Akifumi Asahara

Spectral dynamics of picosecond gain-switched pulses from nitride-based vertical-cavity surface-emitting lasers

Short pulses generated from low-cost semiconductor lasers by a simple gain-switching technique have attracted enormous attention because of their potential usage in wide applications. Therein, reducing the durations of gain-switched pulses is a key technical point for promoting their applications. Therefore, understanding the dynamic characteristics of gain-switched pulses is highly desirable. Herein, we used streak camera to investigate the time- and spectral-resolved lasing characteristics of gain-switched pulses from optically pumped InGaN single-mode vertical-cavity surface-emitting lasers. We found that fast initial components with ultra-short durations far below our temporal resolution of 5.5 ps emerged on short-wavelength sides, while the entire pulses were down-chirped, resulting in the simultaneous broadening of the spectrum and pulse width. The measured chirp characteristics were quantitatively explained using a single-mode rate-equation model, combined with carrier-density-dependent gain and index models. The observed universal fast short-wavelength components can be useful in generating even shorter pulses from gain-switched semiconductor lasers.

Ultraviolet stimulated emission from high-temperature-annealed MgO microcrystals at room temperature

Research on semiconductor nanowires underlies the development of the miniaturization of laser devices with low cost and low energy consumption. In general, nanowire lasers are made of direct band gap semiconductors, e.g., GaN, ZnO and CdS, and their band-edge emissions are used to achieve optically pumped laser emission. In addition to the existing class of nanowire lasers, we here show that air-annealed micrometer-sized MgO cubic crystals with well-defined facets exhibit room-temperature stimulated emission at 394 nm under pulsed laser pumping at ∼350 nm. Surface midgap states are assumed to be responsible for the excitation and emission processes. The present findings will not only provide opportunities for the development of miniaturized lasers composed of insulating oxides, but will also open up functionality in various families of cubic crystalline materials.

Gain-switching dynamics in optically pumped single-mode InGaN vertical-cavity surface-emitting lasers

The gain-switching dynamics of single-mode pulses were studied in blue InGaN multiple-quantum-well vertical-cavity surface-emitting lasers (VCSELs) through impulsive optical pumping. We measured the shortest single-mode pulses of 6.0 ps in width with a method of up-conversion, and also obtained the pulse width and the delay time as functions of pump powers from streak-camera measurements. Single-mode rate-equation calculations quantitatively and consistently explained the observed data. The calculations indicated that the pulse width in the present VCSELs was mostly limited by modal gain, and suggested that subpicosecond pulses should be possible within feasible device parameters.

Direct generation of 2-ps blue pulses from gain-switched InGaN VCSEL assessed by up-conversion technique

Ultra-short pulses in blue region generated from compact and low-cost semiconductor lasers have attracted much attention for a wide variety of applications. Nitride-based vertical-cavity surface-emitting lasers (VCSELs), having intrinsic high material gain and short cavities, favor the generation of ultra-short blue pulses via a simple gain-switching technique. In this study, we fabricated a single-mode InGaN VCSEL consisting of 10-period InGaN/GaN quantum wells (QWs). The output pulses were evaluated accurately with an up-conversion measurement system having time resolution of 0.12 ps. We demonstrated that ultra-short blue pulses, as short as 2.2 ps at 3.4 K and 4.0 ps at room temperature, were generated from the gain-switched InGaN VCSEL via impulsive optical pumping, without any post-processing. The gain-switched pulses we obtained should greatly promote the development of ultra-short blue pulse generation. In addition, this successful assessment demonstrates the up-conversion techniques usefulness for characterizing ultra-short blue pulses from semiconductor lasers.

Purely excitonic lasing in ZnO microcrystals: Temperature-induced transition between exciton-exciton and exciton-electron scattering

Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion , . We show that ZnO microcrystals with a size of a few micrometers exhibit purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission. We then present the distinct experimental evidence that the room-temperature 2 excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. As the temperature is lowered below ~150 K, the lasing mechanism is shifted from the exciton-electron scattering to the exciton-exciton scattering. We also argue that the ease of carrier diffusion plays a significant role in showing room-temperature excitonic lasing. Among other lasing materials, ZnO is one of the most well-studied optical semiconductors because of its relatively large exciton binding energy Eb of about 60 meV [1,2]. Previously, there have been a number of studies on room-temperature laser emission from ZnO nanostructures, such as thin films [37], nanowires [811], nanodisks [1214] and nanoparticles [1518]. Two main mechanisms are generally invoked as responsible for optical gain: excitonic and electron-hole plasma (EHP) recombinations [1,2]. An EHP state is formed when the density of electron-hole pairs exceeds the Mott density nM, where screening reduces the Coulomb interaction sufficiently that no bound excitonic states are present. Room-temperature EHP emission is observed commonly in most ZnO nanostructures under sufficiently high optical excitation [47,11,17,18]. On the other hand, 3 room-temperature excitonic lasing is achieved almost exclusively in high-quality ZnO thin films [47] despite of the fact that the exciton biding energy (Eb = ~60 meV) is larger than the thermal energy at room temperature (~25 meV). Note also that the exciton dominated regime generally overlaps with the EHP dominated regime [47]. This is most likely because rather high excitation density, which will result in the electron-hole density close to nM, is required for excitonic laser action to compensate the heavy optical loss inherent to these nanostructures. Thus, realization of pure excitonic lasing at room temperature would be difficult for ZnO nanostructured materials, as has been proved in the case of ZnO nanowires [11]. To make matters more challenging, the origin of the excitonic gain in ZnO at room temperature is still controversial. Although the room-temperature excitonic lasing was originally attributed to the exciton-exciton (ex-ex) scattering process [37], Klingshirn and his coworkers [19,20] have recently argued that other mechanisms similar to those based on the exciton-LO phonon (ex-LO) or exciton-electron (ex-el) scattering processes are more likely to be responsible for the optical gain at room temperature. Thus, it is still an open question whether the excitonic process, especially the ex-ex scattering process, indeed accounts for the room-temperature lasing in ZnO or not [9,21]. In order to shed new light on the excitonic lasing process in ZnO, we here employ 4 well-annealed, or well-crystallized, micrometer-sized ZnO crystals. Although lasing characteristics of the ZnO microcrystals have hardly been paid attention to previously [2224], we show that these microcrystals have some advantages over the well-studied nanostructured materials to identify the origin of the excitonic optical gain. First, purely excitonic lasing is achieved in the ZnO microcrystals because of their high crystallinity and low optical loss. Second, individual ZnO microcrystals can serve as effective resonators to show random laser action because their size is larger than the emission wavelength [25]. Although the resulting random feedback is incoherent, the lasing line will represent the maximum of the net gain of the medium [25,26]. That is, the emission spectrum narrows continuously towards the center of the amplification line with increasing pumping intensity. As for the coherent random laser based on ZnO nanostructures [1517], it is not straightforward to determine the position of the gain maximum on account of the presence of multiple resonance modes. From a detailed analysis of the temperature dependent lasing spectra of the ZnO microcrystals, we present the convincing experimental evidence that the excitonic lasing in ZnO at room temperature is not induced by the ex-ex scattering but by the ex-el scattering. We also demonstrated that the transition from ex-ex to ex-el process

Transient gain analysis of gain-switched semiconductor lasers during pulse lasing.

We analyzed the transient gain properties of three gain-switched semiconductor lasers with different materials and cavity structures during pulse lasing. All the semiconductor lasers were pumped with impulse optical pumping, and all the generated gain-switched output pulses were well described by exponential functions in their rise parts, wherein the transient gains were derived according to the rate-equation theoretical model. In spite of the different laser structures and materials, the results consistently demonstrated that a higher transient gain produces shorter output pulses, indicating the dominant role of higher transient gain in the generation of even shorter gain-switched pulses with semiconductor lasers.

Probing of local structures of thermal and photoinduced phases in rubidium manganese hexacyanoferrate by resonant Raman spectroscopy.

Resonant couplings of the electronic states and the stretching vibrations of CN(-) ligands, which bridges metal ions, is investigated by resonance Raman spectroscopy for Rb(0.94)Mn[Fe(CN)6](0.98)·0.2H2O. Large excitation wavelength dependences over one order of magnitude were found for Raman peaks corresponding to different valence pairs of metal ions in the excitation wavelength range between 350 and 632 nm. In the thermal low-temperature phase, the CN(-) stretching modes due to the low-temperature-phase configuration (Fe(2+)-Mn(3+)) and the phase-boundary configuration (Fe(3+)-Mn(3+)) are coupled to the Fe(2+)-to-Mn(3+) intervalence transfer band and Jahn-Teller distorted Mn(3+) d-d transition band, respectively. In the photoinduced low-temperature phase, the Fe(3+)-Mn(3+) mode shows strong resonant enhancement with the CN(-)-to-Fe(3+) charge-transfer band, which exists in the high-temperature phase with a cubic structure. From these resonance behaviors, we conclude that the local lattice symmetry of the photoinduced phase is cubic in contrast with the tetragonal symmetry in the thermal low-temperature phase.

Scan-less confocal phase imaging based on dual-comb microscopy

Confocal imaging and phase imaging are powerful tools in life science research and industrial inspection. To coherently link the two techniques with different depth resolutions, we introduce an optical frequency comb (OFC) to microscopy. Two-dimensional (2D) image pixels of a sample were encoded onto OFC modes via 2D spectral encoding, in which OFC acted as an optical carrier with a vast number of discrete frequency channels. Then, a scan-less full-field confocal image with a depth resolution of 62.4 um was decoded from a mode-resolved OFC amplitude spectrum obtained by dual-comb spectroscopy. Furthermore, a phase image with a depth resolution of 13.7 nm was decoded from a mode-resolved OFC phase spectrum under the above confocality. The phase wrapping ambiguity can be removed by the match between the confocal depth resolution and the phase wrapping period. The proposed hybrid microscopy approach will be a powerful tool for a variety of applications.Confocal laser microscopy (CLM) is a powerful tool in life science research and industrial inspection because it offers two-dimensional optical sectioning or three-dimensional imaging capability with micrometer depth selectivity. Furthermore, scan-less imaging modality enables rapid image acquisition and high robustness against surrounding external disturbances in CLM. However, the objects to be measured must be reflective, absorptive, scattering, or fluorescent because the image contrast is given by the optical intensity. If a new image contrast can be provided by the optical phase, scan-less CLM can be further applied for transparent non-fluorescent objects or reflective objects with nanometer unevenness by providing information on refractive index, optical thickness, or geometrical shape. Here, we report scan-less confocal dual-comb microscopy offering a phase image in addition to an amplitude image with depth selectivity by using an optical frequency comb as an optical carrier of amplitude and phase with discrete ultra-multichannels. Our technique encodes confocal amplitude and phase images of a sample onto a series of discrete modes in the optical frequency comb with well-defined amplitude and phase to establish a one-to-one correspondence between image pixels and comb modes. The technique then decodes these images from comb modes with amplitude and phase. We demonstrate confocal phase imaging with milliradian phase resolution under micrometer depth selectivity on the millisecond timescale. As a proof of concept, we demonstrate the quantitative phase imaging of standing culture fixed cells and the surface topography of nanometer-scale step structures. Our technique for confocal phase imaging will find applications in three-dimensional visualization of stacked living cells in culture and nanometer surface topography of semiconductor objects.