We are using organic small molecules as absorbing material to investigate coherent perfect absorption in layered thin-film structures. Therefore we realize strongly asymmetric resonator structures with a high optical quality dielectric distributed Bragg reflector and thermally evaporated wedged organic materials on top. We investigate the optical properties of these structures systematically by selective optical pumping and probing of the structure. By shifting the samples along the wedge, we demonstrate how relations of phase and amplitude of all waves can be tuned to achieve coherent perfect absorption. Thus almost all incident radiation dissipates in the thin organic absorbing layer. Furthermore, we show how these wedged structures on a high-quality reflective dielectric mirror can be used to determine optical dispersion relations of absorbing materials in a broad spectral range. This novel approach does not require any specific a priori knowledge on the absorbing film.
Two of the most successful microcresonator concepts are the vertical cavity surface emitting laser (VCSEL), where light is confined between distributed Bragg reflectors (DBRs), and the distributed feedback (DFB) laser, where a periodic grating provides positive optical feedback to selected modes in an active waveguide (WG) layer. Our work concerns the combination of both into a composite device, facilitating coherent interaction between both regimes and giving rise to novel laser modes in the system. In a first realization, a full VCSEL stack with an organic active layer is evaporated on top of a diffraction grating with a large period (approximately 1 micron), leading to diffraction of waveguided modes into the surface emission of the device. Here, the coherent interaction between VCSEL and WG modes, as observed in an anticrossing of the dispersion lines, facilitates novel hybrid lasing modes with macroscopic in-plane coherence .
In further studies, we decrease the grating period of such devices to realise DFB conditions in a second-order Bragg grating which strongly couples photons via first-order light diffraction to the VCSEL. This efficient coupling can be compared to more classical cascade-coupled cavities and is successfully described by a coupled oscillator model . When both resonators are non-degenerate, they are able to function as independent structures without substantial diffraction losses. The realization of such novel devices provides a promising platform for photonic circuits based on organic microlasers.
 A. Mischok et al., Adv. Opt. Mater., early online, DOI: 10.1002/adom.201600282, (2016)
 T. Wagner et al., Appl. Phys. Lett., accepted, in production, (2016)
Two of the most successful microresonator concepts are the vertical cavity surface emitting laser (VCSEL), comprising a vertical cavity of highly reflective DBRs sandwiching an active layer, and the distributed feedback (DFB) laser, where a periodic optical grating selects laser modes from an active waveguide (WG) layer. Here, an organic microcavity is coupled with in-plane periodic photonic wires or dots to facilitate a coherent interaction between waveguided and vertically emitting modes as well as creating an additional in-plane confinement. The vertical positioning of such patterning plays a crucial role in the observable features. While embedding metallic or dielectric wires directly in the cavity layer leads to a strong lateral confinement as well as the observation of photonic Bloch states [1,2], the deposition of the full VCSEL stack on top of a periodic grating reveals novel features. In such a device, we demonstrate the coherent coupling between parabolic VCSEL and linear WG modes in the angle-resolved far field emission. In this system, lasing occurs not only at the VCSEL parabola apex but also at points of hybridization, when the dispersion of modes cross, showing a drastically enhanced in-plane coherence . The coherent coupling of two conceptually different devices with perpendicular propagation directions paired with the macroscopic coherence facilitate a multitude of new applications.
 Adv. Opt. Mater. 2(8), 746 (2014)
 Phys. Rev. Appl. 3, 064016 (2015).
 Adv. Opt. Mater. under review (2016).
Due to their geometry, optical microcavities allow strong confinement of light between the mirrors and promise single mode operation at lowest possible lasing thresholds. Nevertheless, such devices suffer from losses not only due to parasitic absorption of the active or mirror layers, but especially via outcoupling of leaky and waveguided modes within the active layer. In this work, we present an organic microcavity sandwiched between high quality dielectric distributed Bragg reflectors. A highly conductive silver layer of 40nm thickness is added next to the active layer, leading to the formation of Tamm-Plasmon-Polaritons (TPP), one replacing the original cavity mode and shifting its resonance to the red, another one emerging from the long-wavelength sideband and moving to the blue. To avoid parasitic absorption introduced by such contacts, the silver layer is structured on the micrometer-scale using photolithography, yielding separated areas supporting either original cavity mode or red shifted TPP-resonances. This separation leads to a strong spatial trapping of the modes to only their resonant regions on the sample and can in turn be exploited to achieve complete three-dimensional confinement of photons. In elliptic holes produced in the metal layer, we observe the formation of Mathieu-Modes, leading to a reduction of the lasing threshold by six times. Facilitating triangular cuts in the silver layer, highly confined standing modes develop in the system, allowing a precise optimization of the spatial mode extension and reducing the threshold even further down to one order of magnitude below the threshold of an unstructured organic cavity. These results show that the introduction of absorptive metals, needed for the realization of an electrically driven laser, can in turn be harnessed to improve the characteristics of the device.
We investigate a planar organic microcavity under spatially periodic optical excitation. The host:guest system of
Alq3:DCM is the emitting layer embedded in between two dielectric mirrors. Excitation by an interference field of two
femtosecond laser pulses generates an array of lasers spaced by few microns. The far field of the cavity response shows
conventional stimulated emission at k=0 and, in addition, two stripes of laser emission at oblique angles. The excitation
pattern generates a periodic modification of the optical properties of the cavity, a dynamic diffraction grating with a
period of few microns. This enhances the spontaneous emission in the direction of the Bragg angle, which depends on
the distance of the interference stripes. Via the angle of incidence of the excitation beams, we can optically tune output
angle and the wavelength of lasing. Measurements are confirmed by simulations of the mode dynamics inside a lossy
cavity with small excitation spot sizes, where the local gain exceeds the total mirror and absorptive losses. We find that
adjacent cavity quasimodes couple out of phase at certain separation distances, which critically depend on the quasimode
radius and, thus, on the residual absorption. Thus, we gain insight into the development of coherence and mode-locking
We investigate high finesse organic planar microcavities under different optical pumping conditions. The design
of the cavities is chosen to realize the smallest possible cavity thickness of λ/2. We use different coherent light
sources to pump the structures optically. 120 fs pulses out of an amplifier system and pulses of 1 ns length from
a 532 nm solid state laser are applied. Emission properties of an organic microcavity are further investigated
when incoherent light, emitted from an inorganic light emitting diode, is used to excite the cavity layer. A
monolithically integrated device is realized, where a high quality organic microcavity is deposited directly onto
the surface of a light emitting diode. A set of modified rate equations is applied to simulate input-output
curves of different organic microcavities under different optical pumping conditions. In addition to the modified
spontaneous emission rates due to the resonator, which are described by the standard set of rate equations, our
model takes the finite number of the molecules per mode into account. This limits the upper bound for the
number of photons emitted into the mode since absorption saturation takes place during the pumping process
by short optical pulses. An analysis of the experimental results show that this effect can substantially modify
the lasing characteristics of lambda-half organic microcavities.
The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigate
the characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied here
consists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq3:DCM.
Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq3 at a wavelength
of 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beam
position with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organic
microcavities in a range of 55 nm. The continuously variable cavity thickness allows us to study the thickness
dependence of the input-output characteristics in a single sample. These data are obtained at a certain emission
wavelength, λ, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspond
to different multiples of λ/2. For a decreasing thickness of the active layer, one-dimensional optical confinement
is expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through the
mirrors increases with decreasing thickness resulting in a minimum threshold value for an active layer thickness
of approximately 3/2 λ. This lower threshold limit is set by nonradiative losses as well as residual absorption.
×The lasing threshold of a microcavity is mainly determined by the spontaneous emission factor β, which is
inversely proportional to the mode volume Vc. We demonstrate an experimental way to decrease the mode
volume via lateral structuring of the microcavity. This redistributes both number and density of the transversal
cavity modes, which increases the amplitude of the internal electromagnetic field. Our samples are microcavities
with an active layer of variable thickness (0.2 to 2 μm) made of tris-(8-hydroxyquinoline) aluminium (Alq3) doped
with 4-(dicyanomethylene)-2- methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). With thermal coevaporation
through a shadow mask, this layer is structured into an array of photonic boxes square-shaped microcavities with
an area of 55 μm2. Using a microscope objective, we record the spatial distribution of the cavity transmission
spectra with submicron resolution. The modes of the photonic boxes show a clear discretization, which is due
to the multidimensional optical confinement. Under selective excitation of the DCM molecules via a focused
pulsed laser (532 nm, 1.5 ns, 2 kHz , &diameter; ≈ 3μm), we record the spatially and spectrally resolved emission of
single photonic boxes. The laser pulse energy is varied to obtain input-output curves of the cavity modes. At
an excitation energy of ~30 pJ, we observe superlinear growth as well as a spectral narrowing of the emission
from the lowest energy mode of a single photonic box. For this lasing transition, we determine a spontaneous
emission factor β of ≈0.01.
We present experimental and theoretical study of refractive index modification induced by femtosecond laser
pulses in photorefractive crystals. The single pulses with central wavelength of 800 nm, pulse duration of 150 fs,
and energy in the range of 6-130 nJ, tightly focused into the bulk of Fe-doped LiNbO3 and stoichiometric LiTaO3
crystals induce refractive index change of up to about 10-3 within the volume of about (2.0 x 2.0 x 8.0) μm3.
The photomodification is independent of the polarization orientation with respect to the crystalline c-axis. The
recorded region can be erased optically by a defocused low-intensity single pulse of the same laser. Recording
and erasure can be repeated at the same position many times without loss of quality. These findings demonstrate
the basic functionality of the ultrafast three-dimensional all-optical rewritable memory. Theoretically they are
interpreted by taking into account electron photogeneration and recombination as well as formation of a space-charge
field. The presented analysis indicates dominant role of photovoltaic effect for our experimental conditions,
and suggests methods for controlling various parameters of the photomodified regions.
We present a study of time-resolved transmission and emission properties of optically anisotropic planar microcavity
structures. The structures consist of λ/4-layers of SiO2 and TiO2 for the dielectric mirrors and a cavity
layer of either SiO2 or the organic dye composite AlQ3/DCM. For the SiO2 cavity, we observe a polarization
splitting at normal incidence leading to terahertz oscillations of transmitted coherent light. The polarization
splitting is explained by an optical anisotropy of the dielectric layers caused by the fabrication process. We
apply an up-conversion setup for temporally and spectrally resolved transmission measurements and obtain a
corresponding beating of 1.25 THz. Time resolved measurements yield a Q-value of 1600, corresponding to a
cavity photon lifetime of 0.65 ps. We explain our observations with a transfer-matrix model and introduce a
Fourier-transform based analytical algorithm. The cavity filled with the organic dye composite can act as an
organic microcavity laser. The birefringence of the distributed Bragg reflectors leads to lasing in two perpendicularly
polarized modes. Investigations of the ultrafast dynamics of this laser system show a phase coupling
of the two laser modes leading to the generation of a terahertz optical beat. The oscillation frequency can be
widely tuned by variations in the fabrication process.
We report on the experimental observation of polarization splitting and terahertz oscillations in transmission and laser emission from optically anisotropic microcavities. A guest-host composite of tris-(8-hydroxyquinoline) aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) serves as active laser material. The anisotropy is attributed to oblique columnar structures in the distributed Bragg reflector mirrors of our microcavity, resulting from sample fabrication. A splitting of 0.2 nm occurs in the laser emission from an organic vertical cavity surface emitting laser at a wavelength of 612.6 nm, and a splitting of 2.5nm is obtained from a sample for Ti-Sapphire laser transmission at 781 nm. Split modes are perpendicularly polarized.
An upconversion setup allows temporally resolved studies of transmission and emission behavior, showing an oscillation at a frequency of 1.25THz in transmission, and 0.18THz in emission, respectively. The temporal behavior of laser emission is modelled by a set of rate-equations and extended to account for the resulting oscillations. Our observations suggest that a phase-coupling mechanism between both occuring modes is present in the laser emission from our microcavity.
Laser induced transient gratings are used to study carrier generation and recombination properties via free carrier nonlinearity in differently grown GaAs and CdZnTe samples. Simulation of free carrier, photorefractive, and absorptive optical nonlinearities for 10-ns pulses and various illumination intensities allowed us to reveal conditions for the efficient transient quenching of EL2 defect at room temperature in semi-insulating GaAs. In addition, the straightforward coupling of nonlinear degenerate four wave-mixing signal at 1.06 μm with the steady-state charge states of EL2 defect is shown to allow a rough estimation of a crystal compensation ratio by EL2 defect. This novel method was applied to liquid-encapsulated Czochralski and Bridgeman-grown samples and compensation values ranging from 0.1 to approximately 0.6 have been derived. Also, feasibility of nanosecond- and picosecond-dynamic grating techniques for control of
GaAs wafer quality is shown. The first one allowed fast and highly sensitive mapping of EL2 defect distribution and its charge state; the second one has proved a presence of a fast traps in the vicinity of dislocation conglomerations. Analogous mapping of CdZnTe wafers has shown very high spatial homogeneity of the samples, and revealed areas with the non-photoactive absorption or scattering of light.
We investigate the spatial displacement dynamics of optically excited wave packets in semiconductor superlattices. A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn leads to formation of a time-varying coherent wave packet. The real space oscillation of the excited wave packet identifies these quantum beats of the Wannier-Stark states as Bloch oscillations: We present an experimental technique which measures directly the displacement of the wave packet center- of-mass. The oscillating Bloch wave packets create a microscopic dipole moment which can be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. We show that the Bloch wave packet undergoes harmonic spatial motion, proving for the first time the predictions of Bloch and Zener. The influence of an experimental conditions on displacement of the Bloch wave packet is discussed.
Using transient light-induced grating experiments, we demonstrate important consequences of interaction between photoexcited electrons and EL2 centers in semiinsulating GaAs at room temperature. Carrier lifetime is found to depend on the local density and ionization ratio of the EL2 centers. A substantial slow down of diffusive grating decay due to the interaction between electrons and photoionized EL2 donors is observed.
We demonstrate for the first time the applicability of transient grating technique to study carrier dynamics in porous silicon and to characterize the sublayers with the different structure in an optical way. The parameters of deeper layers of 130 mm thick porous Si structure have been found similar to those in crystalline substrate. The free standing films at high injection levels indicated very fast nonequilibrium carrier recombination with t equals 430 ps, while the carrier lifetime in its crystalline substrate was above 2 ns.
The possibility to combine photorefractive effect and metastability of defect states in GaAs crystals is combined. Peculiarities of free carrier and photorefractive nonlinearities at conditions of deep donor EL2 optical quenching by short laser pulse are analyzed theoretically and experimentally. The enhancement of low temperature photorefractive effect was observed at T less than or equal to 250 K in a good agreement with theoretical simulation and attributed to the temporary EL2 quenching by nanosecond pulses.