequencies when lasing is not easily achievable. They have found applications from sensing to quantum optics and are of particular interest for integrated photonics at communication wavelengths. Decreasing the footprints of nonlinear components while maintaining their high up-conversion efficiency remains a challenge in the miniaturization of integrated photonics. Here we explore lithographically defined AlGaInP nano(micro)structures/Al₂O₃/Ag as a versatile platform to achieve efficient SHG/SFG in both waveguide and resonant cavity configurations in both narrow- and broadband infrared (IR) wavelength regimes (1300–1600 nm). The effective excitation of highly confined hybrid plasmonic modes at fundamental wavelengths allows efficient SHG/SFG to be achieved in a waveguide of a cross-section of 113 nm × 250 nm, with a mode area on the deep subwavelength scale (λ²/135) at fundamental wavelengths. Remarkably, we demonstrate direct visualization of SHG/SFG phase-matching evolution in the waveguides. This together with mode analysis highlights the origin of the improved SHG/SFG efficiency. We also demonstrate strongly enhanced SFG with a broadband IR source by exploiting multiple coherent SFG processes on 1 µm diameter AlGaInP disks/Al₂O₃/Ag with a conversion efficiency of 14.8% MW⁻¹ which is five times the SHG value using the narrowband IR source. In both configurations, the hybrid plasmonic structures exhibit >1000 enhancement in the nonlinear conversion efficiency compared to their photonic counterparts. Our results manifest the potential of developing such nanoscale hybrid plasmonic devices for state-of-the-art on-chip nonlinear optics applications.

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

The ultrafast switching of magnetization in multiferroic materials by a femtosecond laser could provide various advantages in photonics and magnonics. An efficient approach to control the light matter interaction is the modulation of ultrafast coherent magnons and phonons in the high frequency range. Spontaneous Raman and infrared spectra reveal the excitation of magnons and optical phonons in multiferroic BiFeO3 in the sub few terahertz range. However, coherent control of such quasiparticles has not been achieved yet. In this study, we demonstrate that linearly polarized laser pulses simultaneously excite coherent magnons out of plane and in plane cyclone modes and optical phonon E mode in BiFeO3. Experimental results in conjugation with phenomenological theory, by considering three uniformly distributed magnetic domains reveal that impulsive stimulated Raman scattering is responsible for the generation of coherent magnons and phonons in BiFeO3. The observation of these terahertz magnon and optical phonon modes paves the way for the development of ultrafast magneto electro optical devices.

Nanosecond near resonant excitation in As ₅₀ S ₅₀ thin films leads to strong nonlinear optical response, i.e. nonlinear absorption coefficient up to 4 × 10 ⁶ cm GW ⁻¹ and nonlinear refractive index of 8.5 cm ² GW ⁻¹ , both of which are the strongest ever reported in amorphous semiconductors. We propose a three-level energy band model to explain this effect, which indicates that the nonlinear process is reverse saturable absorption in nature, mediated by excited-state absorption from slow interband transition between the conduction and valence band. On the other hand, observation of negative nonlinear refractive index reveals the occurrence of self-defocusing effect. Finally, benefitting from the strong nonlinear response, we demonstrate a promising application of As ₅₀ S ₅₀ thin films as an optical limiter for optoelectronic sensors.