Laser Excitation Research Articles

Optical-domain modulation cancellation method for background-suppression and dual-gas detection in light-induced thermo-elastic spectroscopy

To address the issues of signal saturation and measurement range reduction caused by the background in light-induced thermoelastic spectroscopy (LITES) employing the first harmonic (1 f) modulation, an optical-domain modulation cancellation method (MOCAM) was reported in the 1f-based LITES. A balance laser source with the opposite sine modulation phase as the excitation laser source synchronously excites the quartz tuning fork (QTF) to realize background suppression. By optimizing the modulation amplitude of the balance laser and the modulation phase difference between the two laser sources, the background level was suppressed by two orders of magnitude from ∼ 7 mV to ∼ 18 μV, close to the QTF thermal noise level (∼ 1 μV). Combining the excitation laser with the balance laser enables MOCAM to overcome common mode interferences, e.g. optical path shift, laser power fluctuation, and focus position change. Furthermore, methane and water vapor (CH4/H2O) dual-gas detection based on MOCAM and time-division multiplexing method was demonstrated, and measurement range is enlarged with easy calibration due to zero background. The MOCAM-based 1f-LITES technique delivers effective background suppression and provides robustness to common mode interferences, thus establishes a feasible solution for mobile vehicle or airborne gas monitoring applications with serious environmental disturbances.

An Ultralow‐Cost, Durable, Flexible Substrate for Ultrabroadband Surface‐Enhanced Raman Spectroscopy

Flexible devices have recently seen notable advancements for use in various areas. Surface‐enhanced Raman spectroscopy (SERS) has emerged as a transformative technology in this space. However, practical applications of flexible SERS substrates are often restricted to labs, hindered by challenges such as inconsistent reproducibility, limited scalability (<10 mm2), short lifespan (<1 day), narrowband response (<100 nm), and especially high costs (>10 dollars mm−2). Addressing these barriers, an ultralow‐cost (≈10 cents mm−2), highly durable, flexible substrate that boasts exceptional SERS performance is studied. This highly scalable, adhesive, ultrathin nanomesh, crafted from a 3D network of silver‐deposited PVA nanofibers, ensures a high density of hotspots and consistent measurements (<20% CV). Its protective PVA layer keeps the silver unoxidized for sustained usage (>1 month). Remarkably versatile, this substrate works across a range of excitation laser wavelengths in both the visible and near‐infrared regions, including 488, 532, 633, and 785 nm. Furthermore, the substrate has shown its efficacy in analyzing residual pesticides on fruits for food safety testing and human biofluids (sweat and urine) for health monitoring. This innovation paves the way for transitioning SERS applications from labs to industrial settings.

Atomic-scale view of the photoinduced structural transition to form sp3-like bonded order phase in graphite

Photoexcitation of solids often induces structural phase transitions between different ordered phases, some of which are unprecedented and thermodynamically inaccessible. The phenomenon, known as photoinduced structural phase transition (PSPT), is of significant interest to the technological progress of advanced materials processing and the fundamental understanding of material physics. Here, we applied scanning tunnelling microscopy (STM) to directly characterise the primary processes of the PSPT in graphite to form a sp3-like carbon nano-phase called diaphite. The primary challenge was to provide microscopic views of the graphite-to-diaphite transition. On an atomic scale, STM imaging of the photoexcited surface revealed the nucleation and proliferation processes of the diaphite phase; these were governed by the formation of sp3-like interlayer bonds. The growth mode of the diaphite phase depends strongly on the photon energy of excitation laser light. Different dynamical pathways were proposed to explain the formation of a sp3-like interlayer bonding. Potential mechanisms for photon-energy-dependent growth were examined based on the experimental and calculated results. The present results provide insight towards realising optical control of sp2-to-sp3 conversions and the organisation of nanoscale structures in graphene-related materials.

Coherent light scattering from a telecom C-band quantum dot

Quantum networks have the potential to transform secure communication via quantum key distribution and enable novel concepts in distributed quantum computing and sensing. Coherent quantum light generation at telecom wavelengths is fundamental for fibre-based network implementations, but Fourier-limited emission and subnatural linewidth photons have so far only been reported from systems operating in the visible to near-infrared wavelength range. Here, we use InAs/InP quantum dots to demonstrate photons with coherence times much longer than the Fourier limit at telecom wavelength via elastic scattering of excitation laser photons. Further, we show that even the inelastically scattered photons have coherence times within the error bars of the Fourier limit. Finally, we make direct use of the minimal attenuation in fibre for these photons by measuring two-photon interference after 25 km of fibre, demonstrating finite interference visibility for photons emitted about 100,000 excitation cycles apart.

A coherent phonon-induced hidden quadrupolar ordered state in Ca2RuO4

Ultrafast laser excitation provides a means to transiently realize long-range ordered electronic states of matter that are hidden in thermal equilibrium. Recently, this approach has unveiled a variety of thermally inaccessible ordered states in strongly correlated materials, including charge density wave, ferroelectric, magnetic, and intertwined charge-orbital ordered states. However, more exotic hidden states exhibiting higher multipolar ordering remain elusive owing to the challenge of directly manipulating and detecting them with light. Here we demonstrate a method to induce a dynamical transition from a thermally allowed to a thermally forbidden spin-orbit entangled quadrupolar ordered state in Ca2RuO4 by coherently exciting a phonon that is strongly coupled to the order parameter. Combining probe photon energy-resolved coherent phonon spectroscopy measurements with model Hamiltonian calculations, we show that the dynamical transition is manifested through anomalies in the temperature, pump excitation fluence, and probe photon energy dependence of the strongly coupled phonon. With this procedure, we introduce a general pathway to uncover hidden multipolar ordered states and to control their re-orientation on ultrashort timescales.

Fabrication of amorphous nanoporous ZrO2/SiO2 aerogel enabling nonlinear optical properties

Amorphous oxides have unique physicochemical properties with extensive opto-electronic applications such as the thin-film transistor, light-emitting diode backplanes, and supercontinuum generation. In this contribution, we synthesize the amorphous ZrO2/SiO2 nanoporous aerogel with high structural integrity. With the femtosecond excitation laser at 800–1,064 nm, the broadband second harmonic generation is observed. The nonlinear optical properties of the as-prepared ZrO2/SiO2 aerogel are investigated at 1.0 μm and 1.5 μm for the first time. Subsequently, the amorphous ZrO2/SiO2 saturable absorber is originally applied in the Yb-doped and Er-doped fiber lasers to realize the mode-locking operations. In the Yb-doped fiber laser, the dissipative soliton resonance mode-locking operation is demonstrated with the largest pulse duration of 22 ns at a repetition rate of 7.8 MHz and a high signal-to-noise ratio of 64 dB. In the Er-doped fiber laser, a conventional soliton mod-locking regime is observed with an ultrashort pulse width of 960 fs, a repetition frequency of 6.55 MHz, and a time-bandwidth production of 0.347. Our work shows the good ability of the ZrO2/SiO2 aerogel in generating ultrafast pulses and extends the saturable absorber into the amorphous material realm.

Ultrafast dynamics of a photoinduced phase transition in single-crystal trititanium pentoxide

We investigate photoinduced phase transition of Ti3O5 using the pump-probe method in an ultrafast electron microscope. We measured time-resolved electron diffraction patterns of single-crystal lamellae of Ti3O5 excited by the femtosecond laser at the wavelength of 800 and 400 nm. Under 800 nm laser excitation, the photoinduced phase transition from the β-phase to the λ-phase with a similar timescale to previous measurements conducted using nanocrystals was observed. This indicates that the photoinduced phase transition is not a phenomenon influenced by the material surface, size, or boundary, but an intrinsic property in a bulk crystal of Ti3O5. The photoinduced phase transition was not detected, when excited by the laser at the wavelength of 400 nm. The results are discussed based on the electronic states of β-Ti3O5 and λ-Ti3O5. The density of the dissociated interatomic bonds between Ti at the specific site should be an important factor for triggering the photoinduced phase transition.

Tuning of Chromaticity Coordinates for Ce:YAG Single Crystals by Laser Excitation

Tuning of Chromaticity Coordinates for Ce:YAG Single Crystals by Laser Excitation

Break of thermal equilibrium between optical and acoustic phonon branches of CsPbI3 under continuous-wave light excitation and cryogenic temperature

The kinetic of low-temperature carrier and lattice of lead-halide perovskite is yet to be fully understood. In this work, we investigate the steady-state photoluminescences (PLs) of CsPbI3 at the environmental temperature (Te) ranging from 20 K to 300 K, and observed anomalous behaviors at cryogenic temperatures: The carrier temperature (Tc) of pure CsPbI3 exhibits a negative correlation with Te, accompanied by an expansion in Urbach tails of absorption spectra (Abs.) and excessive red-shifts at peak energy of PLs. These phenomena are also observed in those samples containing a certain amount of Cs4PbI6, but to a lesser extent and occurs at lower temperatures. It is attributed to the intensified hot phonon bottleneck effect (HPB) in CsPbI3 at cryogenic Te, which hinders the energy transfer from hot carriers, via longitudinal optics (LO) phonons to longitudinal acoustic (LA) phonons, to the ambient. For samples under continuous-wave laser excitation, in specific, the barrier induced by the enhanced HPB at low Te prevents the effective thermalization among carriers, LO and LA phonons, which, therefore, form thermally isolated ensembles with different temperatures. At cryogenic Te range, the elevated temperatures of carrier and LO phonon expand the high-energy side of PLs and the low-energy tail of Abs., respectively. For those samples in which the CsPbI3 is mixed with Cs4PbI6, the interfacial LO-LO interaction across them provides a bypass for heat dissipation, mitigating the heat accumulation in LO-phonons of CsPbI3. The results suggest that a strong HPB effect may break the thermal equilibrium among different branches of phonons in the lattice under certain extreme conditions.

Resonant Versus Nonresonant Spin Readout of a Nitrogen-Vacancy Center in Diamond under Cryogenic Conditions.

The last decade has seen an explosive growth in the use of color centers for metrology applications, the paradigm example arguably being the nitrogen-vacancy (NV) center in diamond. Here, we focus on the regime of cryogenic temperatures and examine the impact of spin-selective, narrow-band laser excitation on NV readout. Specifically, we demonstrate a more than fourfold improvement in sensitivity compared to that possible with nonresonant (green) illumination, largely due to a boost in readout contrast and integrated photon count. We also leverage nuclear spin relaxation under resonant excitation to polarize the ^{14}N host, which we then prove beneficial for spin magnetometry. These results open opportunities in the application of NV sensing to the investigation of condensed matter systems, particularly those exhibiting superconducting, magnetic, or topological phases selectively present at low temperatures.

Microfiber knot resonator augmented quartz-enhanced photoacoustic spectroscopy

In this work, a microfiber knot resonator augmented quartz-enhanced photoacoustic spectroscopy (MKR-QEPAS) was developed for the analysis of trace gases. In the proof-of-concept experiment, a 1.39 μm laser diode was utilized for the detection of H2O molecules. Based on the H2O absorption line, the optical fiber-based MKR was meticulously designed to modulate the transmission spectrum, thereby inducing constructive interference with the excitation laser wavelength. The MKR was integrated with a clamp-type tuning fork, leading to a pronounced evanescent resonance enhancement. Consequently, the detection sensitivity of the MKR-QEPAS was improved by approximately one order of magnitude. The micro ring resonator is posited to hold significant promise for hybrid integration in QEPAS on-chip sensors.

Nonlinear optical properties of gold with intense femtosecond laser excitations above the damage threshold

The nonlinear optical properties of gold thin films have been studied under the irradiation of destructive laser pluses. The transmissivity and the reflectivity of a 100-nm-thick gold film are measured with femtosecond laser irradiations up to 106 J/m2 fluence, which is much higher than the damage threshold of gold. The nonlinearity is calculated with Fresnel loss equations. The real part of the complex refractive index is calculated to have a peak at ∼10 kJ/m2, which is close to the damage threshold. The complex third-order nonlinear susceptibility χ3 is estimated to be (4.95 − 2.02i) × 10−21 m2/V2, which agrees with the trend in previous reports that it decreases as the pulse duration decreases. This nonlinearity is further studied with a plasmon-photon exchange (PPE) model, where laser induced plasmons and absorbed photons are strongly correlated. The result of the simulation with the PPE model is in good agreement with the experimental transmissivity above the damage threshold. The model predicted an interaction time between photons and plasmons to be about 500 fs. This model is found to be essential in understanding such nonlinear optical processes under both extremely high and conventionally low laser irradiations.

Optical temperature sensing characteristics of Yb3+/Nd3+ co-doped YAG single crystal fiber based on up-conversion luminescence

YAG: 5 mol%Yb3+/x mol%Nd3+(x = 0, 0.5, 1, 2, 4, 7) and YAG: 2 mol%Nd3+ single crystal (SC) fluorescent materials were prepared using laser heated pedestal growth (LHPG) method. The up-conversion (UC) spectra of YAG: Yb3+/Nd3+ were recorded under 980 nm laser excitation, the results show that the optimal material was obtained at 5 mol%Yb3+/2 mol%Nd3+. An integrated YAG single crystal fiber (SCF) with end 5 mol%Yb3+/2 mol%Nd3+ co-doped was prepared by the LHPG method, the temperature-dependent characteristics were measured in the range from 348 K to 1448 K. It shows that the fluorescence intensity increases and then decreases with the increase of temperature. Fluorescence intensity ratios (FIR) in Nd3+ UC spectra, including FIR1 (4F7/2/4F5/2 → 4I9/2), FIR2 (4F9/2/4F5/2 → 4I9/2) and FIR3 (4G5/2/4F9/2 → 4I9/2), vary monotonically in temperature ranges of 348–1273 K, 523–1273 K, and 673–1273 K, respectively. The maximally absolute and relative sensitivities for FIR1 were, respectively, achieved to be 2.34 × 10−3 K−1 at 738 K and 4.61 × 10−1 %K−1 at 390 K, and these values for FIR2 were accordingly achieved to be 1.19 × 10−3 K−1 at 1273 K and 4.59 × 10−1 %K−1 at 735 K, and for FIR3 they were 4.66 × 10−3 K−1 at 1273 K and 4.09 × 10−1 %K−1 at 760 K. Therefore, we believe that the fabricated YAG: Yb3+/Nd3+ fluorescent materials have favorable application potential for optical thermometer.

Improvement of response bandwidth and sensitivity of Rydberg receiver using multi-channel excitations

We investigate the response bandwidth of a superheterodyne Rydberg receiver at a room-temperature vapor cell, and present an architecture of multi-channel lasers excitation to increase the response bandwidth and keep sensitivity, simultaneously. Two microwave fields, denoted as a local oscillator (LO) E_{text{LO}} and a signal field E_{text{SIG}}, couple two Rydberg states transition of |52D_{5/2}rangle to |53P_{3/2}rangle . In the presence of the LO field, the frequency difference between two fields can be read out as an intermediate frequency (IF) signal using Rydberg electromagnetically induced transparency (EIT) spectroscopy. The bandwidth of the Rydberg receiver is obtained by measuring the output power of IF signal versus the frequency difference between two fields. The bandwidth dependence on the Rabi frequency of excitation lasers is presented, which shows the bandwidth decrease with the probe Rabi frequency, while it is quadratic dependence on the coupling Rabi frequency. Meanwhile, we investigate the effect of probe laser waist on the bandwidth, showing that the bandwidth is inversely proportional to the laser waist. We achieve a maximum response bandwidth of the receiver about 6.8 MHz. Finally, we design an architecture of multi-channel lasers excitation for increasing the response and keeping the sensitivity, simultaneously. Our work has the potential to extend the applications of Rydberg atoms in communications.

Second-Order Nonlinearity of Graphene Quantum Dots Measured by Hyper-Rayleigh Scattering

The first hyperpolarizability of graphene quantum dots (GQDs) suspended in water was determined using the hyper-Rayleigh scattering (HRS) technique. To the best of our knowledge, this is the first application of the HRS technique to characterize GQDs. Two commercial GQDs (Acqua-Cyan and Acqua-Green) with different compositions were studied. The HRS experiments were performed with an excitation laser at 1064 nm. The measured hyperpolarizabilities were (1.0±0.1)×10−27 esu and (0.9±0.1)×10−27 esu for Acqua-Cyan and Acqua-Green, respectively. The results were used to estimate the hyperpolarizability per nanosheet obtained by assuming that each GQD has five nanosheets with 0.3 nm thickness. The two-level model, used to calculate the static hyperpolarizability per nanosheet, provides values of (2.4±0.1)×10−28 esu (Acqua-Cyan) and (0.5±0.1)×10−28 esu (Aqua-Green). The origin of the nonlinearity is discussed on the basis of polarized resolved HRS experiments, and electric quadrupolar behavior with a strong dependence on surface effects. The nontoxic characteristics and order of magnitude indicate that these GQDs may be useful for biological microscopy imaging.

Detection and identification of leukemic and normal cells using Raman spectroscopy and multivariate statistical analysis

Raman spectroscopy technique was successfully employed to study the difference between leukemic cells (Jurkat cell line) and normal human bone marrow mononuclear cells using near-infrared laser (785 nm) excitation in this study. Significant differences in Raman spectra from two kinds of cell lines showed special changes in the percentage of biochemical constituents in different cells. The obtained spectral data were used to develop diagnostic algorithms by multivariate statistical analysis, including principal component analysis combined with linear discriminate analysis. The multivariate statistical analysis method for classification between cancer and normal cells has achieved good differentiation with high diagnostic sensitivity (100%), specificity (100%), and accuracy (100%), respectively. These results showed that Raman spectroscopy is a novel, nondestructive method, that provided interesting information about the changes in biochemical properties of cells. Raman spectroscopy combined with multivariate statistical analysis can serve as a potential clinical diagnosis method of leukemia.

Unique sandwich and microstructure design of phosphor-in-glass film for high brightness laser-driven white lighting

All-inorganic phosphor-in-glass (PiG) converter withstanding high power laser excitation has garnered attention for laser-driven white lighting, while its poor heat dissipation and low fluorescence conversion efficiency has been serious challenges in high-brightness white lighting. Herein, a thermally robust PiG film converter with a sandwich phosphor structure and microstructure interface was designed for ultrahigh-brightness laser-driven white lighting. The yellow-emitting YAG PiG film was thermocompression bonded to microstructure sapphire (MS) and planar alumina (PA) to construct the MS-PiG-PA converter. The YAG PiG film possess higher luminescence intensity than YAG phosphor and was tightly attached to sapphire and alumina without microcracks. Benefiting from the double-sided heat dissipation design and microstructure interface light field strengthening effect, the MS-PiG-PA converter emits ultrahigh luminous flux white light (5197 lm) at power density of 30 W/mm2 and displays excellent CRI and CCT stability. The MS-PiG-PA converter has a lower working temperature, higher saturation threshold and luminous efficacy.

Spectral Tuning of High-Harmonic Generation with Resonance-Gradient Metasurfaces.

High-index dielectric subwavelength structures and metasurfaces are capable of enhancing light-matter interaction by orders of magnitude via geometry-dependent optical resonances. This enhancement, however, comes with a fundamental limitation of a narrow spectral range of operation in the vicinity of one or few resonant frequencies. Here, this limitation is tackled by introducing an innovative and practical approach to achieve spectrally tunable enhancement of light-matter interaction with resonant metasurfaces. Resonance-gradient metasurfaces are designed and fabricated with varying geometrical parameters that translate into resonant frequencies dependence on one of the coordinates of the metasurface. The metasurfaces are composed of bone-like nanoresonators, which are made of germanium and support high-quality optical resonances in the mid-IR spectral range. The concept is applied to observe the resonant enhancement of the third and fifth harmonics generated from the gradient metasurfaces being used in conjunction with a tunable excitation laser to provide a wide spectral coverage of resonantly-enhanced tunable generation of multiple opticalharmonics.

Optical tuning of the terahertz response of black phosphorus quantum dots: effects of weak carrier confinement

Abstract In contrast to few-layer black phosphorus (BP) with a relatively larger area, BP quantum dots (BP-QDs) are expected to have distinctive electromagnetic response and carrier behaviors, especially in low-frequency range such as in the THz regime. Herein, we experimentally investigate the THz properties of BP-QDs as well as the optical control of these properties. It is demonstrated that the effects of weak carrier confinement, which is associated with diffusive restoring current in each BP-QD, contribute significantly to the effective THz conductivity of BP-QDs. Instead, spectral features of discretely spaced energy levels as shown for many kinds of semiconductor QDs in UV-visible range are not observed in the THz regime. This indicates an insignificant contribution of strong quantum confinement here. Based on the modified Drude–Smith formula, we show that the optical excitation/pump of a CW laser can induce photogenerated carriers and enhance the effects of weak carrier confinement in BP-QDs. Thus, a nonlinear enhancement of THz absorption can be observed by increasing the power of the excitation laser. These results not only deepen our understanding of the fundamental physics of BP nanomaterials but also provide an alternative approach to realize active control of BP-based THz devices.

Effect of Zn2+, S2−, Mo4+ and V5+ single doped BiTa7O19:Er3+/Yb3+ on upconversion luminescence intensity under 980 nm laser excitation

On the basis of BiTa7O19 (BTO):0.1Er3+/0.4 Yb3+, Zn2+, S2−, Mo4+ and V5+ single doped upconversion phosphors (UCP) were successfully prepared by solid phase sintering. The lattice structure, particle size and diffuse reflectance spectra were measured by X-ray diffraction, scanning electron microscope and spectrophotometer. Upconversion luminescence (UCL) spectra of these UCP were investigated under 980 nm laser excitation with power density from 1.82 to 124.99 W/cm2. Zn2+ doped UCP can obtain the highest UCL intensity when excitation power density is less than 56.91 W/cm2, and Mo4+ doped UCP can obtain the highest UCL intensity when excitation power density is from 56.91 to 124.99 W/cm2. The UCL intensity of all samples increases first and then decreases with the increase of excitation power. By measuring UCL intensity changes with the excitation power of the UCP mixing with BN and UCP in vacuum and atmosphere, the experimental results show that the increase of temperature caused by laser excitation is the reason for the decrease of UCL intensity under high power excitation. Utilizing LIR technology, it is proven that at high power 90.95 W/cm2, Mo4+ and Zn2+ single doped can decrease and increase the UCP temperature under the same power density excitation compared with BTO:0.1Er3+/0.4 Yb3+, respectively. The maximum relative temperature sensitivity of all UCP is calculated in the range of 0.00767–0.00854 K−1 at 303 K. All experiments show that Zn2+ and Mo4+ single-doped UCP are suitable for temperature sensing and luminescence imaging under low and high-power excitation, respectively.