Silicon nitride SiN_x nanomechanical resonators are central to sensing, quantum technologies, and fundamental physics experiments due to their exceptional mechanical quality factors (Q). However, as resonator thickness approaches the nano-scale, surface-related dissipation limits performance. Here, we investigate the role of surface chemistry in low-stress Si-rich SiNx membranes through a combination of hydrofluoric acid (HF) etching and trimethylchlorosilane (TMCS) silanization, correlated with surface characterization and mechanical measurements. Preliminary analysis by TEM-EELS, XPS, RBS/ERDA, and XRR reveals a native oxide surface layer (1-2 nm). Surface modification by HF and TMCS was subsequently evaluated using XPS, photothermal FTIR, contact-angle measurements, and intrinsic quality factor (Q_int) characterization. While HF etching effectively removes the native oxide and TMCS introduces hydrophobic Si-(CH_3)_3 termination, neither oxide thickness nor surface energy correlates with mechanical dissipation. TMCS treatments produce the largest enhancements, increasing Q_int by up to 50

Nanomechanical photothermal sensing has significantly advanced single-molecule/particle microscopy and spectroscopy, and infrared detection. In this approach, the nanomechanical resonator detects shifts in resonant frequency due to photothermal heating. However, the relationship between photothermal sensitivity, response time, and resonator design has not been fully explored. This paper compares three resonator types - strings, drumheads, and trampolines - to explore this relationship. Through theoretical modeling, experimental validation, and finite element method simulations, we find that strings offer the highest sensitivity (with a noise equivalent power of 280 fW/Hz1/2 for strings made of silicon nitride), while drumheads exhibit the fastest thermal response. The study reveals that photothermal sensitivity correlates with the average temperature rise and not the peak temperature. Finally, the impact of photothermal back-action is discussed, which can be a major source of frequency instability. This work clarifies the performance differences and limits among resonator designs and guides the development of advanced nanomechanical photothermal sensors, benefiting a wide range of applications.

In nanomechanical photothermal absorption spectroscopy and microscopy, the measured substance becomes a part of the detection system itself, inducing a nanomechanical resonance frequency shift upon thermal relaxation. Suspended, nanometer-thin ceramic or 2D material resonators are innately highly-sensitive thermal detectors of localized heat exchanges from substances on their surface or integrated into the resonator itself. Consequently, the combined nanoresonator-analyte system is a self-measuring spectrometer and microscope: responding to a substances transfer of heat over the entire spectrum for which it absorbs, according to the intensity it experiences. Limited by their own thermostatistical fluctuation phenomena, nanoresonators have demonstrated sufficient sensitivity for measuring trace analyte as well as single particles and molecules with incoherent light or focused and unfocused coherent light. They are versatile in their design, support various sampling methods and hyphenation with other spectroscopic methods, and are capable in a wide range of applications including fingerprinting, separation science, and surface sciences.

Understanding optical absorption in silicon nitride is crucial for cutting-edge technologies like photonic integrated circuits, nanomechanical photothermal infrared sensing and spectroscopy, and cavity optomechanics. Yet, the origin of its strong dependence on film deposition and fabrication process is not fully understood. This Letter leverages nanomechanical photothermal sensing to investigate optical extinction κ_ext at 632.8 nm wavelength in LPCVD SiN strings across a wide range of deposition-related tensile stresses (200-850 MPa). Measurements reveal a reduction in κ_ext from 10^3 to 10^1 ppm with increasing stress, correlated to variations in Si/N content ratio. Within the band-fluctuations framework, this trend indicates an increase of the energy bandgap with the stress, ultimately reducing absorption. Overall, this study showcases the power and simplicity of nanomechanical photothermal sensing for low absorption measurements, offering a sensitive, scattering-free platform for material analysis in nanophotonics and nanomechanics.

Understanding optical absorption in silicon nitride is crucial for cutting-edge technologies like photonic integrated circuits, nanomechanical photothermal infrared sensing and spectroscopy, and cavity optomechanics. Yet, the origin of its strong dependence on the film deposition and fabrication process is not fully understood. This Letter leverages nanomechanical photothermal sensing to investigate optical extinction kappa(ext) at a 632.8 nm wavelength in low-pressure chemical vapor deposition (LPCVD) SiN strings across a wide range of deposition-related tensile stresses (200-850 MPa). Measurements reveal a reduction in kappa(ext) from 10(3) to 10(1) ppm with increasing stress, correlated to variations in Si/N content ratio. Within the band-fluctuations framework, this trend indicates an increase of the energy bandgap with the stress, ultimately reducing absorption. Overall, this study showcases the power and simplicity of nanomechanical photothermal sensing for low absorption measurements, offering a sensitive, scattering-free platform for material analysis in nanophotonics and nanomechanics.

We present a novel method for the quantitative analysis of mixtures of semivolatile chemical compounds. For the first time, thermal desorption is integrated directly with nanoelectromechanical infrared spectroscopy (NEMS-IR-TD). In this new technique, an analyte mixture is deposited via nebulization on the surface of a NEMS sensor and subsequently desorbed using heating under vacuum. The desorption process is monitored in situ via infrared spectroscopy and thermogravimetric analysis. The resulting spectro-temporal maps allow for selective identification and analysis of the mixture. In addition, the corresponding thermogravimetric data allow for analysis of the desorption dynamics of the mixture components. As a demonstration, caffeine and theobromine were selectively identified and quantified from a mixture with a detection limit of less than 6 pg (about 30 fmol). With its exceptional sensitivity, NEMS-IR-TD allows for the analysis of low abundance and complex analytes with potential applications ranging from environmental sensing to life sciences.

Understanding light-matter interaction at the nanoscale requires probing the optical properties of matter at the individual nanoabsorber level. To this end, we developed a nanomechanical photothermal sensing platform that can be used as a full spectromicroscopy tool for single molecule and single particle analysis. As a demonstration, the absorption cross-section of individual gold nanorods is resolved from a spectroscopic and polarization standpoint. By exploiting the capabilities of nanomechanical photothermal spectromicroscopy, the longitudinal localized surface plasmon resonance in the NIR range is unraveled and quantitatively characterized. The polarization features of the transversal surface plasmon resonance in the VIS range are also analyzed. The measurements are compared with the finite element method, elucidating the role played by electron surface and bulk scattering in these plasmonic nanostructures, as well as the interaction between the nanoabsorber and the nanoresonator, ultimately resulting in absorption strength modulation. Finally, a comprehensive comparison is conducted, evaluating the signal-to-noise ratio of nanomechanical photothermal spectroscopy against other cutting-edge single molecule and particle spectroscopy techniques. This analysis highlights the remarkable potential of nanomechanical photothermal spectroscopy due to its exceptional sensitivity.

One of the challenges of nanoelectromechanical systems (NEMS) is the effective transduction of the tiny resonators. Vertical structures, such as nanomechanical pillar resonators, which are exploited in optomechanics, acoustic metamaterials, and nanomechanical sensing, are particularly challenging to transduce. Existing electromechanical transduction methods are ill-suited as they put constraints on the pillars' material and do not enable a transduction of freestanding pillars. Here, we present an electromechanical transduction method for single nanomechanical pillar resonators based on surface acoustic waves (SAWs). We demonstrate the transduction of freestanding nanomechanical platinum-carbon pillars in the first-order bending and compression mode. Since the principle of the transduction method is based on resonant scattering of a SAW by a nanomechanical resonator, our transduction method is independent of the pillar's material and not limited to pillar-shaped geometries. It represents a general method to transduce vertical mechanical resonators with nanoscale lateral dimensions.

Nanomechanical silicon nitride (SiN) drum resonators are currently employed in various fields of applications that arise from their unprecedented frequency response to physical quantities. In the present study, we investigate the thermal transport in nanomechanical SiN drum resonators by analytical modeling, computational simulations, and experiments for a better understanding of the underlying heat transfer mechanism causing the thermal frequency response. Our analysis shows that radiative heat loss is a non-negligible heat transfer mechanism in nanomechanical SiN resonators, limiting their thermal responsivity and response time. This finding is important for optimal resonator designs for thermal sensing applications as well as cavity optomechanics.

Photosynthetic organisms had to evolve diverse mechanisms of light-harvesting to supply photosynthetic apparatus with enough energy. Cryptophytes represent one of the groups of photosynthetic organisms combining external and internal antenna systems. They contain one type of immobile phycobiliprotein located at the lumenal side of the thylakoid membrane, together with membrane-bound chlorophyll a/c antenna (CAC). Here we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the CAC proteins of cryptophyte Rhodomonas salina. The major CAC carotenoid, alloxanthin, is a cryptophyte-specific carotenoid, and it is the only naturally-occurring carotenoid with two triple bonds in its structure. In order to explore the energy transfer pathways within the CAC complex, three excitation wavelengths (505, 590, and 640 nm) were chosen to excite pigments in the CAC antenna. The excitation of Chl c at either 590 or 640 nm proves efficient energy transfer between Chl c and Chl a. The excitation of alloxanthin at 505 nm shows an active pathway from the S2 state with efficiency around 50%, feeding both Chl a and Chl c with approximately 1:1 branching ratio, yet, the S1-route is rather inefficient. The 57 ps energy transfer time to Chl a gives ~25% efficiency of the S1 channel. The low efficiency of the S1 route renders the overall carotenoid-Chl energy transfer efficiency low, pointing to the regulatory role of alloxanthin in the CAC antenna.

Thermal detectors are a cornerstone of infrared and terahertz technology due to their broad spectral range. These detectors call for efficient absorbers with a broad spectral response and minimal thermal mass. A common approach is based on impedance-matching the sheet resistance of a thin metallic film to half the free-space impedance. Thereby, one can achieve a wavelength-independent absorptivity of up to 50%. However, existing absorber films typically require a thickness of the order of tens of nanometers, which can significantly deteriorate the response of a thermal transducer. Here, we present the application of ultrathin gold (2 nm) on top of a surfactant layer of oxidized copper as an effective infrared absorber. An almost wavelength-independent and long-time stable absorptivity of 47(3)%, ranging from 2 μm to 20 μm, can be obtained. The presented absorber allows for a significant improvement of infrared/terahertz technologies in general and thermal detectors in particular.

We have developed an experimental set-up allowing to measure transient absorption spectra of carotenoids after two-photon excitation (2PE), thereby exciting the lowest excited state S1 directly. We have compared 2PE and 1PE data of three carotenoids, lycopene, β-carotene, and neurosporene. Compared to ‘standard’ 1PE, the transient absorption spectra of all three carotenoids are broader after 2PE, suggesting larger conformational disorder induced by direct excitation of the S1 state. Also, 2PE generates slightly longer S1 lifetimes compared to 1PE, which is most likely due to increased magnitude of the S* signal after 2PE. The difference in the S* signal magnitude is explained by a different subsets of conformers excited by 1PE and 2PE.

The fucoxanthin-chlorophyll a protein from Emiliania huxleyi (E-FCP) is a member of the LHC family of light-harvesting proteins. It has a rather unusual pigment composition as its binds more Chl-c than Chl-a, and 19 '-hexanoyloxyfucoxanthin (hFx) as the main carotenoid instead of fucoxanthin (Fx) typically found in various FCP complexes. The presence of a hexanoyloxy tail in hFx suppresses the charge transfer character of the S-1/ICT state resulting in almost no effect of polarity on the excited state dynamics of hFx, strongly contrasting with the excited-state properties of Fx. Here we report on the dynamics of the energy transfer between hFx and Chl in E-FCP, and we compare it with Fx-Chl energy transfer in the FCP complex from Phaeodactylum tricornutum. In both complexes, the excited hFx (Fx) transfers energy from the S-2 state with a sub-100 fs time constant and no effect of the hexanoyloxy tail on the efficiency of the S-2 route was found. The energy transfer via the S-1/ICT state has in E-FCP two channels characterized by 1.5 and 11 ps time constants, while for FCP these two channels operate with time constants of 0.8 and 4.5 ps. Thus, minimizing the charge transfer character of S-1/ICT in hFx results in about twice slower energy transfer via the S-1/ICT state, underlining the importance of the ICT state in facilitating carotenoid-Chl energy transfer in systems utilizing keto carotenoids as energy donors.

To demonstrate the value of the multipulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of lowest excited singlet state (S-1) and intramolecular charge transfer (ICT) state equilibration, we performed ultrafast transient absorption pump-dump-probe and pump-repump -probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states, which reveals that lower polarity, proticity, and temperature delay S-1/ICT equilibration. On the basis of a two-state model representing the S-1 and ICT states on the same S-1/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based on this model allowed us to quantify the population transfer rates throughout the life of the S-1/ICT state as well as separate the S-1 and ICT spectral signatures. The results are consistent with the concept that the S-1 and ICT states are part of one potential energy surface.

We used ultrafast transient absorption spectroscopy to study excited-state dynamics of the keto-carotenoid fucoxanthin (Fx) and its two derivatives: 19'-butanoyloxyfucoxanthin (bFx) and 19'-hexanoyloxyfucoxanthin (hFx). These derivatives occur in some light-harvesting systems of photosynthetic microorganisms, and their presence is typically related to stress conditions. Even though the hexanoyl (butanoyl) moiety is not a part of the conjugated system of hFx (bFx), their absorption spectra in polar solvents exhibit more pronounced vibrational bands of the S2 state than for Fx. The effect of the nonconjugated acyloxy moiety is further observed in transient absorption spectra, which for Fx exhibit characteristic features of an intramolecular charge transfer (ICT) state in all polar solvents. For bFx and hFx, however, much weaker ICT features are detected in methanol, and the spectral markers of the ICT state disappear completely in polar, but aprotic acetonitrile. The presence of the acyloxy moiety also alters the lifetimes of the S1/ICT state. For Fx, the lifetimes are 60, 30, and 20 ps in n-hexane, acetonitrile, and methanol, whereas for bFx and hFx, these lifetimes yield 60, 60, and 40 ps, respectively. Testing the S1/ICT state lifetimes of hFx in other solvents revealed that some ICT features can be induced only in polar, protic solvents (methanol, ethanol, and ethylene glycol). Thus, bFx and hFx represent a rather rare example of a system in which a nonconjugated functional group significantly alters excited-state dynamics. By comparison with other carotenoids, we show that a keto group at the acyloxy tail, even though it is not in conjugation, affects the electron distribution along the conjugated backbone, resulting in the observed decrease of the ICT character of the S1/ICT state of bFx and hFx.

We have applied femtosecond transient absorption spectroscopy in pump-probe and pump-dump-probe regimes to study energy transfer between fucoxanthin and Chl a in fucoxanthin-Chl a complex from the pennate diatom Phaeodactylum tricornutum. Experiments were carried out at room temperature and 77 K to reveal temperature dependence of energy transfer. At both temperatures, the ultrafast (< 100 fs) energy transfer channel from the fucoxanthin S-2 state is active and is complemented by the second pathway via the combined S-1/ICT state. The S-1/ICT-Chl a pathway has two channels, the fast one characterized by sub-picosecond energy transfer, and slow having time constants of 4.5 ps at room temperature and 6.6 ps at 77 K. The overall energy transfer via the S-1/ICT is faster at 77 K, because the fast component gains amplitude upon lowering the temperature. The pump dump-probe regime, with the dump pulse centered in the spectral region of ICT stimulated emission at 950 nm and applied at 2 ps after excitation, proved that the S-1 and ICT states of fucoxanthin in FCP are individual, yet coupled entities. Analysis of the pump-dump-probe data suggested that the main energy donor in the slow S-1/ICT-Chl a route is the S-1 part of the S-1/ICT potential surface.

Alloxanthin, which has two triple bonds within its backbone, was studied by steady-state and femtosecond transient absorption spectroscopies. Alloxanthin demonstrates an S2 energy comparable to its non-triple bond homolog, zeaxanthin, while the S1 lifetime of 19 ps is markedly longer than that of zeaxanthin (9 ps). Along with corroborating quantum chemistry calculations, the results show that the long-lived S1 state of alloxanthin, which typically corresponds to the dynamic of a shorter carotenoid backbone, implies the triple bond isolates the conjugation of the backbone, increasing the S1 state energy and diminishing the S1–S2 energy gap.

We studied the impact excitation power on the plasmonic enhancement of Forster resonance energy transfer (FRET) between CdSe/ZnS colloidal dots. We showed that when such quantum dots (QDs) were in the vicinity of metallic nanoparticles (MNPs), with the increase of the excitation laser intensity the spectral width of their emission was initially increased and then decreased, reaching its original value when the laser intensity was low. Such a spectral lineshape recovery happened while the emission peak wavelength of the quantum dots underwent a significant amount of red shift. By tuning the exciton–plasmon coupling strength and including the effects of the heat generated by plasmonic absorption, we showed that these results could be explained in terms of transition from a donor-dominated spectrum at low laser intensity to an acceptor-dominated one at high laser intensity. This suggests that excitation intensity can increase the rate of FRET between colloidal QDs when they are in close proximity to MNPs.

We used photoactive substrates consisting of about 1 nm coating of a metal oxide on glass substrates to investigate the impact of the structures of colloidal quantum dots on their photophysical and photochemical properties. We showed during irradiation these substrates can interact uniquely with such quantum dots, inducing distinct forms of photo-induced processes when they have different cores, shells, or ligands. In particular, our results showed that for certain types of core-shell quantum dot structures an ultrathin layer of a metal oxide can reduce suppression of quantum efficiency of the quantum dots happening when they undergo extensive photo-oxidation. This suggests the possibility of shrinking the sizes of quantum dots without significant enhancement of their non-radiative decay rates. We show that such quantum dots are not influenced significantly by Coulomb blockade or photoionization, while those without a shell can undergo a large amount of photo-induced fluorescence enhancement via such blockade when they are in touch with the metal oxide.

We study the mutual effects of photoinduced processes (irradiation effects) and plasmonic emission enhancement in close-packed CdSe/ZnS colloidal quantum dots in the vicinity of gold metallic nanoparticles with two significantly different size distributions. For this we examine the impact of the heat generated by the metallic nanoparticles, the strength of plasmonic field enhancement, and the rate of energy transfer from the quantum dots to metallic nanoparticles in the presence of a laser field with low and moderate intensities. Our results show that the interplay between the photophysics of the quantum dots and their plasmonic emission enhancement is significantly pronounced when the metallic nanoparticles are large. In such a case we observed large suppression of photoinduced fluorescence enhancement (PFE). For smaller metallic nanoparticles the results suggest mostly an overall time-independent suppression of the quantum dots’ emission with no significant impact on PFE.