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Three-dimensional (3D) printing enables the fabrication of complex, highly customizable structures, which are difficult to fabricate using conventional fabrication methods. Recently, the concept of four-dimensional (4D) printing has emerged, which adds active and responsive functions to 3D-printed structures. Deployable or adaptive structures with desired structural and functional changes can be fabricated using 4D printing; thus, 4D printing can be applied to actuators, soft robots, sensors, medical devices, and active and reconfigurable photonic devices. The shape of 3D-printed structures can be transformed in response to external stimuli, such as heat, light, electric and magnetic fields, and humidity. Light has unique advantages as a stimulus for active devices because it can remotely and selectively induce structural changes. There have been studies on the light activation of nanomaterial composites, but they were limited to rather simple planar structures. Recently, the light activation of 3D-printed complex structures has attracted increasing attention. However, there has been no comprehensive review of this emerging topic yet. In this paper, we present a comprehensive review of the light activation of 3D-printed structures. First, we introduce representative smart materials and general shape-changing mechanisms in 4D printing. Then, we focus on the design and recent demonstration of remote light activation, particularly detailing photothermal activations based on nanomaterial composites. We explain the light activation of 3D-printed structures from the millimeter to sub-micrometer scale.

The introduction of an immobile passivation material leads to the suppression of the deeper defect states of Cl-mixed perovskite nanocrystals, while the inevitable halide migration is hampered, resulting in high efficiency and spectral stability of blue PeLEDs.

Exceptional points (EPs) are degenerate singularities in a non-Hermitian system that can be induced by controlling the interaction between resonant photonic modes. EPs can enable unusual optical phenomena and significantly enhance the optical sensitivity under small perturbations. However, most studies thus far have been limited to static photonic structures. In this study, we propose and experimentally demonstrate electrically addressable EP in a plasmonic structure. Inspired by optical microcavity studies, we employ a localized spoof plasmon structure that supports circulating plasmonic modes in a compact single-resonator geometry. The plasmonic modes are perturbed by an angled metal line, and the interaction between the plasmonic modes is electrically controlled using a varactor. Continuous electrical tuning of the varactor capacitance facilitates simultaneous coalescence of the real and imaginary parts of the eigenfrequency, allowing the direct addressing of EPs. We first investigate the eigenmodes and their coupling in localized plasmonic structures using numerical simulations. We then present experimentally measured spectra that manifest the coalescence of the two resonant modes in both the resonance frequency and linewidth. Electrically addressable EPs in compact plasmonic structures may provide exciting opportunities for highly functional and tunable elements in integrated device platforms., Comment: 25 pages

Momentum space topology can be exploited to manipulate radiation in real space. Here we demonstrate topological control of 2D perovskite emission in the strong coupling regime via polaritonic bound states in the continuum (BICs). Topological polarization singularities (polarization vortices and circularly polarized eigenstates) are observed at room temperature by measuring the Stokes parameters of photoluminescence in momentum space. Particularly, in symmetry-broken structures, a very large degree of circular polarization (DCP) of ∼0.835 is achieved in the perovskite emission, which is the largest in perovskite materials to our knowledge. In the strong coupling regime, lower polariton modes shift to the low-loss spectral region, resulting in strong emission enhancement and large DCP. Our reciprocity analysis reveals that DCP is limited by material absorption at the emission wavelength. Polaritonic BICs based on 2D perovskite materials combine unique topological features with exceptional material properties and may become a promising platform for active nanophotonic devices.

Spoof surface plasmons in corrugated metal surfaces allow tight field confinement and guiding even at low frequencies and are promising for compact microwave photonic devices. Here, we use metal-ink printing on flexible substrates to construct compact spoof plasmon resonators. We clearly observe multipole resonances in the microwave frequencies and demonstrate that they are still maintained even under significant bending. Moreover, by combining two resonators of slightly different sizes, we demonstrate spectral filtering via the Vernier effect. We selectively address a target higher-order resonance while suppressing the other modes. Finally, we investigate the index-sensing capability of printed plasmonic resonators. In the Vernier structure, we can control the resonance amplitude and frequency by adjusting a resonance overlap between two coupled resonators. The transmission amplitude can be maximized at a target refractive index, and this can provide more functionalities and increased design flexibility. The metal-ink printing of microwave photonic structures can be applied to various flexible devices. Therefore, we expect that the compact, flexible plasmonic structures demonstrated in this study may be useful for highly functional elements that can enable tight field confinement and manipulation.

One critical factor for bolometer sensitivity is efficient electromagnetic heating of thermistor materials, which plasmonic nanogap structures can provide through the electric field enhancement. In this report, using finite element method simulation, electromagnetic heating of nanorod dimer antennas with a nanogap filled with vanadium dioxide (VO2) was studied for long-wavelength infrared detection. Because VO2 is a thermistor material, the electrical resistance between the two dimer ends depends on the dimer’s temperature. The simulation results show that, due to the high heating ability of the nanogap, the temperature rise is several times higher than expected from the areal coverage. This excellent performance is observed over various nanorod lengths and gap widths, ensuring wavelength tunability and ultrafast operating speed, thereby making the dimer structures a promising candidate for high sensitivity bolometers.

Organic-inorganic hybrid perovskites hold great potential for various optoelectronic devices with exceptional properties. Although the direct generation of circularly polarized emission from perovskites would enable various compact devices, achieving a large degree of circular polarization (DCP) at room temperature still remains challenging. Herein, we demonstrate that DCP can be strongly enhanced at the narrow mode position of chiral Fano resonances. In our design, a perovskite film is spin-coated on a symmetry-broken structure with a relatively large feature size. A large DCP of more than 0.5 is achieved at room temperature without the direct patterning of the perovskite layer. Reciprocity calculation reveals that chiral field enhancement enables the emission of opposite helicity to couple into counter-propagating slab modes and leads to a large DCP. Our design is very general and scalable. Our work may lead to circularly polarized light sources based on various perovskite materials.

Despite numerous potential advantages of subradiative dark modes due to an extremely high quality ( ${Q}$ ) factor, the practicality still depends on the consistency of excitations for a tight fabrication tolerance. This article proposes a novel approach to adaptive transitions between different dipole moments to allow for consistent excitation of Fano and electric quadrupole resonances that are extremely sensitive to geometric errors. The proposed geometry of each metamolecule consists of four strips printed on a thin dielectric film with four gaps between the strips, and the geometric error is described as structural deformation by adjusting the strip lengths and gap sizes in the vertical direction. In each deformed state, current and magnetic-field distributions are analyzed to interpret operating principles, and multipole moments are calculated based on the multipole expansion using the current distributions. The results demonstrate that the consistent excitation of the electric quadrupole moment is enabled based on a simultaneous transition between toroidal and electric dipole moments, and the Fano resonance is consistently excited by the magnetic-to-electric dipole moment transition. For verification, original and deformed metasurfaces are fabricated, and their transmission and reflection coefficients are measured in a semianechoic chamber to experimentally validate the proposed dipole moment transitions. The results confirm that the proposed approach has great potential to bring advances in recent sensing and energy harvesting applications where an extremely high ${Q}$ -factor is essential.

Four-dimensional (4D) printing can add active and responsive functions to three-dimensional (3D) printed objects in response to various external stimuli. Light, among others, has a unique advantage of remotely controlling structural changes to obtain predesigned shapes. In this study, we demonstrate multicolor 4D printing of shape-memory polymers (SMPs). Using color-dependent selective light absorption and heating in multicolor SMP composites, we realize remote actuation with light illumination. We experimentally investigate the temperature changes in colored SMPs and observe a clear difference between the different colors. We also present simulations and analytical calculations to theoretically model the structural variations in multicolor composites. Finally, we consider a multicolor hinged structure and demonstrate the multistep actuation by changing the color of light and duration of illumination. 4D printing can allow complex, multicolor geometries with predesigned responses. Moreover, SMPs can be reused multiple times for thermal actuation by simply conducting thermomechanical programming again. Therefore, 4D printing of multicolor SMP composites have unique merits for light-induced structural changes. Our study indicates that multicolor 4D printing of SMPs are promising for various structural changes and remote actuation.

Three-dimensional (3D) printing is a new paradigm in customized manufacturing and allows the fabrication of complex optical components and metaphotonic structures that are difficult to realize via traditional methods. Conventional lithography techniques are usually limited to planar patterning, but 3D printing can allow the fabrication and integration of complex shapes or multiple parts along the out-of-plane direction. Additionally, 3D printing can allow printing on curved surfaces. Four-dimensional (4D) printing adds active, responsive functions to 3D-printed structures and provides new avenues for active, reconfigurable optical and microwave structures. This review introduces recent developments in 3D and 4D printing, with emphasis on topics that are interesting for the nanophotonics and metaphotonics communities. In this article, we have first discussed functional materials for 3D and 4D printing. Then, we have presented the various designs and applications of 3D and 4D printing in the optical, terahertz, and microwave domains. 3D printing can be ideal for customized, nonconventional optical components and complex metaphotonic structures. Furthermore, with various printable smart materials, 4D printing might provide a unique platform for active and reconfigurable structures. Therefore, 3D and 4D printing can introduce unprecedented opportunities in optics and metaphotonics and may have applications in freeform optics, integrated optical and optoelectronic devices, displays, optical sensors, antennas, active and tunable photonic devices, and biomedicine. Abundant new opportunities exist for exploration.

Organic-inorganic hybrid perovskites hold great potential for various optoelectronic devices with exceptional properties. Although the direct generation of circularly polarized emission from perovskites would enable various compact devices, achieving a large degree of circular polarization (DCP) at room temperature still remains challenging. Herein, we demonstrate that DCP can be strongly enhanced at the narrow mode position of chiral Fano resonances. In our design, a perovskite film is spin-coated on a symmetry-broken structure with a relatively large feature size. A large DCP of more than 0.5 is achieved at room temperature without the direct patterning of the perovskite layer. Reciprocity calculation reveals that chiral field enhancement enables the emission of opposite helicity to couple into counter-propagating slab modes and leads to a large DCP. Our design is very general and scalable. Our work may lead to circularly polarized light sources based on various perovskite materials.

Three-dimensional (3D) printing is ideal for the fabrication of various customized 3D components with fine details and material-design complexities. However, most components fabricated so far have been static structures with fixed shapes and functions. Here we introduce bistability to 3D printing to realize highly-controlled, reconfigurable structures. Particularly, we demonstrate 3D printing of twisting and rotational bistable structures. To this end, we have introduced special joints to construct twisting and rotational structures without post-assembly. Bistability produces a well-defined energy diagram, which is important for precise motion control and reconfigurable structures. Therefore, these bistable structures can be useful for simplified motion control in actuators or for mechanical switches. Moreover, we demonstrate tunable bistable components exploiting shape memory polymers. We can readjust the bistability-energy diagram (barrier height, slope, displacement, symmetry) after printing and achieve tunable bistability. This tunability can significantly increase the use of bistable structures in various 3D-printed components.

Here we report a novel approach to create tunable and reconfigurable microwave metamaterials using metal ink printing on a paper. Our approach enables easy-to-fabricate but still highly functional and tunable elements. An array of asymmetric split ring resonators was printed on a photopaper to induce Fano resonances in the microwave region (~ 10 GHz). Then, the printed paper was cut line by line and folded, so that a step height between neighboring unit cells can be created and tuned. With the varying step height between neighboring cells, our sample demonstrates significant spectral shifts and resonance tuning despite its simple geometry. Depending on the cutting direction, we observe either spectral redshift or blueshift. We explain our experimental observations based on the interactions between electric/magnetic dipoles in the neighboring unit cell. Moreover, we observe phase singularity at the zero amplitude position of the Fano resonance spectrum. The spectral phase exhibits a drastic change at this singularity point, and the phase spectrum can be also largely tuned with the geometry change in our sample. The drastic changes in phase could be very useful for various applications, such as optical sensing and beam steering.

Understanding the relationships between environmental factors and aquatic communities at the large scale is important for conserving freshwater biodiversity and sustaining ecological integrity. In this study, we evaluated the ecological status of the Han River watershed in Korea with the predictive distribution maps for EPT (Ephemeroptera, Plecoptera, and Trichoptera) taxa. From large-scale surveys, environmental and biological data for 360 sites in 189 streams and rivers in the Han River watershed in 2012 and 2013 were analyzed. Based on correlation analysis and stepwise multiple regression analysis, the factors influencing EPT richness were altitude, percent of coarse substrate, biochemical oxygen demand (BOD), and total phosphorus (TP). DPI (distribution prediction index), indicating the distribution probability of EPT assemblages, was calculated by using a frequency ratio model. This index was high in the mountainous area where altitude and percent of coarse substrate were high, but low in the urbanized area where BOD and TP were high. Verification of the model indicated high reliability of DPIs as demonstrated by the low root-mean-square-error (RMSE) values between the predicted and observed values. This study provides a basis for the sustainable management of stream ecosystems as well as paves the way for future studies such as those on environmental assessment techniques using aquatic insect communities.

Strong light absorption in ultrathin films has been of great interest for both fundamental studies and device applications. Here we demonstrate and analyze controllable superabsorption in excitonic thin films in the visible region. By adjusting the concentration of J-aggregate dyes, we control the dispersion of excitonic films (from optically metallic to nonmetallic ones) and show that this leads to drastic changes in the optical response of organic thin films. We find that planar excitonic films can have various optical features in the visible region, for example, surface polaritons, epsilon-near-pole, asymmetric Fabry–Perot type resonances, and so on. We leverage these diverse features to study perfect absorption in planar films without additional structural patterning. We also demonstrate that strong light absorption can even occur away from an excitonic absorption peak (i.e., maximum optical loss position) due to cavity-like resonances in the high dielectric constant region. Our work demonstrates that...

Atomically thin semiconducting oxide on graphene carries a unique combination of wide band gap, high charge carrier mobility, and optical transparency, which can be widely applied for optoelectronics. However, study on the epitaxial formation and properties of oxide monolayer on graphene remains unexplored due to hydrophobic graphene surface and limits of conventional bulk deposition technique. Here, we report atomic scale study of heteroepitaxial growth and relationship of a single-atom-thick ZnO layer on graphene using atomic layer deposition. We demonstrate atom-by-atom growth of zinc and oxygen at the preferential zigzag edge of a ZnO monolayer on graphene through in situ observation. We experimentally determine that the thinnest ZnO monolayer has a wide band gap (up to 4.0 eV), due to quantum confinement and graphene-like structure, and high optical transparency. This study can lead to a new class of atomically thin two-dimensional heterostructures of semiconducting oxides formed by highly controlled epitaxial growth.

International audience; We demonstrate an efficient ENZ response in the visible spectral range using organic molecular ultrathin films possessing a Lorentz-type dispersion. For this purpose, two polymethine dyes: sodium [5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulphobutyl)- benzimidazol-2-ylidene]-propenyl]-1-ethyl-3-(4-sulphobutyl)-benzimidazolium hydroxide] (TDBC), and [2,4-bis[8-hydroxy-1,1,7,7-tetramethyljulolidin-9-yl]squaraine] (HTJSq) were used in spin-coated polymer films at different doping concentrations. By varying the doping concentration in thin films, the real part of highly dispersive permittivity ε1 can be manipulated and tuned such that the spectral width of ENZ region -1 < ε1

We perform a detailed analysis of perfect absorption in unpatterned thin films by invoking admittance matching conditions. We consider a single, absorptive layer coated on a reflective substrate and investigate perfect absorption conditions for normal and oblique incidences of light. For normal incidence, it is found that we need larger optical losses as the film gets thinner. But, for oblique incidence, perfect absorption can be achieved even for very small losses especially around the epsilon-near-zero frequency of the film. We consider the oblique incidence case in two different configurations (termed Berreman and ATR configurations). In both cases, we show that perfect absorption involves critical coupling to TM modes existing in thin films. We also evaluate actual light absorption and the electric field distribution in ITO films using the transfer matrix method, and show that it agrees well with our analytic theory. Finally, we present a versatile diagram that visualizes the admittance matching process in thin film structures. Our work presents an in-depth analysis of light absorption in thin films and provides design principles for various thin film devices, such as sensors, optical filters, optical modulators, and thermal emitters.

We demonstrate polarization-dependent photo-response enhancement in metamaterial-coupled quantum dots-in-a-well infrared detectors. A gold split-ring resonator metamaterial layer was patterned by electron-beam lithography in the detector aperture. In this integrated structure, the detector spectral response is given by the convolution of the metamaterial field enhancement and the original detector response. Our polarization-resolved measurement unambiguously shows that the spectral response can be strongly modified by metamaterial patterning. When the metamaterial resonance matches the QD absorption peak, we obtain a clear enhancement of generated photocurrent. Various metamaterial designs can be employed to implement multi-functional detector structures.

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

We have succeeded in tuning the localized surface plasmon resonant wavelength in Ag-Al nanoparticles (NPs) by using the Al content. The plasmon resonant wavelength of the Ag-Al NP ensemble shifts toward a shorter wavelength from 590 to 510 nm (approximately 80 nm) with an increase in the Al content from 0 to 7.16 atomic percent (at.%). A theoretical interpretation and model for the experimental results are presented. The effective dielectric functions are modeled by taking into account the variations in both the bulk plasma frequency and the structural information on the Ag-Al NPs with the Al content.

We present a new type of electrically tunable strong coupling between planar metamaterials and epsilon-near-zero modes that exist in a doped semiconductor nanolayer. The use of doped semiconductors makes this strong coupling tunable over a wide range of wavelengths through the use of different doping densities. We also modulate this coupling by depleting the doped semiconductor layer electrically. Our hybrid approach incorporates strong optical interactions into a highly tunable, integrated device platform.

We propose a general, easy-to-implement scheme for broadband coherent perfect absorption (CPA) using epsilon-near-zero (ENZ) multilayer films. Specifically, we employ indium tin oxide (ITO) as a tunable ENZ material, and theoretically investigate CPA in the near-infrared region. We first derive general CPA conditions using the scattering matrix and the admittance matching methods. Then, by combining these two methods, we extract analytic expressions for all relevant parameters for CPA. Based on this theoretical framework, we proceed to study ENZ CPA in a single layer ITO film and apply it to all-optical switching. Finally, using an ITO multilayer of different ENZ wavelengths, we implement broadband ENZ CPA structures and investigate multi-wavelength all-optical switching in the technologically important telecommunication window. In our design, the admittance matching diagram was employed to graphically extract not only the structural parameters (the film thicknesses and incident angles), but also the input beam parameters (the irradiance ratio and phase difference between two input beams). We find that the multi-wavelength all-optical switching in our broadband ENZ CPA system can be fully controlled by the phase difference between two input beams. The simple but general design principles and analyses in this work can be widely used in various thin-film devices.

We propose a broadband coherent perfect absorption thin film device based on epsilon-near-zero indium tin oxide thin films in the near-infrared wavelength region and investigate its optical properties.

Excitonic films such as J-aggregate-doped polymer films can exhibit sharp Lorentzian dispersions and thus have various optical features in the visible region. They can even show an optically metallic response and can be considered as alternative plasmonic materials. However, there were no systematic studies on optical interactions in such excitonic films. Here, we perform theoretical investigations on optical modes and interactions in planar excitonic thin films. We gradually vary the dye concentration and the film thickness and study optical coupling to surface bound waves (surface polariton modes and epsilon-near-zero modes). We also investigate thin-film interferences in high-loss and low-loss regimes. Finally, we discuss the freespace impedance matching that can result from the Lorentzian dispersion in excitonic films. Our work enables in-depth understandings on optical interactions in excitonic films and it can provide guidelines for various nanophotonic applications in the visible region.

The energy relay dye, 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), was used with a near-infrared sensitizing dye, TT1, to increase the overall power conversion efficiency of a dye-sensitized solar cell (DSC) from 3.5% to 4.5%. The unattached DCM dyes exhibit an average excitation transfer efficiency (ETE) of 96% inside TT1-covered, mesostructured TiO(2) films. Further performance increases were limited by the solubility of DCM in an acetonitrile based electrolyte. This demonstration shows that energy relay dyes can be efficiently implemented in optimized dye-sensitized solar cells, but also highlights the need to design highly soluble energy relay dyes with high molar extinction coefficients.

The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light-matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.

We experimentally demonstrate perfect absorption of incident light in an ultrathin, planar organic layer, together with large photoluminescence (PL) enhancement. We find that diverse features appear in the absorption spectra of J-aggregate excitonic films, depending on the incident light angle and the phase controller thickness. We achieve strong absorption even away from the excitonic absorption pole. We explain the angle-dependent perfect absorption by comparing radiative and nonradiative damping rates for different incident angles. Moreover, we achieve large PL enhancement at strong light absorption conditions. This demonstrates that the absorbed light energy in excitonic perfect absorbers can be retrieved, unlike other perfect absorbers based on metal nanostructures where the absorbed energy is mainly dissipated as heat due to ohmic losses. Excitonic perfect absorbers can be useful for energy conversion devices or fluorescence-based optical devices.

A metal−dielectric−metal (MDM) waveguide with a nanoscale gap supports highly confined surface plasmon−polariton modes, termed gap plasmons. The spontaneous emission of an emitter placed in such a metal nanogap is expected to be strongly modified due to coupling to gap plasmons. We investigate the light emission properties of semiconductor quantum dots (QD) in a metal nanoslit, which is a truncated MDM waveguide. More specifically, we measure both the lifetime and the state of polarization of the out-coupled QD emission from a metal nanoslit. We observe clear lifetime and polarization changes of QD emission. As the slit width gets smaller, the QD exciton lifetime gradually decreases, and its emission becomes polarized normal to the slit, as expected for gap plasmon coupled light emission. We also find that the polarization of the collected QD emission is flipped (i.e., becomes parallel to a slit) when the excited emitters are located just outside the slit. We have conducted dipole emission calculations in...

In this study we demonstrate tunable and broadband perfect absorption in epsilon-near-zero ITO thin films and multilayers at near infrared wavelengths and investigate their optical, electrical, and structural properties.

In this study, we report ITO thin film designs for epsilon-near-zero coherent perfect absorption in near infrared wavelength regime using an admittance matching» method and investigate their optical properties.

Perfect absorption (PA) of incident light is important for both fundamental light-matter interaction studies and practical device applications. PA studies so far have mainly used resonant nanostructures that require delicate structural patterning. Here, we realize tunable and broadband PA in the near-infrared region using relatively simple thin film coatings. We adjust the growth condition of an ITO film and control its epsilon-near-zero (ENZ) wavelength. We show that this results in highly tunable PA in the telecommunication window. Then, using an ITO multilayer of different ENZ wavelengths, we demonstrate broadband PA that covers a wide range of near-infrared wavelengths. The use of ENZ coatings makes PA adjustable during the film growth and does not require any structural patterning afterward. It also facilitates the chip-scale integration of perfect absorbers with other device components. Broadband PA relaxes the single wavelength condition in previous PA studies and thus it is suitable for many practical device applications, including sensors, photodetectors and energy harvesting devices.

Soil carbon dioxide (CO2) evolution, litter decomposition, and nitrogen availability was measured four years after thinning in a 19-year-old Japanese larch (Larix leptolepis Gord.) plantation of central Korea. Four different thinning intensities [control (C), 10% (T10), 20% (T20), and 40% (T40)] were applied. There were significant differences in seasonal mean soil temperature, moisture, and CO2 evolution among the thinning intensities. Annual soil CO2 evolution (Mg CO2 ha−1) was 29.8 for C, 27.0 for T10, 24.2 for T20, and 23.8 for T40, respectively, and decreased with the thinning intensity. High soil CO2 evolution in the control and light thinning plots might be related to root respiration from high stand densities. After decomposing for four years, 30 and 23%, 30 and 27%, 21 and 10%, and 28 and 30% of the original needle litter dry mass and nitrogen mass remained for control, T10, T20, and T40, respectively. However, there were no significant trends with the thinning intensity. Needle litter a...

We measured photosynthetically active radiation (PAR) beneath the forest canopy, understory species’ richness and diversity, and biomass in a Larix leptolepis plantation in central Korea 4 years after thinning. Four different thinning intensities (control, 10%, 20%, and 40% stocking reduction) were applied in 1997. Mean annual intercepted PAR at 30 cm and 1 m above ground level was significantly different among the thinning intensities and strongly correlated with the number of stems remaining after thinning. Understory species richness and diversity were lowest in the unthinned stands and increased with thinning intensity, and there were significant correlations between the total number of understory species or diversity index (Shannon’s diversity index, H″) and the number of stems per hectare after thinning. Also understory above-ground biomass was significantly increased with thinning intensity in both sampling months (June and August 2001). Thinning increased light inputs to the ground and resulted in higher species richness and diversity and above-ground biomass. These results suggested that light availability and understory responses to thinning at a comparable intensity are likely to last well beyond the 4 years of this study.

Changes in land use can affect the distribution and cycling of soil organic matter and nutrients. Soil organic carbon (C) and nitrogen (N) concentrations, soil CO2 evolution, litter decomposition, and soil N availability under the three types of conversion of agricultural lands to natural vegetation (rice field conversion to forest, crop field conversion to shrub, and indigenous forest) in central Korea after 20 years of fallow were compared. There was no significant change in soil organic C and N concentrations among the three types at the three soil depths, and soil organic C and N increases were confined to the topsoil (0–10 cm). Soil CO2 evolution tracked seasonal soil temperature, however, there was no difference among conversion types. Annual soil CO2 evolution (Mg CO2/ha/yr) was 36.0±3.7 for the rice field conversion to forest site, 33.8±4.1 for the crop field conversion to shrub site, and 40.7±4.1 for the indigenous forest site, respectively. After decomposing for one year, 55.8% for the rice fiel...