Get posts starting with a letter

posts.php

<?php

add_filter( 'posts_where', 'start_with_letter', 10, 2 );

function start_with_letter( $where, &$wp_query )
{
    global $wpdb;
    if ( $start_with_letter = $wp_query->get( 'start_with_letter' ) ) {
        $where .= ' AND ' . $wpdb->posts . '.post_title LIKE \'' . esc_sql( like_escape( $start_with_letter ) ) . '%\'';
    }
    return $where;
}

$posts = new WP_Query(
	array(
		"start_with_letter" => "a"
	)
);