Optical Pumping Conditions Research Articles

Stabilizing Single‐Source Evaporated Perovskites with Organic Interlayers for Amplified Spontaneous Emission

AbstractMetal halide perovskites are promising semiconductors with promising applications in optoelectronic and photonic technologies. When coherent emission applications are targeted, materials with lower lasing thresholds and increased stabilities must be developed to increase the performance under continuous wave optical pumping condition and finally allow the realization of the long sought‐after electrically pumped lasers. Perovskite multiple‐quantum‐wells (MQWs) can potentially ease the population inversion by confining photoexcitation within the heterostructure's wells, but their fabrication process and structural design still require a delicate optimization to make them valuable photonic platforms. Here, perovskite MQWs are fabricated based on organic semiconductors and CsPbBr3, using a facile and easily scalable sequential single‐source vacuum evaporation method. Perovskite with the organic interlayer shows radically enhanced phase stability, passivated defects, and improved radiative recombination properties. In this way, upon proper design of the heterostructure wells and barriers thicknesses, optically pumped amplified spontaneous emission can be achieved. This work reports an effective fabrication approach for perovskite MQWs, while providing a deeper understanding of their photophysical properties to foster their application as coherent light emitters.

Terahertz photoluminescence in doped nanostructures with spatial separation of donors and acceptors

Terahertz (THz) luminescence in doped nanostructures with GaAs/AlGaAs quantum wells is studied under conditions of interband optical pumping. The study is carried out on compensated quantum wells with different doping profiles. In one structure, compensation is carried out directly in each quantum well by introducing donors and acceptors with the same concentration. In the other, donors and compensating acceptors are spatially separated, namely, donors are located in quantum wells, while acceptors are located in barriers. The spectra of THz photoluminescence are studied at various temperatures and levels of optical excitation. It is shown that the main contributions to the THz emission in both structures are due to the electron transitions from the first electron subband and excited donor states to the ground donor states. It is found that the spatial separation of donors and acceptors provides an approximately twofold increase in the integrated intensity of THz emission. This is directly related to the increase in the rate of dynamic depopulation of the ground donor levels due to the optical transitions from the ground donor states to the first heavy-hole subband, which is confirmed by photoluminescence studies in the near-infrared spectral range.

In-Situ Detection for Atomic Density in the K-Rb-21Ne Co-Magnetometer via an Optical Heterodyne Interferometry

The low-frequency fluctuations of the atomic density within the cell can induce the longterm drift of the K-Rb-21Ne spin-exchange relaxation-free (SERF) co-magnetometer output, such that the accurate measurement of in situ atomic density is of great significance for improving the performance of co-magnetometer. In this paper, the complex refractive index model of the spin ensembles under the hybrid optical pumping condition is established first, according to which the relation between atomic density and its complex refractive index is revealed and an optical heterodyne-based scheme for atomic density detection is proposed. The dependence of the atomic density on the demodulated phase signal from the optical heterodyne-based scheme is provided by numerical simulations. After that, a dual acousto-optics frequency shifter (AOFS)-based optical heterodyne interferometry is constructed with a noise level below 1 mrad/Hz for frequencies > 1 Hz, and a compact SERF co-magnetometer is implemented as the testing medium, by which the atomic density detection with resolution of 0.40 K @ 473 K is reached and the experimental results agree well with theoretical simulations. Moreover, the detection scheme proposed in this paper has the properties of high detection sensitivity and immunity to laser power fluctuation, which are also proved experimentally.

Terahertz Metasurface Modulators Based on Photosensitive Silicon

AbstractMetasurfaces solve the lack of materials in the terahertz (THz) band and control precisely the amplitude, phase, polarization, and transmission characteristics of THz waves, providing an effective way to realize THz functional devices. This article focuses on the design of THz metasurface modulators with a unit structure consisting of metal square rings, including resonance frequency, phase, and amplitude modulators. By embedding photosensitive semiconductor silicon (Si) in the unit structure, the unit structure is built from meta‐atom to molecularization model under the optical pumping condition, and the resonance frequencies are switched between high and low frequencies. The resonance frequency switchable characteristic is demonstrated using the equivalent LC oscillation circuit model, and the theoretical calculation results agree well with the simulations. Through theoretical calculations, the modulators achieve ultrafast switching times of less than 0.141 ps by the optical pumps, which have significant advantages in ultrafast THz modulators. By continuing to change the embedded position of the silicon in the unit structure, not only is a wide range of THz phase modulation achieved, but also multilevel modulation of the phase is realized. It is found that there is a strong relationship between the modulation depth and phase variation of THz waves, and a reasonable analysis is given. Further the amplitude modulator with a larger modulation depth (MD) is developed, and when the conductivity of photosensitive semiconductor silicon (σSi) reaches 2.5 × 106 S m−1, return loss (RL) is ≈0 dB, and the maximum MD reaches ≈100%; in order to gain insight into the nature of modulation, the modulation mechanism of THz waves under optical pumping conditions is analyzed. In addition, graphene‐based THz metasurface amplitude modulators are designed. When the depth of amplitude modulation is achieved by bias voltage modulation of the Fermi energy level of graphene, the maximum modulation amplitude is 23.42 dB, with a minimal modulation accuracy of 0.05 THz eV−1. In the article, the designed modulators have extremely excellent modulation performance. It has great potential applications in silicon‐based THz photonic devices, ultrahigh frequency electronic devices, high sensitivity sensors, and high‐precision imaging.

A Systematic Study of Spin‐Dependent Recombination in GaAs1−xNx as a Function of Nitrogen Content

A systematic study of spin‐dependent recombination (SDR) under steady‐state optical pumping conditions as a function of nitrogen content, x, in dilute nitride alloys of the form GaAs1−xNx is reported. Use of high‐excitation power densities up to 107 W cm−2 allows measurement of the full SDR versus power curves, even at relatively high nitrogen contents of x = 0.039. Alloy contents for are determined within by fitting the photoluminescence (PL) spectra using a Roosbroeck–Shockley relation, and values consistent with those obtained by studying the intensity of the GaN‐like LO2 Raman mode are found. PL intensity increases by a factor known as the SDR ratio when switching from linearly to circularly polarized pump excitation. This factor reaches 5 for x = 0.022 and decreases with increasing x, falling to 1.5 for x = 0.039. Moreover, the excitation power required for maximum SDR increases with increasing x, varying from 0.6 mW for x = 0.022 to 15 mW for x = 0.039. These observations indicate an increase in the density of electronically active defects with increasing nitrogen content, both responsible for the SDR and other, standard Shockley–Read–Hall centers. The result demonstrates the importance of including nonspin‐dependent recombination channels in a complete model of SDR.

Plasmonic distributed feedback lasing in an anodic aluminum oxide/silver/polymer hybrid membrane.

A hybrid membrane is employed as a high-order plasmonic distributed feedback (DFB) cavity to reduce the lasing threshold of polymer lasers. The hybrid membrane consists of an anodic aluminum oxide (AAO) membrane, a 25 nm thick silver layer and a free-standing polymer membrane. The AAO membrane is fabricated by a low-cost, single chemical etching method. Then, a layer of silver with a thickness of 25 nm is sputtered on the surface of the AAO. Subsequently, a polymer membrane is directly attached to the silver-plated AAO membrane, forming an AAO/silver/polymer hybrid membrane. Under optical pumping conditions, low-threshold, three-order DFB lasing is observed. The proposed laser device exhibited a dual-threshold characteristic because of the evolution from amplified spontaneous emission to DFB lasing. And a significant shift from omnidirectional emission to directional emission lasing can be observed while the pump energy density is beyond the second threshold. Furthermore, the plasmonic enhancement sourced from silver corrugation reveals important improvement effects to the DFB lasing of AAO/silver/polymer hybrid membrane for decreasing threshold, narrowing full width at half maximum (FWHM), and an increasing Q factor. This work may promote the design and production of low-cost and large-area high-order plasmonic DFB polymer lasers.

Time-resolved reversible optical switching of the ultralow-loss phase change material Sb2Se3

The antimony-based chalcogenide Sb2Se3 is a rapidly emerging material for photonic phase change applications owing to its ultra-low optical losses at telecommunication wavelengths in both crystalline and amorphous phases. Here, we investigate the dynamical response of these materials from nanoseconds to milliseconds under optical pumping conditions. We apply bichromatic pump-probe transient reflectance spectroscopy which is a widely used method to study the optical performance of optical phase change materials during phase transitions induced by direct pulsed optical switching. Amorphous regions of several hundreds of nanometers in diameter are induced by pulsed excitation of the material using a wavelength of 488 nm above the absorption edge, while the transient reflectance is probed using a continuous wave 980 nm laser, well below the absorption edge of the material. We find vitrification dynamics in the nanosecond range and observe crystallization on millisecond time scales. These results show a large five-orders of magnitude difference in time scales between crystallization and vitrification dynamics in this material. The insights provided in this work are fundamental for the optimisation of the material family and its employment in photonic applications.

Enhanced GeSn Microdisk Lasers Directly Released on Si

AbstractGeSn alloys are promising candidates for complementary metal‐oxide‐semiconductor‐compatible, tunable lasers. Relaxation of residual compressive strain in epitaxial GeSn has recently shown promise in improving the lasing performance. However, the suspended device configuration that is thus far introduced to relax the strain is destined to limit heat dissipation, thus hindering the device performance. Herein is demonstrated that strain‐free GeSn microdisk laser devices fully released on Si outperform the canonical suspended devices. This approach allows to simultaneously relax the limiting compressive strain while offering excellent thermal conduction. Optical simulations confirm that, despite a relatively small refractive index contrast between GeSn and Si, optical confinement in strain‐free GeSn optical cavities on Si is superior to that in conventional strain‐free GeSn cavities suspended in the air. Moreover, thermal simulations indicate a negligible temperature increase in the device. Conversely, the temperature in the suspended devices increases substantially reaching, for instance, 120 K at a base temperature of 75 K under the employed optical pumping conditions. Such improvements enable increasing the operation temperature by ≈40 K and reducing the lasing threshold by 30%. This approach lays the groundwork to implement new designs in the quest for room temperature GeSn lasers on Si.

Dynamic upconversion multicolour editing enabled by molecule-assisted opto-electrochemical modulation

Controlling nonlinear optical signals electrically offers many opportunities for technological developments. Lanthanide-activated nanoparticles have recently emerged as leading platforms for nonlinear upconversion of infra-red excitation within nanometric volumes. However, manipulation of upconversion emission is restricted to varying percentages of component materials, nanocrystal structure, and optical pumping conditions. Here, we report temporal modulation of anti-Stokes luminescence by coupling upconversion nanoparticles with an electrochemically responsive molecule. By electrically tailoring orbital energy levels of the molecules anchored on nanoparticle surfaces, we demonstrate reversible control of molecular absorption, resulting in dynamic colour editing of anti-Stokes luminescence at single-particle resolution. Moreover, we show that a programmable logic gate array based on opto-electrochemical modulation can be constructed to convert information-encrypted electrical signals into visible patterns with millisecond photonic readout. These findings offer insights into precise control of anti-Stokes luminescence, while enabling a host of applications from low-threshold infrared logic switches to multichannel, high-fidelity photonic circuits.

Optical pumping of quantum dot micropillar lasers.

Arrays of quantum dot micropillar lasers are an attractive technology platform for various applications in the wider field of nanophotonics. Of particular interest is the potential efficiency enhancement as a consequence of cavity quantum electrodynamics effects, which makes them prime candidates for next generation photonic neurons in neural network hardware. However, particularly for optical pumping, their power-conversion efficiency can be very low. Here we perform an in-depth experimental analysis of quantum dot microlasers and investigate their input-output relationship over a wide range of optical pumping conditions. We find that the current energy efficiency limitation is caused by disadvantageous optical pumping concepts and by a low exciton conversion efficiency. Our results indicate that for non-resonant pumping into the GaAs matrix (wetting layer), 3.4% (0.6%) of the optical pump is converted into lasing-relevant excitons, and of those only 2% (0.75%) provide gain to the lasing transition. Based on our findings, we propose to improve the pumping efficiency by orders of magnitude by increasing the aluminium content of the AlGaAs/GaAs mirror pairs in the upper Bragg reflector.

Трехуровневое приближение при расчете параметров оптически детектируемого магнитного резонанса в условиях сильной лазерной накачки

An algorithm is proposed for the approximate solution of a purely nonlinear problem of the parameters of optically detected magnetic resonance in the ground state of alkali atoms in an optically dense medium under conditions of intense narrow-band optical pumping, which causes, first, bleaching of the atomic medium, and, second, partial suppression of spin-exchange broadening. The direct solution of the Liouville equation under these conditions is complicated by the fact that the relaxation time of each of the levels of the hyperfine and Zeeman structure of the ground state is determined by the populations of the remaining levels, which leads to the necessity of solving a self-consistent problem in a multilevel system, and, as a rule, requires the use of supercomputers. In this paper, approximations are proposed that make it possible to significantly simplify and speed up the calculation by orders of magnitude, and the results are compared with experimental data using the example of a two-beam Mx scheme of a magnetometric sensor

Optical Modulation of Anti-Stokes Luminescence in CaF2:Er Crystals

Emission and visible photoluminescence excitation spectra of CaF2:Er (0.5 at %) crystals have been studied under different optical pumping conditions. The results demonstrate that anti-Stokes luminescence of the erbium-doped crystals has higher intensity in the case of pumping of the 4I13/2 metastable state of the dopant. Resonance IR photon wavelengths have been determined for two-photon excitation of visible emission in CaF2:Er. A method has been developed for the optical modulation of anti-Stokes luminescence, which allows the nature of the corresponding electron transitions in emission centers to be identified.

GaN micro-chimney cavity laser

A floating micro-chimney cavity is fabricated by photolithography, dry etching and isotropic wet etching technology. A quasi-single mode ultraviolet whispering gallery mode lasing with high quality factor is observed from the fabricated micro-chimney cavity under optical pumping condition. The quasi-single mode lasing features, e.g., the threshold, the lasing mode number, the quality factor, the resonance mechanism are systematically analyzed by experiment and a three-dimensional finite difference time-domain simulation. This work provides a feasible way for realizing single mode lasing with high quality factor.

High Circular Polarized Nanolaser with Chiral Gammadion Metal Cavity

We demonstrate a circularly polarized laser with the metal-gallium-nitride gammadion nanocavities. The ultraviolet lasing signal was observed with the high circular dichroism at room temperature under pulsed optical pump conditions. Without external magnetism which breaks the time-reversal symmetry to favor optical transitions of a chosen handedness, the coherent outputs of these chiral nanolasers show a dissymmetry factor as high as 1.1. The small footprint of these lasers are advantageous for applications related to circularly polarized photons in future integrated systems, in contrast to the bulky setup of linearly-polarized lasers and quarter-wave plates.

Technique for magnetic moment reconstruction of laser-cooled atoms using direct imaging and prospects for measuring magnetic sublevel distributions

We describe a simple time-of-flight technique for measuring the magnetic moment of an optically pumped magneto-optical trap. The technique relies on free-expansion imaging of a cold atom cloud in a small magnetic field gradient without the need to detect spatial separation between magnetic sublevels. We find that the effective acceleration of the cloud can be used to characterize extreme state optical pumping. In the general case, we show that the integrated displacement of the falling cloud can be accurately modeled using rate equation simulations of magnetic sublevel populations, and knowledge of local magnetic fields, field gradients, and light intensities. The agreement between the model and the data allows the reconstruction of magnetic moments and suggests that this technique may be suitable for the measurement of population distributions over a range of optical pumping conditions.

Modeling the generation of optical modes in a semiconductor waveguide with distributed feedback formed by a space charge wave

The amplification and generation of optical TE waves arising on a lattice formed by a space charge wave in a planar waveguide based on a donor-doped semiconductor (gallium arsenide) n-GaAs are considered. The region of interaction is limited by contacts with a constant electric field applied between them, which, while operating in the Gunn oscillations suppression mode, gener-ates a small-signal periodic inhomogeneity. Reflection and transmission regimes for same-index TE modes propagating in the waveguide structure are investigated depending on the phase mis-match and the pump level. It is shown that even with a relatively small modulation depth of the di-electric constant (about 10–5), under conditions of high optical pumping (with a gain of about 150 cm–1) and a corresponding detuning from phase matching there is the possibility of not only ampli-fying the direct and backward (reflected) optical modes, but also their generation. The advantage of the considered effects is that they enable flexible control of parameters of the dynamic lattice. The results obtained can be used to create semiconductor laser generators based on the interaction of the optical modes and a space charge wave.

Tailoring Whispering Gallery Lasing and Random Lasing in A Compound Cavity.

A compound cavity was proposed to achieve both whispering gallery mode (WGM) lasing and random lasing. The WGM-random compound cavity consisted of a random structure with an annular boundary, which was fabricated by a method combining both inkjet printing and metal-assisted chemical etching methods. An ultrathin polymer membrane was attached to the WGM-random compound cavity, forming a polymer laser device. A transformation from WGM lasing to random lasing was observed under optical pumping conditions. The laser performance could be easily tailored by changing the parameter of the WGM-random compound cavity. These results provide a new avenue for the design of integrated light sources for sensing applications.

Perovskite random lasers on fiber facet

Abstract Hybrid lead halide perovskites have made great strides in next-generation photovoltaic and optoelectronic devices. Random lasers based on perovskite materials have been intensively investigated, but the miniaturization of perovskite random lasers has not been achieved up to now. Here, we report the fabrication of perovskite random lasers based on perovskite films deposited on the optical fiber facets using a dip-coating method. Under optical pumping conditions, random lasing was observed with minimum threshold energy of 32.3 μJ/cm2 at ~550 nm; also, the lasing threshold decreased with increasing diameter of the optical fiber. The results show that the random lasing emission originates from the scattering between the perovskite crystal grains, and the decreased threshold is attributed to the increased active area of the perovskite film. The directionality of perovskite random lasers indicates that the divergence angle of the laser beam is less than 60°. We also demonstrate that the perovskite random laser on the fiber facet can prevent speckle formation and improve image quality. These results may promote the applications of random lasers in compact sources and integrated optoelectronic devices.

Development of Digital Holographic Microscopy on an All-Optical Phase Shifter

We propose an all-optical phase-shifting method using Yb-doped optical fibers and demonstrate its potential in on-axis digital holography microscopy (DHM) system. The Yb-doped optical fiber induces a phase change under optical pumping conditions. The induced phase shift shows a good linear response to applied pumping currents, and N-step phase-shifting up to 2p can be properly performed. Four-, six-, and eight-step phase-shifting methods are performed, and the phase retrievals from their phase-shifted interferograms are achieved using an interframe correlation matrix method. An 8-step phase shifting-based reconstruction, which is effective for background noise reduction, is used for 3D imaging of an indium tin oxide (ITO) thin film and an organic light-emitting diode (OLED) thin film. The reconstructed surface profile was in agreement with the step structure of the samples. The proposed all-optical phase-shifting method is expected to be applied to various optical imaging systems using fiber-based phase-shifting interferometers.

Two-frequency laser with distributed feedback formed by a space charge wave

The regimes of amplification and generation of optical TE waves arising on a grating formed by a space charge wave (SCW) in a plane optical waveguide based on an n-GaAs semiconductor are considered. For the perturbed n-GaAs waveguide, the reflectance and transmittance of TE modes with the same indices (m = n = 0) are calculated depending on the pump level and length of interaction between the optical and SCWs. It is shown that even with a relatively small depth of modulation of the dielectric constant $$(\Delta \varepsilon \approx 10^{ - 5} )$$ under conditions of high optical pumping (with an amplification factor $$\gamma \approx 150$$ cm−1) and corresponding SCW-optical interaction length, there is a possibility of not only amplification, but also generation of forward and backward optical modes at a wavelength of 10.6 μm. The results can be used to create tunable semiconductor laser generators based on the SCW-optical interaction and operating in the near and mid-IR range.