279 research outputs found
On the Optimization of Deep Networks: Implicit Acceleration by Overparameterization
Conventional wisdom in deep learning states that increasing depth improves
expressiveness but complicates optimization. This paper suggests that,
sometimes, increasing depth can speed up optimization. The effect of depth on
optimization is decoupled from expressiveness by focusing on settings where
additional layers amount to overparameterization - linear neural networks, a
well-studied model. Theoretical analysis, as well as experiments, show that
here depth acts as a preconditioner which may accelerate convergence. Even on
simple convex problems such as linear regression with loss, ,
gradient descent can benefit from transitioning to a non-convex
overparameterized objective, more than it would from some common acceleration
schemes. We also prove that it is mathematically impossible to obtain the
acceleration effect of overparametrization via gradients of any regularizer.Comment: Published at the International Conference on Machine Learning (ICML)
201
Organic Photodiodes with an Extended Responsivity using Ultrastrong Light-Matter Coupling
In organic photodiodes (OPDs) light is absorbed by excitons, which dissociate
to generate photocurrent. Here, we demonstrate a novel type of OPD in which
light is absorbed by polaritons, hybrid light-matter states. We demonstrate
polariton OPDs operating in the ultra-strong coupling regime at visible and
infrared wavelengths. These devices can be engineered to show narrow
responsivity with a very weak angle-dependence. More importantly, they can be
tuned to operate in a spectral range outside that of the bare exciton
absorption. Remarkably, we show that the responsivity of a polariton OPD can be
pushed to near infrared wavelengths, where few organic absorbers are available,
with external quantum efficiencies exceeding those of a control OPD
Inverting Singlet and Triplet Excited States using Strong Light-Matter Coupling
In organic microcavities, hybrid light-matter states can form with energies
that differ from the bare molecular excitation energies by nearly 1 eV. A
timely question, given recent advances in the development of thermally
activated delayed fluorescence materials, is whether strong light-matter
coupling can be used to invert the ordering of singlet and triplet states and,
in addition, enhance reverse intersystem crossing (RISC) rates. Here, we
demonstrate a complete inversion of the singlet lower polariton and triplet
excited states. We also unambiguously measure the RISC rate in strongly-coupled
organic microcavities and find that, regardless of the large energy level
shifts, it is unchanged compared to films of the bare molecules. This
observation is a consequence of slow RISC to the lower polariton due to the
delocalized nature of the state across many molecules and an inability to
compete with RISC to the dark exciton reservoir, which occurs at a rate
comparable to that in bare molecules
Triplet harvesting in the polaritonic regime: a variational polaron approach
We explore the electroluminescence efficiency for a quantum mechanical model
of a large number of molecular emitters embedded in an optical microcavity. We
characterize the circumstances under which a microcavity enhances harvesting of
triplet excitons via reverse intersystem-crossing (R-ISC) into singlet
populations that can emit light. For that end, we develop a time-local master
equation in a variationally optimized frame which allows for the exploration of
the population dynamics of chemically relevant species in different regimes of
emitter coupling to the condensed phase vibrational bath and to the microcavity
photonic mode. For a vibrational bath that equilibrates faster than R-ISC (in
emitters with weak singlet-triplet mixing), our results reveal that significant
improvements in efficiencies with respect to the cavity-free counterpart can be
obtained for strong coupling of the singlet exciton to a photonic mode, as long
as the singlet to triplet exciton transition is within the inverted Marcus
regime; under these circumstances, we show the possibility to overcome the
detrimental delocalization of the polariton states across a macroscopic number
of molecules. On the other hand, for a vibrational bath that equilibrates
slower than R-ISC (i.e., emitters with strong singlet-triplet mixing), we find
that while enhancemnents in photoluminiscence can be obtained via vibrational
relaxation into polaritons, this only occurs for small number of emitters
coupled to the photon mode, with delocalization of the polaritons across many
emitters eventually being detrimental to electroluminescence efficiency. These
findings provide insight on the tunability of optoelectronic processes in
molecular materials due to weak and strong light-matter coupling
Spiking Optical Patterns and Synchronization
We analyze the time resolved spike statistics of a solitary and two mutually
interacting chaotic semiconductor lasers whose chaos is characterized by
apparently random, short intensity spikes. Repulsion between two successive
spikes is observed, resulting in a refractory period which is largest at laser
threshold. For time intervals between spikes greater than the refractory
period, the distribution of the intervals follows a Poisson distribution. The
spiking pattern is highly periodic over time windows corresponding to the
optical length of the external cavity, with a slow change of the spiking
pattern as time increases. When zero-lag synchronization between the two lasers
is established, the statistics of the nearly perfectly matched spikes are not
altered. The similarity of these features to those found in complex interacting
neural networks, suggests the use of laser systems as simpler physical models
for neural networks
What's Behind the Mask: Estimating Uncertainty in Image-to-Image Problems
Estimating uncertainty in image-to-image networks is an important task,
particularly as such networks are being increasingly deployed in the biological
and medical imaging realms. In this paper, we introduce a new approach to this
problem based on masking. Given an existing image-to-image network, our
approach computes a mask such that the distance between the masked
reconstructed image and the masked true image is guaranteed to be less than a
specified threshold, with high probability. The mask thus identifies the more
certain regions of the reconstructed image. Our approach is agnostic to the
underlying image-to-image network, and only requires triples of the input
(degraded), reconstructed and true images for training. Furthermore, our method
is agnostic to the distance metric used. As a result, one can use -style
distances or perceptual distances like LPIPS, which contrasts with
interval-based approaches to uncertainty. Our theoretical guarantees derive
from a conformal calibration procedure. We evaluate our mask-based approach to
uncertainty on image colorization, image completion, and super-resolution
tasks, demonstrating high quality performance on each
A Neural Space-Time Representation for Text-to-Image Personalization
A key aspect of text-to-image personalization methods is the manner in which
the target concept is represented within the generative process. This choice
greatly affects the visual fidelity, downstream editability, and disk space
needed to store the learned concept. In this paper, we explore a new
text-conditioning space that is dependent on both the denoising process
timestep (time) and the denoising U-Net layers (space) and showcase its
compelling properties. A single concept in the space-time representation is
composed of hundreds of vectors, one for each combination of time and space,
making this space challenging to optimize directly. Instead, we propose to
implicitly represent a concept in this space by optimizing a small neural
mapper that receives the current time and space parameters and outputs the
matching token embedding. In doing so, the entire personalized concept is
represented by the parameters of the learned mapper, resulting in a compact,
yet expressive, representation. Similarly to other personalization methods, the
output of our neural mapper resides in the input space of the text encoder. We
observe that one can significantly improve the convergence and visual fidelity
of the concept by introducing a textual bypass, where our neural mapper
additionally outputs a residual that is added to the output of the text
encoder. Finally, we show how one can impose an importance-based ordering over
our implicit representation, providing users control over the reconstruction
and editability of the learned concept using a single trained model. We
demonstrate the effectiveness of our approach over a range of concepts and
prompts, showing our method's ability to generate high-quality and controllable
compositions without fine-tuning any parameters of the generative model itself.Comment: Project page available at
https://neuraltextualinversion.github.io/NeTI
Redundant Wavelets on Graphs and High Dimensional Data Clouds
In this paper, we propose a new redundant wavelet transform applicable to
scalar functions defined on high dimensional coordinates, weighted graphs and
networks. The proposed transform utilizes the distances between the given data
points. We modify the filter-bank decomposition scheme of the redundant wavelet
transform by adding in each decomposition level linear operators that reorder
the approximation coefficients. These reordering operators are derived by
organizing the tree-node features so as to shorten the path that passes through
these points. We explore the use of the proposed transform to image denoising,
and show that it achieves denoising results that are close to those obtained
with the BM3D algorithm.Comment: 4 pages, 4 figures, 1 table, submitted to IEEE Signal Processing
Letter
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