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Fluorescence recognition of d-glucose in water with excellent sensitivity, selectivity, and chiral selectivity is desired because d-glucose is an essential component in biological and pathological processes. We report an innovative approach that exploits the 1:2 stoichiometric inclusion complexes of γ-cyclodextrin (γ-CyD) with two molecules of fluorescent monoboronic acid-based receptors, which form a

The photo-quenching mechanism of 2-(4-phenylboronic acid)-1-pyrenemethamide (C1-APB), which has potential application as a saccharide-recognition sensor, was investigated. By performing temperature-dependent time-resolved photoluminescence measurements, we determined the mechanism responsible for the photo-quenching properties of C1-APB to be a photoinduced electron transfer (PET). Moreover, the dependence of the electron transfer rate (k

One of the peculiar features in quantum mechanics is that a superposition of macroscopically distinct states can exist. In optical system, this is highlighted by a superposition of coherent states (SCS), i.e. a superposition of classical states. Recently this highly nontrivial quantum state and its variant have been demonstrated experimentally. Here we demonstrate the superposition of coherent states in quantum measurement which is also a key concept in quantum mechanics. More precisely, we propose and implement a projection measurement onto an arbitrary superposition of two weak coherent states in optical system. The measurement operators are reconstructed experimentally by a novel quantum detector tomography protocol. Our device is realized by combining the displacement operation and photon counting, well established technologies, and thus has implications in various optical quantum information processing applications.

The parameters related to the localized states in green-emitting indium gallium nitride (InGaN) have been evaluated by considering the energy diagrams derived by five different methods: (1) the exponential tail of the low-energy side of photoluminescence (PL) spectra, (2) the photon energy dependence of PL decay time, (3) excitation energy dependence of the PL peak energy, (4) the PL excitation spectrum, and (5) the temperature dependence of PL peak energy. The results indicate that the energy diagram of InGaN is divided into four regions: deep localized states, migration region, transition region, and extended states. It is suggested that wider localized states and a narrower transition region are preferable in order to achieve higher PL efficiency. In addition, the dependence of carrier density on PL properties supports the fact of photo-generated carriers forming localized excitons in green-emitting InGaN, although the carriers do not form localized excitons in orange-emitting InGaN and instead exist as localized electrons and holes.

It is well known that the surface trap states and electronic disorders in the solution-processed CH3 NH3 PbI3 perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surface-trap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (Voc ) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed Voc as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5 % in the polymer-modified devices (PCE=15 %) with Voc more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50 % humidity).

The carrier density dependence of photoluminescence (PL) spectra, PL efficiency, and time-resolved PL (TRPL) in InGaN/GaN nanocolumns with orange emission has been measured over a wide range of carrier densities to investigate the carrier recombination dynamics at low and room temperatures. The TRPL decay curves were analyzed by stretched exponential functions to obtain a distribution of the decay times. The results indicate that the emission origin in InGaN with orange emission under low excitation intensity is the recombination of localized electrons and holes, not from localized excitons. Our analysis shows that the total density of localized states is an important factor to increase PL efficiency, which implies that red-emitting InGaN with a high density of localized states due to large indium fluctuation has the potential to achieve high PL efficiency. In addition, we have shown that the stretched exponential function is an effective approach to analyze several carrier relaxation processes with different decay times.

We report the dependence of stimulated emission phenomena on randomness in a collection of regularly arranged InGaN/GaN nanocolumns. By comparing the stimulated emission behavior for two samples with different degrees of randomness, we found that localization effects are inevitable even in the almost perfect sample, and several modes partially overlapping in space will compete with each other. The ultrafast dynamics of stimulated emission under femtosecond pulsed laser excitation were also investigated, and the emission peaks were observed to change with time.

We developed a new and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film for fabricating highly efficient perovskite solar cells (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation–halide perovskite (FAPbI3)0.85(MAPbBr3)0.15. The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold. When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent–voltage performance with high fill factor and suppressed hysteresis, achieving the best conversio...

Formamidine (FA) and cesium (Cs) cations were introduced into quasi-two-dimensional (2D) perovskites as B site cations. The unique crystalline growth of the resulting (n-C6H13NH3)2FAPb2I7, which promotes charge transport in photovoltaic solar cells, was confirmed, as was the stability of this material. The photovoltaic properties of (n-C6H13NH3)2FAPb2I7 were found to be superior to those of other homologous quasi-2D perovskite compounds.

An inclusion complex consisting of a boronic acid fluorophore (C1-APB) and β-cyclodextrin (β-CyD) acts as a supramolecular sugar sensor whose response mechanism is based on photoinduced electron transfer (PET) from the excited pyrene to the boronic acid. We have investigated the PET process in C1-APB/CyD complexes by using time-resolved photoluminescence (TRPL) measurements at room temperature, and have succeeded in estimating the electron-transfer time to be about 1 ns. We have also studied the effects of CyDs on the PET process by comparing two kinds of CyDs (α-CyD, β-CyD) under different water-dimethyisulfoxide (DMSO) concentration conditions. We found that the CyDs interacting with the boronic acid moiety completely inhibits PET quenching and increases the monomer fluorescence intensity.

We have measured the temperature dependence of the reflection and photoluminescence spectra of a lead bromide hybrid material (CH3NH3PbBr3) using high-quality macroscopic single crystals. Single cr...

We consider the phase sensing via a weak optical coherent state at quantum limit precision. A detection scheme for the phase estimation is proposed, which is inspired by the suboptimal quantum measurement in coherent optical communication. We theoretically analyze a performance of our detection scheme, which we call the displaced-photon counting, for phase sensing in terms of the Fisher information and show that the displaced-photon counting outperforms the static homodyne and heterodyne detections in a wide range of the target phase. The proof-of-principle experiment is performed with linear optics and a superconducting nanowire single-photon detector. The result shows that our scheme overcomes the limit of the ideal homodyne measurement, even under practical imperfections.

The optical properties of InGaN/GaN multiple-quantum-well (MQW) nanocolumns whose emission wavelength is in the yellow-to-red region (2.1 ∼ 2.3 eV) have been investigated using time-resolved photoluminescence (PL) measurements. The analysis of the PL decay curves using a stretched exponential function shows that the radiative and nonradiative rates in the red region are almost the same as those in the yellow region. Long-lived migrating carriers also contribute the strong PL intensity in the red region. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)2(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6]4- octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)2PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have measured the optical properties of wurtzite InN nanocolumns and film by photoluminescence (PL) measurements at temperatures from 5 to 300 K and analyzed the PL spectra by fitting with the free-electron recombination bound (FERB) model. For the top-linked InN nanocolumns, we observed strong PL intensity compared to the InN film sample. The PL spectra were asymmetrical with low-energy tails and a red-shift of the PL peak energy position was observed with increasing temperature. However, for the separated InN nanocolumns, we observed weak PL intensity and symmetrical PL spectra. Analyzing the spectra shape of the top-linked InN nanocolumns at 5 K using the FERB model, we evaluated the intrinsic bandgap energy and carrier density of InN nanocolumns to be 0.69eV and 2.5 × 10 17 cm -3 , respectively.

We report the first observations of resonantly excited coherent optical phonons in bulk ZnTe using noncollinear optical parametric amplifiers. As the pulse photon energy approaches the bandgap, the amplitudes of coherent phonons increase. Since pulse fluence and polarization dependencies show that generation mechanism of resonantly excited coherent phonons is impulsive stimulated Raman scattering, this enhancement can be concluded to be the result of the resonant Raman effect.

We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc (SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state.

The coherent dynamics of excitons in InAs quantum dots (QDs) was investigated in the telecommunication wavelength range using a transient four-wave mixing technique. The sample was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for obtaining a high S/N in the four-wave mixing measurements, although no high-sensitive heterodyne detection was carried out. The dephasing time and transition dipole moment were precisely estimated from the polarization dependence of signals, taking into account their anisotropic properties. The population lifetimes of the excitons were also measured by using a polarization-dependent pumpprobe technique. A quantitative comparison of these anisotropies demonstrates that in our QDs, non-radiative population relaxation, polarization relaxation and pure dephasing are considerably smaller than the radiative relaxation. A comparison of the results of the four-wave mixing and pump-probe measurements revealed that the pure dephasing could be directly estimated with an accuracy of greater than 0.1 meV by comparing the results of four-wave mixing and pump-probe measurements. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated coherent LO phonon properties in zinc-based II-VI widegap semiconductors, focusing on phonon-plasma coupled modes. By a careful treatment of the time evolution of the signals in ZnS, ZnSe, and ZnTe, we found a frequency upshift as the pump intensity increases. Using a classical coupled oscillator model, we have explained the pump intensity dependence of both the shift and the decay rates by a mixing of highly damped two-photon generated plasma. From the linear dependence between them we can estimate the photo-excited carrier mobilities, leading to a new powerful estimation method to measure the mobility.

In polyacetylene and polydiacetylene oligomers, photoluminescence (PL) is induced from the dipole-forbidden state by vibronic coupling. The assumption that the PL in polythiophene (PT) and poly( p -phenylene vinylene) (PPV) also includes this induced component, consistently explains many experimental results. In this work, we focus on the temperature dependence of the time-integrated and time-resolved PL spectra of a PT derivative and unsubstituted PPV, and demonstrate the validity of the above model for their temperature dependence. Applying this model to the hitherto unresolved drastic PL spectral change of unsubstituted PPV, we propose that the spectral change and the spectral shape itself can be understood as resulting from a contribution to the PL from the 2 A g state coupled with a change in the ordering of the 2 A g and the 1 B U states at low temperatures.

Optical properties of lead-iodide-based one-dimensional perovskite-type crystals [NH 2 C(I)=NH 2 ] 3 PbI 5 and [CH 3 SC(=NH 2 )NH 2 ] 3 PbI 5 have been investigated theoretically and experimentally. The electronic and excitonic structures are studied based on group theoretical consideration and first-principle band calculation. Strong one-dimensional anisotropy of the optical absorption spectra, large Stokes shifts ( ∼ 1.0 eV) and huge exchange energies ( ∼ 70 meV) indicate that the excitons in these crystals are one-dimensional Frenkel excitons.

We have confirmed biexciton formation in an organic–inorganic hybrid quantum-well material (C4H9NH3)2PbBr4 by photoluminescence and two-photon absorption measurements. The biexciton has extremely large binding energy, 60 meV, which to our knowledge is the largest value ever reported for a semiconductor. By analyzing the spectrum of biexciton luminescence, the biexciton gas temperature was found to be much higher than the bath temperature due to a higher local temperature arising from the large biexciton binding energy.

Wannier series excitons have been identified by electroabsorption and two-photon absorption spectroscopy on a layered-perovskite-type crystal (C 6 H 13 NH 3 ) 2 PbI 4 . 1s, 2s, 2p, 3p excitons are located at 2.34, 2.60, 2.61, and 2.64 eV, respectively. This finding indicates the excitons in this system exhibit a relatively strong two-dimensional character. The large binding energy of the 1s exciton (>300 meV) is considerably enhanced by the dielectric confinement effect.

We systematically investigate and compare the structural and optical properties of InGaN nanocolumns (NCs) and InGaN/GaN single quantum structures (SQSs) fabricated on top of GaN NCs as functions of the GaN column diameter. Each InGaN/GaN SQS comprises an In-rich InGaN single quantum dot at the center of a semipolar Ga-rich InGaN single quantum well. The InGaN NCs and InGaN/GaN SQSs qualitatively exhibit similar optical properties. However, the specific diameters at which the photoluminescence spectra change from single-peak emission patterns to double-peak patterns are different. Based on scanning transmission electron microscopy observations of the InGaN layers formed on the GaN NCs with different growth times, a model for the growth of InGaN on GaN NCs is proposed and the origin of the difference in the specific diameters is elucidated. Finally, when QSs are introduced to the InGaN-based NCs, the radiative recombination probability increases while the nonradiative and surface recombination probabilities decrease, thus leading to high emission efficiency.

We performed pulse-propagation experiments at the Z 3 -exciton resonance in CuCl and observed optical precursors. From the comparison between experimental results and numerical calculations, we obtained the conditions for a clear observation of a precursor in the propagation of a pulse with a finite spectral width.

We report the dependence of randomness in sample configuration on stimulated emission phenomena in two-dimensional nanocolumn arrays. From the wavelength selectivity of the photoluminescence, we found that the stimulated emission is apparently related to a distributed feedback mechanism. By comparing the emission behavior between two regularly arranged InGaN/GaN nanocolumn samples with different degrees of randomness, we found that localization effects become prominent if the sample array has any randomness, even in an almost perfect sample. Several modes are localized at different areas in the nanostructures and partially overlap in space, and they compete with each other, especially in a slightly imperfect sample. In addition to the randomness dependence, by observing the wave number space images of the photoluminescence, we confirmed that the stimulated emission phenomena in the crystal arrays are generated by Bragg diffraction at photonic band edges, though the modes have some degree of variability via the sample randomness.

The effect of the structural properties on the optical characteristics was investigated for In0.3Ga0.7N nanocolumns (NCs) grown on GaN NCs as a function of GaN column diameter, D GaN. With increasing D GaN, the photoluminescence spectra changed from single-peak to double-peak emissions at the diameter D 0 where InGaN axial NCs change to InGaN–InGaN core–shell NCs. For the core–shell NCs, the volume recombination probabilities of the InGaN cores did not change with D GaN. Whereas the surface recombination probability of the InGaN cores exponentially decreased because of the spontaneous formation of InGaN shells for D GaN > D 0, it drastically increased for D GaN ≤ D 0.

Broadband light sources play essential roles in diverse fields, such as high-capacity optical communications, optical coherence tomography, optical spectroscopy and spectrograph calibration. Although a nonclassical state from spontaneous parametric down-conversion may serve as a quantum counterpart, its detection and characterization have been a challenging task. Here we demonstrate the direct detection of photon numbers of an ultrabroadband (110 nm FWHM) squeezed state in the telecom band centred at 1535 nm wavelength, using a superconducting transition-edge sensor. The observed photon-number distributions violate Klyshko's criterion for the nonclassicality. From the observed photon-number distribution, we evaluate the second- and third-order correlation functions and characterize a multimode structure, which implies that several tens of orthonormal modes of squeezing exist in the single optical pulse. Our results and techniques open up a new possibility to generate and characterize frequency-multiplexed nonclassical light sources for quantum info-communications technology.

The time-resolved photoluminescence spectrum of a polythiophene derivative has been analyzed from the viewpoint of the vibronic structure and its dynamics. We found that the photoluminescence (PL) originates from two vibronic coupled states which consist of the odd-parity 1 B u component and the even-parity 2 A g component. Since the fluorescences from the two states have different time constants and different vibronic structures, the PL shows a biexponential decay and a spectral change.

We report a simple method of a serial-to-parallel conversion that does not use a Fourier-transform system. The method is based on directly picking up a narrow temporal range of the skewed input signal by using a nonlinear wave mixing with the reference pulse. We show theoretically that the pick-up method is less influenced by the relaxation times of the nonlinear material. We demonstrate our method experimentally using a self-organized quantum-well material, and confirm that T-bits/s pulses are clearly converted into spatial patterns with conversion rates of 140 GHz.

We measured the time-resolved photoluminescence of highly ordered and disordered films of a polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene). The formation of self-trapped excitons (STE's) within 10 ps after photoexcitation and the subsequent migration of the STE's are common behavior in both films. The spectral narrowing of the phonon sidebands within 10 ps after photoexcitation is associated with the formation process of STE's and the spectral changes on a few hundred ps time scale are explained by the STE migration and the dependence of the electron-phonon coupling on the effective conjugation length.

We measured the absorption and |χ(3)| spectra around the excitonic resonance for Langmuir–Blodgett (LB) film of polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene) at 10 K and room temperature. We derived a theoretical distribution function for the effective conjugation length taking the molecular weight distribution into consideration, and fitted the calculated absorption and |χ(3)| spectra to the experimental ones. From the fits, we estimated the average effective conjugation length to be 21 thiophene rings at 10 K, and 15 thiophene rings at room temperature.

Nanoparticles of lead iodide (PbI2) were prepared by exposure of Langmuir-Blodgett (LB) films of lead arachidate to hydrogen iodide. The photoluminescence (PL) spectrum of the nanoparticles consist...

We have investigated how the morphology and diameter of GaN nanocolumn affect the optical properties of exciton and biexciton. We measured photoluminescence (PL) of excitons in GaN nanocolumns at low temperature and found that the PL spectra depend on the column diameter. Changing the distance between the sample and the collimating lens, it was established that this phenomenon is due to the z -polarized PL radiated from the side surface. This result proves that the nano-scale morphology produces a change in the optical properties. In addition, to elucidate the mechanism causing the increase of the biexciton binding energy observed in a previous experiment, we calculated the exciton binding energies by the variational method and also estimated their sizes. From the calculated results we can expect that the only biexcitons experience spatial confinement effect even when the nanocolumn width is much larger than the exciton Bohr radius. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1 ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree–Fock-type Coulomb interactions between excitons.

Exciton polariton in an organic-inorganic multiple-quantum-well (MQW) single crystal (C 4 H 9 NH 3 ) 2 PbBr 4 at low temperature has been investigated by photoluminescence excitation (PLE), reflection, and time-resolved photoluminescence measurements. Since (C 4 H 9 NH 3 ) 2 PbBr 4 has ideal two-dimensional excitons with an extremely large oscillator strength and forms self-organized MQW with a very short well-period (d∼10A), polaritonic coupling among excitons is strong and extends over a large number of wells. Therefore, observed MQW polariton features were the same as those of bulk polaritons. We have also investigated relaxation dynamics of the MQW polariton in the same framework as discussions on bulk polaritons.

We observed intersubband transition in GaN/AlN multiple quantum disk (MQD) in nanocolumns by absorption spectra. We also investigated ultrafast relaxation dynamics at 1.55 μm by time-resolved saturable absorption. We have found that the temporal response of nanocolumns is almost the same as the multiple-quantum-well (MQW) sample. This indicates that the delay caused by multiple scatterings in MQD nanocolumns is negligible. The saturation intensity for the Brewster configuration in MQD was also estimated.

We measured photoluminescence (PL) from excitons and biexcitons in GaN nanocolumns at low temperature and found that the PL spectra of excitons depend on the nanocolumn diameter. Taking into account the polaritonic effect of the excitons, calculation of the PL spectra revealed that the dependence on diameters causes a difference of PL intensity from side surfaces of the nanocolumns. At high excitation intensities, we also observed biexciton emissions and found that the biexciton binding energies are higher than those in bulk samples. Although the mechanism for the increase in the binding energy is not clear at present, we suppose that it arises from a spatial confinement effect due to the nanocolumn morphology.

This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO2 thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed.

We measured the dephasing time and radiative lifetime of excitons in InAs quantum dots fabricated using the strain compensation technique. The dephasing time at 3 K was as long as 2.86 ns using transient four-wave mixing measurements at an excitation wavelength of 1.468 µm. This ultralong dephasing time was due to the significant suppression of pure dephasing.

The spectra of the third-order optical nonlinearity, $|{\ensuremath{\chi}}^{(3)}(\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{-}\ensuremath{\omega})|,$ around excitonic resonance in a spin-coated film and a Langmuir-Blodgett (LB) film of polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene), were measured using the degenerate four-wave-mixing method. The obtained value of $|{\ensuremath{\chi}}^{(3)}(\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{-}\ensuremath{\omega})|$ at the excitonic resonance was $\ensuremath{\sim}{10}^{\ensuremath{-}7}\mathrm{esu},$ which was the largest value ever obtained in conjugated polymers. From the absorption spectra and $|{\ensuremath{\chi}}^{(3)}|$ spectra, the average effective conjugation lengths of the spin-coated film and the LB film were estimated to be approximately 20- and 36-thiophene rings, respectively.

Third-order optical nonlinearity around the exciton resonance in (C6H13NH3)2PbI4 was measured using time-integrated and spectrally-resolved four-wave-mixing (FWM) techniques. For excitation below the exciton resonance, biexciton contribution to the FWM signals was observed. The dephasing energy of the biexcitons was estimated to be larger than 10 meV.

The third-order nonlinear optical susceptibility, χ (3) (− ω ; ω , − ω , ω ), of the layered perovskite-type material (C 6 H 13 NH 3 ) 2 PbI 4 is measured by a transient four-wave mixing technique using a 200-fs-pulse laser source. The maximum … χ (3) … value is 1.6 × 10 −6 esu at the lowest-exciton resonance at 8 K. Longitudinal and transverse relaxation times of the excitons are 7 ps and 0.2 ps, respectively.

We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.

We propose quantum receivers with optical squeezing and photon-number-resolving detectors (PNRDs) for the near-optimal discrimination of quaternary phase-shift-keyed coherent state signals. The basic scheme is similar to the previous proposals [e.g., Izumi et al., Phys. Rev. A 86, 042328 (2012)] in which displacement operations, on-off detectors, and electrical feedforward operations were used. Here we study two types of receivers, one of which installs optical squeezings and the other uses PNRDs instead of on-off detectors. We show that both receivers can attain lower error rates than that in the previous scheme. In particular, we show the PNRD-based receiver has a significant gain when the ratio between the mean photon number of the signal and the number of the feedforward steps is relatively high, in other words, when the probability of detecting two or more photons at each detector is not negligible. Moreover, we show that the PNRD-based receiver can suppress the errors due to dark counts, which the receiver with the on-off detector cannot do with a small number of feedforwards.

Using a degenerate pump–probe technique on the Brewster configuration at room temperature, we investigate the dynamics of the intersubband transition in GaN/AlN multiple-quantum wells. The relaxation dynamics is found to consist of ultrafast (∼140 fs) and slower (∼1.3 ps) components. We estimate the third-order susceptibility for the Brewster configuration to be ∼5.5×10−18 m2/V2, which indicates that its value becomes ∼2.2×10−16 m2/V2 when a light beam is parallel to the well.