This repository contains all source code used to generate the results in the article Surrogate- and invariance-boosted contrastive learning for data-scarce applications in science.

• The folder generate_datasets contains all numerical programs used to generate the datasets, for both Photonic Crystals (PhC) and the Time-independent Schrodinger Equation (TISE)
• main.py is the main code used to train the neural networks (explained in detail below)

Please install the required Python packages: pip install -r requirements.txt.

A python3 environment can be created prior to this: conda create -n sibcl python=3.8; conda activate sibcl.

For generation of the PhC dataset, assess to MATLAB is required to calculate the density-of-states (DOS) of PhCs.

The PhC datasets used in this work can be downloaded from this link.

Alternatively, to generate (potentially other variants) of the PhC dataset, please refer to relevant code stored in generate_datasets/PhC/ and the descriptions below.

Periodic unit cells are defined using a level set of a Fourier sum; different unit cells can be generated using the get_random() method of the FourierPhC class defined in fourier_phc.py.

To generate the labeled PhC datasets, we first compute their band structures using MPB. This can be executed via:

• Target dataset of random Fourier unit cells:

  python phc_gendata.py --h5filename="mf1-s1" --pol="tm" --nsam=5000 --maxF=1 --seed=1`

• Source dataset of simple cylinders:

  python phc_gencylin.py --h5filename="cylin" --pol="tm" --nsam=10000`

each program will create a dataset with the eigen-frequencies, group velocities, etc, stored in a .h5 file (which can be accessed using the h5py package). We then calculate the DOS using the GRR method provided by the MATLAB code from /boyuanliuoptics/DOS-calculation/. To do so, we first parse the data to create the .txt files required as inputs to the program, compute the DOS using MATLAB and then add the DOS labels back to the original .h5 files. These steps will be executed automatically by simply running the shell script get_DOS.sh and entering the h5 filename identifiers when prompted. Note that for this to run smoothly, python and MATLAB will first need to be added to PATH and the environment with the installed dependencies should be loaded.

Relevant code stored in generate_datasets/TISE/. Example usage:

• To generate target dataset, e.g. in 3D, python tise_gendata.py --h5filename="tise3d" --ndim 3 --nsam 5000.
• To generate low resolution dataset, python tise_gendata.py --h5filename='tise3d_lr' --ndim 3 --nsam 10000 --lowres --orires=32 (--orires defines the resolution of the input to the neural network).
• To generate QHO dataset, python tise_genqho.py --h5filename='tise2d_qho' --ndim 2 --nsam 10000.

Training of the neural networks for all problems introduced in the article (i.e. PhC DOS prediction, PhC Band structure prediction, TISE ground state energy prediction using both low resolution or QHO data as surrogate) can all be executed using main.py by indicating the appropriate flags (see below). This code also allows training via the SIB-CL framework or any of the baselines, again with the use of the appropriate flag. This code also contains other prediction problems not presented in the article, such as predicting higher energy states of TISE, TISE wavefunctions and single band structure.

• --path_to_h5: indicate directory where h5 datasets are located. The h5 filenames defined in the dataset classes in datasets_PhC_SE.py should also be modified according to the names used during dataset generation.

• --predict: defines prediction task. Options: 'DOS', 'bandstructures', 'eigval', 'oneband', 'eigvec'.

• --train: specify if training via SIB-CL or baselines. Options: 'sibcl', 'tl', 'sl', 'ssl' ('ssl' performs regular contrastive learning without surrogate dataset). For invariance-boosted baselines, e.g. TL-I or SL-I, specify 'tl' or 'sl' here and add the relevant invariances flags (see below).

• --iden: required; specify identifier for saving of models, training logs and results.

Invariances flags: --translate_pbc (set this flag to include rolling translations), --pg_uniform (set this flag to uniformly sample the point group symmetry transformations), --scale (set this flag to scale unit cell - used for PhC), --rotate (set this flag to do 4-fold rotations), --flip (set this flag to perform horizontal and vertical mirrors). If --pg_uniform is used, there is no need to include --rotate and --flip.

Other optional flags can be displayed via python main.py --help. Examples of shell scripts can be found in the sh_scripts folder.

By default, running main.py will create 3 subdirectories:

• ./pretrained_models/: state dictionaries of pretrained models at various epochs indicated in the eplist variable will be saved to this directory. These models are used for further fine-tuning.
• ./dicts/: stores the evaluation losses on the test set as dictionaries saved as .json files. The results can then be plotted using plot_results.py.
• ./tlogs/: training curves for pre-training and fine-tuning are stored in dictionaries saved as .json files. The training curves can be plotted using get_training_logs.py. Alternatively, the --log_to_tensorboard flag can be set and training curves can be viewed using tensorboard; in this case, the dictionaries will not be generated.