$params = [

    'index' => 'soso_*',

    'type' => 'links_1',

    'body' => [

        'query' => [

            'match' => [

                'title' => $kw

            ]

        ],

                'highlight' => [

                    'pre_tags'  => '<em>',

                    'post_tags' => '</em>',

                    'fields'    => [

                       'title' => new \stdClass()

                    ]

                ],

    ]

];